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Title: Fragments of Earth Lore - Sketches & Addresses Geological and Geographical
Author: Geikie, James
Language: English
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Transcriber's Note
Text emphasis is displayed as _Italics_ and =Bold= respectively.
Superscript characters are denoted with the carat character (i.e., 8^e).
Whole and fractional parts are displayed as 5-1/2.
FRAGMENTS
OF
EARTH LORE
[Illustration: PLATE I
SCOTLAND]
FRAGMENTS
OF
EARTH LORE
SKETCHES & ADDRESSES
Geological and Geographical
BY
JAMES GEIKIE, D.C.L., LL.D., F.R.S., &c.
MURCHISON-PROFESSOR OF GEOLOGY AND MINERALOGY IN THE
UNIVERSITY OF EDINBURGH
FORMERLY OF H.M. GEOLOGICAL SURVEY OF SCOTLAND
WITH MAPS AND ILLUSTRATIONS
EDINBURGH
JOHN BARTHOLOMEW & CO.
LONDON: SIMPKIN, MARSHALL, HAMILTON, KENT & Co., Ltd.
1893
PREFACE.
The articles in this volume deal chiefly with the history of Glacial
times and the origin of surface-features. As they were not written with
any view to their subsequent appearance in a collected form, each is so
far independent and complete in itself. Under these circumstances some
repetition was unavoidable, if the articles were not to be recast, and
I did not think it advisable to make such radical alteration. With the
exception of verbal changes and some excisions, therefore, the papers
remain substantially in their original state. Here and there a footnote
has been added to indicate where the views expressed in the text have
since been modified; but I have not been careful to insert such notes
throughout. Geologists, like other folk, live and learn, and the reader
will probably discover that the opinions set forth in some of the
later articles are occasionally in advance of those maintained in the
writer's earlier days.
I have to thank the Publishers of _Good Words_ for allowing me to
republish the articles on the Cheviot Hills and the Outer Hebrides. My
acknowledgments are also due to Mr. Bartholomew for the excellent maps
with which the volume is so well illustrated.
Edinburgh, _April 5th, 1893_.
LIST OF MAPS.
Plate I. PHYSICAL FEATURES OF SCOTLAND _Frontispiece_
" II. STRUCTURE OF MOUNTAINS 60
" III. PAST AND PRESENT GLACIATION OF THE WORLD 193
" IV. ICE AGE IN NORTHERN EUROPE 324
" V. THE GEOGRAPHICAL EVOLUTION OF CONTINENTS 348
" VI. BATHY-HYPSOMETRICAL MAP, ILLUSTRATING
DEVELOPMENT OF COAST-LINES 428
CONTENTS.
CHAP. PAGE
I. GEOGRAPHY AND GEOLOGY 1
II. THE PHYSICAL FEATURES OF SCOTLAND 14
III. MOUNTAINS: THEIR ORIGIN, GROWTH, AND DECAY 36
IV. THE CHEVIOT HILLS 62
V. THE LONG ISLAND, OR OUTER HEBRIDES 125
VI. THE ICE AGE IN EUROPE AND NORTH AMERICA 160
VII. THE INTERCROSSING OF ERRATICS IN GLACIAL
DEPOSITS 194
VIII. RECENT RESEARCHES IN THE GLACIAL GEOLOGY
OF THE CONTINENT 220
IX. THE GLACIAL PERIOD AND THE EARTH-MOVEMENT
HYPOTHESIS 248
X. THE GLACIAL SUCCESSION IN EUROPE 288
XI. THE GEOGRAPHICAL EVOLUTION OF EUROPE 326
XII. THE EVOLUTION OF CLIMATE 349
XIII. THE SCIENTIFIC RESULTS OF DR. NANSSEN'S
EXPEDITION 382
XIV. THE GEOGRAPHICAL DEVELOPMENT OF COAST-LINES 393
I.
Geography and Geology.[A]
[A] Portion of a lecture given in 1886 to the Class of Geology in the
University of Edinburgh.
The teaching of Geography naturally occupies a prominent place in
every school curriculum. It is rightly considered essential that we
should from an early age begin to know something of our own and other
countries. I am not sure, however, that Geography is always taught in
the most interesting and effective manner. Indeed, according to some
geographers, who are well qualified to express an opinion, the manner
in which their subject is presented in many of our schools leaves much
to be desired. But a decided advance has been made in recent years,
and with the multiplication of excellent text-books, maps, and other
appliances, I have no doubt that this improvement will continue. When
I attended school the text-books used by my teachers were about as
repellent as they could be. Our most important lesson was to commit to
memory a multitude of place-names, and the maps which were supposed
to illustrate the text-books were, if possible, less interesting and
instructive. Nowadays, however, teachers have a number of more or less
excellent manuals at their service, and the educational maps issued by
our cartographers show in many cases a very great advance on the bald
and misleading caricatures which did duty in my young days as pictures
of the earth's surface.
During the progress of some war we often remark that the task of
following the military operations compels us to brush up our Geography.
I am uncharitable enough to suspect that it would frequently be
truer to say that, before these campaigns commenced, we had no such
knowledge to brush up. The countries involved in the commotion were
probably mere names to many of us. We had no immediate interest in
them or their inhabitants, and had we been asked, before the outbreak
of hostilities, to indicate the precise positions of the places upon a
map, some of us perhaps might have been sorely puzzled to do so. Nor
is such ignorance always discreditable. One cannot know everything;
the land-surface of the globe contains upwards of 50 millions of
square miles, and one may surely be excused for not having a detailed
knowledge of this vast area. I have referred to the subject simply
because I think it gives us a hint as to how the teaching of Political
Geography might be made most instructive and interesting. Historical
narrative might often be interwoven with the subject in such a way
as to fix geographical features indelibly on the memory. Striking
and picturesque incidents, eventful wars, the rise and progress of
particular trades, the routes followed by commerce, the immigration
and emigration of races, the gradual development of the existing
political divisions of the Old World, the story of Columbus and the
early voyagers, the geographical discoveries of later times--all these,
and such as these, might be introduced into our lessons in Political
Geography. The wanderings of a Mungo Park, a Bruce, a Livingstone, a
Stanley, traced on a good map, could not fail to arrest the attention
of the youthful student of African geography. In like manner, the
campaigns of the great Napoleon might be made to do good service
in illustrating the geographical features of large portions of our
own continent. Then, as regards Britain, what a world of poetry and
romantic story clings to every portion of its surface--why, the very
place-names themselves might suggest to any intelligent teacher themes
and incidents, the deft treatment of which would make the acquisition
of Geography a delightful task to the dullest boy or girl.
The intimate relation that obtains between Political Geography and
History has indeed long been recognised, and is in fact self-evident.
And we are all well aware that in our school manuals of Geography it
has been usual for very many years to note the scenes of remarkable
events. Such notes, however, are of necessity extremely brief; and it
need hardly be said that to fully incorporate history in a text-book of
general Geography would be quite impracticable. It might be done to a
certain extent for our own and a few of the more important countries;
but similar detail need not be attempted in regard to regions which are
of less consequence from the political point of view. Indeed, I should
be inclined to leave the proper application of historical knowledge in
the teaching of Geography very much to the teacher himself, who would
naturally select such themes and incidents as seemed best adapted to
attract the attention of his pupils. Be that, however, as it may,
it is enough for my present purpose if I insist upon the fact that
the proper study of Political Geography involves the acquisition of
some historical knowledge. One can hardly conceive the possibility
of an intelligent student taking pains to become acquainted with the
political geography of a country without at the same time endeavouring
to learn something of its history--otherwise, his geographical
attainments would hardly surpass those of a commercial traveller, whose
geographical studies have been confined to the maps and tables of his
Bradshaw.
But if it be impossible to ignore History in the teaching of Political
Geography, it is just as impossible to exclude from our attention
great physical features and characteristics. Surface-configuration,
climate, and natural products all claim our attention. It is obvious,
in fact, that the proper study of Political Geography must give us
at least a general notion of the configuration, the river-systems,
and climatic conditions of many different lands. For has not the
political development of races depended most largely on the physical
conditions and natural resources of the countries occupied by them?
So far, then, as these have sensibly influenced the progress of
peoples, they come naturally under the consideration of Political
Geography. Thus, if Political Geography be closely connected and
interwoven, as it were, with History, not less intimate are its
relations to Physical Geography. It does not embrace all Physical
Geography, but it introduces us to many facts and phenomena, the
causes and mutual relations of which we cannot understand without
first mastering the teachings of Physical Geography. In the study of
this latter science we come more closely into contact with Nature;
we cease to think of the surface of the earth as parcelled out into
so many lots by its human occupants--we no longer contemplate that
surface from the limited point of view of the political geographer--we
are now not merely members of one particular community, but have
become true citizens of the world. To us north and south, east and
west are of equal interest and importance. Our desire now is to
understand, if haply we may, the complex system of which we ourselves
form a part. The distribution of land and water--the configuration of
continental areas and oceanic basins--the circulation of oceanic and
terrestrial waters--earth-movements and volcanoes--ice-formations--the
atmosphere--climatology--the geographical distribution of plants
and animals--in a word, _the world as one organic whole_ now forms
the subject of our contemplation. Such being the scope of Physical
Geography, it is satisfactory to know that its importance as a subject
of study in our schools has been fully recognised. This being admitted,
I shall now proceed to show that Physical Geography, although, like
Political Geography, it is a separate and distinct subject, yet, just
as the study of the latter involves some knowledge of History, so
the prosecution of Physical Geography compels us to make a certain
acquaintance with Geology. We cannot, in fact, learn much about the
atmosphere, about rain and rivers, glaciers and icebergs, earthquakes
and volcanoes, and the causes of climate, without at the same time
becoming more or less familiar with the groundwork on which geological
investigations are based. And just as a knowledge of history enables
us better to understand the facts of Political Geography, so some
acquaintance with the results of geological inquiry are necessary
before we can hope to comprehend many of the phenomena of which
Physical Geography treats. Let me try to make this plain. The physical
geographer, we shall suppose, is considering the subject of terrestrial
waters. He tells us what is meant by the drainage-system of a country,
points out how the various minor water-courses or brooks and streams
unite to form a river, describes for us the shape of the valley through
which a typical river makes its way--how the valley-slope diminishes
from the mountains onwards to the sea-coast--how, at first, in its
upper or mountain-track, the flow of the river is torrential--how,
as the slope of the valley decreases, the river begins to wind about
more freely, until it reaches the head of its plain-track or delta,
when, no longer receiving affluents, it begins to divide, and enters
the sea at last by many mouths. He tells us further what proportion of
the rainfall of the country passes seawards in our river, and he can
measure for us the quantity of water which is actually discharged. All
this is purely Physical Geography; but when we come to ask why some
rivers flow in deep cañons, like those of the Colorado--why valleys
should widen out in one part and contract, as it were, elsewhere--why
the courses of some rivers are interrupted by waterfalls and rapids,
and many other similar questions, the physical geographer must know
something of Geology before he can give an answer. He can describe
the actual existing conditions; without the aid of Geology, he can
tell us nothing of their origin and cause. So the political geographer
can map out for us the present limits of the various countries of
Europe, but History must be invoked if we would know how those
boundaries came to be determined. The moment, therefore, the physical
geographer begins to inquire into the origin of any particular
physical feature, he enters upon the domains of the geologist. And as
he cannot possibly avoid doing so, it is quite common now to find a
good deal of the subject-matter of Geology treated of in text-books
of Physical Geography. I state this merely to show how very closely
the two sciences are interlocked. Take, for example, the configuration
of river valleys just referred to. The physical geographer recognises
the fact that a river performs work; by means of the sediment which
it carries in suspension and rolls along its course, it erodes its
bed in many places, and undermines its banks, and thus its channel is
deepened and widened. He can measure the amount of sediment which it
carries down to the sea, and the quantity of saline matters which its
waters hold in solution: and knowing that all these substances have
been abstracted from the land, he is able to estimate approximately
the amount of material which is annually transferred from the surface
of the drainage-area involved. He discovers this to be so relatively
enormous that he has no difficulty in believing that the valleys
in which rivers flow might have been hollowed out by the rivers
themselves. But, without trespassing further into the geologist's
domains, he cannot go beyond this: and you will at once perceive that
something more is required to prove that any particular valley owes its
origin to the erosive action of running water. Suppose someone were
to suggest to him that his river-valley might be a minor wrinkle in
the earth's crust caused by earth-movements, or that it might indicate
the line of a fissure or dislocation, due to some comparatively recent
convulsion--how could his computation of the amount of material at
present carried seawards by the river prove such suggestions to be
erroneous? And what light could it throw upon the origin of the
varied configuration of the river-valley--how would it explain the
presence or absence of cascades and rapids, of narrow gorges and open
expanses? None of these phenomena can be interpreted and accounted
for without the aid of the geologist: without some knowledge of rocks
and rock-structures, the origin of the earth's surface-features
is quite inexplicable. To give an adequate explanation of all the
surface-features of a country in detail would of course require a
profound study of Geology; but a general acquaintance only with its
elementary facts is quite sufficient to enable us to form a reasonable
and intelligent view of the cause and origin of the main features
of the land as a whole. Thus a few lessons in elementary Geology
would make clear to any child how rivers have excavated valleys, why
cataracts and gorges occur here, and open valleys with gently-flowing
waters elsewhere.
Let me select yet another example to show how dependent Physical
Geography is upon Geology. The physical geographer, in describing the
features of the land, tells us how the great continental areas are
traversed in various directions by what he calls mountain-chains. Thus,
in speaking of America, he tells us that it may be taken as a type
of the continental structure--namely a vast expanse of land, low or
basin-like in the interior, and flanked along the maritime regions by
elevated mountain borders--the highest border facing the deepest ocean.
He points out further that the great continental areas are crossed from
west to east by well-marked depressions, to a large extent occupied
by water. Thus Europe is separated from Africa by the Mediterranean,
a depression which is continued eastward through the Black Sea into
the Aralo-Caspian area. South America is all but cut away from North
America, while Australia is separated from Asia by the East India Seas.
We find, in fact, all over the world that well-marked natural features
are constantly being repeated. Not only do the great land-masses
of the globe bear certain resemblances to each other, but even in
their detailed structure similar parallelisms recur. The physical
geographer notes all these remarkable phenomena, but he can give us
no clue to their meaning. He may describe with admirable skill the
characteristic features of plains and plateaux, of volcanic mountains
and mountain-chains, but he cannot tell us why plains should occur
here and mountains there; nor can he explain why some mountains, such
as those of Scotland or Norway, differ so much in configuration from
the Alps and the Pyrenees. The answer to all these questions can only
be given by Geology. It is from this science we learn how continental
areas and oceanic basins have been evolved. The patient study of the
rocks has revealed the origin of the present configuration of the
land. There is not a hill or valley, not a plateau or mountain-region,
which does not reveal its own history. The geologist can tell you why
continents are bordered by coast-ranges, and why their interiors are
generally comparatively low and basin-shaped. The oceanic basins and
continental areas, we learn, are primeval wrinkles in the earth's
crust, caused by its irregular subsidence upon the gradually cooling
and contracting nucleus. The continents are immense plateau-like areas
rising more or less abruptly above those stupendous depressions
of the earth's crust which are occupied by the ocean. While those
depressions are in progress the maritime borders of the land-areas are
subjected to enormous squeezing and crushing, and coast-ranges are the
result--the elevation of those ranges necessarily holding some relation
to the depth of the contiguous ocean. For, the deeper the ocean the
greater has been the depression under the sea, and, consequently,
the more intense the upheaval along the continental borders. It is
for the same reason that destructive earthquakes are most likely to
occur in the vicinity of coast-ranges which are of comparatively
recent geological age. These, and indeed all, mountains of elevation
are lines of weakness along which earth-movements may continue from
time to time to take place. But all mountains are not mountains of
elevation; many elevated regions owe their mountainous character simply
to the erosive action of sub-aërial agents, such as rain, frost, ice,
and running water, the forms assumed by the mountains being due to
their petrological character and geological structure. There are,
for example, no true mountains of elevation in Scotland; hence to
write of the _chain of the Grampians_ or the _range of the Lowthers_
is incorrect and actually misleading. Without the aid of Geology the
geographer cannot, in fact, discriminate between mountains of elevation
and mountains of denudation; hence geographical terms so constantly in
use as _mountain-range_ and _mountain-chain_ are very often applied by
writers, ignorant of geological structure, to elevated regions which
have no claim to be described either as _chains_ or _ranges_. Some
knowledge of Geology, therefore, is essential to us if we would have
correct views of many of the grandest features of the globe. But it
will be said that, after all, the physical geographer deals with the
earth as we now find it; he does not need to trouble himself with the
origin of the phenomena he describes. Well, as I have just shown, he
cannot, even if he would, escape trenching on Geology; and if he could,
his subject would be shorn of much of its interest. He recognises that
the world he studies has in it the elements of change--the forces of
Nature are everywhere modifying the earth's surface--considerable
changes are sometimes brought about even in one's lifetime, while
within the course of historical ages still greater mutations have
taken place--he becomes conscious, in short, that the existing state
of things is but the latest phase of an interminable series of changes
stretching back into the illimitable past, and destined to be prolonged
into the indefinite future. Thus he gladly welcomes the labours of the
geologist, whose researches into the past have thrown such a flood of
light upon the present. In fact, he can no more divorce his attention
from the results of geological inquiry than the political geographer
can shut his eyes to the facts of History.
Let me, in conclusion, give one further illustration of the close
inter-dependence of the two sciences of which I am speaking. One of the
subjects treated of by Physical Geography is the present geographical
distribution of plants and animals. The land-surface of the globe has
been mapped out into so many biological regions, each of which is
characterised by its special fauna and flora. The greatest changes in
the flora and fauna of a continent are met with as we pass from south
to north, or _vice versa_. Proceeding in the direction of the latitude,
the changes encountered are much less striking. Now, these facts are
readily explained by the physical geographer, who points out that
the distribution is due chiefly to climatic conditions--a conclusion
which is obvious enough. But when we go into details we find that mere
latitude will not account for all the phenomena. Take, for example,
the case of the Scandinavian flora of our own Continent. It is true
that this flora is largely confined to northern latitudes; but isolated
colonies occur in our own mountains and in the mountains of middle
and southern Europe. How are these to be accounted for? The physical
geographer says that the plants grow there simply because they obtain
at high levels in low latitudes the favourable climatic conditions
underneath which they flourish at low levels in high latitudes. He
therefore concludes that the distribution of life-forms is due to
varying climatic and physical conditions. But if we ask him how those
curious colonies of foreigners come to be planted on our mountains,
he cannot tell. To get our answer we must come to the geologist; and
he will explain that they are, as it were, living fossils--monuments
of former great physical and climatic changes. He will prove to us
that the climate of Europe was at a recent geological period so cold
that the Scandinavian flora spread south into middle Europe, where it
occupied the low grounds. When the climate became milder, then the
northern invaders gradually retired--the main body migrating back
to the north--while some stragglers, retreating before the stronger
Germanic flora, took shelter in the mountains, whither the latter could
not or would not follow, and so there our Scandinavians remain, the
silent witnesses of a stupendous climatic revolution. Now, all the
world over, plants and animals have similar wonderful tales to tell of
former geographical changes. The flora and fauna of our country, for
example, prove that the British Islands formed part of the Continent at
a very recent geological period; and so, from similar evidence, we know
that not long ago Europe was joined on to Africa. On the other hand,
the facts connected with the present distribution of life demonstrate
that some areas, such as Australia, have been separated from the
nearest continental land for vastly prolonged periods of time.
It would be a very easy matter to adduce many further illustrations to
show how close is the connection between the studies of the physical
geographer and the geologist. I do not indeed exaggerate when I say
that no one can hope to become a geologist who is not well versed in
Physical Geography; nor, on the other hand, can the physical geographer
possibly dispense with the aid of Geology. The two subjects are as
closely related and interwoven, the one with the other, as History is
with Political Geography. I do not see therefore how educationists who
have admitted the great importance of Physical Geography as a branch
of general education, can logically exclude Geology as a subject of
instruction in schools. Already, indeed, it has been introduced by many
teachers, and I am confident that ere long it will be as generally
taught as Physical Geography. I would not, however, present the subject
to young people as a lesson to be learned from books. A good teacher
should be able to dispense with these helps, or rather hindrances--for
such they really are to a young beginner. His pupils ought to have
previously studied the subject of Physical Geography, and if they have
been well taught they ought to have already acquired no mean store of
geological knowledge. They ought, in fact, to have learned a good deal
about the great forces which are continually modifying the surface of
the globe, and what they have now to do is to study more particularly
the results which have followed from the constant operation of
those forces. We shall suppose, for example, that the teacher has
described how rivers erode their channels, and waves tend to cut back
a coast-line, and how the products of erosion, consisting of gravel,
sand, and mud, are distributed along river-valleys and accumulated in
lakes and seas. He now exhibits to his class good-sized fragments of
conglomerate, sandstone, and shale, and points out how each of these
rocks is of essentially the same character, and must therefore have
had the same origin, as modern sedimentary accumulations. His pupils
should be encouraged to examine the rocks of their own neighbourhood,
whether exhibited in natural sections or artificial exposures, and to
compare these with the products of modern geological action. One hour's
instruction in the field is, in fact, worth twenty hours of reading
or listening to lectures. Knowledge at first hand is what is wanted.
There are many excellent popular or elementary treatises dealing with
Historical Geology, and these have their uses, and may be read with
profit as well as pleasure. But the mere reading of such books, it is
needless to say, will never make us geologists. They help no doubt to
store the mind with interesting and entertaining knowledge, but they do
not cultivate the faculties of observation and reasoning. And unless
geology is so taught as to accomplish this result, I do not see why it
should enter into any school curriculum. Further, I would remark that,
however interesting a geological treatise may be, it cannot possibly
stimulate the imagination as the practical study of the science
is bound to do. One may put into the hands of a youth a clear and
well-written description of some particular fossiliferous limestone,
and he may by dint of slavish toil be able to repeat verbatim all
that he has read. That is how a good deal of book-knowledge of science
is acquired. Only think, however, of the drudgery it involves--the
absolute waste of time and energy. But let us illustrate our lesson
by means of a lump of the limestone itself; let us show him the
character of the rock and the nature of its fossil contents, and his
difficulties disappear. Better still--let us take him, if we can, into
a limestone quarry, and he will be a dull boy indeed if he fails fully
to understand what limestone is, or to realise the fact that the rock
he is looking at accumulated slowly, like existing oceanic formations,
at the bottom of a sea that teemed with animal life. It is unnecessary,
however, that I should illustrate this subject further. I would only
repeat that the beginner should be taught from the very first to use
his own eyes, and to draw logical conclusions from the facts which
he observes. Trained after this manner, he would acquire, not only a
precise and definite knowledge of what geological data really are,
but he would learn also how to interpret those data. He would become
familiar, in fact, with the guiding principles of geological inquiry.
How much or how little of Historical Geology should be given in schools
will depend upon circumstances. Great care, however, should be taken to
avoid wearying the youthful student with strings of mere names. What
good is gained by learning to repeat the names of fifty or a hundred
fossils, if you cannot recognise any one of these when it is put into
your hand? With young beginners I should not attempt anything of that
kind. If the neighbourhood chanced to be rich in fossils, I should
take my pupils out on Saturday to the sections where they were found,
and let them ply their hammers and collect specimens for themselves. I
should describe no fossils which they had not seen and handled. Of the
more remarkable forms of extinct animals and plants, which are often
represented by only fragmentary remains, I should exhibit drawings
showing the creatures as they have been restored by the labours of
comparative anatomists. Such restorations and ideal views of geological
scenes like those given by Heer, Dana, Saporta, and others, convey far
more vivid impressions of the life of a geological period than the
most elaborate description. In fine, the story of our earth should be
told much in the same manner as Scott wrote the history of Scotland
for his grandson. There is no more reason for requiring the juvenile
student to drudge through minute geological data before introducing him
to the grand results of geological investigation, than there is for
compelling him to study the manuscripts in our Record Offices before
allowing him to read the history which has been drawn from these and
similar sources of information. It is enough if at the beginning of
his studies he has already learned the general nature of geological
evidence and the method of its interpretation. Provided with such a
stock of geological knowledge as I have indicated, our youth would
leave school with some intelligent appreciation of existing physical
conditions, and a not inadequate conception of world-history.
II.
The Physical Features of Scotland.[B]
[B] _Scottish Geographical Magazine_, vol. i., 1885.
Scotland, like "all Gaul," is divided into three parts, namely, the
Highlands, the Central Lowlands, and the Southern Uplands. These, as a
correctly drawn map will show, are natural divisions, for they are in
accordance not only with the actual configuration of the surface, but
with the geological structure of the country. The boundaries of these
principal districts are well defined. Thus, an approximately straight
or gently undulating line taken from Stonehaven, in a south-west
direction, along the northern outskirts of Strathmore to Glen Artney,
and thence through the lower reaches of Loch Lomond to the Firth of
Clyde at Kilcreggan, marks out with precision the southern limits of
the Highland area and the northern boundary of the Central Lowlands.
The line that separates the Central Lowlands from the Southern Uplands
is hardly so prominently marked throughout its entire course, but it
follows precisely the same north-east and south-west trend, and may
be traced from Dunbar along the base of the Lammermoor and Moorfoot
Hills, the Lowthers, and the hills of Galloway and Carrick, to Girvan.
In each of the two mountain-tracts--the Highlands and the Southern
Uplands--areas of low-lying land occur, while in the intermediate
Central Lowlands isolated prominences and certain well-defined belts
of hilly ground make their appearance. The statement, so frequently
repeated in class-books and manuals of geography, that the mountains of
Scotland consist of three (some writers say five) "ranges" is erroneous
and misleading. The original author of this strange statement probably
derived his ignorance of the physical features of the country from
a study of those antiquated maps upon which the mountains of poor
Scotland are represented as sprawling and wriggling about like so many
inebriated centipedes and convulsed caterpillars. Properly speaking,
there is not a true mountain-range in the country. If we take this
term, which has been very loosely used, to signify a linear belt of
mountains--that is, an elevated ridge notched by cols or "passes"
and traversed by transverse valleys--then in place of "three" or
"five" such ranges we might just as well enumerate fifty or sixty, or
more, in the Highlands and Southern Uplands. Or, should any number
of such dominant ridges be included under the term "mountain-range,"
there seems no reason why all the mountains of the country should
not be massed under one head and styled the "Scottish Range." A
mountain-range, properly so called, is a belt of high ground which has
been ridged up by earth-movements. It is a fold, pucker, or wrinkle
in the earth's crust, and its general external form coincides more or
less closely with the structure or arrangement of the rock-masses of
which it is composed. A mountain-range of this characteristic type,
however, seldom occurs singly, but is usually associated with other
parallel ranges of the same kind--the whole forming together what
is called a "mountain-chain," of which the Alps may be taken as an
example. That chain consists of a vast succession of various kinds of
rocks, which at one time were disposed in horizontal layers or strata.
But during subsequent earth-movements those horizontal beds were
compressed laterally, squeezed, crumpled, contorted, and thrown, as it
were, into gigantic undulations and sharper folds and plications. And,
notwithstanding the enormous erosion or denudation to which the long
parallel ridges or ranges have been subjected, we can yet see that the
general contour of these corresponds in large measure to the plications
or foldings of the strata. This is well shown in the Jura, the parallel
ranges and intermediate hollows of which are formed by undulations of
the folded strata--the tops of the long hills coinciding more or less
closely with the arches, and the intervening hollows with the troughs.
Now folded, crumpled, and contorted rock-masses are common enough in
the mountainous parts of Scotland, but the configuration of the surface
rarely or never coincides with the inclination of the underlying
strata. The mountain-crests, so far from being formed by the tops of
great folds of the strata, frequently show precisely the opposite kind
of structure. In other words, the rocks, instead of being inclined away
from the hill-tops like the roof of a house from its central ridge,
often dip into the mountains. When they do so on opposite sides the
strata of which the mountains are built up seem arranged like a pile of
saucers, one within another.
There is yet another feature which brings out clearly the fact that the
slopes of the surface have not been determined by the inclination of
the strata. The main water-parting that separates the drainage-system
of the west from that of the east of Scotland does not coincide with
any axis of elevation. It is not formed by an anticlinal fold or
"saddleback." In point of fact it traverses the strata at all angles
to their inclination. But this would not have been the case had the
Scottish mountains consisted of a chain of true mountain-ranges. Our
mountains, therefore, are merely monuments of denudation, they are
the relics of elevated plateaux which have been deeply furrowed and
trenched by running water and other agents of erosion. A short sketch
of the leading features presented by the three divisions of the country
will serve to make this plain.
* * * * *
The Highlands.--The southern boundary of this, the most extensive of
the three divisions, has already been defined. The straightness of
that boundary is due to the fact that it coincides with a great line
of fracture of the earth's crust--on the north or Highland side of
which occur slates, schists, and various other hard and tough rocks,
while on the south side the prevailing strata are sandstones, etc.,
which are not of so durable a character. The latter, in consequence
of the comparative ease with which they yield to the attacks of the
eroding agents--rain and rivers, frost and ice--have been worn away to
a greater extent than the former, and hence the Highlands, along their
southern margin, abut more or less abruptly upon the Lowlands. Looking
across Strathmore from the Sidlaws or the Ochils, the mountains seem
to spring suddenly from the low grounds at their base, and to extend
north-east and south-west, as a great wall-like rampart. The whole area
north and west of this line may be said to be mountainous, its average
elevation being probably not less than 1500 feet above the sea.
A glance at the contoured or the shaded sheets of the Ordnance Survey's
map of Scotland will show better than any verbal description the
manner in which our Highland mountains are grouped. It will be at once
seen that to apply the term "range" to any particular area of those
high grounds is simply a misuse of terms. Not only are the mountains
not formed by plications and folds, but they do not even trend in
linear directions. It is true that a well-trained eye can detect
certain differences in the form and often in the colouring of the
mountains when these are traversed from south-east to north-west. Such
differences correspond to changes in the composition and structure of
the rock-masses, which are disposed or arranged in a series of broad
belts and narrower bands, running from south-west to north-east across
the whole breadth of the Highlands. Each particular kind of rock gives
rise to a special configuration, or to certain characteristic features.
Thus, the mountains that occur within a belt of slate, often show a
sharply cut outline, with more or less pointed peaks and somewhat
serrated ridges--the Aberuchill Hills, near Comrie, are an example. In
regions of gneiss and granite the mountains are usually rounded and
lumpy in form. Amongst the schists, again, the outlines are generally
more angular. Quartz-rock often shows peaked and jagged outlines;
while each variety of rock has its own particular colour, and this in
certain states of the atmosphere is very marked. The mode in which the
various rocks yield to the "weather"--the forms of their cliffs and
corries--these and many other features strike a geologist at once; and
therefore, if we are to subdivide the Highland mountains into "ranges,"
a geological classification seems the only natural arrangement that can
be followed. Unfortunately, however, our geological lines, separating
one belt or "range" from another, often run across the very heart
of great mountain-masses. Our "ranges" are distinguished from each
other simply by superficial differences of feature and structure. No
long parallel hollows separate a "range" of schist-mountains from
the succeeding "ranges" of quartz-rock, gneiss, or granite. And no
degree of careful contouring could succeed in expressing the niceties
of configuration just referred to, unless the maps were on a very
large scale indeed. A geological classification or grouping of the
mountains into linear belts cannot therefore be shown upon any ordinary
orographical map. Such a map can present only the relative heights
and disposition of the mountain-masses, and these last, in the case
of the Highlands, as we have seen, cannot be called "ranges" without
straining the use of that term. Any wide tract of the Highlands, when
viewed from a commanding position, looks like a tumbled ocean in
which the waves appear to be moving in all directions. One is also
impressed with the fact that the undulations of the surface, however
interrupted they may be, are broad--the mountains, however they may
vary in detail according to the character of the rocks, are massive,
and generally round-shouldered and often somewhat flat-topped, while
there is no great disparity of height amongst the dominant points of
any individual group. Let us take, for example, the knot of mountains
between Loch Maree and Loch Torridon. There we have a cluster of eight
pyramidal mountain-masses, the summits of which do not differ much in
elevation. Thus in Liathach two points reach 3358 feet and 3486 feet;
in Beinn Alligin there are also two points reaching 3021 feet and
3232 feet respectively; in Beinn Dearg we have a height of 2995 feet;
in Beinn Eighe are three dominant points--3188 feet, 3217 feet, and
3309 feet. The four pyramids to the north are somewhat lower--their
elevations being 2860 feet, 2801 feet, 2370 feet, and 2892 feet. The
mountains of Lochaber and the Monadhliath Mountains exhibit similar
relationships; and the same holds good with all the mountain-masses of
the Highlands. No geologist can doubt that such relationship is the
result of denudation. The mountains are monuments of erosion--they
are the wreck of an old table-land--the upper surface and original
inclination of which are approximately indicated by the summits of the
various mountain-masses and the direction of the principal water-flows.
If we in imagination fill up the valleys with the rock-material which
formerly occupied their place, we shall in some measure restore the
general aspect of the Highland area before its mountains began to be
shaped out by Nature's saws and chisels.
It will be observed that while streams descend from the various
mountains to every point in the compass, their courses having often
been determined by geological structure, etc., their waters yet tend
eventually to collect and flow as large rivers in certain definite
directions. These large rivers flow in the direction of the average
slope of the ancient table-land, while the main water-partings that
separate the more extensive drainage-areas of the country mark out,
in like manner, the dominant portions of the same old land-surface.
The water-parting of the North-west Highlands runs nearly north and
south, keeping quite close to the western shore, so that nearly all the
drainage of that region flows inland. The general inclination of the
North-west Highlands is therefore easterly towards Glenmore and the
Moray Firth. In the region lying east of Glenmore the average slopes of
the land are indicated by the directions of the rivers Spey, Don, and
Tay. These two regions--the North-west and South-east Highlands--are
clearly separated by the remarkable depression of Glenmore, which
extends through Loch Linnhe, Loch Lochy, and Loch Ness, and the further
extension of which towards the north-east is indicated by the straight
coast-line of the Moray Firth as far as Tarbat Ness. Now, this long
depression marks a line of fracture and displacement of very great
geological antiquity. The old plateau of the Highlands was fissured and
split in two--that portion which lay to the north-west sinking along
the line of fissure to a great but at present unascertained depth. Thus
the waters that flowed down the slopes of the north-west portion of the
broken plateau were dammed by the long wall of rock on the "up-cast,"
or south-east side of the fissure, and compelled to flow off to
north-east and south-west along the line of breakage. The erosion thus
induced sufficed in the course of time to hollow out Glenmore and all
the mountain-valleys that open upon it from the west.
The inclination of that portion of the fissured plateau which lay to
the south-east is indicated, as already remarked, by the trend of
the principal rivers. It was north-east in the Spey district, nearly
due east in the area drained by the Don, east and south-east in that
traversed by the Tay and its affluents, westerly and south-westerly in
the district lying east of Loch Linnhe.[C] Thus, a line drawn from Ben
Nevis through the Cairngorm and Ben Muich Dhui Mountains to Kinnaird
Point passes through the highest land in the South-east Highlands, and
probably indicates approximately the dominant portion of the ancient
plateau. North of that line the drainage is towards the Moray Firth;
east of it the rivers discharge to the North Sea; while an irregular
winding line, drawn from Ben Nevis eastward through the Moor of Rannoch
and southward to Ben Lomond, forms the water-parting between the North
Sea and the Atlantic, and doubtless marks another dominant area of the
old table-land.
[C] The geological reader hardly requires to be reminded that many
of the minor streams would have their courses determined, or greatly
modified, by the geological structure of the ground. Thus, such
streams often flow along the "strike" and other "lines of weakness,"
and similar causes, doubtless, influenced the main rivers during the
gradual excavation of their valleys.
That the valleys which discharge their water-flow north and east
to the Moray Firth and the North Sea have been excavated by rivers
and the allied agents of erosion, is sufficiently evident. All the
large rivers of that wide region are typical. They show the orthodox
three courses--namely, a torrential or mountain-track, a middle or
valley-track, and a lower or plain-track. The same is the case with
some of the rivers that flow east from the great north-and-south
water-parting of the North-west Highlands, as, for example, those that
enter the heads of Beauly Firth, Cromarty Firth, and Dornoch Firth.
Those, however, which descend to Loch Lochy and Loch Linnhe, and the
sea-lochs of Argyllshire, have no lower or plain-track. When we
cross the north-and-south water-parting of the North-west Highlands,
we find that many of the streams are destitute of even a middle or
valley-track. The majority are mere mountain-torrents when they reach
the sea. Again, on the eastern watershed of the same region, a large
number of the valleys contain lakes in their upper and middle reaches,
and this is the case also with not a few of the valleys that open upon
the Atlantic. More frequently, however, the waters flowing west pass
through no lakes, but enter the sea at the heads of long sea-lochs or
fiords. This striking contrast between the east and west is not due to
any difference in the origin of the valleys. The western valleys are as
much the result of erosion as those of the east. The present contrast,
in fact, is more apparent than real, and arises from the fact that
the land area on the Atlantic side has been greatly reduced in extent
by subsidence. The western fiords are merely submerged land-valleys.
Formerly the Inner and Outer Hebrides were united to themselves and the
mainland, the country of which they formed a part stretching west into
the Atlantic, as far probably as the present 100 fathoms line. Were
that drowned land to be re-elevated, each of the great sea-lochs would
appear as a deep mountain-valley containing one or more lake-basins of
precisely the same character as those that occur in so many valleys
on the eastern watershed. Thus we must consider all the islands lying
off the west coast of the Highlands, including the major portions of
Arran and Bute, as forming part and parcel of the Highland division of
Scotland. The presence of the sea is a mere accident; the old lands
now submerged were above its level during a very recent geological
period--a period well within the lifetime of the existing fauna and
flora.
The old table-land of which the Highlands and Islands are the denuded
and unsubmerged relics, is of vast geological antiquity. It was
certainly in existence, and had even undergone very considerable
erosion, before the Old Red Sandstone period, as is proved by the
fact that large tracts of the Old Red Sandstone formation are found
occupying hollows in its surface. Glenmore had already been excavated
when the conglomerates of the Old Red Sandstone began to be laid
down. Some of the low-lying maritime tracts of the Highland area in
Caithness, and the borders of the Moray Firth, are covered with the
sandstones of that age; and there is evidence to show that these
strata formerly extended over wide regions, from which they have since
been removed by erosion. The fact that the Old Red Sandstone deposits
still occupy such extensive areas in the north-east of the mainland,
and in Orkney, shows that the old table-land shelved away gradually
to north and east, and the same conclusion may be drawn, as we have
seen, from the direction followed by the main lines of the existing
drainage-system. We see, in short, in the table-land of the Highlands,
one of the oldest elevated regions of Europe--a region which has been
again and again submerged either in whole or in part, and covered with
the deposits of ancient seas and lakes, only to be re-elevated, time
after time, and thus to have those deposits in large measure swept away
from its surface by the long-continued action of running water and
other agents of denudation.
* * * * *
The Central Lowlands.--The belt of low-lying ground that separates the
Highlands from the Southern Uplands is, as we have seen, very well
defined. In many places the Uplands rise along its southern margin as
abruptly as the Highlands in the north. The southern margin coincides,
in fact, for a considerable distance (from Girvan to the base of the
Moorfoots) with a great fracture that runs in the same direction as the
bounding fracture or fault of the Highlands. The Central Lowlands may
be described, in a word, as a broad depression between two table-lands.
A glance at the map will show that the principal features of the
Lowlands have a north-easterly trend--the same trend, in fact, as the
bounding lines of the division. To this arrangement there are some
exceptions, the principal being the belt of hilly ground that extends
from the neighbourhood of Paisley, south-east through the borders of
Renfrewshire and Ayrshire, to the vicinity of Muirkirk. The major part
of the Lowlands is under 500 feet in height, but some considerable
portions exceed an elevation of 1000 feet, while here and there the
hills approach a height of 2000 feet--the two highest points (2352 and
2335 feet) being attained in Ben Cleugh, one of the Ochils, and in
Tinto. Probably the average elevation of the Lowland division does not
exceed 350 or 400 feet. Speaking generally, the belts of hilly ground,
and the more or less isolated prominences, are formed of more durable
rocks than are met with in the adjacent lower-lying tracts. Thus the
Sidlaws, the Ochil Hills, and the heights in Renfrewshire and Ayrshire,
are composed chiefly of more or less hard and tough volcanic rocks; and
when sandstones enter into the formation of a line of hills, as in the
Sidlaws, they generally owe their preservation to the presence of the
volcanic rocks with which they are associated. This is well illustrated
by the Lomond Hills in Fifeshire, the basal and larger portion of which
consists chiefly of somewhat soft sandstones, which have been protected
from erosion by an overlying sheet of hard basalt-rock. All the
isolated hills in the basin of the Forth are formed of knobs, bosses,
and sheets of various kinds of igneous rock, which are more durable
than the sandstones, shales, and other sedimentary strata by which they
are surrounded. Hence it is very evident that the configuration of the
Lowland tracts of Central Scotland is due to denudation. The softer
and more readily disintegrated rocks have been worn away to a greater
extent than the harder and less yielding masses.
Only in a few cases do the slopes of the hill-belts coincide with folds
of the strata. Thus, the northern flanks of the Sidlaws and the Ochils
slope towards the north-west, and this also is the general inclination
of the old lavas and other rocks of which those hills are composed.
The southern flanks of the same hill-belt slope in Fifeshire towards
the south-east--this being also the dip or inclination of the rocks.
The crest of the Ochils coincides, therefore, more or less closely,
with an anticlinal arch or fold of the strata. But when we follow the
axis of this arch towards the north-east into the Sidlaws, we find it
broken through by the Tay valley--the axial line running down through
the Carse of Gowrie to the north of Dundee. From the fact that many
similar anticlinal axes occur throughout the Lowlands, which yet give
rise to no corresponding features at the surface, we may conclude that
the partial preservation of the anticline of the Ochils and Sidlaws is
simply owing to the greater durability of the materials of which those
hills consist. Had the arch been composed of sandstones and shales it
would most probably have given rise to no such prominent features as
are now visible.
Another hilly belt, which at first sight appears to correspond roughly
to an anticlinal axis, is that broad tract of igneous rocks which
separates the Kilmarnock coal-field from the coal-fields of the Clyde
basin. But although the old lavas of that hilly tract slope north-east
and south-west, with the same general inclination as the surface, yet
examination shows that the hills do not form a true anticline. They
are built up of a great variety of ancient lavas and fragmental tuffs
or "ashes," which are inclined in many different directions. In short,
we have in those hills the degraded and sorely denuded fragments of an
ancient volcanic bank, formed by eruptions that began upon the bottom
of a shallow sea in early Carboniferous times, and subsequently became
sub-aërial. And there is evidence to show that after the eruptions
ceased the volcanic bank was slowly submerged, and eventually buried
beneath the accumulating sediments of later Carboniferous times.
The exposure of the ancient volcanic bank at the surface has been
accomplished by the denudation of the stratified masses which formerly
covered it, and its existence as a dominant elevation at the present
day is solely due to the fact that it is built up of more resistant
materials than occur in the adjacent low-lying areas. The Ochils and
the Sidlaws are of greater antiquity, but have a somewhat similar
history. Into this, however, it is not necessary to go.
The principal hills of the Lowlands form two interrupted belts,
extending north-east and south-west, one of them, which we may call the
Northern Heights, facing the Highlands, and the other, which may in
like manner be termed the Southern Heights, flanking the great Uplands
of the south. The former of these two belts is represented by the
Garvock Hills, lying between Stonehaven and the valley of the North
Esk; the Sidlaws, extending from the neighbourhood of Montrose to the
valley of the Tay at Perth; the Ochil Hills, stretching along the south
side of the Firth of Tay to the valley of the Forth at Bridge-of-Allan;
the Lennox Hills, ranging from the neighbourhood of Stirling to
Dumbarton; the Kilbarchan Hills, lying between Greenock and Ardrossan;
the Cumbrae Islands and the southern half of Arran; and the same line
of heights reappears in the south end of Kintyre. A well-marked hollow,
trough, or undulating plain of variable width, separates these Northern
Heights from the Highlands, and may be followed all the way from near
Stonehaven, through Strathmore, to Crieff and Auchterarder. Between the
valleys of the Earn and Teith this plain attains an abnormal height
(the Braes of Doune); but from the Teith, south-west by Flanders
Moss and the lower end of Loch Lomond to the Clyde at Helensburgh,
it resumes its characteristic features. It will be observed also
that a hollow separates the southern portion of Arran from the much
loftier northern or Highland area. The tract known as the Braes of
Doune, extending from Glen Artney south-east to Strath Allan, although
abutting upon the Highlands, is clearly marked off from that great
division by geological composition and structure, by elevation and
configuration. It is simply a less deeply eroded portion of the long
trough or hollow.
Passing now to the Southern Heights of the Lowlands, we find that these
form a still more interrupted belt than the Northern Heights, and that
they are less clearly separated by an intermediate depression from the
great Uplands which they flank. They begin in the north-east with the
isolated Garleton Hills, between which and the Lammermoors a narrow
low-lying trough or hollow appears. A considerable width of low ground
now intervenes before we reach the Pentland Hills, which are in like
manner separated from the Southern Uplands by a broad low-lying tract.
At their southern extremity, however, the Pentlands merge more or less
gradually into a somewhat broken and interrupted group of hills which
abut abruptly on the Southern Uplands, in the same manner as the
Braes of Doune abut upon the slate hills of the Highland borders. In
this region the greatest heights reached are in Tinto (2335 feet), and
Cairntable (1844 feet), and, at the same time, the hills broaden out
towards north-west, where they are continued by the belt of volcanic
rocks already described as extending between the coal-fields of the
Clyde and Kilmarnock. Although the Southern Heights abut so closely
upon the Uplands lying to the south, there is no difficulty in drawing
a firm line of demarcation between the two areas--geologically and
physically they are readily distinguished. No one with any eye for
form, no matter how ignorant he may be of geology, can fail to see
how strongly contrasted are such hills as Tinto and Cairntable with
those of the Uplands, which they face. The Southern Heights are again
interrupted towards the south-east by the valleys of the Ayr and the
Doon, but they reappear in the hills that extend from the Heads of Ayr
to the valley of the Girvan.
Betwixt the Northern and Southern Heights spread the broad Lowland
tracts that drain towards the Forth, together with the lower reaches
of the Clyde valley, and the wide moors that form the water-parting
between that river and the estuary of the Forth. The hills that occur
within this inner region of the Central Lowlands are usually more or
less isolated, and are invariably formed by outcrops of igneous rock.
Their outline and general aspect vary according to the geological
character of the rocks of which they are composed--some forming more
or less prominent escarpments like those of the Bathgate Hills and the
heights behind Burntisland and Kinghorn, others showing a soft rounded
contour like the Saline Hills in the west of Fifeshire. Of the same
general character as this inner Lowland region is the similar tract
watered by the Irvine, the Ayr, and the Doon. This tract, as we have
seen, is separated from the larger inner region lying to the east by
the volcanic hills that extend from the Southern Heights north-west
into Renfrewshire.
The largest rivers that traverse the Central Lowlands take their rise,
as might be expected, in the mountainous table-lands to the north and
south. Of these the principal are the North and South Esks, the Tay
and the Isla, the Earn, and the Forth, all of which, with numerous
tributaries, descend from the Highlands. And it will be observed
that they have breached the line of the Northern Heights in three
places--namely, in the neighbourhood of Montrose, Perth, and Stirling.
The only streams of importance coming north from the Southern Uplands
are the Clyde and the Doon, both of which in like manner have broken
through the Southern Heights. Now, just as the main water-flows of
the Highlands indicate the average slope of the ancient land-surface
before it was trenched and furrowed by the innumerable valleys that
now intersect it, so the direction followed by the greater rivers that
traverse the Lowlands mark out the primeval slopes of that area. One
sees at a glance, then, that the present configuration of this latter
division has been brought about by the erosive action of the principal
rivers and their countless affluents, aided by the sub-aërial agents
generally--rain, frost, ice, etc. The hills rise above the average
level of the ground, not because they have been ridged up from below,
but simply owing to the more durable nature of their component rocks.
That the Northern and Southern Heights are breached only shows that the
low grounds, now separating those heights from the adjacent Highlands
and Southern Uplands, formerly stood at a higher level, and so allowed
the rivers to make their way more or less directly to the sea. Thus,
for example, the long trough of Strathmore has been excavated out of
sandstones, the upper surface of which once reached a much greater
height, and sloped outwards from the Highlands across what is now the
ridge of the Sidlaw Hills. Here then, in the Central Lowlands, as in
the Highlands, true mountain- or hill-ranges are absent. But if we
are permitted to term any well-marked line or belt of high ground a
"range," then the Northern and Southern Heights of the Lowlands are
better entitled to be so designated than any series of mountains in the
Highlands.
* * * * *
The Southern Uplands.--The northern margin of this wide division having
already been defined, we may now proceed to examine the distribution
of its mountain-masses. Before doing so, however, it may be as well
to point out that considerable tracts in Tweeddale, Teviotdale, and
Liddesdale, together with the Cheviot Hills, do not properly belong
to the Southern Uplands. In fact, the Cheviots bear the same relation
to those Uplands as the Northern Heights do to the Highlands. Like
them they are separated by a broad hollow from the Uplands, which they
face--a hollow that reaches its greatest extent in Tweeddale, and
rapidly wedges out to south-west, where the Cheviots abut abruptly
on the Uplands. Even where this abrupt contact takes place, however,
the different configuration of the two regions would enable any
geologist to separate the one set of mountains from the other. But for
geographical purposes we may conveniently disregard these geological
contrasts, and include within the Southern Uplands all the area lying
between the Central Lowlands and the English Border.
If there are no mountains in the Highlands so grouped and arranged as
to be properly termed "ranges," this is not less true of the Southern
Uplands. Perhaps it is the appearance which those Uplands present when
viewed from the Central Lowlands that first suggested the notion that
they were ranges. They seem to rise like a wall out of the low grounds
at their base, and extend far as eye can reach in an approximately
straight line. It seems more probable, however, that our earlier
cartographers merely meant, by their conventional hill-shading, to mark
out definitely the water-partings. But to do so in this manner now,
when the large contour maps of the Ordnance Survey may be in any one's
hands, is inexcusable. A study of those maps, or, better still, a visit
to the tops of a few of the dominant points in the area under review,
will effectually dispel the idea that the Southern Uplands consist of a
series of ridges zigzagging across the country. Like the Highlands, the
area of the Southern Uplands is simply an old table-land, furrowed into
ravine and valley by the operation of the various agents of erosion.
Beginning our survey of these Uplands in the east, we encounter
first the Lammermoor Hills--a broad undulating plateau--the highest
elevations of which do not reach 2000 feet. West of this come the
Moorfoot Hills and the high grounds lying between the Gala and the
Tweed--a tract which averages a somewhat higher elevation--two points
exceeding 2000 feet in height. The next group of mountains we meet
is that of the Moffat Hills, in which head a number of important
rivers--the Tweed, the Yarrow, the Ettrick, and the Annan. Many points
in this region exceed 2000 feet, others approach 2500 feet; and some
reach nearly 3000 feet, such as Broad Law (2754 feet), and Dollar
Law (2680 feet). In the south-west comes the group of the Lowthers,
with dominant elevations of more than 2000 feet. Then follow the
mountain-masses in which the Nith, the Ken, the Cree, the Doon, and
the Girvan take their rise, many of the heights exceeding 2000 feet,
and a number reaching and even passing 2500 feet, the dominant point
being reached in the noble mountain-mass of the Merrick (2764 feet).
In the extreme south-west the Uplands terminate in a broad undulating
plateau, of which the highest point is but little over 1000 feet. All
the mountain-groups now referred to are massed along the northern
borders of the Southern Uplands. In the south-west the general surface
falls more or less gradually away towards the Solway--the 500 feet
contour line being reached at fifteen miles, upon an average, from
the sea-coast. In the extreme north-east the high grounds descend in
like manner into the rich low grounds of the Merse. Between these low
grounds and Annandale, however, the Uplands merge, as it were, into the
broad elevated moory tract that extends south-east, to unite with the
Cheviots--a belt of hills rising along the English Border to heights of
1964 feet (Peel Fell), and 2676 feet (the Cheviot).
The general configuration of the main mass of the Southern
Uplands--that is to say, the mountain-groups extending along the
northern portion of the area under review, from Loch Ryan to the coast
between Dunbar and St. Abb's Head--is somewhat tame and monotonous. The
mountains are flat-topped elevations, with broad, rounded shoulders
and smooth grassy slopes. Standing on the summits of the Higher hills,
one seems to be in the midst of a wide, gently undulating plain, the
surface of which is not broken by the appearance of any isolated peaks
or eminences. Struggling across the bogs and peat-mosses that cover
so many of those flat-topped mountains, the wanderer ever and anon
suddenly finds himself on the brink of a deep green dale. He discovers,
in short, that he is traversing an elevated undulating table-land,
intersected by narrow and broad trench-like valleys that radiate
outwards in all directions from the dominant bosses and swellings of
the plateau. The mountains, therefore, are merely broad ridges and
banks separating contiguous valleys; in a word, they are, like the
mountains of the Highlands, monuments of erosion, which do not run in
linear directions, but form irregular groups and masses.
The rocks that enter into the formation of this portion of the Southern
Uplands have much the same character throughout. Consequently there
is less variety of contour and colour than in the Highlands. The
hills are not only flatter atop, but are much smoother in outline,
there being a general absence of those beetling crags and precipices
which are so common in the Highland regions. Now and again, however,
the mountains assume a rougher aspect. This is especially the case
with those of Carrick and Galloway, amongst which we encounter a
wildness and grandeur which are in striking contrast to the gentle
pastoral character of the Lowthers and similar tracts extending along
the northern and higher parts of the Southern Uplands. Descending to
details, the geologist can observe also modifications of contour even
among those monotonous rounded hills. Such modifications are due to
differences in the character of the component rocks, but they are
rarely so striking as the modifications that arise from the same cause
in the Highlands. To the trained eye, however, they are sufficiently
manifest, and upon a geologically coloured map, which shows the
various belts of rock that traverse the Uplands from south-west to
north-east, it will be found that the mountains occurring within
each of those separate belts have certain distinctive features. Such
features, however, cannot be depicted upon a small orographical map.
The separation of those mountains into distinct ranges, by reference
to their physical aspect, is even less possible here than in the
Highlands. Now and again, bands of certain rocks, which are of a
more durable character than the other strata in their neighbourhood,
give rise to pronounced ridges and banks, while hollows and valleys
occasionally coincide more or less closely with the outcrops of the
more readily eroded strata; but such features are mere minor details
in the general configuration of the country. The courses of brooks
and streams may have been frequently determined by the nature and
arrangement of the rocks, but the general slope of the Uplands and the
direction of the main lines of water-flow are at right angles to the
trend of the strata, and cannot therefore have been determined in that
way. The strata generally are inclined at high angles--they occur, in
short, as a series of great anticlinal arches and synclinal curves, but
the tops of the grand folds have been planed off, and the axes of the
synclinal troughs, so far from coinciding with valleys, very often run
along the tops of the highest hills. The foldings and plications do
not, in a word, produce any corresponding undulations of the surface.
Mention has been made of the elevated moory tracts that serve to
connect the Cheviots with the loftier Uplands lying to north-west.
The configuration of these moors is tamer even than that of the
regions just described, but the same general form prevails from
the neighbourhood of the Moffat Hills to the head-waters of the
Teviot. There, however, other varieties of rock appear, and produce
corresponding changes in the aspect of the high grounds. Not a few of
the hills in this district stand out prominently. They are more or less
pyramidal and conical in shape, being built up of sandstones often
crowned atop with a capping of some crystalline igneous rock, such as
basalt. The Maiden Paps, Leap Hill, Needs Law, and others are examples.
The heights draining towards Liddesdale and lower reaches of Eskdale,
composed chiefly of sandstones, with here and there intercalated
sheets of harder igneous rock, frequently show escarpments and
terraced outlines, but have a general undulating contour; and similar
features are characteristic of the sandstone mountains that form the
south-west portion of the Cheviots. Towards the north-east, however,
the sandstones give place to various igneous rocks, so that the hills
in the north-east section of the Cheviots differ very much in aspect
and configuration from those at the other extremity of the belt. They
have a more varied and broken outline, closely resembling many parts of
the Ochils and other portions of the Northern and Southern Heights of
the Central Lowlands.
The low-lying tracts of Roxburghshire and the Merse, in like manner,
present features which are common to the inner region of the Central
Lowlands. Occasional ridges of hills rise above the general level of
the land, as at Smailholm and Stitchell to the north of Kelso, while
isolated knolls and prominences--some bald and abrupt, others smooth
and rounded--help to diversify the surface. Bonchester Hill, Rubers
Law, the Dunian, Penielheugh, Minto Hills, and the Eildons may be
mentioned as examples. All of these are of igneous origin, some being
mere caps of basalt resting upon a foundation of sandstone, while
others are the stumps of isolated volcanoes.
In the maritime tracts of Galloway the low grounds repeat, on a smaller
scale, the configuration of the lofty Uplands behind, for they are
composed of the same kinds of rock. Their most remarkable feature is
the heavy mountain-mass of Criffel, rising near the mouth of the Nith
to a height of 1800 feet.
Everywhere, therefore, throughout the region of the Southern Uplands,
in hilly and low-lying tracts alike, we see that the land has been
modelled and contoured by the agents of erosion. We are dealing, as
in the Highlands, with an old table-land, in which valleys have been
excavated by running water and its helpmates. Nowhere do we encounter
any linear banks, ridges, or ranges as we find described in the
class-books, and represented upon many general maps of the country.
In one of those manuals we read that in the southern district "the
principal range of mountains is that known as the Lowther Hills, which
springs off from the Cheviots, and, running in a zigzag direction to
the south-west, terminates on the west coast near Loch Ryan." This
is quite true, according to many common maps, but unfortunately the
"range" exists upon those maps and nowhere else. The zigzag line
described is not a range of mountains, but a water-parting, which is
quite another matter.
The table-land of the Southern Uplands, like that of the Highlands, is
of immense antiquity. Long before the Old Red Sandstone period, it had
been furrowed and trenched by running water. Of the original contour of
its surface, all we can say is that it formed an undulating plateau,
the general slope of which was towards south-east. This is shown by the
trend of the more important rivers, such as the Nith and the Annan,
the Gala and the Leader; and by the distribution of the various strata
pertaining to the Old Red Sandstone and later geological periods. Thus,
strata of Old Red Sandstone and Carboniferous age occupy the Merse
and the lower reaches of Teviotdale, and extend up the valleys of the
Whiteadder and the Leader into the heart of the Silurian Uplands.
In like manner Permian sandstones are well developed in the ancient
hollows of Annandale and Nithsdale. Along the northern borders of the
Southern Uplands we meet with similar evidence to show that even as
early as Old Red Sandstone times the old plateau, along what is now its
northern margin, was penetrated by valleys that drained towards the
north. The main drainage, however, then as now, was directed towards
south-east.
Many geological facts conspire to show that the Silurian table-land of
these Uplands has been submerged, like the Highlands, in whole or in
part. This happened at various periods, and each time the land went
down it received a covering of newer accumulations--patches of which
still remain to testify to the former extent of the submergences. From
the higher portions of the Uplands those accumulations have been almost
wholly swept away, but they have not been entirely cleared out of the
ancient valleys. They still mantle the borders of the Silurian area,
particularly in the north-east, where they attain a great thickness
in the moors of Liddesdale and the Cheviot Hills. The details of the
evolution of the whole area of the Southern Uplands form an interesting
study, but this pertains rather to Geology than to Physical Geography.
It is enough, from our present point of view, to be assured that the
main features of the country were chalked out, as it were, at a very
distant geological period, and that all the infinite variety in the
relief of our land has been brought about directly, not by titanic
convulsions and earth-movements, but by the long-continued working of
rain and rivers--of frost and snow and ice, supplemented from time to
time by the action of the sea.
The physical features more particularly referred to in this paper are
of course only the bolder and more prominent contours--those namely
which can be expressed with sufficient accuracy upon sheets of such
a size as the accompanying orographical map of Scotland (Plate I.).
With larger maps considerably more detail can be added, and many
characteristic and distinguishing features will appear according to
the care with which such maps are drawn. In the case of the Ordnance
Survey map, on the scale of 1 inch to a mile, the varying forms of the
surface are so faithfully delineated as frequently to indicate to a
trained observer the nature of the rocks and the geological structure
of the ground. The artists who sketched the hills must indeed have had
good eyes for form. So carefully has their work been done, that it is
often not difficult to distinguish upon their maps hills formed of such
rocks as sandstone from those that are composed of more durable kinds.
The individual characteristics of mountains of schist, of granite, of
quartz-rock, of slate, are often well depicted: nay, even the varieties
of igneous rock which enter into the formation of the numerous hills
and knolls of the Lowlands can frequently be detected by the features
which the artists have so intelligently caught. Another set of features
which their maps display are those due to glaciation. These are
admirably brought out, even down to the smallest details. A glance at
such maps as those of Teviotdale and the Merse, for example, shows at
once the direction taken by the old _mer de glace_. The long parallel
flutings of the hill-slopes, _roches moutonnées_, projecting knolls and
hills with their "tails," the great series of banks and ridges of stony
clay which trend down the valley of the Tweed--these, and many more
details of interest to specialists, are shown upon the maps. All over
Scotland similar phenomena are common, and have been reproduced with
marvellous skill on the shaded sheets issued by the Ordnance Survey.
And yet the artists were not geologists. The present writer is glad
of this opportunity of recording his obligations to those gentlemen.
Their faithful delineations of physical features have given him many
valuable suggestions, and have led up to certain observations which
might otherwise not have been made.
III.
Mountains: Their Origin, Growth, and Decay.[D]
[D] _Scottish Geographical Magazine_, vol. ii., 1886.
Mountains have long had a fascination for lovers of nature. Time was,
however, when most civilised folk looked upon them with feelings akin
to horror; and good people, indeed, have written books to show that
they are the cursed places of the earth--the ruin and desolation of
their gorges and defiles affording indubitable proof of the evils
which befell the world when man lapsed from his primitive state of
innocence and purity. All this has changed. It is the fashion now to
offer a kind of worship to mountains; and every year their solitudes
are invaded by devotees--some, according to worthy Meg Dods, "rinning
up hill and down dale, knapping the chuckie-stanes to pieces wi'
hammers, like sae mony roadmakers run daft--to see, as they say, how
the warld was made"--others trying to transfer some of the beauty
around them to paper or canvas--yet others, and these perhaps not the
least wise, content, as old Sir Thomas Browne has it, "to stare about
with a gross rusticity," and humbly thankful that they are beyond the
reach of telegrams, and see nothing to remind them of the _fumun et
opes strepitumque Romæ_. But if the sentiment with which mountains are
regarded has greatly changed, so likewise have the views of scientific
men as to their origin and history. Years ago no one doubted that all
mountains were simply the result of titanic convulsions. The crust of
the earth had been pushed up from below, tossed into great billows,
shivered and shattered--the mountains corresponding to the crests of
huge earth-waves, the valleys to the intervening depressions, or to
gaping fractures and dislocations. This view of the origin of mountains
has always appeared reasonable to those who do not know what is meant
by geological structure, and in some cases it is pretty near the
truth. A true mountain-chain, like that of the Alps, does indeed owe
its origin to gigantic disturbances of the earth's crust, and in such
a region the larger features of the surface often correspond more or
less closely with the inclination of the underlying rocks. But in many
elevated tracts, composed of highly disturbed and convoluted strata,
no such coincidence of surface-features and underground structure can
be traced. The mountains do not correspond to great swellings of the
crust--the valleys neither lie in trough-shaped strata, nor do they
coincide with gaping fractures. Again, many considerable mountains are
built up of rocks which have not been convoluted at all, but occur in
approximately horizontal beds. Evidently, therefore, some force other
than subterranean action must be called upon to explain the origin of
many of the most striking surface-features of the land.
Every geologist admits--it is one of the truisms of his science--that
corrugations and plications are the result of subterranean action. Nor
does any one deny that when a true mountain-chain was first upheaved
the greater undulations of the folded strata probably gave rise to
similar undulations at the surface. Some of the larger fractures and
dislocations might also have appeared at the surface and produced
mural precipices. So long a time, however, has elapsed since the
elevation of even the youngest mountain-chains of the globe that the
sub-aërial agents of erosion--rain, frost, rivers, glaciers, etc.--have
been enabled greatly to modify their primeval features. For these
mountains, therefore, it is only partially true that their present
slopes coincide with those of the underlying strata. Such being the
case with so young a chain as the Alps, we need not be surprised to
meet with modifications on a still grander scale in mountain-regions of
much greater antiquity. In many such tracts the primeval configuration
due to subterranean action has been entirely remodelled, so that
hills now stand where deep hollows formerly existed, while valleys
frequently have replaced mountains. And this newer configuration is the
direct result of erosion, guided by the mineralogical composition and
structural peculiarities of the rocks.
It is difficult, or even impossible, for one who is ignorant of
geological structure to realise that the apparently insignificant
agents of erosion have played so important a _rôle_ in the evolution
of notable earth-features. It may be well, therefore, to illustrate
the matter by reference to one or two regions where the geological
structure is too simple to be misunderstood. The first examples I shall
give are from tracts of horizontal strata. Many readers are doubtless
aware of the fact that our rock-masses consist for the most part of the
more or less indurated and compacted sediments of former rivers, lakes,
and seas. Frequently those ancient water-formed rocks have been very
much altered, so as even sometimes to acquire a crystalline character.
But it is enough for us now to remember that the crust of the globe, so
far as that is accessible to observation, is built up mostly of rocks
which were originally accumulated as aqueous sediments. Such being the
case, it is obvious that our strata of sandstone, conglomerate, shale,
limestone, etc., must at first have been spread out in approximately
horizontal or gently inclined sheets or layers. We judge so from what
we know of sediments which are accumulating at present. The wide
flats of our river valleys, the broad plains that occupy the sites of
silted-up lakes, the extensive deltas of such rivers as the Nile and
the Po, the narrow and wide belts of low-lying land which within a
recent period have been gained from the sea, are all made up of various
kinds of sediment arranged in approximately horizontal layers. Now,
over wide regions of the earth's surface the sedimentary strata still
lie horizontally, and we can often tell at what geological period they
became converted into dry land. Thus, for example, we know that the
elevated plateau through which the river Colorado flows is built up
of a great series of nearly horizontal beds of various sedimentary
deposits, which reach a thickness of many thousand feet. It is
self-evident that the youngest strata must be those which occur at the
surface of the plateau, and they, as we know, are of lacustrine origin
and belong to the Tertiary period. Now, American geologists have shown
that since that period several thousands of feet of rock-materials have
been removed from the surface of that plateau--the thickness of rock so
carried away amounting in some places to nearly 10,000 feet. Yet all
that prodigious erosion has been effected since early Tertiary times.
Indeed, it can be proved that the excavation of the Grand Cañon of
the Colorado, probably the most remarkable river-trench in the world,
has been accomplished since the close of the Tertiary period, and is
therefore a work of more recent date than the last great upheaval
of the Swiss Alps. The origin of the cañon is self-evident--it is a
magnificent example of river-erosion, and the mere statement of its
dimensions gives one a forcible impression of the potency of sub-aërial
denudation. The river-cutting is about 300 miles long, 11 or 12 miles
broad, and varies from 3000 to 6000 feet in depth.
Take another example of what denuding agents have done within a recent
geological period. The Faröe Islands, some twenty in number, extend
over an area measuring about 70 miles from south to north, and nearly
50 miles from west to east. These islands are composed of volcanic
rocks--beds of basalt with intervening layers of fine fragmental
materials, and are obviously the relics of what formerly was one
continuous plateau, deeply trenched by valleys running in various
directions. Subsequent depression of the land introduced the sea to
these valleys, and the plateau was then converted into a group of
islands, separated from each other by narrow sounds and fiords. Were
the great plateau through which the Colorado flows to be partially
submerged, it would reproduce on a larger scale the general phenomena
presented by this lonely island-group of the North Atlantic. The
flat-topped "buttes" and "mesas," and the pyramidal mountains of
the Colorado district would form islands comparable to those of the
Faröes. Most of the latter attain a considerable elevation above the
sea--heights of 1700, 2000, 2500, and 2850 feet being met with in
several of the islands. Indeed, the average elevation of the land in
this northern archipelago can hardly be less than 900 feet. The deep
trench-like valleys are evidently only the upper reaches of valleys
which began to be excavated when the islands formed part and parcel
of one and the same plateau--the lower reaches being now occupied by
fiords and sounds. It is quite certain that all these valleys are the
work of erosion. One can trace the beds of basalt continuously across
the bottoms, and be quite sure that the valleys are not gaping cracks
or fractures. Now, as the strata are approximately horizontal, it is
obvious that the hollows of the surface have nothing whatever to do
with undulations produced by earth-movements. The sub-aërial erosion
of the islands has resulted in the development of massive flat-topped
and pyramidal mountains. These stand up as eminences simply because the
rock-material which once surrounded them has been gradually broken up
and carried away. Nothing can well be more impressive to the student
of physical geology than the aspect presented by these relics of
an ancient plateau (Plate II. Fig. 1). Standing on some commanding
elevation, such as Nakkin in Suderöe, one sees rising before him great
truncated pyramids--built up of horizontal beds of basalt rising tier
above tier--the mountains being separated from each other by wide and
profound hollows, across which the basalt-beds were once continuous.
Owing to the parallel and undisturbed position of the strata, it is
not hard to form an estimate of the amount of material which has been
removed during the gradual excavation of the valleys. In order to do so
we have simply to measure the width, depth, and length of the valleys.
Thus in Suderöe, which is 19 miles long and 6 miles broad, the bottoms
of the valleys are 1000 feet at least below the tops of the mountains,
and some of the hollows in question are a mile in width. Now, the
amount of rock worn away from this one little island by sub-aërial
erosion cannot be less than that of a mass measuring 10 miles in
length by 6 miles in breadth, and 800 feet in thickness. And yet the
Faröe Islands are composed of rocks which had no existence when the
soft clays, etc., of the London Basin were being accumulated. All the
erosion referred to has taken place since the great upheaval of the
Eocene strata of the Swiss Alps.
But if the evidence of erosion be so conspicuous in regions composed
of horizontal strata, it is not less so in countries where the rocks
are inclined at various angles to the horizon. Indeed, the very fact
that inclined strata crop out at the surface is sufficient evidence of
erosion. For it is obvious that these outcrops are merely the truncated
ends of beds which must formerly have had a wider extension. But while
the effects produced by the erosion of horizontal strata are readily
perceived by the least-informed observer, it requires some knowledge
of geological structure to appreciate the denudation of curved or
undulating strata. And yet there is really no mystery in the matter.
All we have to do is by careful observation to ascertain the mode of
arrangement of the rocks--this accomplished, we have no difficulty
in estimating the minimum erosion which any set of strata may have
experienced. An illustration may serve to make this plain. Here, for
example, is a section across a region of undulating strata (Fig. 2).
Let the line _A B_ represent the surface of the ground, and _C D_ be
any datum line--say, the sea-level. An observer at _A_, who should
walk in the direction of _B_, would cross successively eight outcrops
of coal; and, were he incapable of reading the geological structure
of the ground, he might imagine that he had come upon eight separate
coal-seams. A glance at the section, however, shows that in reality he
had met with only two coals, and that the deceptive appearances, which
might be misread by an incautious observer, are simply the result of
denudation. In this case the tops of a series of curved or arched beds
have been removed (as at _E_), and, by protracting the lines of the
truncated beds until they meet, we can estimate the minimum amount of
erosion they have sustained. Thus, if the strata between _o_ and _p_
be 300 feet thick, it is self-evident that a somewhat greater thickness
of rock must have been removed from the top of the anticlinal arch or
"saddleback" at _E_.
Again, let us draw a section across strata which have been fractured
and dislocated, and we shall see how such fractures likewise enable us
to estimate the minimum amount of erosion which certain regions have
experienced. In Fig. 3 we have a series of strata containing a bed of
limestone _L_, and a coal-seam _a_. The present surface of the ground
is represented by the line _A B_. At _F_ the strata are traversed by a
fault or dislocation--the beds being thrown down for say 500 feet on
the low side of the fault--so that the coal at _a^2_ occurs now at a
depth of 500 feet below its continuation at _a^1_. At the surface of
the ground there is no inequality of level--the beds overlying the coal
(_a²_) having been removed by denudation. Were the missing rocks to
be replaced, they would occupy the space contained within the dotted
lines above the present surface _A B_. Such dislocations are of common
occurrence in our coal-fields, and it is not often that they give rise
to any features at the surface. We may thus traverse many level or
gently-undulating tracts, and be quite unconscious of the fact that
geologically we have frequently leaped up or dropped down for hundreds
of feet in a single step. Nay, some Scottish streams and rivers flow
across dislocations by which the strata have been shifted up or down
for thousands of feet, and in some places one can have the satisfaction
of sitting upon rocks which are geologically 3000 yards below or
above those on which he rests his feet. In other words, thousands of
feet of strata have been removed by denudation from the high sides of
faults. These, as I have said, often give rise to no feature at the
surface; but, occasionally, when "soft" rocks have been shifted by
dislocations, and brought against "hard" rocks, the latter, by better
resisting denudation than the former, cause a more or less well-marked
feature at the surface, and thus betray the presence of a fault to the
geologist. The phenomena presented by faults, therefore, are just as
eloquent of denudation as is the truncated appearance of our strata;
and only after we have carefully examined the present extension and
mutual relations of our rock-masses, their varied inclination, and
the size of the dislocations by which they are traversed, can we
properly appreciate the degree of erosion which they have sustained.
Before we are entitled to express any opinion as to the origin of
the surface-features of a country, we must first know its geological
structure. Until we have attained such knowledge, all our views as
to the origin of mountains are of less value than the paper they are
written upon.
I have spoken of the evidence of denudation which we find in our
truncated and dislocated rock-masses; there is yet another line of
evidence which I may very shortly point out. As every one knows, there
exist in this and many other countries enormous masses of igneous
rocks, which have certainly been extruded from below. Now, some of
these rocks, such as granite, belong to what is called the _plutonic_
class of rocks; they are of deep-seated origin--that is to say, they
never were erupted at the surface, but cooled and consolidated at great
depths in the earth's crust. I need not go into any detail to show
that this is the case--it is a conclusion based upon incontrovertible
facts, and accepted by every practical geologist. When, therefore, we
encounter at the actual surface of the earth great mountain-masses of
granite, we know that in such regions enormous denudation has taken
place. The granite appears at the surface simply because the thick
rock-masses under which it solidified have been gradually removed by
erosion.
The facts which I have now briefly passed in review must convince us
that erosion is one of the most potent factors with which the geologist
has to deal. We have seen what it has been able to effect in certain
tracts composed of strata which date back to a recent geological
period, such as the plateau of the Colorado and the pyramidal mountains
of the Faröe Islands. If in regions built up of strata so young as
the rocks of those tracts the amount of erosion be so great, we may
well expect to meet with evidence of much more extensive denudation in
regions which have been subjected for enormously longer periods to the
action of the eroding agents.
The study of geological structure, or the architecture of the earth's
crust, has enabled us to group all mountains under these three
principal heads:--
1. _Mountains of Accumulation._
2. _Mountains of Elevation._
3. _Mountains of Circumdenudation._
1. Mountains of Accumulation.--Volcanoes may be taken as the type
of this class of mountains. These are, of course, formed by the
accumulation of igneous materials around the focus or foci of eruption,
and their mode of origin is so generally understood, and, indeed,
so obvious, that I need do no more than mention them. Of course,
they are all subject to erosion, and many long-extinct volcanoes are
highly denuded. Some very ancient ones, as those of our own country,
have been so demolished that frequently all that remains are the now
plugged-up pipes or flues through which the heated materials found a
passage to the surface--all those materials, consisting of lavas and
ashes, having in many cases entirely disappeared. In former times
volcanic eruptions often took place along the line of an extensive
fissure--the lava, instead of being extruded at one or more points,
welled-up and overflowed along the whole length of the fissure, so as
to flood the surrounding regions. And this happening again and again,
vast plateaux of igneous rock came to be built up, such as those of the
Rocky Mountains, Iceland, the Faröes, Antrim and Mull, Abyssinia and
the Deccan. These are called _plateaux of accumulation_ (see Fig. 1),
and all of them are more or less highly denuded, so that in many cases
the plateaux have quite a mountainous appearance. Of course, plateaux
of accumulation are not always formed of igneous rocks. Any area of
approximately horizontal strata of aqueous origin, rising to a height
of a thousand feet or more above the sea, would come under this class
of plateau--the plateau of the Colorado being a good example. Although
that plateau is of recent origin, yet its surface, as we have seen,
has been profoundly modified by superficial erosion; and this is true
to a greater extent of plateaux which have been much longer exposed
to denudation. It is obvious that even mountains and plateaux of
accumulation often owe many of their present features to the action of
the surface-agents of change.
2. Mountains of Elevation.--We have seen that the strata which enter
most largely into the composition of the earth's crust, so far as that
is open to observation, consist of rocks which must originally have
been disposed in horizontal or approximately horizontal layers. But,
as every one knows, the stratified rocks are not always horizontally
arranged. In Scotland they rarely are so. On the contrary, they are
inclined at all angles from the horizon, and not infrequently they even
stand on end. Moreover, they are often traversed by dislocations, large
and small. No one doubts that these tilted and disturbed rocks are
evidence of wide-spread earth-movements. And it has been long known to
geologists that such movements have happened again and again in this
and many other countries where similar disturbed strata occur. Some of
these movements, resulting in the upheaval of enormous mountain-masses,
have taken place within comparatively recent geological times. Others
again date back to periods inconceivably remote. The Pyrenees, the
Alps, the Caucasus, the Himalaya, which form the back-bone of Eurasia,
are among the youngest mountains of the globe. The Highlands of
Scotland and Scandinavia are immeasurably more ancient; they are, in
point of fact, the oldest high grounds in Europe, nor are there any
mountain-masses elsewhere which can be shown to be older. But while the
Alps and other recent mountains of elevation still retain much of their
original configuration, not a vestige of the primeval configuration of
our own Highlands has been preserved; their present surface-features
have no direct connection with those which must have distinguished them
in late Silurian times. Our existing mountains are not, like those of
the Alps, mountains of elevation.
The structure of a true mountain-chain is frequently very complicated,
but the general phenomena can be readily expressed in a simple diagram.
Let Fig. 5 be a section taken across a mountain-chain, _i.e._ at right
angles to its trend or direction. The dominant point of the chain is
shown at _B_, while _A_ and _C_ represent the low grounds. Now, an
observer at _A_, advancing towards _B_, would note that the strata, at
first horizontal, would gradually become undulating as he proceeded on
his way--the undulations getting always more and more pronounced. He
would observe, moreover, that the undulations, at first symmetrical,
as at _a_, would become less so as he advanced--one limb of an arch
or _anticline_, as it is termed, being inclined at a greater angle
than the other, as at _b_. Approaching still nearer to =B=, the arches
or anticlines would be seen eventually to bend over upon each other,
so as to produce a general dip or inclination of the strata towards
the central axis of the chain. Crossing that axis (_B_), and walking
in the direction of the low grounds (_C_), the observer would again
encounter the same structural arrangement, but of course in reverse
order. Thus, in its simplest expression, a true mountain-chain consists
of strata arranged in a series of parallel undulations--the greater
mountain ridges and intervening hollows corresponding more or less
closely to the larger undulations and folds of the strata. Now, could
these plicated strata be pulled out, could the folds and reduplications
be smoothed away, so as to cause the strata to assume their original
horizontal position, it is obvious that the rocks would occupy a
greater superficial area. We see, then, that such a mountain-chain
must owe its origin to a process of tangential or lateral thrusting
and crushing. The originally horizontal strata have been squeezed
laterally, and have yielded to the force acting upon them by folding
and doubling up. It seems most probable that the larger contortions
and foldings which are visible in all true mountain-chains, owe their
origin to the sinking down of the earth's crust upon the cooling and
contracting nucleus. During such depressions of the crust the strata
are necessarily subjected to enormous lateral compression; they are
forced to occupy less space at the surface, and this they can only do
by folding and doubling-back upon themselves. If the strata are equally
unyielding throughout a wide area, then general undulation may ensue;
but should they yield unequally, then folding and contortion will take
place along one or more lines of weakness. In other words, the pressure
will be relieved by the formation of true mountain-chains. Thus,
paradoxical as it may seem, the loftiest mountains of the globe bear
witness to profound depression or subsidence of the crust. The Andes,
for example, appear to owe their origin to the sinking down of the
earth's crust under the Pacific; and so in like manner the Alps would
seem to have been ridged up by depression of the crust in the area of
the Mediterranean. Mountain-chains, therefore, are true wrinkles in the
crust of the earth; they are lines of weakness along which the strata
have yielded to enormous lateral pressure.
A glance at the geological structure of the Alps and the Jura shows us
that these mountains are a typical example of such a chain; they are
mountains of elevation. In the Jura the mountains form a series of long
parallel ridges separated by intervening hollows; and the form or shape
of the ground coincides in a striking manner with the foldings of the
strata. In these mountains we see a succession of symmetrical flexures,
the beds dipping in opposite directions at the same angle from the axis
of each individual anticline. There each mountain-ridge corresponds
to an _anticline_, and each valley to a _syncline_, or trough-shaped
arrangement of strata. But as we approach the Alps the flexures become
less and less symmetrical, until in the Alps themselves the most
extraordinary convolutions and intricate plications appear, the strata
being often reversed or turned completely upside down.
Though it is true that the slopes of this great mountain-chain
not infrequently correspond more or less closely to the slope or
inclination of the underlying rocks, it must not be supposed that this
correspondence is often complete. Sometimes, indeed, we find that the
mountains, so far from coinciding with anticlines, are in reality
built up of synclinal or basin-shaped strata; while in other cases
deep and broad valleys run along the lines of anticlinal axes (Fig.
6). All this speaks to enormous erosion. A study of the geological
structure of the Alps demonstrates that thousands of feet of rock have
been removed from those mountains since the time of their elevation. A
section drawn across any part of the chain would show that the strata
have been eroded to such an extent, and the whole configuration so
profoundly modified, that it is often difficult, or even impossible,
to tell what may have been the original form of the surface when the
chain was upheaved. And yet the Alps, it must be remembered, are of
comparatively recent age, some of their highly-confused and contorted
rocks consisting of marine strata which are of no greater antiquity
than the incoherent clays and sands of the London Tertiary basin.
Now, when we reflect upon the fact that, in the case of so young a
mountain-chain, the configuration due to undulations of the strata
has been so greatly modified, and even in many places obliterated, it
is not hard to believe that after sufficient time has elapsed--after
the Alps have existed for as long a period, say, as the mountains
of middle Germany--every mountain formed of anticlinal strata shall
have disappeared, and those synclines which now coincide with valleys
shall have developed into hills. The reader who may have paid little
or no attention to geological structure and its influence upon the
form of the ground, will probably think this a strange and extravagant
statement; yet I hope to show presently that it is supported by all
that we know of regions of folded strata which have been for long
periods of time subjected to denudation.
* * * * *
3. Mountains of Circumdenudation.--In countries composed of undulating
and folded strata which have been for long ages exposed to the
action of eroding agents, the ultimate form assumed by the ground
is directly dependent on the character of the rocks, and the mode
of their arrangement. The various rock-masses which occur in such a
neighbourhood as Edinburgh, for example, differ considerably in their
power of resisting denudation. Hence the less readily eroded rocks
have come in time to form hills of less or greater prominence. Such
is the case with the Castle Rock, Corstorphine Hill, the Braids, the
Pentlands, etc. These hills owe their existence, as such, to the fact
that they are composed of more enduring kinds of rock than the softer
sandstones and shales by which they are surrounded, and underneath
which they were formerly buried to great depths. Some hills, again,
which are for the most part built up of rocks having the same character
as the strata that occur in the adjacent low grounds, stand up as
prominences simply because they have been preserved by overlying caps
or coverings of harder rocks--rocks which have offered a stronger
resistance to the action of the denuding agents. The Lomond Hills are
good examples. Those hills consist chiefly of sandstones which have
been preserved from demolition by an overlying sheet of basalt-rock.
But the mode in which rocks are arranged is a not less important factor
in determining the shape which the ground assumes under the action of
the agents of erosion. Thus, as we have already seen, flat-topped,
pyramidal mountains, and more or less steep-sided or trench-like
valleys, are characteristic features in regions of horizontal strata.
When strata dip or incline in one general direction, then we have a
succession of escarpments or dip-slopes, corresponding to the outcrops
of harder or less readily eroded beds, and separated from each other
by long valleys, hollows, or undulating plains, which have the same
trend as the escarpments (Fig. 7). This kind of configuration is
well exemplified over a large part of England. The general dip or
inclination of the Mesozoic or Secondary strata throughout that
country, between the shores of the North Sea and the English Channel,
is easterly and south-easterly--so that the outcrops of the more
durable strata form well-defined escarpments that face the west and
north-west, and can be followed almost continuously from north to
south. Passing from the Malvern Hills in a south-easterly direction,
we traverse two great escarpments--the first coinciding with the
outcrop of the Oolite, and forming the Cotswold Hills; and the second
corresponding to the outcrop of the Chalk, and forming the Chiltern
Hills. The plains and low undulating tracts that separate these
escarpments mark the outcrops of more yielding strata--the low grounds
that intervene between the Cotswolds and the Malvern Hills being
composed of Liassic and Triassic clays and sandstones. In Scotland
similar escarpments occur, but owing to sudden changes of the dip,
and various interruptions of the strata, the Scottish escarpments are
not so continuous as those of the sister-country. Many of the belts
of hilly ground in the Scottish Lowlands, however, exemplify the
phenomena of escarpment and dip-slope. Thus, the Sidlaws in Forfarshire
consist of a series of hard igneous rocks and interbedded sandstones
and flags--the outcrops of which form a succession of escarpments with
intervening hollows. The same appearances recur again and again all
over the Lowlands. Wherever, indeed, any considerable bed of hard rock
occurs in a series of less enduring strata--the outcrop of the harder
rock invariably forms a well-marked feature or escarpment. As examples,
I may refer to Salisbury Crags, Craiglockhart Hill, Dalmahoy Crags, the
Bathgate Hills, King Alexander's Crag, etc. All these are conspicuous
examples of the work of denudation--for it can be demonstrated
that each of these rock-masses was at one time deeply buried under
sandstones and shales, and they now crop out at the surface, and form
prominent features simply because the beds which formerly covered and
surrounded them have been gradually removed.
From what has now been said it will be readily understood that in
regions composed of strata the inclination or dip of which is not
constant but continually changing in direction, the surface-features
must be more or less irregular. If the strata dip east the outcrops
of the harder beds will form escarpments facing the west, and the
direction of the escarpments will obviously change with the direction
of the dip. Undulating strata of variable composition will, in short,
give rise to an undulating surface, but the superficial undulations
will not coincide with those of the strata. On the contrary, in
regions consisting of undulating strata of diverse consistency the
hills generally correspond with synclinal troughs--or, in other words,
trough-shaped strata tend to form hills; while, on the other hand,
arch-shaped or anticlinal strata most usually give rise to hollows
(see Fig. 2). This remarkable fact is one of the first to arrest the
attention of every student of physical geology, and its explanation
is simple enough. An anticlinal arrangement of strata is a weak
structure--it readily succumbs to the attacks of the denuding agents; a
synclinal arrangement on the contrary, is a strong structure, which is
much less readily broken up. Hence it is that in all regions which have
been exposed for prolonged periods to sub-aërial denudation synclinal
strata naturally come to form hills, and anticlinal strata valleys or
low grounds. In the case of a mountain-chain so recently elevated as
that of the Alps, the mountain-ridges, as we have seen, often coincide
roughly with the greater folds of the strata. Such anticlinal mountains
are weakly built, and consequently rock-falls and landslips are of
common occurrence among them--far more common, and on a much larger
scale, than among the immeasurably older mountains of Scandinavia and
Scotland. The valleys of the Pyrenees, the Alps, and the Apennines,
are cumbered with enormous chaotic heaps of fallen rock-masses. From
time to time peaks and whole mountain-sides give way, and slide into
the valleys, burying hamlets and villages, and covering wide tracts of
cultivated land. Hundreds of such disastrous rock-falls have occurred
in the Alps within historical ages, and must continue to take place
until every weakly-formed mountain has been demolished. The hills
and mountains of Scotland have long since passed through this phase
of unstable equilibrium. After countless ages of erosion our higher
grounds have acquired a configuration essentially different from that
of a true mountain-chain. Enormous landslips like that of the Rossberg
are here impossible, for all such weakly-constructed mountains have
disappeared.
A little consideration will serve to show how such modifications and
changes have come about. When strata are crumpled up they naturally
crack across, for they are not elastic. During the great movements
which have originated all mountains of elevation, it is evident
that the strata forming the actual surface of the ground would often
be greatly fissured and shattered along the crests of the sharper
anticlinal ridges. In the synclinal troughs, however, although much
fissuring would take place, yet the strata would be compelled by the
pressure to keep together. Now, when we study the structure of such a
region as the Alps, we find that the tops of the anticlines have almost
invariably been removed, so as to expose the truncated ends of the
strata--the ruptured and shattered rock-masses having in the course
of time been carried away by the agents of erosion. Such mountains
are pre-eminently weak structures. Let us suppose that the mountains
represented in the diagram (Fig. 8) consist of a succession of strata,
some of which are more or less permeable by water, while others are
practically impermeable. It is obvious that water soaking down from the
surface will find its way through the porous strata (_p_), and come out
on the slopes of the mountains along the joints and cracks (_c_) by
which all strata are traversed. Under the influence of such springs and
the action of frost, the rock at the surface will eventually be broken
up, and ever and anon larger and smaller portions will slide downwards
over the surface of the underlying impermeable stratum. The undermining
action of rivers will greatly intensify this disintegrating and
disrupting process. As the river deepens and widens its valley (_v_),
it is apparent that in doing so it must truncate the strata that are
inclined towards it. The beds will then crop out upon the slopes of the
valley (as at _b_, _b_), and so the conditions most favourable for a
landslip will arise. Underground water, percolating through the porous
beds (p), and over the surface of the underlying impermeable beds (_i_,
_i_, _i_), must eventually bring about a collapse. The rocks forming
the surface-slopes of the mountain will from time to time give and
slide into the valley, or the whole thickness of the truncated strata
may break away and rush downwards; and this process must continue so
long as any portion of the anticlinal arch remains above the level of
the adjacent synclinal troughs.
Thus it will be seen that an anticlinal arch is a weak structure--a
mountain so constructed falls a ready prey to the denuding agents; and
hence in regions which have been exposed to denudation for as long a
period as the Scottish or Scandinavian uplands, a mountain formed of
anticlinally arranged strata is of very exceptional occurrence. When
it does appear, it is only because the rocks of which it is composed
happen to be of a more enduring character than those of the adjacent
tracts. The Ochil Hills exemplify this point. These hills consist of a
great series of hard igneous rocks, which are arranged in the form of a
depressed anticlinal arch--the low grounds lying to the north and south
being composed chiefly of sandstones and shales. Here it is owing to
the more enduring character of the igneous rocks that the anticlinal
arch has not been entirely removed. We know, however, that these
igneous rocks were formerly buried under a great thickness of strata,
and that their present appearance at the surface is simply the result
of denudation.
If an anticlinal arch be a weak structure, a synclinal arrangement of
strata is quite the opposite. In the case of the former each bed has
a tendency to slip or slide away from the axis, while in a syncline
it is just the reverse--the strata being inclined towards and not
away from the axis. Underground water, springs, and frost are enabled
to play havoc with anticlinal strata, for the structure is entirely
in their favour. But in synclinal beds the action of these powerful
agents is opposed by the structure of the rocks--and great rock-falls
and landslips cannot take place. Synclinal strata therefore endure,
while anticlinal strata are worn more readily away. Even in a true
mountain-range so young as the Alps, denudation has already demolished
many weakly-built anticlinal mountains, and opened up valleys along
their axes; while, on the other hand, synclinal troughs have been
converted into mountains. And if this be true of the Alps, it is still
more so of much older mountain-regions, in which the original contours
due to convolutions of the strata have entirely disappeared (see Fig.
9).
The mountains of such regions, having been carved out and modelled by
denuding agents, are rightly termed _mountains of circumdenudation_,
for they are just as much the work of erosion as the flat-topped and
pyramidal mountains which have been carved out of horizontal strata.
The Scottish Highlands afford us an admirable example of a mountainous
region of undulating and often highly-flexed strata, in which the
present surface-features are the result of long-continued erosion. As
already remarked, this region is one of the oldest land-surfaces in the
world. In comparison with it, the Pyrenees, the Alps, and the Himalayas
are creations of yesterday. The original surface or configuration
assumed by the rocks composing our Highland area at the time when these
were first crushed and folded into anticlines and synclines had already
been demolished at a period inconceivably more remote than the latest
grand upheaval of the Alps. Even before the commencement of Old Red
Sandstone times, our Archæan, Cambrian, and Silurian rocks had been
planed down for thousands of feet, so that the bottom beds of the Old
Red Sandstone were deposited upon a gently undulating surface, which
cuts across anticlines and synclines alike. In late Silurian and early
post-Silurian times the North-west Highlands probably existed as a true
mountain-chain, consisting of a series of parallel ranges formed by the
folding and reduplication of the strata. The recent observations of my
friends, Professor Lapworth and Messrs. Peach and Horne, in Sutherland,
have brought to light the evidence of gigantic earth-movements, by
which enormous masses of strata have been convoluted and pushed
for miles out of place. We see in that region part of a dissected
mountain-chain. The mountain-masses which are there exposed to view
are the basal or lower portions of enormous sheets of disrupted rock,
the upper parts of which have been removed by denudation. In a word,
the mountains of Sutherland are mountains of circumdenudation--they
have been carved out of elevated masses by the long-continued action
of erosion. To prove this, one has only to draw an accurate section
across the North-west Highlands, when it becomes apparent that the
form or shape of the ground does not correspond or coincide with the
convolutions of the strata, and that a thickness of thousands of
feet of rock has been denuded away since those strata were folded and
fractured. All over the Highlands we meet with similar evidence of
enormous denudation. The great masses of granite which appear at the
surface in many places are eloquent of the result produced by erosion
continued for immeasurable periods of time. Every geologist knows that
granite is a rock which could only have been formed and consolidated at
great depths. When, therefore, such a rock occurs at the surface, it is
evidence beyond all doubt of prodigious erosion. The granite has been
laid bare by the removal of the thick rock-masses underneath which it
cooled and consolidated.
A glance at any map of Scotland will show that many river-valleys, and
not a few lakes, of the Highlands have a north-east and south-west
trend. This trend corresponds to what geologists call the _strike_ of
the strata. The rocks of the Highlands have been compressed into a
series of folds or anticlines and synclines, which have the direction
just stated--namely, north-east and south-west. A careless observer
might therefore rashly conclude that these surface-features resembled
those of the Jura--in other words, that the long parallel hollows were
synclinal troughs, and that the intervening ridges and high grounds
were anticlinal arches or saddle-backs. Nothing could be further from
the truth. A geological examination of the ground would show that
the features in question were everywhere the result of denudation,
guided by the petrological character and geological structure of the
rocks. Several of the most marked hollows run along the backs of
anticlinal axes, while some of the most conspicuous mountains are
built up of synclinal or trough-shaped strata. Ben Lawers, and the
depression occupied by Loch Tay, are excellent examples; and since
that district has recently been mapped in detail by Mr. J. Grant
Wilson, of the Geological Survey, I shall give a section (Fig. 10) to
show the relation between the form of the ground and the geological
structure of the rocks. This section speaks for itself. Here evidently
is a case where "valleys have been exalted and mountains made low."
A well-marked syncline, it will be observed, passes through Ben
Lawers, while Loch Tay occupies a depression scooped out of an equally
well-defined anticline--a structure which is just the opposite of
that which we should expect to find in a true mountain-chain. It will
be also noted that Glen-Lyon coincides neither with a syncline nor a
fault; it has been eroded along the outcrops of the strata. Many of the
north-east and south-west hollows of the Highlands indeed run along
the base of what are really great escarpments--a feature which, as we
have seen, is constantly met with in every region where the strata
"strike" more or less steadily in one direction. In the Highlands the
strata are most frequently inclined at considerable angles, so that the
escarpments succeed each other more rapidly than would be the case if
the strata were less steeply inclined. In no case does any north-east
and south-west hollow coincide with a structural cavity. Loch Awe has
been cited as an example of a superficial depression formed by the
inward dip of the strata on either side. But, as was shown many years
ago by my brother, A. Geikie,[E] this lake winds across the _strike_
of the strata. Moreover, if it owed its existence to a great synclinal
fold, why, he asks, does it not run along the same line as far as the
same structure continues? It does not do so: it is not continuous with
the synclinal fold, while vertical strata appear in the middle of the
lake, where, as my brother remarks, they have clearly no business to be
if the sides of the lake are formed by the inward dip of the schists.
[E] _Trans. Edin. Geol. Soc._ vol. ii. p. 267.
The Great Glen, as I mentioned in the preceding article, coincides
with a fracture or dislocation--a line of weakness along which the
denuding agents had worked for many ages before the beginning of Old
Red Sandstone times; and it is possible that smaller dislocations
may yet be detected in other valleys. But in each and every case the
valleys as we now see them are valleys of erosion; in each and every
case the mountains are mountains of circumdenudation; they project
as eminences because the rock-masses which formerly surrounded them
have been gradually removed. We have only to protract the outcrops
of the denuded strata--to restore their continuations--to form some
faint idea of the enormous masses of rock which have been carried away
from the surface of the Highland area since the strata were folded
and fractured. All this erosion speaks to the lapse of long ages. The
mountains of elevation which doubtless at one time existed within
the Highland area had already, as we have seen, suffered extreme
erosion before the beginning of Old Red Sandstone times, much of
the area having been converted into an undulating plateau or plain,
which, becoming submerged in part, was gradually overspread by the
sedimentary deposits of the succeeding Old Red Sandstone period. Those
sediments were doubtless derived in large measure from the denudation
of the older rocks of the Highlands, and since they attain in places
a thickness of 20,000 feet, and cover many square miles, they help
us to realise in some measure the vast erosion the Highland area had
sustained before the commencement of the Carboniferous period. Nor
must we forget that the Old Red Sandstone formation which borders the
Highlands has itself experienced excessive denudation: it formerly had
a much greater extension, and doubtless at one time overspread large
tracts of the Highlands. Again, we have to remember that during the
Carboniferous and Permian periods, and the later Mesozoic and Cainozoic
eras, the Highlands probably remained more or less continuously in the
condition of land. Bearing this in mind, we need not be surprised that
not a vestige of the primeval configuration brought about by the great
earth-movements of late Silurian times has been preserved. Indeed, had
the Highland area, after the disappearance of the Old Red Sandstone
inland seas, remained undisturbed by any movement of elevation or
depression, it must long ago have been reduced by sub-aërial erosion to
the condition of a low-lying undulating plain. But elevation en masse
from time to time took place, and so running water and its numerous
allies have been enabled to carry on the work of denudation.
Thus in the geological history of the Scottish Highlands we may trace
the successive phases through which many other elevated tracts
have passed. The Scandinavian plateau, and many of the mountains of
middle Germany--such, for example, as the Harz, the Erzgebirge, the
Thüringer-Wald, etc.--show by their structure that they have undergone
similar changes. First we have an epoch of mountain-elevation, when the
strata are squeezed and crushed laterally, fractured and shattered--the
result being the production of a series of more or less parallel
anticlines and synclines, or, in other words, a true mountain-chain.
Next we have a prolonged period of erosion, during which running water
flows through synclinal troughs, works along the backs of broken and
shattered anticlines, and makes its way by joints, gaping cracks,
and dislocations, to the low grounds. As time goes on, the varying
character of the rocks and the mode of their arrangement begin to tell:
the weaker structures are broken up; rock-falls and landslips ever and
anon take place; anticlinal ridges are gradually demolished, while
synclines tend to endure, and thus grow, as it were, into hills, by
the gradual removal of the more weakly-constructed rock-masses that
surround them. Valleys continue to be deepened and widened, while the
intervening mountains, eaten into by the rivers and their countless
feeders, and shattered and pulverised by springs and frosts, are
gradually narrowed, interrupted, and reduced, until eventually what
was formerly a great mountain-chain becomes converted into a low-lying
undulating plain. Should the region now experience a movement of
depression, and sink under the sea, new sedimentary deposits will
gather over its surface to a depth, it may be, of many hundreds or even
thousands of feet. Should this sunken area be once more elevated en
masse--pushed up bodily until it attains a height of several thousand
feet--it will form a plateau, composed of a series of horizontal strata
resting on the contorted and convoluted rocks of the ancient denuded
mountain-chain. The surface of the plateau will now be traversed by
streams and rivers, and in course of time it must become deeply cleft
and furrowed, the ground between the various valleys rising into
mountain-masses. Should the land remain stationary, its former fate
shall again overtake it; it will inevitably be degraded and worn down
by the sub-aërial agents of erosion, until once more it assumes the
character of a low-lying undulating plain.
Through such phases our Highlands have certainly passed. At a very
early epoch the Archæan rocks of the north-west were ridged up into
great mountain-masses, but before the beginning of the pre-Cambrian
period wide areas of those highly-contorted rocks had already been
planed across, so that when subsidence ensued the pre-Cambrian
sandstones were deposited upon a gently undulating surface of highly
convoluted strata. Another great epoch of mountain-making took place
after Lower Silurian times, and true mountain-ranges once more appeared
in the Highland area. We cannot tell how high those mountains may have
been, but they might well have rivalled the Alps. After their elevation
a prolonged period of erosion ensued, and the lofty mountain-land
was reduced in large measure to the condition of a plain, wide areas
of which were subsequently overflowed by the inland seas of Old Red
Sandstone times--so that the sediments of those seas or lakes now
rest with a violent unconformity on the upturned and denuded edges
of the folded and contorted Silurian strata. At a later geological
period the whole Highland area was elevated _en masse_, forming an
undulating plateau, traversed by countless streams and rivers, some of
which flowed in hollows that had existed before the beginning of Old
Red Sandstone times. Since that epoch of elevation the Highland area,
although subject to occasional oscillations of level, would appear to
have remained more or less continuously in the condition of dry land.
The result is, that the ancient plateau of erosion has been deeply
incised--the denuding agents have carved it into mountain and glen--the
forms and directions of which have been determined partly by the
original surface-slopes of the plateau, and partly by the petrological
character of the rocks and the geological structure of the ground.
[Illustration:
PLATE II.
INFLUENCE OF ROCK STRUCTURE ON THE FORM OF THE GROUND.
FIG. 1. PLATEAU OF ACCUMULATION: HORIZONTAL STRATA, DENUDED.
FIG. 2. SYNCLINAL (S.O.) AND ANTICLINAL (E.) STRATA, DENUDED.
FIG. 3. FAULTED STRATA, SHOWING DENUDATION.
FIG. 4. MOUNTAIN OF ACCUMULATION,--VOLCANO.
FIG. 5. DIAGRAMMATIC SECTION OF A TYPICAL MOUNTAIN CHAIN,
OR MOUNTAINS OF UPHEAVAL.
FIG. 6. TYPES OF ROCK STRUCTURE IN THE ALPS (AFTER PROF. HEIM)
_The dotted lines show portions of strata, denuded._
FIG. 7. ESCARPMENTS (e) AND DIP SLOPES (d).
FIG. 8. EROSION OF ANTICLINAL MOUNTAINS.
FIG. 9. PLATEAU OF EROSION, SHOWING MOUNTAINS OF CIRCUMDENUDATION (aa).
FIG 10. SECTION ACROSS BEN LAWERS AND LOCH TAY, SHOWING MOUNTAINS
OF CIRCUMDENUDATION.
The Edinburgh Geographical Institute J. G. Bartholomew F.R.G.S.
]
Thus, in the evolution of the surface-features of the earth, the
working of two great classes of geological agents is conspicuous--the
subterranean and the sub-aërial. The sinking down of the crust upon the
cooling nucleus would appear to have given rise to the great oceanic
depressions and continental ridges, just as the minor depressions
within our continental areas have originated many mountain-chains. In
the area undergoing depression the strata are subjected to intense
lateral pressure, to which they yield along certain lines by folding
up. The strata forming the Alps, which are 130 miles broad, originally
occupied a width of 200 miles; and similar evidence of enormous
compression is conspicuous in the structure of all mountains of
elevation. Great elevation, however, may take place with little or
no disturbance of stratification: wide continental areas have been
slowly upheaved _en masse_, and sea-bottoms and low-lying plains
have in this way been converted into lofty plateaux.[F] Many of the
most conspicuous features of the earth's surface, therefore, are due
directly to subterranean action. All those features, however, become
modified by denudation, and eventually the primeval configuration
may be entirely destroyed, and replaced by contours which bear no
direct relation to the form of the original surface. (See Fig. 9.)
In the newer mountain-chains of the globe the surface-features are
still largely those due directly to upheaval; so in some recently
elevated plateaux the ground has not yet been cut up and converted into
irregular mountain-masses. Many of the more ancient mountain-chains
and ranges, however, have been exposed so long to the abrading action
of the denuding agents that all trace of their original contour has
vanished. And in like manner plateaux of great age have been so
highly denuded, so cut and carved by the tools of erosion, that their
plateau character has become obscured. They have been converted into
undulating mountainous and hilly regions. Everywhere throughout the
world we read the same tale of subsidence and accumulation, of upheaval
and denudation. The ancient sedimentary deposits which form the major
portion of our land-surfaces, are the waste materials derived from
the demolition of plains, plateaux, and mountains of elevation. In
some mountain-regions we read the evidence of successive epochs of
uplift, separated by long intervening periods of erosion, followed
by depression and accumulation of newer sediments over the denuded
surface. Thus the Alps began to be elevated towards the close of
Palæozoic times. Erosion followed, and subsequently the land became
depressed, and a vast succession of deposits accumulated over its
surface during the long-continued Mesozoic era into early Cainozoic
times. Again, a great upheaval ensued, and the Mesozoic and Eocene
strata were violently contorted and folded along the flanks of the
chain. Then succeeded another period of erosion and depression, which
was again interrupted by one or more extensive upheavals. Away from
those lines of weakness which we call mountain-chains, we constantly
encounter evidence of widespread movements of elevation, during
which broad areas of sea-bottom have been upheaved to the light of
day, and, after suffering extensive denudation have subsided, to be
again overspread with the spoils of adjacent lands, and then upheaved
once more. And such oscillations of level have occurred again and
again. Looking back through the long vista of the past, we see each
continental area in a state of flux--land alternating with sea, and
sea with land--mountains and plateaux appearing and disappearing--a
constant succession of modifications, brought about by the antagonistic
subterranean and sub-aërial agents.
The hills are shadows, and they flow
From form to form, and nothing stands;
They melt like mists, the solid lands,
Like clouds they shape themselves and go.
[F] This is the generally accepted view of modern geologists. It is
very difficult, however, to understand how a wide continental area can
be vertically upheaved. It seems more probable that the upheaval of the
land is only apparent. The land seems to rise because the sea retreats
as the result of the subsidence of the crust within the great oceanic
basins. See Article xiv. (1892.)
IV.
The Cheviot Hills.[G]
[G] From _Good Words_ for 1876.
I.
The ridge of high ground that separates England from Scotland is not,
like many other hilly districts, the beloved of tourists. No guide-book
expatiates upon the attractiveness of the Cheviots; no cunningly-worded
hotel-puffs lure the unwary vagrant in search of health, or sport,
or the picturesque, to the quiet dells and pastoral uplands of the
Borders. Since the biographer of Dandie Dinmont, of joyous memory,
joined the shades, no magic sentences, either in verse or prose,
have turned any appreciable portion of the annual stream of tourists
in the direction of the Cheviots. The scenery is not of a nature to
satisfy the desires of those who look for something piquant--something
"sensational," as it were. It is therefore highly improbable that
the primeval repose of these Border uplands will ever be disturbed
by inroads of the "travelling public," even should some second Burns
arise to render the names of hills and streams as familiar as household
words. And yet those who can spare the time to make themselves well
acquainted with that region should do so; they will have no reason
to regret their visit, but very much the reverse. For the scenery is
of a kind which grows upon one. It shows no clamant beauties--you
cannot have its charms photographed--the passing stranger may see
nothing in it to detain him; but only tarry for a while amongst these
green uplands, and you shall find a strange attraction in their soft
outlines, in their utter quiet and restfulness. For those who are
wearied with the crush and din of life, I cannot think of a better
retreat. One may wander at will amongst the breezy hills, and inhale
the most invigorating air; springs of the coolest and clearest water
abound, and there are few of the brooks in their upper reaches which
will not furnish natural shower-baths. Did the reader ever indulge in
such a mountain-bath? If not, then let him on a summer day seek out
some rocky pool, sheltered from the sun, if possible, by birch and
mountain-ash, and, creeping in below the stream where it leaps from the
ledges above, allow the cool water to break upon his head, and he will
confess to having discovered a new aqueous luxury. Then from the slopes
and tops of the hills you have some of the finest panoramic views to
be seen in this island. Nor are there wanting picturesque nooks, and
striking rock scenery amongst the hills themselves: the sides of the
Cheviot are seamed with some wild, rugged chasms, which are just as
weird in their way as many of the rocky ravines that eat into the
heart of our Highland mountains. The beauty of the lower reaches of
some of the streams that issue from the Cheviots is well known; and
few tourists who enter the vale of the Teviot neglect to make the
acquaintance of the sylvan Jed. But other streams, such as the Bowmont,
the Kale, the Oxnam, and the Rule will also well repay a visit. In
addition to all these natural charms, the Cheviot district abounds in
other attractions. Those who are fond of Border lore, who love to seek
out the sites of old forays, and battles, and romantic incidents, will
find much to engage them; for every stream, and almost every hill, is
noted in tale and ballad. Or if the visitor have antiquarian tastes, he
may rival old Monkbarns, and do his best to explain the history of the
endless camps, ramparts, ditches, and terraces which abound everywhere,
especially towards the heads of the valleys. To the geologist the
district is not less interesting, as I hope to be able, in the course
of these papers, to show. The geological history of the Cheviots might
be shortly summed up, and given in a narrative form, but it will
perhaps be more interesting, and, at the same time more instructive,
if we shall, instead, go a little into detail, and show first what
the nature of the evidence is, and, second, how that evidence may be
pieced together so as to tell its own story. I may just premise that
my descriptions refer almost exclusively to the Scottish side of the
Cheviots--which is not only the most picturesque, but also the most
interesting, both from an antiquarian and geological point of view.
The Cheviots extend from the head of the Tyne in Northumberland, and
of the Liddel in Roxburghshire, to Yeavering Bell and the heights
in its neighbourhood (near Wooler), a distance of upwards of thirty
miles. Some will have it that the range goes westward so as to include
the heights about the source of the Teviot, but this is certainly a
mistake, for after leaving Peel Fell and crossing to the heights on the
other side of the Liddel Water, we enter a region which, both in its
physical aspect and its geological structure, differs considerably from
the hilly district that lies between Peel Fell and the high-grounds
that roll down to the wide plains watered by the Glen and the Till.
The highest point in the range is that which gives its name to the
hills--namely, the Cheviot--a massive broad-topped hill, which reaches
an elevation of 2767 feet above the sea, and from which a wonderful
panorama can be scanned on a clear day. The top of the hill is coated
with peat, fifteen to twenty feet thick, in some places. A number of
deep ravines trench its slopes, the most noted of which are Hen Hole
and the Bizzle. Peel Fell, at the other extremity of the range, is
only 1964 feet high, while the dominant points between Peel Fell and
the Cheviot are still lower--ranging from 1500 feet to 1800 feet. The
general character of the hills is that of smooth rounded masses, with
long flowing outlines. There are no peaks, nor serrated ridges, such as
are occasionally met with in the northern Highlands; and the valleys
as a rule show no precipitous crags and rocky precipices, the most
conspicuous exceptions being the deep clefts mentioned as occurring in
the Cheviot. The hills fall away with a long gentle slope into England,
while on the Scottish side the descent is somewhat abrupt; so that
upon the whole the northern or Scottish portion of the Cheviots has
more of the picturesque to commend it than the corresponding districts
in England. Indeed, the opposite slopes of the range show some rather
striking contrasts. The long, flat-topped elevations on the English
side, that sweep south and south-west from Carter Fell and Harden Edge,
and which are drained by the Tyne, the Rede Water, and the Coquet,
are covered for the most part with peat. Sometimes, however, when the
slope is too great to admit of its growth, the peat gives place to
rough scanty grass and scrubby heath, which barely suffice to hide the
underlying barren sandstone rocks. One coming from the Scottish side
is hardly prepared, indeed, for the dreary aspect of this region as
viewed from the dominant ridge of the Cheviots. If in their physical
aspect the English slopes of these hills are for the most part less
attractive than the Scottish, it is true also that they offer less
variety of interest to the geologist. Those who have journeyed in
stagecoaching times from England into Scotland by Carter Fell, will
remember the relief they felt when, having surmounted the hill above
Whitelee, and escaped from the dreary barrens of the English border,
they suddenly caught a sight of the green slopes of the Scottish hills,
and the well-wooded vales of Edgerston Burn and Jed Water. On a clear
day the view from this point is very charming. Away to the west stretch
in seemingly endless undulations the swelling hills that circle round
the upper reaches of Teviotdale. To east and north-east the eye glances
along the bright-green Cheviots of the Scottish border, and marks how
they plunge, for the most part somewhat suddenly, into the low grounds,
save here and there, where they sink in gentler slopes, or throw out a
few scattered outposts--abrupt verdant hills that somehow look as if
they had broken away from the main mass of the range. From the same
standpoint one traces the valleys of the Rule and the Jed--sweetest
of border streams--stretching north into the well-clothed vale of the
Teviot. Indeed, nearly the whole of that highly-cultivated and often
richly-wooded country that extends from the base of the Cheviots to
the foot of the Lammermuirs, lies stretched before one. Here and there
abrupt isolated hills rise up amid the undulating low grounds, to hide
the country behind them. Of these the most picturesque are dark Rubers
Law, overlooking the Rule Water; Minto Crags, and Penielheugh with its
ugly excrescence of a monument, both on the north side of the Teviot;
and the Eildon Hills, which, as all the world knows, are near Melrose.
After he has sated himself with the rare beauty of this landscape (and
still finer panoramic views are to be had from the top of Blackhall
Hill, Hownam Law, the Cheviot, as also from various points on the line
of the Roman Road and other paths across the hills into England),
the observer will hardly fail to be struck by the great variety of
outlines exhibited. Some of the hills, especially those to the west and
north-west, are grouped in heavy masses, and present for the most part
a soft, rounded contour, the hills being broad atop and flowing into
each other with long, smooth slopes. Other elevations, such as those
to the east and north-east of Carter Fell, while showing similar long
gentle slopes, yet are somewhat more irregular in form and broken in
outline, the hills having frequently a lumpy contour. Very noteworthy
objects in the landscape also are the little isolated hills of the
low grounds, such as Rubers Law, and the Dunian, above Jedburgh. They
rise, as I have said, quite suddenly out of that low gently undulating
country that sinks softly into the vales of the Teviot and the Tweed.
This variety arises from the geological structure of the district. The
hills vary in outline partly because they are made up of different
kinds of rock, and partly owing to the mode in which these rocks have
been arranged. But notwithstanding all this variety of outline, one
may notice a certain sameness too. Flowing outlines are more or less
conspicuous all over the landscape. Many of the hills, especially as
we descend into Teviotdale, seem to have been smoothed or rounded off,
as it were, so as to present their steepest faces as a rule towards
the south-west. And if we take the compass-bearing of the hill-ridges
of the same district, we shall find that these generally trend from
south-west to north-east So much, then, at present for the surface
configuration of the Cheviot region. When we come to treat of the
various rock-masses, and to describe the superficial accumulations
underneath which these are often concealed, we shall be in a better
position to give an intelligible account of the peculiar form of the
ground, and the causes to which that configuration must be ascribed.
The solid rocks which enter into the composition of the Cheviots
consist mainly of (1) hard grey and blue rocks, called _greywacké_ by
geologists, with which are associated blue and grey shale; (2) various
old igneous rocks; and (3) sandstones, red and white, interbedded
with which occur occasional dark shales. Now, before we can make any
endeavour towards reconstructing in outline the physical geography of
the Cheviot Hills during past ages, it is necessary that we should
discover the order in which the rock-masses just referred to have been
amassed. I shall first describe, therefore, some sections where the
members of the different series are found in juxtaposition, for the
purpose of pointing out which is the lowest-lying, and consequently the
oldest, and which occupy the uppermost and intermediate positions.
[Illustration: FIG 1.--Conglomerate and Red Sandstone, etc., _c_,
resting on Greywacké and Shale, _g_.]
The first section to which reference may be made is exposed in the
course of the River Jed, at Allars Mill, a little above Jedburgh.
This section is famous in its way as having been described and
figured by Dr. Hutton, who may be said to have founded the present
system of physical geology. In the bed of the stream are seen certain
confused ridges of a greyish blue rock running right across the river
course--that is, in a direction a little north of east and south
of west. These ridges are the exposed edges of beds of greywacké
and shale, which are here standing on end. The beds are somewhat
irregular, being inclined from the vertical, now in one direction
and now in another, or, as a geologist would say, the "dip" changes
rapidly, sometimes being up the valley and sometimes down. The same
beds continue up the steep bank of the river for a yard or two, and
are there capped by another set of rocks altogether, namely, by soft
red sandy beds which at the bottom become _conglomeratic_--that is
to say, they are charged with water-worn stones. The annexed diagram
(Fig. 1) will show the general appearances presented: _g_ represents
the vertical greywacké and shale, and _c_ the overlying deposits of
conglomerate and red sandy beds. Now let us see what this section
means. What, in the first place, is greywacké? The term itself has
really no meaning, being a name given by the miners in the Harz
Mountains to the unproductive rocks associated with the vein-stones
which they work. When we break the rock we may observe that it is a
granular mixture of small particles of quartz, to which sometimes
felspar and other minerals are added. The grains are bound together
in a hardened matrix of argillaceous or clayey and silicious matter,
blue, or grey, or green, or brown and yellow, as the case may be.
At Allars Mill, and generally throughout the Cheviot district, the
prevailing colour is a pale greyish blue or bluish grey; but shades of
green and brown often occur. The component particles of the rock are
usually rounded or water-worn. Again, we notice that the ridges and
bands of rock that traverse the course of the Jed at Allars Mill are
merely the outcrops of successive _strata_ or beds. It is clear then
that greywacké and the grey shales that accompany it are _aqueous_
rocks--that is to say, they consist of hardened sediment, which has
undoubtedly been deposited in successive layers of variable thickness
by water in motion. But since the sediments of rivers and currents
are laid down in approximately horizontal planes, it is evident that
if the greywacké and shale be sedimentary deposits they have suffered
considerable disturbance since the time of their formation; for, as
we have seen, the beds, instead of being horizontal or only gently
inclined, actually approach the vertical. The fact is, that the
outcrops which we see are only the truncated portions of what were
formerly rapid undulations or folds of the strata, the tops of the
folds or arches having been cut away by geological agencies, to which
I shall refer by-and-by. What were at one time horizontal strata have
been crumpled up into great folds, the folds being squeezed tightly
together, and their upper portions planed away before the overlying red
sandy beds were laid down. The accompanying diagram (Fig. 2) may serve
to make all this clearer. Let A A represent the present surface of the
ground, and B B a depth of say fifty feet or a hundred feet from the
surface. The continuous lines between A and B represent the greywacké
beds as we now see them in section; the dotted lines above A A indicate
the former extension of the strata, and the dotted lines below B B
their continuation below that datum line. Hence it is obvious that in
a succession of vertical or highly inclined beds, we may have the same
strata repeated many times, the same beds coming again and again to the
surface. Thus the stratum at S is evidently the same bed as that at W,
X, Y, and Z.
[Illustration: Fig 2.]
Such great foldings or redoublings of strata are most probably
originated during subsidence of a portion of the earth's crust. While
the ground is slowly sinking down, the strata underneath are perforce
compelled to occupy less space laterally, and this they can only do by
yielding amongst themselves. All folding or contortion on the large
scale--that, namely, which has affected areas of strata extending
over whole countries--seems to have taken place under great pressure;
in other words, to have been produced at considerable depths from
the earth's surface. We can conceive, therefore, of a wide tract of
land sinking down for hundreds of feet, and producing at the surface
comparatively little change. But a depression of a few hundred feet
at the surface implies a considerably greater depression at a depth
of several thousand feet from the surface, and it is at great depths,
therefore, that the most violent folding must take place. Consequently
considerable contortion, and much folding, and lateral crushing
and reduplication of strata may occur, and yet no trace of this be
observable at the surface, save only a gentle depression. For example,
in Greenland, a movement of subsidence has been going on for many
years--the land has been slowly sinking down. The rocks at the surface
are of course quite undisturbed by this widely-extended movement, but
the strata at great depths may be undergoing much compression and
contortion. It follows from such considerations, that if we now get
highly contorted strata covering wide areas at the surface, we suspect
that very considerable _denudation_ has taken place. That is to say,
large masses of rock have been removed by the geological agents of
change, so as to expose the once deeply-buried tops of the arched or
curved and folded strata. We may therefore infer from a study of the
phenomena in the Jed at Allars Mill, first, that the red sandy beds are
younger than the greywacké and shale, seeing that they rest upon them;
and, second, that a very long period of time must have elapsed between
the deposition of the older and the accumulation of the younger set
of strata; for it is obvious that considerable time was required for
the consolidation and folding of the greywacké, and an incalculable
lapse of ages was also necessary to allow of the gradual wearing away
by rain, frost, and running water of the great thickness of rocks
underneath which the greywacké was crumpled. And all this took place
before the horizontally-bedded red sandstone and conglomerate gathered
over the upturned ends of the underlying strata. The succession of
rocks at Allars Mill is seen in many other places in the Cheviot
district, but enough has been said to prove that the greywacké beds are
the older of the two sets of strata.
There is another class of rocks, the relative position of which we
must now ascertain, for no one shall wander much or far among the
Cheviots without becoming aware of the existence of other kinds of
rock than greywacké and sandstone. Many of the hills east of Oxnam
and Jed Waters, for example, are composed of igneous masses--of
rocks which have had a volcanic origin. As we shall afterwards see,
the whole north-eastern section of the Cheviots is built up of such
rocks. At present, however, we are only concerned with the relation
which these bear to the greywacké and the red sandy beds. Now at
various localities--for example, in Edgerston Burn, on the hill-face
south of Plenderleith, and again along the steep front of Hindhope
and Blackball Hills, which are on the crest of the Cheviots--we find
that the igneous rocks rest upon the greywacké and shale (see Fig. 3)
precisely in the same way as do the red sandy beds. They therefore
belong to a later date than the greywacké. In other places, again, we
meet with the conglomerates and red sandstones (_c_, Fig. 4) resting
upon and wrapping round the igneous rocks, _i_, and thus it becomes
quite obvious that the latter occupy an intermediate position between
the greywacké and shale on the one hand, and the conglomerate and red
sandstone upon the other.
[Illustration: Fig. 3.--Igneous rocks (_i_, _a_) resting on Greywacké
and Shale, _g_.]
[Illustration: Fig. 4.--_c_, Conglomerate and Sandstones, resting on
Igneous rocks, _i_.]
We have now cleared the way so far, preparatory to an attempt to trace
the geological history of the Cheviots. The three sets of rocks,
whose mutual relations we have been studying, are those of which the
district is chiefly composed; but, as we shall see in the sequel,
there are others, not certainly of much extent, but nevertheless
having an interesting story to tell us. Nor shall we omit to notice
the superficial accumulations of clay, gravel, sand, silt, alluvium,
and peat; monuments as they are of certain great changes, climatic and
geographical, which have characterised not the Cheviots only, but a
much wider area.
II.
If we draw a somewhat straight line from Girvan, on the coast of
Ayrshire, in a north-east direction to the shores of the North Sea,
near Dunbar, we shall find that south of that line, up to the English
border, nearly the whole country is composed of various kinds of
greywacké and shale like the basement beds of the Cheviot district.
Here and there, however, especially in certain of the valleys and some
of the low-lying portions of this southern section of Scotland, one
comes upon small isolated patches and occasional wider areas of younger
strata, which rest upon and conceal the greywackés and shales. Such is
the case in Teviotdale, the Cheviot district, and the country watered
by the lower reaches of the Tweed, in which regions the bottom beds
are hidden for several hundreds of square miles underneath younger
rocks. Indeed, the greywacké and shale form but a very small portion of
the surface in the Cheviots, appearing upon a coloured geological map
like so many islands or fragments, as it were, which have somehow been
detached from the main masses of greywacké of which the Lammermuirs and
the uplands of Dumfries and Selkirk shires are composed. Although the
bottom rocks of the Cheviot Hills are thus apparently separated from
the great greywacké area, there can be no doubt that they are really
connected with it, the connection being obscured by the overlying
younger strata. For if we could only strip off these latter, if we
could only lift aside the great masses of igneous rock and sandstone
that are piled up in the Cheviot Hills and the adjoining districts,
we should find that the bottom upon which these rest is everywhere
greywacké and shale. In part proof of this it may be mentioned that
at various places in those districts which are entirely occupied by
sandstone and igneous rock, the streams have cut right down through
the younger rocks so as to expose the bottom beds, as in Jed Water at
Allars Mill. Again, when we trace out the boundaries of any detached
areas of greywacké we invariably find these bottom beds disappearing on
all sides underneath the younger strata by which they are surrounded.
One such isolated area occurs in the basin of the Oxnam Water, between
Littletonleys and Bloodylaws, a section across which would exhibit
the general appearance shown in the accompanying diagram (Fig. 5).
Another similarly isolated patch is intersected by Edgerston Burn and
the Jed Water between Paton Haugh and Dovesford. But the largest of
these detached portions appears, forming the crest of the Cheviots,
at the head of the River Coquet. There the basement beds occupy the
watershed, extending westward, some three or four miles, as far as
the sandstones of Hungry Law, while to the north and east they plunge
under the igneous rocks of Brownhart Law and the Hindhope Hills. Now
it is evident that all those detached and isolated areas of greywacké
and shale are really connected underground, and not only so, but they
also piece on in the same way to the great belt of similar strata
that stretches from sea to sea across the whole breadth of Scotland.
Indeed, we may observe in the Cheviot district how long and massive
promontories of greywacké jut out from that great belt, and extend
often for miles into the areas that are covered with younger strata,
as, for example, in the Brockilaw and Wolfelee Hills. A generalised
section across the greywacké regions of the Cheviot Hills would
therefore present the appearances shown in the annexed diagram (Fig.
6), in which G represents the basement beds, I the igneous rocks, and C
the red sandstones, etc.
[Illustration: Fig. 5.--Section across Greywacké area of Oxnam Water;
G, Greywacké and Shale; C, Sandstone, etc.]
[Illustration: Fig. 6.--Diagram section across Greywacké districts of
Cheviot Hills.]
Throughout the whole of the district under review the bottom beds
are observed to dip at a high angle--the strata in many places being
actually vertical--and the edges or crops of the strata run somewhat
persistently in one direction, namely, from south by west to north
by east; or, as a geologist would express it, the beds have an
approximately south-west and north-east "strike." Now as the dip is
sometimes to north-west and sometimes to south-east, it is evident that
the rocks have been folded up in a series of rapid convolutions, and
that some of the beds must be often repeated.
From the character of the fossils which the bottom beds have yielded
we learn that the strata belong to that division of past time
which is known as the Silurian age. These fossils appear to be of
infrequent occurrence, and the creatures of which they are the relics
occupied rather a humble place in the scale of being. They are called
_graptolites_ (from their resemblance to pens), an extinct group of
hydroid zoophytes, apparently resembling the sertularians of our own
seas.
The general appearance of the Silurian strata of the Cheviots is
indicative of deposition in comparatively quiet water, but how deep
that water was one cannot say. Upon the whole, the beds look not
unlike the sediments that gather in calm reaches of the sea, such as
estuaries, betokening the presence of some not distant land from which
fine mud and sand were washed down. Another proof that some of the
strata at all events were accumulated not far from a shore-line, is
found in certain coarse bands of grit and pebbles, which are not likely
to have been formed in deep water. This evidence, however, cannot be
considered decisive, and in the present state of our knowledge all
that we can assert with anything like confidence is simply this:--That
during the deposition of the Silurian strata the whole of the Cheviot
area lay under water--existed, in short, as a muddy sea-bottom, in the
slime of which flourished here and there, in favourable spots, those
minute hydroid animals called _graptolites_.
Between the deposition of the Silurian and the formation of the rocks
that come next in order a long interval elapsed, during which the mud,
sand, and grit that gathered on the floor of the ancient sea were
hardened into solid masses, and eventually squeezed together into great
folds and undulations. It has already been pointed out that these
changes could hardly have been effected save under extreme pressure,
and this consideration leads us to infer that a great thickness of
strata has been removed entirely from the Cheviot district, so as to
leave no trace of its former existence. Long before the deposition
of the younger strata that now rest upon and conceal the Silurian
rocks, the action of the denuding forces--the sea, frosts, rain, and
rivers--had succeeded in not only sweeping gradually away the strata
underneath which the bottom beds were folded, but in deeply scarping
and carving these bottom beds themselves. Can we form any reasonable
conjecture as to the geological age of the strata underneath which the
bottom beds of the Cheviots were folded, and which, as we have seen,
had entirely disappeared before the younger rocks of the district were
accumulated? Well, it is obvious that the missing strata must have been
of later formation than the bottom beds, and it is equally evident that
they must have been of much more ancient date than the igneous rocks of
the Cheviot Hills. Now, as we shall afterwards see, these igneous rocks
belong to the Old Red Sandstone age, that is to say, to the age that
succeeded the Silurian. How is it then, if the bottom beds be really
of Silurian and the igneous rocks of Old Red Sandstone age, that a
gap is said to exist between them? The explanation of this apparent
contradiction is not far to seek. When we compare the fossils that
occur in the Silurian strata of the Cheviot Hills and the districts
to the west, with the organic remains disinterred from similar strata
elsewhere, as in Wales for example, we find that the bottom beds of
the Cheviots were in all probability accumulated at approximately the
same time as certain strata that occur in the middle division of the
Upper Silurian. In Wales and in Cumberland the strata that approximate
in age to the Silurian of the Cheviots are covered by younger strata
belonging to the same formation which reach a thickness of several
thousand feet. It may quite well be, therefore, that the succession of
Silurian strata in the Cheviots was at one time more complete than it
is now. The upper portions of the formation which are so well developed
in Wales and Cumberland, and which are likewise represented to a small
extent in Scotland, had in all probability their equivalents in what
are our border districts. In other words, there are good grounds for
believing that the existing Silurian rocks of the Cheviots were in
times preceding the Old Red Sandstone age covered with younger strata
belonging to the same great system. The missing Silurian strata of the
Cheviots may have attained a thickness of several thousand feet, and
underneath such a mass of solid rock the lower-lying strata might well
have been consolidated and subsequently squeezed into folds.
We now pass on to consider the next chapter in the geological history
of the Cheviot Hills. As we proceed in our investigations it will
be noticed that the evidence becomes more abundant, and we are thus
enabled to build up the story of the past with more confidence,
and with fuller details. For it is with geological history as with
human records--the further back we go in time the scantier do the
facts become. The rocks upon which Nature writes her own history
are palimpsests, on which the later writing is ever the most easily
deciphered. Nay, she cannot compile her newer records without first
destroying some of those compiled in earlier times. The sediments
accumulating in modern lake and sea are but the materials derived
from the degradation of the rocks we see around us, just as these in
like manner have originated from the demolition of yet older strata.
Thus the further we trace back the history of our earth, the more
fragmentary must we expect the evidence to be; and conversely, the
nearer we approach to the present condition of things the more abundant
and satisfactory must the records become. Accordingly, we find that
the igneous rocks of the Cheviot Hills tell us considerably more than
the ancient Silurian deposits upon which they rest. The surface of the
latter appears to be somewhat irregular underneath the igneous rocks,
showing that hills and valleys, or an undulating table-land, existed in
the Cheviot district prior to the appearance of the younger formation.
But before we attempt to summarise the history of that formation, it is
necessary to give some description, however short, of the rocks that
compose it.
These consist chiefly of numerous varieties of a rock called porphyrite
by geologists, piled in more or less irregular beds, one on top of
another, in a somewhat confused manner. The colour of the freshly
fractured rocks is very variable, being usually some shade of blue or
purple; but pink, red, brown, greenish, and dark grey or almost black
varieties also occur. Some of the rocks are finely crystalline; others,
again, are much coarser, while many are compact, or nearly so, a lens
being required to detect a crystalline texture. The mineral called
felspar is usually scattered more or less abundantly through the matrix
or base, which itself is composed principally of felspathic materials.
Besides distinct scattered crystals of felspar, other minerals often
occur in a similar manner; mica and hornblende being the commonest.
Occasionally the rocks contain numerous circular, oval, or flattened
cavities, which are sometimes so abundant as to give the appearance
of a kind of coarse slag to the porphyrite. These little cavities,
however, are usually filled up with mineral matter--such as calcspar,
calcedony, jasper, quartz, etc. Sometimes also cracks, crannies, and
crevices of some size have been sealed up with similar minerals. Now
nearly all these appearances are specially characteristic of rocks
which have at one time been in a state of igneous fusion; nor can
there be any doubt that the Cheviot porphyrites are merely solidified
lava-beds, which have been poured out from the bowels of the earth.
In modern lavas we may notice not only a crystalline texture, but
frequently also we observe those in our porphyrites. Such cavities are
due to the expansive force of the vapours imprisoned in the molten mass
at the time of eruption. They form chiefly towards the upper surface of
a lava stream, and are often drawn out or flattened in the direction
in which the lava flows. Thus a stream of lava, as it creeps on its
way, becomes slaggy and scoriaceous or cindery above and in front, and
as the molten mass within continues to flow, the slags and cinders
that cover its face tumble down before it, and form the pavement
upon which the stream advances. In this way slags and cinders become
incorporated with the bottom of the lava, and hence it is that so many
volcanic rocks are scoriaceous, as well below as above. The vapours
which produce the cavities usually contain minerals in solution, and
these, as the lava cools, are frequently deposited, partially filling
up the vesicles, so as to form what are called geodes. But many of
the cavities have been filled in another way--by the subsequent
infiltration of water carrying mineral matter in solution. And since
we know that all rocks are so permeated by water, it is clear that
the cavities may have received their contents during many successive
periods, after the solidification of the rock in which they occur. It
is in this manner that the jaspers, calcedony, and beautiful agates of
commerce have been formed. Rocks abundantly charged with cavities are
said to be _vesicular_, and when the vesicles are filled with mineral
matter, then the mass becomes, in geological language, _amygdaloidal_,
from the almond-like shape assumed by the flattened vesicles.
Now all the appearances described above, and many others hardly
less characteristic of true lavas, are to be met with amongst those
porphyrites which, as I have said, form the major portion of the
Cheviot Hills. From the valley of the Oxnam, east by Cessford,
Morebattle, and Hoselaw, and south by Edgerston, Letham, Browndeanlaws,
and Hindhope, the porphyrites extend over the whole area, sweeping
north-east across the border on to the heights above the Rivers Glen
and Till. In the hills at Hindhope we notice a good display of the
oldest beds of the series. At the base occurs a very peculiar rock
resting upon the Silurian, and thus forming the foundation of the
porphyrites. It varies in colour, being pink, grey, green, red, brown,
or variously mottled. Sometimes it is fine-grained and gritty, like
a soft, coarse-grained sandstone; at other times it is not unlike a
granular porphyrite; but when most typically developed it consists of a
kind of coarse angular gravel embedded in a gritty matrix. The stones
sometimes show distinct traces of arrangement into layers; but they are
often heaped rudely together with little or no stratification at all.
They consist chiefly of fragments of porphyrites; but bits of Silurian
rocks also occur amongst them. This peculiar deposit unquestionably
answers to the heaps of dust, sand, stones, and bombs which are shot
out of modern volcanoes; it is a true tuff--that is, a collection of
loose volcanic ejectamenta.
Upon what kind of surface did it fall? Long before the eruptions began,
the Silurian rocks had been sculptured into hills and valleys by the
action chiefly of the sub-aërial forces, and it was upon these hills
and in these valleys that the igneous materials accumulated. It is
difficult to say, however, whether at this period the Cheviot district
was above or under water. The traces of bedding in the tuff would seem
to indicate the assorting power of water; but the evidence is too
slight to found upon, because we know that in modern eruptions, loose
ejectamenta frequently assume a kind of irregular bedded arrangement.
For aught we can say to the contrary, therefore, dry land may have
extended across what is now southern Scotland and northern England
when the first rumblings of volcanic disturbance shook the Cheviot
area. Be that as it may, we know that the volcanic outbursts began in
those old times, as they almost invariably commence now, by a discharge
of sand, small stones, blocks, and cinders. These, we may infer,
covered a wide area round the centre of dispersion--the chief focus
of eruption being probably in the vicinity of the big Cheviot, where
a mass of granite seems to occupy the core or deep-seated portion of
the old volcanic centre. The locality where the tuff occurs is some
nine miles or so distant from this point, and the intervening ground
could hardly have escaped being more or less thickly sprinkled with
the same materials. The whole of that intervening ground, however, now
lies deeply buried under the massive streams of once-molten rock that
followed in succession after the first dispersion of stones and débris.
Although, as I have said, it may be doubted whether at the beginning of
their activity the Cheviot volcanoes were sub-aqueous, yet there are
not a few facts that lead to the inference that the eruption of the
porphyrites took place for the most part, if not exclusively, under
water. The beds are occasionally separated by layers of sandstone,
grit, and conglomerate; but such beds are rare, and true tuffs are
rarer still. If the outbursts had been sub-aërial, we ought surely to
have met with these latter in greater abundance, while we should hardly
have expected to find such evidently water-arranged strata as do occur
here and there. The porphyrites themselves present certain appearances
which lead to the same conclusion. Thus we may observe how the bottoms
of the beds frequently contain baked or hardened sand and mud, showing
that the molten rock had been poured out over some muddy or sandy
bottom, and had caught up and enclosed the soft, sedimentary materials,
which now bear all the marks of having been subjected to the action
of intense heat. Sometimes, indeed, the old lava-streams seem to have
licked up beds of unconsolidated gravel, the water-worn stones being
now scattered through their under portions. As no fossils occur in any
of the beds associated with the porphyrites, one cannot say whether
the latter flowed into the sea or into great freshwater lakes. Neither
can we be certain that towards their close the eruptions were not
sub-aërial. They may quite well have been so. The porphyrites attain
a thickness of probably not less than fifteen hundred or two thousand
feet, and the beds which we now see are only the basal, and therefore
the older portions of the old volcanoes. The upper parts have long
since disappeared, the waste of the igneous masses having been so great
that only the very oldest portions now remain, and these, again, are
hewn and carved into hill and valley. Any loose accumulation of stones
and débris, therefore, which may have been thrown out in the later
stages of the eruptions, must long ere this have utterly disappeared.
We can point to the beds which mark the beginning of volcanic activity
in the Cheviots; we can prove that volcanoes continued in action there
for long ages, great streams of lava being poured out--the eruptions of
which were preceded and sometimes succeeded by showers of stones and
débris; we can show, also, that periods of quiescence, more or less
prolonged, occasionally intervened, at which times water assorted the
sand and mud, and rounded the stones, spreading them out in layers.
But whether this water action took place in the sea or in a lake we
cannot tell. Indeed, for aught one can say, some of the masses of
rounded stones I refer to may point to the action of mountain torrents,
and thus be part evidence that the volcanoes were sub-aërial. If we
are thus in doubt as to some of the physical conditions that obtained
in the Cheviot district during the accumulation of the porphyrites
and their associated beds, we are left entirely to conjecture when
we seek to inquire into the conditions that prevailed towards the
close of the volcanic period. For just as we have proof that before
this period began the Silurian strata had been subjected to the most
intense denudation--had, in short, been worn into hill and valley--so
do we learn from abundant evidence that the rocks representing the old
volcanoes of the Cheviots are merely the wrecks of formerly extensive
masses. Not only have the upper portions of these volcanoes been swept
away, but their lower portions, likewise, have been deeply incised, and
thousands of feet of solid rock have been carried off by the denuding
forces. And by much the greater part of all this waste took place
before the accumulation of those sandstones which now rest upon the
worn outskirts of the old volcanic region.
III.
Some reference has already been made (see p. 64) to the general
appearance presented by the valleys of the Cheviots. In their upper
reaches they are often rough and craggy; narrow dells, in fact, flanked
with steep shingle-covered slopes, and occasionally overlooked by
beetling cliffs, or fringed with lofty scaurs of decomposing rocks. As
we follow down the valleys they gradually widen out; the hill-slopes
becoming less steep, and retiring from the stream so as to leave a
narrow strip of meadow-land through which the clear waters canter gaily
on to the low grounds of the Teviot. In their middle reaches these
upland dales are not infrequently well cultivated to a considerable
height, as in the districts between Hownam and Morebattle, and between
Belford and Yetholm--the former in the valley of the Kale, and the
latter in that of the Bowmont. It is noticeable that all the narrower
and steeper reaches lie among Silurian strata and Old Red Sandstone
porphyrites. No sooner do we leave the regions occupied by these tough
and hard rock-masses than the whole aspect of the scenery changes.
The surrounding hills immediately lose in height and fall away into a
softly undulating country, through which the streams and rivers have
dug for themselves deep romantic channels. Nevertheless, it is a fact,
as we shall see by-and-by, that south-west of the region occupied by
the igneous rocks of the Cheviot Hills, all the higher portions of
the range (Hungry Law, Carter Fell, Peel Fell, etc.) are built up of
sandstones. For the present, however, I confine attention to those
valleys whose upper reaches lie either wholly or in part among igneous
rocks or Silurian strata. A typical and certainly the most beautiful
example is furnished us by the vale of the River Jed. This stream
rises among the sandstone heights which have just been mentioned as
composing the south-west portion of the Cheviot range. The first seven
or eight miles of its course lead us through a broad open valley, which
has been hollowed out almost exclusively in sandstones and shales;
by-and-by, however, we are led into a Silurian tract, and thereupon
the valley contracts and the hill-slopes descend more steeply to the
stream. But we soon leave the grassy glades of this Silurian tract and
enter all at once upon what may be termed the lower reaches of the
Jed. No longer cooped up in the rocky gully, painfully worn for itself
in the hard greywacké and shales, the stream now winds through a much
deeper and broader channel which has evidently been excavated with
greater ease. Precipitous banks and scaurs here overlook the river at
every bend, the banks becoming higher and higher and retiring further
and further from each other, as the water glides on its way, until at
last they fairly open upon the broad vale of the Teviot. Sometimes the
river flows along one side of its valley for a considerable distance,
and whenever this is the case, it gives us a line of bold cliffs which
are usually flanked on the opposite side by sloping ground. This is
the general character of all valleys of erosion, and especially of the
lower reaches of the Jed.
A glance at the cliffs and scaurs of the Jed shows that they consist of
horizontal or gently undulating strata of soft earthy, friable, shaly
sandstone, arranged in thin beds and bands, which alternate rapidly
with crumbling, sandy, and earthy shales; the whole forming a loose
and unconsolidated mass that readily becomes a prey to the action of
the weather, rain, frost, and running water. The prevailing colour is
a dull red, but pale yellow, white, green, and purple discolorations
are visible when the strata are closely scanned. The finest sections
occur between Glen Douglas and Inchbonnie, and at Mossburnford, but
the cliffs throughout present the same general appearance, and are
picturesque in the highest degree. Everywhere the banks are thickly
wooded, and even the steep red scaurs are dashed and flecked with
greenery, which droops and springs from every ledge and crevice in
which a root can fix itself. How vivid and striking is the contrast
between the fresh delicate green of early summer and the rich warm
tint of these rocks, which when lit up by the setting sun seem almost
to glow and burn! Well may the good folk of Jedburgh be proud of the
lovely valley in which their lot is cast. In no similar district in
Scotland will the artist meet with a greater number of such "delicious
bits," in which all the charms of wood and water, of meadow and rock
are so harmoniously combined. It is not with the scenic beauties of
the Jed, however, that we have at present to do. I wish the reader
to examine with me certain appearances visible at the base of the
red beds, where these rest upon those older rocks which have formed
the subject of the preceding papers. In the bed of the river at
Jedburgh, we see the junction between the red beds and the Silurian
strata, and may observe how the bottom portions of the former, which
repose immediately upon the greywackés, are abundantly charged with
well-rounded and water-worn stones. Many of these stones consist of
greywacké, hardened grit, and other kinds of rock, and most of them
undoubtedly have been derived from Silurian strata. In other districts
where the old igneous rocks of the Cheviots form the pavement upon
which the red beds repose, the stones in the lower portions of the
latter are made up chiefly of rounded fragments of the underlying
porphyrites. All which clearly shows that the red beds have been
built out of the ruins of the older strata of the district. This is
unquestionably the origin not only of the conglomerates, but of all
the red beds through which the River Jed cuts its way from the base
of the hills to the Teviot. When we trace out the boundary of these
beds, we find that this leads us along the base of the hills, close to
the hill-foot; and not only so, but it frequently takes us into the
hill-valleys also. And this shows that the Cheviots had already been
deeply excavated by streams before any portion of the red beds was
deposited.
I have said that the red beds are approximately horizontal; sometimes,
however, they have a decided _dip_ or inclination, and when this is
continuous, it is invariably in a direction away from the hills. Thus
as we traverse the ground from the hill-foot to the Teviot, we pass
over the outcrops of the red beds and slowly rise from a lower to a
higher geological position. The strata, however, are generally so
flat that their dip is often not greater than the average slope or
inclination of the ground. Hence when we ascend the valley-slopes
from the stream, we soon reach the higher beds of the series, as,
for example, in the undulating heights that overlook the Jed in the
neighbourhood of Jedburgh. In that district a number of quarries
have been opened, in which the upper beds of the red series are well
exposed, as at Ferniehirst, Tudhope, etc. These consist of thick beds
of greyish white, yellowish, and reddish sandstones, which, unlike
the crumbling earthy deposits below, are quite suitable for building
purposes. Scales of fish and plant remains are often met with in the
thick sandstones, but the underlying earthy, friable red beds appear to
be quite destitute of any organic remains.
Let us now briefly recapitulate the main facts we have just
ascertained. They are these:--1. All the low grounds that abut upon the
hills are composed of horizontal or nearly horizontal strata, which
consist chiefly of red earthy beds, passing down into conglomerates,
and up into whitish and reddish sandstones. 2. The conglomeratic
portion forms the boundary of the series, fringing the outskirts of the
hills, and resting sometimes upon Silurian strata and sometimes upon
Old Red Sandstone igneous rocks. 3. Fossils occur in the white and red
sandstones, but seem to be wanting in the underlying red earthy beds.
[Illustration: Fig. 7.--S, Silurian strata; _i_, Old Red Sandstone
Igneous Rocks; _a^1_, Conglomerate; _a^2_, Red earthy beds; _a^3_,
White and Red Sandstones.]
The accompanying diagram (Fig. 7) gives a generalised view of the
relation borne by the red beds to the older rocks of the Cheviots.
It will be seen that the former rest _unconformably_ upon the Old
Red Sandstone igneous rocks, and also, of course, upon the Silurian
strata. The section shows that the red beds lie upon a worn and
denuded surface. Now this speaks to the lapse of a long period of
time. It may be remembered that we had some grounds for believing that
the latest eruptions of the Cheviot volcanoes were sub-aërial. The
evidence now enables us to advance further, and to state that after
the close of the volcanic period, the whole Cheviot district existed
as an elevated tract of dry land, from which streams flowed north and
south. And for so long a time did these conditions endure, that the
rivulets and streams were enabled to scoop out many channels and broad
valleys before any of the outlying red beds had come into existence.
Before the conglomerate beds were laid down, the ancient volcanic
bank of the Cheviots had thus suffered great erosion. This is what
"unconformability" means. It points to the prolonged continuance of
a land-surface, subject as that must always be to the wearing action
of the sub-aërial forces. Rain and frost disintegrate the rocks, and
running water rolls the débris from higher to lower levels, and piles
it up in the form of gravel, sand, and mud in lakes and the sea. While
the old volcanic country of the Cheviots was being thus denuded,
it would appear that a wide extent of land existed in the Northern
Highlands and Southern Uplands of Scotland, and also in what are now
the lake districts of England and the hilly tracts of Wales. And in all
these regions valleys were formed, which at a subsequent time were more
or less filled up with newer deposits.
The presence of the red beds that sweep round the base of the Cheviot
Hills shows unmistakably that a period of submergence followed these
land conditions. All the low grounds of Southern Scotland disappeared
beneath a wide sheet of water, which stretched from the foot of the
Lammermuirs up to the base of the Cheviots, and here and there entered
the valleys, and so extended into the hills. This water, however, does
not seem to have been that of an open sea; rather was it portion of a
great freshwater lake, brackish lagoon, or inland sea. The lowest beds
of the red series are merely hardened layers and masses of gravel and
rolled shingle, which would seem at first sight to indicate the former
action of waves along a sea-beach. There are certain appearances,
however, which lead one to suspect that these ancient shingle beds
may have had quite another origin. In some places the stones exactly
resemble those which are found so abundantly in glacial deposits. They
are sub-angular and blunted, and, like glaciated stones, occasionally
show striæ or scratches. This, however, is very rarely the case. Most
of the stones appear subsequently to have been rolled about in water,
and in this process they must have lost any ice-markings they may have
had, and become smoothed and rounded like ordinary gravel stones.
The same appearances may be noted in the glacier valleys of Norway
and Switzerland, where at the present day the glaciated stones which
are pushed out at the lower ends of the glaciers are rolled about in
the streams, and soon lose all trace of ice-work. It is impossible,
however, to enter here into all the details of the evidence which lead
one to suspect that glaciers may have existed at this early period
among the Cheviot and Lammermuir Hills. In the latter district, the
conglomerates occur in such masses and so exactly resemble the morainic
débris and ice-rubbish of modern glacial regions, that the late Sir A.
C. Ramsay long ago suggested their ice-origin.
Let us conceive, then, that when the ancient lake or inland sea of
which I have spoken reached the base of the Cheviots, glaciers may
have nestled in the valleys. Streams issuing from the lower ends of
these would sweep great quantities of gravel down the valleys to the
margin of the lake, and it is quite possible that there might be enough
wave-action to spread the gravel out along the shores. It is evident,
however, that the main heaps of shingle would gather opposite what were
at that time the mouths of glacier valleys; and it is just in such
positions that we now meet with the thickest masses of conglomerate.
Ere long, however, the supposed glaciers would seem to have melted
away, and only fine sand and mud, with here and there small rounded
stones and grit, accumulated round the shores of the ancient lake. Of
course, during all this time fine-grained sediment gathered over the
deeper parts of the lake-bottom.
We have no evidence to show what kind of creatures, if any, inhabited
the land at this time; nor do any fossils occur in the red earthy beds
to throw light upon the conditions of life that may have obtained in
the lake. If glaciers really existed and sent down ice-cold water,
the conditions would hardly be favourable to life of any kind; for
glacial lakes are generally barren. But the absence of fossils may
be due to other causes than this. It is a remarkable fact, that red
strata are, as a rule, unfossiliferous, and the few fossils which
they do sometimes yield are generally indicative rather of lacustrine
and brackish-water, than marine conditions. The paucity or absence of
organic remains seems to have been often due to the presence in the
water of a superabundance of salts. Now this excessive salinity may
have arisen in either of two ways. First, we may suppose some wide
reach of the sea to have been cut off from communication with the
open ocean by an elevation of a portion of its bed; and in this case
we should have a lagoon of saltwater, which evaporation would tend to
concentrate to such a degree, that by-and-by nothing would be able to
live in its waters. Or, again, we may have a lake so poisoned by the
influx of springs and streams, carrying various salts in solution,
as to render it uninhabitable by life of any kind, either animal or
vegetable. Many red sandstone deposits, as Sir A. C. Ramsay has pointed
out, are evidently lagoon-formations, which is proved by the presence
of associated beds of rock-salt, gypsum, and magnesian limestone.
They have slowly accumulated in great inland seas or lakes having no
outlet, whose waters were subject to evaporation and concentration,
although now and then they seem to have communicated more or less
freely with the ocean. The red earthy beds of the Jed, however, though
unfossiliferous, yet contain no trace of rock-salt or magnesian
limestone. The only character they have in common with the salt-bearing
strata of the New Red Sandstone of England is their colour, due to
the presence of peroxide of iron, which we can hardly conceive could
have been deposited in the mud of a sea communicating freely with the
ocean. But a quiet lake, fed by rivulets and streams that drained an
old volcanic district, is precisely the kind of water-basin in which
highly ferruginous mud and sand might be expected to accumulate. Such
a lake, tainted with the various salts, etc., carried into it by
streams and springs (some of which may have been thermal; for, as we
shall see presently, the volcanic forces, although quiescent, were yet
not extinct), might well be unfitted for either animal or plant, and
probably this is one reason why the red earthy beds of the Jed are so
unfossiliferous.
After some time, the physical conditions in the regions under review
experienced some further modification. Considerable depression of the
land supervened, and the waters of our inland sea or lake rose high on
the slopes of the Cheviots. Mark now how the character of the sediment
changes. The prevailing red colour has disappeared, and white, yellow,
and pale greenish or grey sand begins to be poured over the bed of
the lake. Even yet, however, ferruginous matter exists in sufficient
quantity to tint the sediment red in some places. With the appearance
of these lighter-coloured sandy deposits, the conditions seem to have
become better fitted to sustain life. Fish of peculiar forms, which,
like the gar-pike of North American lakes, were provided with a strong
scaly armour of tough bone, began to abound, weeds grew in the water,
and the neighbouring land supported a vegetation now very meagrely
represented by the few remains of plants which have been preserved.
In some places fish-scales are found in considerable abundance.
They belong to several genera and species which are more or less
characteristic of the Old Red Sandstone formation. The most remarkable
form was the _Pterichthys_, or wing-finned fish. Its blunt-shaped head
and the anterior portion of its body were sheathed in a solid case
of bone, formed by the union of numerous bony scales or plates. Two
curious curved spine-like arms occupied the place of pectoral fins,
and may have been used by the creature in paddling along the bottom of
the sea or lake in which it lived. The posterior part of the body was
covered with bony scales, but these were not suturally united. Other
kinds of fish were the _Holoptychius_ and _Coccosteus_, both of which
were, like the Pterichthys, furnished with bony scales. The scales of
the former overlapped, and had a curious wrinkled surface. The head of
the Coccosteus was protected by a large bony shield or buckler, and a
similar bony armour covered the ventral region.
The organic remains of these fish-bearing strata are too scanty,
however, to enable us to form any idea of the kind of climate which
characterised the district at this long-past period; but if we rely
upon the fossils which have been met with in strata of the same or
approximately the same age elsewhere, we may be pretty sure the
climate was genial, and nourished on the land an abundant vegetation,
consisting of ferns, great reeds, and club-mosses, which attained the
dimensions of large trees, conifers, and other strange trees which have
no living analogues.
It seems most likely that when the land sank down in the Cheviot
district, so as to allow the old lake to reach as it were a higher
level, some communication with the outlying ocean was effected. Red
ferruginous mud would then cease to accumulate, or gather only now
and then; the deposits would for the most part be white or yellow,
or pale green; and fish would be able to come in from the sea. The
communication with the ocean, however, was probably never very free,
but liable to frequent interruption.
Here, then, ends the third great period of time represented by the
rocks of the Cheviot district. The first period, as we have seen,
closed with the deposition of the Silurian strata. Thereafter
supervened a vast lapse of time, not recorded in the Cheviots by the
presence of any rocks, but represented in other regions by younger
members of the Silurian system. During this unrecorded portion of past
time, the Silurian strata of the Cheviots were hardened, compressed,
folded, upheaved to the light of day, and worn into hills and valleys
by the action of the sub-aërial forces. Then began the second period
of rock-forming in our district. Volcanoes poured out successive beds
of molten matter and showers of stones and ashes, and so built up the
rock-masses of the highest parts of the Cheviot Hills. These eruptions
belong to the Old Red Sandstone age, and form a portion of what we term
the Lower Old Red Sandstone. After the extinction of the volcanoes,
another prolonged period elapsed, which is not accounted for in the
Cheviots by the presence of any rocks. Then it was, as we know, that
the great volcanic bank was denuded and worn into a system of hills
and valleys. Now, since it is evident that the red beds of the Jed and
other places are also of Old Red Sandstone age, it follows that they
must belong to a higher place in the Old Red Sandstone formation than
the much-denuded igneous rocks upon which they rest unconformably.
The reasonable conclusion seems to be that the denudation or wearing
away of the Lower Old Red Sandstone igneous rocks of the Cheviots was
effected during that period which is represented in other districts of
Scotland by what is called the Middle Old Red Sandstone, so that the
Jed beds will thus rank as Upper Old Red Sandstone.
[Illustration: Fig. 8.--_s_, Silurian strata; _i_, Cheviot Igneous
Rocks (Lower Old Red Sandstone); _r_, Upper Old Red Sandstone
series; _c_, Kelso Igneous Rocks (Lower Carboniferous); _d_, Lower
Carboniferous Sandstones, Shales, etc.]
I come now to speak of certain rocks which, although they are developed
chiefly beyond the limits of our district, yet require a little
consideration before we can complete our account of the geological
history of the Cheviots. The rocks referred to consist chiefly of old
lava-beds, which very closely resemble those of the Lower Old Red
Sandstone. They appear on the south side of the Tweed valley below
Kelso, whence they extend south-west and west, crossing the river at
Makerstoun, and sweeping north to form the hills about Smailholm,
Stichill, and Hume (Fig. 8). All to the east of these rocks, the valley
of the Tweed is occupied by a great thickness of grey sandstones,
and grey and blue shales and clays, with which are associated thin
cement-stone bands, and occasional coarse sandy limestones called
cornstone. These strata rest upon the outskirts of the Kelso igneous
rocks, and are clearly of later date than these, since in their lower
beds, which are often conglomeratic, we find numerous rounded fragments
of the igneous rocks upon which the sandstones and shales abut. The
latter have yielded a number of fossils, both animals and plants, to
which I shall refer presently. In the bed of the Teviot near Roxburgh,
and elsewhere, the Kelso igneous rocks are found reposing upon whitish
and reddish sandstones, which are evidently the upper members of the
red beds of the Jed Water and other localities.
Strata closely resembling the grey sandstones and shales of the Tweed
valley appear among the Cheviot Hills at the head of the Jed Water,
where they are marked by the presence of thick massive sandstones,
which form all the tops of the hills between Hungry Law and the heights
that overlook the sources of the Liddel Water--the greatest height
reached being at Carter Fell, which is 1815 feet above the sea-level.
The strata at this place contain some impure limestone and thin seams
of coal, while beds of lava and tuff appear intercalated in the series.
[Illustration: Fig. 9.--Section across old volcanic neck. The dotted
line above suggests the original form of the volcano; _b_, plug of
igneous rock which rose in a molten state and cooled in the vent.]
Now let us rapidly sum up what seem to be the inferences suggested
by these briefly-stated facts. We have seen that the Upper Old Red
Sandstone began to be deposited in a lake which, as time wore on,
probably communicated with the sea, while the land was undergoing a
process of depression, so that the area of deposition was thus widely
increased, and sediment gradually accumulated in places and at levels
which had existed as land when the ancient lake first appeared in the
Cheviot district. The old lava-beds of Kelso show that the volcanic
forces, which had long been quiescent, again became active. Great
floods of molten matter issued from the bowels of the earth, and poured
over the bottom of the inland sea. But all the larger volcanoes of this
period were confined to the centre of the Tweed valley. Not a few
little isolated volcanoes, however, seem to have dotted the sea-bottom
beyond the limits of the Kelso area. From these, showers of stones
were ejected, and sometimes also they poured out molten matter. Their
sites are now represented by rounded hills which stand up, more or
less abruptly, above the level of the undulating tracts in which they
occur (Fig. 9). Among the most marked are Rubers Law, Black Law, the
Dunian, and Lanton Hill. Of course it is only the plugged-up vents or
necks that now remain; all the loose ejectamenta by which these must
at one time have been surrounded have long since been worn and washed
away. At last the Kelso volcanoes became extinct, and the little ones
also probably died out at the same time. Another long period now
ensued, during which the inland sea disappeared, and its dried-up bed
was subjected to the denuding action of the sub-aërial forces. The
volcanic rocks of the Kelso district suffered considerable erosion,
while the softer sandy strata amongst which they were erupted no doubt
experienced still greater waste. Ere long, however, the scene again
changes; and what is now the vale of Tweed becomes a wide estuary,
the shores of which are formed at first by the Kelso igneous rocks.
Into this estuary, rivers and streams carry the spoil of the Southern
Uplands, and strew its bed with sand and mud. Occasionally ferns and
large coniferous trees are floated down, and, getting water-logged,
sink to the bottom, where they become entombed in the slowly
accumulating sediment. The character of these buried plants shows that
the climate must have been genial. They belong to species which are
characteristic of the Carboniferous system, and we look upon them with
interest as the forerunners of that vast plant-growth which by-and-by
was to cover wide areas in Britain, and to give rise to our coal-seams,
the source of so much national wealth. In the waters of the estuary,
minute crustaceous creatures called _cyprides_ abounded, and with these
was associated a number of small molluscs, chiefly univalves. Here
and there considerable quantities of calcareous mud and sand gathered
on the bed of the estuary, and formed in time beds of cement-stone,
and impure limestone or cornstone. How long that condition of things
obtained in the Tweed valley we cannot tell; but we know that after
a very considerable thickness of sediment had accumulated, estuarine
conditions prevailed over the south-west end of what is now the Cheviot
range. This points to a considerable depression of the land. In this
same region volcanic action appeared, and streams of lava and showers
of fragmental materials were ejected--the remains of which are seen
in Hungry Law, Catcleugh Shin, and the head-waters of the Jed. Genial
climatic conditions continued; and here and there, along what were
either low islets or the flat muddy shores of the estuary, plants grew
in sufficient quantity to form masses of vegetation which, subsequently
buried under mud and sand, were compressed and mineralised, and so
became coal. The only place where these are now met with is on the
crest of the Cheviots at Carter Fell. The process of depression still
continuing, thick sand gradually spread over the site of the submerged
forests. To trace the physical history immediately after this, we must
go out of the Cheviot district; and it may suffice if I merely state
that these estuarine or lacustrine conditions, which prevailed for a
long time not only over the Tweed and Cheviot areas but in various
other parts of Scotland, at last gave place to the sea. In this sea,
corals, sea-lilies, and numerous molluscs and fishes abounded--all
pointing to the prevalence of genial climatic conditions. The organic
remains and the geological position of the estuarine beds of the
Tweed and the Cheviots--resting as they do upon the Upper Old Red
Sandstone--prove them to belong to the Lower series of the great
Carboniferous system.
It was some time during the Carboniferous period that wide sheets of
melted matter were forcibly intruded among the Old Red Sandstone and
the Lower Carboniferous strata of the Cheviot district; but although
these are now visible at the surface, as at Southdean, Bonchester,
etc., they never actually reached that surface at the time of their
irruption. They cooled in the crust of the earth amongst the strata
between which they were intruded, and have only been exposed to view by
the action of the denuding forces which have worn away the sedimentary
beds by which they were formerly covered.
A very wide blank next occurs in the geological history of the
Cheviots. We have no trace of the many great systems, comprising
vast series of strata and representing long eras of time, which we
know, from the evidence supplied by other regions, followed after the
deposition of the Lower Carboniferous strata. The Middle and Upper
Carboniferous groups are totally wanting, so likewise is the Permian
system; and all the great series of "Secondary" systems, of which the
major portion of England is composed, are equally absent. Nay, even
Tertiary accumulations are wanting. There is one very remarkable relic,
however, of Tertiary times, and that is a long dyke or vertical wall
of basalt-rock which traverses the country from east to west, crossing
the crest of the Cheviots near Brownhart Law, and striking west by
north through Belling Hill, by the Rule Water at Hallrule Mill, on
towards Hawick. This is one of a series of such dykes, common enough
in some parts of Scotland, which become more numerous as we approach
the west coast, where they are found associated with certain volcanic
rocks of Tertiary age, in such a way as to lead to the belief that they
all belong to the same period. The melted rock seems to have risen
and cooled in great cracks or fissures, and seldom to have overflowed
at the surface. Indeed it is highly probable that many or even most
of the dykes never reached the surface at all, but have been exposed
by subsequent denudation of the rocks that once overlaid them. Such
would appear to have been the case with the great dyke of the Cheviot
district.
We can only conjecture what the condition of this part of southern
Scotland was in the long ages that elapsed between the termination of
the Lower Carboniferous period and the close of the Tertiary ages. It
is more than likely that it shared in some of the submergences that
ensued during the deposition of the upper group of the Carboniferous
system; but after that it may have remained, for aught we can tell, in
the condition of dry land all through those prolonged periods which
are unrecorded in the rocks of the Cheviot Hills, but have left behind
them such noteworthy remains in England and other countries. Of one
thing we may be sure, that during a large part of those unrecorded ages
the Cheviot district could not have been an area of deposition. Rather
must it have existed for untold eras as dry land; and this explains
and accounts for the enormous denudation which the whole country has
experienced; for there can be little doubt that the Lower Carboniferous
strata of Carter Fell were at one time continuous with the similar
strata of the lower reaches of the Tweed valley. Yet hardly a trace of
the missing beds remains in any part of the country between the ridge
of the hills at the head of the Jed Water and the Tweed at Kelso. Only
little patches are found capping the high ground opposite Jedburgh, as
at Hunthill, etc. Thus more than a thousand feet of Lower Carboniferous
strata, and probably not less than five hundred or six hundred feet of
Old Red Sandstone rocks, have been slowly carried away, grain by grain,
from the face of the Cheviot district since the close of the Lower
Carboniferous period.
IV.
In the first of these papers some reference was made to the
configuration of the ground in the Cheviot district. We have seen
that the outlines assumed by the country have been determined in
large measure by the nature of the rocks. Thus where igneous masses
abound, the hills present a more or less irregular, and broken or lumpy
contour, while the valleys are frequently narrow and deep. In the
tracts occupied by Silurian strata, we have, as a rule, broad-topped
hill-masses with a smoothly-rounded outline, whose slopes generally
fall away with a long gentle sweep into soft green valleys, along the
bottoms of which the streams often flow in deep gullies and ravines.
Where the country is formed of sandstones, and other associated strata,
the hills are generally broad and well-rounded, but the outline is
not infrequently interrupted by lines of cliff and escarpment. These
strata, however, are confined chiefly to the low-grounds, where they
form a gently-undulating country, broken here and there, as in Dunian
Hill, Bonchester Hill, Rubers Law, etc., by abrupt cones and knobs of
igneous rock.
It is evident, then, that the diversified character of the Cheviot
Hills and the adjoining low-grounds depends on the character of the
rocks and also, as we shall see presently, upon geological structure.
Each kind of rock has its own peculiar mode of weathering. All do
not crumble away under the action of rain, frost, and running water
in precisely the same manner. Some which yield equally and uniformly
give rise to smooth outlines, others of more irregular composition,
such as many igneous rocks, break up and crumble unequally in a
capricious and eccentric way, and these in the course of time present
a hummocky, lumpy, and rough irregular configuration. And as soft and
readily-weathered rocks must wear away more rapidly than indurated and
durable masses, it follows that the former will now be found most
abundantly at low levels, while the latter will enter most extensively
into the composition of the hills. But the contour of a country depends
not only upon the relative durability of the rocks, but also upon the
mode of their occurrence in the crust of the earth. Strata, as we
have seen, do not all lie in one way; some are horizontal, others are
inclined to the horizon, while yet others are vertical. Again, many
rocks are amorphous; that is to say, they occur in somewhat thick
masses which show no trace of a bedded arrangement. Such differences of
structure and arrangement influence in no small degree the weathering
and denudation of rocks, and cannot be left out of account when we are
seeking to discover the origin of the present configuration of our
hills and valleys. Thus, escarpments and the terraced aspect of many
hill-slopes are due to inequalities in the strata of which such hills
are built up. The softer strata crumble away more rapidly under the
touch of the atmospheric forces than the harder beds which rest upon
them, and hence the latter are undermined, and their exposed ends or
crops, losing support, fall away and roll down the slopes. The igneous
rocks of the Cheviots are arranged in beds; but so massive are these,
that frequently a hill proves to be composed from base to summit of
one and the same sheet of old lava. Hence there is a general absence
of that terraced aspect which is so conspicuous in hills that are
built up of bedded rock-masses. Here and there, however, the beds are
not so massive, several cropping out upon a hill-side; and whenever
this is the case (as near Yetholm) we find the hill-slopes presenting
the usual terraced appearance--a series of cliffs and escarpments,
separated by intervening slopes, rising one above the other. In the
Silurian districts no such terraces or escarpments exist, the general
high dip of the strata, which often approaches the vertical, precluding
any such contour. In a region composed of highly-inclined greywacké
and shale, however, we should expect to find that where the strata are
of unequal durability, the harder beds will stand up in long narrow
ridges, separated by intervening hollows, which have been worn out
along the outcrops of the softer and more easily-denuded beds. And such
appearances do show themselves in some parts of the Silurian area. As a
rule, however, the Silurian strata are not thick-bedded, and harder and
softer bands alternate so rapidly that they yield on the whole a smooth
surface under the action of the atmospheric forces. In the low-lying
districts, which, as I have said, are mostly occupied by sandstones and
shaly beds, all the abrupt isolated hills are formed of igneous rocks,
which are much harder and tougher than the strata that surround them.
It is quite evident that these hills owe their present appearance to
the durable nature of their constituent rocks, which now project above
the general level of the surface, simply because they have been better
able to resist the denuding agents than the softer rocks that once
covered and concealed them.
We see, then, that each kind of rock has its own particular mode of
weathering, and that the configuration of a country depends primarily
upon this and upon geological structure. Indeed, so close is the
connection between the geology and the surface-outline of a country,
that to a practised observer the latter acts as an unfailing index to
the general nature of the underlying rocks, and tells him at a glance
whether these are igneous like basalt and porphyrite, aqueous like
sandstone and shale, or hardened and altered strata like greywacké.
But while one cannot help noticing how in the Cheviot district the
character of the scenery depends largely upon the nature and structure
of the rocks, he shall, nevertheless, hardly fail to observe that
flowing outlines are more or less conspicuous over all the region.
And as he descends into the main valleys, he shall be struck with the
fact that the hill-slopes seem to be smoothed off in a direction that
coincides with the trend of these valleys. In short, he cannot help
noticing that the varied configuration that results from the weathering
of different rock-masses has been subsequently modified by some agent
which seems to have acted universally over the whole country. In the
upper reaches of the Cheviot valleys, the rocks have evidently been
rounded off by some force pressing upon them in a direction coinciding
with that of the valleys; but soon after entering upon their lower
reaches, we notice that the denuding or moulding force must have turned
gradually away to the north-east--the northern spurs of the Cheviots,
and the low-grounds that abut upon these being smoothed off in a
direction that corresponds exactly with the trend of that great strath
through which flow the Teviot and the Tweed, from Melrose downwards.
Throughout this broad strath, which extends from the base of the
Lammermuirs to the foot of the Cheviots, and includes the whole of
Teviotdale, the ground presents a remarkable closely-wrinkled surface,
the ridges and intervening hollows all coinciding in direction with the
general trend of the great strath, which is south-west and north-east;
but turning gradually round to east, as we approach the lower reaches
of the Tweed.
Passing round the north-eastern extremity of the Cheviot range into
Northumberland, we observe that the same series of ridges and hollows
continues to follow an easterly direction until we near the sea-board,
when the trend gradually swings round to the south-east, as in the
neighbourhood of Belford and Bamborough, where the ridges run parallel
with the coast-line.
The ridges and hollows are most conspicuous in the low-grounds of
Roxburghshire and Berwickshire, especially in the regions between
Kelso and Smailholm, and between Duns and Coldstream. The dwellers
along the banks of the Tweed are quite familiar with the fact that the
roads which run parallel with the river are smooth and level, for they
coincide with the trend of the ridges and hollows; whilst those that
cross the country at right angles to this direction must of course
traverse ridge after ridge, and are therefore exceedingly uneven. In
this low-lying district most of the ridges are composed of superficial
deposits of stony and gravelly clay and sand, and the same is the case
with those that sweep round the north-eastern spurs of the Cheviots
by Coldstream and Ancroft. Some ridges, however, consist either of
solid rock alone, as near Stichill, or of rock and overlying masses
of clay and stones. In the hilly regions, again, nearly all the ridges
are of rock alone, especially in the districts lying between Melrose
and Selkirk and between Selkirk and Hawick. Indeed, the hills drained
by the upper reaches of the Teviot and its tributaries are more or
less fluted and channelled, as it were--many long parallel narrow
hollows having been driven out along their slopes and even frequently
across their broad tops. This scolloped and ridged aspect of the
hills, however, disappears as we approach the upper reaches of the
hill-valleys. From Skelfhill Pen (1745 feet) by Windburgh Hill (1662
feet), on through the ridge of the Cheviot watershed, none of the hills
shows any appearance of a uniformly-wrinkled surface.
[Illustration: Fig. 10.--Rounded Rocks, with superficial deposits, _t_
_t_ _t_, heaped up against steep faces. The arrows indicate direction
followed by the smoothing agent.]
A close inspection of the rock-ridges satisfies one that they have
been smoothed off by some agent pressing upon them in a direction that
coincides with their own trend; and not only so, but the smoothing
agent, it is clearly seen, must have come from the watersheds and then
pressed outwards to the low-grounds which are now watered by the Teviot
and the Tweed. This is shown by the manner in which the rocks have
been smoothed off, for their smooth faces look towards the dominant
watersheds, while their rough and unpolished sides point away in the
opposite direction. Sometimes, however, we find that more or less
steeply projecting rocks _face_ the dominant watersheds. When such is
the case, there is usually a long sloping "tail" behind the crag--a
"tail" which is composed chiefly of superficial deposits. The hills
between Hume and Stichill afford some good examples. The two kinds of
appearances are exhibited in the accompanying diagram (Figs. 10, 11.)
The appearance shown in Fig. 10 is of most common occurrence in the
upland parts of the country, while "crag and tail" (as shown in Fig.
11) is seen to greatest advantage in the open low-grounds. In both
cases it will be observed that superficial deposits (_t_) nestle behind
a more or less steep face of rock.
[Illustration: Fig. 11.--"Crag and Tail"; boss of hard rock, _c_;
intersecting sandstones, _s_; superficial deposits heaped up in rear of
crag, _t_. The arrow indicates direction followed by smoothing agent.]
When the rocks have not been much exposed to the action of the
weather, they often show a polished surface covered with long parallel
grooves and striæ or scratches. Such polished and scratched surfaces
are best seen when the superficial deposits have been only recently
removed. Often, too, when we tear away the thick turf that mantles the
hill-slopes, we find the same phenomena. Indeed, wherever the rocks
have not been much acted upon by the weather, and thus broken up and
decomposed, we may expect to meet with more or less well-marked grooves
and stride. Now the remarkable circumstance about these scratches is
this--they agree in direction with the trend of the rock-ridges and the
hollows described above. Nor can we doubt that the superficial markings
have all been produced by one and the same agent. In the upper valleys
of the Cheviots, the scratches coincide in direction with the valleys,
which is, speaking generally, from south to north, but as we approach
the low-grounds they begin to turn more to the east (just, as we have
seen, is the case with the ridges and hollows), until we enter England
to the east of Coldstream, where the striæ point first nearly due east,
but eventually swing round to the south-east, as is well seen upon the
limestone rocks between Lowick and Belford. In Teviotdale the general
trend of the striæ is from south-west to north-east, a direction
which continues to hold good until the lower reaches of the Tweed
are approached, when, as we have just mentioned, they begin to turn
more and more to the east. Thus it becomes evident that the denuding
agent, whatever it was, that gave rise to these ridges and scratched
rock-surfaces must have pressed outwards from all the dominant
watersheds, and, sweeping down through the great undulating strath that
lies between the Cheviots and the Lammermuirs, must have gradually
turned away to the east and south as it rounded the northern spurs of
the former range, so as to pass south-east over the contiguous maritime
districts of Northumberland.
A few words now as to the character of the superficial deposits which
enter so largely into the composition of the long parallel banks and
ridges in the low-grounds of Roxburghshire, Berwickshire, and the
northern part of Northumberland. The most conspicuous and noteworthy
deposit is a hard tough tenacious clay, which is always more or
less well-charged with blunted and sub-angular stones and boulders,
scattered pell-mell through the mass. This clay is as a rule quite
unstratified--it shows no lines of bedding, and although here and
there it contains irregular patches and beds of gravel and sand, yet
it evidently does not owe its origin to the action of water. Its
colour in the upper part of Teviotdale and the Cheviots is generally a
drab-brown, or pale grey and sometimes yellow, while here and there,
as in the upper reaches of the Jed valley, it is a dark dingy bluish
grey. In the lower parts of Teviotdale and in the Tweed district it
is generally red or reddish brown. The stones in the clay have all
been derived from the rocks of the region in which it occurs. Thus
in Teviotdale we find that in the higher reaches of the dale which
are Silurian the stones and boulders consist of various kinds of
greywacké, etc. In the lower reaches, however, when we pass into the
Red Sandstone area, we note that the clay begins to contain fragments
of red sandstone, while the clay itself takes on a reddish tinge, until
we get down to the vale of the Tweed, where not only is the clay very
decidedly red, but its sandstone boulders also are very numerous.
The same appearances present themselves in passing outwards from the
Cheviots. At first the clay contains only stones that have been derived
from the upper parts of the hills, but by-and-by, as we near the
low-grounds, other kinds begin to make their appearance, so that by the
time we reach the Tweed we may obtain from the clay specimens of every
kind of rock that occurs within the drainage-area of the Teviot and the
lower reaches of the River Tweed.
Look at the stones, and you shall observe that all the harder and
finer-grained specimens are well-smoothed and covered with striæ or
scratches, the best marked of which run parallel with the longer axis
of each stone and boulder. These scratches are evidently very similar
to those markings that cover the surface of the underlying solid rock,
and we may feel sure, therefore, that the denuding agent which smoothed
and scratched the solid rocks had also something to do with the stones
and boulders of the clay.
Underneath the stony clay, or _Till_, as it is called, we find here
and there certain old river gravels. We know that these gravels are
river-formations, because not only do they lie at the bottom of the
river-valleys, but the stones, we can see, have been arranged by water
running in one constant direction, and that direction is always _down_
the valley in which the gravels chance to occur. Frequently, however,
there is no trace of such underlying gravels, but the till rests
directly upon the solid rocks.
Now what do all these appearances mean? It is clear that there is no
natural agent in this country engaged in rounding and scratching the
rocks, or in accumulating a stony clay like till. In alpine regions,
however, we know that glaciers, as they slowly creep down their
valleys, grind and polish and scratch the rocks over which they pass,
and that underneath the moving ice one may detect smoothed and striated
stones precisely resembling those that occur in till. Frost in such
alpine regions splits up the rocks of the cliffs and mountain-slopes
that overlook a glacier, and immense masses of angular stones and
débris, thus loosened, roll down and accumulate along the flanks of
the ice-streams. Eventually such accumulations are borne slowly down
the valley upon the back of the glacier, and are dropped at last over
the terminal front of the ice, where they become intermingled with the
stones and rubbish, which are pushed or washed out from underneath the
ice. These heaps and masses of angular débris and stones are called
"moraines," and one can see that in Switzerland the glaciers must at
some time have been much larger, for ancient moraines occur far down
in the low-grounds of that country--the glaciers being now confined
to the uppermost reaches of the deep mountain-valleys. Moreover, we
may note how the mountain-slopes overlooking the present puny glaciers
have been rubbed by ice up to a height of sometimes a thousand feet and
more above the level of the existing ice-streams. Now since the aspect
presented by the glaciated rock-surfaces of Switzerland is exactly
paralleled by the rounded and smoothed rocks of Scotland, there can
be no doubt that the latter have had a similar origin. Again, we find
throughout the low-grounds of Switzerland a deposit of till precisely
resembling that which is so well developed in Teviotdale and the valley
of the Tweed. And as there can be no doubt that the Swiss till has been
produced by the action of glacier ice, we are compelled to believe the
same of the till in Scotland.
Let us further note that in the deep mountain-valleys of Switzerland
the glacial deposits consist for the most part of coarse morainic
débris--of such materials, in short, as the terminal moraines of
existing glaciers are mainly composed. Not infrequently this morainic
débris has been more or less acted upon by the rivers that escape from
the glaciers, and the angular stones have been rounded and arranged in
bedded masses. It is only when we get out of the mountain-valleys and
approach the low-grounds that the till, or stony clay, begins to appear
abundantly. The same phenomena characterise the Cheviot district. In
the upper reaches of the mountain-valleys at the heads of the Teviot,
the Kale, the Bowmont, etc., either till does not occur or it is thin
and often concealed below masses of rude morainic débris and gravel.
Out in the low-grounds, however, till, as we have already remarked, is
the most conspicuous of all the superficial deposits. From these facts
it may be inferred that till indicates the former presence of great
confluent glaciers, while morainic débris in hill-valleys points to the
action of comparatively small local and isolated glaciers.
What, then, are the general conclusions which may be derived from a
study of the rock-ridges, flutings, and striæ, and the till of the
Cheviot district? Clearly this: that the whole country has at one time
been deeply buried under glacier ice. The evidence shows us that the
broad strath stretching between the Lammermuirs and the Cheviots must
have been filled to overflowing with a great mass of ice that descended
from the uplands of Peebles and Selkirk and the broad-topped heights
that overlook the sources of the Teviot. The Cheviots appear to have
been quite buried underneath this wide sea of ice, and so likewise
were the Lammermuirs. At the same time, as we know, all Scotland was
similarly enveloped in a vast sheet of snow and ice, which streamed out
from the main watersheds of the country, and followed the lines of the
chief straths--that is to say, the general slope of the ground. The
track of the ice in the Cheviot district is very distinctly marked. In
Teviotdale it followed the trend of the valley, and, grinding along
the outcrop of the Silurian strata, deepened old hollows and scooped
out new ones in the soft shaly beds, while the intervening harder
strata, which offered greater resistance to the denuding action of
the ice, did not wear so easily, and so were rounded off, and formed
a series of ridges running parallel to the eroded hollows. The stones
and rubbish, dragged along underneath the ice, necessarily increased
as the glacier mass crept on its way. The rocks were scratched and
grooved by the stones that were forced over them, and the polishing
was completed by the finer sand and clay which resulted from the
grinding process. Wherever a rock projected there would be a tendency
for the stones and clay and sand to gather behind it. One may notice
the same kind of action upon the bed of a stream, where the sediment
tends to collect in the rear of prominent stones and boulders. And we
can hardly fail to have observed further that the sediment of a river
often arranges itself under the action of the current in long banks,
which run parallel to the course of the water. Underneath the ice-sheet
the stones, sand, and clay behaved in the same way. Behind projecting
rocks in sheltered nooks and hollows, they accumulated, while in
places exposed to the full sweep of the ice-stream they were piled up
and drawn out into long parallel banks and ridges, the trend of which
coincided with that of the ice-flow. The presence of confused and
irregular patches and lenticular beds of sand, clay, and gravel in the
till is not difficult to understand when we know that there is always
more or less water flowing on underneath a glacier. Such streams must
assort the débris, and roll angular fragments into rounded stones and
pebbles; but the materials thus assorted in layers will ever and anon
be crushed up so as to be either partially or wholly obliterated by the
slowly moving glacier.
As the stones and clay were derived from the underlying rocks, it is no
wonder that the colour of the till should vary. In the Silurian tracts
it is pale yellowish, or bluish grey, and the stones consist chiefly
of fragments of Silurian rocks, all blunted and smoothed, and often
beautifully polished and striated. When we get into the Red Sandstone
region of the low-grounds the colour of the clay begins by-and-by to
change, and fragments of red sandstone become commingled with the
Silurian stones, until ere long the colour of the deposit is decidedly
red, and sandstone fragments abound. Everywhere the stones show that
they have been carried persistently in one direction, and that is _out
from the watershed, and down the main valleys_.
The direction of the ice-marks upon the solid rocks, and the trend of
the "drums," as the parallel ridges of till are termed, show that the
ice-sheet of Teviotdale and Tweed gradually turned away to the east and
south-east as it swept round the north-eastern spurs of the Cheviots.
Now we may well ask why the ice did not go right out into the North
Sea, which is apparently the course it ought to have followed. The
same curious deflection affected the great ice-stream that occupied
the basin of the Forth. When it got past North Berwick, that stream,
instead of flowing directly east into the North Sea, turned away to
the south-east and overflowed the northern spurs of the Lammermuirs,
bringing with it into the valley of the Tweed stones and boulders which
had travelled all the way from the Highlands. It is obvious there
must have been some impediment to the flow of the Scottish ice into
the basin of the North Sea. What could have blocked its passage in
that direction? At the very time that Scotland lay concealed beneath
its ice-sheet, Norway and Sweden were likewise smothered in ice which
attained a thickness of not less than five or six thousand feet.
The whole basin of the Baltic was occupied by a vast glacier which
flowed south into Northern Germany, and this sheet was continuous with
glacier-ice that crossed over Denmark. When we consider how shallow
the North Sea is (it does not average more than forty fathoms between
Scotland and the Continent), we cannot doubt that the immense masses
of ice descending from Norway could not possibly have floated off,
but must actually have crept across the bottom of that sea until they
abutted upon and coalesced with the Scottish ice, so as to form one
vast _mer de glace_.
Thus it was that the Scandinavian ice blocked up the path of the
Scottish glaciers into the basin of the North Sea, and compelled them
to flow south-east into England.[H] Had there been no such obstruction
to the passage of the Scottish glaciers, it is impossible to believe
that snow and ice could ever have accumulated to such a depth in
Scotland. The Scottish ice reached a thickness of some three thousand
feet in its deeper parts. It is evident, however, that had there been
a free course for the glaciers, they would have moved off before they
could have attained this thickness. And we can hardly doubt, therefore,
that it was the damming-up of their outlet by the great Scandinavian
ice-sheet that enabled them to deepen to such an extent in the valleys
and low-grounds of Scotland.
[H] In the extreme north of Scotland we find that the Scottish ice was,
in like manner, compelled to turn aside and overflow Caithness from
south-east to north-west.
When the ice-sheet was at its thickest, the Cheviots were completely
covered, nevertheless they served to divide the ice-flow between
Scotland and England, although here and there one finds that the ice
passed over some of the lower summits, carrying with it boulders and
stones. This is by no means an uncommon circumstance in Scotland and
other glaciated countries. Thus we note that Highland boulders have
been brought into the vale of the Tweed across the Lammermuirs; and in
the same way boulders from the heights overlooking Eskdale have been
carried over some of the lower hill-tops into the vale of the Teviot.
In like manner the Swedish ice occasionally overflowed the lower
mountain-tops of the dividing ridge or watershed into Norway.
What wonder now that the Cheviot area should exhibit so many flowing
outlines, that the hills should be so smoothed and rounded and fluted,
that the low-grounds should be cumbered with such heaps of clay and
striated stones? Long before the great glaciers appeared, the rocks
were weathered and worn by the action of the usual atmospheric forces,
and each had assumed its own peculiar outline; but how greatly has
this been modified by the grinding action of the ice-sheet! To what
an extent have projecting rocks been rubbed, and how great is the
destruction that has befallen the loose accumulations of river gravel,
sand, and clay that gathered in the valleys before the advent of the
Ice Age! All that now remains of these are a few patches preserved here
and their underneath the till. The Cheviots can tell us nothing of the
kinds of plants and animals that clothed and peopled the country in
pre-glacial times. All we learn is that streams and rivers flowed as
they flow now, and that by-and-by everything was changed, and the land
disappeared underneath a vast covering of snow and ice.
In my concluding paper I will show how this ice period passed away, and
how the present condition of things succeeded.
V.
I have described the condition of the Cheviot district during the
climax of the Ice Age as one of intense arctic cold, the whole ridge of
hills being then completely smothered in snow and ice. This excessive
climate, however, did not last continuously throughout the so-called
glacial period, but was interrupted by more than one mild interglacial
epoch. We have evidence in Scotland, as in other countries, to show
that the great confluent ice-masses melted away so as to uncover all
the low-grounds and permit the reappearance of plants and animals.
Rivers again watered the land, and numerous lakes diversified the face
of the country. Willows, hazels, and alders grew in the sheltered
valleys, oak-trees flourished in the low-grounds, and Scots firs
clustered upon the hill-slopes. A strong, grassy vegetation covered
wide areas, and sedges and rushes luxuriated in marshy places and
encroached upon the margins of the lakes. The mammoth, or woolly-coated
elephant, roamed over the land, and among its congeners were the
extinct ox, the horse, the Irish elk, and the reindeer. After such a
temperate condition of things had continued for some time--perhaps
for thousands of years--the land, during the last interglacial epoch,
became gradually submerged to a depth of several hundred feet, and a
cold, ungenial sea, in which flourished species of northern and arctic
shells, covered the low-grounds of Scotland. The cold continuing to
increase, our glaciers descended for the last time from the mountains
and encroached upon the bed of the sea, until they became confluent,
fairly usurping the floor of the German Ocean, and pushing back the
western seas as far as, and even beyond, the islands of the Outer
Hebrides. There is good reason to believe that such great changes of
climate occurred several times during the glacial period, which thus
seems to have consisted of an alternation of cold and genial epochs.
But as the last phase in this extraordinary series of changes was a
cold one, during which great glaciers scoured the face of the country,
we now obtain only a few scattered traces of the genial conditions
that characterised the preceding mild interglacial epochs. Vegetable
accumulations, lake and river deposits with mammalian remains, marine
beds and their shelly contents, were all ploughed up by the ice, and to
a very large extent demolished. Here and there, however, we find in the
till or boulder-clay that marks the last cold epoch, wasted fragments
of trees, tusks of mammoths, and broken sea-shells; while underneath
the till we occasionally come upon old lake deposits with vegetable and
mammalian remains, or, as the case may be, beds of marine origin well
stocked with sea-shells of arctic species. And these freshwater and
marine beds repose, in many cases, upon an older accumulation of till,
which belongs to an earlier cold epoch of the glacial period. In the
Cheviot district proper, the traces of mild, interglacial conditions
are very slight, but in the immediate neighbourhood we find them more
strongly marked. Thus, in the valley of the Slitrig, near Hawick, we
notice freshwater beds with peaty matter lying between a lower and
an upper till or boulder-clay; and interglacial freshwater beds also
appear in the neighbouring county of Peebles, particularly in the
valley of the Leithan Water. Again, in the valley of the Tweed near
Carham, there occur interglacial beds in which I detected numerous
bones of water-rats and frogs. These interglacial remains acquire a
peculiar interest when we come to view the "superficial deposits" of
Scotland in connection with those of England and the Continent; for,
as I have endeavoured to show elsewhere,[I] it is most likely that the
ancient gravels of England, which contain the earliest traces of man,
belong for the most part to interglacial times; and the extraordinary
changes of climate described above may therefore have been actually
witnessed by human eyes. Indeed, I believe it was the advent of the
last cold epoch of the Ice Age that drove out the old tribes who used
the rude flint implements that are now found in the gravel deposits
and caves of England, and who occupied the British area along with
hippopotami, rhinoceroses, elephants, lions, hyænas, and other animals.
The men who entered Britain after the final disappearance of arctic
conditions, were more advanced in civilisation, and were accompanied by
a very different assemblage of animals--by a group represented by oxen,
sheep, dogs, and other creatures, most of which are still indigenous to
Britain.
[I] _Great Ice Age._
But to return to the Cheviots. When the final cold epoch had reached
its climax, and the ice-sheet began to melt away for the last time,
the tops of the hills then once more became uncovered, and large
blocks, detached by the action of the frost, fell upon the surface of
the glaciers, and were borne down the valleys, some of them to become
stranded here and there on hill-slopes, others to be carried far away
from the Cheviot area and dropped at last over Northumberland and
Durham, or even further south. As the melting of the ice continued, and
the glacier of the Tweed ceased to reach the sea, great accumulations
of gravel and sand were formed. Underneath the ice, sub-glacial streams
ploughed out the till, and paved their hidden courses with gravel
and sand. In summer-time, the whole surface of the Tweed glacier was
abundantly washed with water, which, pouring down by clefts and holes
in the ice, swelled all the sub-glacial streams and rivers. At the
same time, floods descending from the Lammermuirs and the Cheviots,
pushed with them vast quantities of shingle, gravel, and sand, part of
which was swept upon the surface of the Tweed glacier, while much seems
to have gathered along its flanks, forming banks and ridges running
parallel with the course of the valley.
At last the time came when the ice had fairly vanished from the lower
reaches of the Tweed, and we now walk over its bed and mark the
long ridges and banks of shingle and gravel that were formed by the
sub-glacial streams and rivers, and the somewhat similar accumulations
that gathered along the sides of the glacier at the foot of the
Lammermuir Hills. Here and there, also, we note the heaps (_i.e._
moraines) of shingle, earth, clay, and débris, with large erratics
which travelled on the surface of the ice, and were dropped upon the
ground as that ice melted away. All the loose erratics that lie at
the surface in the lower reaches of the Tweed valley have come from
the west. Some of them rest upon hard rock, others upon till, and yet
others crown the tops and slopes of gravel and sand hillocks, or appear
in low mounds of morainic origin.
In the valleys of the Cheviot Hills one traces the footsteps of the
retiring glaciers in mounds and hummocks of rude earthy débris, blocks,
and rock-rubbish. These are terminal moraines, and they indicate
certain pauses in the recession of the ice. The most remarkable
examples occur in the valley of the Kale Water at Blinkbonny, a
mile or so above the village of Eckford. At that place a bank of
morainic matter at one time blocked up the valley of the Kale, and
thus formed a wide and extensive lake that stretched up to and beyond
Morebattle. Numerous curious hillocks of gravel and sand are banked
against the moraine, and point to the action of the flood-waters
that escaped from the melting glacier. Other gravelly moraine mounds
occur higher up the same valley, as near Grubbit Mill. These last
tell us of a time when the Kale glacier had retreated still further,
so as to have its terminal front near where Morebattle now is.
Wreaths and hummocks of gravel and sand, extending from Grubbit to
the north-east, along the hollow in the hills that leads to Yetholm
Loch, indicate the course taken by a portion of the torrents that
escaped from the ice in summer-time. In other hill-valleys, similar
indications of ancient local glaciers may be seen. Some of the most
conspicuous of these appear upon the slopes and in the high valleys
within the drainage-areas of the Jed and the Kale. They consist
chiefly of mounds and hillocks, made up of coarse earthy débris and
rock-rubbish; sometimes these are solitary and rest in the throat of
a valley, at other times they are scattered all over the hill-slopes
and valley-bottom. One can have no doubt as to what they mean: they
indicate clearly the presence of insignificant glaciers that were
soon to vanish away. The larger and better-defined mounds are true
terminal moraines, while the scattered heaps of rubbish point out
for us the beds in which the glaciers lay. Thus, from the sea-coast
up to the highest ridge of this border country, we follow the spoor
of the melting ice; passing from massive and wide-spread deposits of
till, gravel, and sand, and angular débris in the low-grounds, up
to insignificant heaps and scatterings of rock-rubbish and angular
boulders at the higher levels of the country.
Several more or less extensive flats in the hill-valleys indicate the
former presence of lakes which have become obliterated by the action of
the streams. But by far the most conspicuous example of such silted-up
lakes is that of the Kale valley, to which reference has already
been made. In the later stages of the Ice Age that river-valley must
have existed as a lake from Marlfield up to and beyond Morebattle.
Indeed, there is evidence to show that even within historical times a
considerable lake overspread the flat grounds in this neighbourhood.
The name _Morebattle_ is supposed to mean the "village by the lake,"
and, up to a few years ago, there was a sheet of water called Linton
Loch a little to the east of Morebattle. But this has been drained by
the proprietor, and is now represented by only two insignificant pools.
The present course of the Kale between Marlfield and Kalemouth is of
post-glacial age--the old pre-glacial and interglacial course being
filled up with drifted materials. As the appearances at this place are
somewhat typical of many of the valleys of the Cheviot district, I may
briefly summarise the history of the Morebattle lake.
Before the advent of the last great age of ice the Kale would seem
to have flowed from Marlfield, close to the line now followed by the
turnpike road as far as Easter Wooden, after which it passed near the
present sites of Blinkbonny and Mosstower, and so on to the Teviot,
which it joined some little distance above Kalemouth. During the Ice
Age many of the old river-courses were completely choked up with clay,
stones, and gravel, so that when the ice melted away the rivers did
not always or even often regain their old channels. Thus, in the
case of the Kale, we find that the present course of the river below
Marlfield is of recent or post-glacial age, having been excavated by
the river since the close of the glacial epoch. The old or pre-glacial
course lies completely choked up and concealed under the rubbish shot
into it at a time when glacier-ice filled all the valley of the Kale
down to Marlfield. At this latter place the Kale glacier seems to have
made a considerable pause--it ceased for some time to retreat--and
thus a heavy bank of gravel, sand, shingle, earth, blocks, and angular
rubbish gathered in front of it, and obliterated the old river-course
into which they were dropped. When the glacier at last disappeared, a
lake was formed above the morainic dam that closed the valley below
Marlfield, and the outflow of the lake took place at a point lying
some little distance to the north of the old or pre-glacial course of
the Kale. By slow degrees the river excavated a new channel for itself
in the Old Red Sandstone rocks, and in doing so gradually lowered the
level of the waters. This and the silting action of the Kale and its
feeders slowly converted the lake-hollow into a broad alluvial flat
through which the river now winds its way.
Another extensive lake seems to have occupied the vale of the Teviot
between Jedfoot and Eckford, and similar old lake-beds occur in several
of the hill-valleys. One good example is seen in the valley of the
Oxnam Water, where the flat tract that extends from the old village of
Oxnam up to the foot of the Row Hill indicates the former presence of
a lake which has been drained by the stream cutting for itself a gorge
in Silurian greywackés and shales. In many other valleys it is easy to
see that the streams do not always occupy their pre-glacial courses,
and some of the old forsaken courses are still patent enough. Thus,
a glance at the hollow that extends from Mossburnford on the Jed to
Hardenpeel on the Oxnam is enough to convince one that in pre-glacial,
and probably in early post-glacial times also, a considerable stream
has flowed from what is now the vale of the Jed into the valley of the
Oxnam.
In all the valleys we meet with striking evidence to show that the
streams and rivers must formerly have been larger than they are
now. Certain banks and ridges of gravel fringe the valley-slopes at
considerable heights, and indicate the action of deeper and broader
currents than now make their way towards the sea. It is probable that
these high-level gravel terraces date their existence back to the close
of the Ice Age, when local glaciers still lingered in some of the
mountain-valleys, and when in summer-time great floods and torrents
descended from the hills.
An extremely humid climate seems to have characterised Scotland even
in post-glacial times, as may be gathered from the phenomena of her
peat-mosses. Very little peat occurs on the Scottish side of the
Cheviots, and it is conspicuous chiefly on the very crest of the hills,
where it attains a thickness that varies from a foot or two up to five
or six yards. Here and there we detect the remains of birch under the
peat, but the peat itself is composed chiefly of bog-moss and heather.
The evidence so abundantly supplied by the peat-mosses in other parts
of Scotland shows that after the Ice Age had passed away the Scottish
area became clothed with luxuriant forests of oak, pine, and other
trees. At that time the British Islands appear to have been joined to
themselves and the Continent across the upraised beds of the Irish Sea
and the German Ocean. Races of men who used polished stone implements
and sailed in canoes that were hollowed out of single oaks inhabited
the country, together with certain species of oxen (now either extinct
or domesticated), the elk, the beaver, the wolf, and other animals,
such as the dog and the sheep, which are still indigenous. The climate
was more excessive then than it is now--the summers being warmer and
the winters colder. By-and-by, however, submergence ensued, the great
wooded plain that seems once to have extended between Britain and the
Continent disappeared below the waves, and the climate of this country
became more humid. The old forests began to decay and the peat-mosses
to increase, until by-and-by large areas in the low-grounds passed
into the condition of dreary moor and morass, and even the brushwood
and stunted trees of the hills died down and became enveloped in a
mantle of bog-moss. A study of the present condition of the Scottish
peat-mosses leads one to believe that the rate of increase is now much
exceeded by the rate of decay, and that the eventual disappearance of
the peat that clothes hill-tops and valley-bottoms is only a question
of time. Draining and other agricultural operations have no doubt
influenced to some extent this general decay of the peat-mosses; but
there is reason to suspect that the change of climate, to which the
decay of the peat is due, may really be owing to some cosmical cause.
Quite recently an accomplished Norwegian botanist has come to similar
conclusions regarding the peat-mosses of the Scandinavian peninsula.
We have now traced the geological history of the Cheviot district
down to the "Recent Period." From this point the story of the past
must be continued by the archæologist, and into his province I will
not trespass further than to indicate some of the more remarkable
traces which the early human occupants of the upland valleys left
behind them. Before doing so, however, I may briefly recapitulate the
general results we have obtained from our rapid review of the glacial
and post-glacial deposits. A study of these has taught us that the
Cheviot Hills and the adjoining low-grounds participated in those
arctic conditions under the influence of which all Scotland and a
large portion of England were buried beneath a wide-spread _mer de
glace_. The Cheviots themselves were completely smothered under a mass
of glacier-ice which extended across the vale of the Tweed, and was
continuous over the Lammermuirs with the vast sheet that filled all the
great lowlands of central Scotland. But although the Cheviots were thus
overwhelmed, they yet served to divide the ice-flow, for we find that
the gelid masses moved outwards from the hills towards the valley of
the Tweed, turning gradually away to east and south-east to creep over
the north part of England. How far south the ice-sheet reached has not
yet been determined, but its _moraine profonde_ or till may be traced
to the edge of the Thames valley; and I have picked up in Norfolk
ice-worn fragments of igneous rock, which have been derived from
the Cheviots themselves, showing that Scottish ice actually invaded
the low-grounds south of the Wash. Such severe glacial conditions,
after continuing for a long time, were interrupted more than once by
intervening periods characterised by a milder and more genial climate.
The great _mer de glace_ then melted out of the valleys, and for
aught that we can say the snow and ice may even have vanished from
the hills themselves. Vegetation now covered the country, and herds
of the mammoth, the old extinct ox, the Irish elk, the reindeer, the
horse, and probably other creatures, roamed over the now deserted
beds of the glaciers. It was probably at this time that Palæolithic
man lived in Britain. He was contemporaneous with lions, elephants,
rhinoceroses, hippopotami, mammoths, reindeer, and other animals of
southern and northern habitats, the former living in England when the
climate was genial, but being replaced by the northern species when
the temperature began again to fall, and snow and glaciers once more
reappeared and crept downwards and outwards from the hills. Towards
the close of the interglacial period the land became submerged to a
considerable extent, and species of arctic shells lived over the sites
of the drowned land where the mammoth and its congeners had flourished.
By-and-by the cold so far increased that another great ice-sheet filled
up the shallow sea, and as it slowly ground over the face of the land
and the sea-bottom, it scoured out and demolished to a large extent all
loose fluviatile, lacustrine, and marine accumulations. When at last
the ice melted away, it left the ground cumbered with stony clay, and
with much gravel and sand and morainic débris. It is underneath these
deposits that we yet obtain now and again fragments of the life of that
interglacial epoch. But in all the regions visited by the last great
incursion of the _mer de glace_, such relics are comparatively rare; it
is only when we get beyond the districts that were overwhelmed that the
ancient interglacial remains are well preserved. Beyond the southern
extremity reached by the latest general ice-sheet--that is to say, in
the regions south of the Humber, we find the country often sprinkled
with tumultuous heaps and wide-spread sheets of gravel and brick-earth,
which seem to owe their origin to the floods and torrents that escaped
from the melting ice. These waters, sweeping over the land, carried
along with them such relics of man and beast as lay at the surface,
washing away interglacial river-deposits, and scattering the materials
far and wide over the undulating low-grounds of central and eastern
England. Mr. S. B. J. Skertchly, of the Geological Survey of England,
has shown that such is the origin of the so-called "river-gravels"
with ancient flint implements and mammalian remains in the districts
watered by the Little Ouse, the Waveney, and other rivers in that part
of England. These gravels could not possibly have been deposited by
the present rivers, for they are found capping the hills at a height
of more than eighty feet above the sources of the streams. The whole
aspect of the gravels, indeed, betokens the action of rapid floods and
torrents, such as must have been discharged abundantly in summer-time
from the melting ice-sheet that lay at no great distance to the north.
When the ice-sheet vanished away, it left the ground covered thickly
in many places with its various deposits. Rivers and streams were thus
often debarred from their old channels, and were forced to cut out for
themselves new courses, partly in drifted materials, and partly in
solid rock. A number of lakes then existed which have since been silted
up. So long as glaciers lingered in the hill-valleys, the rivers seem
to have flowed in greater volume than they now do. By-and-by the bare
and treeless country became clothed with a luxuriant forest-growth,
and was tenanted by animals, many of which are still indigenous to
our country, while others have become locally extinct, such as wolf,
beaver, and wild boar. In certain of the old lake-beds of the Cheviot
district numerous remains of red-deer and other animals have been
turned out in the search for marl, and in land drainage and reclamation
operations--the red-deer antlers being sometimes of noble dimensions.
It seems probable that in early post-glacial times our country was
joined to the Continent and shared in a continental climate, the
summers being then warmer and the winters colder than now.
The men who lived in Britain after the final disappearance of the great
glaciers used stone implements, which were often polished and highly
finished, and they sailed in canoes, being probably a race of active
hunters and fishers. They belong to the archæologist's "Neolithic" or
new-stone period--the "Palæolithic" or old-stone period being of much
older date, and separated, as I believe, from Neolithic times by the
intervention of the last cold epoch of the Ice Age.
To the forest epoch succeeded a time when the climate became very
humid, a result which may have been due in large part to the separation
of Britain from the Continent. It was then that the ancient forests
began to decay, and peat-mosses to increase. How long such humid
conditions of climate characterised the country we can hardly say, but
we know that nowadays our peat-mosses do not grow so rapidly as they
once did, and indeed almost everywhere the rate of decay is greater
than the rate of increase. This points to a further change of climate,
and brings us at once face to face with the present.
And now a few words, in conclusion, as to the old camps and other
remains that occur so abundantly in the valleys of the Cheviot Hills.
In many of the hill-valleys, especially towards their upper reaches,
as in the valleys of the Kale and the Bowmont, almost every hill is
marked by the presence of one or more circular or oval camps or forts.
They are generally placed in the most defensible positions, on the very
tops of the hills or on projecting spurs and ridges. Most of them are
of inconsiderable dimensions, and could not have afforded protection
to any large number of men, for many hardly exceed one hundred feet in
diameter. Not a few consist of only a single circular or oval rampart
with an external ditch--the rampart being composed of the rude débris
which was dug out to form the ditch. Others, however, are not only
much larger (five to six hundred feet in diameter), but surrounded, in
whole or in part, with two or more ramparts separated by intervening
ditches; and I have noticed that as a rule the side which must have
been most easily assailable was protected by several ramparts rising
one above the other. From the extraordinary number of these hill-forts
one has the impression that the upper valleys of the Cheviots must
at one time have been thickly peopled, probably in pre-Roman times.
It is easy to see that the camps or forts overlooking a valley often
bear a certain relation to each other, as if the one had been raised
to support the other, and not infrequently we can trace well-marked
intrenchments extending across a hill-ridge, or along a hill-slope
for a distance of not much short of a mile, and evidently having some
strategic connection with the forts or camps in their vicinity. I found
no trace of any "dwellings," either near the forts or in the vicinity
of the terraces. The only indications of what may have been the walls
of such appear within a fortified camp, called the Moat Hill, at
Buchtrig. This is an isolated knoll of rock, which has been strongly
fortified--large slabs and blocks of the porphyrite of which it is
composed having been wedged out with infinite pains to form circular
ramparts. The "walls" are of course nearly level with the ground and
grassed over, but they indicate little square enclosures, which may
very possibly have been huts closely huddled together. This fort is
oval, and measures five hundred feet by two hundred and seventy.
In the same neighbourhood we also meet with plentiful marks of ancient
cultivation and with places of sepulture--all of which may without
much doubt be referred to the same period as the camps and forts. The
slopes of the hills are often marked with broad horizontal terraces,
that remind one strongly of the "lazy-beds" of the Hebrides. They are
evidently the "cultivated grounds" of the hill-men, and doubtless the
hill-slopes were selected for various reasons, chief among which would
be their retired and somewhat inaccessible position. The ease with
which they could be drained and irrigated would be another of their
recommendations; and we must bear in mind that at this early date the
low-grounds were covered with forests and morasses, and therefore not
so easily cultivated as the hill-slopes.
Here and there we notice also little conical hillocks or tumuli. They
were formerly much more numerous, and by-and-by they will doubtless
all disappear. Numbers, even within recent years, have been pulled
down, partly to clear the ground, and partly for the sake of the stones
of which they are composed. This is much to be regretted; for their
destruction simply means the obliteration of historical records, the
loss of which can never be made good. I asked a farmer what had become
of the tumuli which at one time, according to the Ordnance Survey
map, were dotted over the hill behind his house. "If it's the wee
knowes (knolls) you mean, I pu'd them down, for they were jist in the
way. There was naething o' importance below the stanes, only a wheen
worthless bits o' pottery!" And the worthy pointed to a heap of stones
behind a neighbouring "dyke," where I afterwards found some fragments
of the pottery which had been so ruthlessly demolished. These tumuli
are no doubt old burial-places, and much information concerning the
habits of our ancient predecessors might often be obtained by a careful
examination of the mounds, when it is deemed essential to remove them.
But, surely, after all, they might be spared, for they can seldom be so
very much "in the way"; and, at all events, if they must be removed,
might it not be well to communicate the fact of their approaching
demolition to some local archæological society, or to any member of the
Berwickshire Naturalists' Club, who for the sake of science would, I
feel certain, do what was possible to preserve an accurate account of
their contents?
"Standing-stones" are met with now and again, either singly or in
groups, and sometimes they form circles. It is most likely that they
were raised by the same people who made the forts and tilled the
horizontal "lazy-beds." One can only conjecture that they may have
been designed as memorial stones, to mark the place where a chief
or person of consequence was slain in battle. They may also mark
burial-places, or indicate the site of some deed of prowess or other
action or circumstance worthy of being remembered. Antiquarians at one
time considered that all these stones were relics of druidical worship;
but it is needless to say that this view has long been abandoned.
That the ancient inhabitants of the Cheviots may have had some kind
of religion is exceedingly probable, but it must have been of a very
primitive kind, not more advanced than that of the North American
Indians.
Such are some of the more notable relics of the people who lived in the
valleys of the Cheviot Hills in pre-Roman times. These valleys, as I
have said, seem to have supported a numerous population, who tilled the
slopes and probably hunted in the forests of the adjoining low-grounds.
That they lived in fear of foes is sufficiently evident from the number
of their intrenchments and fortified camps, to which they would betake
themselves whenever their enemies appeared.
What effect the Roman occupation had on the dwellers among these hills
we cannot tell. The great "Watling Street" passes across the Cheviots,
and there are some old circular forts and camps quite close to that
wonderful road, along which many a battalion of Roman soldiers must
have marched; and these forts, if of pre-Roman age, were not at all
likely to have been held by the natives after Watling Street was made.
In the remoter fastnesses of the hills, however, the old tribes may
have continued to crop their "lazy-beds," to hunt, and tend their
herds, during the Roman occupation, and the old forts may have been in
requisition long after the last Roman had disappeared over the borders.
But I have already, I fear, delayed too long over the old history of
the Cheviot Hills, and must now draw my meagre sketches to a close.
In my first paper I said that these hills were a _terra incognita_ to
the tourist. Those who visit the district must not therefore expect
to meet with hotel accommodation. But "knowing" pedestrians will not
be much disturbed with this information, and will probably find, after
they have concluded their wanderings, that the hospitality and general
heartiness for which our stalwart Borderers were famous in other days
are still as noteworthy characteristics as they used to be.
V.
The Long Island, or Outer Hebrides.[J]
[J] _Good Words_, 1879.
I.
That long range of islands and islets which, extending from latitude
56° 47' N. to latitude 58° 32' N., acts as a great natural breakwater
to protect the north-west coast of Scotland from the rude assaults
of the Atlantic billows is not much visited by the ordinary tourist.
During "the season" the steamers now and again, it is true, deposit a
few wanderers at Tarbert and Stornoway, some of whom may linger for a
shorter or longer time to try a cast for salmon in Loch Laxdail, while
others, on similar piscatorial deeds intent, may venture inland as far
as Gearaidh nah Aimhne (Garrynahine). Others, again, who are curious
in the matter of antiquities, may visit the weird standing-stones of
Callernish, or even brave the jolting of a "trap" along the somewhat
rough road that leads from Tarbert to Rodel, in order to inspect the
picturesque little chapel there, and take rubbings of its quaint
tombstones with their recumbent effigies of knights, and Crusaders'
swords, and somewhat incomprehensible Latinity. Occasionally a few
bolder spirits may be tempted by the guide-books to visit Barra Head,
with its ruddy cliffs and clouds of noisy sea-birds, or even to run
north to the extremity of the Long Island to view the wonders of
the Butt of Lewis. But, as a rule, the few summer visitants who are
landed at Stornoway content themselves with a general inspection of
the grounds about Sir James Mathieson's residence, while those who
are dropped at Tarbert on Saturday are usually quite ready to depart
on Monday with the steamer that brought them. The fact is that hotel
accommodation in the Outer Hebrides is rather limited, and the means
of locomotion through the islands is on the same slender scale. Those,
therefore, who are not able and willing to rough it had better not
venture far beyond Tarbert and Stornoway.
When the islands are first approached they present, it must be
confessed, a somewhat forbidding aspect. Bare, bleak rocks, with a
monotonous rounded outline, crowd along the shore, and seem to form
all but the very highest portions of the land that meet our view,
while such areas of low-ground as we can catch a glimpse of appear
to be everywhere covered with a dusky mantle of heath and peat. But,
although the general character of the scenery is thus tame and sombre,
yet there are certain districts which in their wild picturesqueness are
hardly surpassed by many places in the northern Highlands, while one
may search the coast-line of the mainland in vain for cliffs to compare
with those gaunt walls of rock, against which the great rollers of the
Atlantic continually surge and thunder. It is wonderful, too, how,
under the influence of a light-blue sky, flecked with shining silvery
clouds, the sombre peat-lands lighten up and glow with regal purple and
ruddy brown. With such a sky above him, and with a lively breeze fresh
from the Atlantic and laden with the sweetness of clover and meadow-hay
and heather-bloom sweeping gaily past him, what wanderer in the Outer
Hebrides need be pitied? And such days are by no means so rare in these
islands as many a jaundiced Lowlander has maintained. It is true that
heavy mists and drizzling rain are often provokingly prevalent, and I
cannot forget the experience of a sad-hearted exile, who had resided
continuously for a year in Lewis, and who, upon being asked what kind
of climate that island enjoyed, replied: "Sir, it has no climate. There
are nine months of winter, and three months of very bad weather." For
myself, I can say that my experience of the climate in June, July, and
early August of several years has been decidedly favourable. During
those months I found comparatively few days in which a very fair
amount of walking and climbing could not be accomplished with ease and
pleasure, and that is a good deal more than one could venture to say
of Skye and many parts of the west coast of the mainland. The greatest
drawback to one's comfort are the midges, which in these islands
are beyond measure bloodthirsty, and quite as obnoxious as the most
carnivorous mosquitoes. Smoking, and all the other arts and devices by
which the designs of these tiny pests are usually circumvented, have
no effect upon the Hebridean vampires. In the low-grounds especially
they make life a burden. But those who have already become acquainted
with the Ross-shire midges, and yet have preserved their equanimity,
may feel justified in braving the ferocity of the Hebridean hosts. And
if they do so I believe they will be well repaid for their courage. To
the hardy pedestrian, especially, who likes to escape from the beaten
track laid down in guide-books, it will be a pleasure in itself to
roam over a region which has not yet come entirely under the dominion
of Mr. Cook. If he be simply a lover of the picturesque he will yet
not be disappointed, and possibly he may pick up a few hints in these
notes as to those districts which are most likely to repay him for his
toil in reaching them. But if to his love of the picturesque he joins
a taste for archæological pursuits, then I can assure him there is a
rich and by no means exhausted field of study in the antiquities of the
Long Island. Interesting, however, as are the relics of prehistoric and
later times which one meets with, yet it is the geologist, perhaps, who
will be most rewarded by a visit to these islands.
The physical features of the Outer Hebrides are, as already stated,
somewhat monotonous, but this is quite consistent with considerable
variety of scenic effect. All the islands are not equally attractive,
although the configuration of hills and low-grounds remains
persistently the same from the Butt of Lewis to Barra Head. The most
considerable island is that of which Lewis and Harris form the northern
and southern portions respectively. By far the larger part of the
former is undulating moorland, the only really mountainous district
being that which adjoins Harris in the south. A good general idea
of the moorlands is obtained by crossing the island from Stornoway
to Garrynahine. What appeared at first to be only one vast extended
peat-bog is then seen to be a gently-undulating country, coated, it
is true, with much peat in the hollows, but clad for the most part
with heath, through which ever and anon peer bare rocks and rocky
débris. Now and again, indeed, especially towards the centre of the
island, the ground rises into rough round-topped hills, sprinkled
sparingly with vegetation. One of the most striking features of the
low-grounds, however, is the enormous number of freshwater lakes,
which are so abundant as to form no small proportion of the surface.
They are, as a rule, most irregular in outline, but have a tendency to
arrange themselves in two directions--one set trending from south-east
to north-west, while another series is drawn out, as it were, from
south-west to north-east. I am sure that I am within the mark in
estimating the freshwater lakes in the low-grounds of Lewis to be at
least five hundred in number. In the mountain-district the lakes are,
of course, confined to the valleys, and vary in direction accordingly.
Harris and the southern part of Lewis are wholly mountainous, and show
hardly a single acre of level ground. The mountains are often bold and
picturesque, especially those which are over 1600 feet in height. They
are also exceedingly bare and desolate, the vegetation on their slopes
being poor and scanty in the extreme. Some of the hills, indeed, are
absolutely barren. In North Harris we find the highest peaks of the
Outer Hebrides: these are the Clisham, 2622 feet, and the Langa, 2438
feet. The glens in this elevated district are often wild and rugged,
such as the Bealach-Miavag and the Bealach-na-Ciste, both of which open
on West Loch Tarbert. But amid all this ruggedness and wild disorder
of broken crag and beetling precipice, even a very non-observant eye
can hardly fail to notice that the general contour or configuration of
the hills is smooth, rounded, and flowing, up to a rather well-marked
level, above which the outline becomes broken and interrupted, and all
the rounded and smoothed appearance vanishes. The contrast between the
smoothly-flowing contour of the lower elevations and the shattered and
riven aspect of the harsh ridges, sharp peaks, and craggy tors above,
is particularly striking. The mammillated and dome-shaped masses have
a pale, ghastly grey hue, their broad bare surfaces reflecting the
light freely, while at higher elevations the abundant irregularities
of the rocks throw many shadows, and impart a darker aspect to the
mountain-tops.
The appearances now described are very well seen along the shores of
West Loch Tarbert. All the hills that abut upon that loch show smoothed
and rounded faces, and this character prevails up to a height of 1600
feet, or thereabout, when all at once it gives way, and a broken,
interrupted contour succeeds. Thus the top of the Tarcall ridge in
South Harris is dark, rough, and irregular, while the slopes below are
grey, smooth, and flowing. The same is conspicuously the case with the
mountains in North Harris, the ruinous and sombre-looking summits of
the Langa and the Clisham soaring for several hundred feet above the
pale grey mammillated hills that sweep downwards to the sea.
After having familiarised themselves with the aspect of the hills
as seen from below, the lover of the picturesque, not less than the
geologist, will do well to ascend some dominant point from which an
extensive bird's-eye view can be obtained. For such purpose I can
recommend the Tarcall and Roneval in South Harris, the Clisham and the
Langa in North Harris, and Suainabhal in Lewis. The view from these
hills is wonderfully extensive and very impressive. From Suainabhal one
commands nearly all Lewis; and what a weird picture of desolation it
is! An endless succession of bare, grey, round-backed rocks and hills,
with countless lakes and lakelets nestling in their hollows, undulates
outwards over the districts of Uig and Pairc. Away to the north spread
the great moorlands with their lochans, while immediately to the south
one catches a fine panoramic view of the mountains of Harris. And then
those long straggling arms of the sea, reaching into the very heart of
the island--how blue, and bright, and fresh they look! I suppose the
natives of the Lewis must have been fishermen from the very earliest
times. It seems hardly possible otherwise to believe that the bare
rocks and peat-bogs, which form the major portion of its surface,
could ever have supported a large population; and yet there is every
evidence to show that this part of the Long Island was tolerably well
populated in very early days. The great standing-stones of Callernish
and the many other monoliths, both solitary and in groups, that are
scattered along the west coast of Lewis, surely betoken as much. And
those curious round towers, or places of refuge and defence, which are
so well represented in the same district, although they may be much
younger in date than the monoliths of Callernish, tell the same tale.
From the summits of the Clisham and the Langa the view is finer than
that obtained from Suainabhal. The former overlook all the high-grounds
of Harris and Lewis, and the monotonous moors with their countless
straggling lakes and peaty tarns. Indeed, they dominate nearly the
whole of the Long Island, the hills of distant Barra being quite
distinguishable. Of course, the lofty island of Rum, and Skye with
its Coolins, are both clearly visible, the whole view being framed in
to eastward by the mountains of Ross and Sutherland. On a clear day,
which, unfortunately, I did not get, one should be quite able to see
St. Kilda. Hardly less extensive is the view obtained from Roneval
(1506 feet) in the south of Harris. Far away to the west lie St. Kilda
and its little sister islet of Borerey. Southwards stretch the various
islands of the Outer Hebrides--North Uist, Benbecula, South Uist, and
Barra. How plainly visible they all are--a screen of high mountains
facing the Minch, and extending, apparently, along their whole eastern
margin--with broad lake-dappled plains sweeping out from the foot-hills
to the Atlantic. In the east, Skye with its spiky Coolins spreads
before one, and north of Skye we easily distinguish Ben Slioch and
the mountains of Loch Maree and Loch Torridon. South Harris lies, of
course, under our feet, and it is hard to give one who has not seen it
an adequate notion of its sterile desolation. Round-backed hills and
rocks innumerable, scraped bare of any soil, and supporting hardly a
vestige of vegetation; heavy mountain-masses with a similar rounded
contour, and equally naked and desolate; blue lakelets scattered in
hundreds among the hollows and depressions of the land: such is the
general appearance of the rocky wilderness that stretches inland from
the shores of the Minch. Then all around lies the great blue sea,
shining like sapphire in the sun, and flecked with tiny sails, where
the fishermen are busy at their calling.
From what has now been said, it will readily be understood that there
is not much cultivable land in Harris and the hilly parts of Lewis.
What little there is occurs chiefly along the west coast, a character
which we shall find is common to most of the islands of the Outer
Hebrides. In the neighbourhood of Stornoway, and over considerable
areas along the whole west coast of Lewis, the moorlands have been
broken in upon by spade and plough, with more or less success. But
natural meadow-lands, such as are frequently met with on the west side
of many of the islands both of the Outer and Inner Hebrides, are not
very common in Lewis.
One of the most notable features of the hillier parts of the Long
Island are the enormous numbers of loose stones and boulders which are
everywhere scattered about on hill-top, hill-side, and valley-bottom.
Harris is literally peppered with them, and they are hardly less
abundant in the other islands. They are of all shapes and sizes--round,
sub-angular, and angular. One great block in Barra I estimated to
weigh seven hundred and seventy tons. Many measure over three or
four yards across, while myriads are much smaller. These boulders are
sometimes utilised in a singular way. In Harris, there being only one
burial-place, the poor people have often to carry their dead a long
distance, and this of course necessitates resting on the journey. To
mark the spot where they have rested, the mourners are wont to erect
little cairns by the road-side, many of which are neatly built in the
form of cones and pyramids, while others are mere shapeless heaps of
stones thrown loosely together. Instead of raising cairns, however,
they occasionally select some boulder, and make it serve the purpose by
canting it up and inserting one or more stones underneath. Occasionally
I have seen in various parts of the mainland great boulders cocked
up at one end in the same way. Some of these may be in their natural
position, but as they often occupy conspicuous and commanding
situations, I am inclined to think that the cromlech-builders may have
tampered with them for memorial purposes. The present custom of the
Harris men may therefore be a survival from that far-distant period
when Callernish was in its glory.
North Uist is truly a land of desolation and dreariness. Bare, rocky
hills, which are remarkable for their sterile nakedness even in the
Long Island, form the eastern margin, and from the foot of these the
low, undulating rocky and peaty land stretches for some ten or twelve
miles to the Atlantic. The land is everywhere intersected by long,
straggling inlets of sea-water, and sprinkled with lakes and peaty
tarns innumerable. Along the flat Atlantic coast, which is overlooked
by some sparsely-clad hills, are dreary stretches of yellow sand
blown up into dunes. Near these are a few huts and a kirk and manse.
Not a tree, not even a bush higher than heather, is to be seen.
Peat, and water, and rock; rock, and water, and peat--that is North
Uist. The neighbourhood of Lochmaddy, which is the residence of a
sheriff-substitute, and rejoices besides in the possession of a jail,
is depressing in the extreme. It is made up of irregular bits of flat
land all jumbled about in a shallow sea, so that to get to a place one
mile in direct distance you may have to walk five or six miles, or even
more. I could not but agree with the natives of the more coherent parts
of the Long Island, who are wont to declare that Lochmaddy is only "the
clippings of creation"--the odds and ends and scraps left over after
the better lands were finished. North Uist, however, boasts of some
interesting antiquities--Picts' houses, and a great cairn called the
Barp, inside of which, according to tradition, rest the remains of a
wicked prince of the "good old days." Notwithstanding these, there are
probably few visitors who will not pronounce North Uist to be a dreary
island.
Benbecula is precisely like North Uist, but it lacks the bare mountains
of the latter. There is only one hill, indeed, in Benbecula; all the
rest is morass, peat, and water.
Massive mountains fringe all the eastern shores of South Uist, and
send westward numerous spurs and foot-hills that encroach upon the
"machars," or good lands, so as to reduce then to a mere narrow strip,
bordering on the Atlantic. Save the summits of Beinn Mhor (2033 feet)
and Hecla (1988 feet), which are peaked and rugged, all the hills show
the characteristic flowing outline which has already been described in
connection with the physical features of Harris. The low-grounds are,
as usual, thickly studded with lakes, and large loose boulders are
scattered about in all directions.
Barra is wholly mountainous, and, except that it is somewhat less
sterile, closely resembles Harris in its physical features, the hills
being smoothed, rounded, and bare, especially on the side of the island
that faces the Minch. Of the smaller islands that lie to the south,
such as Papey, Miuley, and Bearnarey, the most noteworthy features are
the lofty cliffs which they present to the Atlantic. For the rest, they
show precisely the same appearances as the hillier and barer portions
of the larger islands--rounded rocks with an undulating outline, dotted
over with loose stones and boulders, and now and again half-smothered
in yellow sand, which the strong winds blow in upon them.
There is thus, as I have said, considerable uniformity and even
monotony throughout the whole range of the Outer Hebrides. I
speak, however, chiefly as a geologist. An artist, no doubt, will
find infinite variety, and as he wends his way by moorland, or
mountain-glen, or sea-shore, scenes are constantly coming into
view which he will be fain to transfer to his sketch-book. The
colour-effects, too, are often surprisingly beautiful. When the rich
meadow-lands of the west coast are in all their glory, they show many
dazzling tints and shades, the deep tender green being dashed and
flushed with yellow, and purple, and scarlet, and blue, over which the
delighted eye wanders to a belt of bright sand upon the shore, and the
vast azure expanse of the Atlantic beyond. Inland are the heath-clad
moors, sprinkled with grey boulders and masses of barren rock, and
interspersed with lakes, some of which are starred with clusters of
lovely water-lilies. Behind the moorlands, again, rise the grim,
bald mountains, seamed and scarred with gullies, and in their very
general nakedness and sterility offering the strongest contrast to the
variegated border of russet moor, and green meadow, and yellow beach
that fringe the Atlantic coast.
All through the islands, indeed, the artist will come upon interesting
subjects. A most impressive scene may sometimes be witnessed on
crossing the North Ford, between North Uist and Benbecula. At
low-water, the channel or sound between these two islands, which is
five miles in breadth, disappears and leaves exposed a wide expanse
of wet sand and silt, dotted with black rocks and low tangle-covered
reefs and skerries. On the morning I passed over, ragged sheets of mist
hung low down on the near horizon, half-obscuring and half-revealing
the stony islets, and crags, and hills that lay between the ford and
the Minch. Seen through such a medium, the rocks assumed the most
surprising forms, sometimes towering into great peaks and cliffs,
at other times breaking up, as it were, into low reefs and shoals,
and anon dissolving in grey mist and vapour. At other times the thin
cloud-curtain would lift, and then one fancied one saw some vast city
with ponderous walls and battlements, and lofty towers and steeples,
rising into the mist-wreaths that hung above it, while from many points
on the Benbecula coast, where kelp was being prepared, clouds of smoke
curled slowly upwards, as if from the camp-fires of some besieging
army. The track of the ford winds round and about innumerable rocks,
upon which a number of "natives," each stooping solitary and silent to
his or her work, were reaping the luxuriant seaweed for kelp-making.
Their silence was quite in keeping with the general stillness, which
would have been unbroken but for the harsh scream of the sea-birds, as
they ever and anon rose scared from their favourite feeding-grounds
while we plodded and plashed on our way. The artist who could
successfully cope with such a scene would paint a singularly weird and
suggestive picture.
But, to return to the physical features of the Long Island, what, we
may ask, is the cause of that general monotony of outline to which
reference has so frequently been made? At first we seem to get an
answer to our question when we are told that the islands of the Outer
Hebrides are composed chiefly of one and the same kind of rock.
Everyone nowadays has some knowledge of the fact that the peculiar
features of any given district are greatly due to the character and
arrangement of the rock-masses. For example, who is not familiar with
the outline of a chalk country, as distinguished from the contour of a
region the rocks of which are composed, let us say, of alternating beds
of limestone and sandstone and masses of old volcanic material? The
chalk country, owing to the homogeneousness of its component strata,
has been moulded by the action of weather and running water into an
undulating region with a softly-flowing outline, while the district
of composite formation has yielded unequally to the action of Time's
workers--rains, and frosts, and rivers--and so is diversified with
ridge, and escarpment, and knolls, and crags. When, therefore, we learn
that the Outer Hebrides are composed for the most part of the rock
called _gneiss_ and its varieties, we seem to have at once found the
meaning of the uniformity and monotony. It is true that although pink
and grey gneiss and schistose rocks prevail from the Butt of Lewis
to Barra Head, yet there are some other varieties occasionally met
with--thus soft red sandstone and conglomerate rest upon the gneissic
rocks near Stornoway, but they occur nowhere else throughout the Long
Island. Now and again, however, the gneiss gives place to granite, as
on the west coast of Lewis near Carloway; and here and there the strata
are pierced by vertical dykes and curious twisted and reticulated veins
of basalt-rock. All these, however, hold but a minor and unimportant
place as constituents of the islands. Gneiss is beyond question the
most prevalent rock, and we seem justified in assigning the peculiar
monotony of the Outer Hebridean scenery to that fact.
But when we come to examine the matter more attentively, we find that
there is still some important factor wanting. We have not got quite to
the solution of the question. When we study the manner in which the
gneiss and gneissic rocks disintegrate and break up at the sea-coast
or along the flanks of some rugged mountain-glen, we see they give
rise to an irregular uneven surface. They do not naturally decompose
and exfoliate into rounded dome-shaped masses, such as are so commonly
met with all through the islands, but rather tend to assume the aspect
of rugged tors, and peaks, and ridges. The reason for this will be
more readily understood when it is learned that the gneissic rocks of
the Outer Hebrides are for the most part arranged in strata, which,
notwithstanding their immense antiquity--(they are the oldest rocks
in Europe)--and the many changes they have undergone, are yet, as a
rule, quite distinguishable. The strata are seldom or never horizontal,
but are usually inclined at a high angle, either to north-east or
south-west, although sometimes, as in the vicinity of Stornoway, the
"dip" or inclination of the beds is to south-east. Throughout the
major portion of the Long Island, however, the outcrop of the strata
runs transversely across the land from south-east to north-west. Now
we know that when this is the case strata of variable composition and
character give rise to long escarpments and intervening hollows--the
escarpments marking the outcrops of the harder and more durable beds,
and the hollows those strata that are softer and more easily eroded by
the action of the denuding forces, water and frost. When the dip of the
strata is north-east we expect the escarpments to face the south-west,
and the reverse will be the case when the strata incline in the
opposite direction.
Seeing then that the Outer Hebrides are composed chiefly of gneissic
rocks and schists which yield unequally to the weather, and which, in
the course of time, would naturally give rise to lines of sharp-edged
escarpments or ridges and intervening hollows, with now and again
massive hills and mountains showing great cliffs and a generally
broken and irregular outline, why is it that such rugged features are
so seldom present at low levels, and are only conspicuous at the very
highest elevations? The rocks of the Outer Hebrides are of immense
antiquity, and there has therefore been time enough for them to assume
the irregular contour which we might have expected. But in place of
sharp-rimmed escarpments, and tors, and broken shattered ridges, we see
everywhere a rounded and smoothly-flowing configuration which prevails
up to a height of 1600 feet or thereabout, above which the rocks take
on the rugged appearance which is natural to them. By what magic have
the strata at the lower levels escaped in such large measure from the
action of rain and frost, which have furrowed and shattered the higher
mountain-tops?
I have said that long lines of escarpment and ridges, corresponding to
the outcrops of the harder and more durable strata, are not apparent in
these islands. A trained eye, however, is not long in discovering that
such features, although masked and obscured, are yet really present.
The round-backed rocks are drawn out, as it were, in one persistent
direction, which always agrees with the _strike_ or outcrop of the
strata; and in many districts one notices also that long hollows
traverse the land from south-east to north-west in the same way.
Such alternating hollows and rounded ridges are very conspicuous in
Barra and the smaller islands to the south, and they may likewise be
noted in most of the larger islands also. Looking at these and other
features, the geologist has no hesitation in concluding that the whole
of the islands have been subjected to some powerful abrading force,
which has succeeded to a large extent in obliterating the primary
configuration of the land. The rough ridges have been rounded off,
the sharp escarpments have been bevelled, the abrupt tors and peaks
have been smoothed down. Here and there, it is true, the dome-shaped
rock-masses are beginning again to break up under the action of the
weather so as to resume their original irregular configuration. And,
doubtless, after the lapse of many ages, rain and frost will gradually
succeed in destroying the present characteristic flowing outlines, and
the islands will then revert to their former condition, and rugged
escarpments, sharp peaks, and rough broken hummocks and tors will again
become the rule. But for a long time to come these grey Western Islands
will continue to present us with some of the most instructive examples
of rounded and mammillated rock-masses to be met with in Europe.
From Barra Head in Bearnarey to the Butt of Lewis we are constantly
confronted by proofs of the former presence of that mysterious abrading
power, which has accommodated itself to all the sinuosities of the
ground, so that from the sea-level up to a height of 1600 feet at
least, the eye rests almost everywhere upon bare round-backed rocks and
smoothed surfaces.
II.
In the preceding article I have described the peculiar configuration
of the Long Island--rounded and flowing for the most part--and have
pointed out how that softened outline is not such as the rocks would
naturally assume under the influence of the ordinary agents of erosion
with which we are familiar in this country. The present contour has
superseded an older set of features, which, although highly modified
or disguised, and often well-nigh obliterated, are yet capable of
being traced, and are, no doubt, the conformation assumed by the rocks
under the long-continued action of rain and frost and running water.
We have now to inquire what it was that removed or softened down the
primal configuration I refer to, and gave to the islands their present
monotonous, undulating contour.
Any one fresh from the glacier-valleys of Switzerland or Norway could
have little doubt as to the cause of the transformation. The smoothed
and rounded masses of the Outer Hebrides are so exactly paralleled by
the ice-worn, dome-shaped rocks over which a glacier has flowed, that
our visitor would have small hesitation in ascribing to them a similar
origin; and the presence of the countless perched blocks and boulders
which are scattered broadcast over the islands would tend to confirm
him in his belief. A closer inspection of the phenomena would soon
banish all doubt from his mind; for, on the less-weathered surfaces,
he would detect those long parallel scratches and furrows which are
the sure signs of glacial action, while, in the hollows and over the
low-grounds, he would be confronted with that peculiar deposit of
clay and sand and glaciated stones and boulders which are dragged on
underneath flowing ice.
Having satisfied ourselves that the rounded outline of the ground is
the result of former glacial action, our next step is to discover, if
we can, in what direction the abrading agent moved. Did the ice, as
we might have supposed, come out of the mountain-valleys and overflow
the low country? If that had been the case, then we should expect to
find the glacial markings radiating outwards in all directions from
the higher elevations. Thus the low-grounds of Uig, in Lewis, should
give evidence of having been overflowed by ice coming from the Forest
of Harris; the undulating, rocky, and lake-dappled region that extends
between Loch Roag and Loch Erisort should be abraded and striated
from south-west to north-east. Instead of this, however, the movement
has clearly been from south-east to north-west. All the prominent
rock-faces that look towards the Minch have been smoothed off and
rounded, while in their rear the marks of rubbing and abrading are much
less conspicuous. It is evident that the south-east exposure has borne
the full brunt of the ice-grinding--the surfaces that are turned in the
opposite direction, or towards the Atlantic, having been in a measure
protected or sheltered by their position. The striations or scratches
that are seen upon the less-weathered surfaces point invariably towards
the north-west, and from their character and the mode in which they
have been graved upon the rock, we are left in no doubt as to the trend
of the old ice-plough--which was clearly from south-east to north-west.
Nor is it only the low-grounds that are marked in this direction.
Ascend Suaina (1300 feet), and you shall find it showing evident signs
of having been abraded all over, from base to summit. The same, indeed,
is the case with all the hills that stretch from sea to sea between
Uig and Loch Seaforth. Beinn Mheadonach, Ceann Resort, Griosamul, and
Liuthaid, are all strongly glaciated from south-east to north-west.
North and South Harris yield unequivocal evidence of having been
overflowed by ice which did not stream out of the mountain-valleys,
but crossed the island from the Minch to the Atlantic. A number of
mountain-glens, coming down from the Forest of Harris, open out
upon West Loch Tarbert, and these we see have been crossed at right
angles by the ice--the mountains between them being strongly abraded
from south-east to north-west. It is the same all over South Harris,
which affords the geologist every evidence of having been literally
smothered in ice, which has moved in the same persistent direction. The
rock-faces that look towards the Minch are all excessively naked; they
have been terribly ground down and scraped, and the same holds good
with every part of the island exposed to the south-east.
Now, the mode in which the rocks have been so ground, scraped, rounded,
and smoothed betokens very clearly the action of land-ice, and not of
floating-ice or icebergs. The abrading agent has accommodated itself
to all the sinuosities of the ground, sliding into hollows and creeping
out of them, moulding itself over projecting rocks, so as eventually to
grind away all their asperities, and convert rugged tors and peaks into
round-backed, dome-shaped masses. It has carried away the sharp edges
of escarpments and ridges, and has deepened the intervening hollows in
a somewhat irregular way, so that now these catch the drainage of the
land and form lakes. Steep rocks facing the Minch have been bevelled
off and rounded atop, while in their rear the ice-plough, not being
able to act with effect, has not succeeded in removing the primeval
ruggedness of the weathered strata.
I have said that the movement of the ice was from south-east to
north-west. But a close examination of the ice-markings will show that
the flow was very frequently influenced by the form of the ground.
Minor features it was able to disregard, but some prominent projecting
rock-masses succeeded in deflecting the ice that flowed against them.
For example, if we study the rocks in North Harris, we shall find that
the Langa and the Clisham have served as a wedge to divide the ice,
part of which flowed away into Lewis, while the other current or stream
crept out to sea by West Loch Tarbert. The Langa and the Clisham,
indeed, raised their heads above the glacier mass--they were islets in
a sea of ice. It is for this reason that they and the Tarcull ridge in
South Harris have not been smoothed and abraded, but still preserve
their weathered outline. All surfaces below a height of 1600 feet which
are exposed to the south-east, and which have not been in recent times
broken up by the action of rain and frost, exhibit strongly-marked
glaciation. But above that level no signs of ancient ice-work can be
recognised.
We see now why it is that the hill-slopes opposite the Minch should,
as a rule, be so much more sterile than those which slope down to the
Atlantic. The full force of the ice was exerted upon the south-east
front, in the rear of which there would necessarily be comparatively
"quiet" ice. For the same reason we should expect to find much of the
rock débris which the ice swept off the south-east front sheltering on
the opposite side. Neither clay nor sand nor stones would gather under
the ice upon the steep rocks that face the Minch. The movement there
was too severe to permit of any such accumulation. But stones and clay
and sand were carried over and swept round the hills, and gradually
accumulated in the rear of the ice-worn rocks, just in the same way as
gravel and sand are heaped up behind projecting stones and boulders
in the bed of a stream. Hence it is that the western margin of Harris
is so much less bleak than the opposite side. Considerable taluses
of "till," as the sub-glacial débris is called, gather behind the
steeper crags, and ragged sheets of the same material extend over the
low-grounds. All the low-grounds of Lewis are in like manner sprinkled
with till. Over that region the ice met with but few obstacles to its
course, and consequently the débris it forced along underneath was
spread out somewhat equally. But wherever hills and peaks and hummocks
of rock broke the regularity of the surface, there great abrasion took
place and no till was accumulated.
Thus the position and distribution of this sub-glacial débris or
bottom-moraine tell the same tale as the abraded rocks and glacial
striæ, and clearly indicate an ice-flow from the south-east. This is
still further proved by the manner in which the upturned ends of the
strata are frequently bent over underneath the till in a north-westerly
direction, while the fragments dislodged from them and enclosed in the
sub-glacial débris stream away as it were to the same point of the
compass. Not only so, but in the west of Lewis, where no red sandstone
occurs, we find boulders of red sandstone enclosed in the till, which
could not have been derived from any place nearer than Stornoway. In
other words, these boulders have travelled across the island from the
shores of the Minch to the Atlantic sea-board.
Having said so much about the glaciation of Lewis and Harris, I need
not do more than indicate very briefly some of the more interesting
features of the islands further south.
I spent some time cruising up and down the Sound of Harris, and found
that all the islets there had been ground and scraped by ice flowing
in the normal north-west direction, and sub-glacial débris occurs on
at least one of the little islands--Harmetrey. But all the phenomena
of glaciation are met with in most abundance in the dreary island of
North Uist. The ridge of mountains that guards its east coast has been
battered, and ground down, and scraped bare in the most wonderful
manner, while the melancholy moorlands are everywhere sprinkled with
till, full of glaciated stones, many of which have travelled west
from the coast range. Benbecula shows in like manner a considerable
sprinkling of till, and the trend of the glacial striæ is the same
there as in North Uist, namely, a little north of west. There are no
hills of any consequence in Benbecula, but the highly-abraded and
barren-looking mountains that fringe the eastern margin of North Uist
are continued south in the islands of Roney and Fuiey, either of which
it would be hard to surpass as examples of the prodigious effect of
land-ice in scouring, scraping, and grinding the surface over which it
moves.
South Uist presents the same general configuration as North Uist,
its east coast being formed of a long range of intensely glaciated
mountains, in the rear of which ragged sheets and heaps of sub-glacial
débris are thrown and scattered over the low, undulating tract that
borders the Atlantic. No part of either Benbecula or North Uist has
escaped the action of ice, but in South Uist that knot of high-ground
which is dominated by the fine mountains of Beinn Mhor and Hecla
towered above the level of the glacier-mass, and have thus been the
cause of considerable deflection of the ice-flow. The ice-stream
divided, as it were, part flowing round the north flank of Hecla,
and part streaming past the southern slopes of Beinn Mhor. But the
ice-flow thus divided speedily reunited in the rear of the mountains,
the southern stream creeping in from the south-east, and the northern
stream stealing round Hecla towards the south-west. The track of this
remarkable deflection and reunion is clearly marked out by numerous
striæ all over the low-grounds that slope outwards to the Atlantic
coast. The till, it need hardly be added, affords the same kind of
evidence as the sub-glacial deposits of the other islands, and points
unmistakably to a general ice-movement across South Uist from the Minch
to the Atlantic.
The influence which an irregular surface has in causing local
deflections of an ice-flow is also well seen in Barra, where the striæ
sometimes point some 5° or 10°, and sometimes 25° and even 35° north of
west--these variations being entirely due to the configuration of the
ground. This island is extremely bare in many places, more especially
over all the region that slopes to the Minch. The Atlantic border is
somewhat better covered with soil, as is the case with South Uist and
the other islands already described.
Vatersey, Saundry, Papey, Miuley, and Bearnarey, are all equally well
glaciated; but as they show little or no low-ground with gentle slopes,
they have preserved few traces of sub-glacial débris. In this respect
they resemble the rockier and hillier parts of the large islands to
the north. Till, however, is occasionally met with, as for example on
the low shores of Vatersey Bay, and on the southern margin of Miuley.
Doubtless, if it were carefully looked for it would be found sheltering
in patches in many nooks and hollows, protected from the grind of the
ice that advanced from the south-east. I saw it in several such places
in the islet of Bearnarey, where the striæ indicated an ice-flow as
usual towards the north-west.
We have now seen that the whole of the Long Island has been ground,
and rubbed, and scraped by land- or glacier-ice which has traversed
the ground in a prevalent south-east and north-west direction. We have
seen also that this ice attained so great a thickness that it was able
to overflow all the hills up to a height of 1600 feet above the sea.
It is needless to say that such a mass could not have been nurtured on
the islands themselves. They have no gathering grounds of sufficient
extent, and if they had, the ice would not have taken the peculiar
direction it did. Instead of flowing across the islands it would have
radiated outwards from the mountain-valleys. Where, then, did the ice
come from?
Looking across the Minch we see Skye and the mountains of the
north-west Highlands, and those regions, as we know, have also been
subjected to extreme glaciation. From the appearances presented by
the mountains of Ross-shire we are compelled to believe that all that
region was buried in ice up to a height of not less than 3000 feet--the
ice-sheet was probably even as much as 3500 feet in thickness. The
evidence shows that the under portion of this vast ice-sheet flowed
slowly off the country into the Minch by way of the great sea-lochs.
Thus we know that an enormous mass crept down Loch Carron and united
with another great stream stealing out from the mountains of Skye, to
flow north through the hollows of Raasay Sound and the Inner Sound
into the Minch. So deep was the ice that it completely smothered the
island of Raasay (1272 feet high) and overflowed all the lofty trappean
table-lands of Skye. From the Coolins, as a centre-point, another
movement of the ice-sheet was towards the south-west, against the
islands of Rum, Cannay, and Eigg. Further north similar vast masses of
ice streamed out into the Minch, from Loch Torridon, Gairloch, Loch
Ewe, and Loch Broom. The direction of the glaciation in the north of
Skye, which is towards north-west, shows that the glacier-mass which
overflowed that area must eventually have reached the shores of the
Long Island. In short, there cannot be a reasonable doubt that the
immense sheet of ice that streamed off the north-west Highlands must
have filled up entirely the basin of the Minch, and thereafter streamed
across the Outer Hebrides. But it may be objected that if the Outer
Hebrides were overflowed by ice that streamed from the mainland across
the north end of Skye, we ought to get many fragments of Skye rocks
and Ross-shire rocks too in the sub-glacial débris or till of Lewis
and Harris, and the north end of North Uist. But all such fragments
are apparently wanting. True, there are bits of stone like the igneous
rocks of Skye often met with in the Hebridean till, but as veins or
dykes of precisely the same kind of rock occur in the Long Island
itself, we cannot say that the stones referred to are other than
native. A little reflection will show us, however, that it is extremely
improbable indeed that stones derived from Skye and the mainland should
ever have been dragged on under the ice, and deposited amongst the till
of the Long Island. There is only one part of the whole Outer Hebrides
where we might have anticipated that fragments from the mainland should
occur; and there, sure enough, they put in an appearance.
But before I attempt to explain the non-occurrence of Skye rocks in
the till of the Outer Hebrides, let me show in a few words what the
glaciation of the Long Island, Skye, and the north-west Highlands
teaches us as to the general aspect presented by the ice-sheet. The
height reached by the surface of the ice in Ross-shire and the Long
Island respectively indicates of course that the main movement was
from the mainland. We must conceive of an immense sheet of solid ice
filling up all the inequalities of the land, obliterating the glens,
and sweeping across the hill-tops; and not only so, but occupying the
wide basin of the Minch to the entire exclusion of the sea, the surface
of the ice rising so high that it overtopped the whole of the Outer
Hebrides, and left only the tips of a few of the higher mountains
uncovered. The slope of the surface was persistently outwards from the
mainland, and the striation of the Long Island indicates clearly that
the dip or inclination of that surface was towards the north-west. Nay,
more than this, we are now enabled for the first time to say with some
approach to certainty what was the precise angle of that inclination.
If we take the upper surface of the ice in Ross-shire to have been 3000
feet (and it was not less), then the slope between the mainland and
the Outer Hebrides was only 25 feet in the mile, or about 1 in 210. It
is quite possible, however, and even probable, that the actual height
attained by the ice-sheet in the north-west Highlands was more than
3000 feet. I think it may yet turn out to have been 3500 feet, and if
this were so it would give an inclination for the surface of the ice of
about 35 feet in the mile. In either case the slope was so very gentle
that to the eye it would have appeared like a level plain. Over the
surface of this plain would be scattered here and there a solitary big
erratic or two, while in other places long trains of large and small
angular boulders would stream outwards. All these would be derived from
such mountain in Skye and the mainland as were able to keep their heads
above the level of the ice-flow; while a few also might be dislodged by
the frost and rolled down upon the glacier from the tips of the Clisham
and the Langa in Harris, and Hecla and Beinn Mhor in South Uist. Every
such block, it is evident, would be carried across the buried Hebrides,
out into the Atlantic in the direction indicated by the glaciation of
the Long Island--that is, towards the north-west.
But while the upper strata of the ice doubtless followed that
particular course, it is obvious that this could not be the case
with the under portion of the great sheet, the path of which would
be controlled in large measure by the form of the ground over which
the ice moved. The upper strata that overflowed the Outer Hebrides,
as we have seen, were locally deflected again and again by important
obstacles, and it is quite certain that the same would take place with
the deeper portions of the ice-flow.
It is well known that the sea along the inner margin of the Long Island
is very deep. In many places it reaches a depth of 600 feet, and
occasionally the sounding-lead plunges down for upwards of 700 feet.
It would seem, however, that these great depths did not exist before
the advent of the ice-sheet, but that the bottom of the Minch along
the eastern borders of the Long Island was then some 250 or 300 feet
shallower than now, the floor of the sea having since been excavated
in the manner I shall presently describe. It is quite apparent,
therefore, that the long ridge of the Outer Hebrides must have offered
an insuperable obstacle to the direct passage of the bottom-ice out
to the Atlantic. Here was a great wall of rock shooting up from the
floor of the Minch, at a high angle, to a height ranging in elevation
from 400 feet to upwards of 3000 feet. It is simply impossible that
the lower strata of the ice that occupied the bed of the Minch could
climb that precipitous barricade. They were necessarily deflected, one
portion creeping to north-east and another to south-west, but both
hugging the great wall of rock all the way. We see precisely the same
result taking place in the bed of every stream. Let us stand upon an
almost submerged boulder, and note how the water is deflected to right
and left, and we shall observe at the same time that the boulder, by
obstructing the current, forces the water downwards upon the bed of the
stream, the result being that a hollow is dug out in front. Now, in a
similar manner, the ice, squeezed and pressed against the Hebridean
ridge by the steady flow of the great current that crossed the Minch,
necessarily acted with intense erosive force upon its bed. Hence in the
course of time it scooped out a series of broad deep trenches along the
whole inner margin of the Long Island, the amount of the excavation
reaching from 200 to 300 feet. Similar excavated basins occur in like
positions opposite all the precipitous islands of the Inner Hebrides.
Wherever, indeed, the ice-sheet met with any great obstruction to
its flow, there excessive erosion took place, and a more or less
deep hollow was dug out in front of the opposing cliff, or crag,
or precipitous mountain. While, therefore, the upper strata of the
ice-sheet overflowed the Outer Hebrides from south-east to north-west,
the under portions of the same great ice-flow were compelled by the
contour of the ground to creep away to north-east and south-west, until
they could steal round the ridge and so escape outwards to the Atlantic.
This being the case, we have a very simple and obvious explanation of
the absence of Skye rocks in the till of the Long Island. One sees
readily enough that the sub-glacial débris dragged across the Minch
would naturally be carried away to south-west and north-east by the
"under-tow" or deflected ice. It is quite impossible that any Skye
fragments or bits of rock from the mainland could travel over the bed
of the Minch, and then be pushed up the precipitous rock wall of the
Long Island. There is only one place in all the Outer Hebrides where
we might expect to meet with extraneous boulders in the till, and
that is in the north of Lewis, where the land shelves gently into the
sea, and the great rocky ridge terminates. Here the under-strata of
the ice would begin to steal up upon the land, favoured by its gentle
inclination, and in that very place accordingly we meet with a deposit
of till in which are found many boulders of a hard red sandstone, and
some of various porphyries which are quite alien to the Long Island.
Moreover, the till itself in that locality is much more of a clay than
the usual sub-glacial débris in other parts of Lewis, and contains
numerous fragments of sea-shells. All this is quite in keeping with the
other evidence. The extreme north end of Lewis was overflowed by the
under-current that crept up the bed of the Minch, hugging the Hebridean
ridge, and dragging along with it a muddy mass interspersed with the
shells and other marine exuviæ that lay in its path, and numerous
stones, some of which may have come from Skye, while others were
derived from the mainland.
I have already said enough, perhaps, about the abrasion of the
Hebrides, but I may add a few words upon the origin of the freshwater
lakes. Many of these rest in complete rock-basins; others, again, seem
to lie partly upon solid rock and partly upon till; while yet others
appear to occupy mere shallow depressions in the surface of the till.
All of them thus owe their origin to the action of the ice-sheet. As
one might have expected, the great majority lie along the outcrop of
the gneissic strata, which, as a rule, corresponds pretty closely to
the flow of the ice. Hence the general trend of the lakes is from
south-east to north-west. In many cases in fashioning these rock-basins
the ice has merely deepened in an irregular manner previously existing
hollows, which are now, of course, filled with water. In not a few
places, however, the lakes are drawn out in other directions--this
being due usually to changes in the strike or outcrop of the strata.
For example, over a considerable district in the south of Lewis many
lake-hollows extend from south-west to north-east, or at right angles
to the direction of the ice-flow. Such lakes are usually dammed up at
one or both extremities by glacial débris.
Thus most of the features characteristic of the Outer Hebrides owe
their origin directly or indirectly to the action of that great sheet
of ice which swept over the islands during what is called the Glacial
Period. And there is no region in northern Europe where the immensity
of the abrading agent can be more vividly realised. From a study of
the phenomena there exhibited we for the first time obtain a definite
idea of the surface-slope, and are able to plumb the old ice-sheet, and
ascertain with some approach to accuracy its exact thickness. In the
deeper parts of the area, between the mainland and the Long Island, its
thickness was not less than 3800 feet. Of course this great depth of
ice could not have been derived exclusively from the snow that fell on
the mountains of the north-west Highlands. Doubtless the precipitation
took place over its whole surface, just as is the case in Greenland and
over the Antarctic continent. The winter cold must have been excessive,
but the precipitation necessary to sustain such a mass of ice implies
great evaporation; in other words, the direct heat of the sun _per
diem_ in summer-time was probably considerably in excess of what it is
now in these latitudes. The west and south-west winds must have been
laden with moisture, the greater portion of which would necessarily
fall in the form of snow. We see something analogous to this taking
place in the Antarctic regions at the present day. That quarter of the
globe has its summer in perihelion, and, therefore, must be receiving
then more heat _per diem_ than our hemisphere does in its summer
season, which, as every one knows, happens when the earth is furthest
removed from the sun. But, notwithstanding this, the summer of the
Antarctic continent is cold and ungenial--the presence of the great
ice-sheet there cooling the air and causing most of the moisture to
fall as snow. Paradoxical as it may seem, therefore great summer heat
is almost, if not quite, as necessary as excessive winter cold for the
production and maintenance of a wide continental glacier.
III.
When we last took a peep at the Outer Hebrides we found those luckless
islands all but obliterated under an immense sheet of ice extending
from the mainland out into the Atlantic. How far west the great glacier
spread itself we cannot as yet positively say; but if the known slope
of its surface between the north-west Highlands and the Long Island
continued, as there is every reason to believe it would, then it is
extremely probable that the ice flowed out to the edge of the great
Scottish submarine plateau. Here the sudden deepening of the Atlantic
would arrest its progress and cause it to break up into icebergs. In
those old times, therefore, a steep wall of ice would extend all along
the line of what is now the edge of the 100-fathoms plateau. From this
wall large tabular masses would ever and anon break away and float off
into the Atlantic--a condition of things which is closely paralleled at
present along the borders of the ice-drowned Antarctic continent.
By-and-by, however, a great change took place, and the big ice-sheet
melted off the Long Island and vanished from the Minch. We read the
evidence for this change of climate in certain interesting deposits
which occur in considerable bulk at the northern extremity of Lewis,
and in smaller patches in the Eye peninsula of the same island. In
those districts the old sub-glacial débris or till is covered with
beds of clay and sand in which many marine exuviæ are found--shells of
molluscs, entomostraca, foraminifera, etc. They clearly prove, then,
that after the ice-sheet had vanished Lewis was submerged in the sea
to a depth of not less than 200 feet, and they also prove that the
temperature of the sea was much the same then as now, for the shells
all belong to species that are still living in these northern waters.
It is very remarkable that the marine deposits in question seem to
occur nowhere else in any part of the Long Island. We cannot believe
that the submergence was restricted to the very limited areas where
the shell-beds are met with: it must, on the contrary, have affected
a very large portion, if not the whole, of the Outer Hebrides. Why,
then, do not we meet with shelly sands and clays, with raised beaches
and other relics of the former occupation of these islands by the sea,
covering wide areas in the low-grounds? How can we explain the absence
of such relics from all those districts which, being much under the
level of 200 feet, must necessarily have at one time formed part of the
sea-floor? The explanation is not difficult to discover.
Resting upon the surface of the shell-beds at Ness and Garabost we
find an upper or overlying accumulation of sub-glacial débris or till.
At Ness this upper till closely resembles, in general appearance, the
lower deposit that rests directly upon the rocks. It is a pell-mell
accumulation of silty clay, crammed with glaciated stones, amongst
which are many fragments of red sandstone and some extra-Hebridean
rocks, and interspersed through it occur also broken fragments of
sea-shells. The marine deposits lying below are usually much confused
and contorted, and here and there they are even violently commingled
with the upper till. They show, generally, a most irregular surface
under that accumulation, and are evidently only the wreck of what they
must at one time have been. Now the presence of this upper till proves
beyond doubt that the intense arctic conditions of climate once more
supervened. A big ice-sheet again filled up the basin of the Minch and
flowed over the Long Island--its under-tow creeping along the inner
margin of the lofty rock-barrier as before, and eventually stealing
over the low-ground at the Butt, where its bottom-moraine or till was
dragged over the marine deposits, and confusedly commingled with them.
The upper strata of the ice that streamed across the islands renewed
the work of abrasion, and succeeded in scraping away all traces of the
late occupation by the sea. If any such now exist they must lie buried
under the till that cloaks the low-ground on the western margins of
the islands. Hence it is that we find not a vestige of shelly beds in
any part of the Long Island which was exposed to the full brunt of
the ice-flow. At Garabost they have been ploughed through in the most
wonderful manner, and only little patches remain. At Ness, however,
they are more continuous. This is owing to the circumstance that the
ground in that neighbourhood is low-lying and offered no obstacle
to the passage of the ice out to sea. Hence the shell-beds were not
subjected to such excessive erosion as overtook them along the whole
eastern border of the Long Island.
Eventually, however, this later advance of the ice-sheet ceased. The
climate grew less arctic, and the great glacier began to melt away,
until the time came that its upper strata ceased to overflow the
islands. They then passed away to north and south, along the hollow
now occupied by the Minch, following the same path as the bottom-ice.
Considerable snow-fields, however, still covered the Outer Hebrides,
and large local glaciers occupied all the mountain-valleys, and,
descending to low levels, piled up their terminal moraines. Some of
these local glaciers appear to have gone right out into the Minch,
as in South Uist, and may have coalesced with the great glacier that
still filled that basin. It was during this condition of things that
most of the great perched blocks that are scattered so profusely over
the islands began to be dropt into their present positions. During
the climax of glacial cold, when the upper strata of the ice-sheet
streamed across the Hebrides, large fragments of rock would certainly
be wrenched off and carried on underneath the ice; but as only a few of
the Hebridean mountain-tops were then exposed, there would be a general
absence of such enormous erratics as are detached by frost and rolled
down upon the surface of a glacier, and any such superficially-borne
erratics would be transported, of course, far beyond the Long Island
into the Atlantic. When the ice had ceased to overflow the islands,
boulders derived from Skye and the mainland would no longer be carried
so directly out to the Atlantic, but would travel thither by the more
circuitous route, which the now diminished ice-sheet was compelled to
follow.
As the snow and ice melted off the Hebrides, the rocks would begin
to be exposed to the action of intense frost, and many fragments,
becoming dislodged and falling upon _névé_, small local ice-sheets,
and glaciers, would be stranded on hill-slopes and sprinkled over the
low-grounds, along with much broken débris and rock-rubbish. Eventually
all the lower-grounds would be deserted by the ice, glaciers would die
out of the less elevated valleys, and linger in only a few of the glens
that drain the higher mountain-masses. Such local glaciers have flowed
often at right angles to the direction followed by the great ice-sheet.
Thus, the ice-markings in the glens that come down from the Forest
of Harris to West Loch Tarbert, run from north to south, while the
trend of the older glaciation on the intervening high-grounds is from
south-east to north-west.
The morainic rubbish and erratics of this latest phase in the glacial
history of the Long Island may be traced down almost to the water's
edge, showing plainly that there has been no great submergence of that
region since the disappearance of glacial conditions. This is somewhat
remarkable, because along the shores of central and southern Scotland
we have indisputable evidence to show that the land was drowned to
the depth of at least fifty feet in post-glacial times. In the Outer
Hebrides, however, there are no traces of any post-glacial submergence
exceeding a dozen feet or so; that is to say, there is no proof that
the Outer Hebrides have been of much less extent than they are now. On
the contrary, we have many reasons for believing that they were within
comparatively recent times of considerably larger size, and were even
in all probability united to the mainland. The abundance of large
trees in the peat-mosses, and the fact that these ancient peat-covered
forests extend out to sea, are alone sufficient to convince one that
the Outer Hebrides have been much reduced in area since the close of
the glacial period. These now bleak islands at one time supported
extensive forests, although nowadays a tree will hardly grow unless
it be carefully looked after. That old forest period coincided in
all probability with the latest continental condition of the British
Islands--when the broad plains which are now drowned under the German
Ocean formed part of a great forest-land, that included all the
British Islands, and extended west for some distance into tracts over
which now roll the waves of the Atlantic. The palmy days of the great
British forests, however, passed away when the German Ocean came into
existence. The climatic conditions were then not so favourable for the
growth of large trees; and in the uplands of our country, and what are
now our maritime districts, the forests decayed, and were gradually
overgrown by and buried under peat-mosses. The submergence of the land
continued after that, until central and southern Scotland were reduced
to a considerably smaller size than now, and then by-and-by the process
was reversed, and the sea once more retreated, leaving behind it a
number of old raised beaches to mark the levels at which it formerly
stood.
The greatest submergence that overtook central and southern Scotland
in times posterior to the latest continental condition of Britain did
not exceed fifty feet, or thereabout; and the extreme limits reached by
the sea in the period that supervened between the close of the glacial
epoch and the "age of forests" was not more than one hundred feet. The
Outer Hebrides, however, were certainly not smaller in post-glacial
times than they are now, and we have no evidence to show that after the
"age of forests" had passed away the sea rose higher than a dozen feet
or so above its present level. Now there are only two ways in which all
this can be accounted for. Either the Hebrides remained stationary, or
stood at a level higher than now, while the central and southern parts
of Scotland were being submerged; or else there has been a very recent
depression within the Hebridean area, which has carried down below the
sea all traces of late glacial and post-glacial raised beaches. All we
know for certain is, that the only raised beaches in the Long Island
are met with in low maritime regions at only a few feet above the
present high-water mark. My own impression is that the whole district
has been submerged within comparatively recent times; for if the
present coast-line had endured since the close of the glacial period,
or even since the last continental condition of Britain, I should have
expected the sea to have done more than it has in the way of excavation
and erosion.
In a former article I have spoken of the sand-dunes and sandy flats
of the west coast of the Long Island. These receive their greatest
development in North Uist, Benbecula, and South Uist. Along the whole
western margin of these islands stretch wide shoals and banks of
yellow sand and silt, and similar shoals and banks cover the bed of
the shallow sounds or channels. In the middle of the Sound of Harris
one may often touch the bottom with an oar, and even run one's boat
aground. It is the same in the Sound of Barra, while, as I have already
mentioned, one may walk at low-water from Benbecula into the adjacent
islands of North and South Uist. Where does all this sand come from?
Certainly not from the degradation of the islands by the sea, for the
sounds appear to be silting up, and the general appearance of the sandy
flats along the west coast indicates that the land is upon the whole
gaining rather than losing. I have no doubt at all that this sand and
silt are merely the old sub-glacial débris which the ice-sheet spread
over the low shelving plateau that extends west under the Atlantic to
the 100-fathoms line. That plateau must have been thickly covered with
till, and with heaps and sheets of gravel and sand and silt, and it is
these deposits, sifted and winnowed by the sea, which the tides and
waves sweep up along the Atlantic margin of the islands.
There are many other points of interest to that I might touch upon,
but I have said enough perhaps to indicate to any intelligent observer
the kind of country he may be led to expect in the Long Island. Of
course the history of the glacial period is very well illustrated
in many parts of the mainland, which are much easier of access than
the Outer Hebrides. But these islands contain, at least, one bit of
evidence which does not occur anywhere else in Britain. In them we
obtain, for the first time, data for measuring the actual slope of the
ice-sheet. It does not follow, however, that the inclination of the
surface towards the Atlantic was the same all over the area covered
by the ice-sheet. The slope of the sheet that flowed east into the
basin of the German Ocean, for example, may have been, and probably
was, less than that of the Hebridean ice-flow. But apart altogether
from this particular point, I think there is no part of the British
Islands where the evidence for the former action of a great ice-sheet
is more abundant and more easily read, or where one may realise with
such vividness the conditions that obtained during that period of
extraordinary climatic vicissitudes, which geologists call the Glacial
Epoch.
Leaving these old arctic scenes, and coming down to the actual present,
no one, I think, can wander much about the Outer Hebrides without
pondering over the fate of the islanders themselves. Many writers
have asserted that the Celt of these rather out-of-the-way places
is a lazy, worthless creature, whom we Saxons should do our best to
weed out. One cannot help feeling that this assertion is unfair and
cruel. The fact is, we judge him by a wrong standard. He is by nature
and long-inherited habits a fisherman, and has been wont to cultivate
only so much land as should suffice for the sustenance of himself and
those immediately dependent upon him. In old times he was often enough
called upon to fight, wrongly or rightly, and thus acquired that proud
bearing which it has taken so many long years of misery to crush out.
He is, as a rule, totally unfit for the close confinement and hard work
which are the lot of the great mass of our mechanics--does not see the
beauty of that, and has rather a kind of contempt for the monotonous
drudgery of large manufacturing towns. One of the few situations in
town that he cares to fill is that of police-constable. Give him a
life in the open air, however trying it may be, and he will be quite
content if he can make enough to feed himself and family. If the
fishing chance to be very profitable he does not, as a rule, think of
saving the surplus he has made, but looks forward rather to a spell of
idleness, when he can smoke his pipe and talk interminable long talks
with his neighbours. No doubt this, judged by our own standard, is all
very shocking. Why doesn't he put his money in the savings-bank, and
by-and-by die and leave it to those who come after him? Simply because
he is a Celt, and not a Saxon.
Of course one knows how it will all end. Ere long the unadulterated
Celt will be driven or improved out of these islands, and will retire
to other lands, where, mingling and intermarrying with Teutons, he
will eventually disappear, but not without leavening the races amongst
which he is destined to vanish. And who will take his place in the
Long Island? Probably a few farmers, a few shepherds, and a sprinkling
of gamekeepers; and it is just possible that a few fishermen also may
be allowed to settle down here and there upon the coast. One may see
the process going on at present. Large tracts that once supported many
villages are now quite depopulated. The time will come when somebody in
Parliament will move for the reduction of the Civil Service estimates
by the amount of the sheriff-substitute's salary, and when the jail at
Lochmaddy will have nothing higher in the scale of being to imprison
than some refractory ram. One may be pardoned for wishing that he could
foretell for the islands another fate than this. It is sad to think
that a fine race of people is thus surely passing away from amongst
us, for, despite all that can be urged against them, they are what I
say. The fishermen of Lewis and Barra are bold, stalwart fellows, whom
it would be difficult to peer amongst any similar class of men on the
mainland. And all through the island one meets with equally excellent
specimens of our kind. Many a brave soldier who fought our battles in
the great French wars hailed from these outer islands. Pity it is that
no feasible plan to prevent the threatened scattering of the race has
yet been brought forward. Some day we may regret this, and come to
think that though mutton and wool in the Long Island are desirable, yet
islanders would have been better.
[Postscript.--On pages 153.4 I have described the second general
ice-sheet that overflowed the Outer Hebrides as having eventually
become resolved into a series of local ice-sheets and glaciers.
Subsequent research, however, has since led me to believe that the
district ice-sheets and local glaciers referred to were not the
direct descendants of the last great ice-sheet. They appear to
have come into existence long after that ice-sheet had entirely
disappeared. _See_ Article X.]
VI.
The Ice Age in Europe and North America.[K]
[K] Address to the Geological Society of Edinburgh, 1884.
In casting about for a subject upon which to address you this evening,
I thought I could hardly do better than give you the result of a
comparison which I have recently been able to make between the glacial
phenomena of Europe and North America. The subject of glaciation seems
to be now somewhat worn; but I gather from the fact that writers can
still be found who see in our superficial deposits strong evidence of
the Deluge, that a short outline of what we really do know may not be
unacceptable. In the short time at our disposal, it is obvious that
I cannot enter into much detail, and that many interesting questions
must remain untouched. It will be as well, therefore, that I should at
the outset define the limits of the present inquiry, and state clearly
what are the chief points to which I wish to direct your attention. My
main object, then, will be to bring into prominence such evidence as
seems to betoken in a special manner the uniformity of conditions that
obtained in the northern hemisphere during the Ice Age. In other words,
I shall confine myself to a description of certain characteristic and
representative phenomena which are common to Europe and North America,
with the view of showing that the physical conditions of the glacial
period were practically the same in both continents.
The phenomena which might be considered under this head embrace nearly
all the facts with which glacialists are familiar, but I purpose
restricting myself to three questions only, viz.:--
1st. _The extent of glaciation._
2nd. _Changes of climate during the Ice Age._
3rd. _The results of fluvio-glacial action._
The consideration of these questions, even if it were exhaustive
(which it cannot be on this occasion), would still leave the general
subject very incomplete, for we must forego the discussion of all
such interesting topics as the "connection between glaciation and
submergence," "the formation of rock-basins," and the "origin of the
geographical distribution of our faunas and floras." Confining my
inquiry within the limits just specified, I shall begin by sketching
broadly the general results obtained by glacialists in Europe, and
thereafter I shall proceed to give an outline of the corresponding
conclusions arrived at by American observers.
I.
_The Extent of Glaciation in Europe._
To what extent, then, let us ask, has Europe been glaciated? What areas
have been covered with perennial snow and ice? Owing to the fulness and
clearness of the evidence, we are able to give a very definite answer
to this question. It is hardly too much to say that we are as well
acquainted with the distribution of glacier-ice in Europe during the
Ice Age as we are with that of existing snow-fields and glaciers.
The nature of the evidence upon which our knowledge is based
is doubtless familiar to many whom I have the pleasure of now
addressing, but for the sake of those who have not such familiarity
with the subject I may be allowed to indicate very briefly its
general character. A rock-surface over which ice has flowed for any
considerable time exhibits either an abraded, worn, and smoothed
appearance, or the rocks are disrupted and broken, and larger or
smaller fragments are found to have been removed and carried forward
in the direction followed by the ice. Now, ice-worn and shattered
rock-surfaces of this description, such as can be seen underneath
existing glaciers, occur more or less abundantly over vast regions in
Europe. They are met with from the North Cape south as far as Leipzig,
and from the Outer Hebrides east to the valley of the Petchora and the
foot-slopes of the Ural Mountains. Nor are they confined to northern
Europe. They appear again and again in France and Spain and Italy,
and in the low-grounds of middle Europe, where they occupy positions
now far removed from the influence of glacial action. Such ice-worn
and disrupted rock-surfaces not only prove that glacier-ice formerly
covered large portions of our Continent, but they also indicate for
us the directions in which that enveloping ice moved. The smoother
surfaces in question are very frequently marked with coarse and fine
parallel scratches and grooves of precisely the same nature and origin
as the scratches and grooves which characterise the rocky bed of a
modern glacier. And these markings, having been produced by the sand,
grit, and stones which are pushed and dragged over the rocks by flowing
ice, necessarily discover for us the path of glacial movement. But
all rocks subjected to glacial action are not necessarily smoothed
and polished. Sometimes, owing to structural peculiarities, and for
various other reasons, rocks cannot resist the pressure of the ice,
but are crushed and broken, and the resulting fragments are rolled and
dragged forward in the direction of ice-flow. In this manner the path
of a glacier becomes strewed with débris which has from time to time
been forced from its rocky bed. There is really no mystery, therefore
in tracking the spoor of extinct glaciers; for we have two sets of
facts to aid us, either of which might suffice to indicate the extent
and direction of glaciation. Consider, however, for a moment, what one
observes in connection with rock-striation. We have, in the first
place, the rounding and smoothing, and the parallel ruts and striæ. Not
only so, but we frequently find that one side of prominent projecting
knolls and hills is more highly worn and abraded than the other. Often,
indeed, one side may show no trace whatsoever of abrasion. Here, again,
we have clear evidence of the direction of ice-flow. Who can doubt that
the worn and abraded rocks look towards the point whence the ice came,
and that the non-glaciated rocks in the rear have been sheltered by the
rocks in front? It is for this reason that in the mountainous regions
of northern Europe the striated and smoothed rock-surfaces invariably
look up the valleys, while the broken and unworn rock-ledges face in
the opposite direction.
Once more, note the manner in which the sub-glacial rock-rubbish,
consisting of clay, sand, grit, stones, and boulders, has been amassed.
In places where the ice must have moved more or less rapidly, as on
considerable slopes, no accumulation took place, while in the rear
of projecting crags and knobs of rock, sub-glacial materials often
gathered deeply. Again, over low-lying tracts, where the motion of
the ice would necessarily be retarded, clay, sand, and stones tended
to collect. And this particularly appears to have been the case in
those regions where the slow-creeping and gradually thinning ice-sheet
approached its terminal line. Hence it is that we encounter such thick
and wide-spread sheets of sub-glacial detritus upon the undulating
low-grounds and plains of southern Sweden, Denmark, Schleswig-Holstein,
Holland, northern Germany, Poland, and Russia.
The sub-glacial débris to which I specially refer is known as _Till_
or _Boulder-clay_ in this country, as _Krosstenslera_ in Sweden, as
_Geschiebelehm_ or _Geschiebemergel_ in Germany, and as _Grundmoräne_
or _Moraine profonde_ in Switzerland. Its general characters are too
well known to require more than the briefest summary. In general this
peculiar accumulation is an unstratified clay, containing, scattered
higgledy-piggledy through it, stones and boulders of all shapes and
sizes. Many of these rock-fragments are smoothed and striated, and
even the smallest particles, when viewed under the microscope, often
show delicate scratches. Frequently, too, the clay is excessively hard
and tough, and in many places it shows a kind of pseudo-lamination,
which is generally more or less crumpled, and often highly involved.
These appearances prove that the clay has not only been subjected to
intense pressure, but has actually been rolled over upon itself. I
need only refer to the plentiful occurrence of "slickensides" in such
clays--the joints by which the clay is often traversed showing such
polishing clearly on their faces. These, and many other facts which
time forbids me to mention, have received an explanation which has now
been generally adopted by European glacialists. The boulder-clay or
till is considered by them to represent the ground- or bottom-moraine
of glacier-ice. There used to be a notion prevalent amongst geologists
in our country that this clay was almost peculiar to these islands.
It occurs, however, in most countries of Europe. Vast regions in the
north are more or less continuously covered by it, and we meet with
it abundantly also upon the low-grounds of Switzerland, from which it
may be followed far down the great valley of the Rhone into the sunny
plains of France. The lower valleys of the Pyrenees and other Spanish
ranges show it well, and it is conspicuous likewise in northern Italy,
especially over the low tracts at the mouths of the great lake-valleys.
In all those places one can see boulder-clay of as pronounced a
character as any to be met with in Scotland.
Danish, Dutch, German, and Russian geologists have of late years
devoted much attention to the study of this clay, which is so
remarkably developed in their respective countries. It has been long
well known that a large proportion of the stones and boulders contained
in the till are of northern derivation, but it is only of recent years
that we have ascertained the particular routes by which those wanderers
or erratics have travelled. The rock-fragments in question have been
tracked back, as it were, to their parent masses, and thus, partly
in this way, and partly by the evidence of ice-worn surfaces, we have
been enabled to follow the spoor of the great northern ice-sheet in a
most satisfactory manner. Let one or two examples suffice. Boulders
derived from Lapland and Finland occur in the till at St. Petersburg,
and have been traced south-east to Moscow. Again, fragments carried
from Gottland, in the Baltic, are met with in the boulder-clay of east
Prussia, and have been followed south to beyond Berlin. In like manner
boulders of well-known Scanian rocks appear in the boulder-clay of
Leipzig. So also Swedish and Norwegian rock-fragments are seen in the
boulder-clay of Denmark, Hanover, and Holland.
Very wide areas in northern Germany are covered with an almost
continuous sheet of glacial detritus, so that it is only occasionally
that the underlying rocks crop out at the surface. Striated
rock-surfaces are therefore by no means so commonly exposed as in
regions like the Lowlands of Scotland. They are not wanting, however,
and their evidence is very striking. Thus, in the neighbourhood of
Leipzig and Dresden, we find glacial striæ impressed upon certain
highly-abraded and ice-worn hillocks of porphyry, the striæ being the
work of ice which flowed into Saxony from the north. Similar striæ;,
having a general southerly trend, occur at Rüdersdorf, near Berlin,
at Gommern, near Magdeburg, at Velpke in Brunswick, at Osnabrück in
Hanover, and at other places. Again, we encounter remarkable evidence
of the powerful pressure exerted by the ice in the displacement and
removal of huge blocks of strata. In Saxony, for example, the Tertiary
strata are turned up, pushed out of place, and involved in boulder-clay
to such an extent that the brown coals have often been mined for in
this strange position. Witness also the extensive displacements and
dislocations of the Cretaceous formation in the Danish islands of the
Baltic. So great are the contortions and displacements of the Chalk in
Moen, that these disturbances were formerly attributed to subterranean
action. Along the north-east coast of that island, cliffs 400 feet in
height exhibit the Cretaceous beds thrown upon end, twisted, bent,
and even inverted, boulder-clay being squeezed into and between the
disjointed and ruptured rock-masses.
From a study of these and similar phenomena, it has been demonstrated
that during the climax of the Ice Age a very large part of northern
Europe was buried under a thick covering of glacier-ice. And it has
been conclusively shown that this ice-sheet streamed outwards in all
directions from the high-grounds of Scandinavia, for which reason it is
often spoken of as the Scandinavian ice-sheet. But as it was fed, not
from the snow-fields of Scandinavia alone, but from the precipitation
of snow over its whole surface, it is better, I think, to speak of it
as the northern ice-sheet. In the extreme north of Scandinavia the ice
flowed northward into the Arctic Ocean, while south of the dominant
watershed of Lapland and Sweden its course in those high latitudes was
east and south-east. It filled up the depressions of the White Sea, the
Gulf of Bothnia, and the Baltic, extending east to the valley of the
Petchora and the base of the Ural Mountains, and south-east to Kazan,
some 200 miles east of Nijnii-Novgorod. From this point its terminal
front trended a little west of south, until it reached the fiftieth
parallel of latitude. Undulating a few miles south and north of this
parallel, it swept directly west through Russia into Galicia, till it
touched the foot-hills of the Carpathian range. After this we follow it
along the northern base of the Riesen Gebirge, the Erz Gebirge, and the
Harz, and thence westward through Hanover, and into the Low Countries,
as far south at least as the mouth of the Rhine. Throughout the vast
regions lying west and north of this terminal line, the track followed
by the ice has been well ascertained. It was east and south-east in
Russia, southerly in east Prussia, south-westerly in Denmark, Hanover,
and Holland.
The action of a mass of glacier-ice, reaching a thickness of several
thousand feet, must necessarily have resulted in extensive erosion of
the rocks over which it passed. Everywhere, therefore, throughout the
vast area just indicated, we meet with evidence of severe erosion.
But, as one should expect, such erosion is most marked in the hilly
regions--in those areas where steep slopes induced more rapid motion of
the ice, and where projecting crags and hills opposed the advance of
the eroding agent. All such prominent obstructions were energetically
assailed--abraded, rounded, worn, and smoothed, or crushed, shattered,
dislocated, and displaced. The high-grounds of Scandinavia and Finland,
formed for the most part of tough, crystalline rocks, or of more or
less durable strata, show everywhere _roches moutonnées_--smoothed and
rounded rocks--while innumerable rock-basins have been scooped out in
front of prominent crags and hills. In Denmark and other countries,
where less durable rocks prevail, the strata have often been broken
and disrupted, and pushed out of place. But as regions formed of such
rocks are generally gently-undulating, and seldom show abrupt crags and
hills, they oppose few obstructions to the advance of an ice-sheet.
When the northern ice-sheet flowed into Russia and Germany, it crept
over a low-lying and, for the most part, gently-undulating surface;
and although here and there the form of the ground favoured glacial
erosion and disruption, and extensive displacements of rock-masses
took place, yet, upon the whole the low-lying regions referred to
became areas of accumulation. The sub-glacial detritus--ground out
or wrenched away from the rough Scandinavian plateau and the uplands
of Finland--was dragged on underneath the ice, and spread over the
great plains lying to the south-east and south, as the gradually
attenuated ice-sheet crawled to its terminal line. My friend Dr. Amund
Helland, the well-known Norwegian geologist, has made an estimate of
the amount of rock-débris derived from Scandinavia and Finland which
lies scattered over the low-grounds of northern Europe. According to
him, the area in Denmark, Holland, Germany, and Russia (exclusive of
Finland), over which northern detritus is scattered, contains about
2,100,000 square kilometres, and the average thickness of the deposits
is about 150 feet, of which, however, only two-thirds, or 100 feet, are
of northern origin, the remaining third consisting of local materials.
Taking, then, 100 feet as fairly representing the average thickness
of the rock-rubbish derived from Finland and Scandinavia, the area of
which is given as 800,000 square kilometres, there is enough of this
material to raise the general surface of those lands by 255 feet. The
same amount of material would suffice to fill up all the numerous lakes
of Finland and Sweden sixteen or seventeen times over. Or, if tumbled
into the Baltic, it would fill the basin of that sea one and a half
times. In short, enough northern rock-débris lies upon the low-grounds
of northern Europe, which, were it restored to the countries from which
it has been taken, would obliterate all the lake-hollows of Finland
and Sweden, raise the level of those lands by 80 feet, and fill up the
entire basin of the Baltic, with all its bays. And yet this estimate
leaves out of account all the material which the ice-sheet carried away
from Norway and the British Islands.
Of the glaciation of our own land I need say very little. The
configuration of our country necessarily made it a centre of dispersion
during the Ice Age, and the ice which covered Ireland, Scotland, and
the major portion of England radiated outwards from the dominant
elevations of the land. But as the ice creeping outwards from those
centres became confluent, the directions which it followed were often
considerably modified, especially upon the low-grounds. We know that
the British ice-sheet not only covered the land up to near the tops of
our higher mountains, but filled up all our seas and extended into the
Atlantic beyond the coasts of Ireland and the Outer Hebrides--these
latter islands having been glaciated from the east by the ice that
flowed outwards from the mainland. Another point upon which we are now
well assured is the fact that the British and Scandinavian ice-sheets
coalesced, so that the basin of the North Sea completely brimmed over
with glacier-ice.
Finally, then, in contemplating the physical conditions that obtained
in northern Europe at the climax of the Ice Age, we have to picture to
ourselves the almost total obliteration under a vast ice-sheet of all
the land-features of the British Islands, Scandinavia, and Finland, and
the adjacent low-lying tracts of Denmark, Holland, Germany, Poland, and
Russia. If at that distant date a prehistoric man could have stood on
the summit of Snaehatten, he would have seen an apparently interminable
plain of snow and ice, bounded only by the visible horizon. Could
he have followed the plain southwards in hopes of escaping from it,
he would have descended its gently-sloping surface by imperceptible
gradations for a distance of 700 miles, before he reached its
termination at the foot of the mountains of middle Germany. Or, could
he have set out upon an easterly course, he would have crossed the Gulf
of Bothnia, buried several thousand feet beneath him, and touched the
foot-slopes of the Ural Mountains before he gained the terminal front
of the ice-cap, a distance of 1600 miles. On the other hand, had he
walked south-west in the direction of Ireland, he would have traversed
the area of the North Sea at a height of several thousand feet above
its bed, and, crossing the British area, would only have reached the
ice-front at a point some 50 miles beyond the coast of Ireland. Here he
would have seen the ice-sheet presenting a steep face to the assaults
of the Atlantic, and breaking away in massive tabular bergs, like those
which are calved by the ice-cap of the Antarctic regions.
I must now pass rapidly in review the facts relating to the glaciation
of the mountainous regions which lay outside of the area covered by
the northern ice-sheet. The glaciers of the Alps of Switzerland, about
which so much has been written, and the study of which first gave
Venetz, Charpentier, and Agassiz the clue to the meaning of striated
rocks, boulder-clay, and erratics, are, as is well known, the puny
descendants of former gigantic ice-flows. At the culmination of the Ice
Age all the mountain-valleys of Switzerland and northern Italy were
choked with glaciers that streamed out upon the low-grounds. Along
the northern slopes of the Alps, as in Bavaria and Würtemberg, these
glaciers coalesced to form a considerable ice-sheet, and so likewise
did the glaciers that descended from Switzerland, Savoy, and Dauphiny,
into the great valley of the Rhone. Even in north Italy the same was
the case with the glaciers that occupied the valleys in which now lie
Lakes Orta, Maggiore, Varese, Lugano, and Como--the united ice-flows of
those valleys forming a glacier which deployed upon the plains of the
Po, with a frontage of not less than 40 miles.
To the north of the Alps, the Vosges Mountains and the Black Forest,
the Harz, the Erz Gebirge, the Riesen Gebirge, and the Böhmer-Wald--all
had their perennial ice and glaciers, although none of those elevated
tracts now reaches the snow-line. It was the same with the Carpathians
and the Urals, amongst which we meet with relics of much larger
ice-streams than any that now exist in the Alps. Considerably further
south were the glaciers of the Despoto Dagh of Roumelia. Great glaciers
also in former times descended from the Caucasus, and in many hilly
regions of Asia Minor indubitable traces of similar large ice-flows
have been detected. The high-grounds of central France, and the
mountains of Beaujolais and Lyonnais supported considerable glaciers,
while from the Pyrenees numerous glaciers of the first class flowed out
upon the low-grounds of France, and considerable ice-streams occupied
the mountain-valleys on the Spanish side. Other Peninsular chains--the
Serra da Estrella, the Sierra Guadarama, and the Sierra Nevada--had
likewise their snow-fields and ice-streams. The same was the case
with the Apennines and the Apuan Alps of Italy, the traces of former
glacial action being conspicuous over a considerable part of Tuscany.
Even in Corsica we encounter the same evidence of glaciation--striated
rock-surfaces and moraines--which point to the former descent of
considerable glaciers from Monte Rotondo.
But rock-striæ and moraines are not the only proofs of former cold and
humid conditions having prevailed over middle and southern Europe at
the climax of the glacial period. The limestone-breccias of Gibraltar
have been described by Professor Ramsay and myself, and we have shown
that these could only have been formed under the influence of excessive
frost and melting snows. The limestone of the Rock has been broken up
along the ridge, and its fragments showered down the slopes, at a time
when these were more or less thickly covered with snow. Resting upon
and imbedded in this snow, the rock-rubbish would be carried downward
and outward during the gradual melting that took place in summer. And
in this way immense accumulations of débris were borne forwards over
the low-grounds that extended from the base of the Rock into regions
which are now partially submerged. Breccias which have probably had a
similar origin occur also in Corsica, Malta, and Cyprus, and doubtless
they will yet be recognised in many other places. Again, over wide
areas in northern France and the south of England, we meet with
extensive sheets of earthy clay and rock-rubbish, which have certainly
been heaped up under very different conditions of climate than obtain
now. This stony earth has evidently travelled down the gentle slopes
of the land, under the influence of frost and melting snow, in much
the same way as ice-driven rock-rubbish and soil move slowly down the
slopes of such dreary regions as Patagonia and certain low-lying tracts
within the Arctic Circle.
II.
_Changes of Climate in Europe during the Ice Age._
We come next to the very interesting question of alternations
of climate during the Ice Age. The evidence under this head has
accumulated to such an extent within recent years as to convince most
students of Pleistocene geology that very extensive changes of climate
characterised the glacial period. How many such changes took place we
are not yet in a position to say, but we know that the intensely arctic
condition of things which has just been described was interrupted more
than once by what have been termed "interglacial epochs," during which
a mild and genial climate prevailed over middle and northern Europe.
For some time it was believed that such "interglacial epochs" had only
a local significance, that they bespoke mere transitory retreats of the
ice-fields, such as are known to have taken place within historical
times in the glacier-valleys of the Alps. But increased observation
and reflection have shown that this explanation of the phenomena of
"interglacial beds" will not suffice. It is impossible to enter here
upon details, but I may briefly state that the evidence in question
is two-fold. _First_, we have the stratigraphical evidence. We have
ascertained the existence, over wide areas in this and other glaciated
countries, of several successive sheets of boulder-clay, which are
often separated from each other by fossiliferous aqueous strata. It has
been demonstrated that each of these sheets of sub-glacial detritus
is the accumulation of a separate and distinct ice-flow. _Second_, we
have the evidence of fossil organic remains. We find, for example, that
the flora which covered the low-grounds of middle and temperate Europe
during a certain stage of the glacial or Pleistocene period, consisted
of plants which are now restricted to the tops of our mountains and to
northern Scandinavia. The characteristic fauna associated with that
flora embraced the reindeer, glutton, mammoth, woolly rhinoceros,
Arctic fox lemming, chamois, and so forth. We know, indeed, that man
hunted the reindeer and the mammoth in the south of France. Similar
testimony to the coldness and humidity of the climate is borne by the
land- and freshwater shells which occur in certain Pleistocene deposits
in Italy, Corsica, southern France, Switzerland, Germany, etc. That
this flora and fauna were contemporaneous with the great glaciation of
our Continent has been as well ascertained as the fact of the Roman
occupation of Britain. But if the evidence of organic remains strongly
confirms and supports that supplied by the distribution of glacial
deposits in Europe, no less forcibly does it corroborate the physical
evidence as to the former existence of a warm and genial interglacial
climate. During interglacial times a most abundant mammalian fauna
roamed over all temperate Europe--a fauna comprising such animals as
Irish deer, urus, bison, horse, stag, saiga, brown bear, grisly bear,
several species of elephant, rhinoceros, and hippopotamus, hyæna,
lion, leopard, etc. A like tale of genial conditions is told by the
land- and freshwater shells, which occur in some of the Pleistocene
deposits of England, France, Belgium, Germany, Switzerland, and
Italy. The testimony of the associated flora is just as striking. How
genial and equable must have been the climate which permitted plants
like the Canary laurel, the Judas-tree, the fig-tree, and others to
flourish side by side in the north of France, with such forms as the
hazel, willow, ash, and sycamore! The most noteworthy additions to our
knowledge of interglacial conditions which have recently been made
are the results obtained by M. Gaudry in the valley of the Seine, and
by Dr. Penck in Bavarian Tyrol, the latter of whom has shown that
there have been at least three great advances of the Alpine glaciers,
separated by long-continued mild conditions, during which the glaciers
receded far into the mountains.
It is interesting to observe that we have, especially in our own
islands, good evidence to show that during the glacial period
considerable oscillations of the relative level of land and sea took
place. Thus, it has been ascertained, that just before the latest epoch
of extensive glaciation, the British Islands were largely submerged
in the sea. To what depth this remarkable submergence was carried
we do not know, because any marine deposits which may have been
accumulated at that time over the drowned country were for the most
part obliterated by the action of the ice-sheet which subsequently
covered and reglaciated our lands.[L] But the few fragments of such
marine deposits as have been preserved show us that the depression
reached more than 500 feet in Scotland (_i.e._, measured from the
present sea-level), and exceeding 1000 feet in Wales and Ireland. We
note, then, in passing, that the only great Pleistocene submergence of
these lands of which geologists have any knowledge took place before
the appearance of the last general ice-sheet that overflowed our
low-grounds. The submergences of a later date were of inconsiderable
importance, hardly exceeding 100 feet or thereabouts below the present
sea-level. The latest occupant of our islands and of northern Europe
was not the sea, but ice. The "Palæocrystic Sea," which we have been
recently assured would account for our glacial phenomena, is of
"the stuff that dreams are made of." There is not a jot or tittle
of evidence for the former existence of such a sea over any part of
Britain or the continent of Europe.
[L] I no longer believe in this "great submergence." The marine shells
in the high-level drift-deposits of our islands are "erratics," carried
by the ice-sheet which occupied the basin of the Irish Sea. That the
low-grounds were submerged but the amount of the submergence has not
been ascertained; probably it did not exceed a few hundred feet.
It is not necessary for my present purpose to enter further into the
evidence of interglacial conditions. The latest northern ice-sheet was
preceded by a long epoch of mild and genial conditions, during which
elephants and hippopotami ranged north as far at least as Yorkshire;
while middle Germany, as we know from the testimony of its interglacial
deposits, enjoyed a similar delightful climate. And yet the immediately
preceding glacial epoch had seen all those fertile regions covered
with an ice-sheet that extended south as far as the fiftieth parallel
of latitude. Now the question with which I am at present concerned
is the extent of the latest general glaciation. Did the last great
ice-sheet reach as far south as its predecessor? It certainly did not.
Its bottom-moraine has now been mapped out and distinguished from that
of the older ice-sheet, and we know that it does not extend so far
south as the latter. It is entirely absent over all the region to the
west of the River Elbe, from near Dresden to Hamburg and the coast of
Holland.[M] So that western Germany and Holland, which were covered
by ice during the epoch of greatest glaciation, were not invaded by
the ice-sheet underneath which the upper boulder-clay was accumulated.
This latest ice-sheet, however, overwhelmed all Mecklenburg and Mark
Brandenburg, and streamed south nearly as far as Saxony; its southern
margin extended east through Silesia, by Liegnitz and Breslau, into
Poland and Russia. But the precise line it followed in the latter
country has yet to be ascertained. We may surmise, however, that it
nowhere reached so far south or east as the ice-flow of the earlier
epoch. I may add that the southern termination of the latest ice-sheet
is in many places marked out by heaps, mounds, and ridges of earthy
sand, gravel, rolled stones, and erratics; in short, by terminal
moraines. These, however, are frequently highly degraded and washed
down.
[M] Klockmann, _Jahrb. der k. preuss. geol. Landesanstalt für 1883_, p.
330.
Of the extension of glacier-ice in the British Islands at the epoch
in question I shall only say that the glaciation of Scotland was
hardly, if at all, less extensive than during the climax of the Ice
Age. Ireland, too, appears to have been almost as thickly mantled;
but the ice-sheet that covered England and Wales did not extend so
far south as that of the penultimate glacial epoch, a considerable
area in East Anglia and the midland counties remaining apparently free
from invasion. The Scandinavian and British ice-sheets, however, again
coalesced upon the floor of the North Sea.
III.
_The Results of Fluvio-glacial Action in Europe._
The third question which I now proceed to consider is the result
produced by the rivers and torrents of the Ice Age. This, I am aware,
is a wide subject, and one upon which much has been written. But there
are a few points which may be advantageously discussed for the purpose
of bringing into prominent view the conditions which obtained in the
river-valleys of Europe during the last great extension of glacier-ice.
A little consideration will serve to convince one that the intense
glacial conditions that obtained in our Continent during the cold
epochs of the glacial period were due to a low temperature, combined
with excessive snow-fall. The winters, we can have no doubt, must have
been prolonged and severe. But mere low temperature will not account
for the enormous precipitation of snow. For this, great evaporation
was required. And we are therefore forced to admit that the direct
heat of the sun in summer must have been greater than it is in the
same regions at the present day. Now, if this were really the case
(and I do not see how otherwise the facts can be explained), then we
ought to meet with evidence of swollen rivers, torrents, and widespread
inundations everywhere outside of the glaciated areas. And this is
precisely what we do find. Immense accumulations of coarse gravels
are widely spread over all the valleys that head in regions which
were formerly the sites of snow-fields and glaciers. These gravels
are of such a character and are so distributed as to make it certain
that they could not have been transported to and deposited in their
present positions by rivers like those which now wind their way down
the valleys of middle Europe. Still more remarkable are the enormous
sheets of loam which are spread over much wider areas and reach to more
considerable heights than the gravels. The origin of the gravels is
sufficiently evident; they are simply the coarser detritus, swept along
by the enormously flooded rivers of the glacial period, and meet with
their analogues in the torrential gravels of modern glacier-valleys
in the Alps and other elevated regions. The more widely-spread loams,
according to the opinion of most glacialists, represent the finer mud
and silt deposited from the muddy waters of the same period. But the
height to which such gravels and loams ascend is so great that those
who hold them to be of fluvio-glacial origin have found it difficult
to maintain this view. Some writers, indeed, who have not sufficiently
considered the weight of the evidence in its favour, have set it aside,
and boldly suggested all kinds of wonderful hypotheses in its place.
One imaginative author, for example, believes the wide-spread loams to
be of volcanic origin, while another finds in the same deposits strong
evidence of the Deluge. By a well-known and experienced observer,
the famous löss of middle Europe is considered to be an Æolian
accumulation--that is to say, a wind-blown deposit--the result of
long-continued or frequently-repeated dust-storms. This is the opinion
of Baron Richthofen, whose great work on China is so justly esteemed.
He infers that at the time of the formation of our löss central Europe
was a dry desiccated region, just as wide areas in central Asia are
in our own day. He does not attempt to show us, however, how such
climatic conditions could ever obtain in Europe. In point of fact, the
geographical conditions of our Continent have not changed materially
since Pleistocene times, and the presence of the wide Atlantic Ocean,
that laves all our western shores, is of itself sufficient to preclude
the possibility of such a climate having obtained in middle Europe.
Richthofen's theory likewise fails to account for the geographical
distribution of the löss, and for many facts relating to its geology.
Only one of these last shall I mention. The löss is intimately
associated with accumulations, the glacial and fluvio-glacial origin of
which cannot be doubted. It belongs, in fact, to the glacial series,
and was laid down at a time when vast snow-fields and ice-sheets
existed, and when it is quite impossible that a dry climate could
have characterised any part of our Continent. In common with most
geologists, I believe that the löss is simply an inundation-mud,
deposited in temporary lakes and over flooded areas during the summer
meltings of the snow- and ice-fields; and I shall now try to show how
the occurrence at high levels of gravels and such loams as the löss
may be accounted for without having recourse to volcanic action or to
winds, or even to the Deluge. I shall invoke no agencies other than
those which we are perfectly well assured were in full operation during
the Ice Age.
Now, I ask you, in the first place, to bear in mind that while a
glacial epoch continued, extreme conditions could not have been
restricted to the areas undergoing glaciation. There is abundant
evidence, indeed, to show that heavy, snows occasionally covered other
regions, and that in such places severe frosts acted upon the rocks
and soils even of the low-grounds. Need we wonder if at a time when
the northern ice-sheet approached the fiftieth parallel of latitude
in middle Europe, when almost every mountain-group of central and
southern Europe had its snow-fields and glaciers--need we wonder if at
such a time the climate of wide areas outside of the glaciated tracts
was extremely ungenial? The more closely the superficial accumulations
of such areas are studied, the more clearly do we perceive in them
the evidence of cold and humid conditions. Try, then, to picture to
yourselves the probable aspect of those regions during a glacial epoch.
Immediately south of the northern ice-sheet deep snows must have buried
large tracts of country, and such snows may have endured often for long
years, notwithstanding the great melting that took place in summer.
Even much further south, as in Spain and Italy, deep snows would cover
the lesser hills and hill-ranges, while frost would act energetically
in many a district where such action is now either inconsiderable or
unknown. Such being the general conditions that must have obtained in
the non-glaciated areas, let us very briefly consider what the results
of such conditions must necessarily have been. Every one has noticed,
during the more or less rapid melting of snow in winter and early
spring, that our streams and rivers are then much muddier than when in
summer and autumn they are swollen by heavy rains. This of course is
due to the action of frost, by means of which rocks are disintegrated
and soils are broken up and pulverised, so that when thaw supervenes,
the superficial covering becomes soaked with moisture like a sponge.
To such an extent does this take place, that one may often see the
saturated soil creeping, slipping, and even flowing down the slopes.
The effect of mere thaw is of course much intensified when the water
derived from melting snows is present. Rills and tiny brooks then
become converted into dark muddy torrents, and enormous quantities of
fine-grained detritus are eventually swept into the rivers. The rivers
rise in flood and inundate their plains, over the surface of which
considerable deposits of loam and silt often accumulate. We cannot
doubt that similar but much more intense action must have taken place
over very wide regions in Europe during a glacial epoch. Such having
necessarily been the case, we are not required to suppose that the
löss and similar loams have been deposited entirely by rivers flowing
from glaciers. It is doubtless true that most of the rivers headed in
those days in glacier regions, and must in consequence have been highly
discoloured with glacial mud, and probably a very large proportion of
the loams in question consists of the fine flour of rocks--the result
of glacial grinding. But the action of frost and thaw and melting snow
upon the low-grounds, such as I have described, cannot be ignored,
and seems to have played a more important _rôle_ than has yet been
recognised. I think it helps us better to explain the well-known fact
that land-shells are more or less commonly distributed through the
löss. One can readily understand, at all events, how snail-shells might
be swept down the slopes of the land at the time of the spring thaws,
and how large numbers might find their way eventually into the swollen
glacial rivers. I have often observed, during the melting of snow and
the thawing of soils, quantities of snail-shells in the very act of
being swept into our brooks and rills. And we are all familiar with the
fact that, after a spring-flood has subsided, snail-shells, along with
vegetable débris, are often plentifully stranded upon the valley-slopes
and flood-plains of our rivers.
Admitting, then, that the löss and similar accumulations are simply
inundation-loams formed at a time when glaciers were discharging
immense volumes of muddy water, and when the low-grounds were liable
every summer to the denuding action of melting snows, and so forth, I
have yet to account for the fact that these supposed inundation-loams
sometimes occur at a height of 100 feet, or even of 300 feet, above
the present levels of the rivers. Two theories have been advanced in
explanation, each of which seems to me to contain an element of truth.
It has, in the first place, been maintained, as by Prestwich, that the
löss at the higher levels was probably deposited long before the rivers
had excavated their channels to their present depths. Thus, during
flood, they would be enabled to overflow tracts which they could not
possibly have reached when they had deepened their valleys to a much
greater degree. But while we must fully admit that the erosion effected
by the rivers of the Pleistocene or Glacial period was excessive, yet
we find it difficult or impossible to believe that great valleys,
several miles in width, and two or three hundred feet in depth, were
excavated in hard Devonian and other equally durable rocks by the
swollen and active rivers of the Ice Age. And although it is extremely
probable that the löss at the highest levels is older than the similar
deposit at the lowest levels of such a valley as the Rhine, yet this
does not get us out of our difficulty.
The other view to which I have alluded takes little or no account
of river-erosion, but maintains that the floods of the Ice Age were
sufficiently great to reach the highest levels at which river-gravels
and loams occur. It is likely enough that, under present conditions, we
can form but a very inadequate idea of the vast bodies of freshwater
which formerly swept down our valleys, but we may be pardoned if we
express our inability to conceive of our European rivers flowing with a
breadth of many miles, and a depth of two or three hundred feet.
A few years before his death, Mr. Darwin made a suggestion to me, which
I think gives us the true solution of the problem. He thought that
during an Ice Age great beds of frozen snow might have accumulated
over the low-grounds outside of the glaciated areas (in the manner I
have already described), and that many valleys might have been filled
to a considerable depth during a large part of the year with blown
snow, afterwards congealed. In autumn, when the running water failed,
the lines of drainage might in many cases be more or less choked, and
it would be a mere chance whether the drainage, together with gravel,
sand, and mud, would follow precisely the same lines during the next
summer. Such action being repeated year after year, it might well
happen that many river-valleys might become largely filled with rudely
alternating layers of frozen snow and fluviatile detritus. And if this
were so, the flooded rivers in summer would be enabled to overflow much
wider and more elevated tracts than they could otherwise have reached.
As the climate became less excessive, we can conceive of the frozen
snows gradually melting, and of river-detritus being deposited at lower
and lower levels in the valleys.
The probability of such frozen masses having choked up valleys and
impeded the drainage during the Ice Age is not a mere plausible
conjecture. In the far north of Alaska--in a region which was certainly
not overflowed by the North American ice-cap--extensive sheets of ice
occur, more or less deeply buried under thick soil. Nor can there
be much doubt that these ice-masses date back to the Glacial period
itself, seeing that in the soils which overlie them we meet with
remains of the mammoth and other contemporaneous mammalian forms.
Here, then, we have direct proof of the fact of frozen snow and ice
having accumulated in the hollows of the land outside of the glaciated
areas.[N]
[N] I have given Mr. Darwin's views, and discussed the origin of the
Pleistocene fluvio-glacial deposits at some length in _Prehistoric
Europe_, chaps, viii. and ix. To this work I refer for detailed
geological evidence in support of the view advocated above.
Now, if such conditions existed in the valleys of middle Europe,
the widespread loss of those regions is readily accounted for. The
occurrence of irregular sheets and shreds of gravel and loam at
heights of more than a hundred feet above a valley-bottom offers no
difficulty--it is in fact precisely the kind of phenomenon we might
have expected. We are therefore not required to go out of our way to
dream about impossible volcanic action, or to call upon the winds
of heaven to help us, or upon the waters of the Deluge to float us
out of our difficulties. But while I believe the views I have now
advocated sufficiently account for the appearances presented by the
ancient valley-gravels and loams of central Europe, there are two very
considerable areas of löss which require some further explanation. The
first of these is that broad belt of löss which extends from west to
east across the plains of northern Germany, and the northern boundary
of which coincides with the limits reached by the last great ice-sheet,
from which it spreads south to the foot-hills of the Harz, and other
mountains of middle Europe. Here we have a sheet of löss which bears
no apparent relation to the valley-systems of the region in which it
occurs. But the fact of its northern boundary being coincident with the
terminal front of the last great northern ice-sheet at once suggests
its origin. It is evident that this ice-sheet must have blocked the
rivers flowing north, and dammed back their waters.[O] A wide sheet
of muddy water must therefore have extended east and west over the
very area which is now covered by the belt of löss in question. This
temporary lake would doubtless be subject to great alternations of
level--a portion draining away perhaps under the ice-sheet--but the
water would for the most part make its way westward, and eventually
escape into the English Channel. From the waters of this great lake,
fed by many large glacial rivers, abundant precipitation of loam and
silt must have taken place.
[O] The late Mr. Belt, as is well known, was of opinion that all the
rivers flowing north in Europe and Asia were dammed back by a great
Polar glacier, and that all the low-tracts in the northern portions of
the two continents were thus covered by wide inland seas of freshwater.
As I do not believe that such a Polar ice-cap existed during the
Glacial period, I cannot agree with Mr. Belt that the alluvial plains
of northern Siberia mark the sites of ice-dammed lakes.
The second and by far the most extensive sheet of löss in Europe is
the so-called "black earth," or "tchernozem," with which such enormous
tracts in southern Russia are covered. This widespread löss--for such
it really is--I have elsewhere tried to show consists of the flood-loam
and inundation-muds laid down by the water escaping along the margin
of the northern ice-sheet, which discharged its drainage in the
direction of the Black Sea, its black colour being due to the grinding
down and pulverising of the black Jurassic shales which extend over
such wide regions in middle Russia.
IV.
_The Extent of Glaciation in North America._
The various phenomena of glaciation which go to prove that a great
ice-sheet formerly covered a wide region in northern Europe are
developed on a still more extensive scale in North America. Smoothed
and striated rock-surfaces, crushed and dislocated rock-masses, and
enormous accumulations of morainic débris and fluvio-glacial detritus,
all combine to tell the same tale. The morainic accumulations of North
America have been distributed upon the same principles as the similar
deposits of our own Continent. Boulder-clay of precisely the same
character as that of Scotland and Scandinavia, of Switzerland and north
Italy, covers vast tracts in the low-grounds of the British Possessions
and the northern States of the Union, where it forms enormous sheets,
varying in thickness from 30 or 50 up to 100 feet or more. In the rough
Laurentian high-lands, however, it is more sparingly developed, and
the same is the case in the hilly regions of New England. In short,
it thickens out upon the low-grounds, and thins off upon the steeper
slopes, while it attains its greatest thickness and forms the most
continuous sheets in the country that lies south of the great lakes.
The southern limits of this deposit form a kind of rude semi-circle.
From New York the boundary-line has been followed north-west through
New Jersey and Pennsylvania to beyond the forty-second parallel,
after which it turns to the south-west, passing down through Ohio to
Cincinnati (39°); then, striking west and south-west through Indiana,
it traverses the southern portion of Illinois. Its course after
it reaches the valley of the Missouri has been only approximately
determined, but it turns at last rather abruptly to the north-west,
sweeping away in that direction through Kansas, Nebraska, Dakota, and
Montana.
The general course followed by the ice-sheet underneath which this
boulder-clay was formed has been well ascertained, partly by the
evidence of the clay and its contents, and partly by that of _roches
moutonnées_ and striated rocks. The observations of geologists in
Canada and the States leave it in no doubt that an enormous sheet
of ice flowed south over all the tracts which are now covered with
boulder-clay. During a recent visit to Canada and the States, I had
opportunities of examining the glacial deposits at various points
over a somewhat extensive area, and everywhere I found the exact
counterparts of our own accumulations. In Minnesota, Wisconsin, Iowa,
Illinois, Indiana, and Ohio, and again in New York, Connecticut, and
Massachusetts, and the low-grounds of Canada, I recognised boulder-clay
of precisely the same character as that with which we are familiar at
home. The glacial phenomena of the Hudson valley and of the lower part
of the Connecticut River were especially interesting. In those regions
the evidence of a southward flow of the ice is most conspicuous, and
the phenomena, down to the smallest details, exactly recalled those of
many parts of Europe. Professor Dana, under whose guidance I visited
the Connecticut valley, showed me, at a considerable height upon the
valley-slope, an ancient water-course, charged with gravel and shingle,
which could not possibly have been laid down under present conditions.
It was, in fact, a sub-glacial water-course, and resembled the similar
water-courses which are associated with boulder-clay in our own country.
If I met with only familiar glacial phenomena in the low-lying tracts
traversed by me, I certainly saw nothing strange or abnormal in the
hillier tracts. In passing over the dreary regions between the valley
of the Red River and Lake Superior I was constantly reminded of the
bleak tracts of Archæan gneiss in the north-west of Scotland, and
of the similar rough broken uplands in many parts of Scandinavia and
Finland. The whole of that wild land is _moutonnée_. Rough tors and
crags are smoothed off, while boulder-clay nestles on the lee-side. In
the hollows between the _roches moutonnées_ are straggling lakes and
pools and bogs innumerable. Frequently, too, one comes upon rounded
cones and smooth banks of morainic gravel and sand, and heaps of coarse
shingle and boulders, while erratics in thousands are scattered over
the whole district. If you wish to have a fair notion of the geological
aspect of the region I refer to, you will find samples of it in many
parts of the Outer Hebrides and western Ross-shire and Sutherland.
Cover those latter districts with scraggy pines, and their resemblance
to the uplands of Canada will be complete.
From descriptions given by travellers it would appear that morainic
detritus--mounds and sheets of stony clay, gravel and sand, shingle,
boulders, and erratics--are more or less plentifully sprinkled over
all the British Possessions and the islands of the Arctic Archipelago;
so that we have every reason to believe that the ice-sheet which left
its moraines at New York and Cincinnati extended northwards to the
Arctic Ocean. Nor can there be much doubt that this same _mer de glace_
became confluent in the west with the great glaciers that streamed
outwards from the Rocky Mountains; while we know for a certainty that
the southern portion of Alaska, together with British Columbia and
Vancouver Island, were buried in ice that flowed outwards into the
Pacific.
Along the eastern sea-board north of New York city there is no tract
which has not been overflowed by ice. The islands in Boston Harbour are
made up for the most part of tough boulder-clay; and boulder-clay and
striated rocks occur also in Maine, New Brunswick, Nova Scotia, and
Newfoundland.
Thus we may say that the ice-covered region of North America was
bounded on the north by the Arctic, on the west by the Pacific, and on
the east by the Atlantic Oceans. The Rocky Mountains, however, divided
the great _mer de glace_ that overflowed Canada and the States from
the ice that streamed outwards to the Pacific. Measured from the base
of the Rockies to the Atlantic, the _mer de glace_ of Canada and the
States must have exceeded 2500 miles in width, and it stretched from
north to south over 40 degrees of latitude.
Outside of this vast region and the great mountain-ranges of the
far west, there are few hilly areas in the States which reach any
considerable elevation. South of the _mers de glace_ of the north and
west, no such mountain-groups as those of middle and southern Europe
occur, and consequently we do not expect to meet with many traces
of local glaciation. Nevertheless, these have been recognised in
the Alleghany Mountains, West Virginia, and in the Unaka Mountains,
between Tennessee and North Carolina. But the glaciers of those minor
hill-ranges were of course mere pigmies in comparison with the enormous
ice-streams that flowed down the valleys of the Rocky Mountains and
the Sierra Nevada. Even as far south as the Sierra Madre of Mexico
glaciers seem formerly to have existed; and Mr. Belt has described the
occurrence of what he considered to be boulder-clays at a height of
2000 to 3000 feet in Nicaragua.
I have mentioned the fact that in Europe we have, outside of the
glaciated areas, certain accumulations (such as the Gibraltar breccias)
which could only have been formed under the influence of extreme cold.
Similar accumulations occur in North Carolina, where they have been
carefully studied by Mr. W. C. Kerr. According to Mr. Kerr, these
deposits have crept down the declivities of the ground under the
influence of successive freezings and thawings; and now that attention
has been called to such phenomena, our American friends will doubtless
detect similar appearances in many other places.
The facts which I have now briefly indicated suffice to show that
during the climax of glaciation North America must have presented
very much the same appearance as Europe. Each continent had its great
northern ice-sheet, south of which local glaciers existed in hilly
districts, many of which are now far below the limits of perennial
snow. We may note, also, that in each continent the _mers de glace_
attained their greatest development over those regions which at the
present day have the largest rainfall. Following the southern limits
of glaciation in Europe, we are led at first directly east, until we
reach central Russia, when the line we follow trends rapidly away to
the north-east. The like is the case with North America. Trace the
southern boundary of the ice-sheet west of New York, and you find,
when you reach the valley of the Missouri, that it bends away to the
north-west. Now we can hardly doubt that one principal reason for
the non-appearance of the _mer de glace_ in the far east of Europe
and the far west of America was simply a diminishing snow-fall.
Those non-glaciated regions which lay north of the latitudes reached
by the ice-sheets were dry regions in glacial times for the same
reasons that they are dry still. The only differences between glacial
Europe and America were differences due to geographical position and
physical features. The glaciation of the Urals was comparatively
unimportant, because those mountains, being flanked on either side by
vast land-areas, could have had only a limited snow-fall; while the
mountain-ranges of western North America, on the other hand, being
situated near the Pacific, could not fail to be copiously supplied. For
obvious reasons, also, the North American ice-sheet greatly exceeded
that of Europe. In all other respects the conditions were similar in
both continents.
V.
_Changes of Climate in North America during the Ice Age._
American geologists are now pretty well agreed that their "interglacial
deposits"--the existence of which is not disputed--have precisely the
same meaning as the similar deposits which occur in Europe. They tell
of great climatic changes. At present, however, there is no certain
evidence in the American deposits of more than one interglacial epoch;
but the proofs of such an epoch having obtained are overwhelming.
The occurrence again and again of fossiliferous beds intercalated
between two separate and distinct sheets of boulder-clay and morainic
accumulations, leaves us in no doubt that we are dealing with precisely
the same phenomena which confront us in Europe. No mere partial
recession and re-advance of the _mer de glace_ will account for the
facts. We have seen that during the culmination of the Glacial period
the American ice-sheet overflowed Ohio, Indiana, and Illinois. Now
interglacial deposits occur as far north as the Canadian shores of
Lakes Ontario and Superior, so that all the country to the south must
have been uncovered by ice before those interglacial deposits were
laid down. But the evidence entitles us to say much more than this.
The interglacial beds of Ohio, Indiana, Illinois, and other States,
afford abundant evidence of a great forest-growth having covered the
regions vacated by the ice of the penultimate glacial epoch. The trees
of this forest-land included sycamore, beech, hickory, red-cedar, and
others; and amongst the plants were grape vines of enormous growth,
which, according to Professor Cox, "indicate perhaps the luxuriance
of a warmer climate." At all events, the climate that nourished such
a forest-growth could not have been less genial than the present. And
such being the case, we may reasonably infer that the vast regions to
the north of the lakes were no more inhospitable then than they are now.
To this genial interglacial epoch succeeded the last glacial epoch,
when a great ice-sheet once more enveloped a wide area. In the extreme
east this latest _mer de glace_ appears to have reached as far south
as that of the earlier epoch; but as we follow its terminal moraines
westward they lead us further and further away from the southern
limits attained by the preceding ice-sheet. These great terminal
moraines form an interesting study, and the general results obtained by
American observers have been very carefully put together by Professor
Chamberlin. I traversed wide regions of those moraines in Indiana,
Illinois, Wisconsin, and Minnesota, and, so far as my observations
went, I could only confirm the conclusions arrived at by Professor
Chamberlin and others. The mounds, banks, cones, and ridges are
unquestionably moraines--of enormous dimensions, no doubt, but in all
their phenomena strictly analogous to similar gravelly moraines in our
own country and the Continent. Many of the American moraines consist
almost entirely of water-worn material--sand, gravel, shingle, and
boulders, together with large angular and sub-angular erratics. These
deposits are generally stratified, and frequently show diagonal or
false-bedding. In this and other respects they exactly reproduce--but
of course on a much larger scale--our Scottish kames, and the similar
accumulations of north Germany and Finland, and the low-grounds of
Italy opposite the mouths of the great Alpine lakes. The kames of
Wisconsin again and again reminded me of the gravelly moraines that
cover the ground for many miles round the lower end of Lake Garda.
It is this gravelly and sandy aspect of the American moraines that
is most conspicuous, water-assorted materials seeming everywhere to
form their upper and outer portions. Now and again, however, a deep
cutting discloses underneath and behind such water-worn detritus a
mass of confused materials, consisting of clay, sand, gravel, shingle,
and boulders, which are angular and sub-angular, often smoothed and
striated, and of all shapes and sizes. According to Mr. Chamberlin,
this unstratified material "is indistinguishable from true till, and
is doubtless to be regarded as till pushed up into corrugations by the
mechanical action of the ice."
This grand series of moraines stretches from the peninsula of Cape Cod
across the northern States, and passes in a north-westerly direction
into the British Possessions, over which it has been followed for some
400 miles. The disposition of the moraines, forming as they do a series
of great loops, shows that the ice-sheet terminated in a number of
lobes or gigantic tongue-like processes. Nothing seen by me suggested
any marine action; on the contrary, every appearance, as I have said,
betokened the morainic origin of the mounds; and Mr. Chamberlin assured
me that their peculiar distribution was everywhere suggestive of this
origin. No one who has traversed the regions I refer to is at all
likely to agree with Sir W. Dawson's view, that the American mounds,
etc., are the shore-accumulations of an ice-laden sea.
The morainic origin of these accumulations having been demonstrated
by American geologists, we are now able to draw another parallel
between the European and American glacial deposits. We have seen that
in Europe the ice-sheet of the latest glacial epoch was by no means
so extensive as that of the preceding glacial epoch. The same was
the case in North America. Moreover, in America, just as in Europe,
the latest occupant of the land was not the sea, but glacier-ice.
In Scotland and Scandinavia the gradual disappearance of the latest
ice-sheets was marked by a partial submergence, which in the former
country did not greatly exceed 100 feet, and in the latter 700 feet.
In America, in like manner, we find traces of a similar partial
submergence. In Connecticut this did not exceed 40 or 50 feet, but
increased to some 500 feet in the St. Lawrence, and to over 1000 feet
in the Arctic regions. If there ever was during the Glacial period a
greater submergence than this in North America it must have taken place
in earlier glacial or interglacial times, but of such a submergence
no trace has yet been recognised. In this respect the American record
differs somewhat from our own, for in Britain we have evidence of a
submergence of over 1000 feet, which supervened in times immediately
preceding the latest great extension of continental ice.[P] But nowhere
in middle Europe, and nowhere in North America, in the region south
and west of the great lakes, is there any trace of a general marine
submergence. The "Palæocrystic Sea" is as idle a dream for the northern
States of America as it is for any part of Europe.
[P] See footnote, p. 173.
VI.
_The Results of Fluvio-glacial Action in North America._
The close analogies which obtain between the glacial and interglacial
deposits of Europe and North America are equally characteristic
of the fluvio-glacial accumulations of the two continents. As in
Europe, so in America we meet with considerable sheets of gravel and
shingle, sand, fine clay, and loam, which are evidently of freshwater
origin. In the gently-undulating tracts of the northern States those
deposits often spread continuously over wide regions; in the hillier
districts, however, they are most characteristic of the valleys. They
are very well represented, for example, in the Connecticut valley,
where they have been carefully studied by Professor Dana. Like the
similar deposits of our own Continent, they have been laid down by
the torrents and swollen rivers of the Glacial period. The great
range of moraines which marks the extreme limits reached by the
latest ice-sheet is generally associated with sheets of gravel and
sand, which one can see at a glance are of contemporaneous origin,
having been spread out by the water escaping from the melting ice.
Nor can one doubt that the vast sheets of löss in the Missouri and
Mississippi valleys are strictly analogous in origin, as they are in
structure and disposition, to the löss of Europe. I have spoken of the
probable existence of a glacial lake formed by the damming back of
the Rhine and other rivers by the European ice-sheet. Now, in North
America we meet with evidence of the same phenomenon. When the last
ice-sheet of that continent attained its maximum development, all the
water escaping from its margin in the north States necessarily flowed
south into the Mississippi and Missouri rivers. But in course of time
the ice melted away beyond the drainage-area of those rivers, and
disappeared from the valley of the Red River of the north, which, it
will be remembered, empties itself northward into Lake Winnipeg. When
the ice-front had retired so far it naturally impeded the drainage of
the Red River basin, and thus formed a vast glacial lake, the limits
of which have been approximately mapped out by Mr. Upham, by whom the
ancient lake has been designated Lake Agassiz. The deposits laid down
in this lake consist of finely laminated clays, etc., which resemble in
every particular the similar unfossiliferous clays so frequently found
associated with glacial accumulations in Europe. Had the drainage of
the Red River valley been south instead of north, the clays and loams
of the far north-west would not have been arrested and spread out where
they now are, and Manitoba would have been covered for the most part
with loose shingle, gravel, and sand.
Thus the final disappearance of the American ice-sheet was marked
by the formation not only of moraines, but of flood-gravels and
torrential- and inundation-deposits of the same character as those with
which we are familiar at home. Wherever similar geographical conditions
prevailed, there similar geological results followed.
VII.
_Conclusion._
There are many other points of resemblance between the glacial and
fluvio-glacial accumulations of the two continents, but to these time
forbids any reference. Indeed, I cannot recall any signal difference.
Such differences as do occur are due simply to the varying conditions
of the two continental areas. The glacial phenomena of North America
are a repetition of those of Europe, but upon a much grander scale. The
boulder-clays of the former continent, in their composition, structure,
and distribution, exactly recall our own. Interglacial beds occur
under similar circumstances in both continents; and the same is the
case with the gravelly moraines and fluvio-glacial accumulations.
We are driven, then, to the conclusion that the physical conditions
of the Glacial period were practically the same in Europe and North
America. What those conditions were I have already indicated, and have
shown that the results arrived at by geologists are not vague dreams
and speculations, but a logical induction from well-ascertained facts.
Before we can believe that volcanic eruptions, a general deluge, or a
Palæocrystic Sea have produced the many varied phenomena of our glacial
formations, either in whole or in part, we must first shut our eyes and
then erase from our minds all knowledge of the facts which have been so
laboriously gathered by a long succession of competent observers.
[Illustration:
PLATE III
DISTRIBUTION OF ICE
PAST AND PRESENT.
POLAR VIEW
OF THE
WORLD
ON LAMBERTS EQUAL AREA PROJECTION
]
VII.
The Intercrossing of Erratics in Glacial Deposits.[Q]
[Q] _The Scottish Naturalist_, 1881.
Among the many phenomena connected with the glacial deposits of this
country which have puzzled geologists there is none more remarkable
than the "intercrossing of erratics." The fact that such wandered
blocks have apparently crossed each other's tracks in their journeys
appears at first sight inexplicable on the assumption that their
transport has been effected by land-ice. The phenomena in question,
therefore, have always been appealed to by those who uphold the
iceberg origin of our boulder-clays, etc., as evidence decisively in
favour of their views. No one can deny that any degree and amount of
intercrossing might take place in the case of icebergs. We can readily
conceive how floating ice, detached from a long line of coast, might be
compelled by shifting winds and changing currents to tack about again
and again, so as to pursue the most devious course, and scatter their
stony burdens in the most erratic manner over the sea-bottom; while,
on the other hand, it is quite impossible to understand how a similar
irregular distribution of erratics could take place under one and the
same glacier flowing in a determinate direction. It is little wonder,
then, that the curious phenomena of the intercrossing of erratics
should have had much importance attached to it by the upholders of
the iceberg theory, seeing that all the other proofs which have been
adduced in favour of this theory have only served to demonstrate its
insufficiency. Upon the facts connected with the intercrossing of
erratics, the supporters of this time-honoured theory are now making
what I must believe is their last stand. I purpose therefore, in this
paper, to give a short outline of those facts, with the view of showing
that so far from being antagonistic to the land-ice theory, they are in
complete harmony with it; and indeed must be considered as affording an
additional demonstration of its truth.
Some years ago I called attention to the fact that in the middle
districts of Scotland the boulder-clay not infrequently contains a
curious commingling of northern and southern erratics.[R] I showed
that this was the case throughout a belt of country extending from the
sea-coast near Ayr, north-east to the valley of the Irvine, and thence
across the watershed into the Avon, and east to Lesmahagow, then down
the valley of the Clyde to Carluke, stretching away to the east by
Wilsontown, and thereafter continuing along the crest of the Pentlands
and the northern slopes of the Lammermuir Hills, by Reston and Ayton,
to the sea. "All along this line," I remarked, "we have a 'debatable
ground' of variable breadth, throughout which we find a commingling in
the till of stones which have come from north and from south. South of
it, characteristic Highland stones do not occur, and north of it stones
derived from the south are similarly absent." The explanation of these
facts is obvious. The belt of ground referred to was evidently the
meeting-place of the Highland and southern _mers de glace_. Here the
two opposing ice-flows coalesced and became deflected by their mutual
pressure to right and left--one great current going east and another
west. It is evident that the line of junction between the two _mers de
glace_ could not be rigorously maintained in one and the same position
during a period of glaciation, but would tend to oscillate backwards
and forwards, according as one or the other ice-sheet prevailed.
Sometimes the southern ice-sheet would be enabled to push back the
northern _mer de glace_, while at other times the converse would take
place. Nor is it necessary to suppose that the advance of one ice-sheet
was general along the whole line. On the contrary, it is most likely
that the movement was quite irregular--an ice-sheet advancing in some
places, while at other points its line of junction with the opposing
ice-sheet remained stationary, or even retrograded. Such movements
would obviously give rise to oscillations in the sub-glacial débris of
clay and stones; and thus we have a simple and natural explanation of
those intercrossings of erratics which are so characteristic of that
region which I have termed the "debatable ground." And this conclusion
is borne out by the fact that the glacial striæ of the same "debatable
ground" afford like evidence of oscillation in the trend of the
ice-flow.
[R] _Great Ice Age_, 2nd edit., p. 609.
Along the base of the Highland mountains in Forfarshire, etc., we
meet with similar intercrossings of erratics. Thus we occasionally
encounter in the boulder-clays overlying the Silurian regions erratics
of Old Red Sandstone rocks which have come from the east or south-east;
while the abundant presence of erratics of Silurian origin, on the
other hand, bespeak an ice-flow from the west towards the low-grounds.
In some places within the Silurian area we encounter a greyish-blue
boulder-clay containing Silurian fragments only, while in other places
within the same area the boulder-clay becomes reddish, and is charged
with many boulders of Old Red Sandstone rocks. Now the greyish-blue
till could only have been laid down by glacier-ice descending from
the Silurian high-grounds to Strathmore, while the red boulder-clay
points to a partial invasion of the Silurian regions by land-ice,
which had previously traversed the lower-lying Old Red Sandstone
areas. These apparently contradictory movements are readily accounted
for by the former presence in the area of the North Sea of the great
Scandinavian _mer de glace_. Dr. James Croll was the first to point out
that the glacial phenomena of Caithness and the Shetlands could only
be accounted for by the advance of the Scandinavian ice-sheet towards
our coasts, where it encountered and deflected the Scottish ice-sheet
out of its normal course--a sagacious induction, which the admirable
and exhaustive researches of my colleagues, Messrs. B. N. Peach and
J. Horne, have now firmly established. The lower blue boulder-clay
was evidently accumulated at a time when the Scottish ice was able
to flow more or less directly east or south-east towards what is now
the coast-line; while the overlying red boulder-clay points to a
subsequent period when the presence of the Scandinavian _mer de glace_
was sufficiently great to compel the Scottish ice out of its normal
course, and cause it to flow in a north-easterly direction. In doing
so it now and again passed from tracts of Old Red Sandstone to invade
the Silurian area, and thus an overlying red boulder-clay was here and
there accumulated upon the surface of a greyish-blue till in which not
a single fragment of any Old Red Sandstone rock occurs.
Recently Messrs. B. N. Peach and J. Horne, in a most instructive paper
on the "Glaciation of Caithness,"[S] have described some remarkable
comminglings of material which occur in a region where the glacial
striæ afford equally striking evidence of conflicting ice-movements.
These phenomena are developed here and there along a line which
indicates the meeting-place of two rival ice-streams, on each side
of which the boulder-clay presents different characteristics--the
one boulder-clay being the _moraine profonde_ of the ice that flowed
ENE. and NNE. towards the Caithness plain, while the other is an
accumulation formed underneath the ice that streamed across that plain
from SE. to NW. These phenomena are thus, as my colleagues remark,
quite analogous to those met with in the middle districts of Scotland,
as described by me, and referred to in a preceding paragraph. Now it
is obvious that while these examples of "intercrossings" of erratics
and "cross-hatching" of striæ all go strongly to support the land-ice
theory of the glacial phenomena, they at the same time negative the
notion of floating-ice having had anything to do with the production of
the phenomena under review.
[S] _Proceedings Royal Physical Society_, Edinburgh, 1881.
Before considering the evidence adduced by Mr. Mackintosh and others
as to the intercrossings of erratics in the drift-deposits of England,
I shall mention some of the more remarkable examples of the same
phenomena which have been noticed by continental geologists. The first
cases I shall cite are those which have been observed in the glacial
accumulations of the Rhone valley in eastern France. The land-ice
origin of these accumulations has never been called in question, and
as the intercrossings of erratics in that region are not only more
common, but much more striking and apparently inexplicable than any
which have been noticed elsewhere, it will be admitted that they of
themselves afford a strong presumption that the conflicting courses
followed by the erratics in certain regions of our own country are
the result rather of oscillations in the flow of land-ice than of the
random and eccentric action of icebergs. The researches of Swiss and
French glacialists have proved that during the climax of the Glacial
period an enormous area in the low-grounds of eastern France was
covered with a huge _mer de glace_, formed by the union of the great
Rhone glacier with the glaciers descending from the mountains of Savoy
and Dauphiny. A line drawn from Bourg by way of Chatillon, Villeneuve,
Trévoux, and Lyons to Vienne, and thence south-east by Beaurepaire to
the valley of the Isère, a few miles above St. Marcellin, indicates
roughly the furthest limits reached by the _mer de glace_. Over all
the low-grounds between that terminal line and the mountains are found
widespread sheets of boulder-clay and sand and gravel, together with
loose erratics. Now and again, too, well-marked terminal moraines make
their appearance, while the rock-surfaces, when these are visible and
capable of bearing and retaining glacial markings, present the usual
aspect of _roches moutonnées_. The same kinds of morainic materials
and ice-markings may of course be followed up into the valleys not
only of the Alps properly so-called, but also into those of the hills
of Bugey and the secondary mountain-chain of Savoy and Dauphiny. It
has indeed long been known that local glaciers formerly occupied the
mountain-valleys of Bugey. For example, a number of small glaciers have
descended from the slopes of the mountains west of Belley (such as Bois
de la Morgue, Bois de Lind, etc.) to the Rhone, and again from Mont du
Chat to the north-west. These glaciers were quite independent of the
greater ice-streams of the neighbouring Alps of Savoy, and the same was
the case with the glaciers of that mountainous tract which extends from
Nantua south to Culoz, between the valleys of the Ain and the Rhone.
From this elevated region many local glaciers descended, such as that
of the Valromey, which flowed for a distance of some twenty miles from
north to south. Again, similar local glaciers have left abundant traces
of their former presence throughout the mountainous belt of land that
stretches between Chambery and Grenoble to the west of the valley of
the Isère. The moraines of all those local glaciers, charged as they
are with the débris of the neighbouring heights, clearly indicate that
the local glaciers flowed each down its own particular valley. There
are certain other appearances, however, which seem at first sight to
contradict this view. Sometimes, for example, we encounter in the same
valleys erratics which do not belong to the drainage-system within
which they occur, but have without doubt been derived from the higher
Alps of Switzerland and Savoy. And the course followed by these foreign
erratics has crossed at all angles that which the local glaciers have
certainly pursued--occasionally, indeed, the one set of erratics has
travelled in a direction exactly opposed to the trend taken by the
others. As examples, I may cite the case of the erratics which occur
in Petit Bugey. In this district we encounter many locally-derived
erratics which have come from Mont du Chat to the west of the Lac
du Bourget--that is to say, they have travelled in a north-westerly
direction. But in the same neighbourhood are found many erratics of
Alpine origin which have been carried from north-east to south-west, or
at right angles to the course followed by the local erratics. Again,
in the valley of the Seran we have evidence in erratics and terminal
moraines of a local glacier which flowed south as far as the Lyons and
Geneva Railway, in the neighbourhood of which, a few miles to the west
of Culoz, its terminal moraines may be observed. This is the extinct
Glacier du Valromey of MM. Falsan and Chantre. Now it is especially
worthy of note that in the same valley we have distinct evidence of an
ice-flow from south to north--_i.e._, _up_ the valley. Erratics and
morainic materials which are unquestionably of Alpine origin have been
followed a long way up the Seran valley--for two-thirds of its length
at least. Before they could have entered that valley and approached the
slopes of Romey, they must have travelled down the valley of the Rhone
from the higher Alps of Savoy in a _south-west_ and _south_ direction
until they rounded the Montagne du Grand Colombier. It was only after
they had rounded this massive mountain-ridge that they could pursue
their course up the valley of the Seran, in a direction precisely
opposite to that which they had previously followed. These and many
similar and even more remarkable examples of the "intercrossings"
of streams of erratics are described by MM. Falsan and Chantre, and
graphically portrayed in their beautiful and instructive work on the
"Ancient Glaciers and Erratic Deposits of the Basin of the Rhone"; and
the explanation of the phenomena given by them is extremely simple
and convincing. The local erratics and moraines pertain partly to the
commencement and partly to the closing stage of the Glacial period.
Long before the south branch of the great glacier of the Rhone had
united with the glacier of the Arve, and this last with the glaciers
of Annecy and Beaufurt, and before these had become confluent with the
glacier of the Isère, etc., the secondary mountain-ranges of Savoy and
Dauphiny and the hills of Bugey were covered with very considerable
snow-fields, from which local glaciers descended all the valleys to the
low-ground. But when the vast ice-flows of Switzerland, Upper Savoy,
etc., at last became confluent, they completely overflowed many of the
hilly districts which had formerly supported independent snow-fields
and glaciers, and deposited their bottom-moraines over the morainic
débris of the local glaciers. In other cases, where the secondary
hill-ranges were too lofty to be completely drowned in the great _mer
de glace_, long tongues of ice dilated into the valleys, and compelled
the local ice out of its course; sometimes, as in the case of the
Valromey, forcing it backward up the valleys down which it formerly
flowed. But when once more the mighty _mer de glace_ was on the wane,
then the local glaciers came again into existence, and reoccupied
their old courses. And thus it is that in the hilly regions at the
base of the higher Alps, and even out upon the low-grounds and plains,
we encounter that remarkable commingling of erratics which has been
described above. Not infrequently, indeed, we find one set of moraines
superposed upon another, just as in the low-grounds of northern
Germany, etc., we may observe one boulder-clay overlying another, the
erratics in which give evidence of transport in different directions.
The observations recorded by MM. Falsan and Chantre, and their
colleagues, thus demonstrate that "intercrossings" of erratics of the
most pronounced character have been brought about solely by the action
of glaciers. In the case of the erratics and morainic accumulations of
the basin of the Rhone, the action of icebergs is entirely precluded.
I may now mention some of the more remarkable examples of
intercrossings of erratics which have been recorded from the glacial
accumulations of north Germany, etc. An examination of the glacial
striæ, _roches moutonnées_, and boulder-clays of Saxony leads to
the conviction, according to Credner, Penck, Torell, Helland, and
others, that the whole of that region has been invaded by the great
Scandinavian _mer de glace_ which flowed into Saxony from NNE. to SSW.
Erratics from southern Sweden and Gothland occur in the boulder-clay,
and the presence of these, taken in connection with the direction
of the glaciation, leaves us no alternative but to agree with the
conclusions arrived at by the Saxon geologists. But, apparently
in direct contradiction of this conclusion, we have evidence to
show that boulders of the same kinds of rock occur in Denmark and
Holland, pointing to a former ice-flow from north-east to south-west
and west. Thus boulders derived from Gothland occur at Gröningen
in Holland, while fragments from the island of Öland are met with
in Faxö; and erratics from the borders of the Gulf of Finland are
encountered at Hamburg. Indeed, when geologists come to examine the
erratics in north Germany and Poland generally, they find evidence of
apparently two ice-flows--one of which went south-south-west, south,
and south-east--spreading out, as it were, in a fan-shape towards the
southern limits reached by the great "Northern Drift,"--while the other
seems to have followed the course of the Baltic depression, overflowing
the low-grounds of northern Prussia, Holland, etc., in a south-west and
west direction. Now, it is quite evident that no one _mer de glace_
could have followed these various directions at one and the same time.
The explanation of the apparent anomaly, however, is not far to seek.
It is reasonable to infer that long before the _mer de glace_ had
attained its maximum dimensions, when as yet it was confined to the
basin of the Baltic and was only able to overflow the northern regions
of Prussia, etc., its course would be determined by the contour of the
pavement upon which it advanced. It would, therefore, be compelled
to follow the Baltic depression, and for a long time it would carry
erratics from Finland, the Baltic islands, and eastern Sweden in a
south-west and west-south-west direction. And this would continue to
be the direction even after a considerable portion of the low-grounds
of Prussia, etc., had been overflowed. But when the ice-sheet was
enabled to advance south into Saxony, Poland, and Lithuania, erratics
from Finland, the Baltic islands, etc., would necessarily cease to
travel towards the west, and hold on a south-south-east, south, and
south-south-west course. Again, when the _mer de glace_ was on the
decline, a time would return when the ice, as before, would be
controlled in its flow by the Baltic depression, and this would give
rise to a further distribution of erratics in a prevalent west-by-south
direction.[T]
[T] For a fuller discussion of the distribution of erratics on
the Continent, I may refer to Appendix, Note B, in _Prehistoric
Europe_, where the reader will find references to the literature of
this interesting subject. [Continental geologists now recognise a
distinct stage of the Ice Age, during which their "Upper Diluvium" was
deposited by a great glacier that occupied the basin of the Baltic.
This "Great Baltic Glacier" appears to have been contemporaneous with
the local ice-sheets and valley-glaciers of the Highlands and other
mountain-tracts of our island. See Article X. 1892.]
No one of late years has been more assiduous in the collection of facts
relating to the intercrossing of erratics in the drift-deposits of
England than Mr. D. Mackintosh.[U] He has written many instructive and
interesting descriptions of the phenomena in question, which he justly
thinks are of prime importance from a theoretical point of view. In a
recent paper[V] he presents us with the results of a systematic survey
of the direction and limits of dispersion of the erratics of the west
of England and east of Wales, which he evidently is of opinion afford
strong support to the iceberg theory, while at the same time they
are directly opposed to the theory of transport by land-ice. I have
attentively considered all the arguments advanced by Mr. Mackintosh in
favour of his views--the one upon which he apparently lays most stress
being that of the intercrossings of erratics observed by him--and I
shall now proceed to point out how the phenomena described by him are
most satisfactorily explained by the land-ice theory. They seem to
me, indeed, to lend additional support to that theory, in the same
manner as the intercrossings of boulders observed in Scotland, northern
Germany, etc., and sub-alpine regions of France. Mr. Mackintosh calls
attention to the fact that erratics of the well-known Criffel granite
are found scattered over a large part of the plain of Cumberland,
from which they extend south along the coast to near the mouth of the
estuary of the Duddon. They reappear on the coast in the neighbourhood
of Blackpool and Liverpool, and again at intervals on the coasts of
north Wales from Flint to Colwyn Bay, and thence to Penmaenmawr and
the neighbourhood of Beaumaris. They are dispersed over the peninsula
of Wirral and the Cheshire plain, etc., and they have been followed
south-east as far as the neighbourhood of Cardington, near Church
Stretton, Burton, Wolverhampton, Stafford, Hare Castle, Macclesfield,
and Manchester. This great stream of boulders, therefore, spreads
out to south-east, south, and south-west: the erratics, to quote
Mr. Mackintosh, "have radiated from an area much smaller than their
terminal breadth." The same is the case, I may remark in passing, with
erratics in the boulder-clays of Scotland, Scandinavia, north Germany,
etc., as also with those in the drift-deposits of the great Rhone
glacier and other ancient glaciers both on the north and south side of
the Alps. Now, the course followed by the Criffel erratics is crossed
at an acute angle by the path pursued by many boulders of Eskdale
granite, and various felspathic rocks derived from the Cumberland
mountains. For example, Cumberland erratics of the kinds mentioned
occur near St. Asaph and Moel-y-Tryfane and in Anglesey, and they
have been followed over a wide district in Cheshire, etc., extending
as far south as Church Stretton and Wolverhampton, and as far east as
Rochdale. More than this, we find that numerous erratics of felstone,
derived from the mountain of Great Arenig, in north Wales, have gone
to north-east as far as Halkin Mountain, in Flintshire, Eryrys, near
Llanarmon, and Chirk, from which last-named place they have been traced
in a south-easterly direction to Birmingham, Bromsgrove, etc. A glance
at the map of England will show that this south-easterly drift of
erratics crosses at an acute angle the paths followed by the Criffel
granite boulders and the erratics derived from Cumberland, so that we
have now several intercrossings to account for. How can this be done by
the land-ice theory?
[U] This enthusiastic geologist died in 1891.
[V] _Quart. Journ. Geol. Soc._, vol. xxxv. p. 425
The explanation seems to me obvious, for the phenomena are, after all,
less striking than similar appearances which have been observed in
Scotland, especially by my colleagues, Messrs. Peach and Horne, in
Caithness and the Orkney and Shetland Islands; and they are certainly
less intricate than the facts recorded by MM. Falsan and Chantre
concerning the intercrossing, interosculation, and direct opposition
of erratic paths in Savoy and Dauphiny. We have only to reflect that
the great _mer de glace_--to which, as I believe, all the English
phenomena are due--did not come into existence and attain its maximum
dimensions in the twinkling of an eye, nor could it afterwards have
disappeared in the same sudden manner. On the contrary, a period
of local glaciation must have preceded the appearance of the great
ice-sheet. At first, and for a long time, permanent snow would be
confined to the higher elevations of the land, and glaciers would
be limited to mountain-valleys; but as the temperature fell the
snow-line would gradually descend, until at last, probably after a
prolonged period, it reached what is now the sea-level. Thus the
formation of _névé_ and glacier-ice would eventually take place over
what are now our low-grounds, and other tracts also, which are now
submerged. It is quite impossible that the vast sheets of ice which
can be demonstrated to have covered Scotland, a large part of England,
Ireland, Scandinavia, and north Germany, and even the limited area of
the Faröe Islands, could possibly have been fed by the snow-fields of
mountain-heights only. The precipitation and accumulation of snow, and
the formation of _névé_ and glacier-ice, must have taken place over
enormous regions in what are now the temperate latitudes of Europe.
It is obvious that the direction of ice-flow in the basin of the Irish
Sea opposite the south of Scotland and the west of England, while
preserving a general southerly trend, would vary at different periods.
Before the _mer de glace_ in that basin had attained its climax there
must have been a time when the ice, streaming outwards from the
high-grounds of Cumberland, was enabled to push its way far westward
out into the basin of the Irish Sea. At that time it was still able
to hold its own against the pressure exerted by the Scottish ice. But
as the general _mer de glace_ increased in thickness, the course of
the Cumberland ice would be diverted ever further and further to the
south-east, until, eventually, the Scottish ice came to hug the coast
of Cumberland, and to overflow Lancashire in its progress towards the
south-east. So gorged with ice did the basin of the Irish Sea become,
that a portion of the Scottish ice was forced over the plain of
Cumberland and up the valley of the Eden, where it coalesced with the
ice coming north from the Shap district, and thereafter flowed in an
easterly direction to join the great _mer de glace_ of the North Sea
basin.
Thus the intercrossings of the Criffel and Cumberland erratics
described by Mr. Mackintosh receive a ready explanation by the land-ice
theory. Nor do the intercrossings of the Welsh erratics with those
derived from Scotland and Cumberland offer any difficulty. The ice
coming from the Welsh mountains would naturally be deflected towards
south-east by the _mer de glace_ that streamed in that direction, and
might quite well have carried its characteristic boulders as far as
Birmingham before the general _mer de glace_ had attained its greatest
dimensions. But when that period of maximum glaciation arrived, the
Welsh boulders would be unable to travel so far towards the east, and
the Scottish and Cumberland boulders would then cross the path formerly
followed by the felstone erratics from Great Arenig.
Again, it is evident that when the _mer de glace_ was gradually
decreasing similar oscillations of the ice-flow would take place,
but in reverse order, and thus would give rise to a second series of
intercrossings. Moreover, we must remember that the Glacial period
was characterised by several great changes of climate. It was not one
continuous and prolonged period of cold conditions, but consisted
rather of a succession of arctic and genial climates; so that the same
countries were overrun at different epochs by successive _mers de
glace_, each of which would rework, denude, and redistribute to a large
extent the morainic materials of its predecessor, and thus might well
cause even greater complexity in the dispersion of erratics than has
yet been recognised anywhere in these islands.
Mr. Mackintosh refers to the occurrence of chalk-flints and Lias
fossils associated with northern erratics in the drift-deposits of
the west of England, the presence of which, he thinks, is fatal to
the theory of transport by land-ice. Thus, he says, chalk-flints,
etc., have been met with at Lillieshall (east of Wellington), at
Strethill (near Ironbridge), at Seisdon (between Wolverhampton and
Bridgenorth), at Wolverhampton, near Stafford, and near Bushbury.
Chalk-flints have also been found as far west as Malvern and Hatfield
Camp, south of Ledbury. All these erratics have crossed England from
the east, according to Mr. Mackintosh and other observers. Not only
so, but, as Mr. Mackintosh remarks, those found at Wolverhampton,
Birmingham, etc., "must have _crossed the course_ of the northern
boulders near its southerly termination." And since both northern and
eastern erratics are found associated in the same drift-deposit, it
seems to him "impossible to explain the intercrossing by land-ice or
glaciers." Now, on the contrary, those eastern erratics are scattered
over the very districts where I should have expected to find them. The
observations of geologists in East Anglia have shown that that region
has been invaded by the _mer de glace_ of the North Sea basin.[W]
This remarkable glacial invasion is proved not only by the direction
followed by stones of local derivation, and by boulders which have come
south from Scotland and the northern counties, but by the occurrence
in the boulder-clay at Carnelian Bay and Holderness of erratics of
certain well-known Norwegian rocks, which have been recognised by
Mr. Amund Helland. The occurrence of chalk-flints and fragments of
Oolitic rocks in the neighbourhoods mentioned by Mr. Mackintosh thus
only affords additional evidence in favour of the land-ice origin of
the drift-deposits described by him. The _mer de glace_ that flowed
down the east coast of England seems to have encroached more and more
upon the land, until eventually it swept over the low-lying Midlands
in a south-westerly direction, and coalesced with the _mer de glace_
that streamed inland from the basin of the Irish Sea, and the ice that
flowed outwards from the high-grounds of Wales. The united ice-stream
would thereafter continue on its south-westerly course down the Severn
valley to the Bristol Channel. I have no doubt that Mr. Mackintosh will
yet chronicle the occurrence of chalk-flints and other eastern erratics
from localities much further to the south than Ledbury.
[W] See Mr. Skertchly's description of East Anglian deposits in _Great
Ice Age_, 2nd edit., p. 358.
Again, considerable stress has been laid by Mr. Mackintosh upon the
occurrence of chalk-flints in the drift-deposits of Blackpool, Dawpool,
Parkgate, Halkin Mountain, Wrexham, the peninsula of Wirral, Runcorn,
Delamere, Crewe, Leylands, Piethorne (near Rochdale), and other places.
"All these flints," Mr. Mackintosh remarks, "belong to the basin of the
Irish Sea, and have almost certainly crossed the general course of the
northern boulders on their way from Ireland." Here, unfortunately, the
Irish Sea intervenes to conceal the evidence that is needed to enable
us to track the exact path followed by the erratics in question. I am
not so certain as Mr. Mackintosh that the chalk-flints he refers to
came from the north of Ireland. Chalk-flints occur pretty numerously in
the drift-deposits in the maritime districts of north-eastern Scotland,
which we have every reason to believe have been derived from an area
of Cretaceous rocks covering the bottom of the adjacent sea; and for
aught one can say to the contrary, patches of chalk-with-flints may
occur in like manner in the bed of the Irish Sea. I cannot at present
remember whether any boulders of the basalt-rocks, which are associated
with the Chalk in the north of Ireland, have been recognised in the
drifts of the west of England; but if the chalk-flints really came from
Antrim, it is more than probable that they would be accompanied by
fragments of the hard igneous rocks which overlie the Cretaceous strata
of north Ireland. Chalk and chalk-flints occur in the boulder-clay of
the Isle of Man, where they are associated, Mr. Horne tells us, with
Criffel granite and fragments of a dark trap-rock.[X] Possibly these
last are basalt-rocks from Antrim. It seems reasonable, therefore,
to believe that erratics of Irish origin have found their way to the
Isle of Man; and if this be so, it may be permissible to assume that
the chalk-flints of Blackpool, etc. (and perhaps also some of the
basalt-rocks), have come from the same quarter. Mr. Horne has no doubt
that the Irish erratics were brought to the Isle of Man by land-ice.
Referring to the conclusion arrived at by Mr. Close that the Irish _mer
de glace_ "was probably not less than 3000 feet in depth," he remarks:
"It is highly probable that this great mass of Irish ice succeeded,
after a hard battle (_i.e._, with the Scottish ice-sheet), in reaching
the Manx coast-line. It is not to be supposed that the normal momentum
of the respective ice-sheets remained constant. The moving force must
have varied with changing conditions. On the other hand, it is quite
possible that there may have been an 'under-tow' of the ice from the
north-east coast of Ireland, which would easily account for Antrim
chalk and chalk-flints in the Manx till." I would go further, and state
my conviction that before the united ice-sheets had attained their
maximum development, it is almost certain that the ice flowing into the
Irish Sea basin by the North Channel would for a long time exceed in
mass the coalescent glaciers that descended from the Southern Uplands
of Scotland, and would therefore be enabled to extend much further to
the east than it could at a later date, when the general _mer de glace_
had reached its climax. It might thus have advanced as far as and even
beyond the Isle of Man. This inference is based upon the simple fact
that the area drained by the _mer de glace_ of the North Channel was
very much greater than the area extending from the watershed of the
Southern Uplands of Scotland to the Isle of Man. Erratics from the
north of Ireland would thus travel down the bed of the North Channel,
and eventually be distributed over a wide area up to and possibly even
some distance beyond the Isle of Man. But as the Scottish and Cumbrian
ice-flows gradually increased in importance, the _mer de glace_ coming
from the North Channel would be forced further and further to the west,
until the ice-flow issuing from the high-grounds of Kirkcudbright at
last succeeded in reaching the middle of the Irish Sea basin. This
gradual modification of the general ice-flow in that basin would of
course give rise to a redistribution of the ground-moraine, and the
Irish erratics would then travel onwards underneath the Scottish ice,
and eventually reach the low-grounds of Lancashire and Cheshire,
along with erratics from Criffel and the Cumbrian mountains. It is,
therefore, quite unnecessary to suppose that the _mer de glace_ of
the North Channel actually crossed the whole breadth of the basin of
the Irish Sea to invade Lancashire, Cheshire, and north Wales. Had
this been the case, chalk-flints, chalk, and many other kinds of rock
derived from the north of Ireland, and even from Arran and Argyll,
would have abounded in the drifts of the west of England. Erratics
coming from Ireland could not possibly have travelled underneath
Irish ice further east than the Isle of Man. There or thereabouts, as
I have said, the _mer de glace_ of the North Channel would begin to
encounter the ice streaming down from the uplands of Galloway and the
mountains of Cumberland: and as the ice from these quarters increased
in thickness, it would gradually override what had formerly been the
bottom-moraine or till of the North Channel _mer de glace_. Thus Irish
erratics would become commingled with erratics from Criffel, etc., and
be carried forward in a southerly and south-easterly direction. The
chalk-flints in the drifts of Lancashire, Cheshire, etc., are probably
therefore _remaniés_--the relics of the bottom-moraine of the North
Channel _mer de glace_ rearranged and redistributed. And this is why
they and other Irish rocks are so comparatively rare in the glacial
accumulations of the west of England.
[X] _Trans. Edin. Geol. Soc._, vol. ii., 1874.
Thus all the instances of intercrossings adduced by Mr. Mackintosh
as favouring the iceberg theory, and condemning its rival, I would
cite as proving exactly the opposite. So far from presenting any real
difficulty to an upholder of the land-ice theory, they, in point of
fact, as I have already remarked, lend that view additional support.
It is not my purpose to criticise all the arguments and reasons
advanced by Mr. Mackintosh in favour of his special views, but I may be
allowed a few remarks on the somewhat extraordinary character of the
agents which, according to him, were mainly instrumental in producing
the drift-phenomena of western England. Before doing so, however, I
may point out that, in ascribing the transport of erratics in that
region (and, by implication, the formation of the boulder-clays, etc.,
with which most of these erratics are associated) to floating-ice and
sea-currents, Mr. Mackintosh has failed to furnish us with any "fossil
evidence" to show that western England was under water at the time
the boulder-clays and erratics were being accumulated. He speaks of
cold and warm currents, but where do we find any traces of the marine
organisms which must have abounded in those waters? Where are the
raised sea-beaches which must have marked the retreat of the sea? Where
do we encounter any organic relics that might help us to map out the
zones of shallow and deep water? The sea-shells, etc., which occur in
the boulder-clays are undeniably _remaniés_; they are erratics just
as much as the rock-fragments with which they are associated. Similar
assemblages of organic remains are met with in the till of Caithness,
where shallow-water and deep-sea shells, and shells indicative of
genial and again of cold conditions, are all confusedly distributed
throughout one and the same deposit. The same or analogous facts are
encountered in the _Blocklehm_ of some parts of Prussia, marine and
freshwater shells occurring commingled in the boulder-clay. Nay,
even in the _moraine profonde_ of the ancient Rhone glacier, broken
and well-preserved shells of Miocene and Pliocene species appear
enclosed in the tumultuous accumulation of clay, sand, and erratics.
And precisely similar phenomena confront us in the glacial deposits
of the neighbourhood of Lago Lugano. Mr. Mackintosh refers to the
so-called "stratification" of the boulder-clay, as if that were a proof
of accumulation in water. But a rude kind of bedding, generally marked
by differences of colour, and sometimes by lines of stones, was the
inevitable result of the sub-glacial formation of the boulder-clay.
The "lines of bedding" are due to the shearing of the clay under great
pressure, and may be studied in the boulder-clay of Switzerland and
Italy, and in the till not only of the Lowlands but of the Highlands
of Scotland. Occasionally the "lines" are so close that the clay
sometimes presents the appearance of rude and often wavy and irregular
lamination--a section of such a boulder-clay reminding one sometimes
of that of a gnarled gneiss or crumpled schist. And these appearances
may be noted in boulder-clays which occupy positions that preclude
the possibility of their being marine--as in certain valleys of the
Highlands, such as Strathbraan, and in the neighbourhood of Como, in
Italy. This "lamination" is merely indicative of the intense pressure
to which the till was subjected during its gradual accumulation under
the ice. It is assuredly not the result of aqueous action. Aqueous
lamination is due to sifting and winnowing--the coarser or heavier
and finer or lighter particles being separated in obedience to their
different specific gravity, and arranged in layers of more or less
regularity according to circumstances. There is nothing of this kind of
arrangement, however, in the so-called stratified boulder-clay. If the
clay of an individual lamina be washed and carefully sifted, it will be
found to be composed of grains of all shapes, sizes, and weights, down
to the finest and most impalpable flour. It is impossible to believe
that such a heterogeneous assemblage of grains could have been dropt
into water without the particles being separated and sifted in their
progress to the bottom. Of course, every one knows that patches and
beds of laminated clay and sand of veritable aqueous origin occur now
and again in boulder-clay. I suppose there is no boulder-clay without
them. I have seen them in the till of Italy and Switzerland, where
they show precisely the same features as the similar laminated clays in
the till of our own islands. But these included patches and beds point
merely to the action of sub-glacial waters, such as we know circulate
under the glaciers of the Alps, of Norway, and of Greenland.
Again, I would remark that Mr. Mackintosh has ignored all the evidence
which has been brought forward from time to time to demonstrate
the sub-glacial origin of boulder-clay, and to prove the utter
insufficiency of floating-ice to account for the phenomena. And he
adduces no new facts in support of the now discredited iceberg theory,
unless it be his statement that _flat_ striated rock-surfaces (such
as those near Birkenhead) have been caused by floating-ice--the
dome-shaped _roches moutonnées_ being, on the other hand the work
of land-ice. As a matter of personal observation, I can assure Mr.
Mackintosh that _flat_ striated surfaces are by no means uncommonly
associated in one and the same region with _roches moutonnées_.
What are _roches moutonnées_ but the rounded relics of what were
formerly rough uneven tors, projecting bosses, and prominent rocks?
The general tendency of glacial action is to reduce the asperities of
a land-surface; hence projecting points are rounded off, while flat
surfaces are simply, as a rule, planed smoother. Mr. Mackintosh might
traverse acres of such smoothed rock-surfaces in regions where the
strata are comparatively horizontal--for example, in the case of the
basaltic plateaux of the Faröes and of Iceland, which have certainly
been glaciated by land-ice. Similar flat glaciated surfaces are met
with again and again both in the Highlands and Lowlands of Scotland,
occupying positions and associated with _roches moutonnées_ and till
of such a character as to prove beyond any doubt that they no less
certainly are the result of the action of land-ice. But it is needless
to discuss the probability or possibility of glaciation of any kind
being due to floating-ice. We know that glaciers can and do polish and
striate rock-surfaces; no one, however, can say the same of icebergs:
and until some one can prove to us that icebergs have performed this
feat, or can furnish us with well-considered reasons for believing
them to be capable of it, glacialists will continue sceptical.
But leaving these and other points which serve to show the
weakness of the cause which Mr. Mackintosh supports with such keen
enthusiasm, I may, in conclusion, draw attention to certain very
remarkable theoretical views of his which seem to me to be not
only self-contradictory, but opposed to well-known natural laws.
Briefly stated, his general view is that the erratics of the west
of England have been distributed by floating-ice during a period of
submergence--the scattering of erratics and the accumulation of the
associated glacial deposits having commenced at or about the time
when the land began to sink, and continued until the submergence
reached some 2000 feet below the present sea-level. In applying this
hypothesis to explain the phenomena, Mr. Mackintosh makes rather free
use of sea-currents and winds. For example, he holds that a current
coming from Criffel carried with it boulder-laden ice which flowed
south-west to the Isle of Man, south to north Wales, and south-east in
the direction of Blackpool and Manchester, Liverpool and Wolverhampton,
Dawpool and Church Stretton. Now, in the first place, it is very
strange that there is not a vestige or trace of any such submergence,
either in the neighbourhood of Criffel itself or in the region to the
north of it. The whole of that region has been striated and rubbed by
land-ice coming down from the watershed of the Galloway mountains, to
the north of which the striæ, _roches moutonnées_, and tracks followed
by erratics, indicate an ice-flow _towards_ the north-west, north,
and north-east. It is, therefore, absolutely certain that at the time
the granite erratics are supposed to have sailed away from Criffel
on floating-ice, the whole of the Southern Uplands of Scotland were
covered with a great ice-field extending from Wigtown to Berwickshire;
so that, according to Mr. Mackintosh's hypothesis, we should be forced
to believe that an ocean-current originated in Criffel itself! But
waiving this and other insuperable objections which will occur to any
geologist who is familiar with the glacial phenomena of the south
of Scotland, and confining myself to the evidence supplied by the
English drifts, I would remark that Mr. Mackintosh's hypothesis is not
consistent with itself. A current flowing in the direction supposed
could not possibly have permitted floating-ice to sail from Cumbria
to the Isle of Man, to Moel-y-Tryfane and Colwyn Bay. Mr. Mackintosh
admits this himself, but infers that the transport of the Cumbrian
erratics may have taken place at a different time. But how could this
be, seeing that the Criffel and Cumbrian erratics occur side by side
in one and the same deposit? Again, the hypothesis of an ocean-current
coming from Criffel is inconsistent with the presence of the Irish
chalk-flints in the drifts of the west of England. Did these also
come at a different time? And what about the dispersion of erratics
from Great Arenig, which have gone north-east and north-north-east,
almost exactly in the face of the supposed Criffel current? Here an
ocean-current is obviously out of the question; and accordingly we are
told that this dispersion of Welsh boulders was probably the result
of wind. But why should this wind have propelled the floating-ice so
far and no further in an easterly direction? Surely if floating-ice
was swept outwards from Great Arenig as far as Eryrys, bergs must
have been carried now and again much further to the east. And if they
did not sail eastwards, what became of them? Did they all melt away
immediately when they came into the ice-laden current that flowed
towards the south-east?[Y] A still greater difficulty remains. The
Criffel and Cumbrian erratics suddenly cease when they are followed
to the south, great quantities of them being accumulated over a belt
of country extending from beyond Wolverhampton to Bridgenorth. What
was it that defined the southern limits of these northern boulders?
It is clear that it could not have been high-ground, for the Severn
valley, not to speak of low-lying regions further to the north-east,
must have been submerged according to Mr. Mackintosh's hypothesis.
There was therefore plenty of sea-room for the floating-ice to escape
southwards. And yet, notwithstanding this, vast multitudes of bergs and
floes, as soon as they arrived at certain points, suddenly melted away
and dropt their burdens! In what region under the sun does anything
like that happen at the present day? Mr. Mackintosh thinks that the
more or less sharply-defined boundary-line reached by the erratics
"could only have resulted from close proximity to a persistent current
of water (or air?) sufficiently warm to melt the boulder-laden ice."
He does not tell us, however, where this warm current of water or
air came from, or in what direction it travelled. He forgets some of
his own facts connected with the appearance of erratics of eastern
derivation, and which, according to him, point to an ocean-current
that flowed across from Lincolnshire into the very sea in which the
Criffel granite and Cumbrian boulders were being dropt. The supposed
warm ocean-current, then, if such it was rather than air, could hardly
have come from the east. Neither is it at all likely that it could
have come from the west, sheltered as the region of the Severn valley
must have been by the ice-laden mountains of Wales. Again, the south
is shut to us; for there are no erratics in the south of England from
which to infer a submergence of that district. If it be true that
all the northern erratics which are scattered over the low-grounds
of England, Denmark, Holland, Germany, Poland, and Russia, owe their
origin to boulder-laden ice carried by ocean-currents, no such warm
water as Mr. Mackintosh desiderates could possibly have come from the
east or south-east. We are left, then, to infer that the supposed warm
current[Z] must have flowed up the Severn valley directly in the face
of the Criffel current, underneath which it suddenly plunged at a high
temperature, the line of junction between it and the cold water being
sharply defined, and retaining its position unchanged for a long period
of time! However absurd this conclusion may be, it is forced upon us if
we admit the hypothesis at present under review. For we must remember
that the floating-ice is supposed to have melted whenever it came into
contact with the warm current. The erratics occur up to a certain
boundary-line, where they are concentrated in enormous numbers, and
south of which they do not appear. Here, then, large and small floes
alike must have vanished at once! Certainly a very extraordinary case
of dissolution.
[Y] Mr. Mackintosh says nothing about the "carry" or direction of the
erratics in west and south Wales. Were the paths of these erratics
delineated upon a map, we should find it necessary to suppose that the
wind- or sea-current by which the floating-ice was propelled had flowed
outwards in all directions from the dominant heights!
[Z] It must have likewise flowed in more or less direct opposition
to the current which, in accordance with the iceberg hypothesis,
transported boulders southwards from the high-grounds of south Wales!
If we dismiss the notion of a warm ocean-current for that of a warm
wind, we do not improve our position a whit. Where did the warm wind
come from? Not, certainly, from the ice-laden seas to the east. Are
we to suppose, then, that it flowed in from the south or south-west?
If so, we might well ask how it came to pass that in the immediate
proximity of such a very warm wind as the hypothesis demands, great
snow-fields and glaciers were allowed to exist in Wales? Passing that
objection, we have still to ask how this wind succeeded in melting
large and small masses of floating-ice with such rapidity that it
prevented any of them ever trespassing south of a certain line? It is
obvious that it must have been an exceedingly hot wind; and that, just
as the hypothetical warm ocean-current must have suddenly dived under
the cold water coming from the north, so the hot wind, after passing
over the surface of the sea until it reached a certain more or less
well-defined line, must have risen all at once and flowed vertically
upwards into the cold regions above.
Thus, in seeking to escape from what he doubtless considers the
erroneous and extravagant views of "land-glacialists," Mr. Mackintosh
adopts a hypothesis which lands him in self-contradictions and a
perfect "sea of troubles"--a kind of chaos, in fact. In attempting to
explain the drifts of western England and east Wales he has ignored
the conditions that must have obtained in contiguous regions--thus
forgetting that "nothing in the world is single," and that one ought
not to infer physical conditions for one limited area without stopping
to inquire whether these are in consonance with what is known of
adjacent districts, or in harmony with the existing phenomena of nature.
I have so strongly opposed Mr. Mackintosh's explanation of the
sudden termination of the northern erratics in the neighbourhood
of Wolverhampton and elsewhere, that perhaps I ought to offer an
explanation of my own, that it may, in its turn, undergo examination.
I labour under the disadvantage, however, of not having studied the
drifts in and around Wolverhampton, etc., and the suggestion which I
shall throw out must therefore be taken for what it is worth. It seems
to me, then, that the concentration of boulders in the neighbourhood
of Wolverhampton, and the limits reached by the northern erratics
generally, mark out, in all probability, the line of junction between
the _mer de glace_ coming from the basin of the Irish Sea and that
flowing across the country from the vast _mer de glace_ that occupied
the basin of the German Ocean. Along this line the southerly transport
of the northern boulders would cease, and here they would therefore
tend to become concentrated. But it is most likely that now and again
they would get underneath the ice-flow that set down the Severn valley,
and I should anticipate that they will yet be detected, along with
erratics of eastern origin, as far south even as the Bristol Channel.
If it be objected to this view that erratics from Great Arenig have
been met with south of Wolverhampton, at Birmingham and Bromsgrove,
I would reply that these erratics were probably carried south either
before or after the general _mer de glace_ had attained its climax--at
a period when the Welsh ice was able to creep out further to the east
than it could when the invasion of the North Sea ice was at its height.
I cannot conclude this paper without expressing my admiration for the
long-continued and successful labours of the well-known geologist
whose views I have been controverting. Although I have entered my
protest against his iceberg hypothesis, and have freely criticised his
theoretical opinions, I most willingly admit that the practical results
of his unwearied devotion to the study of those interesting phenomena
with which he is so familiar have laid all his fellow-workers under a
debt of gratitude.
VIII.
Recent Researches in the Glacial Geology of the Continent.[AA]
[AA] Presidential Address to the Geological Section of the British
Association, Newcastle, 1889.
THE President of this section must often have some difficulty in
selecting a subject for his address. It is no longer possible to give
an interesting and instructive summary of the work done by the devotees
of our science during even one year. So numerous have the students of
geological science become--so fertile are the fields they cultivate--so
abundant the harvests they reap, that one in my present position may
well despair of being able to take stock of the numerous additions to
our knowledge which have accumulated within the last twelve months.
Neither is there any burning question which at this time your President
need feel called upon to discuss. True, there are controversies that
are likely to remain unsettled for years to come--there are still not a
few matters upon which we must agree to differ--we do not yet see eye
to eye in all things geological. But experience has shown that as years
advance truth is gradually evolved, and old controversies die out, and
so doubtless it will continue to be. The day when controversies shall
cease, however, is yet, I hope, far in the future; for should that dull
and unhappy time ever arrive, it is quite certain that mineralogists,
petrologists, palæontologists, and geologists shall have died out of
the world. Following the example of many of my predecessors, I shall
confine my remarks to certain questions in which I have been specially
interested; and in doing so I shall endeavour to steer clear, as far
as I can, of controversial matters. My purpose, then, is to give an
outline of some of the results obtained during the last few years by
Continental workers in the domain of glacial geology.
Those who are not geologists will probably smile when they hear one
declare that wielders of the hammer are extremely conservative--that
they are slow to accept novel views, and very tenacious of opinions
which have once found favour in their eyes. Nevertheless, such is the
case, and well for us that it is so. However captivating, however
imposing, however strongly supported by evidence a new view may appear
to be, we do well to criticise, to sift the evidence, and to call for
more facts and experiments, if such are possible, until the proofs
become so strong as to approach as near a demonstration as geologists
can in most cases expect such proofs to go. The history of our science,
and indeed of most sciences, affords abundant illustration of what
I say. How many long years were the views of sub-aërial erosion, as
taught by Hutton and Playfair, canvassed and controverted before
they became accepted! And even after their general soundness had
been established, how often have we heard nominal disciples of these
fathers of physical geology refuse to go so far as to admit that the
river-valleys of our islands have been excavated by epigene agents!
If, as a rule, it takes some time for a novel view to gain acceptance,
it is equally true that views which have long been held are only with
difficulty discarded. Between the new and the old there is a constant
struggle for existence, and if the latter should happen to survive,
it is only in a modified form. I have often thought that a history of
the evolution of geological theories would make a very entertaining
and instructive work. We should learn from it, amongst other things,
that the advance of our science has not always been continuous--now
and again, indeed, it has almost seemed as if the movement had been
retrograde. Knowledge has not come in like an overwhelming flood--as
a broad majestic river--but rather like a gently-flowing tide, now
advancing, now retiring, but ever, upon the whole, steadily gaining
ground. The history I speak of would also teach us that many of
the general views and hypotheses which have been from time to time
abandoned as unworkable, are hardly deserving of the reproach and
ridicule which we in these latter days may be inclined to cast upon
them. As the Scots proverb says: "It is easy to be wise behindhand."
It could be readily shown that not a few discarded notions and
opinions have frequently worked for good, and have rather stimulated
than checked inquiry. Such reflections should be encouraging to every
investigator, whether he be a defender of the old or an advocate of the
new. Time tries all, and each worker may claim a share in the final
establishment of the truth.
Perhaps there is no department of geological inquiry that has given
rise to more controversy than that which I have selected for the
subject of this address. Hardly a single step in advance has been
taken without vehement opposition. But the din of contending sides is
not so loud now--the dust of the conflict has to some extent cleared
away, and the positions which have been lost or maintained, as the case
may be, can be readily discerned. The glacialist who can look back
over the last twenty-five years of wordy conflict has every reason to
be jubilant and hopeful. Many of those who formerly opposed him have
come over to his side. It is true he has not had everything his own
way. Some extreme views have been abandoned in the struggle; that of a
great Polar ice-sheet, for example, as conceived of by Agassiz. I am
not aware, however, that many serious students of glacial geology ever
adopted that view. But it was quite an excusable hypothesis, and has
been abundantly suggestive. Had Agassiz lived to see the detailed work
of these later days, he would doubtless have modified his notion and
come to accept the view of large continental glaciers which has taken
its place.
The results obtained by geologists who have been studying the
peripheral areas of the drift-covered regions of our Continent,
are such as to satisfy us that the drifts of those regions are not
iceberg-droppings, as we used to suppose, but true morainic matter
and fluvio-glacial detritus. Geologists have not jumped to this
conclusion--they have only accepted it after laborious investigation
of the evidence. Since Dr. Otto Torell, in 1875, first stated his
belief that the Diluvium of north Germany was of glacial origin a
great literature on the subject has sprung up, a perusal of which will
show that with our German friends glacial geology has passed through
much the same succession of controversial phases as with us. At first
icebergs are appealed to as explaining everything--next we meet with
sundry ingenious attempts at a compromise between floating-ice and a
continuous ice-sheet. As observations multiply, however, the element
of floating-ice is gradually eliminated, and all the phenomena are
explained by means of land-ice and "Schmelz-wasser" alone. It is a
remarkable fact that the iceberg hypothesis has always been most
strenuously upheld by geologists whose labours have been largely
confined to the peripheral areas of drift-covered countries. In the
upland and mountainous tracts, on the other hand, that hypothesis has
never been able to survive a moderate amount of accurate observation.
Even in Switzerland--the land of glaciers--geologists at one time
were of opinion that the boulder-clays of the low-grounds had a
different origin from those which occur in the mountain-valleys.
Thus, it was supposed that at the close of the Pleistocene period the
Alps were surrounded by great lakes or by gulfs of some inland sea,
into which the glaciers of the high valleys flowed and calved their
icebergs--these latter scattering erratics and earthy débris over
the drowned areas. Sartorius von Waltershausen[AB] set forth this
view in an elaborate and well-illustrated paper. Unfortunately for
his hypothesis no trace of the supposed great lakes or the inland
sea has ever been detected: on the contrary, the character of the
morainic accumulations, and the symmetrical grouping and radiation of
the erratics and perched blocks over the foot-hills and low-grounds,
show that these last have been invaded and overflowed by the glaciers
themselves. Even the most strenuous upholders of the efficacy of
icebergs as originators of some boulder-clays, admit that the
boulder-clay or till, of what we may call the inner or central region
of a glaciated tract is the product of land-ice. Under this category
comes the boulder-clay of Norway, Sweden, and Finland, and of the
Alpine Lands of central Europe, not to speak of the hilly parts of our
own islands.
[AB] "Untersuchungen über die Klimate der Gegenwart und der Vorwelt,"
etc.--_Natuurkundige Verhandelingen v. d. Holland. Maatsch. d.
Wetensch. te Haarlem_, 1865.
When we come to study the drifts of the peripheral areas, it is
not difficult to see why these should be considered to have had a
different origin. They present certain features which, although not
absent from the glacial deposits of the inner region, are not nearly
so characteristic of such upland tracts. I refer especially to the
frequent interstratification of boulder-clays with well-bedded deposits
of clay, sand, and gravel; and to the fact that these boulder-clays are
often less compressed than those of the inner region, and have even
occasionally a silt-like character. Such appearances do seem at first
to be readily explained on the assumption that the deposits have been
accumulated in water opposite the margin of a continental glacier or
ice-sheet--and this was the view which several able investigators in
Germany were for some time inclined to adopt.
But when the phenomena came to be studied in greater detail, and over
a wider area, this preliminary hypothesis did not prove satisfactory.
It was discovered, for example, that "giants' kettles"[AC] were more
or less commonly distributed under the glacial deposits, and such
"kettles" could only have originated at the bottom of a glacier. Again,
it was found that pre-glacial accumulations were plentifully developed
in certain places below the drift, and were often involved with the
latter in a remarkable way. The "brown-coal formation" in like manner
was violently disturbed and displaced, to such a degree that frequently
the boulder-clay is found to underlie it. Similar phenomena were
encountered in regions where the drift overlies the Chalk--the latter
presenting the appearance of having been smashed and shattered--the
fragments having often been dragged some distance, so as to form a kind
of friction-breccia underlying the drift, while large masses are often
included in the clay itself. All the facts pointed to the conclusion
that these disturbances were due to tangential thrusting or crushing,
and were not the result of vertical displacements, such as are produced
by normal faulting, for the disturbances in question die out from above
downwards. Evidence of similar thrusting or crushing is seen in the
remarkable faults and contortions that so often characterise the clays
and sands that occur in the boulder-clay itself. The only agent that
could produce the appearances, now briefly referred to, is land-ice,
and we must therefore agree with German geologists that glacier-ice
has overflowed all the drift-covered regions of the peripheral area.
No evidence of marine action in the formation of the stony clays is
forthcoming--not a trace of any sea-beach has been detected. And yet,
if these clays had been laid down in the sea during the retreat of
the ice-sheet from Germany, surely such evidence as I have indicated
ought to be met with. To the best of my knowledge the only particular
facts which have been appealed to, as proofs of marine action, are the
appearance of bedded deposits in the boulder-clays, and the occasional
occurrence in the clays themselves of a sea-shell. But other organic
remains are also met with now and again in similar positions, such as
mammalian bones and freshwater shells. All these, however, have been
shown to be derivative in their origin--they are just as much erratics
as the stones and boulders with which they are associated. The only
phenomena, therefore, that the glacialist has to account for are the
bedded deposits which occur so frequently in the boulder-clays of the
peripheral regions, and the occasional silty and uncompressed character
of the clays themselves.
[AC] These appear to have been first detected by Professor Berendt and
Professor E. Geinitz.
The intercalated beds are, after all, not hard to explain. If
we consider for a moment the geographical distribution of the
boulder-clays, and their associated aqueous deposits, we shall find
a clue to their origin. Speaking in general terms, the stony clays
thicken out as they are followed from the mountainous and high-lying
tracts to the low-grounds. Thus they are of inconsiderable thickness
in Norway, the higher parts of Sweden, and in Finland, just as we find
is the case in Scotland, northern England, Wales, and the hilly parts
of Ireland. Traced south from the uplands of Scandinavia and Finland,
they gradually thicken out as the low-grounds are approached. Thus in
southern Sweden they reach a thickness of 43 metres or thereabout,
and of 80 metres in the northern parts of Prussia, while over the
wide low-lying regions to the south they attain a much greater
thickness--reaching in Holstein, Mecklenburg, Pomerania, and west
Prussia, a depth of 120 to 140 metres, and still greater depths in
Hanover, Mark Brandenburg, and Saxony. In those regions, however,
a considerable portion of the diluvium consists, as we shall see
presently, of water-formed beds.
The geographical distribution of the aqueous deposits, which are
associated with the stony clays, is somewhat similar. They are very
sparingly developed in districts where the boulder-clays are thin. Thus
they are either wanting, or only occur sporadically in thin irregular
beds, in the high-grounds of northern Europe generally. Further
south, however, they gradually acquire more importance, until in the
peripheral regions of the drift-covered tracts they come to equal and
eventually to surpass the boulder-clays in prominence. These latter,
in fact, at last cease to appear, and the whole bulk of the diluvium,
along the southern margin of the drift area, appears to consist of
aqueous accumulations alone.
The explanations of these facts advanced by German geologists
are quite in accordance with the views which have long been held
by glacialists elsewhere, and have been tersely summed up by Dr.
Jentzsch.[AD] The northern regions, he says, were the feeding-grounds
of the inland-ice. In those regions melting was at a minimum, while the
grinding action of the ice was most effective. Here, therefore, erosion
reached its maximum--ground-moraine or boulder-clay being unable to
accumulate to any thickness. Further south melting greatly increased,
while ground-moraine at the same time tended to accumulate--the
conjoint action of glacier-ice and sub-glacial water resulting in
the complex drifts of the peripheral area. In the disposition and
appearance of the aqueous deposits of the diluvium we have evidence
of an extensive sub-glacial water-circulation--glacier-mills that
gave rise to "giants' kettles"--chains of sub-glacial lakes in which
fine clays gathered--streams and rivers that flowed in tunnels under
the ice, and whose courses were paved with sand and gravel. Nowhere
do German geologists find any evidence of marine action. On the
contrary, the dovetailing and interosculation of boulder-clay with
aqueous deposits are explained by the relation of the ice to the
surface over which it flowed. Throughout the peripheral area it did
not rest so continuously upon the ground as was the case in the inner
region of maximum erosion. In many places it was tunnelled by rapid
streams and rivers, and here and there it arched over sub-glacial
lakes, so that accumulation of ground-moraine proceeded side by side
with the formation of aqueous sediments. Much of that ground-moraine
is of the usual tough and hard-pressed character, but here and there
it is somewhat less coherent and even silt-like. Now a study of the
ground-moraines of modern glaciers affords us a reasonable explanation
of such differences. Dr. Brückner[AE] has shown that in many places the
ground-moraine of the Alpine glaciers is included in the bottom of
the ice itself. The ground-moraine, he says, frequently appears as an
ice-stratum abundantly impregnated with silt and rock-fragments--it is
like a conglomerate or breccia which has ice for its binding material.
When this ground-moraine melts out of the ice--no running water being
present--it forms a layer of unstratified silt or clay, with stones
scattered irregularly through it. Such being the case in modern
glaciers, we can hardly doubt that over the peripheral areas occupied
by the old northern ice-sheet boulder-clay must frequently have been
accumulated in the same way. Nay, when the ground-moraine melted out
and dropt here and there into quietly-flowing water it might even
acquire in part a bedded character.
[AD] _Jahrb. d. königl. preuss. geologischen Landesanstalt für 1884_,
p. 438.
[AE] "Die Vergletscherung des Salzachgebietes, etc.": _Geographische
Abhandlungen herausgegeben v. A. Penck_, Band i. Heft 1.
The limits reached by the inland-ice during its greatest extension are
becoming more and more clearly defined, although its southern margin
will probably never be so accurately determined as that of the latest
epoch of general glaciation. The reasons for this are obvious. When
the inland-ice flowed south to the Harz and the hills of Saxony it
formed no great terminal moraines. Doubtless many erratics and much
rock-rubbish were showered upon the surface of the ice from the higher
mountains of Scandinavia, but owing to the fanning-out of the ice on
its southward march, such superficial débris was necessarily spread
over a constantly-widening area. It may well be doubted, therefore,
whether it ever reached the terminal front of the ice-sheet in
sufficient bulk to form conspicuous moraines. It seems most probable
that the terminal moraines of the great inland-ice would consist of
low banks of boulder-clay and aqueous materials-the latter, perhaps,
strongly predominating, and containing here and there larger and
smaller angular erratics which had travelled on the surface of the
ice. However that may be, it is certain that the whole region in
question has been considerably modified by subsequent denudation, and
to a large extent is now concealed under deposits belonging to later
stages of the Pleistocene period. The extreme limits reached by the ice
are determined rather by the occasional presence of rock-striæ and
_roches moutonnées_, of boulder-clay and northern erratics, than by
recognisable terminal moraines. The southern limits reached by the old
inland-ice appear in this way to have been tolerably well ascertained
over a considerable portion of central Europe. Some years ago I
published a small sketch-map[AF] showing the extent of surface formerly
covered by ice. On this map I did not venture to draw the southern
margin of the ice-sheet in Belgium further south than Antwerp, where
northern erratics were known to occur, but the more recent researches
of Belgian geologists show that the ice probably flowed south for some
little distance beyond Brussels.[AG] Here and there in other parts of
the Continent the southern limits reached by the northern drift have
also been more accurately determined, but, so far as I know, none of
these later observations involves any serious modification of the
sketch-map referred to.
[AF] _Prehistoric Europe_, 1881.
[AG] See a paper by M. E. Delvaux: _Ann. de la. Soc. géol. de Belg._,
t. xiii. p. 158.
I have now said enough, however, to show that the notion of a general
ice-sheet having covered so large a part of Europe, which a few years
ago was looked upon as a wild dream, has been amply justified by the
labours of those who are so assiduously investigating the peripheral
areas of the "great northern drift." And perhaps I may be allowed
to express my own belief that the drifts of middle and southern
England, which exhibit the same complexity as the Lower Diluvium of
the Continent, will eventually be generally acknowledged to have had
a similar origin. I have often thought that whilst politically we are
happy in having the sea all round us, geologically we should have
gained perhaps by its greater distance. At all events we should have
been less ready to invoke its assistance to explain every puzzling
appearance presented by our glacial accumulations.
I now pass on to review some of the general results obtained by
continental geologists as to the extent of area occupied by inland-ice
during the last great extension of glacier-ice in Europe. It is
well known that this latest ice-sheet did not overflow nearly so
wide a region as that underneath which the lowest boulder-clay was
accumulated. This is shown not only by the geographical distribution
of the youngest boulder-clay, but by the direction of rock-striæ, the
trend of erratics, and the position of well-marked terminal moraines.
Gerard de Geer has given a summary[AH] of the general results obtained
by himself and his fellow-workers in Sweden and Norway; and these have
been supplemented by the labours of Berendt, E. Geinitz, Hauchecorne,
Keilhack, Klockmann, Schröder, Wahnschaffe, and others in Germany, and
by Sederholm in Finland.[AI] From them we learn that the end-moraines
of the ice circle round the southern coasts of Norway, from whence
they sweep south-east by east across the province of Gottland in
Sweden, passing through the lower ends of Lakes Wener and Wetter, while
similar moraines mark out for us the terminal front of the inland-ice
in Finland--at least two parallel frontal moraines passing inland from
Hango Head on the Gulf of Finland through the southern part of that
province to the north of Lake Ladoga. Further north-east than this
they have not been traced; but, from some observations by Helmersen,
Sederholm thinks it probable that the terminal ice-front extended
north-east by the north of Lake Onega to the eastern shores of the
White Sea. Between Sweden and Finland lies the basin of the Baltic,
which at the period in question was filled with ice, forming a great
Baltic glacier which overflowed the Öland Islands, Gottland, and Öland,
and which, fanning-out as it passed towards the south-west, invaded, on
the south side, the Baltic provinces of Germany, while, on the north,
it crossed the southern part of Scania in Sweden and the Danish islands
to enter Jutland.
[AH] _Zeitschrift d. deutsch. geolog. Ges._, Bd. xxxvii., p. 177.
[AI] For papers by Berendt and his associates see especially the
_Jahrbuch d. k. preuss. geol. Landesanstalt_, and the _Zaitschr. d.
deutsch. geol. Ges._ for the past few years. Geinitz: _Forsch. z. d.
Lands- u. Volkskunde_, i. 5; _Leopoldina_, xxii., p. 37; I. _Beitrag z.
Geologie Mecklenburgs_, 1880, pp. 46, 56. Sederholm: _Fennia_, I. No.
7.
The upper boulder-clay of those regions is now recognised as the
ground-moraine of this latest ice-sheet. In many places it is separated
from the older boulder-clay by interglacial deposits--some of which are
marine, while others are of freshwater and terrestrial origin. During
interglacial times the sea that overflowed a considerable portion of
north Germany was evidently continuous with the North Sea, as is shown
not only by the geographical distribution of the interglacial marine
deposits, but by their North Sea fauna. German geologists generally
group all the interglacial deposits together, as if they belonged to
one and the same interglacial epoch. This perhaps we must look upon
as only a provisional arrangement. Certain it is that the freshwater
and terrestrial beds which frequently occur on the same or a lower
level, and at no great distance from the marine deposits, cannot in
all cases be contemporaneous with the latter. Possibly, however, such
discordances may be accounted for by oscillations in the level of the
interglacial sea--land and water having alternately prevailed over the
same area. Two boulder-clays, as we have seen, have been recognised
over a wide region in the north of Germany. In some places, however,
three or more such boulder-clays have been observed overlying one
another throughout considerable areas, and these clays are described
as being distinctly separate and distinguishable the one from the
other.[AJ] Whether they, with their intercalated aqueous deposits,
indicate great oscillations of one and the same ice-sheet--now
advancing, now retreating--or whether the stony clays may not be the
ground-moraines of so many different ice-sheets, separated the one from
the other by true interglacial conditions, future investigations must
be left to decide.
[AJ] H. Schröder: _Jahrb. d. k. preuss. geol. Landesanstalt für 1887_ ,
p. 360.
The general conclusions arrived at by those who are at present
investigating the glacial accumulations of northern Europe may be
summarised as follows:--
1. Before the invasion of northern Germany by the inland-ice the
low-grounds bordering on the Baltic were overflowed by a sea which
contained a boreal and arctic fauna. These marine conditions are
indicated by the presence under the lower boulder-clay of more or
less well-bedded fossiliferous deposits. On the same horizon occur
also beds of sand, containing freshwater shells, and now and again
mammalian remains, some of which imply cold and others temperate
climatic conditions. Obviously all these deposits may pertain to one
and the same period, or more properly to different stages of the
same period--some dating back to a time when the climate was still
temperate, while others clearly indicate the prevalence of cold
conditions, and are therefore probably somewhat younger.
2. The next geological horizon in ascending order is that which
is marked by the Lower Diluvium--the glacial and fluvio-glacial
detritus of the great ice-sheet which flowed south to the foot of
the Harz Mountains. The boulder-clay on this horizon now and again
contains marine, freshwater, and terrestrial organic remains--derived
undoubtedly from the so-called pre-glacial beds already referred
to. These latter, it would appear, were ploughed up and largely
incorporated with the old ground-moraine.
3. The interglacial beds which next succeed contain remains of a
well-marked temperate fauna and flora, which point to something
more than a mere partial or local retreat of the inland-ice. The
geographical distribution of the beds, and the presence in these of
such forms as _Elephas antiquus_, _Cervus elephas_, _C. megaceros_,
and a flora comparable to that now existing in northern Germany,
justify geologists in concluding that the interglacial epoch was one
of long duration, and characterised in Germany by climatic conditions
apparently not less temperate than those that now obtain. One of the
phases of that interglacial epoch, as we have seen, was the overflowing
of the Baltic provinces by the waters of the North Sea.
4. To this well-marked interglacial epoch succeeded another epoch of
arctic conditions, when the Scandinavian inland-ice once more invaded
Germany, ploughing through the interglacial deposits, and working
these up in its ground-moraine. So far as I can learn, the prevalent
belief among geologists in north Germany is that there was only one
interglacial epoch; but, as already stated, doubt has been expressed
whether all the facts can be thus accounted for. There must always be
great difficulty in the correlation of widely-separated interglacial
deposits, and the time does not seem to me to have yet come when we can
definitely assert that all those interglacial beds belong to one and
the same geological horizon.
I have dwelt upon the recent work of geologists in the peripheral areas
of the drift-covered regions of northern Europe, because I think the
results obtained are of great interest to glacialists in this country.
And for the same reason I wish next to call attention to what has been
done of late years in elucidating the glacial geology of the Alpine
Lands of central Europe--and more particularly of the low-grounds that
stretch out from the foot of the mountains. Any observations that
tend to throw light upon the history of the complex drifts of our own
peripheral areas cannot but be of service. It is quite impossible to do
justice in this brief sketch to the labours of the many enthusiastic
geologists who within recent years have increased our knowledge of the
glaciation of the Alpine Lands. At present, however, I am not so much
concerned with the proofs of general glaciation as with the evidence
that goes to show how the Alpine ground-moraines have been formed,
and with the facts which have led certain observers to conclude that
the Alps have endured several distinct glaciations within Pleistocene
times. Swiss geologists are agreed that the ground-moraines which
clothe the bottoms of the great Alpine valleys, and extend outwards
sometimes for many miles upon the low-grounds beyond, are of true
glacial origin. Now these ground-moraines are closely similar to the
boulder-clays of this country and northern Europe--like them, they are
frequently tough and hard-pressed, but now and again somewhat looser,
and less firmly coherent. Frequently also they contain lenticular
beds, and more or less thick sheets of aqueous deposits--in some places
the stony clays even exhibiting a kind of stratification--and ever and
anon such water-assorted materials are commingled with stony clay in
the most complex manner. These latter appearances are, however, upon
the whole best developed upon the low-grounds that sweep out from the
base of the Alps. The only question concerning the ground-moraines
that has recently given rise to much discussion is the origin of the
materials themselves. It is obvious that there are only three possible
modes in which those materials could have been introduced to the
ground-moraine: either they consist of superficial morainic débris
which has found its way down to the bottom of the old glaciers by
crevasses; or they may be made up of the rock-rubbish, shingle, gravel,
etc., which doubtless strewed the valleys before these were occupied by
ice; or, lastly, they may have been derived in chief measure from the
underlying rocks themselves by the action of the ice that overflowed
them. The investigations of Penck, Blaas, Böhm, and Brückner appear to
me to have demonstrated that the ground-moraines are composed mostly
of materials which have been detached from the underlying rocks by
the erosive action of the glaciers themselves. Their observations
show that the regions studied by them in great detail were almost
completely buried under ice--so that the accumulation of superficial
moraines was for the most part impossible; and they advance a number
of facts which prove positively that the ground-moraines were formed
and accumulated under ice. I cannot here recapitulate the evidence,
but must content myself by a reference to the papers in which this is
fully discussed.[AK] These geologists do not deny that some of the
material may occasionally have come from above, nor do they doubt that
pre-existing masses of rock-rubbish and alluvial accumulations may also
have been incorporated with the ground-moraines; but the enormous
extent of the latter, and the direction of transport and distribution
of the erratics which they contain cannot be thus accounted for, while
all the facts are readily explained by the action of the ice itself,
which used its sub-glacial débris as tools with which to carry on the
work of erosion.
[AK] Penck: _Die Vergletscherung der deutschen Alpen._ Blaas:
_Zeitschrift d. Ferdinandeums_, 1885. Böhm: _Jahrb. d. k. k. geol.
Reichsanstalt_, 1885, Bd. xxxv., Heft 3. Brückner: _Die Vergletscherung
d. Salzachgebietes, etc._, 1886.
Professor Heim and others have frequently asserted that glaciers
have little or no eroding power, since at the lower ends of existing
glaciers we find no evidence of such erosion being in operation. But
the chief work of a glacier cannot be carried on at its lower end,
where motion is reduced to a minimum, and where the ice is perforated
by sub-glacial tunnels and arches, underneath which no glacial erosion
can possibly take place; and yet it is upon observations made in
just such places that the principal arguments against the erosive
action of glaciers have been based. If all that we could ever know of
glacial action were confined to what we can learn from peering into
the grottoes at the terminal fronts of existing glaciers, we should
indeed come to the conclusion that glaciers do not erode their rocky
beds to any appreciable extent. But as we do not look for the strongest
evidence of fluviatile erosion at the mouth of a river, but in its
valley--and mountain-tracks, so if we wish to learn what glacier-ice
can accomplish, we must study in detail some wide region from which the
ice has completely disappeared. When this plan has been followed, it
has happened that some of the strongest opponents of glacial erosion
have been compelled by the force of the evidence to go over to the
other camp. Dr. Blaas, for example, has been led by his observations
on the glacial formations of the Inn valley to recant his former
views, and to become a formidable advocate of the very theory which
he formerly opposed. To his work and the memoirs by Penck, Brückner,
and Böhm already cited, and especially to the admirable chapter on
glacier-erosion by the last-named author, I would refer those who may
be anxious to know the last word on this much-debated question.
The evidence of interglacial conditions within the Alpine lands
continues to increase. These are represented by alluvial deposits of
silt, sand, gravel, conglomerate, breccia, and lignites. Penck, Böhm,
and Brückner find evidence of two interglacial epochs, and maintain
that there have been three distinct and separate epochs of glaciation
in the Alps. No mere temporary retreat and re-advance of the glaciers,
according to them, will account for the various phenomena presented by
the interglacial deposits and associated morainic accumulations. During
interglacial times the glaciers disappeared from the lower valleys
of the Alps--the climate was temperate, and probably the snow-fields
and glaciers approximated in extent to those of the present day. All
the evidence conspires to show that an interglacial epoch was of
prolonged duration. Dr. Brückner has observed that the moraines of
the last glacial epoch rest here and there upon löss, and he confirms
Penck's observations in south Bavaria that this remarkable formation
never overlies the morainic accumulations of the latest glacial epoch.
According to Penck and Brückner, therefore, the löss is of interglacial
age. There can be little doubt, however, that löss does not belong to
any one particular horizon. Wahnschaffe[AL] and others have shown that
throughout wide areas in north Germany it is the equivalent in age of
the Upper Diluvium, while Schumacher[AM] points out that in the Rhine
valley it occurs on two separate and distinct horizons. Professor
Andreæ has likewise shown[AN] that there is an upper and lower löss in
Alsace--each characterised by its own special fauna.
[AL] _Abhandl. z. geol. Specialkarte v. Preussen_, etc., Bd. vii. Heft
1; _Zeitschr. d. Zeutsch. geol. Ges._, 1885, p. 904; 1886, p. 367.
[AM] _Hygienische Topographie von Strassburg i. E._, 1885.
[AN] _Abhandl. z. geol. Specialkarte a. Elsass-Lothringen_, Bd. iv.
Heft 2.
There is still considerable difference of opinion as to the mode of
formation of this remarkable accumulation. By many it is considered to
be an aqueous deposit; others, following Richthofen, are of opinion
that it is a wind-blown accumulation; while some incline to the
belief that it is partly the one and partly the other. Nor do the
upholders of these various hypotheses agree amongst themselves as to
the precise manner in which water or wind has worked to produce the
observed results. Thus, amongst the supporters of the aqueous origin
of the löss, we find this attributed to the action of heavy rains
washing over and rearranging the material of the boulder-clays.[AO]
Many, again, have held it probable that löss is simply the finest loam
distributed over the low-grounds by the flood-waters that escaped from
the northern inland-ice and the _mers de glace_ of the Alpine lands of
central Europe. Another suggestion is that much of the material of the
löss may have been derived from the denudation of the boulder-clays by
flood-water, during the closing stages of the last cold period. It is
pointed out that in some regions, at least, the löss is underlaid by a
layer of erratics, which are believed to be the residue of the denuded
boulder-clay. We are reminded by Klockmann[AP] and Wahnschaffe[AQ] that
the inland-ice must have acted as a great dam, and that wide areas
in Germany, etc., would be flooded, partly by water derived from the
melting inland-ice, and partly by waters flowing north from the hilly
tracts of middle Germany. In the great basins thus formed there would
be a commingling of fine silt material derived from north and south,
which would necessarily come to form a deposit having much the same
character throughout.
[AO] Laspeyres: _Erläuterungen z. geol Specialkaret v Preussen_, etc.,
_Blatt. Gröbzig, Zörbig, und Petersberg_.
[AP] Klockmann: _Jahrb. d. k. preuss. geol. Landesanstalt für 1883_, p.
262.
[AQ] Wahnschaffe: _Op. cit._, and _Zeitschr. d. deutsch. geol. Ges._,
1886, p. 367.
From what I have myself seen of the löss in various parts of Germany,
and from all that I have gathered from reading and in conversation
with those who have worked over löss-covered regions, I incline to
the opinion that löss is for the most part of aqueous origin. In many
cases this can be demonstrated, as by the occurrence of bedding and
the intercalation of layers of stones, sand, gravel, etc., in the
deposit; again, by the not infrequent appearance of freshwater shells;
but, perhaps, chiefly by the remarkable uniformity of character which
the löss itself displays. It seems to me reasonable also to believe
that the flood-waters of glacial times must needs have been highly
charged with finely-divided sediment, and that such sediment would be
spread over wide regions in the low-grounds--in the slackwaters of the
great rivers and in the innumerable temporary lakes which occupied,
or partly occupied, many of the valleys and depressions of the land.
There are different kinds of löss or löss-like deposits, however,
and all need not have been formed in the same way. Probably some may
have been derived, as Wahnschaffe has suggested, from denudation of
boulder-clay. Possibly also, some löss may owe its origin to the
action of rain on the stony clays, producing what we in this country
would call "rain-wash." There are other accumulations, however, which
no aqueous theory will satisfactorily explain. Under this category
comes much of the so-called _Berglöss_, with its abundant land-shells,
and its generally unstratified character. It seems likely that such
löss is simply the result of sub-aërial action, and owes its origin
to rain, frost, and wind acting upon the superficial formations, and
rearranging their finer-grained constituents. And it is quite possible
that the upper portion of much of the löss of the lower-grounds may
have been re-worked in the same way. But I confess I cannot yet find in
the facts adduced by German geologists any evidence of a dry-as-dust
epoch having obtained in Europe during any stage of the Pleistocene
period. The geographical position of our Continent seems to me to
forbid the possibility of such climatic conditions, while all the
positive evidence we have points to humidity rather than dryness as
the prevalent feature of Pleistocene climates. It is obvious, however,
that after the flood-waters had disappeared from the low-grounds of
the Continent, sub-aërial action would come into play over the wide
regions covered by the glacial and fluvio-glacial deposits. Thus, in
the course of time, these deposits would become modified,--just as
similar accumulations in these islands have been top-dressed, as it
were, and to some extent even rearranged. I am strengthened in these
views by the conclusions arrived at by M. Falsan--the eminent French
glacialist. Covering the plateaux of the Dombes, and widely spread
throughout the valleys of the Rhone, the Ain, the Isère, etc., in
France there is a deposit of löss, he says, which has been derived from
the washing of the ancient moraines. At the foot of the Alps, where
black schists are largely developed, the löss is dark grey, but west of
the secondary chain the same deposit is yellowish, and composed almost
entirely of silicious materials, with only a very little carbonate of
lime. This _limon_ or löss, however, is very generally modified towards
the top by the chemical action of rain--the yellow löss acquiring a
red colour. Sometimes it is crowded with calcareous concretions, but
at other times it has been deprived of its calcareous element and
converted into a kind of pulverulent silica or quartz. This, the true
löss, is distinguished from another _lehm_, which Falsan recognises as
the product of atmospheric action--formed, in fact, _in situ_, from the
disintegration and decomposition of the subjacent rocks. Even this lehm
has been modified by running water--dispersed or accumulated locally,
as the case may be.[AR]
[AR] Falsan: _La Période glaciaire_, p. 81.
All that we know of the löss and its fossils compels us to include
this accumulation as a product of the Pleistocene period. It is not
of post-glacial age--even much of what one may call the "remodified
löss" being of late Glacial or Pleistocene age. I cannot attempt to
give here a summary of what has been learned within recent years as
to the fauna of the löss. The researches of Nehring and Liebe have
familiarised us with the fact that, at some particular stage in the
Pleistocene period, a fauna like that of the alpine steppe-lands
of western Asia was indigenous to middle Europe, and the recent
investigations by Woldrich have increased our knowledge of this fauna.
At what horizon, then, does this steppe-fauna make its appearance?
At Thiede Dr. Nehring discovered in so-called löss three successive
horizons, each characterised by a special fauna. The lowest of these
faunas was decidedly arctic in type; above that came a steppe-fauna,
which last was succeeded by a fauna comprising such forms as mammoth,
woolly rhinoceros, _Bos_, _Cervus_, horse, hyæna, and lion. Now, if
we compare this last fauna with the forms which have been obtained
from true post-glacial deposits--those deposits, namely, which overlie
the younger boulder-clays and flood-accumulations of the latest
glacial epoch, we find little in common. The lion, the mammoth, and
the rhinoceros are conspicuous by their absence from the post-glacial
beds of Europe. In place of them we meet with a more or less arctic
fauna, and a high-alpine and arctic flora, which as we all know
eventually gave place to the flora and fauna with which Neolithic man
was contemporaneous. As this is the case throughout north-western and
central Europe, we seem justified in assigning the Thiede beds to the
Pleistocene period, and to that interglacial stage which preceded and
gradually merged into the last glacial epoch. That the steppe-fauna
indicates relatively drier conditions of climate than obtained when
perennial snow and ice covered wide areas of the low-ground goes
without saying, but I am unable to agree with those who maintain
that it implies a dry-as-dust climate, like that of some of the
steppe-regions of our own day. The remarkable commingling of arctic-
and steppe-faunas discovered in the Böhmer-Wald[AS] by Woldrich shows,
I think, that the jerboas, marmots, and hamster-rats were not incapable
of living in the same regions contemporaneously with lemmings,
arctic hares, Siberian social voles, etc. But when a cold epoch was
passing away the steppe-forms probably gradually replaced their
arctic congeners, as these migrated northwards during the continuous
amelioration of the climate.
[AS] Woldrich: _Sitzungsb. d. kais. Akad. d. W. math. nat. Cl._, 1880,
p. 7; 1881, p. 177; 1883, p. 978.
If the student of the Pleistocene faunas has certain advantages in the
fact that he has to deal with forms many of which are still living,
he labours at the same time under disadvantages which are unknown to
his colleagues who are engaged in the study of the life of far older
periods. The Pleistocene period was distinguished above all things
by its great oscillations of climate--the successive changes being
repeated and producing correlative migrations of floras and faunas.
We know that arctic and temperate faunas and floras flourished during
interglacial times, and a like succession of life-forms followed the
final disappearance of glacial conditions. A study of the organic
remains met with in any particular deposit will not necessarily,
therefore, enable us to assign these to their proper horizon. The
geographical position of the deposit, and its relation to Pleistocene
accumulations elsewhere, must clearly be taken into account. Already,
however, much has been done in this direction, and it is probable that
ere long we shall be able to arrive at a fair knowledge of the various
modifications which the Pleistocene floras and faunas experienced
during that protracted period of climatic changes of which I have been
speaking. We shall even possibly learn how often the arctic, steppe-,
prairie-, and forest-faunas, as they have been defined by Woldrich,
replaced each other. Even now some approximation to this better
knowledge has been made. Dr. Pohlig,[AT] for example, has compared the
remains of the Pleistocene faunas obtained at many different places
in Europe, and has presented us with a classification which, although
confessedly incomplete, yet serves to show the direction in which we
must look for further advances in this department of inquiry.
[AT] Pohlig: _Sitzungsb. d. niederrheinischen Gesellschaft zu Bonn_,
1884; _Zeitschr. d. deutsch. geolog. Ges._, 1887, p. 798. For a
very full account of the diluvial European and northern Asiatic
mammalian faunas by Woldrich, see _Mém. de l'Acad. des Sciences de St.
Pétersbourg_, vii^e sér., t. xxxv., 1887.
During the last twenty years the evidence of interglacial conditions
both in Europe and America has so increased that geologists generally
no longer doubt that the Pleistocene period was characterised by great
changes of climate. The occurrence at many different localities on the
Continent of beds of lignite and freshwater alluvia, containing remains
of Pleistocene mammalia, intercalated between separate and distinct
boulder-clays has left us no other alternative. The interglacial
beds of the Alpine Lands of central Europe are paralleled by similar
deposits in Britain, Scandinavia, Germany, and France. But opinions
differ as to the number of glacial and interglacial epochs--many
holding that we have evidence of only two cold stages and one general
interglacial stage. This, as I have said, is the view entertained
by most geologists who are at work on the glacial accumulations of
Scandinavia and north Germany. On the other hand, Dr. Penck and others,
from a study of drifts of the German Alpine Lands, believe that they
have met with evidence of three distinct epochs of glaciation, and two
epochs of interglacial conditions. In France, while some observers
are of opinion that there have been only two epochs of general
glaciation, others, as, for example, M. Tardy, find what they consider
to be evidence of several such epochs. Others again, as M. Falsan,
do not believe in the existence of any interglacial stages, although
they readily admit that there were great advances and retreats of
the ice during the Glacial period. M. Falsan, in short, believes in
oscillations, but is of opinion that these were not so extensive as
others have maintained. It is, therefore, simply a question of degree,
and whether we speak of oscillations or of epochs, we must needs
admit the fact that throughout all the glaciated tracts of Europe,
fossiliferous deposits occur intercalated among glacial accumulations.
The successive advance and retreat of the ice, therefore, was not a
local phenomenon, but characterised all the glaciated areas. And the
evidence shows that the oscillations referred to were on a gigantic
scale.
The relation borne to the glacial accumulations by the old river
alluvia which contain relics of palæolithic man early attracted
attention. From the fact that these alluvia in some places overlie
glacial deposits, the general opinion (still held by some) was that
palæolithic man must needs be of post-glacial age. But since we have
learned that all boulder-clay does not belong to one and the same
geological horizon--that, in short, there have been at least two,
and probably more, epochs of glaciation--it is obvious that the mere
occurrence of glacial deposits underneath palæolithic gravels does not
prove these latter to be post-glacial. All that we are entitled in such
a case to say is simply that the implement-bearing beds are younger
than the glacial accumulations upon which they rest. Their horizon
must be determined by first ascertaining the relative position in the
glacial series of the underlying deposits. Now, it is a remarkable
fact that the boulder-clays which underlie such old alluvia belong,
without exception, to the earlier stages of the Glacial period. This
has been proved again and again, not only for this country but for
Europe generally. I am sorry to reflect that some twenty years have now
elapsed since I was led to suspect that the palæolithic deposits were
not of post-glacial but of glacial and interglacial age. In 1871-72 I
published a series of papers in the _Geological Magazine_ in which were
set forth the views I had come to form upon this interesting question.
In these papers it was maintained that the alluvia and cave-deposits
could not be of post-glacial age, but must be assigned to pre-glacial
and interglacial times, and in chief measure to the latter. Evidence
was led to show that the latest great development of glacier-ice
in Europe took place after the southern pachyderms and palæolithic
man had vacated England--that during this last stage of the Glacial
period man lived contemporaneously with a northern and alpine fauna
in such regions as southern France--and lastly, that palæolithic man
and the southern mammalia never revisited north-western Europe after
extreme glacial conditions had disappeared. These conclusions were
arrived at after a somewhat detailed examination of all the evidence
then available--the remarkable distribution of the palæolithic and
ossiferous alluvia having, as I have said, particularly impressed me.
I coloured a map to show at once the areas covered by the glacial and
fluvio-glacial deposits of the last glacial epoch, and the regions in
which the implement-bearing and ossiferous alluvia had been met with,
when it became apparent that the latter never occurred at the surface
within the regions occupied by the former. If ossiferous alluvia did
here and there appear within the recently glaciated areas it was always
either in caves, or as infra- or interglacial deposits. Since the date
of these researches our knowledge of the geographical distribution of
Pleistocene deposits has greatly increased, and implements and other
relics of palæolithic man have been recorded from many new localities
throughout Europe. But none of this fresh evidence contradicts the
conclusions I had previously arrived at; on the contrary, it has
greatly strengthened my general argument.
Professor Penck was, I think, the first on the Continent to adopt the
views referred to. He was among the earliest to recognise the evidence
of interglacial conditions in the drift-covered regions of northern
Germany, and it was the reflections which those remarkable interglacial
beds were so well calculated to suggest that led him into the same path
as myself. Dr. Penck has published a map[AU] showing the areas covered
by the earlier and later glacial deposits in northern Europe and the
Alpine Lands, and indicating at the same time the various localities
where palæolithic finds have occurred, and in not a single case do any
of the latter appear within the areas covered by the accumulations of
the last glacial epoch.
[AU] _Archiv für Anthropologie_, Bd. xv. Heft 3, 1884.
A glance at the papers which have been published in Germany within
the last few years will show how greatly students of the Pleistocene
ossiferous beds have been influenced by what is now known of
the interglacial deposits and their organic remains. Professors
Rothpletz[AV] and Andreæ,[AW] Dr. Pohlig[AX] and others, do not
now hesitate to correlate with those beds the old ossiferous and
implement-bearing alluvia which lie altogether outside of glaciated
regions.
[AV] Rothpletz: _Denkschrift d. schweizer. Ges. für d. gesammt. Nat._,
Bd. xxviii. 1881.
[AW] Andreæ: _Abhandl. z. geolog. Specialkarte v. Elsass-Lothringen_,
Bd. iv. Heft 2, 1884.
[AX] Pohlig: _op. cit._
The relation of the Pleistocene alluvia of France to the glacial
deposits of that and other countries has been especially canvassed.
Rothpletz, in the paper I have cited, includes these alluvia amongst
the interglacial deposits, and in the present year (1889) we have an
interesting essay on the same subject by the accomplished secretary of
the Anthropological and Archæological Congress which met recently in
Paris. M. Boule[AY] correlates the palæolithic cave- and river-deposits
of France with those of other countries, and shows that they must be
of interglacial age. His classification, I am gratified to find, does
not materially differ from that given by myself a number of years ago.
He is satisfied that in France there is evidence of three glacial
epochs and two well-marked interglacial horizons. The oldest of the
palæolithic stages of Mortillet (Chelléenne) culminated according
to Boule during the last interglacial epoch, while the more recent
palæolithic stages (Moustérienne, Solutréenne, and Magdalénienne)
coincided with the last great development of glacier-ice. The
Palæolithic age, so far as Europe is concerned, came to a close during
this last cold phase of the Glacial period.
[AY] Boule: _Revue d'Anthropologie_, 1889, t. 1.
There are many other points relating to glacial geology which have
of late years been canvassed by Continental workers, but these I
cannot discuss here. I have purposely indeed restricted my remarks
to such parts of a wide subject as I thought might have interest for
glacialists in this country, some of whom may not have had their
attention directed to the results which have recently been attained
by their fellow-labourers in other lands. Had time permitted I should
gladly have dwelt upon the noteworthy advances made by our American
brethren in the same department of inquiry. Especially should I have
wished to direct attention to the remarkable evidence adduced in
favour of the periodicity of glacial action. Thus Messrs. Chamberlin
and Salisbury, after a general review of that evidence, maintain
that the Ice Age was interrupted by one chief interglacial epoch and
also by three interglacial sub-epochs or episodes of deglaciation.
These authors discuss at some length the origin of the löss, and come
to the general conclusion that while deposits of this character may
have been formed at different stages of the Glacial period, and under
different conditions, yet upon the whole they are best explained by
aqueous action. Indeed a perusal of the recent geological literature of
America shows a close accord between the theoretical opinions of many
Transatlantic and European geologists.
Thus as years advance the picture of Pleistocene times becomes more and
more clearly developed. The conditions under which our old palæolithic
predecessors lived--the climatic and geographical changes of which they
were the witnesses--are gradually being revealed with a precision that
only a few years ago might well have seemed impossible. This of itself
is extremely interesting, but I feel sure that I speak the conviction
of many workers in this field of labour when I say that the clearing
up of the history of Pleistocene times is not the only end which they
have in view. One can hardly doubt that when the conditions of that
period and the causes which gave rise to these have been more fully
and definitely ascertained we shall have advanced some way towards
the better understanding of the climatic conditions of still earlier
periods. For it cannot be denied that our knowledge of Palæozoic,
Mesozoic, and even early Cainozoic climates is unsatisfactory. But
we may look forward to the time when much of this uncertainty will
disappear. Meteorologists are every day acquiring a clearer conception
of the distribution of atmospheric pressure and temperature and the
causes by which that distribution is determined, and the day is
approaching when we shall be better able than we are now to apply this
extended meteorological knowledge to the explanation of the climates
of former periods in the world's history. One of the chief factors in
the present distribution of atmospheric temperature and pressure is
doubtless the relative position of the great land- and water-areas;
and if this be true of the present, it must be true also of the past.
It would almost seem, then, as if all one had to do to ascertain the
climatic conditions of any particular period, was to prepare a map
depicting with some approach to accuracy the former relative position
of land and sea. With such a map could our meteorologists infer what
the climatic conditions must have been? Yes, provided we could assure
them that in other respects the physical conditions did not differ
from the present. Now there is no period in the past history of our
globe the geographical conditions of which are better known than the
Pleistocene. And yet, when we have indicated these upon a map, we find
that they do not give the results which we might have expected. The
climatic conditions which they seem to imply are not such as we know
did actually obtain. It is obvious, therefore, that some additional
and perhaps exceptional factor was at work to produce the recognised
results. What was this disturbing element, and have we any evidence of
its interference with the operation of the normal agents of climatic
change in earlier periods of the world's history? We all know that
various answers have been given to such questions. Whether amongst
these the correct solution of the enigma is to be found, time will
show. Meanwhile, as all hypothesis and theory must starve without facts
to feed on, it behoves us as working geologists to do our best to add
to the supply. The success with which other problems have been attacked
by geologists forbids us to doubt that ere long we shall have done much
to dispel some of the mystery which still envelopes the question of
geological climates.
IX.
The Glacial Period and the Earth-Movement Hypothesis.[AZ]
[AZ] This article contains the substance of two papers, one read before
the Victoria Institute, in 1892; the other an address delivered to the
Geological Society of Edinburgh, in 1891.
Perhaps no portion of the geological record has been more assiduously
studied during the last quarter of a century than its closing chapters.
We are now in possession of manifold data concerning the interpretation
of which there seems to be general agreement. But while that is the
case, there remain, nevertheless, certain facts or groups of facts
which are variously accounted for. Nor have all the phenomena of
the Pleistocene period received equal attention from those who have
recently speculated and generalised on the subject of Pleistocene
climate and geography. Yet, we may be sure, geologists are not likely
to arrive at any safe conclusions as to the conditions that obtained
in Pleistocene times, unless the evidence be candidly considered
in all its bearings. No interpretation of that evidence which does
not recognise every outstanding group of facts can be expected to
endure. It may be possible to frame a plausible theory to account for
some particular conspicuous phenomena, but should that theory leave
unexplained a residuum of less conspicuous but nevertheless well-proved
facts, then, however strongly it may be fortified, it must assuredly
fall.
As already remarked, there are many phenomena in the interpretation of
which geologists are generally agreed. It is, for example, no longer
disputed that in Pleistocene times vast sheets of ice--continental
_mers de glace_--covered broad areas in Europe and North America, and
that extensive snow-fields and large local glaciers existed in many
mountain-regions where snow-fields and glaciers are now unknown, or
only meagrely developed. It is quite unnecessary, however, that I
should give even the slightest sketch of the aspect presented by the
glaciated tracts of our hemisphere at the climax of the Ice Age. The
geographical distribution and extent of the old snow-fields, glaciers,
and ice-sheets is matter now of common knowledge. It will be well,
however, to understand clearly the nature of the conditions which
obtained at the climax of glacial cold--at that stage, namely, when the
Alpine glaciers reached their greatest development, and when so much of
Europe was cased in snow and ice. This we shall best do by comparing
the present with the past. Now in our day the limits of perennial snow
are attained at heights that necessarily vary with the latitude. This
is shown as follows:--
_Region._ _N. Lat._ _Height of Snow-Line._
Iceland, 65° 3,070 feet.
Norway, 61° 5,180-5,570 "
N. Urals, 59° 30' 4,790 "
Alps, 46° 8,884 or 9,000 "
Caucasus, 43° 10,600-11,000 "
Apennines, 42° 30' 9,520 "
Etna, 37° 30' 9,530 "
Sierra Nevada, 37° 11,187 "
Thus in traversing Europe from north to south the snow-line may be
said to rise from 3000 feet to 11,000 feet in round numbers. It is
possible from such data to draw across the map a series of isochional
lines, or lines of equal perennial snow, and this has been done by my
friend, Professor Penck of Vienna.[BA] It will be understood that each
isochional line traverses those regions above which the line of névé
is estimated to occur at the same height. Thus the isochional line of
1000 metres (3280 feet) runs from the north of Norway down to lat. 64°
on the west coast, whence it must pass west to the south of Iceland.
The line of 1500 metres (4920 ft.) is traced from the north end of the
Urals in a westerly direction. It then follows the back-bone of the
Scandinavian peninsula, passes over to Scotland, and thence strikes
west along lat. 55°. For each of these lines good data are obtainable.
The line of 2000 metres (6560 ft.) is, however, hypothetical. It is
estimated to extend from the Ural Mountains, about the lat. of 57°,
over the mountains of middle Germany and above the north of France. The
line of 2500 metres (8200 ft.) passes from the southern termination of
the Urals, in lat. 51°, to the east Carpathians, thence along the north
face of the Alps, thereafter south-west across the Cevennes to the
north-west end of the Pyrenees; and thence above the Cantabrian and the
Portuguese Highlands to the coast in lat. 39°. The line of 3000 metres
(9840 ft.) is estimated to occur above the Caspian Sea, near lat. 44°,
and extends west through the north end of the Caucasus to the Balkans.
Thence it is traced north-west to the Alps, south-west to the Pyrenees,
which range it follows to the west, and thereafter sweeps south above
the coast at Cadiz. The line of 3500 metres (11,480 ft.) runs from
the Caucasus south-west across Asia Minor to the Lebanon Mountains;
thence it follows the direction of the Mediterranean, and traverses
Morocco above the north face of the Atlas range. Finally the line of
4000 metres (13,120 feet) is estimated to trend in the same general
direction as the last-mentioned line, but, of course, further to the
south. Although these isochional lines are to some extent conjectural,
yet the data upon which they are based are sufficiently numerous and
well-known to prevent any great error, and we may admit that the lines
represent with tolerable accuracy the general position of the snow-line
over our Continent. So greatly has our knowledge of the glaciation of
Europe increased during recent years, that the height of the snow-line
of the Glacial period has been determined by MM. Simony, Partsch,
Penck, and Höfer. Their method is simple enough. They first ascertain
the lowest parts of a glaciated region from which independent glaciers
have flowed. This gives the maximum height of the old snow-line. Next
they determine the lowest point reached by such glaciers. It is obvious
that the snow-line would occur higher up than that, but at a lower
level than the actual source of the glaciers; and thus the minimum
height of the former snow-line is approximately ascertained. The
lowest level from which independent glaciers formerly flowed, and the
terminal point reached by the highest-lying glaciers having been duly
ascertained, it is possible to determine with sufficient accuracy the
mean height of the old snow-line. The required data are best obtained,
as one might have expected, in the Pyrenees and amongst the mountains
of middle and southern Europe. In those regions the snow-line would
seem to have been some 3000 feet or so lower than now. From such
data Professor Penck has constructed a map showing the isochional
lines of the Glacial period. These lines are, I need hardly say, only
approximations, but they are sufficiently near the truth to bring out
the contrast between the Ice Age and the present. Thus the isochional
of 1000 metres, which at present lies above northern Scandinavia,
was pushed south to the latitude of southern France and north Italy;
while the isochional of 2000 metres (now overlying the extreme north
of France and north Germany) passed in glacial times over the northern
part of the Mediterranean.[BB]
[BA] "Geographische Wirkungen der Eiszeit," _Verhandl. d. vierten
deutschen Geographentages zu München_, 1884.
[BB] It is interesting to note that while in the Tatra (north
Carpathians) the snow-line was depressed in glacial times to the extent
of 2700 feet only, in the Alps it descended some 4000 feet or more
below its present level. With the snow-line of that great chain at such
an elevation it is obvious that only a few of the higher points of the
Apennines could rise into the region of _névé_. This is the reason why
moraines are met with in only the higher valleys of that range.
Isochional lines are not isotherms. Their height and direction are
determined not only by temperature, but by the amount and distribution
of the snow-fall. Nevertheless, the position of the snow-line in
Europe during the Ice Age enables us to form a rough estimate of the
temperature. At present in middle Europe the temperature falls 1° F.
for every 300 feet of ascent. Hence if we take the average depression
of the snow-line in glacial times at 3000 feet, that would correspond
approximately to a lowering of the temperature by 10°.[BC] This may
not appear to be much, but, as Penck points out, were the mean annual
temperature to be lowered to that extent it would bring the climate of
northern Norway down to southern Germany, and the climate of Sweden to
Austria and Moravia, while that of the Alps would be met with over the
basin of the Mediterranean.
[BC] Professor Brückner thinks the general lowering of temperature
may not have exceeded 5-1/2° to 7° F. _Verhandlungen der 73
Jahresversammlung der schweizerischen Naturforschenden Gesellschaft in
Davos_, 1890.
Let it be noted further that this lowering of the temperature--this
displacement of climatic zones, was experienced over the whole
continent--extending on the one hand south into Africa, and on the
other east into Asia. But while the conditions in northern and central
Europe were markedly glacial, further south only more or less isolated
snow-capped mountains and local glaciers appeared--such, for example,
as those of the Sierra Nevada, the Apennines, Corsica, the Atlas, the
Lebanon, etc. In connection with these facts we may note also that
the Azores were reached by floating ice; and I need only refer in a
word to the evidence of cold wet conditions as furnished by the plant
and animal remains of the Pleistocene tufas, alluvia, and peat of
southern Europe. Again in north Africa and Syria we find, in desiccated
regions, widespread fluviatile accumulations, which, in the opinion of
a number of competent observers, are indicative of rainy conditions
contemporaneous with the Glacial period of Europe.
When we compare the conditions of the Ice Age with those of the present
we are struck with the fact that the former were only an exaggeration
of the latter. The development of glaciation was in strict accordance
with existing conditions. Thus in Pleistocene times North America
was more extensively glaciated than northern Europe, just as to-day
Greenland shows more snow and ice than Scandinavia. No traces of
glaciation have been observed as yet in northern Asia or in northern
Alaska, and to-day the only glaciers and ice-sheets that exist in
northern regions are confined to the formerly glaciated areas. Again,
in Pleistocene Europe glacial phenomena were more strongly developed
in the west than in the east. Large glaciers, for example, existed in
central France, and a considerable ice-flow poured into the basin of
the Douro. But in the same latitudes of eastern Europe we meet with
few or no traces of ice-action. Again, the Vosges appear to have been
more severely glaciated than the mountains of middle Germany; and so
likewise the old glaciers of the western Alps were on a much more
extensive scale than those towards the east end of the chain. Similar
contrasts may be noted at the present day. Thus we find glaciers in
Norway under lat. 60°, while in the Ural Mountains in the same latitude
there is none. The glaciers of the western Alps, again, are larger than
those in the eastern part of the chain. The Caucasus region, it is
true, has considerable glaciers, but then the mountains are higher.
Now turn for a moment to North America. The eastern area was covered
by one immense ice-sheet, while in the mountainous region of the west
gigantic glaciers existed. In our own day we see a similar contrast.
In the north-east lies Greenland well-nigh drowned in ice, while the
north-west region on the other hand, although considerably higher and
occurring in the same latitude, holds only local glaciers. We may
further note that at the present day very dry regions, even when these
are relatively lofty and in high latitudes, such as the uplands of
Siberia, contain no glaciers. And the same was the case in the Glacial
period. These facts are sufficient to show that the conditions of
glacial times bore an intimate relation to those that now obtain. Could
the requisite increase of precipitation and lowering of temperature
take place, we cannot doubt that ice-sheets and glaciers would reappear
in precisely the same regions where they were formerly so extensively
developed. No change in the relative elevation of the land would be
required--increased precipitation accompanied by a general lowering of
the snow-line for 3000 or 3500 feet would suffice to reintroduce the
Ice Age.
From the foregoing considerations we may conclude:--(1) That the
cold of the Glacial period was a general phenomenon, due to some
widely-acting cause--a cause sufficient to influence contemporaneously
the climate of Europe and North America; (2) that glaciation in our
continent increased in intensity from east to west, and from south to
north; (3) that where now we have the greatest rainfall, in glacial
times the greatest snow-fall took place, and the snow tended most to
accumulate; (4) that in the extreme south of Europe, and in north
Africa and west Asia, increased rain precipitation accompanied lowering
of temperature, from which it may be inferred that precipitation in
glacial times was greater generally than it is now.
Having considered the climatic conditions that obtained at the climax
of the Glacial period, I have next to recapitulate what is known
as to the climatic changes of Pleistocene times. It is generally
admitted that the glacial conditions of which I have been speaking
were repeated twice, some say three times, during the Pleistocene
period; while others maintain that even a larger number of glacial
episodes may have occurred. Two glacial epochs, at all events, have
been recognised generally both in Europe and North America. These were
separated by an interglacial stage of more genial conditions, the
evidence for which is steadily increasing. No one now calls in question
the existence of interglacial deposits, but, as their occurrence is
rather a stumbling-block in the way of certain recently resuscitated
hypotheses, some attempt has been made to minimise their importance--to
explain them away, in fact. It has been suggested, for example--(and
the suggestion is by no means new)--that the deposits in question
only show that there were local oscillations during the advance and
retreat of the old ice-sheets and glaciers. This, however, is not the
view of those who have observed and described interglacial beds--who
know the nature of the organic remains which they have yielded, and
the conditions under which the beds must have been accumulated. I need
not refer to the interglacial deposits of our own country further
than to remark that they certainly cannot be explained away in that
summary fashion. The peat and freshwater beds that lie between the
lower and upper tills in the neighbourhood of Edinburgh, for example,
are of themselves sufficient to prove a marked and decided change of
climate. No mere temporary retreat and re-advance of the ice-sheet
will account for their occurrence. The lower till is unquestionably
the bottom-moraine of an ice-sheet which, in that region, flowed
towards the east. When the geographical position of the deposits in
question is considered it becomes clear that an easterly flow of ice
in Mid-Lothian proves beyond gainsaying that during the accumulation
of the lower till all Scotland was drowned in ice. But when water once
more flowed over the land-surface--when a temperate flora, composed
of hazels and other plants, again appeared, it is obvious that the
ice-sheet had already vanished from central Scotland. This is not the
case of a mere temporary recession of the ice-front. It is impossible
to believe that a temperate or even cold-temperate flora could have
flourished in central Scotland at a period when thick glacier-ice
mantled any portion of our Lowlands. Again, in the upper till we read
the evidence of a recurrence of extreme glacial conditions--when
central Scotland was once more overwhelmed by confluent ice-streams
coming from the Highlands and the southern Uplands. Similar evidence
of recurrent glacial conditions, I need hardly remind you, has been
detected in other parts of the country. We are justified, then, in
maintaining that our interglacial beds point to distinct oscillations
of climate--oscillations which imply a long lapse of time. Continental
observers are equally convinced that the interglacial epoch, of which
so many interesting relics have been preserved over a wide region, was
marked at its climax by a temperate climate and endured for a long
period. The interglacial beds of northern and central Europe form
everywhere marked horizons in the glacial series.
Geologists sometimes forget that in every region where glacial
accumulations are well developed, good observers had recognised an
upper and lower series of "drift-deposits" long before the idea of two
separate glacial epochs had presented itself. Thus, in north Germany,
so clearly is the Upper differentiated from the Lower Diluvium that
the two series had been noted and mapped as separate accumulations
for years before geologists had formulated the theory of successive
ice-epochs.[BD] The division of the German Diluvium into an upper
and a lower series is as firmly established as any other well-marked
division in historical geology. The stratigraphical evidence has been
much strengthened, however, by the discovery between upper and lower
boulder-clays of true interglacial beds, containing lignite, peat,
diatomaceous earth, and marine, brackish, and freshwater molluscs,
fish, etc., and now and again bones of Pleistocene mammals.[BE]
A similar strongly-marked division characterises the glacial
accumulations of Sweden, as has been clearly shown by De Geer,[BF] who
thinks that the older and younger epochs of glaciation were separated
by a protracted period of interglacial conditions. In short, evidence
of a break in the glacial succession has been traced at intervals
across the whole width of the Continent, from the borders of the North
Sea to central Russia. M. Krischtafowitsch has recently detected in the
neighbourhood of Moscow[BG] certain fossiliferous interglacial beds,
the flora and fauna of which indicate a warmer and moister climate than
the present. The interglacial stage, he says, must have been of long
duration, and separated in Russia as in western Europe two distinct
epochs of glaciation.
[BD] Wahnschaffe: _Forschungen zur deutschen Landes- und Volkskunde von
Dr. A. Kirchhoff_, Bd. vi., Heft 1.
[BE] For interglacial beds of north Germany see Helland: _Zeitschr.
d. deutsch. geol._ Ges., xxxi., 879; Penck: _Ibid._, xxxi., 157;
_Länderkunde von Europa_ (Das deutsche Reich), 1887, 512; Dames:
_Samml. gemeinverständl. wissensch. Vorträge, von Virchow u.
Holtzendorff:_ xx. Ser., 479 Heft; Schröder: _Jahrb. d. k. geol.
Landensanst. f._ 1885, p. 219. For further references see Wahnschaffe,
_op. cit._ I have not thought it worth while in this paper to refer
to the interglacial deposits of our own islands. A general account of
them will be found in my _Great Ice Age_, and _Prehistoric Europe_. The
interglacial phenomena of the Continent seem to be less known here than
they ought to be.
[BF] _Zeitschrift d. deutsch. geolog. Gesellschaft_, Bd. xxxvii, p. 197.
[BG] _Anzeichen einer interglaziären Epoche in Central-Russland_,
Moskau, 1891.
No mere temporary retreat and re-advance of the ice-front can account
for these phenomena. The occurrence of remains of the great pachyderms
at Rixdorf, near Berlin, and the character of the flora met with in
the interglacial beds of north Germany and Russia are incompatible
with glacial conditions in the low-grounds of northern Europe. The
interglacial beds, described by Dr. C. Weber[BH] as occurring near
Grünenthal, in Holstein, are among the more recent discoveries of
this kind. These deposits rest upon boulder-clay, and are overlaid by
another sheet of the same character, and belong, according to Weber,
to "that great interglacial period which preceded the last ice-sheet
of northern Europe." The section shows 8 feet of peat resting on
freshwater clay, 2 feet thick, which is underlaid by some 10 feet of
"coral sand," with bryozoa. The flora and fauna have a distinctly
temperate facies. It is no wonder, then, that Continental geologists
are generally inclined to admit that north Germany and the contiguous
countries have been invaded at least twice by the ice-sheets of two
separate and distinct glacial epochs. This is not all, however. While
every observer acknowledges that the Diluvium is properly divided
into an upper and a lower series, there are some geologists who have
described the occurrence of three, and even more boulder-clays--the
one clearly differentiated from the other, and traceable over wide
areas. Is each of these to be considered the product of an independent
ice-sheet, or do they only indicate more or less extensive oscillations
of the ice-front? The boulder-clays are parted from each other by thick
beds of sand and clay, in some of which fossils have occasionally
been detected. It is quite possible that such stratified beds were
deposited during a temporary retreat of the ice-front, which when it
re-advanced covered them up with its bottom-moraine. On the other
hand, the phenomena are equally explicable on the assumption that
each boulder-clay represents a separate epoch of glaciation. Until
the stratified beds have yielded more abundant traces of the life of
the period, our judgment as to the conditions implied by them must be
suspended. It is worthy of note in this connection, however, that in
North America the existence of one prolonged interglacial epoch has
been well established, while distinct evidence is forthcoming of what
Chamberlin discriminates as "stages of deglaciation and re-advancing
ice."[BI]
[BH] _Neues Jahrbuch f. Mineralogie, Geologie, u. Palæontologie_, 1891,
Bd. ii., pp. 62, 228; 1892, Bd. i., p. 114.
[BI] _Sixth Annual Report, U. S. Geol. Survey_, 1884-5, P. 315.
When we turn to the Alpine Lands, we find that there also the
occurrence of former interglacial conditions has been recognised.
The interglacial deposits, as described by Heer and others, are well
known. These form as definite a geological horizon as the similar
fossiliferous zone in the Diluvium of northern Germany. The lignites,
as Heer pointed out, represent a long period of time, and this is still
further illustrated by the fact that considerable fluviatile erosion
supervened between the close of the first and the advent of the later
glacial epoch. No mere temporary retreat and re-advance of the ice will
account for the phenomena. Let us for a moment consider the conditions
under which the accumulations in question were laid down. The glacial
deposits underlying the lignite beds contain, amongst other erratics,
boulders which have come from the upper valley of the Rhine. This
means, of course, that the ancient glacier of the Rhine succeeded in
reaching the Lake of Zurich; and it is well known that it extended at
the same time to Lake Constance. That glacier, therefore exceeded sixty
miles in length. One cannot doubt that the climatic conditions implied
by this great extension were excessive, and quite incompatible with
the appearance in the low-grounds of Switzerland of such a flora as
that of the lignites. The organic remains of the lignite beds indicate
a climate certainly not less temperate than that which at present
characterises the district round the Lake of Zurich. We may safely
infer, therefore, that during interglacial times the glaciers of the
Alps were not more extensively developed than at present. Again, as
the lignites are overlaid by glacial deposits, it is obvious that the
Rhine glacier once more reached Lake Zurich--in other words, there was
a return of the excessive climate that induced the first great advance
of that and other Swiss glaciers. That these advances were really due
to extreme climatic conditions is shown by the fact that it was only
under such conditions that the Scandinavian flora could have invaded
the low-grounds of Europe, and entered Switzerland. It is impossible,
therefore, that the interglacial flora could have flourished in
Switzerland while the immigration of these northern plants was taking
place.
Lignites of the same age as those of Dürnten and Utznach occur
in many places both on the north and south sides of the Alpine
chain. At Imberg, near Sonthofen, in Bavaria, for example, they are
described by Penck[BJ] as being underlaid and overlaid by thick
glacial accumulations. The deposits in question form a terrace along
the flanks of the hills, at a height of 700 feet above the Iller.
The flora of the lignite has not yet been fully studied, but it is
composed chiefly of conifers, which must have grown near where their
remains now occur--that is at 3000 feet, or thereabout, above the sea.
It is incredible that coniferous forests could have flourished at
that elevation during a glacial epoch. A lowering of the mean annual
temperature by 3° C. only would render the growth of trees at that
height almost impossible, and certainly would be insufficient to cause
the glaciers of Algau to descend to the foot of the mountains, as we
know they did--a distance of at least twenty-four miles. The Imberg
lignites, therefore, are evidence of a climate not less temperate than
the present. More than this, there is clear proof that the interglacial
stage was long continued, for during that epoch the Iller had time to
effect very considerable erosion. The succession of changes shown by
the sections near Sonthofen are as follows.
1. The Iller Valley is filled with glacier-ice which flows out upon the
low-grounds at the base of the Alps.
2. The glacier retreats, and great sheets of shingle and gravel are
spread over the valley.
3. Coniferous forests now grow over the surface of the gravels; and as
the lignite formed of their remains attains a thickness of ten feet in
all, it obviously points to the lapse of some considerable time.
4. Eventually the forests decay, and their débris is buried under new
accumulations of shingle and gravel.
5. The Iller cuts its way down through all the deposits to depths of
680 to 720 feet.
6. A glacier again descends and fills the valley, but does not flow so
far as that of the earlier glacial stage.
[BJ] _Die Vergletscherung der deutschen Alpen_, 1882, p. 256.
In this section, as in those at Dürnten and Utznach, we have
conclusive evidence of two glacial epochs, sharply marked off the one
from the other. Nor does that evidence stand alone, for at various
points between Lake Geneva and the lower valley of the Inn similar
interglacial deposits occur. Sometimes these appear at the foot of
the mountains, as at Mörschweil on Lake Constance; sometimes just
within the mountain area, as at Imberg; sometimes far in the heart
of the Alpine Lands, as at Innsbruck. Professor Penck has further
shown, and his observations have been confirmed by Brückner, Blaas,
and Böhm, that massive sheets of fluviatile gravel are frequently
met with throughout the valleys of the Alps, occupying interglacial
positions. These gravels are exactly comparable to the interglacial
gravels of the Sonthofen sections. And it has been demonstrated that
they occur on two horizons, separated the one from the other by
characteristic ground-moraine, or boulder-clay. The lower gravels
rest on ground-moraine, and the upper gravels are overlaid by sheets
of the same kind of glacial detritus. In short, three separate and
distinct ground-moraines are recognised. The gravels, one cannot doubt,
are simply the torrential and fluviatile deposits laid down before
advancing and retreating glaciers; and it is especially to be noted
that each sheet of gravel, after its accumulation, was much denuded
and cut through by river-action. In a word, as Penck and others have
shown, the valleys of Upper Bavaria have been occupied by glaciers at
three successive epochs--each separated from the other by a period
during which much river-gravel was deposited and great erosion of the
valley-bottoms was effected.
On the Italian side of the Alps, similar evidence of climatic changes
is forthcoming. The lignites and lacustrine strata of Val Gandino,
and of Val Borlezza, as I have elsewhere shown,[BK] are clearly of
interglacial age. From these deposits many organic remains have been
obtained--amongst the animals being _Rhinoceros hemitoechus_ and _R.
leptorhinus_. According to Sordelli, the plants indicate a climate
as genial as that of the plains of Lombardy and Venetia, and warmer
therefore than that of the upland valleys in which the interglacial
beds occur. Professor Penck informs me that some time ago he detected
evidence in the district of Lake Garda of three successive glacial
epochs--the evidence being of the same character as that recognised in
the valleys of the Bavarian Alps.
[BK] _Prehistoric Europe_, p. 303.
In the glaciated districts of France similar phenomena are met with.
Thus in Cantal, according to M. Rames,[BL] the glacial deposits belong
to two separate epochs. The older morainic accumulations are scattered
over the surface of the plateau of Archæan schistose rocks, and extend
up the slopes of the great volcanic cone of that region to heights
of 2300 to 3300 feet. One of the features of these accumulations
are the innumerable gigantic erratics, known to the country folk as
_cimetière des enragés_. Sheets of fluvio-glacial gravel are also
associated with the moraines, and it is worthy of note that both have
the aspect of considerable age--they have evidently been subjected to
much denudation. In the valleys of the same region occurs a younger
series of glacial deposits, consisting of conspicuous lateral and
terminal moraines, which, unlike the older accumulations, have a very
fresh and well-preserved appearance. With them, as with the older
moraines, fluvio-glacial gravels are associated. M. Rames shows that
the interval that supervened between the formation of the two series of
glacial deposits must have been prolonged, for the valleys during that
interval were in some places eroded to a depth of 900 feet. Not only
was the volcanic _massif_ deeply incised, but even the old plateau of
crystalline rocks on which the volcanic cone reposes suffered extensive
denudation in interglacial times. M. Rames further recognises that the
second glacial epoch was marked by two advances of the valley-glaciers,
separated by a marked episode of fusion, the evidence for which is
conspicuous in the valley of the Cère.
[BL] _Bull. Soc. Géol. de France_, 1884.
The glacial and interglacial phenomena of Auvergne are quite analogous
to those of Cantal. Dr. Julien has described the morainic accumulations
of a large glacier that flowed from Mont Dore. After that glacier had
retreated a prolonged period of erosion followed, when the morainic
deposits were deeply trenched, and the underlying rocks cut into.
In the valleys and hollows thus excavated freshwater beds occur,
containing the relics of an abundant flora, together with the remains
of elephant (_E. meridionalis_), rhinoceros (_R. leptorhinus_),
hippopotamus, horse, cave-bear, hyæna, etc.--a fauna comparable to
that of the Italian interglacial deposits. After the deposition of the
freshwater beds, glaciers again descended the Auvergne valleys and
covered the beds in question with their moraines.[BM]
[BM] _Des Phénomènes glaciaires dans le Plateau central de France, etc._
According to the researches of Martins, Collomb, Garrigou,
Piette, and Penck, there is clear evidence in the Pyrenees of two
periods of glaciation, separated by an interval of much erosion
and valley-excavation. Penck, indeed, has shown that the valleys
of the Pyrenees have been occupied at three successive epochs by
glaciers--each epoch being represented by its series of moraines and by
terraces of fluvio-glacial detritus, which occur at successively lower
levels.
I have referred in some detail to these discoveries of interglacial
phenomena because they so strongly corroborate the conclusions
arrived at a number of years ago by glacialists in our own country.
Many additional examples might be cited from other parts of Europe,
but those already given may serve to show that at least one epoch of
interglacial conditions supervened during the Pleistocene period.
Before leaving this part of my subject, however, I may point out the
significant circumstance that long before much was known of glaciation,
and certainly before the periodicity of ice-epochs had been recognised,
Collomb had detected in the Vosges conspicuous evidence of two
successive glaciations.[BN]
[BN] _Preuves de l'existence d'anciens glaciers dans les vallées des
Vosges_, 1847, p. 141.
Having shown that alike in the regions formerly occupied by the great
northern ice-sheet, and in the Alpine Lands of central and southern
Europe, alternations of cold and genial conditions characterised the
so-called Glacial period, we may now glance at the evidence supplied
by those Pleistocene deposits that lie outside of the glaciated
areas. Of these we have a typical example in the river-accumulations
of the Rhine Valley between Bâle and Bingen. Here and there these
deposits have yielded remains of extinct and no longer indigenous
mammals and relics of Palæolithic man--one of the most interesting
deposits from which mammalian remains have been obtained being the
Sands of Mosbach, between Wiesbaden and Mayence. The fauna in question
is characteristically Pleistocene, nor can it be doubted that the
Mosbach Sands belong to the same geological horizon as the similar
fluviatile deposits of the Seine, the Thames, and other river-valleys
in western Europe. Dr. Kinkelin has shown,[BO] and with him Dr.
Schumacher agrees,[BP] that the Mosbach deposits are of interglacial
age; while Dr. Pohlig has no hesitation in assigning them to the same
horizon.[BQ] It is true there are no glacial accumulations in the
region where they occur, but they rest upon a series of unfossiliferous
gravels which are recognised as the equivalents of the fluvio-glacial
and glacial deposits of the Vosges, the Black Forest, the Alps, etc.
These gravels are traced at intervals up to considerable heights above
the Rhine, and contain numerous erratics, some of which are several
feet in diameter, while a large proportion are not at all water-worn,
but roughly and sharply angular. The blocks have unquestionably been
transported by river-ice, and imply therefore cold climatic conditions.
The overlying Mosbach Sands have yielded not only _Elephas antiquus_
and _Hippopotamus major_, but the reindeer, the mammoth, and the
marmot--two strongly contrasted faunas, betokening climatic changes
similar to those that marked the accumulation of the river-deposits
of the Thames, the Seine, etc. Of younger date than the Mosbach Sands
is another series of unfossiliferous gravels, which, like the older
series, are charged with ice-floated erratics. The beds at Mosbach are
thus shown to be of interglacial age: they occupy the same geological
horizon as the interglacial beds of Switzerland and other glaciated
tracts in central and northern Europe.
[BO] Kinkelin: _Bericht über die Senckenberg. naturf. Ges. in Frankfurt
a. M._, 1889.
[BP] Schumacher: _Mittheilungen d. Commission für d. geolog.
Landes-Untersuch. v. Elsass-Lothringen_, Bd. ii., 1890, p. 184.
[BQ] _Zeitschr. d. deutsch. geolog. Ges._, 1887, p. 806.
To this position must likewise be assigned the Pleistocene
river-alluvia of other districts. There is no other horizon, indeed,
on which these can be placed. That they are not of post-glacial age
is shown by the fact that in many places the angular gravels and
flood-loams of the Glacial period overlie them. And that they cannot
all belong to pre-glacial times is proved by the frequent occurrence
underneath them of glacial or fluvio-glacial accumulations. It is
quite possible, of course, that here and there in the valleys of
western and southern Europe some of the Pleistocene alluvia may be of
pre-glacial age. But in the main these alluvia must be regarded as
the equivalents of the glacial and interglacial deposits of northern
and Alpine districts. This will appear a reasonable conclusion
when we bear in mind that long before the Pliocene period came to a
close the climate of Europe had begun to deteriorate. In England, as
we know, glacial conditions supervened almost at the advent of the
Pleistocene period. And the same was the case in the Alpine Lands of
the south. Again, in the glaciated areas of north and south alike, the
closing stage of the Pleistocene was characterised by cold climatic
conditions. And thus in those regions the glacial and interglacial
epochs were co-extensive with that period. It follows, therefore,
that the Pleistocene deposits of extra-glacial areas must be the
equivalents of the glacial and interglacial accumulations elsewhere.
If we refused to admit this we should be puzzled indeed to tell what
the rivers of western and southern Europe were doing throughout the
long-continued Glacial period. There is no escape from the conclusion
that the Pleistocene river-alluvia and cave-accumulations must be
assigned to the same general horizon as the glacial and interglacial
deposits. This is now admitted by Continental palæontologists who find
in the character of Pleistocene organic remains abundant proof that
the old river-alluvia and cave-accumulations were laid down under
changing climatic conditions. Did neither glacial nor interglacial
deposits exist, the relics of the Pleistocene flora and fauna met with
in extra-glacial regions would yet lead us to the conclusion that
after the close of the Pliocene period, extremely cold and very genial
climates alternated up to the dawn of the present. Thus during one
stage of the Pleistocene "clement winters and cool summers permitted
the wide diffusion and intimate association of plants which have
now a very different range. Temperate and southern species like the
ash, the poplar, the sycamore, the fig-tree, the judas-tree, etc.,
overspread all the low-grounds of France as far north at least as
Paris. It was under such conditions that the elephants, rhinoceroses,
hippopotamuses, and the vast herds of temperate cervine and bovine
species ranged over Europe, from the shores of the Mediterranean up to
the latitude of Yorkshire, and probably even further north still, and
from the borders of Asia to the western ocean. Despite the presence of
numerous fierce carnivora--lions, hyænas, tigers, and others--Europe
at that time, with its shady forests, its laurel-margined streams,
its broad and deep-flowing rivers--a country in every way suited
to the needs of a race of hunters and fishers--must have been no
unpleasant habitation for Palæolithic man." But during another stage
of the Pleistocene period, the climate of our continent presented the
strongest contrast to those genial conditions. At that time "the dwarf
birch of the Scottish Highlands, and the Arctic willow, with their
northern congeners, grew upon the low-grounds of middle Europe. Arctic
animals, such as the musk-sheep and the reindeer, lived then, all the
year round, in the south of France; the mammoth ranged into Spain and
Italy; the glutton descended to the shores of the Mediterranean; the
marmot came down to the low-grounds at the foot of the Apennines; and
the lagomys inhabitated the low-lying maritime districts of Corsica
and Sardinia. The land and freshwater molluscs of many Pleistocene
deposits tell a similar tale: high alpine, boreal, and hyperborean
forms are characteristic of those deposits in central Europe; even in
the southern regions of our continent the shells testify to a former
colder and wetter climate. It was during the climax of these conditions
that the caves of Aquitaine were occupied by those artistic men who
appear to have delighted in carving and engraving."[BR] Such, in brief,
is the testimony of the Pleistocene flora and fauna of extra-glacial
regions. It is from the deposits in these regions, therefore, that
we derive our fullest knowledge of the life of the period. But a
comparison of their organic remains with those that occur in the
glacial and interglacial deposits of alpine and northern lands shows
us that the Pleistocene accumulations of glacial and extra-glacial
countries are contemporaneous--for there is not a single life-form
obtained from interglacial beds which does not also occur in the
deposits of extra-glacial regions. The converse is not true--nor
is that to be wondered at, for interglacial deposits have only been
sparingly preserved. In regions liable to glaciation such superficial
accumulations must frequently have been ploughed up and incorporated
with ground-moraine. It was only in the extra-glacial tracts that
alluvia of interglacial age were at all likely to be preserved in
any abundance. To appreciate fully the climatic conditions of the
Pleistocene period, therefore, it is necessary to combine the evidence
derived from the glaciated areas with that obtained from the lands
that lay beyond the reach of the ice-plough. The one is the complement
of the other, and this being so, it is obvious that any attempted
explanation of the origin of the Glacial period which does not fully
realise the importance of the interglacial phase of that period cannot
be accepted.
[BR] _Prehistoric Europe_, p. 67.
But if the climatic changes of Pleistocene times are the most
important phenomena which the geologist who essays to trace the
history of that period is called upon to consider, he cannot ignore
the evidence of contemporaneous geographical mutations. These are so
generally admitted, however, that it is only necessary here to state
the well-known fact that everywhere throughout the maritime tracts of
the glaciated lands of Europe and North America frequent changes in
the relative level of land and sea took place during Pleistocene and
post-glacial times.
I must now very briefly review the evidence bearing on the climatic
conditions of post-glacial times. And first, let it be noted that the
closing stage of the Pleistocene period was one of cold conditions,
accompanied in north-western Europe by partial depression of the land
below its present level. This is shown by the late-glacial marine
deposits of central Scotland and the coast-lands of Scandinavia. The
historical records of the succeeding post-glacial period are furnished
chiefly by raised beaches, river- and lake-alluvia, calcareous tufas,
and peat-bogs. An examination of these has shown that the climate, at
first cold, gradually became less ungenial, so that the Arctic-alpine
flora and northern fauna were eventually supplanted in our latitude by
those temperate forms which, as a group, still occupy this region. The
amelioration of the climate was accompanied by striking geographical
changes, the British Islands becoming united with themselves and the
opposite coasts of the continent. The genial character of the climate
at this time is shown by the great development of forests, the remains
of which occur under our oldest peat-bogs. Not only did trees then grow
at greater altitudes in these regions than is at present the case,
but forests ranged much further north, and flourished in lands where
they cannot now exist. In Orkney and Shetland, in the far north of
Norway, and even in the Faröe Islands and in Iceland relics of this old
forest-epoch are met with. In connection with these facts reference may
be made to the evidence obtained from certain raised beaches on both
sides of the N. Atlantic, and from recent dredgings in the intervening
sea. The occurrence of isolated colonies of southern molluscs in our
northern seas, and the appearance in raised beaches of many forms which
are now confined to the waters of more southern latitudes, seem to show
that in early post-glacial times the seas of these northern latitudes
were warmer than now. And it is quite certain that the southern forms
referred to are not the relics of any pre-glacial or interglacial
immigration. They could only have entered our northern seas after the
close of the Glacial period, and their evidence taken in connection
with that furnished by the buried trees of our peat-bogs, leads to the
conclusion that a genial climate supervened after the cold of the last
glacial epoch and of earliest post-glacial times had passed away.
To this genial stage succeeded an epoch of cold humid conditions,
accompanied by geographical changes which resulted in the insulation
of Britain and Ireland--the sea encroaching to some extent on what are
now our maritime regions. The climate was less favourable to the growth
of forests, which began to decay and to become buried under widespread
accumulations of growing peat. At this time glaciers reappeared in the
glens of the Scottish Highlands, and here and there descended to the
sea. The evidence for these is quite conspicuous, for the moraines are
found resting on the surface of post-glacial beaches. Thus my friend
Mr. L. Hinxman, of the Geological Survey, tells us that at the foot
of Glen Thraill well-formed moraines are seen in section reposing on
beach-deposits at the distance of about three-quarters of a mile above
the head of Loch Torridon.[BS] The evidence of this recrudescence of
glacial conditions in post-glacial times is not confined to Scotland. I
believe it will yet be recognised in many other mountain-regions; but
already Prof. Penck has detected it in the valleys of the Pyrenees.[BT]
Dr. Kerner von Marilaun has also described similar phenomena in the
higher valleys of Tyrol, while Professor Brückner has obtained like
evidence in the Salzach region.[BU]
[BS] For Scottish post-glacial glaciers see J. Geikie: _Scottish
Naturalist_, Jan., 1880; _Prehistoric Europe_, pp. 386,407; Penck:
_Deutsche geographische Blätter_, Bd. vi., p. 323; _Verhand. d. Ges. f.
Erdkunde, Berlin_, 1884, Heft 1; Hinxman: _Trans. Edin. Geol. Soc._,
vol. vi., p. 249.
[BT] "Die Eiszeit in den Pyrenäen": _Mitth. d. Vereins. f. Erdkunde_,
Leipzig, 1883.
[BU] Kerner: _Mitth. k. k. geograph. Ges. Wien_, 1890, p. 307;
_Sitzungsb. d. kais. Akad. d. Wissensch. in Wien_, Bd. c., Abth. i.,
1891; Brückner: _X. Jahresbericht d. geograph. Ges. v. Bern_, 1891.
I have elsewhere traced the history of the succeeding stages of the
post-glacial period, and brought forward evidence of similar but less
strongly-marked climatic changes having followed upon those just
referred to, and my conclusions, I may add, have been supported by
the independent researches of Professor Blytt in Norway. But these
later changes need not be considered here, and I shall leave them out
of account in the discussion that follows. It is sufficient for my
present purpose to confine attention to the well-proved conclusion
that in early post-glacial times genial climatic conditions obtained,
and that these were followed by cold and humid conditions, during the
prevalence of which considerable local glaciers reappeared in certain
mountain-valleys.[BV]
[BV] For a full statement of the evidence see _Prehistoric Europe_,
chaps. xvi., xvii.
We speak of Pleistocene or Glacial and of Post-glacial periods as if
the one were more or less sharply marked off from the other. Of course,
that is not the case, and in point of fact it would be for many reasons
preferable to include them under some general term. Taken together they
form one tolerably well-defined cycle of time, characterised above all
by its remarkable climatic changes--by alternations of cold and genial
conditions, which were most strongly contrasted in the earlier stages
of the period. It is further worthy of note that various oscillations
of the sea-level appear to have taken place again and again both in the
earlier and later stages of the cycle.
We may now proceed to inquire whether the phenomena we have been
considering can be accounted for by movements of the earth's crust--a
view which has recently received considerable support, more especially
in America. I need hardly say that the view in question is no novelty.
Many years ago, while our knowledge of the Pleistocene phenomena was
somewhat rudimentary, it was usual to infer that glaciation had been
induced by elevation of the land. This did not seem an unreasonable
conclusion, for above our heads, at a less or greater elevation,
according to latitude, an Arctic climate prevails. One could not doubt,
therefore, that if a land-surface were only sufficiently uplifted
it would reach the snow-line, and become more or less extensively
glaciated. But with the increase of our knowledge of Pleistocene
and post-glacial conditions, such a ready interpretation failed to
satisfy, although not a few geologists have continued to defend
the "earth-movement hypothesis," as accounting fairly well for the
phenomena of the Glacial period. By these staunch believers in the
adequacy of that view, it has been pointed out that elevation might not
only lift lands into the region of eternal snow, but, by converting
large areas of the sea-bed into land, would greatly modify the
direction of ocean-currents, and thus influence the climate. What might
not be expected to happen were the Gulf Stream to be excluded from
northern regions? What would be the fate of the temperate latitudes of
North America and Europe were that genial ocean-river to be deflected
into the Pacific across a submerged Isthmus of Panama? The possibility
of such changes having supervened in Pleistocene times has often been
present to my mind, but I long ago came to the conclusion that they
could not account for the facts. Moreover, I have never been able to
meet with any evidence in favour of the postulated "earth-movements."
Having carefully studied all that has been advanced of late years in
support of the hypothesis in question I find myself more than ever
constrained to oppose it, not only because it is grounded on no basis
of fact, but because it altogether fails to explain the conditions that
obtained in Pleistocene and post-glacial times.
There are various forms in which the hypothesis has appeared, and these
I shall now consider seriatim, and with such brevity as may be. It
has been maintained, for example, that at the advent of the Glacial
period vast areas of northern and north-western Europe, together with
enormous regions in the corresponding latitudes of North America, stood
several thousand feet higher than at present. But when we ask what
evidence can be adduced to prove this we get no satisfactory reply.
We are simply informed that a glacial climate must have resulted from
great elevation, and that the latter, therefore, must have taken place
at the beginning of the Glacial period. Some writers, however, have
ventured to give reasons for their faith. Thus Mr. W. Upham, pointing
to the evidence of the fiords of North America, and to the fact that
drowned river-valleys have been traced outwards across the 100-fathoms
line of the marginal plateau to depths of over 3500 feet, maintains
that the whole continent north of the Gulf of Mexico stood at the
commencement of the Glacial period some 3000 feet at least higher than
now. Of course he cites the fiords of Europe as evidence of a similar
great upheaval for the northern and north-western regions of our
Continent. Mr. Upham even favours the notion that during glacial times
a land-connection probably existed between North America and Europe,
by way of the British Islands, Iceland, and Greenland. When "this
uplifting attained its maximum, and brought on the Glacial period," he
says, "North America and north-western Europe stood 2500 to 3000 feet
above their present height."[BW]
[BW] _American Geologist_, vi., p. 327.
That fiords are simply submerged land-valleys has long been recognised:
that they have been formed mainly by the action of running water--just
in the same way as the mountain-valleys of Norway and Scotland--has
been the belief for many years of most students of physical geology.
But it is hard to understand why they should have been cited by Mr.
Upham in support of his contention, seeing that their evidence seems to
militate strongly against the very hypothesis he strives to maintain.
No one acquainted with the physical features and geological structure
of Scotland and Norway can doubt that the valleys which terminate
in fiords are of great geological antiquity. Their excavation by
fluviatile action certainly dates back to a period long anterior to
the advent of the Ice Age. And a like tale is told by the fiords and
drowned valley-troughs of North America, which cannot be referred to
so recent a period as post-Tertiary times. Those who are convinced
that our continental areas have persisted throughout long æons of
geological time, and that rivers frequently have survived great
geological revolutions--cutting their way across mountain-elevations
as fast as these were uplifted--will readily believe that some of the
submarine river-troughs of North America, such as that of the Hudson,
may belong even to Secondary times.[BX] It would be hard to say at
what particular date the excavation of the Scottish Highland valleys
commenced--but it was probably during the later part of the Palæozoic
era. The process has doubtless been retarded and accelerated frequently
enough, during successive movements of depression and elevation, but it
was practically completed before the beginning of Pleistocene times,
and that is all that we may trouble about here. Precisely the same
conclusion holds good for Norway: and such being the case it is obvious
that the question of the origin and age of the fiords has no bearing
on the problem of the glacial climate and its cause. In point of fact
the evidence, as already remarked, tells against the "earth-movement
hypothesis," for it shows us that, during a period when Europe and
North America stood several thousand feet higher, and extended much
further seawards, rivers, and not glaciers, were the occupants of our
mountain-valleys. It was not until all those valleys had come to assume
much the appearance they now present that general glaciation supervened.
[BX] Professor Dana inclines to date the erosion of the Hudson trough
so far back as the Jura-Trias period.--_American Journ. Science_, xl.,
p. 435.
We are not without direct evidence, however, as to the geographical
conditions that obtained in the ages that immediately preceded the
Pleistocene period. The distribution of the Pliocene marine beds of
Britain entitles us to assume that at the time of their accumulation
our lands did not extend quite so far to the south and east as now.
The absence of similar deposits from the coast-lands of North America
is supposed to support the view of great continental elevation in
pre-glacial times. All it seems to prove, however, is that in Pliocene
times the North American continent was not less extensive than it is at
present. It is even quite possible that in glacial times pre-existing
Pliocene beds may have been ploughed out by the ice, just as seems to
have been the case in the north-east of Scotland. But without going so
far back as Pliocene times, we meet with evidence almost everywhere
throughout the maritime regions of the glaciated areas of Europe and
North America, to show that immediately before those tracts became
swathed in ice the geographical conditions were much the same as at
present. The shelly boulder-clays in various parts of our islands,
and the similar occurrence of marine and brackish-water shells in and
underneath the Diluvium of north Germany, etc., prove clearly enough
that just before the coming-on of glacial conditions neither Britain
nor the present maritime lands of the Continent were far removed
from the sea. It is true that the buried river-channels of Scotland
indicate a pre-glacial elevation of some 200 or 300 feet above the
existing sea-level, but it is quite certain that the Minch, St.
George's Channel, the Irish Sea, the North Sea, and the Baltic were all
in existence at the commencement of the Glacial period. And we are led
to similar conclusions with regard to the geographical conditions of
North America at that time, from the occurrence of marine shells in the
boulder-clays of Canada and New England. We note indeed that there is
abundant evidence of land-submergence during glacial times. Indeed, we
may say that the Pleistocene marine deposits of northern latitudes are
almost invariably indicative of colder conditions than now obtain.
If it be true that cold climatic conditions were contemporaneous in
our latitude with submergence, it is equally true that an extensive
land-surface in north-west Europe has, sometimes at least, co-existed
with markedly genial conditions. In Tertiary times, for example, as
the Oligocene deposits of Scotland, the Faröe Islands, Iceland, and
Greenland testify, a land-connection existed between Europe and the
North American continent. Again, it has been shown that during the
interglacial phase of the Pleistocene period Britain was continental,
and enjoyed at the time a peculiarly genial climate. And somewhat
similar geographical and climatic conditions again supervened in
post-glacial times. In other words, when the land was more elevated
and extensive than now, it enjoyed a warmer climate. Nor can we escape
the conclusion that the excavation of the fiord-valleys of northern
latitudes, which is a very old story (far older than the Pleistocene),
was the work not of glaciers but of running water, at a time when
north-western Europe and the corresponding regions of America were much
more elevated than they are now.
Thus there appears to be no evidence either direct or indirect in
favour of the view that glacial conditions were superinduced by
great continental elevation. But it may be argued that even although
no evidence can be cited in proof of such elevation, still, if
the glacial phenomena can be well explained by its means, we may
be justified in admitting it as a working hypothesis. Movements
of elevation and depression have frequently taken place--the
Pleistocene marine deposits themselves testify to oscillations of
the sea-level--and there can be no objection, therefore, to such
postulations as are made by the hypothesis under review. All this
is readily granted, but I deny that the conditions that obtained in
Pleistocene times can be accounted for by elevation and depression. Let
us see how the desiderated elevation of northern lands would work. Were
north-western Europe and the corresponding latitudes of North America
to be upheaved for 3000 feet, and a land-passage to obtain between the
two continents by way of the Faröe Islands, Iceland, and Greenland,
how would the climate be affected? It is obvious that under such
changed conditions the elevated lands in higher latitudes might well
be subjected to more or less extensive glaciation. Norway would become
uninhabitable and glaciers might well appear in the mountain-valleys
of Scotland. But it may be doubted whether the climate of France and
Spain, or the corresponding latitudes of North America, would be
much affected. For were a land-passage to appear between Britain and
Greenland no Arctic current would flow into the North Atlantic, while
no portion of the Gulf Stream would be lost in Arctic seas. The North
Atlantic would then form a great gulf round which a warm ocean-current
would circulate. The temperature of that sea, therefore, would be
raised and the prevailing westerly and south-westerly winds of Europe
would be warmer than now. However much such warm moist winds might
increase the snow-fall in North Britain and Scandinavia, we cannot
suppose they could have much influence in central and southern Europe,
and in North Africa; and still less could they affect the climate of
Asia Minor and the mountainous regions of the far east, in most of
which evidence of extensive glaciation occurs. And how, we may ask,
could the postulated geographical changes bring about the glaciation
of the mountainous tracts on the Pacific sea-board? In fine, we may
conclude that however much the geographical changes referred to might
affect north-western Europe and north-eastern America, they are wholly
insufficient to account for the glacial phenomena of other regions.
The continuous research of recent years has shown that the lowering of
temperature of glacial times was not limited to the lands which would
be affected by any such elevation as that we are considering. A marked
and general displacement of climatic zones took place over the whole
continent of Europe; and similar changes supervened in North America
and Asia. Are we then to suppose that all the lands within the Northern
Hemisphere were extensively and contemporaneously upheaved?
We may now consider another form of the "earth-movement hypothesis."
It has frequently been suggested that our glacial phenomena may have
been caused by the submergence of the Isthmus of Panama, and the
deflection of the Equatorial Current into the Pacific. But it may be
doubted whether a submergence of that isthmus, unless very extensive
indeed, would result in more than a partial escape of Atlantic water
into the Pacific basin. The Counter Current of the Pacific which now
strikes against the isthmus might even sweep into the Caribbean Sea,
and join the Equatorial on its way to the Gulf of Mexico. But putting
that consideration aside, what evidence have we that the Isthmus of
Panama was submerged during the glacial epoch? None whatsoever, it
may be replied. It is only a pious opinion. Considerable movements of
elevation and depression of the islands in the Caribbean Sea would seem
to have taken place at a comparatively recent date, but those movements
may quite well belong to Pliocene times. Whether they be of Pliocene
or Pleistocene age, however, no one has yet proved that the Isthmus of
Panama was sufficiently submerged, either at the one time or the other,
to permit the escape of the Atlantic Equatorial into the Pacific basin.
But let it be supposed that the isthmus has become so deeply submerged
that the Equatorial Current is wholly deflected, and that no Gulf
Stream issues through the Straits of Florida to temper the climate of
higher latitudes. What would result from such an unhappy change? Can
any one conversant with the geographical distribution of the glacial
phenomena imagine that the conditions of the Glacial period could be
thus reproduced? Norway might indeed become a second south Greenland,
and perennial snow and ice might appear in the mountainous tracts of
the British Islands. The climate of Hudson's Bay and the surrounding
lands might be experienced in the Baltic and its neighbourhood, and
what are now the temperate latitudes of Europe, north of the 50th
parallel, would possibly approach Siberia in character. But surely
these changes are not comparable to the conditions of the Glacial
period. The absence of a Gulf Stream would not sensibly affect the
climate of south-eastern Europe and Asia, and could not have the
smallest influence on that of the Pacific coast-lands of North America.
Yes, but if we conceive the submergence of the Isthmus of Panama
to coincide with great elevation of northern lands, would not such
geographical conditions bring about a glacial epoch comparable to
that of Pleistocene times? It is hard to see how they could. No
doubt the climate of all those regions that would be affected by
the withdrawal of the Gulf Stream alone would become still more
deteriorated if they stood some 3000 feet higher than now. A vast area
in the north-west of Europe would certainly be uninhabitable, but it
is for the advocates of the "earth-movement hypothesis" to explain
why those inhospitable regions should necessarily be covered with an
ice-sheet. For the production of great snow-fields and continental
ice-sheets, considerable precipitation, no less than a low temperature,
is requisite. Under the conditions we have been imagining, however,
precipitation would probably be much less than it is at present. But
to whatever extent north-west Europe might be glaciated, it is obvious
that the geographical revolutions referred to could have little
influence on the climate of south-eastern Europe, not to mention
central and eastern Asia. Nor could they possibly influence the climate
of the Pacific coast-lands of North America. And yet, as is well
known, the climate of all those regions was more or less profoundly
affected during the Glacial period. To account for the widespread
evidences of glaciation by means of elevation it would therefore seem
necessary to infer that all the affected areas were in Pleistocene
times uplifted _en masse_ into the Arctic zone that stretches above our
heads. Now it seems easier to believe that the snow-line was lowered
by several thousand feet than that the continents were elevated to the
same extent. Glaciation, as we have seen, was developed in the same
directions and over the same areas as we should expect it to be were
the snow-line to be generally depressed. To put it in another way,
were the snow-line by some means or other to be lowered over Europe,
Asia, and North America, then, with sufficient precipitation, great
ice-fields and glaciers would reappear in the very regions which they
visited during Pleistocene times. Neither elevation nor depression
of the land would be required to bring about such a result. Certain
advocates of the "earth-movement hypothesis," however, do not maintain
that all the glaciated areas were uplifted at one and the same time.
The glaciation of the Alps, they think, may have taken place earlier
or later than that of north-western Europe, while the ice-period of
the Rocky Mountains may not have coincided with that of eastern North
America. It is not impossible, they suppose, that the glaciation of the
Himalayas may have been caused by an uplifting of that great chain,
quite independent of similar earth-movements in other places. It can be
demonstrated, however, that the glaciation of the Alps and of northern
Europe were contemporaneous, and the facts go far to prove that the
glaciers of the Rocky Mountains and the inland-ice of north-east
America likewise co-existed. At all events all the old glacial
accumulations of our hemisphere are of Pleistocene age, and it is for
the advocates of the hypothesis under review to prove that they are
not contemporaneous. Their doubts on the subject probably arise from
the simple fact that they are well aware how highly improbable or even
impossible it is that all those glaciated lands could have been pushed
up within the snow-line at one and the same time.
Let me, however, advance to another objection. We know that the
Glacial period was interrupted by at least one interglacial epoch of
temperate and even genial conditions. Two glacial epochs with one
protracted interglacial epoch are now generally admitted. How do the
supporters of the "earth-movement hypothesis" explain this remarkable
succession of climatic changes? Their views as to the cause of glacial
conditions we have considered. If we can believe that the glacial
phenomena were due to elevation of the land, then we need have no
difficulty in understanding how glacial conditions would disappear
when the continents again subsided to a lower level. Not only did
North America and Europe lose all their early glacial elevation, but
by a lucky coincidence the Isthmus of Panama reappeared, and the Gulf
Stream resumed its beneficent course into the North Atlantic. This we
are to suppose was the cause of the interglacial epoch. But I would
point out that the geographical conditions which are thus inferred to
have brought about the disappearance of the glacial climate, and to
have ushered in the interglacial epoch, are precisely those that now
obtain--and, nevertheless, we are not yet in the enjoyment of a climate
like that of interglacial times. The strangely equable conditions that
permitted the development of the remarkable Pleistocene flora and fauna
are not experienced in the Europe of our day. And what about the second
glacial epoch? Are we to suppose that once more the lands were greatly
uplifted, and that convenient Isthmus of Panama was again depressed?
Did the Alps, the Pyrenees, and the plateau of central France--in all
of which we have distinct evidence of at least two glacial epochs--did
these heights, one may ask, rise up to bring about their earlier
glaciation, sink down again to induce interglacial conditions, and
once more become uplifted at the succeeding cold epoch, to subside
eventually in order to cause a final retreat of their glaciers?
But the climatic changes to be accounted for were in all probability
more numerous and complex than those just referred to. Competent
observers have adduced unmistakable evidence of three epochs of
glaciation in the Alpine Lands of Europe. And we are not without
distinct hints that similar changes have taken place in northern and
western Europe. Nor in this connection can we ignore the evidence of
several interglacial episodes which Mr. Chamberlin and others have
detected in the glaciated tracts of North America. Even this is not
all, for the upholders of the "earth-movement hypothesis" have still
further to account for the climatic oscillations of post-glacial times.
If it be hard enough to allow the possibility of one great movement
of elevation having affected so enormous an area of our hemisphere,
if we find it extremely difficult to believe either that one such
widespread movement, or that a multitude of local movements, each more
or less independent of the other, could have lifted the glaciated
regions successively within reach of the snow-line--we shall yet find
it impossible to admit that such remarkable upheavals could be repeated
again and again.
We seem driven to conclude, therefore, that the "earth-movement
hypothesis" fails to explain the phenomena of Pleistocene times. One
cannot deny, indeed, that glaciation might be induced locally by
elevation of the land. It is quite conceivable that mountains now
below the limits of perennial snow might come to be ridged up to such
an extent as to be capable of sustaining snow-fields and glaciers.
And such local movements may possibly have happened here and there
during the long-continued Pleistocene period. But the glacial phenomena
of that period are on much too grand a scale and far too widely
distributed to be accounted for in that way. And if the occurrence
of even one glacial epoch cannot be thus explained, we may leave the
supporters of the "earth-movement hypothesis" to show us what light
is thrown by their urim and thummim on the origin of succeeding
interglacial and glacial climates.
There is yet another physical condition of the Pleistocene and
post-glacial periods which any adequate explanation must embrace. I
refer to the oscillation of sea-level, of which so many proofs are
forthcoming. It is very remarkable that almost everywhere throughout
the maritime regions of formerly glaciated areas we find evidence of
submergence. So commonly is this the case, that geologists have long
suspected that the connection between glaciation and submergence might
be one of cause and effect. The possible influence of great ice-sheets
in disturbing the relative level of land and sea is a question,
therefore, of very great importance. It is one, however, which must be
solved by physicists. Croll and others have advocated the view that the
great accumulations of ice of the Glacial period may have displaced
the earth's centre of gravity, and thus caused the sea to rise upon
the glaciated hemisphere. The various results arrived at by physicists
are hardly comparable, because each has used different data, but it
seems probable that we have in this view a _vera causa_ of oscillations
of the sea-level. Another hypothesis would explain the rise of the
sea as due to the attractive influence of the great ice-masses, but
Dr. Drygalski's and Mr. Woodward's elaborate investigations would
seem to have demonstrated that this notion does not account for the
facts. Yet another speculation has been advanced. Mr. Jamieson has
suggested that the mere weight of the ice-sheets would suffice to
press down the earth's crust into a supposed liquid substratum, and
this explanation has met with much acceptance. Unfortunately our
knowledge of the condition of the earth's interior is so very limited
that we cannot be certain as to how the crust would be affected by
the weight of an ice-sheet. No doubt Mr. Jamieson's hypothesis gives
a specious explanation of certain geological phenomena, but if there
be no liquid substratum underlying a thin crust it cannot be true.
At present the prevalent view of physicists appears to be that the
earth is substantially solid. Professor George Darwin has shown
that the prominent inequalities of the earth's surface could not be
sustained unless the crust be as rigid as granite for a depth of 1000
miles. "If the earth be solid throughout," he remarks, "then at 1000
miles from the surface the material must be as strong as granite. If
it be fluid or gaseous inside, and the crust 1000 miles thick, that
crust must be stronger than granite, and if only 200 or 300 miles in
thickness, much stronger than granite." This conclusion is obviously
strongly confirmatory of Sir William Thomson's view, that the earth
is solid throughout. But many geologists find it hard to account for
the convolutions of strata and other structural phenomena on the
supposition that the earth is entirely solid, and they are inclined,
therefore, to adopt the hypothesis of a sub-crust layer of liquid
matter. Whether this be actually the condition or not physicists must
be left to determine. All that we need note is, that if there be any
force in Professor Darwin's argument, it is obvious that the crust is
possessed of great rigidity, and could not be readily deformed by the
mere weight of an ice-sheet. According to Dr. Drygalski, however, the
presence of an ice-sheet, by reducing the temperature of the underlying
crust, would bring about contraction, and in this way cause the surface
to sink. When the ice-sheet had disappeared, then free radiation of
earth-heat would be resumed, the depressed isogeotherms would rise,
and a general warming of the upper portion of the lithosphere would
take place. But the space occupied by the depressed section, owing to
the spheroidal form of the earth, would be smaller than that which it
occupied before sinking had commenced, and consequently when the ice
vanished expansion of the crust would follow, and the land-surface
would then rise again. The whole question is one for physicists to
decide upon, but I may point out that if Drygalski's explanation be
well founded, then it is obvious that it throws no light upon the
origin and subsequent disappearance of an ice-sheet. Somehow or other
this ice-sheet comes into existence, and the cooling and contracting
crust sinks below it; and that depressed condition of the glaciated
area must continue so long as the ice-sheet remains unmelted.
Re-elevation can only take place when, owing to some other cause or
causes, the climate changes and the ice-sheet vanishes.
Those who advocate the "earth-movement hypothesis" as an explanation of
the origin of extensive glaciation have welcomed Mr. Jamieson's view as
harmonising well with their conclusions. They contend, as we have seen,
that glacial conditions were induced by an extensive upheaval of the
crust in northern latitudes, accompanied by a depression of the Isthmus
of Panama. They then proceed to point out that the ice-sheets brought
about their own dissolution by pressing down the crust, and introducing
with submergence a disappearance of glacial conditions. See now how
much they take for granted. In the first place, they assume an amount
of pre-glacial or early glacial elevation of northern regions for which
not a scrap of evidence can be adduced, while they can give no proof of
contemporaneous depression of the Isthmus of Panama. Next, relying on
Mr. Jamieson's hypothesis, they take for granted that the ice-sheets,
called into existence by their postulated earth-movements, succeeded
in depressing the earth's surface even below its present level. That
is to say, the land, which, according to them, was in glacial times
some 3000 feet higher than now, sank down under the weight of its
glacial covering for, say, 3600 feet in north-western Europe. In North
America, in like manner, all the pre-glacial elevation was lost--the
land sinking below its present level for some 200 feet in New England,
for 520 feet at Montreal, for 1000 to 1500 feet in Labrador, and for
1000 to 2000 feet in the Arctic regions. Now, even if we concede the
reasonableness of Mr. Jamieson's hypothesis, and admit that a certain
degree of deformation may take place under the mere weight of an
ice-sheet, it is difficult to believe that the crust can be so readily
deformed as the supporters of the "earth-movement hypothesis" seem to
imply. If it could yield so readily to pressure, one is at a loss to
understand how a great ice-sheet could accumulate--the ice would simply
float off as the land subsided. Take the case of north-western Europe.
The ice-sheet that covered Scotland did not attain, on the average,
3000 feet in thickness, and yet we are to suppose that it was able to
depress the land for some 600 feet below its present level--that is to
say, for 3600 feet below its assumed pre-glacial elevation. Either the
ice depressed the crust to that remarkable extent, or the land upon
which the ice accumulated was not nearly so high as the advocates of
the "earth-movement hypothesis" have supposed. But the average I have
taken for the thickness of the Scottish ice-sheet is excessive, for it
was only in the low-grounds that the _mer de glace_ attained such a
depth. A large part of our country, however, is mountainous, and the
mountain-tops were, of course, not nearly so thickly mantled with ice
as the valleys. And the same to even a larger extent holds good for the
Scandinavian peninsula. If we take the thickness of the Scandinavian
ice-sheet that coalesced with that of Scotland as 4000 feet, we shall
be over the mark. Now, I ask, is it possible to believe that a sheet
of ice of that thickness actually pressed down the crust of the earth
for not less than 3600 feet? But if we accept the "earth-movement
hypothesis," as it has been recently advocated, that is what we must
believe. If we cannot do so, then we cannot accept the assumption of
great elevation of the land in pre-glacial and glacial times. Let me
put the case shortly: if the glacial marine beds and raised beaches
of the Atlantic borders of Europe and North America owe their origin
to depression induced by the weight of an ice-sheet, then it is quite
certain that at the advent of glacial conditions the land could not
have been so highly elevated as the advocates of the "earth-movement
hypothesis" suppose. But if we are to accept the notion of great
elevation of the land, then we must conclude that the submergence
to which the raised beaches testify cannot have been caused by the
pressure of ice-sheets.
It is hardly necessary to pursue this particular subject further, but
before leaving it, attention may be drawn for a moment to the curious
conclusion that the ice-sheets were self-destructive. One is left to
guess at what particular stage the sinking process began, but if the
earth's crust were as readily deformed as the extreme views I have
been examining would compel one to imply, then depression must have
commenced almost immediately with the accumulation of snow and ice. The
several ice-sheets must soon have attained their maximum thickness,
and their disappearance must have been correspondingly rapid. And
yet all the evidence goes to show that a glacial epoch endured for
a comparatively long time--for a time sufficient to account for a
prodigious amount of rock-erosion, and for the accumulation of vast
sheets of glacial débris and fluvio-glacial detritus.[BY]
[BY] It must not be inferred from the above remarks that I deny the
possibility of deformation of the crust having been induced by the
old ice-sheets. The geological evidence is certainly suggestive of
such having been the case. But I much doubt whether the sinking of the
surface was brought about by the mere weight of the ice pressing the
crust down into a subjacent liquid layer. Dr. Drygalski's explanation
would better account for the geological phenomena, but, according to
Rev. Osmond Fisher, it cannot be maintained.
If it be difficult to understand how the "earth-movement hypothesis"
can account for the origin of one glacial epoch, the difficulty is
not lessened when we remember that there are two or more such epochs
to account for. And until the advocates of that hypothesis can
furnish us with some reliable evidence, they can hardly expect us to
believe in their mysterious upheavals and depressions of northern
and temperate regions, and in the no less wonderfully rhythmic
movements of the Isthmus of Panama. In fine, the views which I have
been controverting seem to me to be untenable, inasmuch as they are
founded on mere assumptions, and do not even give a reasonable and
intelligible explanation of the phenomena of glaciated regions, while
they practically ignore or leave unsolved the problem of interglacial
conditions.
Some five-and-twenty years have now elapsed since my lamented friend
and colleague, James Croll, published his well-known physical theory of
the Glacial period. That theory, as you all know, has been frequently
criticised by physicists and others, to whose objections Croll made
a final reply in his _Climate and Cosmology_. In that work he has
successfully defended his views, and even added considerably to the
strength of his general argument. I am not aware that since then any
serious objections to Croll's theory have appeared. The only one indeed
that seems to have attracted attention is that which has been urged
especially by certain American geologists. Their belief is that the
close of the Glacial period must have taken place at a much more recent
date than Croll has inferred. And this belief of theirs is based upon
various estimates which have been made as to the time required for the
erosion of valleys and the accumulation of alluvial deposits since the
Glacial period. Thus, according to Mr. Gilbert, the post-glacial gorge
of Niagara, at the present rate of erosion, must have been excavated
within 7000 years; while Mr. Winchell, from similar measurements of
the post-glacial erosion of the Falls of St. Anthony, concludes that
8000 years have elapsed since the close of the Ice Age. I might cite
a number of similar estimates that tend to show that since the close
of the Glacial period only 7000 or 10,000 years have elapsed. What
will archæologists say to this conclusion? We know that Egypt was
already occupied by a civilised people nearly 6000 years ago, and
their marvellously advanced civilisation at that time presupposes,
according to Egyptologists, many thousands of years of development. Are
we, then, prepared to admit that the close of the Ice Age coincided
with the dawn of Egyptian civilisation? But all American observers are
not so parsimonious with regard to post-glacial time. Thus Professor
Spencer has given the age of the Falls of Niagara as 24,000 years, and
he informed me recently that this does not represent half of the time
since the formation of the third great series of glacial deposits of
the Canadian uplands. In our own Continent similar estimates have been
based on the rate of erosion of river-valleys, the rate of accumulation
of alluvial deposits, of peat-bogs, of stalagmite in caves, and what
not, with results that, to say the least, are rather discordant. The
fact is that all such measurements and estimates, however carefully
conducted and cautiously made, are in the nature of things unreliable.
We are insufficiently acquainted with all the factors of the problem
to be solved, and I cannot therefore agree with those who attribute
much weight to conclusions based on such uncertain data. Dr. Croll's
theory may eventually be modified, but I feel sure that it will not be
overturned by the inconclusive and unsatisfactory estimates to which I
have referred. Moreover, opponents of that theory may be reminded that
its truth does not rest on the accuracy of its author's conclusion as
to the date of the last Ice Age. That periods of high eccentricity of
the earth's orbit have occurred is beyond all doubt, but whether the
formulæ employed by Croll in calculating the date of the last great
cycle can be relied upon for that purpose is quite another question.
At present, so far as I understand the facts, the glacial and the
interglacial phenomena are explained by the astronomical theory, and by
no other. It gives a simple, coherent, and consistent interpretation of
the climatic vicissitudes of the Pleistocene and post-glacial periods,
and in especial it is the only theory that throws any light on the very
remarkable climates of interglacial times.
X.
The Glacial Succession in Europe.[BZ]
[BZ] _Trans. Royal Soc. Edinburgh_, vol. xxxvii. (1892).
For many years geologists have recognised the occurrence of at least
two boulder-clays in the British Islands and the corresponding
latitudes of the Continent. It is no longer doubted that these are the
products of two separate and distinct glacial epochs. This has been
demonstrated by the appearance of intercalated deposits of terrestrial,
freshwater, or, as the case may be, marine origin. Such interglacial
accumulations have been met with again and again in Britain, and they
have likewise been detected at many places on the Continent, between
the border of the North Sea and the heart of Russia. Their organic
contents indicate in some cases cold climatic conditions; in others,
they imply a climate not less temperate or even more genial than
that which now obtains in the regions where they occur. Nor are such
interglacial beds confined to northern and north-western Europe. In
the Alpine Lands of the central and southern regions of our Continent
they are equally well developed. Impressed by the growing strength of
the evidence, it is no wonder that geologists, after a season of doubt,
should at last agree in the conclusion that the glacial conditions of
the Pleistocene period were interrupted by at least one protracted
interglacial epoch. Not a few observers go further, and maintain that
the evidence indicates more than this. They hold that three or even
more glacial epochs supervened in Pleistocene times. This is the
conclusion I reached many years ago, and I now purpose reviewing the
evidence which has accumulated since then, in order to show how far it
goes to support that conclusion.
In our islands we have, as already remarked, two boulder-clays, of
which the lower or older has the wider extension southwards, for it has
been traced as far as the valley of the Thames. The upper boulder-clay,
on the other hand, does not extend south of the midlands of England.
In the north of England, and throughout Scotland and the major portion
of Ireland, it is this upper boulder-clay which usually shows at the
surface. The two clays, however, frequently occur together, and are
exposed again and again in deep artificial and natural sections, as
in pits, railway-cuttings, quarries, river-banks, and sea-cliffs.
Sometimes the upper clay rests directly upon the lower; at other times
they are separated by alluvial and peaty accumulations or by marine
deposits. The wider distribution of the lower till, the direction of
transport of its included erratics, and the trend of the underlying
_roches moutonnées_ and rock-striæ, clearly show that the earlier _mer
de glace_ covered a wider area than its successor, and was confluent
on the floor of the North Sea with the Scandinavian ice-sheet. It was
during the formation of the lower till, in short, that glaciation in
these islands attained its maximum development.
The interglacial beds, which in many places separate the lower from the
upper till, show that after the retreat of the earlier _mer de glace_
the climate became progressively more temperate, until eventually the
country was clothed with a flora essentially the same as the present.
Wild oxen, the great Irish deer, and the horse, elephant, rhinoceros,
and other mammals then lived in Britain. From the presence of such a
flora and fauna we may reasonably infer that the climate during the
climax of interglacial times was as genial as now. The occurrence of
marine deposits associated with some of the interglacial peaty beds
shows that eventually submergence ensued; and as the shells in some
of the marine beds are boreal and arctic forms, they prove that cold
climatic conditions accompanied the depression of the land. To what
extent the land sank under water we cannot tell. It may have been 500
feet or not so much, for the evidence is somewhat unsatisfactory.
The upper boulder-clay of our islands is the product of another _mer de
glace_, which in Scotland would seem to have been hardly less thick and
extensive than its predecessor. Like the latter, it covered the whole
country, overflowed the Outer Hebrides, and became confluent with the
Scandinavian inland-ice on the bed of the North Sea. But it did not
flow so far to the south as the earlier ice-sheet.
It is well known that this later _mer de glace_ was succeeded in our
mountain-regions by a series of large local glaciers, which geologists
generally believe were its direct descendants. It is supposed, in
short, that the inland-ice, after retreating from the low-grounds,
persisted for a time in the form of local glaciers in mountain-valleys.
This view I also formerly held, although there were certain appearances
which seemed to indicate that, after the ice-sheet had melted away from
the Lowlands and shrunk far into the mountains, a general advance of
great valley-glaciers had taken place. I had observed, for example,
that the upper boulder-clay is often well developed in the lower
reaches of our mountain-valleys--that, in fact, it may be met with more
or less abundantly up to the point at which large terminal moraines
are encountered. More than this, I had noticed that upland valleys,
in which no local or terminal moraines occur, are usually clothed and
paved with boulder-clay throughout. Again, the aspect of valleys which
have been occupied by large local glaciers is very suggestive. Above
the point at which terminal moraines occur only meagre patches of till
are met with on the bottoms of the valleys. The adjacent hill-slopes
up to a certain line may show bare rock, sprinkled perchance with
erratics and superficial morainic detritus; but above this line, if
the acclivity be not too great, boulder-clay often comes on again.
These appearances are most conspicuously displayed in the southern
Uplands of Scotland, particularly in south Ayrshire and Galloway, and
long ago they led me to suspect that the local glaciers into which
our latest _mer de glace_ was resolved, after retreating continuously
towards the heads of their valleys, so as to leave the boulder-clay in
a comparatively unmodified condition, had again advanced and ploughed
this out, down to the point at which they dropped their terminal
moraines. Subsequent observations in the Highlands and the Inner and
Outer Hebrides confirmed me in my suspicion, for in all those regions
we meet with phenomena of precisely the same kind. My friends and
colleagues, Messrs. Peach and Horne, had independently come to a
similar conclusion; and the more recent work of the Geological Survey
in the north-west Highlands, as they inform me, has demonstrated that
after the dissolution of the general ice-sheet underneath which the
upper boulder-clay was accumulated, a strong recrudescence of glacial
conditions supervened, and a general advance of great valley-glaciers
took place--the glaciers in many places coalescing upon the low-grounds
to form united _mers de glace_ of considerable extent.
The development of these large glaciers, therefore, forms a distinct
stage in the history of the Glacial period. They were of sufficient
extent to occupy all the fiords of the northern and western Highlands,
at the mouths of which they calved their icebergs, and they descended
the valleys on the eastern slopes of the land into the region of the
great lakes, at the lower ends of which we encounter their outermost
terminal moraines. The Shetland and Orkney Islands and the Inner and
Outer Hebrides at the same time nourished local glaciers, not a few
of which flowed into the sea. Such, for example, was the case in
Skye, Harris, South Uist, and Arran. The broad Uplands of the south
were likewise clothed with snow-fields that fed numerous glaciers.
These were especially conspicuous in the wilds of Galloway, but they
appeared likewise in the Peeblesshire hills; and even in less elevated
tracts they have left more or less well-marked traces of their former
presence.
It is to this third epoch of glaciation that I would assign the
final scooping out of our lake-basins and the completion of the deep
depressions in the beds of our Highland fiords. All the evidence,
indeed, leads to the conviction that the epoch was one of long duration.
It goes without saying that what holds good for Scotland must, within
certain limits, hold good also for Ireland and England. In Wales and
the Cumberland lake district, and in the mountain-regions of the sister
island, we meet with evidence of similar conditions. Each of those
areas has obviously experienced intense local glaciation subsequent to
the disappearance of the last big ice-sheet.
Attention must now be directed to another series of facts which help
us to realise the general conditions that obtained during the epoch
of local glaciation. In the basin of the estuary of the Clyde, and at
various other places both on the west and east coasts of Scotland,
occur certain clays and sands, which overlie the upper boulder-clay,
and in some places are found wrapping round the kames and osar of
the last great ice-sheet. These beds are charged with the relics of
a boreal and arctic fauna, and indicate a submergence of rather more
than 100 feet. In the lower reaches of the rivers Clyde, Forth, and
Tay the clays and sands form a well-marked terrace, and a raised
sea-beach, containing similar organisms, occurs here and there on the
sea-coast, as between Dundee and Arbroath, on the southern shores of
the Moray Firth, and elsewhere. When the terraces are traced inland
they are found to pass into high-level fluviatile gravels, which may
be followed into the mountain-valleys, until eventually they shade off
into fluvio-glacial detritus associated with the terminal moraines of
the great local glaciers. It is obvious, in short, that the epoch of
local ice-sheets and large valley-glaciers was one also of partial
submergence. This is further shown by the fact that in some places the
glaciers that reached the sea threw down their moraines on the 100-feet
beach. It must have been an epoch of much floating ice, as the marine
deposits contain now and again many erratics, large and small, and are,
moreover, frequently disturbed and contorted as if from the grounding
of pack-ice.
The phenomena which I have thus briefly sketched suffice to show that
the epoch of local glaciation is to be clearly distinguished from that
of the latest general _mer de glace_. I have long suspected, indeed,
that the two may have been separated by as wide an interval of time
as that which divided the earlier from the later epoch of general
glaciation. Again and again I have searched underneath the terminal
moraines, in the faint hope of detecting interglacial accumulations. My
failure to discover these, however, did not weaken my conviction, for
it was only by the merest chance that interglacial beds could ever have
been preserved in such places. I feel sure, however, that they must
occur among the older alluvia of our Lowlands. Indeed, as I shall point
out in the sequel, it is highly probable that they are already known,
and that we have hitherto failed to recognise their true position in
the glacial series.
Although we have no direct evidence to prove that a long interglacial
epoch of mild conditions immediately preceded the advent of our local
ice-sheets and large valley-glaciers, yet the indirect evidence is so
strong that we seem driven to admit that such must have been the case.
To show this I must briefly recapitulate what is now known as to the
glacial succession on the Continent. It has been ascertained, then,
that the Scandinavian ice has invaded the low-grounds of Germany on
two separate occasions, which are spoken of by Continental geologists
as the "first" and "second" glacial epochs. The earlier of these was
the epoch of maximum glaciation, when the inland ice flowed south into
Saxony, and overspread a vast area between the borders of the North
Sea and the base of the Ural Mountains. This ice-sheet unquestionably
coalesced with the _mer de glace_ of the British Islands. Its
bottom-moraine and the associated fluvio-glacial detritus are known
in Germany as "Lower Diluvium," and the various phenomena connected
with it clearly show that the inland-ice radiated outwards from the
high-grounds of Scandinavia. The terminal front of that vast _mer de
glace_ is roughly indicated by a line drawn from the south coast of
Belgium round the north base of the Harz, and by Leipzig and Dresden
to Krakow, thence north-east to Nijnii Novgorod, and further north to
the head-waters of the Dvina and the shores of the Arctic Sea near the
Tcheskaia Gulf.
The lower diluvium is covered in certain places by interglacial
deposits and an overlying upper diluvium--a succession clearly
indicative of climatic changes. In the interglacial beds occur remains
of _Elephas antiquus_ and other Pleistocene mammals, and a flora which
denotes a genial temperate climate. One of the latest discoveries
of interglacial remains is that of two peat-beds lying between the
lower and upper diluvium near Grünenthal in Holstein.[CA] Among the
abundant plant-relics are pines and firs (no longer indigenous to
Schleswig-Holstein), aspen, willow, white birch, hazel, hornbeam, oak,
and juniper. Associated with these are _Ilex_ and _Trapa natans_, the
presence of which, as Dr. Weber remarks, betokens a climate like that
of western middle Germany. Amongst the plants is a water-lily, which
occurs also in the interglacial beds of Switzerland, but is not now
found in Europe. The evidence furnished by this and other interglacial
deposits in north Germany shows that, after the ice-sheet of the lower
diluvium had melted away, the climate became as temperate as that now
experienced in Europe. Another recent find of the same kind[CB] is
the "diluvial" peat, etc., of Klinge, in Brandenburg, described by
Professor Nehring. These beds have yielded remains of elk (_Cervus
alces_), rhinoceros (species not determined), a small fox (?), and
Megaceros. This latter is not the typical great Irish deer, but a
variety (_C. megaceros_, var. _Ruffii_, Nehring). The plant-remains
include pine, fir (_Picea excelsa_), hornbeam, warty birch (_Betula
verrucosa_), various willows (_Salix repens_, _S. aurita_, _S. caprea_
[?], _S. cinerea_), hazel, poplar (?), common holly, etc. It is worthy
of note that here also the interglacial water-lily (_Cratopleura_) of
Schleswig-Holstein and Switzerland makes its appearance. Dr. Weber
writes me that the facies of this flora implies a well-marked temperate
insular climate (Seeklima). The occurrence of holly in the heart of the
Continent, where it no longer grows wild, is particularly noteworthy.
The evidence furnished by such a flora leads one to conclude that
at the climax of the genial interglacial epoch, the Scandinavian
snow-fields and glaciers were not more extensive than they are at
present.
[CA] _Neues Jahrbuch f. Min. Geol. u. Palæont._, 1891, ii., pp. 62,
228; _Ibid._, 1892, i., p. 114.
[CB] _Naturwissenschaftliche Wochenschrift_, Bd. vii. (1892), No. 4,
p. 31. The plants were determined by Dr. Weber, Professor Wittmack,
and Herr Warnstorf. [More recent investigations have considerably
increased our knowledge of this flora. See _Naturwissenschaftliche
Wochenschrift_, Bd. vii. (1892), Nr. 24, 25. _Ausland_, 1892, Nr. 20;
_Neues Jahrb. f. Min., etc._, 1893, Bd. i., p. 95.]
The presence of the upper diluvium, however, proves that such genial
conditions eventually passed away, and that an ice-sheet again invaded
north Germany. But this later invasion was not on the same scale as
that of the preceding one. The geographical distribution of the upper
diluvium and the position of large terminal moraines put this quite
beyond doubt. The boulder-clay in question spreads over the Baltic
provinces of Germany, extending south as far as Berlin,[CC] and west
into Schleswig-Holstein and Denmark. At the climax of this later
cold epoch glaciers occupied all the fiords of Norway, but did not
advance beyond the general coast-line. Norway at that time must have
greatly resembled Greenland--the inland-ice covering the interior of
the country, and sending seawards large glaciers that calved their
icebergs at the mouths of the great fiords. In the extreme south,
however, the glaciers did not quite reach the sea, but piled up large
terminal moraines on the coast-lands, which may be followed thence
into Sweden in an easterly direction by the lower end of Lake Wener
and through Lake Wetter. A similar belt of moraines marks out the
southern termination of the ice-sheet in Finland. Between Sweden and
Finland lies the basin of the Baltic, which, at the epoch in question
was filled with ice, forming a great Baltic glacier. This glacier
overflowed the Öland Islands, Gottland, and Öland, fanning out as it
passed towards the south-west and west, so as to invade on the south
the Baltic provinces of Germany, while in the north it traversed the
southern part of Scania, and overwhelmed the Danish islands as it
spread into Jutland and Schleswig-Holstein. The course of this second
ice-sheet is indicated by the direction of transport of erratics, etc.,
and by the trend of rock-striæ and _roches moutonnées_, as well as by
the position of its terminal and lateral moraines.
[CC] Not quite so far south. There is no reason to believe that the
ice-sheet of the so-called Great Baltic Glacier advanced beyond the
Baltic ridge. The upper boulder-clay south of that ridge is the
ground-moraine of an earlier glaciation--the equivalent of our upper
boulder-clay. See note, page 324. Nov. 1, 1892.
Such, then, is the glacial succession which has been established by
geologists in Scandinavia, north Germany, and Finland. The occurrence
of two glacial epochs, separated by a long interval of temperate
conditions, has been proved. The evidence, however, does not show
that there may not have been more than two glacial epochs. There are
certain phenomena, indeed, connected with the glacial accumulations
of the regions in question which strongly suggest that the succession
of changes was more complex than is generally understood. Several
years ago Dr. A. G. Nathorst adduced evidence to show that a great
Baltic glacier, similar to that underneath which the upper diluvium
was amassed, existed before the advent of the vast _mer de glace_ of
the so-called "first glacial epoch,"[CD] and his observations have
been confirmed and extended by H. Lundbohm.[CE] The facts set forth by
them prove beyond doubt that this early Baltic glacier smoothed and
glaciated the rocks in southern Sweden in a direction from south-east
to north-west, and accumulated a bottom-moraine whose included
erratics are equally cogent evidence as to the trend of glaciation.
That old moraine is overlaid by the lower diluvium--_i.e._, the
boulder-clay, etc., of the succeeding vast _mer de glace_ that flowed
south to the foot of the Harz--the transport of the stones in the
superjacent clay indicating a movement from NNE. to SSW., or nearly
at right angles to the trend of the earlier Baltic glacier. It is
difficult to avoid the conclusion that we have here to do with the
products of two distinct ice-epochs. But hitherto no interglacial
deposits have been detected between the boulder-clays in question. It
might, therefore, be held that the early Baltic glacier was separated
by no long interval of time from the succeeding great _mer de glace_,
but may have been merely a stage in the development of the latter.
It is at all events conceivable that before the great _mer de glace_
attained its maximum extension, it might have existed for a time
as a large Baltic glacier. I would point out, however, that if no
interglacial beds had been recognised between the lower and the upper
diluvium, geologists would probably have considered that the last great
Baltic glacier was simply the attenuated successor of the preceding
continental _mer de glace_. But we know this was not so; the two were
actually separated by a long epoch of genial temperate conditions.
[CD] "Beskrifning. till geol. Kartbl. Trolleholm": _Sveriges Geologiska
Undersökning_, Ser. Aa., Nr. 87.
[CE] "Om de äldre baltiska isströmmen i södra Sverige": _Geolog.
Förening. i Stockholm Förhandl._, Bd. x., p. 157.
There are certain other facts that may lead us to doubt whether in the
glacial phenomena of the Baltic coast-lands we have not the evidence
of more than two glacial epochs. Three, and even four, boulder-clays
have been observed in east and west Prussia. They are separated, the
one from the other, by extensive aqueous deposits, which are sometimes
fossiliferous. Moreover, the boulder-clays in question have been
followed continuously over considerable areas. It is quite possible,
of course, that all those boulder-clays may be the product of one
epoch, laid down during more or less considerable oscillations of an
ice-sheet. In this view of the case the intercalated aqueous deposits
would indicate temporary retreats, while the boulder-clays would
represent successive re-advances of one and the same _mer de glace_. On
the other hand, it is equally possible, if not more probable, that the
boulder-clays and intercalated beds are evidence of so many separate
glacial and interglacial epochs. We cannot yet say which is the true
explanation of the facts. But these being as they are, we may doubt if
German glacialists are justified in so confidently maintaining that
their lower and upper diluvial accumulations are the products of the
"first" and "second" glacial epochs. Indeed, as I shall show presently,
the upper diluvium of north Germany and Finland cannot represent the
second glacial epoch of other parts of Europe.
For a long time it has been supposed that the glacial deposits of the
central regions of Russia were accumulated during the advance and
retreat of one and the same ice-sheet. In 1888, however, Professor
Pavlow brought forward evidence to show that the province of Nijnii
Novgorod had been twice invaded by a general _mer de glace_. During the
first epoch of glaciation the ice-sheet overflowed the whole province,
while only the northern half of the same region was covered by the
_mer de glace_ of the second invasion. Again, Professor Armachevsky
has pointed out that in the province of Tchernigow two types of
glacial deposits appear, so unlike in character and so differently
distributed that they can hardly be the products of one and the same
ice-sheet. But until recently no interglacial deposits had been
detected, and the observations just referred to failed, therefore, to
make much impression. The missing link in the material evidence has
now happily been supplied by M. Krischtafowitsch.[CF] At Troïzkoje, in
the neighbourhood of Moscow, occur certain lacustrine formations which
have been long known to Russian geologists. These have been variously
assigned to Tertiary, lower glacial, post-glacial, and pre-glacial
horizons. They are now proved, however, to be of interglacial age,
for they rest upon and are covered by glacial accumulations. Amongst
their organic remains are oak (_Quercus pedunculata_), alder (_Alnus
glutinosa_, _A. incana_), white birch, hazel, Norway maple (_Acer
platanoides_), Scots fir, willow, water-lilies (_Nuphar_, _Nymphæa_),
mammoth, pike, perch, _Anadonta_, wing-cases of beetles, etc. The
character of the plants shows that the climate of central Russia was
milder and more humid than it is to-day.
[CF] _Bull. de la Soc. Impér. des Naturalistes de Moskau_, No. 4, 1890.
It is obvious that the upper and lower glacial deposits of central
Russia cannot be the equivalents of the upper and lower diluvium of
the Baltic coast-lands. The upper diluvium of those regions is the
bottom-moraine of the so-called great Baltic glacier. At the time that
glacier invaded north Germany, Finland was likewise covered with an
ice-sheet, which flowed towards the south-east, but did not advance
quite so far as the northern shores of Lake Ladoga. A double line of
terminal moraines, traced from Hango Head on the Gulf of Finland,
north-east to beyond Joensuu, puts this beyond doubt.[CG] The morainic
deposits that overlie the interglacial beds of central Russia cannot,
therefore, belong to the epoch of the great Baltic glacier. They are
necessarily older. In short, it is obvious that the upper and lower
glacial accumulations near Moscow must be on the horizon of the lower
diluvium of north Germany. And if this be so, then it is clear that
the latter cannot be entirely the product of one and the same _mer de
glace_. When the several boulder-clays described by Schröder and others
as occurring in the Baltic provinces of Germany are reinvestigated,
they may prove to be the bottom-moraines of as many distinct and
separate glacial epochs.
[CG] Sederholm, _Fennia_, i., No. 7; Frosterus, _ibid._, iii., No. 8;
Ramsay, _ibid._, iv., No. 2.
It may be contended that the glacial and interglacial deposits of
central Russia are perhaps only local developments--that their evidence
may be accounted for by the oscillations of one single _mer de glace_.
This explanation, as already pointed out, has been applied to the
boulder-clays and intercalated aqueous beds of the lower diluvium of
north Germany, and the prevalent character of the associated organic
remains makes it appear plausible. It is quite inapplicable, however,
to the similar accumulations in central Russia. During the formation
of the freshwater beds of Troïzkoje, no part of Russia could have
been occupied by an ice-sheet; the climate was more genial and less
"continental" than the present. Yet that mild interglacial epoch was
preceded and succeeded by extremely arctic conditions. It is impossible
that such excessive changes could have been confined to central Russia.
Germany, and indeed all northern and north-western Europe, must have
participated in the climatic revolutions.
So far, then, as the evidence has been considered, we may conclude
that three glacial and two interglacial epochs at least have been
established for northern Europe. If this be the case, then a
similar succession ought to occur in our own islands; and a little
consideration of the evidence already adduced will suffice to show that
it does. It will be remembered that the lower and upper boulder-clays
of the British Islands are the bottom-moraines of two separate and
distinct ice-sheets, each of which in its time coalesced on the floor
of the North Sea with the inland-ice of Scandinavia. It is obvious,
therefore, that our upper boulder-clay cannot be the equivalent of
the upper diluvium of the Baltic coast-lands, of Sweden, Denmark, and
Schleswig-Holstein. De Geer and others have shown that while the great
Baltic glacier was accumulating the upper diluvium of North Germany,
etc., the inland-ice of Norway calved its icebergs at the mouths of
the great fiords. Thus, during the so-called "second" glacial epoch of
Scandinavian and German geologists, the Norwegian inland-ice did not
coalesce with any British _mer de glace_. The true equivalent in this
country of the upper diluvium is not our upper boulder-clay, but the
great valley-moraines of our mountain-regions. It is our epoch of large
valley-glaciers which corresponds to that of the great Baltic ice-flow.
Our upper and lower boulder-clays are on the horizon of the lower
diluvium of Germany and the glacial deposits of central Russia.
It will now be seen that the evidence in Britain is fully borne out
by what is known of the glacial succession in the corresponding
latitudes of the Continent. I had inferred that our epoch of large
valley-glaciers formed a distinct stage by itself, and was probably
separated from that of the preceding ice-sheet by a prolonged interval
of interglacial conditions. One link in the chain of evidence, however,
was wanting: I could not point to the occurrence of interglacial
deposits underneath the great valley-moraines. But these, as we have
seen, form a well-marked horizon on the Continent, and we cannot doubt
that a similar interglacial stage obtained in these islands. We may
feel confident, in fact, that genial climatic conditions supervened on
the dissolution of the last great _mer de glace_ in Britain, and that
the subsequent development of extensive snow-fields and glaciers in our
mountain-regions was contemporaneous with the appearance of the last
great Baltic glacier.
We need not be surprised that interglacial beds should be well
developed underneath the bottom-moraine of that great glacier, while
they have not yet been recognised below the corresponding morainic
accumulations of our Highlands and Uplands. The conditions in the
low-grounds of the Baltic coast-lands favoured their preservation,
for the ice in those regions formed a broad _mer de glace_, under
the peripheral areas of which sub-glacial erosion was necessarily
at a minimum and the accumulation at a maximum. In our Scottish
mountain-valleys, however, the very opposite was the case. The
conditions obtaining there were not at all comparable to those that
characterised the low-grounds of northern Germany, etc., but were quite
analogous to those of Norway, where, as in our own mountain-regions,
interglacial beds are similarly wanting. It is quite possible,
however, that patches of such deposits may yet be met with underneath
our younger moraines, and they ought certainly to be looked for. But
whether they occur or not in our mountain-valleys, it is certain
that some of the older alluvia of our Lowlands must belong to this
horizon. Hitherto all alluvial beds that overlie our upper boulder-clay
have been classified as post-glacial; but since we have ascertained
that our latest _mer de glace_ was succeeded by genial interglacial
conditions, we may be sure that records of that temperate epoch will
yet be recognised in such Lowland tracts as were never reached by the
glaciers of the succeeding cold epoch. Hence, I believe that some of
our so-called "post-glacial" alluvia will eventually be assigned to
an interglacial horizon. Amongst these may be cited the old peat and
freshwater beds that rest upon the upper boulder-clay at Hailes Quarry,
near Edinburgh. To the same horizon, in all probability, belong the
clays, with Megaceros, etc., which occur so frequently underneath
the peat-bogs of Ireland. An interesting account of these was given
some years ago by Mr. Williams,[CH] who, as a collector of Megaceros
remains, had the best opportunity of ascertaining the nature of the
deposits in which these occur. He gives a section of Ballybetagh Bog,
nine miles south-east of Dublin, which is as follows:--
1. Boulder-clay.
2. Fine tenacious clay, without stones.
3. Yellowish clay, largely composed of vegetable matter.
4. Brownish clay, with remains of Megaceros.
5. Greyish clay.
6. Peat.
[CH] _Geol. Mag._, 1881, p. 354.
The beds overlying the boulder-clay are evidently of lacustrine
origin. The fine clay (No. 2), according to Mr. Williams, is simply
reconstructed boulder-clay. After the disappearance of the _mer de
glace_ the land would for some time be practically destitute of any
vegetable covering, and rain would thus be enabled to wash down the
finer ingredients of the boulder-clay that covered the adjacent slopes,
and sweep them into the lake. The clay formed in this way is described
as attaining a considerable thickness near the centre of the old
lake, but it thins off towards the sides. The succeeding bed (No. 3)
consists so largely of vegetable débris that it can hardly be called
a clay. Mr. Williams describes it as a "bed of pure vegetable remains
that has been ages under pressure." He notes that there is a total
absence in this bed of any tenacious clay like that of the underlying
stratum, and infers, therefore, that the rainfall during the growth
of the lacustrine vegetation was not so great as when the subjacent
clay was being accumulated. The remains of Megaceros occur resting on
the surface of the plant-bed and at various levels in the overlying
brownish clay, which attains a thickness of three to four feet. The
latter is a true lacustrine sediment, containing a considerable
proportion of vegetable matter, interstratified with seams of clay
and fine quartz-sand. According to Mr. Williams, it was accumulated
under genial or temperate climatic conditions like the present.
Between this bed and the overlying greyish clay (from 30 inches to
3 feet thick) there is always in all the bog deposits examined by
Mr. Williams a strongly-marked line of separation. The greyish clay
consists exclusively of mineral matter, and has evidently been derived
from the disintegration of the adjacent granitic hills. Mr. Williams
is of opinion that this clay is of aqueo-glacial formation. This he
infers from its nature and texture, and from its abundance. "Why,"
he asks, "did not this mineral matter come down in like quantity all
the time of the deposit of the brown clay which underlies it? Simply
because, during the genial conditions which then existed, the hills
were everywhere covered with vegetation; when the rain fell it soaked
into the soil, and the clay being bound together by the roots of the
grasses, was not washed down, just as at the present time, when there
is hardly any degradation of these hills taking place." He mentions,
further, that in the grey clay he obtained the antler of a reindeer,
and that in one case the antlers of a Megaceros, found embedded in
the upper surface of the brown clay, immediately under the grey clay,
were scored like a striated boulder, while the under side showed no
markings. Mr. Williams also emphasises the fact that the antlers of
Megaceros frequently occur in a broken state--those near the surface
of the brown clay being most broken, while those at greater depths are
much less so. He shows that this could not be the result of tumultuous
river-action--the elevation of the valley precluding the possibility
of its receiving a river capable of producing such effects. Moreover,
the remains show no trace of having been water-worn, the edges of the
teeth of the great deer being as sharp as if the animal had died but
yesterday. Mr. Williams thinks that the broken state of the antlers
is due to the "pressure of great masses of ice on the surface of the
clay in which they were embedded, the wide expanse of the palms of the
antlers exposing them to pressure and liability to breakage; and even,
in many instances, when there was 12 or 14 inches in circumference of
solid bone almost as hard and sound as ivory, it was snapped across."
It is remarkable that in this one small bog nearly one hundred heads of
Megaceros have been dug up.
Mr. Williams' observations show us that the Megaceros-beds are
certainly older than the peat-bogs with their buried timber. When he
first informed me of the result of his researches (1880), I did not
believe the Megaceros-beds could be older than the latest cold phase
of the Ice Age. I thought that they were later in date than our last
general _mer de glace_, and I think so still, for they obviously rest
upon its ground-moraine. But since I now recognise that our upper
boulder-clay is not the product of the last glacial epoch, it seems
to me highly probable that the Megaceros-beds are of interglacial
age--that, in short, they occupy the horizon of the interglacial
deposits of north Germany, etc. The appearances described by Mr.
Williams in connection with the "grey clay" seem strongly suggestive of
ice-action. Ballybetagh Bog occurs at an elevation of 800 feet above
the sea, in the neighbourhood of the Three Rock Mountain (1479 feet),
and during the epoch of great valley-glaciers the climatic conditions
of that region must have been severe. But without having visited the
locality in question I should hesitate to say that the phenomena
necessarily point to local glaciation. Probably frost, lake-ice, and
thick accumulations of snow and _névé_ might suffice to account for the
various facts cited by Mr. Williams.
I have called special attention to these Irish lacustrine beds,
because it is highly probable that the post-glacial age of similar
alluvia occurring in many other places in these islands has hitherto
been assumed and not proved. Now that we know, however, that a long
interglacial stage succeeded the disappearance of the last general
_mer de glace_, we may feel sure that the older alluvia of our Lowland
districts cannot belong exclusively to post-glacial times. The local
ice-sheets and great glaciers of our "third" glacial epoch were
confined to our mountain-regions; and in the Lowlands, therefore,
which were not invaded, we ought to have the lacustrine and fluviatile
accumulations of the preceding interglacial stage. A fresh interest now
attaches to our older alluvia, which must be carefully re-examined in
the new light thus thrown upon them.
Turning next to the Alpine Lands of central Europe, we find that
geologists there have for many years recognised two glacial epochs.
Hence, like their _confrères_ in northern Europe, they speak of
"first" and "second" glacial epochs.[CI] Within recent years, however,
Professor Penck has shown that the Alps have experienced at least
three separate periods of glaciation. He describes three distinct
ground-moraines, with associated river-terraces and interglacial
deposits in the valleys of the Bavarian Alps, and his observations
have been confirmed by Professor Brückner and Dr. Böhm.[CJ] The same
glacialists, I understand, have nearly completed an elaborate survey
of the eastern Alps, of which they intend shortly to publish an
extended account. The results obtained by them are very interesting,
and fully bear out the conclusions already arrived at from their
exploration of the Bavarian Alps.[CK] A similar succession of
glacial epochs has quite recently been determined by Dr. Du Pasquier
in north Switzerland.[CL] Nor is this kind of evidence confined to
the north side of the Alps. On the shores of Lake Garda, between
Salò and Brescia, three ground-moraines, separated by interglacial
accumulations, are seen in section. The interglacial deposits consist
chiefly of loams--the result of sub-aërial weathering--and attain a
considerable thickness. From this Penck infers that the time which has
elapsed since the latest glaciation is less than that required for the
accumulation of either of the two interglacial series--a conclusion
which, he says, is borne out by similar observations in other parts of
the Alpine region.[CM]
[CI] Morlot: _Bulletin de la Soc. Vaud. d. Sciences nat._, 1854, 1858,
1860. Deicke: _Bericht. d. St. Gall. naturf. ges._, 1858. Heer: _Urwelt
der Schweiz._ Mühlberg: _Festschrift d. aarg. naturf. Ges. z. Feier
ihrer 500 Sitz._, 1869. Rothpletz: _Denkschr. d. schweizer. Ges. f.
d. ges. Naturwissensch._, Bd. xxviii., 1881. Wettstein: _Geologie v.
Zurich u. Umgebung_, 1885. Baltzer: _Mitteil. d. naturf. Ges. Bern_,
1887. Renevier: _Bull. de la Soc. helvèt. d. Sciences nat._, 1887.
[CJ] Penck: _Die Vergletscherung d. deutschen Alpen_, 1882. Brückner:
"Die Vergletscherung des Salzachgebietes," _Geogr. Abhandl. Wien_, Bd.
i. Böhm: _Jahrb. der k. k. geol. Reichsanst._, 1884, 1885. See also O.
Fraas, _Neues Jahrb. f. Min. Geol. u. Palæont._, 1880, Bd. i. p. 218;
E. Fugger and C. Kastner, _Verhandl. d. k. k. geol. Reichsanst._, 1883,
p. 136.
[CK] _Mittheil. des deutsch. u. oesterreich. Alpenvereins_, 1890, No.
20 u. 23.
[CL] _Beiträge z. geolog. Karte der Schweiz_, 31 Lief., 1891; _Archiv.
d. Sciences phys. et nat._, 1891, p. 44.
[CM] "Die grosse Eiszeit," _Himmel u. Erde_.
Although the occurrence of such sub-äerial products intercalated
between separate morainic accumulations is evidence of climatic
changes, still it does not tell us how far the glaciers retreated
during an interglacial stage. Fortunately, however, lignite beds and
other deposits charged with plant remains are met with occupying a
similar position, and from these we gather that during interglacial
times the glaciers sometimes retired to the very heads of the
mountain-valleys, and must have been smaller than their present
representatives. Of such interglacial plant-beds, which have been met
with in some twenty localities, the most interesting, perhaps, is the
breccia of Hötting, in the neighbourhood of Innsbruck.[CN] This breccia
rests upon old morainic accumulations, and is again overlaid by the
later moraines of the great Inn glacier. From the fact that the breccia
yielded a number of supposed extinct species of plants, palæontologists
were inclined to assign it to the Pliocene. Professor Penck, however,
prefers to include it in the Pleistocene system, along with all the
glacial and interglacial deposits of the Alpine Lands. According to
Dr. von Wettstein, the flora in question is not Alpine but Pontic. At
the time of the formation of the breccia the large-leaved _Rhododendron
ponticum_ flourished in the Inn Valley at a height of 1200 metres above
the sea; the whole character of the flora, in short, indicates a warmer
climate than is now experienced in the neighbourhood of Innsbruck. It
is obvious, therefore, that in interglacial times the glaciers must
have shrunk back, as Professor Penck remarks, to the highest ridges of
the mountains.
[CN] Penck: _Die Vergletscherung der deutschen Alpen_, p. 228.
_Verhandl. d. k. k. geol. Reichsanst._, 1887, No. 5; _Himmel und Erde_,
1891. Böhm: _Jahrb. d. k. k. geol. Reichsanst._, 1884, p. 147. Blaas:
_Ferdinandeums Zeitschr._, iv. Folge; _Bericht. d. naturwissensch.
Vereins_, 1889, p. 97.
We may now glance at the glacial succession which has been established
for central France. More than twenty years ago Dr. Julien brought
forward evidence to show that the region of the Puy de Dôme had
witnessed two glacial epochs.[CO] During the first of these epochs a
large glacier flowed from Mont Dore. After its retreat a prolonged
interglacial epoch followed, during which the old morainic deposits
and the rocks they rest upon were much eroded. In the valleys and
hollows thus excavated freshwater beds occur which have yielded
relics of an abundant flora, together with the remains of _Elephas
meridionalis_, _Rhinoceros leptorhinus_, etc. After the deposition of
these freshwater alluvia, glaciers again descended the valleys and
covered the interglacial beds with their moraines. Similar results have
been obtained by M. Rames from a study of the glacial phenomenon of
Cantal, which he shows belong to two separate epochs.[CP] The interval
between the formation of the two series of glacial accumulations must
have been prolonged, for the valleys during that interval were in some
places eroded to a depth of 900 feet. M. Rames further recognises
that the second glacial epoch was distinguished by two advances of
valley-glaciers, separated by a marked episode of fusion. Dr. Julien
has likewise noted the evidence for two episodes of fusion during the
first extension of the glaciers of the Puy de Dôme.
[CO] _Des Phénomènes glaciaires dans le Plateau central de la France_,
&c.; Paris, 1869.
[CP] Bull. _Soc. géol. de France_, 1884; see also M. Boule, _Bull. de
la Soc. philomath. de Paris_, 8^e sér. i., p. 87.
Two glacial epochs have similarly been admitted for the Pyrenees;[CQ]
but Dr. Penck some years ago brought forward evidence to show that
these mountains, like the Alps, have experienced three separate and
distinct periods of glaciation.[CR]
[CQ] Garrigou: _Bull. Soc. géol. de France_, 2^e sér. xxiv., p. 577.
Jeanbernat: _Bull. de Soc. d'Hist. nat. de Toulouse_, iv., pp. 114,
138. Piette: _Bull. Soc. géol. de France_, 3^e sér. ii., pp. 503, 507.
[CR] _Mitteilungen d. Vereins f. Erdkunde zu Leipzig_, 1883.
We may now return to Scotland, and consider briefly the changes that
followed upon the disappearance of the local ice-sheets and large
valley-glaciers of our mountain-regions. The evidence is fortunately
clear and complete. In the valley of the Tay, for example, at and below
Perth, we encounter the following succession of deposits:--
6. Recent alluvia.
5. Carse-deposits, 45 feet above sea-level.
4. Peat and forest bed.
3. Old alluvia.
2. Clays, etc., of 100-feet beach.
1. Boulder-clay.
The old alluvia (3) are obviously of fluviatile origin, and show us
that after the deposition of the clays, etc., of the 100-feet beach
the sea retreated, and allowed the Tay and its tributaries to plough
their way down through the marine and estuarine deposits of the
"third" glacial epoch. These deposits would appear to have extended
at first as a broad and approximately level plain over all the lower
reaches of the valleys. Through this plain the Tay and the Earn cut
their way to a depth of more than 100 feet, and gradually removed all
the material over a course which can hardly be less than 2 miles in
breadth below the Bridge of Earn, and considerably exceeds that in the
Carse of Gowrie. No organic remains occur in the "old alluvia," but
the deposits consist principally of gravel and sand, and show not a
trace of ice-action. Immediately overlying them comes the well-known
peat-bed (4). This is a mass of vegetable matter, varying in thickness
from a few inches up to 3 or 4 feet. In some places it seems to be
made up chiefly of reed-like plants and sedges and occasional mosses,
commingled with which are abundant fragments of birch, alder, willow,
hazel, and pine. In other places it contains trunks and stools of oak
and hazel, with hazel-nuts--the trees being rooted in the subjacent
deposits. It is generally highly compressed and readily splits into
laminæ, upon the surface of which many small reeds, and now and again
wing-cases of beetles, may be detected. A large proportion of the woody
débris--twigs, branches, and trunks--appears to have been drifted. A
"dug-out" canoe of pine was found, along with trunks of the same tree,
in the peat at Perth. The Carse-deposits (5), consisting principally of
clay and silt, rest upon the peat-bed. The occurrence in these deposits
of _Scrobicularia piperata_ and oyster-shells leaves us in no doubt as
to their marine origin. They vary in thickness from 10 up to fully 40
feet.[CS]
[CS] For a particular account of the Tay-valley Succession, see
_Prehistoric Europe_, p. 385.
A similar succession of deposits is met with in the valley of the
Forth,[CT] and we cannot doubt that these tell precisely the same tale.
I have elsewhere[CU] adduced evidence to show that the peat-bed, with
drifted vegetable débris, which underlies the Carse accumulations of
the Forth and Tay is on the same horizon as the "lower buried forest"
of our oldest peat-bogs, and the similar bogs that occur in Norway,
Sweden, Denmark, Schleswig-Holstein, Holland, etc. Underneath the
"lower buried forest" of those regions occur now and again freshwater
clays, charged with the relics of an arctic-alpine flora; and quite
recently similar plant remains have been detected in old alluvia at
Corstorphine, near Edinburgh. When the beds below our older peat-bogs
are more carefully examined, traces of that old arctic flora will
doubtless be met with in many other parts of these islands. It was
this flora that clothed north-western Europe during the decay of the
last district ice-sheets of Britain and the disappearance of the great
Baltic glacier.
[CT] _Proc. Roy. Soc. Edin._ 1883-84, p. 745; _Mem. Geol. Survey,
Scotland_, Explanation of Sheet 31.
[CU] _Prehistoric Europe_, chaps. xvi., xvii.
The dissolution of the large valley-glaciers of this country was
accompanied by a general retreat of the sea--all the evidence leading
to the conviction that our islands eventually became united to the
Continent. The climatic conditions, as evidenced by the flora of the
"lower buried forest," were decidedly temperate--probably even more
genial than they are now, for the forests attained at that time a much
greater horizontal and vertical range. This epoch of mild climate and
continental connection was eventually succeeded by one of submergence,
accompanied by colder conditions. Britain was again insulated--the
sea-level in Scotland reaching a height of 45-50 feet above present
high-water. To this epoch pertain the Carse-clays of the Forth and Tay.
A few erratics occur in these deposits, probably betokening the action
of floating ice, but the beds more closely resemble the modern alluvial
silts of our estuaries than the tenacious clays of the 100-feet
terrace. When the Carse-clays are followed inland, however, they pass
into coarse river-gravel and shingle, forming a well-marked high-level
alluvial terrace of much the same character as the yet higher-level
fluviatile terrace which is associated in like manner with the marine
deposits of the 100-feet beach.
Of contemporaneous age with the Carse-clays, with which indeed they are
continuous, are the raised beaches at 45-50 feet. These beaches occur
at many places along the Scottish coasts, but they are seldom seen at
the heads of our sea-lochs. When the sea stood at this level, glaciers
of considerable size occupied many of our mountain-valleys. In the west
they came down in places to the sea-coast, and dropped their terminal
moraines upon the beach-deposits accumulating there. Thus, in Arran[CV]
and in Sutherland,[CW] these moraines are seen reposing on the raised
beaches of that epoch. And I think it is probable that the absence of
such beaches at the heads of many of the sea-lochs of the Highland
area is to be explained by the presence there of large glaciers, which
prevented their formation.
[CV] _British Association Reports_ (1854): Trans. of Sections, p. 78.
[CW] L. Hinxman: Paper read before Edin. Geol. Soc., April 1892.
Thus, there is clear evidence to show that after the genial epoch
represented by the "lower buried forest," a recrudescence of glacial
conditions supervened in Scotland. Many of the small moraines that
occur at the heads of our mountain-valleys, both in the Highlands and
Southern Uplands, belong in all probability to this epoch. They are
characterised by their very fresh and well-preserved appearance.[CX] It
is not at all likely that these later climatic changes could have been
confined to Scotland. Other regions must have been similarly affected.
But the evidence will probably be harder to read than it is with
us. Had it not been for the existence of our "lower buried forest,"
with the overlying Carse-deposits, we could hardly have been able to
distinguish so readily between the moraines of our "third" glacial
epoch and those of the later epoch to which I now refer. The latter, we
might have supposed, simply marked a stage in the final retreat of the
antecedent great valley-glaciers.
[CX] _Prehistoric Europe_ (chaps. xvi., xvii.) gives a fuller statement
of the evidence.
I have elsewhere traced the history of the succeeding stages of
the Pleistocene period, and adduced evidence of similar, but less
strongly-marked, climatic changes having followed upon those just
referred to, and my conclusions have been supported by the independent
researches of Professor Blytt in Norway. But these later changes need
not be considered here. It is sufficient for my general purpose to
confine attention to the well-proved conclusion that after the decay
of the last district ice-sheets and great glaciers of our "third"
glacial epoch genial conditions obtained, and that these were followed
by cold and humid conditions, during the prevalence of which glaciers
reappeared in many mountain-valleys.
We have thus, as it seems to me, clear evidence in Europe of four
glacial epochs, separated the one from the other by protracted
intervals of genial temperate conditions. So far, one's conclusions
are based on data which cannot be gainsaid, but there are certain
considerations which lead to the suspicion that the whole of the
complex tale has not yet been unravelled, and that the climatic
changes were even more numerous than those that I have indicated. Let
it be noted that glacial conditions attained their maximum during
the earliest of our recognised glacial epochs. With each recurring
cold period the ice-sheets and glaciers successively diminished in
importance. That is one of the outstanding facts with which we have
to deal. Whatever may have been the cause or causes of glacial and
interglacial conditions, it is obvious that those causes, after
attaining a maximum influence, gradually became less effective in their
operation. Such having been the case, one can hardly help suspecting
that our epoch of greatest glaciation may have been preceded by an
alternation of cold and genial stages analogous to those that followed
it. If three cold epochs of progressively diminished severity succeeded
the epoch of maximum glaciation, the latter may have been preceded by
one or more epochs of progressively increased severity. That something
of the kind may have taken place is suggested by the occurrence
of the old moraine of that great Baltic glacier that preceded the
appearance of the most extensive _mer de glace_ of northern Europe.
The old moraine in question, it will be remembered, underlies the
lower diluvium. Unfortunately, the very conditions that attended the
glaciation of Europe render it improbable that any conspicuous traces
of glacial epochs that may have occurred prior to the period of maximum
glaciation could have been preserved within the regions covered by the
great inland-ice. Their absence, therefore, cannot be held as proving
that the lower boulder-clays of Britain and northern Europe are the
representatives of the earliest glacial epoch. The lowest boulder-clay,
I believe, has yet to be discovered.
It is in the Alpine Lands that we encounter the most striking evidence
of glacial conditions anterior to the epoch of maximum glaciation.
The famous breccia of Hötting has already been referred to as of
interglacial age. From the character of its flora, Ettinghausen
considered this accumulation to be of Tertiary age. The assemblage of
plants is certainly not comparable to the well-known interglacial flora
of Dürnten. According to the researches of Dr. R. von Wettstein,[CY]
the Hötting flora has most affinity with that of the Pontic Mountains,
the Caucasus, and southern Spain, and implies a considerably warmer
climate than is now experienced in the Inn Valley. This remarkable
deposit, as Dr. Penck pointed out some ten years ago, is clearly of
interglacial age. His conclusions were at once challenged, on the
ground that the flora had a Tertiary and not a Pleistocene facies;
consequently, it was urged that, as all glacial deposits were of
Pleistocene age, this particular breccia could not be interglacial.
But in this, as in similar cases, the palæontologist's contention
has not been sustained by the stratigraphical evidence, and Dr.
Penck's observations have been confirmed by several highly-competent
geologists, as by MM. Böhm and Du Pasquier. The breccia is seen in
several well-exposed sections resting upon the moraine of a local
glacier which formerly descended the northern flanks of the Inn
Valley, opposite Innsbruck, where the mountain-slopes under existing
conditions are free from snow and ice. Nor is this all, for certain
erratics appear in the breccia, which could only have been derived
from pre-existing glacial accumulations, and their occurrence in this
accumulation at a height of 1150 metres shows that before the advent of
the Hötting flora the whole Inn Valley must have been filled with ice.
The plant-bearing beds are in their turn covered by the ground-moraine
of a later and more extensive glaciation. To bring about the glacial
conditions that obtained before the formation of the breccia, the
snow-line, according to Penck, must have been at least 1000 metres
lower than now; while, to induce the succeeding glaciation, the
depression of the snow-line could not have been less than 1200 metres.
These observations have been extended to many other parts of the Alps,
and the conclusion arrived at by Professor Penck and his colleagues,
Professor Brückner and Dr. Böhm, is briefly this--that the maximum
glaciation of those regions did not fall in the "first" but in the
"second" Alpine glacial epoch.
[CY] _Sitzungsberichte d. Kais. Acad. d. Wissensch. in Wien,
mathem.-naturw. Classe_, Bd. xcvii. Abth. i., 1888.
The glacial phenomena of northern and central Europe are so
similar--the climatic oscillations which appear to have taken place
had so much in common, and were on so grand a scale--that we cannot
doubt they were synchronous. We may feel sure, therefore, that the
epoch of maximum glaciation in the Alps was contemporaneous with the
similar epoch in the north. And if this be so, then in the oldest
ground-moraines of the Alps we have the records of an earlier glacial
epoch than that which is represented by the lower boulder-clays of
Britain and the corresponding latitudes of the Continent. In other
words, the Hötting flora belongs to an older stage of the Glacial
period than any of the acknowledged interglacial accumulations of
northern Europe. The character of the plants is in keeping with this
conclusion. The flora has evidently much less connection with the
present flora of the Alps than the interglacial floras of Britain
and northern Europe have with those that now occupy their place. The
Hötting flora, moreover, implies a considerably warmer climate than
now obtains in the Alpine regions, while that of our interglacial beds
indicates a temperate insular climate, apparently much like the present.
The high probability that oscillations of climate preceded the advent
of the so-called "first" _mer de glace_ of northern Europe must lead to
a re-examination of our Pliocene deposits, with a view to see whether
these yield conclusive evidence against such climatic changes having
obtained immediately before Pleistocene times. By drawing the line of
separation between the Pleistocene and the Pliocene at the base of our
glacial series, the two systems in Britain are strongly marked off
the one from the other. There is, in short, a distinct "break in the
succession." From the Cromer Forest-bed, with its abundant mammalian
fauna and temperate flora, we pass at once to the overlying arctic
freshwater bed and the superjacent boulder-clay that marks the epoch of
maximum glaciation.[CZ] Amongst the mammalian fauna of the Forest-bed
are elephants (_Elephas meridionalis_, _E. antiquus_), hippopotamus,
rhinoceros, (_R. etruscus_), horses, bison, boar, and many kinds of
deer, together with such carnivores as bears, _Machærodus_, spotted
hyæna, etc. The freshwater and estuarine beds which contain this
extensive fauna rest immediately upon marine deposits (Weybourn
Crag), the organic remains of which have a decidedly arctic facies.
Here, then, we have what at first sight would seem to be another
break in the succession. The Forest-bed, one might suppose, indicated
an interglacial epoch, separating two cold epochs. But Mr. Clement
Reid, who has worked out the geology of the Pliocene with admirable
skill,[DA] has another explanation of the phenomena. It has long been
known that the organic remains of the marine Pliocene of Britain
denote a progressive lowering of temperature. The lower member of the
system is crowded with southern forms, which indicate warm-temperate
conditions. But when we leave the Older and pass upwards into the
Newer Pliocene those southern forms progressively disappear, while at
the same time immigrants from the north increase in numbers, until
eventually, in the beds immediately underlying the Forest-bed, the
fauna presents a thoroughly arctic facies. During the formation of
the Older Pliocene with its southern fauna our area was considerably
submerged, so that the German Ocean had then a much wider communication
with the seas of lower latitudes. At the beginning of Newer Pliocene
times, however, the land emerged to some extent, and all connection
between the German Ocean and more southern seas was cut off. When at
last the "Forest-bed series" began to be accumulated, the southern half
of the North Sea basin had become dry land, and was traversed by the
Rhine in its course towards the north, the Forest-bed representing the
alluvial and estuarine deposits of that river.
[CZ] In some places, however, certain marine deposits (_Leda myalis_
bed) immediately overlie the Forest-bed.
[DA] _Mem. of Geol. Survey_, "Pliocene Deposits of Britain." _See
postea_, footnote, p. 317.
Mr. Reid, in referring to the progressive change indicated by the
Pliocene marine fauna, is inclined to agree with Professor Prestwich
that this was not altogether the result of a general climatic change.
He thinks the successive dying out of southern forms and the continuous
arrival of boreal species was principally due to the North Sea
remaining fully open to the north, while all connection with southern
seas was cut off. Under such conditions, he says, "there was a constant
supply of arctic species brought by every tide or storm, while at the
same time the southern forms had to hold their own without any aid
from without; and if one was exterminated it could not be replaced."
Doubtless the isolation of the North Sea must have hastened the
extermination of the southern forms, but the change could not have been
wholly due to such local causes. Similar, if less strongly-marked,
changes characterise the marine Pliocene of the Mediterranean area,
while the freshwater alluvia of France, etc., furnish evidence in the
same direction.
The Cromer Forest-bed overlies the Weybourn Crag, the marine fauna of
which has a distinctly Arctic facies. The two cannot, therefore, be
exactly contemporaneous: the marine equivalents of the Forest-bed are
not represented. But Mr. Reid points out that several arctic marine
shells of the Weybourn Crag occur also in the Forest-bed, while certain
southern freshwater and terrestrial shells common in the latter are
met with likewise in the former, commingled with the prevailing arctic
marine species. He thinks, therefore, that we may fairly conclude that
the two faunas occupied adjacent areas. One can hardly accept this
conclusion without reserve. It is difficult to believe that a temperate
flora and mammalian fauna like those of the Forest-bed clothed and
peopled eastern England when the adjacent sea was occupied by arctic
molluscs, etc. Surely the occurrence of a few forms, which are common
to the Forest-bed and the underlying Crag, does not necessarily prove
that the two faunas occupied adjacent districts. Mr. Reid, indeed,
admits that some of the marine shells in the Forest-bed series may have
been derived from the underlying Crag. Were the marine equivalents of
the Forest-bed forthcoming we might well expect them to contain many
Crag forms, but the facies of the fauna would most probably resemble
that of the existing North Sea fauna. Again, the appearance in the
Weybourn Crag of a few southern shells common to the Forest-bed does
not seem to prove more than that such shells were contemporaneous
somewhere with an arctic marine fauna. But it is quite possible that
they might have been carried for a long distance from the south;
and, even if they actually existed in the near neighbourhood of an
arctic marine fauna, we may easily attach too much importance to their
evidence.[DB] I cannot think, therefore, that Mr. Reid's conclusion is
entirely satisfactory. After all, the Cromer Forest-bed rests upon the
Weybourn Crag, and the evidence as it stands is explicable in another
way. It is quite possible, for example, that the Forest-bed really
indicates an epoch of genial or temperate conditions, preceded, as it
certainly was eventually succeeded, by colder conditions.
[DB] The inference that the Forest-bed occupies an interglacial
position is strengthened by the evidence of certain marine deposits
which immediately overlie it. These (known collectively as the _Leda
myalis_ bed) occur in irregular patches, which, from the character of
their organic remains, cannot all be precisely of the same age. In one
place, for example, they are abundantly charged with oysters, having
valves united, and with these are associated other species of molluscs
that still live in British seas. At another place no oysters occur, but
the beds yield two arctic shells, _Leda myalis_ and _Astarte borealis_,
and some other forms which have no special significance. Professor Otto
Torell pointed out to Mr. Reid that these separate deposits could not
be of the same age, for the oyster is sensitive to cold and does not
inhabit the seas where _Leda myalis_ and _Astarte borealis_ flourish.
From a consideration of this and other evidence Mr. Reid concludes that
it is possible that the deposits indicate a period of considerable
length, during which the depth of water varied and the climate changed.
Two additional facts may be noted: _Leda myalis_ does not occur in
any of the underlying Pliocene beds, while the oyster is not found in
the Weybourn and Chillesford Crag, though common lower down in the
Pliocene series. These facts seem to me to have a strong bearing on
the climatic conditions of the Forest-bed epoch. They show us that the
oyster flourished in the North Sea before the period of the Weybourn
Crag--that it did not live side by side with the arctic forms of that
period--and that it reappeared in our seas when favourable conditions
returned. When the climate again became cold an arctic fauna (including
a new-comer, _Leda myalis_) once more occupied the North Sea.
If it be objected that this would include as interglacial what has
hitherto been regarded by most as a Pliocene mammalian fauna,[DC] I
would reply that the interglacial age of that fauna has already been
proved in central France. The interglacial beds of Auvergne, with
_Elephas meridionalis_, rest upon and are covered by moraines,[DD] and
with these have been correlated the deposits of Saint-Prest. Again, in
northern Italy the lignites of Leffe and Pianico, which, as I showed a
number of years ago,[DE] occupy an interglacial position, have likewise
yielded _Elephas meridionalis_ and other associated mammalian forms.
[DC] _Elephas meridionalis_ is usually regarded as a type-form of the
Newer Pliocene, but long ago Dr. Fuchs pointed out that in Hungary
this species is of quaternary age: _Verhandl. d. k. k. geolog.
Reichsanstalt_, 1879, pp. 49, 270. It matters little whether we
relegate to the top of the Pliocene or to the base of the Pleistocene
the beds in which this species occurs. That it is met with upon an
interglacial horizon is certain; and if we are to make the Pleistocene
co-extensive with the glacial and interglacial series we shall be
compelled to include in that system some portion of the Newer Pliocene.
[DD] Julien: _Des Phènoménes glaciaires dans le Plateau central_, etc.,
1869. Boule: _Revue d'Anthropologie_, 1879.
[DE] _Prehistoric Europe_, p. 306. Professor Penck writes me that he
and the Swiss glacialist, Dr. Du Pasquier, have recently examined
these deposits, and are able to confirm my conclusion as to their
interglacial position.
There can be no doubt, then--indeed it is generally admitted--that
the cold conditions that culminated in our Glacial period began to
manifest themselves in Pliocene times. Moreover, as it can be shown
that _Elephas meridionalis_ and its congeners lived in central Europe
after an epoch of extensive glaciation, it is highly probable that
the Forest-bed, which contains the relics of the same mammalian
fauna, is equivalent in age to the early interglacial beds of France
and the Alpine Lands. We seem, therefore, justified in concluding
that the alternation of genial and cold climates that succeeded the
disappearance of the greatest of our ice-sheets was preceded by
analogous climatic changes in late Pliocene times.
I shall now briefly summarise what seems to have been the glacial
succession in Europe:--
{1. Weybourn Crag; ground-moraine of great Baltic
{ glacier underlying lower diluvium; the oldest
{ recognised ground-moraines of central Europe.
{
Glacial { These accumulations represent the earliest
{ glacial epoch of which any trace has been
{ discovered. It would appear to have been one of
{ considerable severity, but not so severe as the
{ cold period that followed.
{2. Forest-bed of Cromer; Hötting breccia; lignites
{ of Leffe and Pianico; interglacial beds of
Interglacial { central France.
{
{ Earliest recognised interglacial epoch; climate
{ very genial.
{3. Lower boulder-clays of Britain; lower diluvium
{ of Scandinavia and north Germany (in part);
{ lower glacial deposits of south Germany and
{ central Russia; ground-moraines and high-level
{ gravel-terraces of Alpine Lands, etc.;
Glacial { terminal moraines of outer zone.
{
{ The epoch of maximum glaciation; the
{ British and Scandinavian ice-sheets confluent;
{ the Alpine glaciers attain their greatest development.
{4. Interglacial freshwater alluvia, peat, lignite, etc.,
{ with mammalian remains (Britain, Germany,
{ etc., central Russia, Alpine Lands, etc.); and
{ marine deposits (Britain, Baltic coast-lands).
Interglacial {
{ Continental condition of British area; climate
{ at first cold, but eventually temperate. Submergence
{ ensued towards close of the period,
{ with conditions passing from temperate to
{ arctic.
{5. Upper boulder-clay of Britain; lower diluvium
{ of Scandinavia, Germany, etc., in part; upper
{ glacial series in central Russia; ground-moraines
{ and gravel-terraces in Alpine Lands.
{
{ Scandinavian and British ice-sheets again
Glacial { confluent, but _mer de glace_ does not extend
{ quite so far as that of the preceding cold epoch.
{ Conditions, however, much more severe than
{ those of the next succeeding cold epoch.
{ Alpine glaciers deposit the moraines of the
{ inner zone.
{6. Freshwater alluvia, lignite, peat, etc. (some of the
{ so-called post-glacial alluvia of Britain;
{ interglacial beds of north Germany, etc.; Alpine
{ Lands(?); marine deposits of Britain and Baltic
{ coast-lands).
Interglacial {
{ Britain probably again continental; climate at
{ first temperate and somewhat insular; submergence
{ ensues with cold climatic conditions--Scotland
{ depressed for 100 feet; Baltic provinces
{ of Germany, etc., invaded by the waters of
{ the North Sea.
{7. Ground-moraines, terminal moraines, etc., of the
{ mountain regions of Britain; upper diluvium
{ of Scandinavia, Finland, north Germany, etc.;
{ great terminal moraines of same regions; terminal
{ moraines in the large longitudinal valleys
{ of the Alps (Penck).
{
{ Major portion of Scottish Highlands covered
Glacial { by ice-sheet; local ice-sheets in Southern Uplands
{ of Scotland and mountain districts in
{ other parts of Britain; great valley-glaciers
{ sometimes coalesce on low-grounds; icebergs
{ calved at mouths of Highland sea-lochs; terminal
{ moraines dropped upon marine deposits
{ then forming (100-feet beach). Scandinavia
{ shrouded in a great ice-sheet, which broke
{ away in icebergs along the whole west coast of
{ Norway. Epoch of the last great Baltic glacier.
{8. Freshwater alluvia (with arctic plants); "lower
{ buried forest and peat" (Britain and north-west
{ Europe generally). Carse-clays and raised
{ beaches of 45 to 50-feet level in Scotland.
Interglacial {
{ Britain again continental; climate at first
{ cold, subsequently becoming temperate: great
{ forests. Eventual insulation of Britain; climate
{ humid, and probably colder than now.
{9. Local moraines in mountain-valleys of Britain,
{ here and there resting on 45 to 50-feet beach;
{ so-called "post-glacial" moraines in the upper
{ valleys of the Alps.
{
{ Probably final appearance of glaciers in our
Glacial { islands. Some of these glaciers attained a
{ considerable size, reaching the sea and shedding
{ icebergs. It may be noted here that the decay
{ of these latest glaciers was again followed by
{ emergence of the land and a recrudescence of
{ forest-growth ("upper buried forest").
A word of reference may now be made to that remarkable association
of evidence of submergence, with proofs of glacial conditions, which
has so frequently been noted by geologists. Take, for example, the
succession in Scotland, and observe how each glacial epoch was preceded
and apparently accompanied by partial submergence of the land:--
1. _Epoch of Greatest Mer de Glace_ (lower boulder-clay); British and
Scandinavian ice-sheets coalescent. Followed by wide land-surface
= Continental Britain, with genial climate. Submergence of land--to
what extent is uncertain, but apparently to 500 feet or so.
2. _Epoch of Lesser Mer de Glace_ (upper boulder-clay); British and
Scandinavian ice-sheets coalescent. Followed by wide land-surface
= Continental Britain, with genial climate. Submergence of land for
100 feet or thereabout.
3. _Epoch of Local Ice-sheets in Mountain Districts;_ glaciers here
and there coalesce on the low-grounds; icebergs calved at mouths of
Highland sea-lochs (moraines on 100-feet beach). Followed by wide
land-surface = Continental Britain, with genial climate. Submergence
of land for 50 feet or thereabout.
4. _Epoch of Small Local Glaciers_, here and there descending to sea
(moraines on 50-feet beach).
These oscillations of the sea-level did not terminate with the
emergence of the land after the formation of the 50-feet beach. There
is evidence to show that subsequent to the retreat of the small local
glaciers (4) and the emergence of the land, our shores extended
seawards beyond their present limits, but how far we cannot tell. With
this epoch of re-emergence the climate again became more genial, our
forests once more attaining a greater vertical and horizontal range.
Submergence then followed (the 25 to 30-feet beach), accompanied
by colder and more humid conditions, which, while unfavourable to
forest-growth, tended greatly to increase the spread of peat-bogs. We
have no evidence, however, to show that small local glaciers again
appeared. Finally the sea retired, and the present conditions ensued.
It will be seen that the submergence which preceded and probably
accompanied the advent of the lesser _mer de glace_ (2) was greater
than that which heralded the appearance of the local ice-sheets (3),
as that in turn exceeded the depression that accompanied the latest
local glaciers (4). There would seem, therefore, to be some causal
connection between cold climatic conditions and submergence. This is
shown by the fact that not only did depression immediately precede and
accompany the appearance of ice-sheets and glaciers, but the degree of
submergence bore a remarkable relation to the extent of glaciation.
Many speculations have been indulged in as to the cause of this curious
connection between glaciation and depression; these, however, I will
not consider here. None of the explanations hitherto advanced is
satisfactory, but the question is one well deserving the attention of
physicists, and its solution would be of great service to geology.
A still larger question which the history of these times suggests
is the cause of climatic oscillations. I have maintained that the
well-known theory advanced by James Croll is the only one that seems
to throw any light upon the subject, and the observations which have
been made since I discussed the question at length, some fifteen years
ago, have added strength to that conviction. As Sir Robert Ball has
remarked, the astronomical theory is really much stronger than Croll
made it out to be. In his recently-published work, _The Cause of an Ice
Age_, Sir Robert says that the theory is so thoroughly well based that
there is no longer any ground for doubting its truth. "We have even
shown," he continues, "that the astronomical conditions are so definite
that astronomers are entitled to direct that vigorous search be
instituted on this globe to discover the traces of those vast climatic
changes through which astronomy declares that our earth must have
passed." In concluding this paper, therefore, I may shortly indicate
how far the geological evidence seems to answer the requirements of the
theory.
Following Croll, we find that the last period of great eccentricity
of the earth's orbit extended over 160,000 years--the eccentricity
reaching its highest value in the earlier stages of the cycle. It is
obvious that during this long cycle the precession of the equinox must
have completed seven revolutions. We might therefore expect to meet
with geological evidence of recurrent cold or glacial and genial or
interglacial epochs; and not only so, but the records ought to show
that the earlier glacial epoch or epochs were colder than those that
followed. Now we find that the epoch of maximum glaciation supervened
in early Pleistocene times, and that three separate and distinct
glacial epochs of diminished severity followed. Of these three, the
first would appear to have been almost as severe as that which preceded
it, and it certainly much surpassed in severity the cold epochs of the
later stages. But the epoch of maximum glaciation, or the first of
the Pleistocene series, was not the earliest glacial epoch. It seems
to have been preceded by one of somewhat less severity than itself,
but which nevertheless, as we gather from the observations of Penck
and his collaborators, was about as important as that which came
after the epoch of maximum glaciation. Hence it would appear that the
correspondence of the geological evidence with the requirements of
the astronomical theory is as close as we could expect it to be. Four
glacial with intervening genial epochs appear to have fallen within
Pleistocene times; while towards the close of the Pliocene, or at the
beginning of the Pleistocene period, according as we choose to classify
the deposits, an earlier glacial epoch followed by genial interglacial
conditions, supervened.
In this outline of a large subject it has not been possible to do more
than indicate very briefly the general nature of the evidence upon
which the chief conclusions are based. I hope, however, to have an
opportunity ere long of dealing with the whole question in detail.
[Note.--Since the original publication of this Essay, renewed
investigation and study have led me to conclude that the correlation
of the British and Continental glacial series is even more simple
than I had supposed. I believe the use of the terms "Lower"
and "Upper" in connection with the "Diluvial" deposits of the
Continent has hitherto blinded us to the obvious succession of
the boulder-clays. In Britain we have, as shown above, a "lower
boulder-clay," an "upper boulder-clay," and the still younger
boulder-clays (ground-moraines), and terminal moraines of our
district ice-sheets and valley-glaciers. In the low-grounds of
the Continent the succession is precisely similar. Thus the lower
boulder-clay that sweeps south into Saxony represents the lower
boulder-clay of Britain. In like manner, the upper boulder-clay of
western and middle Germany, of Poland, and western and north-western
Russia, is the equivalent of our own upper boulder-clay. Lastly, the
so-called "upper diluvium" and the great terminal moraines of the
Baltic coast-lands are on the horizon of the younger boulder-clays
and terminal moraines of the mountainous areas of the British
Islands. The so-called "lower diluvium" of the Baltic coast-lands
thus represents not the _lower_ but the _upper_ diluvium of western
and middle Germany, Poland, etc. German geologists are of opinion
that the upper boulder-clays of the Baltic coast-lands and of the
valley of the Elbe are the ground-moraines of one and the same
ice-sheet, which, on its retreat, piled up the terminal moraines
of the Baltic Ridge. I believe the two boulder-clays in question
are quite distinct, and that the terminal moraines referred to
mark the furthest advance of the last great Baltic glacier. The
contemporaneity of the two boulder-clays has been taken for granted
simply because they are each underlaid by a lower boulder-clay. But,
as we have seen, the upper boulder-clay of the Baltic coast-lands is
underlaid not by one only, but by two, and in some places even by
three other boulder-clays--phenomena which never present themselves
in the regions not invaded by the last great Baltic glacier. Three
or four boulder-clays occur in the coast-lands of the Baltic because
those regions were overflowed successively by three or four separate
ice-sheets. Only two boulder-clays are met with south and east of
the Baltic Ridge, because the tracts lying south and south-east of
that ridge were traversed by only two _mers de glace_--namely, by
that of the epoch of maximum glaciation and by the less extensive
ice-sheet of the next succeeding cold period. In the region between
the Elbe and the mountains of middle Germany only one boulder-clay
appears, because that region has never been invaded by more than
one ice-sheet. The succession thus indicated may be tabulated as
follows:--
1. _Epoch of Earliest Baltic Glacier._ Lowest boulder-clay of
southern Sweden; lowest boulder-clay of Baltic provinces of
Prussia; horizon of the Weybourn Crag.
2. _Epoch of Greatest Mer de Glace._ Lower boulder-clays of middle
and southern Germany, central Russia, British Islands; second
boulder-clay of Baltic provinces of Prussia.
3. _Epoch of Lesser Mer de Glace._ Upper boulder-clay of western and
middle Germany, Poland, and west central Russia; upper boulder-clay
of Britain; third boulder-clay of Baltic provinces of Prussia.
4. _Epoch of Last Great Baltic Glacier._ Upper boulder-clay and
terminal moraines of Baltic coast-lands; district and valley-moraines
of Highlands and Uplands of British Islands.
5. _Epoch of Small Local Glaciers._ Valley-moraines in mountainous
regions of Britain, etc.
The evidence on which these conclusions are based is set forth at some
length in a forthcoming re-issue of my _Great Ice Age_.--Nov. 1, 1892.]
[Illustration:
PLATE IV
SKETCH MAP OF NORTHERN EUROPE SHOWING AREAS COVERED BY
ICE DURING THE EPOCH OF MAXIMUM GLACIATION, AND BY THE GREAT BALTIC
GLACIER AND THE LOCAL ICE-SHEETS OF BRITAIN AT A LATER DATE.
The Edinburgh Geographical Institute J. G. Bartholomew F.R.G.S
]
* * * * *
Explanation of Plate IV.
Map of Europe showing the areas occupied by ice during the Epoch of
Maximum Glaciation (Second Glacial Epoch), and the extent of glaciation
in Scandinavia, Finland, Baltic coast-lands, etc., and the British
Islands during the Fourth Glacial Epoch. For the limits of the greater
glaciation on the Continent, Habenicht, Penck, Nikitin, and Nathorst
have been followed. The Great Baltic Glacier is chiefly after De Geer.
XI.
The Geographical Evolution of Europe.[DF]
[DF] _The Scottish Geographical Magazine_, vol. ii., 1886.
It is one of the commonplaces of geology that the Present is built
up out of the ruins of the Past. Every rock beneath our feet has its
story of change to tell us. Mountains, valleys, and plains, continents
and islands, have passed through vicissitudes innumerable, and bear
within them the evidence of a gradual development or evolution. Looking
back through the vista of the past one sees the dry lands gradually
separating from the ocean, and gathering together into continental
masses according to a definite plan. It is this slow growth, this
august evolution, carried on through countless æons, which most
impresses the student of physical geology. The earth seems for the time
as if endowed with life, and like a plant or animal to pass through its
successive stages of development until it culminates in the present
beautiful world. This conception is one of comparatively recent growth
in the history of geological science. Hutton, the father of physical
geology, had indeed clearly perceived that the dry lands of the globe
were largely composed of the débris of former land-surfaces--that
there had been alternate elevations and depressions of the earth's
crust, causing now sea and now land to predominate over given areas.
But the facts known in this day could not possibly have suggested
those modern ideas of geographical evolution, which are the outcome
of the multifarious observation and research of later years. It is to
Professor Dana, the eminent American geologist, that we are indebted
for the first clear enunciation of the views which I am now about to
illustrate. According to him the great oceanic basins and continental
ridges are of primeval antiquity--their origin is older than that
of our oldest sedimentary formations. It is not maintained that the
present lands have always continued above the level of the sea. On the
contrary, it can be proved that many oscillations of level have taken
place within each continental area, by which the extent and outline
of the land have been modified again and again. Notwithstanding such
changes, however, the great continental ridges would appear to have
persisted from the earliest geological times as dominant elevations of
the earth's crust. Some portions of these, as Dana remarks, may have
been submerged for thousands of feet, but the continents have never
changed places with the oceans.
I shall presently indicate the nature of the evidence by which it is
sought to prove the vast age of our continental masses, but before
doing so it will be well to give an outline of the facts which go to
show that the oceanic depressions of the globe are likewise of primeval
antiquity.
The memorable voyage of the _Challenger_ has done much to increase
our knowledge of the deep seas and the accumulations forming therein.
The researches of the scientific staff of the expedition, and more
particularly those of Mr. Murray, have indeed given a new impulse
to the study of the larger questions of physical geology, and have
lent strong support to the doctrine of the permanence of the oceanic
basins and continental ridges. One of the most important facts brought
before our attention by Mr. Murray is the absence of any land-derived
materials from the sediments now gathering in the deeper abysses of the
ocean. The coasts of continents and continental islands are strewn, as
every one knows, with the wreck of the land--with gravel, sand, and
mud, derived from the demolition of our rocks and soils. The coarser
débris accumulates upon beaches and in shallow littoral waters, while
the finer materials are swept further out to sea by tidal and other
currents--the sediment being gradually sifted as it is borne outwards
into deeper water, until only the finest mud and silt remain to be
swept forward. As the floor of the ocean shelves down to greater depths
the transporting power of currents gradually lessens, and finally
land-derived sediment ceases to appear. Such terrigenous materials
may be said to extend from the littoral zone down to depths of 2000
feet and more, and to a distance of 60 to 300 miles from shore. They
are confined, therefore, to a comparatively narrow belt round the
margins of continents and islands. And thus there are vast regions
of the oceanic depressions over which no terrigenous or land-derived
materials are accumulating. Instead of these we meet with a remarkable
red clay and various kinds of ooze, made up largely of the shells of
foraminifera, pelagic mollusca, and radiolarians, and the frustules
of diatoms. The red clay is the most widely distributed of abysmal
deposits. Indeed, it seems to form a certain proportion of all the
deep-sea organic oozes, and may be said, therefore, to exist everywhere
in the abysmal regions of the oceanic basins. It is extremely
fine-grained, and owes its deep brown or red colour to the presence of
the oxides of manganese and iron. Scattered through the deposit occur
particles of various minerals of volcanic origin, together with lapilli
and fragments of pumice, _i.e._, volcanic _ejectamenta_. Such materials
may have been thrown out from terrestrial volcanoes and carried by
the winds or floated by currents until they became water-logged and
sank; or they may to some extent be the relics of submarine eruptions.
Whatever may have been their immediate source, they are unquestionably
of volcanic origin, and are not associated with any truly terrigenous
sediment. The red clay is evidently the result of the chemical action
of sea-water on volcanic products; and many facts conspire to show
that its formation is an extremely slow process. Thus, remains of
vertebrates, consisting of the ear-bones of whales, beaks of ziphius,
and teeth of sharks, are often plentifully present, and there is no
reason to suppose, as MM. Murray and Renard point out, that the parts
of the ocean where these remains occur are more frequented by whales
and sharks than other regions where similar relics are rarely or never
dredged up. Of these remains some have all the appearance of having
lain upon the sea-bottom for a very long time, for they belong to
extinct species, and are either partially coated or entirely surrounded
with thick layers of manganese-iron. In the same red clay occur small
metallic spherules which are of cosmic origin--in other words, meteoric
dust. The accumulation of all these substances in such relatively great
abundance shows us that the oceanic basins have remained unchanged for
a vast period of time, and assures us that the formation of the abysmal
red clay is extremely slow.
When we come to examine the rocks which enter into the framework of
our continents, we find that they may be roughly classed under these
heads:--
1st, Terrestrial and Aqueous Rocks.
2d, Igneous Rocks.
3d, Crystalline Schists.
By far the largest areas of land are composed of rocks belonging to
the first class. These consist chiefly of the more or less indurated
sediments of ancient rivers, lakes, and seas--namely, conglomerate,
sandstone, shale, limestone, etc. And now and again, interstratified
with such aqueous beds, we meet with rocks of terrestrial origin, such
as lignite, coal, and the débris of former glacial action. Now, most of
our aqueous rocks have been accumulated in the sea, and thus we arrive
at the conclusion that the present continental areas have from time to
time been largely submerged--that the sea has frequently covered what
are now the dry lands of the globe. But one remarkable fact stands out,
and it is this: Nowhere amongst the sedimentary rocks of the earth's
crust do we meet with any ancient sediments which can be likened to
the red clay now slowly accumulating in the deeper abysses of the
ocean. There are no rocks of abysmal origin. Many of our limestones
have undoubtedly formed in deep, clear water, but none of these is
abysmal. Portions of Europe may now and again have been submerged for
several thousand feet, but no part of this or any other continent,
so far as we yet know, has within geological time been depressed to
depths comparable to those of the present oceanic basins. Nay, by
far the larger portions of our marine formations have accumulated in
comparatively shallow water--sandstones and shales (sand and mud) being
by far the most common kinds of rock that we encounter. In short,
aqueous strata have, as a rule, been deposited at no great depth and
at no great distance from dry land; the rocks are built up mostly of
terrigenous material; and even the purer limestones and chalks, which
we may suppose accumulated in seas of moderate depth, not infrequently
contain some terrestrial relic which has been drifted out to sea, and
afford other evidence to show that the nearest land was never very far
away. Followed along their outcrop such rocks sooner or later become
mixed and interbedded with ordinary sedimentary matter. Thus, for
example, the thick carboniferous limestone of Wales and the Midlands of
England must have accumulated in the clear water of a moderately deep
sea. But when this limestone is traced north into Northumberland it
begins to receive intercalations of sandstone and shale, which become
more and more important, until in Scotland they form by much the larger
portion of the series--the enormous thick limestones of the south being
represented by only a few inconsiderable beds, included, along with
seams of ironstone and coal, in a thick succession of sandstones and
shales.
Of the igneous rocks and the crystalline schists I need not speak at
present, but I shall have something to say about them before I have
done.
Having learned that no truly abysmal rocks enter into the composition
of our continents, of what kind of rocks, we may ask, are the islands
composed? Well, some of those islands are built up of precisely the
same materials as we find in the continents. This is the case with most
islands which are not separated from continental areas by profoundly
deep seas. Thus our own islands with their numerous satellites are
geologically one with the adjacent continent. Their present separation
is a mere accident. Were the European area, with the adjacent sea-bed,
to be elevated for a few hundred feet we should find that Britain and
Ireland form geologically part and parcel of the continent. And the
same is the case with Nova Zembla and Spitzbergen in the north, and
with the Mediterranean islands in the south. There is another large
class of islands, however, which are characterised by the total absence
of any of those sedimentary rocks of which, as I have just said, our
continents and continental islands are chiefly built up. The islands
referred to are scattered over the bosom of the great ocean, and are
surrounded by profoundly deep water. Some are apparently composed
entirely of coral, others are of volcanic origin, and yet others are
formed partly of volcanic rock and partly of coral. Thus we have two
distinct kinds of island:--
1st, Islands which have at one time evidently been connected with
adjacent continents, and which are therefore termed
_continental islands_; and
2d, _Oceanic islands_, which rise, as it were, from profound depths
in the sea, and which have never formed part of the continents.
The fauna and flora of the former class agree with those of the
neighbouring continents, although some modifications are met with,
especially when the insulation has been of long standing. When such has
been the case the species of plants and animals may be almost entirely
distinct. Nevertheless, such ancient continental islands agree with
those which have been separated in more recent geological times in
containing both indigenous amphibians and mammals. Oceanic islands,
on the other hand, contain no indigenous mammals or amphibians,
their life consisting chiefly of insects and birds, and usually some
reptiles--just such a fauna as might have been introduced by the
influence of winds and of oceanic currents carrying driftwood.
Such facts, as have now been briefly summarised, point clearly to
the conclusion that the oceanic basins and continental areas are of
primeval antiquity. All the geological facts go to prove that abysmal
waters have never prevailed over the regions now occupied by dry land;
nor is there any evidence to show that continental land-masses ever
existed in what are now the deepest portions of the ocean. The islets
dotted over the surface of the Pacific and the other great seas are
not the relics of a vast submerged continent. They are either the tops
of submarine volcanic mountains, or they are coral structures founded
upon the shoulders of degraded volcanoes and mountain-chains, and
built up to the surface by the indefatigable labours of the humble
polyp. We come then to the general conclusion that oceanic basins and
intervening continental ridges are great primeval wrinkles in the
earth's crust--that they are due to the sinking down of that crust
upon the cooling and contracting nucleus. These vast wrinkles had come
into existence long before the formation of our oldest geological
strata. All our rocks may, in short, be looked upon as forming a mere
superficial skin covering and concealing the crystalline materials
which no doubt formed the original surface of the earth's crust.
Having premised so much, let me now turn to consider the geological
history of our own Continent, and endeavour to trace out the various
stages in its evolution. Of course I can only do so in a very brief
and general manner; it is impossible to go into details. We shall
find, however, that the history of the evolution of Europe, even
when sketched in outline, is one full of instruction for students
of physical geography, and that it amply bears out the view of the
permanency of the greater features of the earth's surface.
The oldest rocks that we know of are the crystalline schists and
gneiss, belonging to what is called the Archæan system. The origin
of these rocks is still a matter of controversy--some holding them
to be part of the primeval crust of the globe, or the chemical
precipitates of a primeval ocean, others maintaining that they are
altered or metamorphosed rocks of diverse origin, a large proportion
having consisted originally of aqueous or sedimentary rocks, such
as sandstone and shale; while not a few are supposed to have been
originally eruptive igneous rocks. According to some geologists,
therefore, the Archæan rocks represent the earliest sediments
deposited over the continental ridges. It is supposed that here and
there those ridges rose above the surface of what may have been a
boiling or highly-heated ocean, from whose waters copious chemical
precipitations took place, while gravel and shingle gathered around
the shores of the primeval lands. According to other writers, however,
the Archæan rocks were probably accumulated under normal conditions.
They consist, it is contended, partly of sediment washed down from
some ancient land-surface, and distributed over the floor of an old
sea (just as sediments are being transported and deposited in our own
day), and partly of ancient igneous rocks. Their present character is
attributed to subsequent changes, superinduced by heat and pressure,
at a time when the masses in question were deeply buried under later
formations, which have since been washed away. In a word, we are
still quite uncertain as to the true origin of the Archæan rocks. Not
infrequently they show a bedded structure, and in that respect they
simulate the appearance of strata of sedimentary origin. It is very
doubtful, however, whether this "bedded structure" is any evidence of
an original aqueous arrangement. We know now that an appearance of
bedding has been induced in originally amorphous rocks during great
earth-movements. Granite masses, for example, have been so crushed
and squeezed as to assume a bedded aspect, and a similar structure
has been developed in many other kinds of rock subjected to enormous
pressure. Whatever may have been the origin of the bedded structure
of the Archæan rocks, it is certain that the masses have been tilted
up and convoluted in the most remarkable manner. Hitherto they have
yielded no unequivocal trace of organic remains--the famous _Eozoon_
being now generally considered as of purely mineral origin. The
physical conditions under which the Archæan gneiss and schist came into
existence are thus quite undetermined, but geologists are agreed that
the earliest land-surfaces, of the former existence of which we can
be quite certain, were composed of rocks. And this brings us to the
beginning of reliable geological history.
All subsequent geological time--that, namely, of which we have any
record preserved in the fossiliferous strata--is divided into four
great eras, namely the Palæozoic, the Mesozoic, the Cainozoic, and
the Post-Tertiary eras, each of which embraces various periods, as
follows:--
Post-Tertiary {Recent.
{Pleistocene.
{Pliocene.
Tertiary or {Miocene.
Cainozoic {Oligocene.
{Eocene.
{Cretaceous.
Secondary or {Jurassic.
Mesozoic {Triassic.
{Permian.
{Carboniferous.
Primary or {Devonian and Old Red Sandstone.
Palæozoic {Silurian.
{Cambrian.
Archæan, Fundamental Gneiss.
Leaving the Archæan, we find that the next oldest strata are those
which were accumulated during the Cambrian period, to which succeeded
the Silurian, the Devonian and Old Red Sandstone, the Carboniferous,
and the Permian periods--all represented by great thicknesses of
strata, which overspread wide regions.
Now, at the beginning of the Cambrian period, we have evidence to show
that the primeval continental ridge was still largely under water,
the dry land being massed chiefly in the north. At that distant date
a broad land-surface extended from the Outer Hebrides north-eastwards
through Scandinavia, Finland, and northern Russia. How much further
north and north-west of the present limits of Europe that ancient
land may have extended we cannot tell, but it probably occupied wide
regions which are now submerged in the shallow waters of the Arctic
Ocean. In the north of Scotland a large inland sea or lake existed in
Cambrian times,[DG] and there is some evidence to suggest that similar
lacustrine conditions may have obtained in the Welsh area at the
beginning of the period. South of the northern land lay a shallow sea
covering all middle and southern Europe. That sea, however, was dotted
here and there with a few islands of Archæan rocks, occupying the site
of what are now some of the hills of middle Germany, such as the Riesen
Gebirge, the Erz Gebirge, the Fichtel Gebirge, etc., and possibly some
of the Archæan districts of France and the Iberian peninsula.
[DG] The Red Sandstones of the north-west Highlands are now believed to
be of pre-Cambrian age.
The succeeding period was one of eminently marine conditions, the wide
distribution of Silurian strata showing that during the accumulation
of these, enormous tracts of our Continent were overflowed by the sea.
None of these deposits, however, is of truly oceanic origin. They
appear for the most part to have been laid down in shallow seas, which
here and there may have been moderately deep. During the formation of
the Lower Silurian the whole of the British area, with the exception
perhaps of some of the Archæan tracts of the north-west, seems to have
been under water. The submergence had commenced in Cambrian times, and
was continued up to the close of the Lower Silurian period. During
this long-continued period of submergence volcanic activity manifested
itself at various points--our country being represented at that time
by groups of volcanic islands, scattered over the site of what is now
Wales, and extending westward into the Irish region, and northwards
into the districts of Cumberland and south Ayrshire. Towards the close
of the Lower Silurian period considerable earth-movements took place,
which had the effect of increasing the amount of dry land, the most
continuous mass or masses of which still occupied the northern and
north-western part of our Continent. In the beginning of Upper Silurian
times a broad sea covered the major portion of middle and probably all
southern Europe. Numerous islands, however, would seem to have existed
in such regions as Wales, and the various tracts of older Palæozoic
and Archæan rocks of middle Germany. Many of these islands, however,
were partially and some entirely submerged before the close of Silurian
times.
The next great period--that, namely, which witnessed the accumulation
of the Devonian and Old Red Sandstone strata--was in some respects
strongly contrasted to the preceding period. The Silurian rocks, as I
have said, are eminently marine. The Old Red Sandstones, on the other
hand, appear to have been accumulated chiefly in great lakes or inland
seas, and they betoken therefore the former existence of extensive
lands, while the contemporaneous Devonian strata are of marine origin.
Towards the close of the Upper Silurian period, then, we know that
considerable upheavals ensued in western and north-western Europe,
and wide stretches of the Silurian sea-bottom were converted into dry
land. The geographical distribution of the Devonian in Europe, and
the relation of that system to the Silurian, show that the Devonian
sea did not cover so broad an expanse as that of the Upper Silurian.
The sea had shallowed, and the area of dry land had increased when
the Devonian strata began to accumulate. In trying to realise the
conditions that obtained during the formation of the Devonian and the
Old Red Sandstone, we may picture to ourselves a time when the Atlantic
Ocean extended eastwards over the south of England and the north-east
of France, and occupied the major portion of central Europe, sweeping
north-east into Russia, and how much further we cannot tell. North
of that sea stretched a wide land-surface, in the hollows of which
lay great lakes or inland seas, which seem now and again to have had
communication with the open ocean. It was in these lakes that the Old
Red Sandstone was accumulated, while the Devonian or marine rocks were
formed in the wide waters lying to the south. Submarine volcanoes were
active at that time in Germany; and similarly in Scotland numerous
volcanoes existed, such as those of the Sidlaw Hills and the Cheviots.
The Carboniferous system contains the record of a long and complex
series of geographical changes, but the chief points of importance
in the present rapid review may be very briefly summed up. In the
earlier part of the period marine conditions prevailed. Thus we find
evidence to show that the sea extended further north than it did
during the preceding Devonian period. During the formation of the
mountain-limestone, a deep sea covered the major portion of Ireland and
England, but shallowed off as it entered the Scottish area. A few rocky
islets were all that represented Ireland and England at that time.
Passing eastwards, the Carboniferous sea appears to have covered the
low-grounds of middle Europe and enormous tracts in Russia. The deepest
part of the sea lay over the Anglo-Hibernian and Franco-Belgian areas;
towards the east it became shallower. Probably the same sea swept over
all southern Europe, but many islands may have diversified its surface,
as in Brittany and central France, in Spain and Portugal, and in the
various areas of older Palæozoic and Archæan rocks in central and
south-west Europe. In the latter stages of the Carboniferous period,
the limits of the sea were much circumscribed, and wide continental
conditions supervened. Enormous marshes, jungles, and forests now
overspread the newly-formed lands. Another feature of the Carboniferous
was the great number of volcanoes--submarine and sub-aërial--which were
particularly abundant in Scotland, especially during the earlier stages
of the period.
The rocks of the Permian period seem to have been deposited chiefly in
closed basins. When, owing to the movement of elevation or upheaval
which took place in late Carboniferous times, the carboniferous
limestone sea had been drained away from extensive areas in central
Europe, wide stretches of sea still covered certain considerable
tracts. These, however, as time went on, were cut off from the main
ocean and converted into great salt lakes. Such inland seas overspread
much of the low-lying tracts of Britain and middle Germany, and they
also extended over a broad space in the north-east of Russia. It was in
these seas that the Permian strata were accumulated. The period, it may
be added, was marked by the appearance of volcanic action in Scotland
and Germany.
So far, then, as our present knowledge goes, that part of the European
continent which was the earliest to be evolved lay towards the
north-west and north. All through the Palæozoic era a land-surface
would seem to have endured in that direction--a land-surface from the
denudation or wearing down of which the marine sedimentary formations
of the bordering regions were derived. But when we reflect on the great
thickness and horizontal extent of those sediments, we can hardly
doubt that the primeval land must have had a much wider range towards
the north and north-west than is the case with modern Europe. The
lands, from which the older Palæozoic marine sediments of the British
Islands and Scandinavia were obtained, must, for the most part, be
now submerged. In later Palæozoic times land began to extend in the
Spanish peninsula, northern France, and middle Europe, the denudation
of which doubtless furnished materials for the elaboration of the
contemporaneous strata of those regions. Southern Europe is so largely
composed of Mesozoic and Cainozoic rocks that we can say very little as
to the condition of that area in Palæozoic times, but the probabilities
are that it continued for the most part under marine conditions.
In few words, then, we may conclude that while after Archæan times
dry land prevailed in the north and north-west, marine conditions
predominated further south. Ever and anon, however, the sea vanished
from wide regions in central Europe, and was replaced by terrestrial
and lacustrine conditions. Further, as none of the Palæozoic marine
strata indicates a deep ocean, but all consist for the most part of
accumulations formed at moderate depths, it follows that there must
have been a general subsidence of our area to allow of their successive
deposition--a subsidence, however, which was frequently interrupted by
long pauses, and sometimes by movements in the opposite direction.
The first period of the Mesozoic era, namely, the Triassic, was
characterised by much the same kind of conditions as obtained towards
the close of Palæozoic times. A large inland sea then covered a
considerable portion of England, and seems to have extended north into
the south of Scotland, and across the area of the Irish Sea into the
north-east of Ireland. Another inland sea extended westward from the
Thüringer-Wald across the Vosges into France, and stretched northwards
from the confines of Switzerland over what are now the low-grounds of
Holland and northern Germany. In this ancient sea the Harz Mountains
formed a rocky island. While terrestrial and lacustrine conditions
thus obtained in central and northern Europe, an open sea existed in
the more southerly regions of the continent. Towards the close of the
period submergence ensued in the English and German areas, and the salt
lakes became connected with the open sea.
During the Jurassic period the regions now occupied in Britain and
Ireland by the older rocks appear to have been chiefly dry land.
Scotland and Ireland, for the most part, stood above the sea-level,
while nearly all England was under water--the hills of Cumberland
and Westmoreland, the Pennine chain, Wales, the heights of Devon and
Cornwall, and a ridge of Palæozoic rocks which underlies London, being
the chief lands in south Britain. The same sea overflowed an extensive
portion of what is now the Continent. The older rocks in the north-west
and north-east of France, and the central plateau of the same country,
formed dry land; all the rest of that country was submerged. In like
manner the sea covered much of eastern Spain. In middle Europe it
overflowed nearly all the low-grounds of north Germany, and extended
far east into the heart of Russia. It occupied the site of the Jura
Mountains, and passed eastward into Bohemia, while on the south side of
the Alps it spread over a large part of Italy, extending eastward so as
to submerge a broad area in Austria-Hungary and the Turkish provinces.
Thus the northern latitudes of Europe continued to be the site of the
chief land-masses, what are now the central and southern portions of
the Continent being a great archipelago with numerous islands, large
and small.
The Jurassic rocks, attaining as they do a thickness of several
thousand feet, point to very considerable subsidence. The movement,
however, was not continuous, but ever and anon was interrupted by
pauses. Taken as a whole, the strata appear to have accumulated in a
comparatively shallow sea, which, however, was sufficiently deep in
places to allow of the growth, in clear water, of coral-reefs.
Towards the close of the Jurassic period a movement of elevation
ensued, which caused the sea to retreat from wide areas, and thus when
the Cretaceous period began the British region was chiefly dry land.
Middle Europe would seem also to have participated in this upward
movement. Eventually, however, subsidence again ensued. Most of what
are now the low-grounds of Britain were submerged, the sea stretching
eastwards over a vast region in middle Europe, as far as the slopes
of the Urals. The deepest part of this sea, however, was in the west,
and lay over England and northern France. Further east, in what are
now Saxony and Bohemia, the waters were shallow, and gradually became
silted up. In the Mediterranean basin a wide open sea existed, covering
large sections of eastern Spain and southern France, overflowing the
site of the Jura Mountains, drowning most of the Alpine Lands, the
Italian peninsula, the eastern borders of the Adriatic, and Greece.
In short, there are good grounds for believing that the Cretaceous
Mediterranean was not only much broader than the present sea, but
that it extended into Asia, overwhelming vast regions there, and
communicated with the Indian Ocean.
Summing up what we know of the principal geographical changes that
took place during the Mesozoic era, we are impressed with the fact
that, all through those changes, a wide land-surface persisted in the
north and north-west of the European area, just as was the case in
Palæozoic times. The highest grounds were the Urals and the uplands
of Scandinavia and Britain. In middle Europe the Pyrenees and the
Alps were as yet inconsiderable heights, the loftiest lands being
those of the Harz, the Riesen Gebirge, and other regions of Palæozoic
and Archæan rocks. The lower parts of England and the great plains of
central Europe were sometimes submerged in the waters of a more or
less continuous sea; but ever and anon elevation ensued, and the sea
was divided, as it were, into a series of great lakes. In the south of
Europe a Mediterranean Sea would appear to have endured all through the
Mesozoic era--a Mediterranean of considerably greater extent, however,
than the present. Thus we see the main features of our Continent were
already clearly outlined before the close of the Cretaceous period. The
continental area then, as now, consisted of a wide belt of high-ground
in the north, extending roughly from south-west to north-east; south of
this a vast stretch of low-grounds, sweeping from west to east up to
the foot of the Urals, and bounded on the south by an irregular zone of
elevated land having approximately the same trend; still further south,
the maritime tracts of the Mediterranean basin. During periods of
depression the low-grounds of central Europe were invaded by the sea,
and the Mediterranean at the same time extended north over many regions
which are now dry land. It is in these two low-lying tracts, therefore,
and the country immediately adjoining them, that the Mesozoic strata of
Europe are chiefly developed.
A general movement of upheaval[DH] supervened at the close of the
Cretaceous period, and the sea which, during that period, overflowed
so much of middle Europe had largely disappeared before the beginning
of Eocene times. The southern portions of the continent, however, were
still mostly under water, while great bays and arms of the sea extended
northwards now and again into central Europe. On to the close of the
Miocene period, indeed, southern and south-eastern Europe consisted
of a series of irregular straggling islands and peninsulas washed
by the waters of a genial sea. Towards the close of early Cainozoic
times, the Alps, which had hitherto been of small importance, were
greatly upheaved, as were also the Pyrenees and the Carpathians. The
floor of the Eocene sea in the Alpine region was ridged up for many
thousands of feet, its deposits being folded, twisted, inverted, and
metamorphosed. Another great elevation of the same area was effected
after the Miocene period, the accumulations of that period now forming
considerable mountains along the northern flanks of the Alpine
chain. Notwithstanding these gigantic elevations in south-central
Europe--perhaps in consequence of them--the low-lying tracts of what
is now southern Europe continued to be largely submerged, and even the
middle regions of the continent were now and again occupied by broad
lakes which sometimes communicated with the sea. In Miocene times, for
example, an arm of the Mediterranean extended up the Rhone valley, and
stretched across the north of Switzerland to the basin of the Danube.
After the elevation of the Miocene strata these inland stretches of
sea disappeared, but the Mediterranean still overflowed wider areas in
southern Europe than it does in our day. Eventually, however, in late
Pliocene times, the bed of that sea experienced considerable elevation,
newer Pliocene strata occurring in Sicily up to a height of 3000 feet
at least. It was probably at or about that period that the Black Sea
and the Sea of Asov retreated from the wide low-grounds of southern
Russia, and that the inland seas and lakes of Austria-Hungary finally
vanished.
[DH] I now doubt whether any vertical upheaval of a wide continental
area is possible. The so-called "continental uplifts" are probably in
most cases rather negative than positive elevations. In other words,
the land seems to rise simply because the sea retreats owing perhaps to
the sinking of the crust within the great oceanic basins. See on this
subject, Article XIII.
The Cainozoic era is distinguished in Europe for its volcanic
phenomena. The grandest eruptions were those of Oligocene times. To
that date belong the basalts of Antrim, Mull, Skye, the Faröe Islands,
and the older series of volcanic rocks in Iceland. These basalts speak
to us of prodigious fissure eruptions, when molten rock welled up along
the lines of great cracks in the earth's crust, flooding wide regions,
and building up enormous plateaux, of which we now behold the merest
fragments. The ancient volcanoes of central France, those of the Eifel
country and many other places in Germany, and the volcanic rocks of
Hungary, are all of Cainozoic age; while, in the south of Europe, Etna,
Vesuvius, and other Italian volcanoes date their origin to the later
stages of the same great era.
Thus before the beginning of Pleistocene times all the main features
of Europe had come into existence. Since the close of the Pliocene
period there have been many great revolutions of climate; several very
considerable oscillations of the sea-level have taken place, and the
land has been subjected to powerful and long-continued erosion. But
the greater contours of the surface which began to appear in Palæozoic
times, and which in Mesozoic times were more strongly pronounced,
had been fully evolved by the close of the Pliocene period. The most
remarkable geographical changes which have taken place since then have
been successive elevations and depressions, in consequence of which the
area of our Continent has been alternately increased and diminished. At
a time well within the human period our own islands have been united to
themselves and the Continent, and the dry land has extended north-west
and north, so as to include Spitzbergen, the Faröe Islands, and perhaps
Iceland. On the other hand, our islands have been within a recent
period largely submerged.
The general conclusion, then, to which we are led by a review of the
greater geographical changes through which the European continent
has passed is simply this--that the substructure upon which all our
sedimentary strata repose is of primeval antiquity. Our dry lands are
built up of rocks which have been accumulated over the surface of a
great wrinkle of the earth's crust. There have been endless movements
of elevation and depression, causing minor deformations, as it were,
of that wrinkle, and inducing constant changes in the distribution
of land and water; but no part of the continental ridge has ever
been depressed to an abysmal depth. The ridge has endured through
all geological time. We can see also that the land has been evolved
according to a definite plan. Certain marked features begin to appear
very early in Palæozoic times, and become more and more pronounced
as the ages roll on. All the countless oscillations of level, all
the myriad changes in the distribution of land and water, all the
earthquake disturbances and volcanic eruptions--in a word, all the
complex mutations to which the geological record bears witness--have
had for their end the completion of one grand design.
A study of the geological structure of Europe--an examination of
the manner in which the highly folded and disturbed strata are
developed--throws no small light upon the origin of the larger or
dominant features of our Continent. The most highly convoluted rocks
are those of Archæan and Palæozoic age, and these are developed chiefly
in the north-western and western parts of the Continent. Highly
contorted strata likewise appear in all the mountain-chains of central
Europe--some of the rocks being of Palæozoic, while others are of
Mesozoic and of Cainozoic age. Leaving these mountains for the moment
out of account, we find that it is along the western and north-western
sea-board where we encounter the widest regions of highly-disturbed
rocks. The Highlands of Scandinavia and Britain are composed, for
the most part, of highly-flexed and convoluted rocks, which speak to
titanic movements of the crust; and similar much-crushed and tilted
rock-masses occur in north-west France, in Portugal, and in western
Spain. But when we follow the highly-folded Palæozoic strata of
Scandinavia into the low-grounds of the great plains, they gradually
flatten out, until in Russia they occur in undisturbed horizontal
positions. Over thousands of square miles in that country the Palæozoic
rocks are just as little altered and disturbed as strata pertaining to
Mesozoic and Cainozoic times.
These facts can have but one meaning. Could we smooth out all the
puckerings, creases, foldings, and flexures which characterise the
Archæan and Palæozoic rocks of western and north-western Europe, it
is certain that these strata would stretch for many miles out into
the Atlantic. Obviously they have been compressed and crumpled up by
some force acting upon them from the west. Now, if it be true that the
basin of the Atlantic is of primeval origin, then it is obvious that
the sinking down of the crust within that area would exert enormous
pressure upon the borders of our continental area. As cooling and
contracting of the nucleus continued, subsidence would go on under the
oceanic basin, depression taking place either slowly and gradually,
during protracted periods, or now and again more or less suddenly. But
whether gradually or suddenly effected, the result of the subsidence
would be the same upon the borders of our Continent; the strata along
the whole western and north-western margins of the European ridge would
necessarily be flexed and disturbed. Away to the east, however, the
strata, not being subject to the like pressure, would be left in their
original horizontal positions.
Now it can be shown that the mountains of Scandinavia and the British
Islands are much older than the Alps, the Pyrenees, and many other
conspicuous ranges in central and southern Europe. Our mountains and
those of Scandinavia are the mere wrecks of their former selves.
Originally they may have rivaled--they probably exceeded--the Alps
in height and extent. It is most likely, indeed, that the areas
of Palæozoic rocks in France, Portugal, and Spain also attained
mountainous elevations. But the principal upheaval of the western
margins of our Continent was practically completed before the close of
the Palæozoic period, and since that time those elevated regions have
been subjected to prodigious erosion, the later formations being in
large measure composed of their débris. I do not, of course, wish it
to be understood that there has been no upheaval affecting the west
of Europe since Palæozoic times. The tilted position of many of our
Mesozoic strata clearly proves the contrary. But undoubtedly the main
disturbances which produced the folding, fracturing, and contortion
of the Palæozoic strata of western Europe took place before the close
of the Palæozoic period. The mountains of Britain and Scandinavia are
amongst the oldest in Europe.
When we come to inquire into the origin of the mountains of central
Europe we have little difficulty in detecting the chief factors
in their formation. An examination of the Pyrenees, the Alps, and
other hill-ranges having the same general trend shows us that they
consist of flexed and convoluted rocks. They are, in short, mountains
of elevation, ridged up by tangential thrusts. Of this we need not
have the slightest doubt. If, for example, we approach the Alps from
the low-grounds of France, we observe the strata as we come towards
the Jura beginning to undulate--the undulations becoming more and
more marked, and passing into sharp folds and plications, until, in
the Alps, the beds become twisted, convoluted, and bent back upon
themselves in the wildest confusion. Now, speaking in general terms, we
may say that similar facts confront us in connection with every true
mountain-range in central Europe. Let it be noted, further, that all
those ranges have the same trend, which we may take to be approximately
east and west, or nearly at right angles to the trend of the Palæozoic
high-grounds of western and north-western Europe. Looked at broadly,
our continental ridge may be said to be traversed from west to east
by two wide depressions or troughs, separated by the intervening
belt of higher grounds just referred to. The former of these troughs
corresponds to the great central plain, which passes through the south
of England, north-east France, the Low Countries, and Denmark, whence
it sweeps east through Germany, and expands into the wide low-grounds
of Russia. The southern trough or depression embraces the maritime
tracts of the Mediterranean and the regions which that sea covers.
Such, then, are the dominant features of our Continent, to which all
others are of subordinate importance. Now it cannot be doubted that
the two great troughs are belts of subsidence in the continental ridge
itself. And their existence explains the origin of the mountain-ranges
which separate them. We know that the northern trough is of extreme
antiquity; it is older, at all events, than the Silurian period. Even
at that distant date its southern limits were marked out by ridges
of Archæan rocks, which seem to have formed islands in what is now
middle Germany, and probably also in Switzerland and central France.
The appearance of those Archæan rocks in central Europe was doubtless
due to a ridging up of the crust induced by those parallel movements
of subsidence which produced the northern and southern troughs. The
northern trough was probably always the shallower depression of the
two, for we have evidence to show that, again and again in Mesozoic and
later times, the seas which overflowed what are now the central plains
of Europe were of less considerable depth than that which occupied
the Mediterranean trough. As time rolled on, therefore, the northern
trough eventually became silted up; but so low even now is the level of
that trough that a relatively slight depression would cause the sea to
inundate most extensive regions in middle Europe.
In Cainozoic times, as we have seen, the last great elevation of the
Alps was effected--an elevation which can hardly have been due to any
other cause than the more or less abrupt depression of the earth's
crust under the Mediterranean basin. The area of that sea is now much
less considerable than it was in Tertiary times--a change due in part
to silting up, but chiefly perhaps to the sinking down of its bed to
profounder depths.
Thus we may conclude that from a very early period--a period
ante-dating the formation of our oldest fossiliferous strata--the
physical structure of our Continent had already been planned. The
dominant features of the primeval continental ridge are those
which have endured through all geological time. They are the
lines along which the beautiful lands in which we dwell have been
constructed. Tilted and convoluted, broken and crushed by myriad
earth-movements--scarred, furrowed, worn and degraded by the frosts,
the rains, the rivers, and the seas of countless ages--the rocks of our
Continent are yet eloquent of design. Where the ignorant sees nothing
save confusion and discord, the thoughtful student beholds everywhere
the evidence of a well-ordered evolution. Such is the conclusion to
which we are led by all geological research.
[Illustration:
SKETCH-MAPS ILLUSTRATING THE GEOGRAPHICAL EVOLUTION
OF CONTINENTAL AREAS
By PROFESSOR JAMES GEIKIE, LL.D., D.C.L., F.R.S.
PLATE V
+-----------------------------+------------------------------+
| MAP SHOWING THE | MAP SHOWING THE |
| AREA OF CONTINENTAL PLATEAU | AREA OF CONTINENTAL PLATEAU |
| OCCUPIED BY SEA IN | OCCUPIED BY SEA IN |
| PALÆOZOIC TIMES. | TERTIARY TIMES. |
+-----------------------------+------------------------------+
| MAP SHOWING THE | MAP SHOWING THE |
| AREA OF CONTINENTAL PLATEAU | AREAS OF DOMINANT DEPRESSION |
| OCCUPIED BY SEA IN | AND ELEVATION. |
| MESOZOIC TIMES. | (Below & Above the 1000 |
| | fathom Contour Line) |
+-----------------------------+------------------------------+
The Edinburgh Geographic Institute J. G. Bartholomew, F.R.G.S.
]
XII.
The Evolution of Climate.[DI]
[DI] Address delivered before the Royal Physical Society at the opening
of the Session 1889-90.
One of the most interesting questions with which geological science
has to deal is that of the evolution of climate. Although there is no
general agreement as to how former climatic fluctuations came about,
yet the prevalent opinion is that in the past, just as in the present,
the character of the climate must have depended mainly on latitude
and the relative position of the great land- and water-areas. This
was the doctrine taught by Lyell, and its cogency none will venture
to dispute. It is true he postulated a total redistribution of oceans
and continents--a view which the progress of science has shown to be
untenable. We can no longer speculate with him on the possibility of
all the great land-areas having been grouped at one time round the
equator, and at some other period about the poles. On the contrary, the
evidence goes to show that the continents have never changed places
with the ocean--that the dominant features of the earth's crust are
of primeval antiquity, and ante-date the oldest of the fossiliferous
formations. The whole question of climatic changes, therefore, must
be reconsidered from the point of view of the modern doctrine of the
permanency of continental and oceanic areas.
But before proceeding to this discussion, it may be well to glance
for a moment at the evidence from which it has been inferred that the
climate of the world has varied. Among the chief proofs of climatic
fluctuations are the character and the distribution of former floras
and faunas. It is true, fossils are, for the most part, relics of
extinct forms, and we cannot assert of any one of these that its
environment must have been the same as that of some analogous living
type. But, although we can base no argument on individual extinct
forms, it does not follow that we are precluded from judging of
the conditions under which a whole suite of extinct organisms may
have lived. Doubtless, we can only reason from the analogy of the
present; but, when we take into account all the forms met with in some
particular geological system, we seem justified in drawing certain
conclusions as to the conditions under which they flourished. Thus,
should we encounter in some great series of strata many reef-building
corals, associated with large cephalopods and the remains of tree-ferns
and cycads, which last from their perfect state of preservation could
not have drifted far before they became buried in sediment, we should
surely be entitled to conclude that the strata in question had been
deposited in the waters of a genial sea, and that the neighbouring
land likewise enjoyed a warm climate. Again should a certain system,
characterised by the presence of some particular and well-marked flora
and fauna, be encountered not only in sub-tropical and temperate
latitudes but also far within the Arctic Circle, we should infer that
such a flora and fauna lived under climatic conditions of a very
different kind from any that now exist. The very presence, in the
far north, of fossils having such a geographical distribution would
show that the temperature of polar seas and lands could not have been
less than temperate. When such broad methods of interpretation are
applied to the problems suggested by former floras and faunas, we
seem compelled to conclude that the conditions which determined the
distribution of life in bygone ages must have been, upon the whole,
more uniform and equable than they are now. It is unnecessary that I
should go into detailed proof; but I may refer, by way of illustration,
to what is known of the Silurian and Carboniferous fossils of the
arctic regions. Most of these occur also in the temperate latitudes
of Europe and North America, while many are recognised as distinctive
types of the same strata nearly all the world over. As showing how
strongly the former broad distribution of life-forms is contrasted
with their present restricted range, Professor Heilprin has cited the
Brachiopoda. Taking existing species and varieties as being 135 in
number, he remarks that "there is scarcely a single species which can
be said to be strictly cosmopolitan in its range, although not a few
are very widely distributed; and, if we except boreal and hyperboreal
forms, but a very limited number whose range embraces opposite sides
of the same ocean. On the other hand, if we accept the data furnished
by Richthofen concerning the Chinese Brachiopoda we find that out
of a total of thirteen Silurian and twenty-four Devonian species,
no less than ten of the former and sixteen of the latter recur in
the equivalent deposits of western Europe: and, further, that the
Devonian species furnish eleven, or nearly 50 per cent. of the entire
number, which are cosmopolitan or nearly so. Again, of the twenty-five
Carboniferous species, North America holds fully fifteen, or 60 per
cent., and a very nearly equal number are cosmopolitan." The same
palæontologist reminds us that by far the greater number of fossils
which occur in the Palæozoic strata of Australia are present also in
regions lying well within the limits of the north temperate zone. "In
fact," he continues, "the relationship between this southern fauna and
the faunas of Europe and North America is so great as to practically
amount to identity."
But, side by side with such evidence of broad distribution, we are
confronted with facts which go to show that, even at the dawn of
Palæozoic times, the oceanic areas at all events had their more or less
distinct life-provinces. While many of the old forms were cosmopolitan,
others were apparently restricted in their range. It would be strange,
indeed, had it been otherwise; for, however uniform the climatic
conditions may have been, still that uniformity was only comparative.
An absolutely uniform world-climate is well-nigh inconceivable. All we
can maintain is that the conditions during certain prolonged periods
were so equable as to allow of the general diffusion of species over
vastly greater areas than now; and that such conditions extended
from low latitudes up to polar regions. Now, among the chief factors
which in our day determine the limitation of faunas and floras, we
must reckon latitude and the geographical position of land and water.
What, then, it may be asked, were the causes which allowed of the much
broader distribution of species in former ages?
It is obvious that before a completely satisfactory answer to that
question can be given, our knowledge of past geographical conditions
must be considerably increased. If we could prepare approximately
correct maps and charts to indicate the position of land and sea
during the formation of the several fossiliferous systems, we should
be able to reason with some confidence on the subject of climate.
But, unfortunately, the preparation of such correct maps and charts
is impossible. The data for compilations of the kind required are
still inadequate, and it may well be doubted whether, in the case of
the older systems, we shall ever be able to arrive at any detailed
knowledge of their geographical conditions. Nevertheless, the
geological structure of the earth's crust has been so far unravelled as
to allow us to form certain general conceptions of the conditions that
must have attended the evolution of our continents. And it is with such
general conceptions only that I have at present to deal.
I said a little ago that the question of geological climates must now
be considered from the point of view of the permanency of the great
dominant features of the earth's crust. I need not recapitulate the
evidence upon which Dana and his followers have based this doctrine
of the primeval antiquity of our continental and oceanic areas. It is
enough if I remind you that by continental areas we simply mean certain
extensive regions in which elevation has, upon the whole, been in
excess of depression; by oceanic area, on the other hand, is meant that
vast region throughout which depression has exceeded elevation. Thus,
while the area of permanent or preponderating depression has, from
earliest geological times, been occupied by the ocean, the continental
areas have been again and again invaded by the sea--and even now
extensive portions are under water. It is not only the continental dry
land, therefore, but all the bordering belt of sea-floor which does not
exceed 1000 fathoms or so in depth, that must be included in the region
of dominant elevation. Were the whole of this region to be raised above
the level of the sea, the present continents would become connected so
as to form one vast land-mass, or continental plateau. (D, Plate IV.)
All the sedimentary strata with which we are acquainted have been
accumulated over the surface of that great plateau, and consequently
are of comparatively shallow-water origin. They show us, in fact, that
at no time in geological history has that plateau ever been drowned in
depths at all comparable to those of the deeper portions of our oceanic
troughs. The stratified rocks teach us, moreover, that the present
land-areas have been gradually evolved, and that, notwithstanding
many oscillations of level, these areas have continued to increase
in extent--so that there is probably more land-surface now than at
any previous era in the history of our globe. To give even a meagre
outline of the evidence bearing upon this interesting subject is here
impossible. All that I can do is to indicate very briefly some of the
general results to which that evidence seems to lead.
The oldest rocks with which we are acquainted are the so-called Archæan
schists[DJ] But these have hitherto yielded no unequivocal traces
of organic life, and as their origin is still doubtful, it would
obviously be futile to speculate upon the geographical conditions of
the earth's surface at the time of their formation. Reliable geological
history only begins with the fossiliferous strata of the Palæozoic
era. From these we learn that in the European area the Archæan rocks
of Britain, Scandinavia, and Finland formed, at that time, the most
extensive tract of dry land in our part of the world. How far beyond
the present limits of Europe that ancient northern land extended we
cannot tell; but it probably occupied considerable regions which are
now submerged in the waters of the Arctic Ocean. Further south, the
continental plateau appears to have been, for the most part, overflowed
by a shallow sea, the surface of which was dotted by a few islands of
Archæan rocks, occupying the sites of what are now some of the hills
of middle Germany and the Archæan districts of France and the Iberian
Peninsula. Archæan rocks occur likewise in Corsica and Sardinia,
and again in Turkey: they also form the nuclei of most of the great
European mountain-chains, as the Pyrenees, the Alps, the Carpathians,
and the Urals. These areas of crystalline schists may not, it is true,
have existed as islands at the beginning of Palæozoic times, for they
were doubtless ridged up by successive elevations at later dates; but
their very presence as mountain-nuclei is sufficient to show that at
a very early geological period, the continental plateau could not
have been covered by any great depth of sea. We can go further than
this--for all the evidence points to the conclusion that, even so far
back as Cambrian times, the dominant features of the present European
continent had been, as it were, sketched out. Looked at broadly, that
part of the great continental plateau upon which our European lands
have been gradually built up may be said to be traversed from west to
east by two wide depressions, separated by an intervening elevated
tract. The former of these depressions corresponds to the great Central
Plain which passes through the south of England, north-east of France,
and the Low Countries, whence it sweeps through Germany, to expand into
the extensive low-grounds of central and northern Russia. The southern
depression embraces the maritime tracts of the Mediterranean, and the
regions which that sea covers. To these dominant features all the
others are of subordinate importance. The two great troughs are belts
of depression in the continental plateau itself. The northern one is of
extreme antiquity--it is older, at all events, than the Cambro-Silurian
period. Even at that distant date its southern limits were marked
out by ridges of Archæan rock, which, as I have said, seem to have
formed islands in what is now central Europe. It was probably always
the shallower depression of the two, for we have evidence to show that
again and again, in Mesozoic and later times, the sea that overflowed
what are now the central lowlands of Europe was of less considerable
depth than that which occupied the Mediterranean trough.
[DJ] I need hardly remind geologists that some of the so-called
"Archæan schists" may really be the highly altered accumulations of
later geological periods.
If we turn to North America, we find similar reason to conclude, with
Professor Dana, that the general topography of that region had likewise
been foreshadowed as far back as the beginning of the Palæozoic era.
Dana tells us that even then the formation of its chief mountain-chains
had been commenced, and its great intermediate basins were already
defined. The oldest lands of North America were built up, as in
Europe, of azoic rocks, and were grouped chiefly in the north. Archæan
masses extend over an enormous region, from the shores of the Arctic
Ocean down to the great lake country, and they are seen likewise in
Greenland and many of the Arctic islands. They appear also in the
long mountain-chains that run parallel with the coast-lines of the
Continent. In a word, the present distribution of the Archæan rocks,
and their relation to overlying strata, lead to the belief that in
North America, just as in Europe, they form the foundation-stones of
that continent, and stretch continuously throughout its whole extent.
We know comparatively little of the geology of the other great
land-masses of the globe, but from such evidence as we have there is
reason to believe that these in their general structure have much the
same story to tell as Europe and North America. In South America,
Archæan rocks extend over vast areas in the east and north-east, and
reappear in the lofty mountain-chains of the Pacific border. They have
been recognised also in various parts of Africa, alike in the north
and east, in the interior, and in the west and south. In Asia, again,
they occupy wide areas in the Indian Peninsula; they are well developed
in the Himalaya, while in China and the mountains and plateaux of
central Asia, azoic rocks, which are probably of Archæan age, are well
developed. The crystalline schists, which cover extensive tracts in
Australia and in the northern island of New Zealand, have also been
referred to the same age. Thus, all the world over, Archæan rocks seem
to form the surface of the ancient continental plateau upon which all
other sedimentary strata have been accumulated. And in every region
where Palæozoic rocks occur, we have evidence to prove that at the time
these last were formed vast areas of the old continental plateau were
under water.
The geological structure of the Palæozoic tracts of Europe and America
has shown us that, during the protracted period of their accumulation,
and notwithstanding many oscillations of level, the land-surface
continued to increase. The same growth of dry land characterised
Mesozoic and Cainozoic times--the primeval depressions that traverse
the continental plateau became more and more silted up, and the sea
eventually disappeared from extensive regions which it had overflowed
in Palæozoic ages. This land-growth, of course, was not everywhere
continuous. Again and again, throughout wide tracts, depression was
in excess of sedimentation and elevation. Even at the present time,
broad tracts of what was once dry land are submerged. But the simple
fact that the younger fossiliferous strata do not extend over such wide
areas as the older systems, is sufficient proof that our land-masses
have all along tended to grow, and to become more and more consolidated.
Reference has already been made to the remarkable fact that no abysmal
accumulations have yet been detected amongst the stratified rocks of
the earth's crust. Ordinary clastic rocks, such as shale, sandstone,
and conglomerate--altered or unaltered, as the case may be--form by
far the largest proportion of our aqueous strata, and speak to us only
of shallow waters. It is true that some of our limestones must have
accumulated in moderately deep clear seas, yet none of these limestones
is of abysmal origin. They prove that portions of the continental
plateau have now and again been submerged for several thousand feet,
but afford no evidence of depths comparable to those of the present
oceanic basins. The enormous thickness obtained by the sedimentary
strata can only be explained on the supposition that deposition took
place over a gradually sinking area. And thus it can be shown that,
within the continental plateau, movements of depression have been
carried on more or less continuously during vast periods of time--and
yet so gradually, that sedimentation was able to keep pace with them.
Take, for example, the Cambrian strata of Wales and Shropshire--all,
apparently, shallow-water deposits--which attain a thickness of 30,000
feet, or thereabout; or the Silurian strata of the same regions, which
are not much less than 20,000 feet thick; and similar great depths of
sedimentary rocks might be cited from North America. Passing on to
later periods, we find like evidence of long-continued depression in
the thick sediments of the younger Palæozoic systems. It is noteworthy,
however, that when we come down to still later ages, the movements of
depression, as measured by the depths of the strata, appear to have
become less and less extensive and profound. Each such movement of
depression was eventually brought to a close by one or more movements
of upheaval--slowly or more rapidly effected, as the case may have
been. Here, then, we are confronted with the striking fact that the
continental plateau has, from time to time, sunk down over wide areas
to depths exceeding those of existing oceans, and yet at so slow a
rate, that sedimentation prevented the depressed regions from becoming
abysmal. It is obvious, then, that such areas are now dry land simply
because, in the long-run, sedimentation and upheaval have been in
excess of depression.
And yet, notwithstanding the numerous upheavals which have taken place
over the continental plateau, these have succeeded in doing little
more than drain away the sea more or less completely from the great
primeval depressions by which that plateau is traversed. If it be true,
therefore, that the continental plateau owes its existence to the
sinking down of the earth's crust within the oceanic basins--if the
continents have been squeezed up by the tangential thrusts exerted by
the sinking areas that surround them--then it follows that while lands
have been gradually extending over the continental plateau, the bed of
the ocean has been sinking to greater and greater depths.
If this general conclusion holds good, it is obvious that the oceanic
troughs of early geological times could not have been so deep as they
are now. During the Palæozoic period, the most continuous areas of
dry land, as we have seen, were distributed over the northern parts
of our hemisphere, while, further south, groups of islands indicated
the continuation of the continental plateau. Doubtless South America,
Africa, Asia, and Australia were, at that distant date, represented by
similar detached areas of dry land. In a word, the primeval continental
plateau was still largely under water. Judging from the character and
broad distribution of the Palæozoic marine faunas the temperature of
the sea was wonderfully uniform. There is certainly nothing to indicate
the existence of such climatic zones as those of the present. We know
very little of the terrestrial life of early Palæozoic times--the
Cambro-Silurian strata are essentially marine. Land-plants, however,
become more numerous in the Old Red Sandstone, and, as every one knows,
they abound in the succeeding Carboniferous and Permian systems. And
the testimony of these floras points to the same conclusion as that
furnished by the marine faunas. The Carboniferous floras of the arctic
regions, and of temperate Europe and America, not only have the same
_facies_, but a considerable number of the species is common to both
areas; while many European species occur in the Carboniferous strata
of Australia and other distant lands. This common _facies_, and the
presence of numerous cosmopolitan forms, surely indicate the former
prevalence of remarkably uniform climatic conditions. The conditions,
of course, need not--indeed, could not--have been absolutely uniform.
At present the various climates which our globe experiences depend
upon the amount of heat received directly and indirectly from the
sun--oceanic and aërial currents everywhere modifying the results that
are due to latitude. It cannot have been otherwise in former times.
In all ages the tropics must have received more direct sun-heat than
temperate and polar regions; and however much the climatic conditions
of the Palæozoic era may have differed from the present--however
uniformly temperature may have been distributed--still, as I have
said, absolute uniformity was impossible. It was doubtless owing to
the fact that the dry lands of Palæozoic times were not only much less
extensive than now, but more interrupted, straggling, and insular,
that the climate of the globe was so equable. Under such geographical
conditions, great oceanic currents would have a much freer course than
is now possible, and warm water would find its way readily across
wide regions of the submerged continental plateau into the highest
latitudes. The winds blowing athwart the land would everywhere be moist
and warm, and no such marked differences of temperature, such as now
obtain, would distinguish the arctic seas from those of much lower
latitudes. At the same time, the comparatively shallow water overlying
the submerged areas of the continental plateau would favour the
distribution of species, and thus bring about that wide distribution of
cosmopolitan forms and general similarity of _facies_, which are such
marked features of the Palæozoic faunas. It is even quite possible that
migration may have taken place here and there across the great oceanic
depression itself; for it may well be doubted whether, at so early a
period, the depression had sunk down to its present depth below the
level of the continental plateau.
Yet, notwithstanding such facilities for migration, and the consequent
similarity of _facies_ I have referred to, the Palæozoic faunas of
different regions have usually certain distinctive characters. Even at
the very dawn of the era the marine faunas were already grouped into
provinces, sometimes widely separated from one another, at other times
closely adjacent, so that it is evident that barriers to migration
here and there existed. It could hardly have been otherwise; for local
and more widely-spread movements of elevation and depression took place
again and again during Palæozoic times.
While the younger Palæozoic systems were being accumulated, excess
of upheaval over depression resulted in the gradual increase of the
land.[DK] The continental plateau came more and more to the surface,
in spite of many oscillations of level. It is quite possible, nay,
even probable that this persistent growth of land, and consequent
modification of oceanic currents may have rendered the climatic
conditions of later Palæozoic times less uniform: but, if so, such
diminished uniformity has left no recognisable impress on either faunas
or floras; for fossils characteristic of the Devonian and Carboniferous
strata of temperate latitudes occur far within the Arctic Circle.
[DK] See footnote p. 341.
Descending to the Mesozoic era, we find that the character and
distribution of marine faunas are still indicative of uniformity.
There could have been little difference of temperature at that time
between arctic seas and those of our own latitude. Cosmopolitan species
abounded in the Jurassic waters, but were relatively less numerous
in those of the Cretaceous period. Professor Neumayr maintains that
already, in the Jurassic period, the climate had become differentiated
into zones. This, he thinks, is indicated by the fact that coral reefs
abound in the Jurassic strata of central Europe, while they are wanting
in the contemporaneous deposits of boreal regions. Dr. Heilprin, on
the other hand, is of opinion that this and certain other distinctive
features of separate Jurassic life-provinces may not have been due to
differences of temperature, but rather to varying physical conditions,
such as character of the sea-bottom, depth of water, and so forth.
Perhaps the safest conclusion we can come to, in the present state
of the evidence, is that the climatic conditions of the Mesozoic era
were, upon the whole, less obviously uniform than those of earlier
ages, but that marked zones of climate like the present had not as
yet been evolved. At the same time, when we consider how many great
geographical revolutions took place during the period in question, we
must be prepared to admit that these could hardly fail to influence the
climate, and thus to have induced modifications in the distribution of
faunas and floras. And probably evidence of such modifications will yet
be recognised, if indeed the phenomena referred to by Neumayr be not
a case in point. It may be noted, further, that while, according to
many botanists, the plants of the Palæozoic periods bespeak not only
uniform climatic conditions but the absence of marked seasonal changes,
those of late Mesozoic times are indicative of less uniformity. The
Cretaceous conifers, for example, show regular rings of growth, and
betoken the existence of seasons, which were less marked, however, than
is now the case.
The geographical changes of Mesozoic times were notable in many
respects. The dominant features of Europe, already foreshadowed in
early Palæozoic times, had become more clearly outlined before the
close of the Cretaceous period. Notwithstanding many movements of
depression, the chief land-areas continued to show themselves in the
north and north-west. The highest grounds were the Urals, and the
uplands of Scandinavia and Britain. In middle Europe the Pyrenees and
the Alps were as yet inconsiderable heights, the loftiest lands in that
region being those of the Harz, the Riesen Gebirge, and other tracts of
Archæan and Palæozoic rocks. The lower parts of England and the great
lowland plains of central Europe were sometimes submerged in the waters
of a wide, shallow sea, but ever and anon elevation ensued, new lands
appeared, and these waters became divided into a series of large inland
seas and lakes. In the south, a deep Mediterranean sea would appear
to have persisted all through the Mesozoic era--a sea of considerably
greater extent, however, than the present.
While in Europe the dominant features of the continental plateau run
approximately east and west, in North America they follow nearly the
opposite direction. In early Mesozoic times, vast tracts of dry land
extended across the northern and eastern sections of the latter area.
Over the Rocky Mountain region, low lands and saline lakes appear
to have stretched, while further west the area of the Great Plateau
and the Pacific slope were covered by the sea. Towards the end of
the Mesozoic era, the land in the far west became more continuous--a
broad belt extending in the direction of the Pacific coast-line from
Mexico up to high northern latitudes. In short, before the Cretaceous
period closed, the major portion of North America had been evolved. A
considerable tract of what is now the western margin of the continent,
however, was still under water, while from the Gulf of Mexico (then
much wider than now) a broad Mediterranean sea swept north and
north-west through Texas and the Rocky Mountain region to communicate
with the Arctic Ocean. All to the east of this inland sea was then, as
it is now, dry land. Thus, up to the close of the Cretaceous period,
in America and Europe alike, oceanic currents coming from the south
had ready access across the primeval continental plateau to the higher
latitudes. Southern Europe indeed, during Mesozoic times, was simply a
great archipelago, having free communication on the one hand across the
low-grounds of central and northern Russia with the arctic seas, and,
on the other, across vast regions in Asia with the Indian Ocean.
Of the other great land-masses of the globe our knowledge is too
limited to allow us to trace their geographical evolution with any
confidence. But from the very wide distribution of Mesozoic strata
in South America, Africa, Asia, and Australia, there can be no doubt
that, at the time of their accumulation, enormous tracts in those
regions were then under water. The land-masses, in short, were not so
continuous and compact as they are at present. And although we must
infer that considerable areas of Mesozoic land are now submerged, yet
these cannot but bear a very small proportion to the wide regions which
have been raised above the sea-level since Mesozoic times. In short,
from what we do know of the geological structure of the continents
in question, we can hardly doubt that they have passed through
geographical revolutions of a like kind with those of Europe and North
America. Everywhere over the great continental plateau elevation
appears, in the long-run, to have been in excess of depression, so
that, in spite of many subsidences, the tendency of the land throughout
the world has been to extend its margins, and to become more and
more consolidated. The Mesozoic lands were larger than those of the
preceding Palæozoic era, but they were still penetrated in many places
by the sea, and warm currents could make their way over wide tracts
that are now raised above the sea-level. Under such circumstances
approximately uniform conditions of climate could not but obtain.
Great geographical changes supervened upon the close of the Cretaceous
period. North America then acquired nearly its present outline.
Its Mediterranean sea had vanished, but the Gulf of Mexico still
overflowed a considerably wider region than now, while a narrow margin
of the Pacific border of the continent continued submerged. In Europe
elevation ensued, and the sea which had overspread so much of the
central and eastern portions of our Continent disappeared. Southern
Europe, however, was still largely under water, while bays and inlets
extended northwards into what are now the central regions of the
Continent. On to the close of the Miocene period, indeed, the southern
and south-eastern tracts of Europe were represented by straggling
islands. In middle Cainozoic times the Alps, which had hitherto been of
small importance, were considerably upheaved, as were also the Pyrenees
and the Carpathians; and a subsequent great elevation of the Alpine
area was effected after the Miocene period. Notwithstanding these
gigantic movements, the low-lying tracts of what is now southern Europe
continued to be largely submerged, and even the central regions of the
Continent were now and again occupied by broad lakes, which sometimes
communicated with the sea. After the elevation of the Miocene strata,
these inland seas disappeared, but the Mediterranean still overflowed
wider areas than it does to-day. Eventually, however, in late Pliocene
times, the bed of that sea experienced considerable elevation; and it
was probably at or about this stage that the Black Sea and the Sea of
Asov retreated from the broad low-grounds of southern Russia, and that
the inland seas and lakes of Austria-Hungary finally vanished.
The movements of upheaval, which caused the Cretaceous seas to
disappear from such broad areas of the continental plateau, induced
many changes in the floras and faunas of the globe. A notable break in
the succession occurs between the Cretaceous and the Eocene, hardly one
species of higher grade than the protozoa passing from one system to
the other. In the Cainozoic deposits we are no longer confronted with
numerous cosmopolitan species--the range of marine forms has become
much more restricted. Nevertheless, the faunas and floras continue
to be indicative of much warmer climates for arctic and temperate
latitudes than now obtain. But, at the same time, differentiation
of climate into zones is distinctly marked. In the early Cainozoic
period, our present temperate latitudes supported a flora of decidedly
tropical affinities, while the fauna of the adjacent seas had a
similar character. Later on the climate of the same latitudes appears
to have passed successively through sub-tropical and temperate
stages. In short, a gradual lowering of the temperature is evinced
by the character and distribution both of floras and faunas. The
differentiation of the climate during one stage of the Cainozoic era is
well illustrated by the Miocene flora. Thus, at a time when Italy was
clothed with a tropical vegetation, in which palm-trees predominated,
middle Europe had its extensive forests of evergreens and conifers,
while in the region of the Baltic conifers and deciduous trees were the
prevalent forms.
When one takes into consideration the fact that, notwithstanding many
oscillations of level, the land during Cainozoic times was gradually
extending, and the sea disappearing from wide regions which it had
formerly covered, one can hardly doubt that the seemingly gradual
change from tropical to temperate conditions was due, in large measure,
to that persistent continental growth. I confess, however, that it is
difficult to account for the very genial climate which continued to
prevail over the arctic regions. So far as one can gather from the
evidence at present available, some of the marine approaches to those
latitudes had been cut off by the movements of elevation which brought
the Cainozoic era to a close, while the arctic lands were perhaps more
extensive than they are now. The Cretaceous Mediterranean Sea of North
America had vanished, and we cannot prove that the Tertiary Sea of
southern Europe communicated across the low-grounds of Russia with the
Arctic Ocean. We know, however, that the archipelago of southern Europe
was in direct connection with the Indian Ocean, and it is most probable
that a wide arm of the same sea stretched north from the Aralo-Caspain
area through Siberia. Indeed, much of what are now the lowlands of
western and northern Asia was probably sea in Tertiary times. It seems
likely, therefore, that, even at this late period, marine currents
continued to reach the Arctic Zone across the continental plateau. When
the warm waters of the Indian Ocean eventually ceased to invade Europe,
and the Mediterranean became much restricted in area, the climate of
the whole Continent could not fail to be profoundly affected.
There is yet another line of evidence to which brief reference may be
made. I have spoken of the remarkable uniformity of climatic conditions
which obtained in Palæozoic times, and of the gradual modification
of these conditions which subsequently supervened. Now, it is worthy
of note that in their lithological characters the oldest sedimentary
strata themselves likewise exhibit a prevalent uniformity which in
later systems becomes less and less conspicuous. The Cambro-Silurian
mechanical sediments, for example, maintain much the same character
all the world over; and the like is true, although in a less degree,
of the marine accumulations of the Devonian period. The corresponding
mechanical deposits of later Palæozoic ages continue to show more
and more diversity, but at the same time they preserve a similarity
of character over much more extensive areas than is found to be the
case with the analogous sediments of the Mesozoic era. Finally, these
last are more or less strongly contrasted with the marine mechanical
accumulations of Cainozoic times, which are altogether more local in
character. This increasing differentiation is quite in keeping with
what we know of the evolution of our land-areas. In early Palæozoic
ages, when insular conditions prevailed and the major portion of the
primeval continental plateau was covered by shallow seas, it is obvious
that mechanical sediments would be swept by tidal and other currents
over enormous areas, and that these sediments would necessarily assume
a more or less uniform character. Indeed, I suspect that much of the
sediment of those early seas may have been the result of tidal scour,
and that marine erosion was more generally effective then than it is
now. With the gradual growth of the land and the consequent deflection
and limitation of currents, marine mechanical sediments would tend
to become more and more local in character. Thus the increasing
differentiation which we observe in passing from the earlier to the
later geological systems is just what might have been expected.
Summing up, now, the results of this rapid review of the evidence, we
seem justified in coming to the following conclusions:--
(1.) In Palæozoic times, Europe and North America were represented
by considerable areas of dry land, massed chiefly in the higher
latitudes, while further south groups of smaller islands were scattered
over the submerged surface of the primeval continental plateau. The
other continents appear, in like manner, to have been represented
by islands--some of which may have reached continental dimensions.
A very remarkable uniformity of climate accompanied these peculiar
geographical conditions.
(2.) In Mesozoic times, the primeval continental plateau came more and
more to the surface, but the land-areas were still much interrupted, so
that currents from tropical regions continued to have ready access to
high latitudes. The climate of the whole globe, therefore, was still
uniform, but apparently not so markedly as in the preceding era.
(3.) In Cainozoic times, the land-masses continued to extend, and
the sea to retreat from hitherto submerged areas of the continental
plateau; and this persistent land-growth was accompanied by a gradual
lowering of the temperature of northern and temperate latitudes, and a
more and more marked differentiation of climate into zones.
Having thus very briefly sketched the geographical evolution of the
land during Palæozoic, Mesozoic, and Tertiary times, and come to the
general conclusion that climate has varied according to the relative
position of land and sea, I have next to consider the geographical
and climatic conditions of the Quaternary period. These, however,
are now so well known, that I need to no more than remind you that,
so far as the chief features of our lands are considered, all these
had come into existence before the dawn of the Ice Age. The greater
contours of the surface, which were foreshadowed in Palæozoic times,
and which in Mesozoic times were more clearly indicated, had been fully
evolved by the close of the Pliocene period. The connection between the
Mediterranean and the Indian Ocean probably ceased in late Pliocene
times. The most remarkable geographical changes which have taken place
since then within European regions have been successive elevations
and depressions, in consequence of which the area of our Continent
has been alternately increased and diminished. At a time well within
the human period, our own islands have been united to themselves and
the Continent, and the dry land has extended north-west and north, so
as to include Spitzbergen, the Faröe Islands, and perhaps Iceland. On
the other hand, our islands have been within a recent period largely
submerged. Similarly, in North America, we are furnished with many
proofs of like oscillations of level having taken place in Quaternary
times. Is it possible, then, to explain the climatic vicissitudes of
the Pleistocene period by means of such oscillations? Many geologists
have tried to do so, but all these attempts have failed. It is quite
true that a general elevation of the land in high latitudes would
greatly increase the ice-fields of arctic regions, and might even give
rise to perennial snow and glaciers in the mountain-districts of our
islands. But it is inconceivable that any such geographical change
could have brought about that general lowering of temperature over the
whole northern hemisphere which took place in Pleistocene times. For we
have to account not only for the excessive glaciation of northern and
north-western Europe, and of the northern parts of North America, but
for the appearance of snow-fields and glaciers in much more southern
latitudes, and in many parts of Asia where no perennial snow now
exists. Moreover, we have to remember that arctic conditions of climate
obtained in north-western Europe even when the land was relatively much
lower than it is at present. The arctic shell-beds of our own and other
temperate regions sufficiently prove that geographical conditions were
not the only factor concerned in bringing about the peculiar climate of
the Pleistocene period. Then, again, we must not forget that at certain
stages of the same period genial conditions of climate were coincident
with a much wider land-surface in north-western Europe than now exists.
The very fact that interglacial deposits occur in every glaciated
region is enough of itself to show that the arctic conditions of the
Pleistocene could not have resulted entirely from a mere elevation of
land in the northern parts of our hemisphere.
The only explanation of the peculiar climatic vicissitudes in question
which seems to meet the facts, so far as these have been ascertained,
is the well-known theory advanced by Dr. Croll. After carefully
considering all the objections which have been urged against that
theory, there is only one, as it seems to me, that is deserving of
serious attention. This objection is not based on any facts connected
with the Pleistocene deposits themselves, but on evidence of quite
another kind. It is admitted that were the Pleistocene deposits alone
considered, Croll's theory would fully account for the phenomena.
But, it is argued, we cannot take the Pleistocene by itself, for
if that theory be true, then climatic conditions similar to those
of the Pleistocene must have supervened again and again during the
past. Where, then, we are asked, is there any evidence in Palæozoic,
Mesozoic, or Cainozoic strata of former widespread glacial conditions?
If continental ice-sheets, comparable to those of the Pleistocene,
ever existed in the earlier ages, surely we ought to find more or
less unmistakable traces of them. Now, at first sight, this looks
a very plausible objection, but it has always seemed to me to be
based upon an assumption that is not warranted by our knowledge of
geographical evolution. Dr. Croll always admitted implicitly that high
eccentricity of the earth's orbit might have happened again and again
without inducing glacial conditions like those of the Pleistocene. The
objection takes no account of the fact that the excessive climate of
the Glacial period was only possible because of special geographical
conditions--conditions that do not appear to have been fully evolved
before Pliocene times. No one has seen this more clearly than Mr.
Wallace,[DL] with the general drift of whose argument I am quite at
one. In earlier ages, the warm water of the tropics overflowed wide
areas of our present continents--most of the dry land was more or
less insular, and the seas within the Arctic Circle were certainly
not cold as at present, but temperate and even genial. If we go back
to Cambro-Silurian times, we find only the nuclei, as it were, of our
existing continents appearing above the surface of widespread shallow
seas. It is quite impossible, therefore, that under such geographical
conditions, great continuous ice-sheets, like those of the Pleistocene,
could have existed--no matter how high the eccentricity of the earth's
orbit may have been. The most that could have happened during such
a period of eccentricity would be the accumulation of snow-fields on
mountains and plateaux of sufficient height, the formation here and
there of local glaciers, and the descent of these in some places to the
sea. And what evidence of such local glaciation might we now expect
to find? No old land-surface of that far-distant period has come down
to us: we look in vain for Cambro-Silurian _roches moutonnées_ and
boulder-clay or moraines. The only evidence we could expect is just
that which actually occurs, namely, erratics (some of them measuring
five feet and more in diameter) embedded in marine deposits. It may
be said that a few erratics are hardly sufficient to prove that a
true Glacial period supervened in Cambro-Silurian times, and I do not
insist that they are. But I certainly maintain that if any lowering
of the temperature were induced by high eccentricity of the earth's
orbit during Cambro-Silurian times, then ice-floated erratics are the
only evidence of refrigeration that we need ever hope to find. The
geographical conditions of early Palæozoic times forbade the formation
of enormous ice-sheets like those of the Pleistocene period. Extreme
climatic changes were then impossible, and periods of high eccentricity
might have come and gone without inducing any modifications of
flora and fauna which we could now recognise. We are ignorant of
the terrestrial life of the globe at that distant period, and our
knowledge of the marine fauna is not sufficient to enable us to deny
the possibility of moderate fluctuations in the temperature of the seas
of early Palæozoic times. Moreover, we must not forget there were then
no such barriers to migration as now exist. If the conditions became
temporarily unsuitable, marine organisms were free to migrate into
more genial waters, and to return to their former habitats when the
unfavourable conditions had passed away.
[DL] See _Island Life_.
The uniform climate so characteristic of the Cambro-Silurian period
appears to have prevailed likewise during the later stages of the
Palæozoic era. This we gather from a general consideration of the
floras and faunas, and their geographical distribution. The dry land,
as we have seen, continued to increase in extent; but vast areas of
the primeval continental plateau of the globe still continued under
water, and currents from southern latitudes flowed unrestricted into
polar regions. During the protracted lapse of time required for the
formation of the later Palæozoic systems several periods of high
eccentricity must have occurred. But, so far as one can judge, the
disposition of the larger land-areas was never such as to induce a
true Ice Age. Nevertheless we are not without evidence of ice-action
in Old Red Sandstone, Carboniferous, and Permian strata. And it seems
to me probable that the erratic accumulations referred to may really
indicate local action, of more or less intensity, brought about by such
lowering of the temperature as would supervene during a period of high
eccentricity. It is true we may explain the phenomena by inferring the
existence of mountains of sufficient elevation--and this, indeed, is
the usual explanation. But it is doubtful whether those who adopt that
view have fully considered what it involves. Take, for example, the
case of the breccias and conglomerates of the Lammermuir Hills, which
have all the appearance of being glacial and fluvio-glacial detritus.
These deposits overlie the highly-denuded Silurian greywackés of
Haddingtonshire in the north and of Berwickshire in the south, and have
evidently been derived from the intervening high-grounds--the width of
which between the Old Red Sandstone accumulations in question does not
exceed eight or nine miles. The breccias reach a height of 1300 feet,
while the dominating point of the intervening uplands is 1700 feet.
Under present geographical conditions it is doubtful whether perennial
snow and glaciers of any size at all could exist in the region of the
Lammermuirs at a less altitude than 7000 feet or more. But between
the breccias of Haddingtonshire and the equivalent deposits in
Berwickshire there is no space for any intermediate range of mountains
of circumdenudation of such a height. Moreover, we must remember that
under the extremely uniform conditions which obtained in Palæozoic
times the snow-line could not possibly have been attained even at that
elevation. When the Devonian coral-reefs described by Dupont were
growing in the sea that overflowed western Europe, to what height must
the southern uplands of Scotland have been elevated in order to reach
the snow-line! We may make what allowance we choose for the denudation
which the Silurian rocks of the Lammermuirs must have experienced since
the deposition of the Old Red Sandstone, but it is simply a physical
impossibility that mountains of circumdenudation of the desiderated
height could ever have existed in the Lammermuir region at the time the
coarse breccias were being accumulated.[DM] It seems to me, then, that
these breccias are in every way better accounted for by a lowering of
temperature due to increased eccentricity of the orbit. This view frees
us from the necessity of postulating excessive upheavals over very
restricted areas, and of creating Alps where no Alps could have existed.
[DM] It may be objected that the conglomerates were probably not
marine, but deposited in lakes, the beds of which may have been much
above sea-level. But from all that we know of the Old Red Sandstone of
Scotland it would appear that the lakes of the period now and again
communicated with the sea, and were probably never much above its level.
When we consider the enormous thickness of the strata that constitute
any of our larger coal-fields, we can hardly doubt that one or
more periods of high eccentricity must have occurred during their
accumulation. It does not follow, however, that we should be able
to detect in these strata any evidence of alternating cold and warm
epochs. So long as ocean-currents from the tropics found ready entrance
to polar regions across vast tracts of what is now dry land, extreme
and widespread glacial conditions were impossible. Any lowering of
temperature due to cosmical causes might indeed induce new snow-fields
and glaciers to appear, or existing ones to extend themselves in
northern regions and the most elevated lands of lower latitudes; but
such local glaciation need not have seriously affected any of the
areas in which coal-seams were being formed. For nothing appears more
certain than this--that our coal-seams as a rule were formed over
broad, low-lying alluvial lands, and in swamps and marshes, along
the margins of estuaries or shallow bays of the sea. Some seams, it
is true, are evidently formed of drifted vegetable débris, but the
majority point to growth _in situ_. The strata with which they are
associated are shallow-water sediments which could only have been
deposited at some considerable distance from any mountain-regions in
which glaciers were likely to exist. It is idle, therefore, to ask for
evidence of glacial action amongst strata formed under such conditions.
The only evidence of ice-work we are likely to get is that of erratics.
And these are not wanting, although it is probable that most of those
which are found embedded in coals have been transported by rafts of
vegetable matter or in the roots of trees. The same explanation,
however, will not account for the boulders which Sir William Dawson
has recorded from the coal-fields of Nova Scotia. He describes them as
occurring on the outside of a gigantic esker of Carboniferous age, and
thinks they were probably dropped there by floating-ice at a time when
coal-plants were flourishing in the swamps on the other side of the
gravel embankment.
If the disposition of the land-areas in Carboniferous times rendered
such an ice-age as that of the Pleistocene impossible--in other words,
if the effects flowing from high eccentricity of the orbit must to
a large extent have been neutralised--the flora and fauna of the
period can hardly be expected to yield any recognisable evidence of
fluctuating climatic conditions. When our winter happened in aphelion
new snow-fields might have appeared, or already existing glaciers
might have increased in size; while, with the winter in perihelion,
the temperature in northern latitudes would doubtless be raised. But
the general result would simply be an alternation of warm and somewhat
cooler conditions. And such fluctuations of climate might readily have
taken place without materially modifying; the life of the period.
The breccias of the Permian system have been described by Ramsay as
of glacial origin. Some geologists agree with him, while others do
not--and many have been the ingenious suggestions which these last
have advanced in explanation of the phenomena. Some have tried to
show how the stones and blocks in the breccias may have been striated
without having recourse to the agency of glacier-ice, but they cannot
explain away the fact that many of the stones (which vary in size
from a few inches to three or four feet in diameter) have travelled
distances of thirty or forty miles from the parent rocks. Similar
erratic accumulations, which may belong to the same system or to the
Carboniferous, occur in India and Australia. According to Dr. Blanford,
the Indian boulder-beds are clearly indicative of ice-action, and
he does not think that they can be explained by an assumed former
elevation of the Himalaya. On the contrary, he is of opinion that the
facts are best accounted for by a general lowering of the temperature,
due probably to the action of cosmical causes. Daintree, Wilkinson, R.
Oldham, and others who have studied the Australian erratic beds have
likewise stated their belief that these are of true glacial origin.
I may pass rapidly over the Mesozoic systems, taking note, however,
of the fact that in them we encounter evidence of ice-action of much
the same kind as that met with in Palæozoic strata. While, on the
one hand, the Mesozoic floras and faunas bespeak climatic conditions
similar to those of earlier ages, but probably not quite so uniform; on
the other, the occurrence of erratics in various marine accumulations
is sufficient to show that now and again ice floated across seas,
the floors of which were tenanted by reef-building corals. The
geographical conditions continued unfavourable to the formation of
extensive ice-sheets in temperate latitudes, no matter how high the
eccentricity of the orbit might have been. The erratics which occur
in certain Jurassic and Cretaceous deposits are admitted by most
geologists to have been ice-borne. Now, it is highly improbable that
the transporting agent could have been coast-ice, for it is hardly
possible to conceive of ice forming on the surface of a sea in which
flourished an abundant Mesozoic fauna. The erratics, therefore, seem
to imply the existence in Mesozoic times of local glaciers, which here
and there descended to the sea, as in the north-east of Scotland. The
erratics in the Scottish Jurassic are evidently of native origin, and
it is most improbable that those which have been met with in the Chalk
of England and France could have floated from any very great distance.
How, then, can we explain the appearance of local glaciers in these
latitudes during Mesozoic times? The geographical conditions of the
period could not have favoured the formation of perennial snow and ice
in our area, unless our lands were at that time much more elevated than
now. And this is the usual explanation. It is supposed that mountains
much higher than any we now possess probably existed in such regions
as the Scottish Highlands. It is easy to imagine the former existence
of such mountains. So long a time has elapsed since the Jurassic
period, that the Archæan and Palæozoic areas cannot but have suffered
prodigious denudation in the interval. But, when one considers how
very lofty, indeed, those mountains must have been, in order to reach
the snow-line of Jurassic times, one may be excused for expressing a
doubt as to whether the suggested explanation is reasonable. At all
events, the phenomena are, to say the least, as readily explicable on
the supposition that the snow-line was temporarily lowered by cosmical
causes. Even with eccentricity at a high value, no great ice-sheets,
indeed, could have existed, but local snow-fields and glaciers might
have appeared in such mountain-regions as were of sufficient height.
And this might have happened without producing any great difference
in the temperature of the sea, or any marked modification in the
distribution of life. In short, we should simply have, as before,
an alternation of warm and somewhat cooler climates, but nothing
approaching to the glacial and interglacial epochs of the Pleistocene.
These conclusions seem to me to be strongly supported by the evidence
of ice-action during Tertiary times. The gigantic erratics of the
Alpine Eocene do not appear to have been derived from the Alps, but
rather from the Archæan area of southern Bohemia. The strata in which
they occur are, for the most part, unfossiliferous; they contain only
fucoidal remains, and are presumably marine. How is it possible to
account for the appearance of these erratics in marine deposits in
central Europe at a time when, as evidenced by the Eocene flora and
fauna the climate was warm? Are we to infer the former existence of an
extremely lofty range of Bohemian Alps which has since vanished? Is
it not more probable that here, too, we have evidence of a lowering
of the snow-line, induced by cosmical causes, which brought about the
appearance of snow-fields and glaciers in a mountain-tract of much
less elevation than would have been required in the absence of high
eccentricity of the orbit? If it be objected that such cosmical causes
must have had some effect upon the distribution of life, I reply
that very probably they had, although not to any extreme extent. The
researches of Mr. Starkie Gardner have shown that the flora of the
English Eocene affords distinct evidence of climatic changes. But as
the geographical conditions of that period precluded the possibility of
extensive glaciation, and could only, at the most, have induced local
glaciers to appear in elevated mountain-regions, it seems idle to cite
the non-occurrence of erratics and morainic accumulations in the Eocene
of England and France as an argument against the application of Croll's
theory to the case of the erratics of the Flysch. I repeat, then, that
under the geographical conditions of the Eocene, all the more obvious
effects likely to have resulted from the passage of a period of high
eccentricity would be the appearance of a few local glaciers, the
existence of which could have had no more influence on the climate of
adjacent lowlands than is notable in similar circumstances in our own
day. It is absurd, therefore, to expect to find evidence in Eocene
strata of as strongly contrasted climates as those of the glacial and
interglacial deposits of the Pleistocene. There must, doubtless, have
been alternations of climate in our hemisphere; but these would consist
simply of passages from warm to somewhat cooler conditions--just such
changes, in fact, as are suggested by the plants of the English Eocene.
The evidence of ice-action in the Miocene strata is even more striking
than that of which I have just been speaking. The often-cited case
of the erratics of the Superga near Turin I need do little more
than mention. These erratics were undoubtedly carried by icebergs,
calved from Alpine glaciers at a time when northern Italy was largely
submerged. The erratic deposits are unfossiliferous, and are underlaid
and overlaid by fossiliferous strata, in none of which are any erratics
to be found. What is the meaning of these intercalated glacial
accumulations? Can we believe it possible that the Miocene glaciers
were enabled to reach the sea in consequence of a sudden movement
of elevation, which must have been confined to the Alps themselves?
Then, if this be so, we must go a step further, and suppose that,
after some little time, the Alps were again suddenly depressed, so
that the glaciers at once ceased to reach the sea-coast. For, as Dr.
Croll has remarked, "had the lowering of the Alps been effected by
the slow process of denudation, it must have taken a long course of
ages to have lowered them to the extent of bringing the glacial state
to a close." And we should, in such a case, find a succession of beds
indicating a more or less protracted continuance of glacial conditions,
and not one set of erratic accumulations intercalated amongst strata,
the organic remains in which are clearly suggestive of a warm climate.
The occurrence of erratics in the Miocene of Italy is all the more
interesting from the fact that in the Miocene of France and Spain
similar evidence of ice-action is forthcoming.
Opponents of Dr. Croll's theory have made much of Baron Nordenskiöld's
statement that he could find no trace of former glacial action in any
of the fossiliferous formations within the Arctic regions. He is
convinced that "an examination of the geognostic condition, and an
investigation of the fossil flora and fauna of the polar lands, show
no signs of a glacial era having existed in those parts before the
termination of the Miocene period." Well, as we have seen, there is no
reason to believe that the geographical conditions in our hemisphere,
at any time previous to the close of the Pliocene period, could have
induced glacial conditions comparable to those of the Pleistocene Ice
Age. The strata referred to by Nordenskiöld, are, for the most part,
of marine origin, and their faunas are sufficient to show us that the
Arctic seas were formerly temperate and genial. If any ice existed
then, it could only have been in the form of glaciers on elevated
lands. And it is quite possible that these, during periods of high
eccentricity, may have descended to the sea and calved their icebergs;
and, if so, erratics may yet be found embedded here and there in the
Arctic fossiliferous formations, although Nordenskiöld failed to see
them. One might sail all round the Palæozoic coast-lines of Scotland
without being able to observe erratics in the strata, and yet, as we
know, these have been encountered in the interior of the country.
The wholesale scattering of erratics at any time previous to the
Pleistocene, must have been exceptional even in arctic regions, and
consequently one is not surprised that they do not everywhere stare the
observer in the face.
The general conclusion, then, to which I think we may reasonably
come, is simply this:--That geological climate has been determined
chiefly by geographical conditions. So long as the lands of the
globe were discontinuous and of relatively small extent, warm
ocean-currents reaching polar regions produced a general uniformity
of temperature--the climate of the terrestrial areas being more or
less markedly insular in character. Under these conditions, the sea
would nowhere be frozen. But when the land-masses became more and more
consolidated, when owing to the growth of the continents the warm
ocean-currents found less ready access to arctic regions, then the
temperature of those regions was gradually lowered, until eventually
the seas became frost-bound, and the lands were covered with snow and
ice. But while the chief determining cause of climate has been the
relative distribution of land and water, it is impossible to doubt that
during periods of high eccentricity of the orbit, the climate must
have been modified to a greater or less extent. In our own day the
geographical conditions are such that, were eccentricity to attain a
high value, the climate of the Pleistocene would be reproduced, and our
hemisphere would experience a succession of alternating cold and genial
epochs.
But in earlier stages of the world's history, the geographical
conditions were not of a kind to favour the accumulation of vast
ice-fields. During a period of extreme eccentricity, there would
probably be fluctuations of temperature in high latitudes; but nothing
like the glacial and interglacial epochs of the Pleistocene could
have occurred. At most, there would be a general lowering of the
temperature, sufficient to render the climate of arctic seas and lands
somewhat cooler, and probably to induce the appearance in suitable
places of local glaciers; and, owing to precession of the equinox,
these cooler conditions would be followed by a general elevation of
the temperature above the normal for the geographical conditions of
the period. In Palæozoic and Mesozoic times, the effects of high
eccentricity of the orbit appear to have been, in a great measure,
neutralised by the geographical conditions, with a possible exception
in the Permian period. But in Tertiary times, when the land-masses had
become more continuous, the cosmical causes of change referred to must
have had greater influence. And I cannot help agreeing with Dr. Croll
that the warm climates of the Arctic regions during that era were, to
some extent, the result of high eccentricity.
In concluding this discussion, I readily admit that our knowledge
of geographical evolution is as yet in its infancy. We have still
very much to learn, and no one will venture to dogmatise upon the
subject. But I hope I have made it clear that the evidence, so far
as it goes, does not justify the confident assertions of Dr. Croll's
opponents, that his theory is contradicted by what we know of the
climatic conditions of Palæozoic, Mesozoic, and Cainozoic times. On
the contrary, it seems to me to gain additional support from the very
evidence to which Nordenskiöld and others have appealed.
Note.--The accompanying sketch-maps (Plate IV.) require a few words
of explanation. The geology of the world is still so imperfectly
known that any attempt at graphic representation of former
geographical conditions cannot but be unsatisfactory. The approximate
positions of the chief areas of predominant elevation and depression
during stated periods of the past may have been ascertained in a
general way; but when we try to indicate these upon a map, such
provisional reconstructions are apt to suggest a more precise and
definite knowledge than is at present attainable. For it must be
confessed that there is hardly a line upon the small maps (A, B,
C) which might not have been drawn differently. This, of course,
is more especially true of South America, Africa, Asia--of large
areas of which the geological structure is unknown. But although the
boundaries of the land-masses shown upon the maps referred to are
thus confessedly provisional, the maps nevertheless bring out the
main fact of a gradual growth and consolidation of the land-areas--a
passage from insular to continental conditions. I need hardly say
this is no novel idea. It was clearly set forth by Professor Dana
upwards of forty years ago (_Silliman's Journal_, 1846, p. 352; 1847,
pp. 176, 381), and it received some years later further illustration
from Professor Guyot, who insisted upon the insular character of
the climate during Palæozoic times (_The Earth and Man_, 1850). It
must be understood that the maps (A, B, C) are not meant to exhibit
the geographical conditions of the world at any one point of time.
In Map A, for example, the area coloured blue was not necessarily
covered by sea at any particular stage in the Palæozoic era. It
simply represents approximately the regions tended. But, as already
stated, numerous oscillations of level occurred in Palæozoic times,
so that many changes in the distribution of land and water must have
taken place down to the close of the Permian period. The land-areas
shown upon the map are simply those which appear to have been more
or less persistent through all the geographical changes referred to.
Similar remarks apply to the other maps representing the more or
less persistent land-areas of Mesozoic and Tertiary times. Thus, for
example, there are reasons for believing that Madagascar was joined
to the mainland of Africa at some stage of the Mesozoic era, but was
subsequently insulated before Tertiary times. Again, as Mesozoic
era a land-connection obtained between New Zealand and Australia.
The same naturalist also points out that a chain of islands, now
represented by numerous islets and shoals, served in Tertiary times
to link Madagascar to India. Map D shows the areas of predominant
elevation and depression. The area coloured brown represents the
great continental plateau, which extends downwards to 1000 fathoms or
so below the present sea-level. The area tinted blue is the oceanic
depression. From the present distribution of plants and animals, we
infer that considerable tracts which are now submerged have formerly
been dry land--some of these changes having taken place in very
recent geological times. And the same conclusions are frequently
suggested by geological evidence. There can be little doubt that
Europe in Tertiary times extended further into the Northern Ocean
than it does now. And it is quite possible that in the Mesozoic and
Palæozoic eras considerable land-areas may likewise have appeared
here and there in those northern regions which are at present under
water. There is, indeed, hardly any portion of the continental
plateau which is now submerged that may not have been land at some
time or other. But after making all allowance for such possibilities,
the geological evidence, as far as it goes, nevertheless leads to
the conclusion that upon the whole a wider expense of primeval
continental plateau has come to the surface since Tertiary times than
was ever exposed during any former period of the world's history.
[Mr. Marcou states (_American Geologist_, 1890, p. 229) that the idea
of a gradual growth of land-areas originated with Elie de Beaumont,
who was in the habit of showing such maps, and used them in his
lectures at Paris as early as 1836. Professor Beudant published three
of these same maps for the Jurassic, Cretaceous, and Tertiary seas in
his _Cours élémentaire de Géologie_ (1841); and Professor Carl Vogt
in his _Lehrbuch der Geologie und Petrefactenkunde_ (1845), which
was confessedly based on Elie de Beaumont's lectures during 1844-46,
gives four maps of the Carboniferous, Jurassic, Cretaceous, and
Tertiary seas.]
XIII.
The Scientific Results of Dr. Nansen's Expedition.[DN]
[DN] From _The Scottish Geographical Magazine_, 1891.
In the Appendix to his most interesting and instructive work, _The
First Crossing of Greenland_, Dr. Nansen treats of the scientific
results of his remarkable journey. The detailed enumeration of these
results, he tells us, would have been out of place in a general
account of his expedition, but will appear in due time elsewhere.
Hence he confines attention in his present work to such questions as
are of most obvious interest, such as the extent, outward form, and
elevation of the inland-ice of Greenland. By way of introduction his
readers are presented with some account of the geological history of
the country, which, although it contains nothing that was not already
familiar to geologists, will doubtless prove interesting to others.
After indicating that Greenland would appear to be composed almost
exclusively of Archæan schists and granitoid eruptive rocks, the author
glances at the evidence which the Mesozoic and Cainozoic strata of
the west coast have supplied as to the former prevalence of genial
climatic conditions. Heer is cited to show that during the formation
of the Cretaceous beds the mean temperature of north Greenland was
probably between 70° and 72° F., while in later Cainozoic times it
could not have been less than 55° F., in 70° N.L. These conclusions
are based on the character of the fossil floras. Now the mean annual
temperature on the west coast of Greenland, where the relics of these
old floras occur, is about 15° F., from which it is inferred that there
has been a decrease of 40° since Cainozoic times. In those times,
says Dr. Nansen, "the country must have rejoiced in a climate similar
to that of Naples, while in the earlier Cretaceous period it must
have resembled that of Egypt." He then refers to the well-known fact
that, long after the deposition of the Cainozoic beds of Greenland,
intensely arctic conditions supervened, when the inland-ice of that
country extended much beyond its present limits. This was the Glacial
period of geologists, during which all the northern regions of America
and Europe, down to what are now temperate latitudes were likewise
swathed in ice. Various hypotheses have been advanced in explanation of
these strange climatic vicissitudes, and some of them are very briefly
discussed by Dr. Nansen. None of the suggested solutions of the problem
quite satisfies him; but he appears to look with most favour on the
view that great climatic revolutions in what are now polar regions
may have resulted from movements of the earth's axis. He admits,
however, that there are certain strong objections to this hypothesis,
and concludes that we have not yet got any satisfactory explanation
to cover all the facts of the case. In discussing the question of a
possible wandering of the pole, the author cites certain astronomical
observations to show that the position of the axis is even now slowly
changing, the movement amounting to half a second in six months. This
is not much; but if the change, as he remarks, were to continue at the
same rate for 3600 years, the shift would amount to one degree. Thus
in a period of no more than 72,000 to 108,000 years Greenland might be
brought into the latitude required for the growth of such floras as
those of Cainozoic and Mesozoic times. Geologists will readily concede
these or longer periods if they be required, but they will have graver
doubts than Dr. Nansen as to whether any such great changes in the
axis are possible. The astronomical observations referred to, even if
they were fully confirmed, do not show that the movement is constant
in one direction. They indicated, as he mentions, a slight increase
of latitude during the first quarter of 1889, followed in the second
quarter of the same year by a decrease, which continued to January,
1890. Since the publication of Professor George Darwin's masterly paper
on the influence of geological changes on the earth's axis of rotation,
geologists have felt assured that the great climatic revolutions to
which the stratified rocks bear witness must be otherwise explained
than by a wandering of the pole. Indeed, the geological evidence alone
is enough to show that profound climatic changes have taken place
while the pole has occupied its present position. Thus, there is no
reasonable grounds for doubting that during the Glacial period the pole
was just where we find it to-day. For, under existing geographical
conditions, could a sufficient lowering of temperature be brought
about, snow-fields and ice-sheets would gather and increase over
the very same areas as we know were glaciated in Pleistocene times.
Still further, we have only to recall the fact that several extreme
revolutions of climate supervened during the so-called Glacial period,
to see how impossible it is to account for the phenomena by movements
of the earth's axis.
If it be true that the great climatic changes of the Pleistocene
period did not result from a wandering to and fro of the pole, then
it is not at all likely that the Mesozoic and Cainozoic climates of
Greenland were induced by any such movement. But does the geological
evidence justify us in believing that the climates in Greenland
during Cretaceous and Tertiary times really resembled those of
Egypt and southern Italy? It may be strongly doubted if it does.
Palæontologists, like other mortals, find it hard to escape the
influence of environment. They are apt to project the actual present
into the past, without, perhaps, fully considering how far they are
justified in doing so. Because there occur in Cretaceous and Tertiary
strata, within Arctic regions, certain assemblages of plants which find
their nearest representatives in southern Italy and Egypt, surely it is
rather rash to conclude that Greenland has experienced climates like
those now characteristic of Mediterranean lands. All that the evidence
really entitles us to assume is simply that the _winter temperature_ of
Greenland was formerly much higher than it is now. That great caution
is required in comparing past with present climatological conditions
may be seen by glancing for a moment at the character of the flora
which lived in Europe during the interglacial phase of the Pleistocene
period. The plants of that period are for the most part living species,
so that while dealing with these we are on safer ground than when we
are treating of the floras of periods so far removed from us as those
of Tertiary and Cretaceous times. Now, in the Pleistocene flora of
Europe we find a strange commingling of species, such as we nowhere
see to-day over any equally wide area of the earth's surface. During
Pleistocene times many plants which are still indigenous to southern
France flourished side by side in that area with species which are no
longer seen in the same region; some of these last having retreated
because unable to support the cold of winter, while others have retired
to the mountains to escape the dryness of the summer. Similar evidence
is forthcoming from the Pleistocene accumulations of Italy, northern
France, and Germany. In a word, clement winters and relatively cool and
humid summers permitted the wide diffusion and intimate association
of plants which have now a very different distribution, temperate and
southern species formerly flourishing together over vast areas of
southern and central Europe. And similarly we find that during the same
period the regions in question were tenanted by southern and temperate
forms of animal life--elephants, rhinoceroses, and hippopotamuses,
together with cervine, bovine, and other forms, not a few of which are
still indigenous to our Continent--that ranged from the shores of the
Mediterranean up to our own latitudes. We cannot doubt, indeed, that
the present geographical distribution of plants and animals differs
markedly from anything that has yet been disclosed by the researches
of geologists. The climatic conditions of our day are exceptional as
compared with those of earlier times, and the occurrence in Greenland
of southern types of plants, therefore, does not justify us in
concluding that climates like those of southern Italy and Egypt were
ever characteristic of arctic regions. It is a low winter temperature
rather than a want of great summer heat that restricts the range
northward of southern floras. If Greenland could be divested of its
inland-ice--if its winter temperature never fell below that of our own
island--it would doubtless become clothed in time with an abundant
temperate flora.
Judging from what is known of the various floras and faunas that
have successively clothed and peopled the world, from Palæozoic down
to the close of Cainozoic times, the general climatic conditions
of the globe, prior to the Glacial period, would seem to have been
prevalently insular rather than continental as they are now. The
lands appear to have been formerly much less continuous, and ocean
currents from southern latitudes had consequently freer access to high
northern regions than is at present possible. In no other way can we
account for the facts connected with the geographical distribution
and extent of the fossiliferous formations. But are we to infer, from
the occurrence of similar assemblages of marine organic remains in
arctic, temperate, and tropical latitudes, that the shores of primeval
Greenland were washed by waters as warm as those of the tropics? Surely
not: an absence of very cold water in the far north is all that we
seem justified in assuming. And so, in like manner, the presence in
Greenland of fossil floras having the same general facies as those that
occur in the corresponding strata of more southern latitudes, does
not compel us to believe that conditions at all similar to what are
now met with in warm-temperate and sub-tropical lands ever obtained
in arctic regions. A relatively high winter temperature alone would
permit the range northward of many tribes of plants which are now
restricted to southern latitudes. Yet, under the most uniform insular
climatic conditions that we can conceive of, there must always have
been differences due to latitude--although such differences were never
apparently so marked as they are now.
In order to appreciate the character of the climate which must have
prevailed when the lands of the globe were much more interrupted
and insular than at present, we have only to consider how greatly
isothermal lines, even under existing continental conditions, are
deflected by ocean-currents. In the North Atlantic, for example, the
winter isotherm of 32° F. is deflected northward from the parallel
of New York to that of Hammerfest--a displacement of at least 30° of
latitude. The Arctic Sea now occupies a partially closed basin, into
which only one considerable current enters from the south. But in
earlier ages the case was otherwise, and there was often communication
across what are now our continental areas. Instead of being girdled,
as at present, by an almost continuous land-mass, the Arctic Sea seems
to have formed with the circumjacent ocean one great archipelago. Thus
freely open to the influx of southern currents, it is not difficult to
believe that the seas of the far north might never be frozen, and that
an "inland-ice" like that of Greenland would be impossible. The present
cold summers of that country, as the late Dr. Croll has insisted,
are due not so much to high latitude as to the presence of snow and
ice. Could these be removed, the summers would be as warm at least as
those of England. Now the occurrence in arctic regions of Palæozoic
and Mesozoic marine faunas is strongly suggestive of the former
presence there of genial waters having free communication with lower
latitudes; and it is to the presence of these warm currents, flowing
uninterruptedly through polar regions, that we would attribute the high
winter temperature and uniform climate to which the fossil floras and
faunas of Greenland bear testimony.
If these views be at all reasonable, it seems unnecessary to call to
our aid hypothetical changes in the position of the earth's axis. It
may be admitted, however, that the climate of the Arctic regions must
have been from time to time more or less affected by those cosmical
causes to which Croll has appealed. So long, however, as insular
conditions prevailed, the changes induced by a great increase in the
eccentricity of the earth's orbit would not necessarily be strongly
marked. Dr. Nansen objects to Croll's well-known theory that "it cannot
account for the recurrence of conditions so favourable as to explain
the existence in Greenland of a climate comparable to what we now find
in tropical regions." No doubt it cannot, but, as we maintain, there
is no good reason for supposing that tropical or sub-tropical climates
ever characterised any area within the Arctic Circle. The remarkable
association in Europe, during so recent a period as the Pleistocene,
of southern and temperate species of plants and animals, ought to warn
us against taking the present distribution of life-forms as an exact
type of the kind of distribution which characterised earlier ages. It
is safe to say that were our present continental areas to become broken
up into groups of larger and smaller islands, so as to allow of a much
less impeded oceanic circulation, the resulting climatic conditions
would offer the strongest contrast to the present. And as the lands
of the globe were apparently in former times more insular than they
are now, it is hazardous to compare the climates of the present with
those of the past. It is reasonable to infer, from the occurrence in
Greenland of fossil floras which find their nearest representatives in
southern Europe and north Africa, that the winters of the far north
were formerly mild and clement. But we cannot conclude, from the same
evidence, that the Arctic summers were ever as hot as those of our
present warm-temperate and sub-tropical zones.
But if the recent expedition has thrown no new light on the disputed
question as to the cause of the high temperature which formerly
prevailed in Greenland, it is needless to say that it has added
considerably to our knowledge of the present physical conditions of
that country. The view held by many that Greenland must be wrapped in
ice has been amply justified, and we can now no longer doubt that the
inland-ice covers the whole country from the 75th parallel southwards.
A section of Greenland in the latitude at which it was crossed by
Nansen and his comrades "gives an almost exact mathematical curve,
approximating very closely to the arc of a circle described with a
radius of about 6500 miles. The whole way across the surface coincides
tolerably accurately with this arc, though it falls away somewhat
abruptly at the coasts, and a little more abruptly on the east side
than the west." Taking the observations of other Arctic travellers
with his own, Nansen is led to the conclusion that "the surface of the
inland-ice forms part of a remarkably regular cylinder, the radius
of which nevertheless varies not a little at different latitudes,
increasing markedly from the south, and consequently making the arc of
the surface flatter and flatter as it advances northwards." He points
out that this remarkable configuration must to a certain extent be
independent of the form of the underlying land-surface, which, to judge
from the character of the wild and mountainous coast-lands, probably
resembles Norway in its general configuration--if, indeed it be not a
group of mountainous islands. The buried interior of Greenland must
in fact be a region of high mountains and deep valleys, all of which
have totally disappeared under the enveloping _mer de glace_. It is
obvious, as Dr. Nansen remarks, that the minor irregularities of the
land "have had no influence whatever upon the form of the upper surface
of the ice-sheet." That surface-form has simply been determined by
the force of pressure--the quasi-viscous mass attaining its maximum
thickness towards the central line of the country, where resistance to
the movement due to pressure must necessarily have been greatest. Thus
although the larger features of the ice-drowned land may have had some
influence in determining the position of the ice-shed, it is not by
any means certain that this central line coincides with the dominant
ridge or watershed of the land itself. For, as Nansen reminds us, the
ice-shed of the Scandinavian inland-ice of glacial times certainly lay
about 100 miles to the east of the main water-parting of Norway and
Sweden. Similar facts, we may add, have been noticed in connection with
the old ice-sheets of Scotland and Ireland.
The greatest elevation attained by the expedition was 9000 feet. How
deeply buried the dominating parts of the land-surface may be at that
elevation one cannot tell. It is obvious, however, that the _mer de
glace_ must be very unequal in thickness. According to Dr. Nansen the
average elevation of the valleys in the interior cannot much exceed
2300 or 3300 feet, so that the ice lying above such depressions must
have a thickness of 5700 to 6700 feet. It cannot, of course, lie so
deeply over mountain-ridges. The eroding power of such a glacier-mass
must be enormous, and Dr. Nansen does not doubt that the buried valleys
of Greenland are being widened and deepened by the grinding of the
great ice-streams that are ever advancing towards the sea.
The expedition met with no streams of surface-water on the inland-ice;
indeed, the amount of superficial melting in the interior was quite
insignificant. And yet, as is well known, many considerable streams
and rivers flow out from underneath the inland-ice all the year
round. It is obvious, therefore, that this water-supply does not come
from superficial sources, as, according to Dr. Nansen, it is usually
supposed to do. But surely it has long been recognised that such rivers
as the Mary Minturn must be derived from sub-glacial melting. And the
various causes to which our author attributes this melting have already
frequently been pointed out. Earth-heat--the influence of pressure in
lowering the melting-point of ice--and the friction induced by the
movement of the ice itself have all long ago been recognised as factors
tending to produce the sub-glacial water-drainage of an ice-sheet.
Dr. Nansen's speculations on the origin of the "drumlins" and "kames"
of formerly glaciated areas will interest geologists, but are not so
novel as he supposes. His description of what are known as "drumlins"
is not quite correct. These long lenticular banks cannot be said
to lie upon boulder-clay, but are merely a structural form of that
accumulation. And it is hardly the case that geologists have "performed
the most acrobatic feats" in trying to explain the origin of the
banks in question. The usual explanation is that they have been formed
underneath the ice as ground-moraine--the upper surface of which
varies in configuration--being sometimes approximately even, as in
broad mountain-valleys; at other times ridged and corrugated, as in
open lowlands. And these modifications of surface are supposed to
have resulted from the varying movement and pressure of the overlying
ice-sheet. The drumlins, in fact, would appear to be analogous to the
banks that accumulate in the beds of rivers. Many drumlins, indeed,
are composed partly of solid rock and partly of boulder-clay, which
would seem to have accumulated in the lee of the projecting rock, much
in the same way as gravel and sand gather behind any large boulder
in a stream-course. Dr. Nansen, apparently, to some extent confounds
drumlins with "kames" and "åsar," of which certainly many strange and
conflicting explanations have been hazarded. These, however, differ
essentially from drumlins, for they consist exclusively, or almost
exclusively, of water-worn and more or less water-assorted materials.
And one widely-accepted view of their origin is that they have
accumulated in tunnels underneath an ice-sheet. This is practically
the same view as Dr. Nansen's. He thinks that when an ice-sheet has
its under-surface furrowed by running water, the ground-moraine will
tend to be pressed up into the river-channels. The water will, in this
way, be compelled to hollow out the roof of its tunnel to a greater
degree, and as the stream continues to work upwards the moraine will
follow it, so as to partially fill the tunnel and form a ridge along
the back of which the sub-glacial stream will run. The material forming
the upper portion of the ridge will thus come to be composed mainly
of water-worn and stratified detritus, derived from the erosion of
the ground-moraine. This is an ingenious suggestion which may be of
good service in some cases, but it is certainly inapplicable to most
kames and åsar. If it were a complete explanation we ought to find
these ridges consisting of an upper water-assorted portion and a lower
unmodified morainic portion (boulder-clay). But this is not the case,
for most kames consist entirely, from top to bottom, of water-assorted
materials. They are found running across an even or gently-undulating
surface of boulder-clay, and sometimes they rest not on boulder-clay
but solid rock.
Dr. Nansen considers another geological question which has given rise
to much controversy, and is still far from being settled--namely,
whether the oscillations of level which have left such conspicuous
traces in northern regions are in any way connected with the appearance
and disappearance of great ice-sheets. Can a big ice-sheet push down
the earth's crust by its weight? and does the crust rise again as the
ice melts away? Could a thick ice-sheet exercise sufficient attraction
upon the sea to cause it to rise upon the land, and thus explain the
origin of some of the so-called raised beaches of this and other
formerly glaciated lands? Can the weight of a great ice-sheet shift
the earth's centre of gravity, and, if so, to what extent? Each of
these questions has been answered in the affirmative and the negative
by controversialists, and, until the geological evidence has been
completely sifted, each, doubtless, will continue to be alternately
affirmed and denied. All that need be pointed out here is that some of
the movements which occurred during the Pleistocene period were on much
too large a scale to be explicable by any of the hypotheses referred
to.
XIV.
The Geographical Development of Coast-lines.[DO]
[DO] Presidential Address to the Geographical Section of the British
Association, Edinburgh, 1892.
Amongst the many questions upon which of late years light has been
thrown by deep-sea exploration and geological research, not the least
interesting is that of the geographical development of coast-lines. How
is the existing distribution of land and water to be accounted for? Are
the revolutions in the relative position of land and sea, to which the
geological record bears witness, due to movements of the earth's crust
or of the hydrosphere? Why are coast-lines in some regions extremely
regular, while elsewhere they are much indented? About 150 years ago
the prevalent belief was that ancient sea-margins indicated a formerly
higher ocean-level. Such was the view held by Celsius, who, from an
examination of the coast-lands of Sweden, attributed the retreat of
the sea to a gradual drying up of the latter. But this desiccation
hypothesis was not accepted by Playfair, who thought it much more
likely that the land had risen. It was not, however, until after Von
Buch had visited Sweden (1806-1808), and published the results of his
observations, that Playfair's suggestion received much consideration.
Von Buch concluded that the apparent retreat of the sea was not due
to a general depression of the ocean-level, but to elevation of the
land--a conclusion which subsequently obtained the strong support of
Lyell. The authority of these celebrated men gained for the elevation
theory more or less complete assent, and for many years it has been the
orthodox belief of geologists that the ancient sea-margins of Sweden
and other lands have resulted from vertical movements of the crust.
It has long been admitted, however, that highly-flexed and disturbed
strata require some other explanation. Obviously such structures are
the result of lateral compression and crumpling. Hence geologists have
maintained that the mysterious subterranean forces have affected the
crust in different ways. Mountain-ranges, they conceive, are ridged up
by tangential thrusts and compression, while vast continental areas
slowly rise and fall, with little or no disturbance of the strata. From
this point of view it is the lithosphere that is unstable, all changes
in the relative level of land and sea being due to crustal movements.
Of late years, however, Trautschold and others have begun to doubt
whether this theory is wholly true, and to maintain that the sea-level
may have changed without reference to movements of the lithosphere.
Thus Hilber has suggested that sinking of the sea-level may be due, in
part at least, to absorption, while Schmick believes that the apparent
elevation and depression of continental areas are really the results
of grand secular movements of the ocean. The sea, according to him,
periodically attains a high level in each hemisphere alternately, the
waters being at present heaped up in the southern hemisphere. Professor
Suess, again, believing that in equatorial regions the sea is, on the
whole, gaining on the land, while in other latitudes the reverse would
appear to be the case, points out this is in harmony with his view
of a periodical flux and reflux of the ocean between the equator and
the poles. He thinks we have no evidence of any vertical elevation
affecting wide areas, and that the only movements of elevation that
take place are those by which mountains are upheaved. The broad
invasions and transgressions of the continental areas by the sea, which
we know have occurred again and again, are attributed by him to secular
movements of the hydrosphere itself.
Apart from all hypothesis and theory, we learn that the surface of
the sea is not exactly spheroidal. It reaches a higher level on the
borders of the continents than in mid-ocean, and it varies likewise
in height at different places on the same coast. The attraction of
the Himalaya, for example, suffices to cause a difference of 300
feet between the level of the sea at the delta of the Indus and on
the coast of Ceylon. The recognition of such facts has led Penck to
suggest that the submergence of the maritime regions of north-west
Europe and the opposite coasts of North America, which took place
at a recent geological date, and from which the lands in question
have only partially recovered, may have been brought about by the
attraction exerted by the vast ice-sheets of the Glacial period.
But, as Drygalski, Woodward, and others have shown, the heights at
which recent marine deposits occur in the regions referred to are
much too great to be accounted for by any possible distortion of the
hydrosphere. The late James Croll had previously endeavoured to show
that the accumulation of ice over northern lands during glacial times
would suffice to displace the earth's centre of gravity, and thus cause
the sea to rise upon the glaciated tracts. More recently other views
have been advanced to explain the apparently causal connection between
glaciation and submergence, but these need not be considered here.
Whatever degree of importance may attach to the various hypotheses of
secular movements of the sea, it is obvious that the general trends
of the world's coast-lines are determined in the first place by the
position of the dominant wrinkles of the lithosphere. Even if we
concede that all "raised beaches," so-called, are not necessarily
the result of earth-movements, and that the frequent transgressions
of the continental areas by oceanic waters in geological times may
possibly have been due to independent movements of the sea, still we
must admit that the solid crust of the globe has always been subject to
distortion. And this being so, we cannot doubt that the general trends
of the world's coast-lines must have been modified from time to time
by movements of the lithosphere.
As geographers we are not immediately concerned with the mode of origin
of those vast wrinkles, nor need we speculate on the causes which may
have determined their direction. It seems, however, to be the general
opinion that the configuration of the lithosphere is due simply to the
sinking-in and doubling-up of the crust on the cooling and contracting
nucleus. But it must be admitted that neither physicists nor geologists
are prepared with a satisfactory hypothesis to account for the
prominent trends of the great world-ridges and troughs. According to
the late Professor Alexander Winchell, these trends may have been
the result of primitive tidal action. He was of opinion that the
transmeridional progress of the tidal swell in early incrustive times
on our planet would give the forming crust structural characteristics
and aptitudes trending from north to south. The earliest wrinkles to
come into existence, therefore, would be meridional or submeridional,
and such, certainly, is the prevalent direction of the most conspicuous
earth-features. There are many terrestrial trends, however, as
Professor Winchell knew, which do not conform to the requirements of
his hypothesis; but such transmeridional features, he thought, could
generally be shown to be of later origin than the others. This is
the only speculation, so far as I know, which attempts, perhaps not
altogether unsuccessfully, to explain the origin of the main trends
of terrestrial features. According to other authorities, however, the
area of the earth's crust occupied by the ocean is denser than that
over which the continental regions are spread. The depressed denser
part balances the lighter elevated portion. But why these regions
of different densities should be so distributed no one has yet told
us. Neither does Le Conte's view, that the continental areas and the
oceanic depressions owe their origin to unequal radial contraction of
the earth in its secular cooling, help us to understand why the larger
features of the globe should be disposed as they are.
Geographers must for the present be content to take the world as
they find it. What we do know is that our lands are distributed
over the surface of a great continental plateau of irregular form,
the bounding slopes of which plunge down more or less steeply into
a vast oceanic depression. So far as geological research has gone,
there is reason to believe that these elevated and depressed areas
are of primeval antiquity--that they ante-date the very oldest of
the sedimentary formations. There is abundant evidence, however, to
show that the relatively elevated or continental area has been again
and again irregularly submerged under tolerably deep and wide seas.
But all historical geology seems to assure us that the continental
plateau and the oceanic hollows have never changed places, although
from time to time portions of the latter have been ridged up and added
to the margins of the former, while ever and anon marginal portions
of the plateau have sunk to very considerable depths. We may thus
speak of the great world-ridges as regions of dominant elevation, and
of the profound oceanic troughs as areas of more or less persistent
depression. From one point of view, it is true, no part of the earth's
surface can be looked upon as a region of dominant elevation. Our
globe is a cooling and contracting body, and depression must always
be the prevailing movement of the lithosphere. The elevation of the
continental plateau is thus only relative. Could we conceive the crust
throughout the deeper portions of the oceanic depression to subside to
still greater depths, while at the same time the continental plateau
remained stationary, or subsided more slowly, the sea would necessarily
retreat from the land, and the latter would then appear to rise. It is
improbable, however, that any extensive subsidence of the crust under
the ocean could take place without accompanying disturbance of the
continental plateau; and in this case the latter might experience in
places not only negative but positive elevation. During the evolution
of our continents, crustal movements have again and again disturbed the
relative level of land and sea; but since the general result has been
to increase the land-surface and to contract the area occupied by the
sea, it is convenient to speak of the former as the region of dominant
elevation, and of the latter as that of prevalent depression. Properly
speaking, both are sinking regions, the rate of subsidence within the
oceanic trough being in excess of that experienced over the continental
plateau. The question of the geographical development of coast-lines is
therefore only that of the dry lands themselves.
The greater land-masses are all situated upon, but are nowhere
co-extensive with, the area of dominant elevation, for very
considerable portions of the continental plateau are still covered by
the sea. Opinions may differ as to which fathoms-line we should take as
marking approximately the boundary between that region and the oceanic
depression; and it is obvious, indeed, that any line selected must be
arbitrary and more or less misleading, for it is quite certain that
the true boundary of the continental plateau cannot lie parallel to
the surface of the ocean. In some regions it approaches within a few
hundreds of fathoms of the sea-level; in other places it sinks for
considerably more than 1000 fathoms below that level. Thus, while a
very moderate elevation would in certain latitudes cause the land to
extend to the edge of the plateau, an elevation of at least 10,000 feet
would be required in some other places to bring about a similar result.
Although it is true that the land-surface is nowhere co-extensive with
the great plateau, yet the existing coast-lines may be said to trend
in the same general direction as its margins. So abruptly does the
continental plateau rise from the oceanic trough, that a depression
of the sea-level, or an elevation of the plateau, for 10,000 feet,
would add only a narrow belt to the Pacific coast between Alaska and
Cape Horn, while the gain of land on the Atlantic slope of America
between 30° N.L. and 40° S.L. would not be much greater. In the higher
latitudes of the northern hemisphere, however, very considerable
geographical changes would be accomplished by a much less amount of
elevation of the plateau. Were the continental plateau to be upheaved
for 3000 feet, the major portion of the Arctic Sea would become land.
Thus, in general terms, we may say that the coast-lines of arctic and
temperate North America and Eurasia are further withdrawn from the edge
of the continental plateau than those of lower latitudes.
In regions where existing coast-lines approach the margin of the
plateau, they are apt to run for long distances in one determinate
direction, and, whether the coastal area be high or not, to show a
gentle sinuosity. Their course is seldom interrupted by bold projecting
headlands or peninsulas, or by intruding inlets, while fringing or
marginal islands rarely occur. To these appearances the northern
regions, as every one knows, offer the strongest contrast. Not only do
they trend irregularly, but their continuity is constantly interrupted
by promontories and peninsulas, by inlets and fiords, while fringing
islands abound. But an elevation of some 400 or 500 fathoms only
would revolutionise the geography of those regions, and confer upon
the northern coast-lines of the world the regularity which at present
characterises those of western Africa.
It is obvious, therefore, that the coast-lines of such lands as Africa
owe their regularity primarily to their approximate coincidence with
the steep boundary-slopes of the continental plateau, while the
irregularities characteristic of the coast-line of north-western Europe
and the corresponding latitudes of North America are determined by the
superficial configuration of the same plateau, which in those regions
is relatively more depressed. I have spoken of the general contrast
between high and low northern latitudes; but it is needless to say
that in southern regions the coast-lines exhibit similar contrasts.
The regular coast-lines of Africa and South America have already been
referred to; but we cannot fail to recognise in the much-indented
sea-board and the numerous coastal islands of southern Chile a complete
analogy to the fiord regions of high northern latitudes. Both are areas
of comparatively recent depression. Again, the manifold irregularities
of the coasts of south-eastern Asia, and the multitudes of islands
that serve to link that continent to Australia and New Zealand, are all
evidence that the surface of the continental plateau in those regions
is extensively invaded by the sea.
A word or two now as to the configuration of the oceanic trough. There
can be no doubt that this differs very considerably from that of the
land-surface. It is, upon the whole, flat or gently-undulating. Here
and there it swells gently upwards into broad elevated banks, some of
which have been traced for great distances. In other places narrower
ridges and abrupt mountain-like elevations diversify its surface,
and project again and again above the level of the sea, to form the
numerous islets of Oceania. Once more, the sounding-line has made us
acquainted with the notable fact that numerous deep depressions--some
long and narrow, others relatively short and broad--stud the floor
of the great trough. I shall have occasion to refer again to these
remarkable depressions, and need at present only call attention to
the fact that they are especially well-developed in the region of
the western Pacific, where the floor of the sea, at the base of the
bounding slopes of the continental plateau, sinks in places to depths
of three and even of five miles below the existing coast-lines. One may
further note the fact that the deepest areas of the Atlantic are met
with in like manner close to the walls of the plateau--a long ridge,
which rises midway between the continents and runs in the same general
direction as their coast-lines, serving to divide the trough of the
Atlantic into two parallel hollows.
But, to return to our coast-lines and the question of their
development, it is obvious that their general trends have been
determined by crustal movements. Their regularity is in direct
proportion to the closeness of their approach to the margin of the
continental plateau. The more nearly they coincide with the edge of
that plateau, the fewer irregularities do they present; the further
they recede from it, the more highly are they indented. Various
other factors, it is true, have played a more or less important part
in their development, but their dominant trends were undoubtedly
determined at a very early period in the world's history--their
determination necessarily dates back, in short, to the time when the
great world-ridges and oceanic troughs came into existence. So far as
we can read the story told by the rocks, however, it would seem that
in the earliest ages of which geology can speak with any confidence,
the coast-lines of the world must have been infinitely more irregular
than now. In Palæozoic times, relatively small areas of the continental
plateau appeared above the level of the sea. Insular conditions
everywhere prevailed. But as ages rolled on, wider and wider tracts of
the plateau were exposed, and this notwithstanding many oscillations of
level. So that one may say there has been, upon the whole, a general
advance from insular to continental conditions. In other words, the
sea has continued to retreat from the surface of the continental
plateau. To account for this change, we must suppose that depression
of the crust has been in excess within the oceanic area, and that now
and again positive elevation of the continental plateau has taken
place, more especially along its margins. That movements of elevation,
positive or negative, have again and again affected our land-areas can
be demonstrated, and it seems highly probable, therefore, that similar
movements may have been experienced within the oceanic trough.
Two kinds of crustal movement, as we have seen, are recognised by
geologists. Sometimes the crust appears to rise, or, as the case may
be, to sink over wide regions, without much disturbance or tilting
of strata, although these are now and again more or less extensively
fractured and displaced. It may conduce to clearness if we speak of
these movements as regional. The other kind of crustal disturbance
takes place more markedly in linear directions, and is always
accompanied by abrupt folding and mashing together of strata, along
with more or less fracturing and displacement. The plateau of the
Colorado has often been cited as a good example of regional elevation,
where we have a wide area of approximately horizontal strata apparently
uplifted without much rock-disturbance, while the Alps or any other
chain of highly-flexed and convoluted strata will serve as an example
of what we may term axial or linear uplifts. It must be understood
that both regional and axial movements result from the same cause--the
adjustment of the solid crust to the contracting nucleus--and that the
term _elevation_, therefore, is only relative. Sometimes the sinking
crust gets relief from the enormous lateral pressure to which it is
subjected by crumpling up along lines of weakness, and then mountains
of elevation are formed; at other times, the pressure is relieved by
the formation of broader swellings, when wide areas become uplifted
relatively to surrounding regions. Geologists, however, are beginning
to doubt whether upheaval of the latter kind can affect a broad
continental area. Probably, in most cases, the apparent elevation of
continental regions is only negative. The land appears to have risen
because the floor of the oceanic basin has become depressed. Even the
smaller plateau-like elevations which occur within some continental
regions may in a similar way owe their dominance to the sinking of
contiguous regions.
In the geographical development of our land, movements of elevation
and depression have played an important part. But we cannot ignore the
work done by other agents of change. If the orographical features of
the land everywhere attest the potency of plutonic agents, they no less
forcibly assure us that the inequalities of surface resulting from such
movements are universally modified by denudation and sedimentation.
Elevated plains and mountains are gradually demolished, and the hollows
and depressions of the great continental plateau become slowly filled
with their detritus. Thus inland-seas tend to vanish, inlets and
estuaries are silted up, and the land in places advances seaward. The
energies of the sea, again, come in to aid those of rain and rivers,
so that under the combined action of all the superficial agents of
change, the irregularities of coast-lines become reduced, and, were no
crustal movement to intervene, would eventually disappear. The work
accomplished by those agents upon a coast-line is most conspicuous in
regions where the surface of the continental plateau is occupied by
comparatively shallow seas. Here full play is given to sedimentation
and marine erosion, while the latter alone comes into prominence upon
shores that are washed by deeper waters. When the coast-lines advance
to the edge of the continental plateau, they naturally trend, as we
have seen, for great distances in some particular direction. Should
they preserve that position, undisturbed by crustal oscillation, for a
prolonged period of time, they will eventually be cut back by the sea.
In this way a shelf or terrace will be formed, narrow in some places,
broader in others, according to the resistance offered by the varying
character of the rocks. But no long inlets or fiords can result from
such action. At most the harder and less readily demolished rocks will
form headlands, while shallow bays will be scooped out of the more
yielding masses. In short, between the narrower and broader parts
of the eroded shelf or terrace a certain proportion will tend to be
preserved. As the shelf is widened, sedimentation will become more and
more effective, and in places may come to protect the land from further
marine erosion. This action is especially conspicuous in tropical and
sub-tropical regions, which are characterised by well-marked rainy
seasons. In such regions immense quantities of sediment are washed down
from the land to the sea, and tend to accumulate along shore, forming
low alluvial flats. All long-established coast-lines thus acquire a
characteristically sinuous form, and perhaps no better examples could
be cited than those of western Africa.
To sum up, then, we may say that the chief agents concerned in the
development of coast-lines are crustal movements, sedimentation, and
marine erosion. All the main trends are the result of elevation and
depression. Considerable geographical changes, however, have been
brought about by the silting up of those shallow and sheltered seas
which, in certain regions, overflow wide areas of the continental
plateau. Throughout all the ages, indeed, epigene agents have striven
to reduce the superficial inequalities of that plateau, by levelling
heights and filling up depressions, and thus, as it were, flattening
out the land-surface and causing it to extend. The erosive action
of the sea, from our present point of view, is of comparatively
little importance. It merely adds a few finishing touches to the work
performed by the other agents of change.
A glance at the geographical evolution of our own Continent will render
this sufficiently evident. Viewed in detail, the structure of Europe
is exceedingly complicated, but there are certain leading features in
its architecture which no profound analysis is required to detect. We
note, in the first place, that highly-disturbed rocks of Archæan and
Palæozoic age reach their greatest development along the north-western
and western borders of our Continent, as in Scandinavia, the British
Islands, north-west France, and the Iberian peninsula. Another belt
of similarly disturbed strata of like age traverses central Europe
from west to east, and is seen in the south of Ireland, Cornwall,
north-west France, the Ardennes, the Thüringer-Wald, the Erz Gebirge,
the Riesen Gebirge, the Böhmer-Wald, and other heights of middle and
southern Germany. Strata of Mesozoic and Cainozoic age rest upon the
older systems in such a way as to show that the latter had been much
folded, fractured, and denuded before they came to be covered with
younger formations. North and north-east of the central belt of ancient
rocks just referred to, the sedimentary strata that extend to the
shores of the Baltic and over a vast region in Russia, range in age
from Palæozoic down to Cainozoic times, and are disposed for the most
part in gentle undulations--they are either approximately horizontal
or slightly inclined. Unlike the disturbed rocks of the maritime
regions and of central Europe, they have obviously been subjected to
comparatively little folding since the time of their deposition. To
the south of the primitive back-bone of central Europe succeeds a
region composed superficially of Mesozoic and Cainozoic strata for the
most part, which, along with underlying Palæozoic and Archæan rocks,
are often highly-flexed and ridged up, as in the chains of the Jura,
the Alps, the Carpathians, etc. One may say, in general terms, that
throughout the whole Mediterranean area Archæan and Palæozoic rocks
appear at the surface only when they form the nuclei of mountains of
elevation, into the composition of which rocks of younger age largely
enter.
From this bald and meagre outline of the general geological structure
of Europe, we may gather that the leading orographical features of our
Continent began to be developed at a very early period. Unquestionably
the oldest land-areas are represented by the disturbed Archæan
and Palæozoic rocks of the Atlantic sea-board and central Europe.
Examination of those tracts shows that they have experienced excessive
denudation. The Archæan and Palæozoic masses, distributed along the
margin of the Atlantic, are the mere wrecks of what, in earlier ages,
must have been lofty regions, the mountain-chains of which may well
have rivalled or even exceeded in height the Alps of to-day. They,
together with the old disturbed rocks of central Europe, formed for a
long time the only land in our area. Between the ancient Scandinavian
tract in the north and a narrow interrupted belt in central Europe,
stretched a shallow sea, which covered all the regions that now form
our Great Plain; while immediately south of the central belt lay the
wide depression of the Mediterranean--for as yet the Pyrenees, the
Alps, and the Carpathians were not. Both the Mediterranean and the
Russo-Germanic sea communicated with the Atlantic. As time went on land
continued to be developed along the same lines, a result due partly
to crustal movements, partly to sedimentation. Thus the relatively
shallow Russo-Germanic sea became silted up, while the Mediterranean
shore-line advanced southwards. It is interesting to note that the
latter sea, down to the close of Tertiary times, seems always to have
communicated freely with the Atlantic, and to have been relatively
deep. The Russo-Germanic sea, on the contrary, while now and again
opening widely into the Atlantic, and attaining considerable depths in
its western reaches, remained on the whole shallow, and ever and anon
vanished from wide areas to contract into a series of inland-seas and
large salt lakes.
Reduced to its simplest elements, therefore, the structure of Europe
shows two primitive ridges--one extending with some interruptions
along the Atlantic sea-board, the other traversing central Europe
from west to east, and separating the area of the Great Plain from
the Mediterranean basin. The excessive denudation which the more
ancient lands have undergone, and the great uplifts of Mesozoic and of
Cainozoic times, together with the comparatively recent submergence
of broad tracts in the north and north-west, have not succeeded in
obscuring the dominant features in the architecture of our Continent.
I now proceed to trace, as rapidly as I can, the geographical
development of the coast-lines of the Atlantic as a whole, and to
point out the chief contrasts between them and the coast-lines of the
Pacific. The extreme irregularity of the Arctic and Atlantic shores of
Europe at once suggests to a geologist a partially-drowned land, the
superficial inequalities of which are accountable for the vagaries of
the coast-lines. The fiords of Norway and Scotland occupy what were
at no distant date land-valleys, and the numerous marginal islands of
those regions are merely the projecting portions of a recently-sunken
area. The continental plateau extends up to and a little beyond the
one hundred fathoms line, and there are many indications that the
land formerly reached as far. Thus the sunken area is traversed by
valley-like depressions, which widen as they pass outwards to the
edge of the plateau, and have all the appearance of being hollows
of sub-aërial erosion. I have already mentioned the fact that the
Scandinavian uplands and the Scottish Highlands are the relics of what
were at one time true mountains of elevation, corresponding in the mode
of their formation to those of Switzerland, and, like these, attaining
a great elevation. During subsequent stages of Palæozoic time, that
highly-elevated region was subjected to long-continued and profound
erosion--the mountain-country was planed down over wide regions to
sea-level, and broad stretches of the reduced land-surface became
submerged. Younger Palæozoic formations then accumulated upon the
drowned land, until eventually renewed crustal disturbance supervened,
and the marginal areas of the continental plateau again appeared as
dry land, but not, as before, in the form of mountains of elevation.
Lofty table-lands now took the place of abrupt and serrated ranges
and chains--table-lands which, in their turn, were destined in the
course of long ages to be deeply sculptured and furrowed by sub-aërial
agents. During this process the European coast-line would seem to
have coincided more or less closely with the edge of the continental
plateau. Finally, after many subsequent movements of the crust in
these latitudes, the land became partially submerged--a condition from
which north-western and northern Europe would appear in recent times
to be slowly recovering. Thus the highly-indented coast-line of those
regions does not coincide with the edge of the plateau, but with those
irregularities of its upper surface which are the result of antecedent
sub-aërial erosion.
Mention has been made of the Russo-Germanic plain and the Mediterranean
as representing original depressions in the continental plateau, and
of the high-grounds that extend between them as regions of dominant
elevation, which, throughout all the manifold revolutions of the past,
would appear to have persisted as a more or less well-marked boundary,
separating the northern from the southern basin. During certain periods
it was no doubt in some degree submerged, but never apparently to
the same extent as the depressed areas it served to separate. From
time to time uplifts continued to take place along this central belt,
which thus increased in breadth, the younger formations, which were
accumulated along the margins of the two basins, being successively
ridged up against nuclei of older rocks. The latest great crustal
movements in our Continent, resulting in the uplift of the Alps and
other east and west ranges of similar age, have still further widened
that ancient belt of dominant elevation which in our day forms the most
marked orographical feature of Europe.
The Russo-Germanic basin is now for the most part land, the Baltic
and the North Sea representing its still submerged portions. This
basin, as already remarked, was probably never so deep as that of the
Mediterranean. We gather as much from the fact that, while mechanical
sediments of comparatively shallow-water origin predominate in the
former area, limestones are the characteristic features of the
southern region. Its relative shallowness helps us to understand why
the northern depression should have been silted up more completely
than the Mediterranean. We must remember also that for long ages it
received the drainage of a much more extensive land-surface than the
latter--the land that sloped towards the Mediterranean in Palæozoic
and Mesozoic times being of relatively little importance. Thus the
crustal movements which ever and anon depressed the Russo-Germanic area
were, in the long-run, counterbalanced by sedimentation. The uplift
of the Alps, the Atlas, and other east and west ranges, has greatly
contracted the area of the Mediterranean, and sedimentation has also
acted in the same direction, but it is highly probable that that sea
is now as deep as, or even deeper than, it has ever been. It occupies
a primitive depression in which the rate of subsidence has exceeded
that of sedimentation. In many respects, indeed, this remarkable
transmeridional hollow--continued eastward in the Red Sea, the Black
Sea, and the Aralo-Caspian depression--is analogous, as we shall see,
to the great oceanic trough itself.
In the earlier geological periods linear or axial uplifts and
volcanic action again and again marked the growth of land on the
Atlantic sea-board. But after Palæozoic times, no great mountains of
elevation came into existence in that region, while volcanic action
almost ceased. In Tertiary times, it is true, there was a remarkable
recrudescence of volcanic activity, but the massive eruptions of
Antrim and western Scotland, of the Faröe Islands and Iceland, must
be considered apart from the general geology of our Continent. From
Mesozoic times onwards it was along the borders of the Mediterranean
depression that great mountain uplifts and volcanoes chiefly presented
themselves; and as the land-surface extended southwards from central
Europe, and the area of the Mediterranean was contracted, volcanic
action followed the advancing shore-lines. The occurrence of numerous
extinct and of still existing volcanoes along the borders of this
inland-sea, the evidence of recent crustal movements so commonly
met with upon its margins, the great irregularities of its depths,
the proximity of vast axial uplifts of late geological age, and the
frequency of earthquake phenomena, all indicate instability, and remind
us strongly of similarly constructed and disturbed regions within the
area of the vast Pacific.
Let us now look at the Arctic and Atlantic coast-lines of North
America. From the extreme north down to the latitude of New York the
shores are obviously those of a partially-submerged region. They are
of the same type as the coasts of north-western Europe. We have every
reason to believe also that the depression of Greenland and north-east
America, from which these lands have only partially recovered,
dates back to a comparatively recent period. The fiords and inlets,
like those of Europe, are merely half-drowned land-valleys, and the
continental shelf is crossed by deep hollows which are evidently
only the seaward continuations of well-marked terrestrial features.
Such, for example, is the case with the valleys of the Hudson and
the St. Lawrence, the submerged portions of which can be followed
out to the edge of the continental plateau, which is notched by them
at depths of 474 and 622 fathoms respectively. There is, in short, a
broad resemblance between the coasts of the entire Arctic and North
Atlantic regions down to the latitudes already mentioned. Everywhere
they are irregular and fringed with islands in less or greater
abundance--highly-denuded and deeply-incised plateaux being penetrated
by fiords, while low-lying and undulating lands that shelve gently
seaward are invaded by shallow bays and inlets. Comparing the American
with the opposite European coasts one cannot help being struck with
certain other resemblances. Thus Hudson Bay at once suggests the
Baltic, and the Gulf of Mexico, with the Caribbean Sea, recalls the
Mediterranean. But the geological structure of the coast-lands of
Greenland and North America betrays a much closer resemblance between
these and the opposite shores of Europe than appears on a glance at
the map. There is something more than a mere superficial similarity.
In eastern North America and Greenland, just as in western Europe,
no grand mountain uplifts have taken place for a prodigious time.
The latest great upheavals, which were accompanied by much folding
and flexing of strata, are those of the Appalachian chain and of
the coastal ranges extending through New England, Nova Scotia, and
Newfoundland, all of which are of Palæozoic age. Considerable crustal
movements affected the American coast-lines in Mesozoic times, and
during these uplifts the strata suffered fracture and displacement,
but were subjected to comparatively little folding. Again, along
the maritime borders of north-east America, as in the corresponding
coast-lines of Europe, igneous action, more or less abundant in
Palæozoic and early Mesozoic times, has since been quiescent. From
the mouth of the Hudson to the Straits of Florida the coast-lines are
composed of Tertiary and Quaternary deposits. This shows that the land
has continued down to recent times to gain upon the sea--a result
brought about partly by quiet crustal movements, but to a large extent
by sedimentation, aided, on the coasts of Florida, by the action of
reef-building corals.
Although volcanic action has long ceased on the American sea-board,
we note that in Greenland, as in the west of Scotland and north
of Ireland, there is abundant evidence of volcanic activity at
so late a period as the Tertiary. It would appear that the great
plateau-basalts of those regions, and of Iceland and the Faröe Islands,
were contemporaneous, and were possibly connected with an important
crustal movement. It has long been suggested that at a very early
geological period Europe and North America may have been united. The
great thickness attained by the Palæozoic rocks in the eastern areas
of the latter implies the existence of a wide land-surface from which
ancient sediments were derived. That old land must have extended
beyond the existing coast-line, but how far we cannot tell. Similarly
in north-west Europe, during early Palæozoic times, the land probably
stretched further into the Atlantic than at present. But whether,
as some think, an actual land-connection subsisted between the two
continents it is impossible to say. Some such connection was formerly
supposed necessary to account for life common to the Palæozoic strata
of both continents, and which, as they were probably denizens of
comparatively shallow water, could only have crossed from one area
to another along a shore-line. It is obvious, indeed, that if the
oceanic troughs in those early days were of an abysmal character, a
belt of shallow water would be required to explain the geographical
distribution of cosmopolitan marine life-forms. But if it be true that
subsidence of the crust has been going on through all geological time,
and that the land-areas have nothwithstanding continued to extend over
the continental plateau, then it follows that the oceanic trough must
be deeper now than it was in Palæozoic times. There are, moreover,
certain geological facts which seem hardly explicable on the assumption
that the seas of past ages attained abysmal depths over any extensive
areas. The Palæozoic strata which enter so largely into the framework
of our lands have much the same appearance all the world over, and
were accumulated for the most part in comparatively shallow water.
A petrographical description of the Palæozoic mechanical sediments
of Europe would serve almost equally well for those of America, of
Asia, or of Australia. Take in connection with this the fact that
Palæozoic faunas had a very much wider range than those of Mesozoic and
later ages, and were characterised above all by the presence of many
cosmopolitan species, and we can hardly resist the conclusion that it
was the comparative shallowness of the ancient seas that favoured that
wide dispersal of species, and enabled currents to distribute sediments
the same in kind over such vast regions. As the oceanic area deepened
and contracted, and the land-surface increased, marine faunas were
gradually restricted in their range, and the cosmopolitan marine forms
diminished in numbers, while sediments, gathering in separate regions,
became more and more differentiated. For these and other reasons which
need not be entered upon here, I see no necessity for supposing that a
Palæozoic Atlantis connected Europe with North America. The broad ridge
upon which the Faröe Islands and Iceland are founded seems to pertain
as truly to the oceanic depression as the long Dolphin Ridge of the
South Atlantic. The trend of the continental plateau in high latitudes
is shown, as I think, by the general direction of the coast-lines of
north-western Europe and east Greenland, the continental shelf being
submerged in those regions for a few hundred fathoms only. How the
Icelandic ridge came into existence, and what its age may be, we can
only conjecture. It may be a wrinkle as old as the oceanic trough
which it traverses, or its origin may date back to a much more recent
period. We may conceive it to be an area which has subsided more slowly
than the floor of the ocean to the north and south; or, on the other
hand, it may be a belt of positive elevation. Perhaps the latter is
the more probable supposition, for it seems very unlikely that crustal
disturbances, resulting in axial and regional uplifts, should have been
confined to the continental plateau only. Be that as it may, there is
little doubt that land-connection did obtain between Greenland and
Europe in the Cainozoic times along this Icelandic ridge, for relics
of the same Tertiary flora are found in Scotland, the Faröe Islands,
Iceland, and Greenland. The deposits in which these plant-remains
occur are associated with great sheets of volcanic rocks, which in
the Faröe Islands and Iceland reach a thickness of many thousand feet.
Of the same age are the massive basalts of Jan Mayen, Spitzbergen,
Franz-Joseph Land, and Greenland. These lavas seem seldom to have
issued from isolated foci in the manner of modern eruptions, but rather
to have welled up along the lines of rectilineal fissures. From the
analogy of similar phenomena in other parts of the world it might be
inferred that the volcanic action of these northern regions may have
been connected with a movement of elevation, and that the Icelandic
ridge, if it did not come into existence during the Tertiary period,
was at all events greatly upheaved at that time. It would seem most
likely, in short, that the volcanic action in question was connected
mainly with crustal movements in the oceanic trough. Similar phenomena,
as is well known, are met with further south in the trough of the
Atlantic. Thus the volcanic Azores rise like Iceland from the surface
of a broad ridge which is separated from the continental plateau by
wide and deep depressions. And so again, from the back of the great
Dolphin Ridge, spring the volcanic islets of St. Paul's, Ascension, and
Tristan d'Acunha.
I have treated of the Icelandic bank at some length for the purpose of
showing that its volcanic phenomena do not really form an exception to
the rule that such eruptions ceased after Palæozoic or early Mesozoic
times to disturb the Atlantic coast-lines of Europe and North America.
As the bank in question extends between Greenland and the British
Islands, it was only natural that both those regions should be affected
by its movements. But its history pertains essentially to that of the
Atlantic trough; and it seems to show us how transmeridional movements
of the crust, accompanied by vast discharges of igneous rock, may come
in time to form land-connections between what are now widely-separated
areas.
Let us next turn our attention to the coast-lines of the Gulf of
Mexico and the Caribbean Sea. These enclosed seas have frequently been
compared to the Mediterranean, and the resemblance is self-evident.
Indeed, it is so close that one may say the Mexican-Caribbean Sea
and the Mediterranean are rather homologous than simply analogous.
The latter, as we have seen, occupies a primitive depression, and
formerly covered a much wider area. It extended at one time over
much of southern Europe and northern Africa, and appears to have
had full communication across Asia Minor with the Indian Ocean, and
with the Arctic Ocean athwart the low-lying tracts of north-western
Asia. Similarly, it would seem, the Mexican-Caribbean Sea is the
remaining portion of an ancient inland-sea which formerly stretched
north through the heart of North America to the Arctic Ocean. Like
its European parallel, it has been diminished by sedimentation and
crustal movements. It resembles the latter also in the greatness and
irregularity of its depths, and in the evidence which its islands
supply of volcanic action as well as of very considerable crustal
movements within recent geological times. Along the whole northern
borders of the Gulf of Mexico the coast-lands, like those on the
Atlantic sea-board of the Southern States, are composed of Tertiary
and recent accumulations, and the same is the case with Yucatan;
while similar young formations are met with on the borders of the
Caribbean Sea and in the Antilles. The Bahamas and the Windward
Islands mark out for us the margin of the continental plateau, which
here falls away abruptly to profound depths. One feels assured that
this portion of the plateau has been ridged up to its present level
at no distant geological date. But notwithstanding all the evidence
of recent extensive crustal movements in this region, it is obvious
that the Mexican-Caribbean depression, however much it may have been
subsequently modified, is of primitive origin.[DP]
[DP] Professor Suess thinks it is probable that the Caribbean Sea and
the Mediterranean are portions of one and the same primitive depression
which traversed the Atlantic area in early Cretaceous times. He further
suggests that it may have been through the gradual widening of the
central Mediterranean that the Atlantic in later times came into
existence.
Before we leave the coast-lands of North America, I would again point
out their leading geological features. In a word, then, they are
composed for the most part of Archæan and Palæozoic rocks; no great
linear or axial uplifts marked by much flexure of strata have taken
place in those regions since Palæozoic times; while igneous action
virtually ceased about the close of the Palæozoic or the commencement
of the Mesozoic period. It is not before we reach the shores of the
Southern States and the coast-lands of the Mexican-Caribbean Sea that
we encounter notable accumulations of Mesozoic, Tertiary, and younger
age. These occur in approximately horizontal positions round the Gulf
of Mexico; but in the Sierra Nevada of northern Colombia and the
Cordilleras of Venezuela the Tertiary strata enter into the formation
of true mountains of elevation. Thus the Mexican-Caribbean depression,
like that of the Mediterranean, is characterised not only by its
irregular depths and its volcanic phenomena, but by the propinquity
of recent mountains of upheaval, which bear the same relation to the
Caribbean Sea as the mountains of north Africa do to the Mediterranean.
We may now compare the Atlantic coasts of South America with those
of Africa. The former coincide in general direction with the edge
of the continental plateau, to which they closely approach between
Cape St. Roque and Cape Frio. In the north-east, between Cape Paria,
opposite Trinidad, and Cape St. Roque, the continental shelf attains
a considerably greater breadth, while south of Cape Frio it gradually
widens until, in the extreme south, it runs out towards the east in the
form of a narrow ridge, upon the top of which rise the Falkland Islands
and south Georgia. Excluding from consideration for the present all
recent alluvial and Tertiary deposits, we may say that the coast-lands
from Venezuela down to the south of Brazil are composed principally
of Archæan rocks; the eastern borders of the continent further south
being formed of Quaternary and Tertiary accumulations. So far as we
know, igneous rocks are of rare occurrence on the Atlantic sea-board.
Palæozoic strata approach the coast-lands at various points between
the mouths of the Amazons and La Plata, and these, with the underlying
and surrounding Archæan rocks, are more or less folded and disturbed,
while the younger strata of Mesozoic and Cainozoic age (occupying
wide regions in the basin of the Amazons, and here and there fringing
the sea-coast) occur in approximately horizontal positions. It would
appear, therefore, that no great axial uplifts have taken place in
those regions since Palæozoic times. The crustal movements of later
ages were regional rather than axial; the younger rocks are not flexed
and mashed together, and their elevation (negative or positive) does
not seem to have been accompanied by conspicuous volcanic action.
The varying width of the continental shelf is due to several causes.
The Orinoco, the Amazons, and other rivers descending to the north-east
coast, carry enormous quantities of sediment, much of which comes to
rest on the submerged slopes of the continental plateau, so that the
continental shelf tends to extend seawards. The same process takes
place on the south-east coast, where the Rio de la Plata discharges
its muddy waters. South of latitude 40° S., however, another cause
has come into play. From the mouth of the Rio Negro to the terminal
point of the continent the whole character of the coast betokens a
geologically recent emergence, accompanied and followed by considerable
marine erosion. So that in this region the continental shelf increases
in width by the retreat of the coast-line, while in the north-east it
gains by advancing seawards. It is to be noted, however, that even
there, in places where the shores are formed of alluvia, the sea tends
to encroach upon the land.
The Atlantic coast of Africa resembles that of South America in certain
respects, but it also offers some important contrasts. As the northern
coasts of Venezuela and Colombia must be considered in relation rather
to the Caribbean depression than to the Atlantic, so the African
sea-board between Cape Spartel and Cape Nun pertains structurally
to the Mediterranean region. From the southern limits of Morocco to
Cape Colony the coastal heights are composed chiefly of Archæan and
Palæozoic rocks, the low shore-lands showing here and there strata of
Mesozoic and Tertiary age together with still more recent deposits.
The existing coast-lines everywhere advance close to the edge of
the continental plateau, so that the submarine shelf is relatively
narrower than that of eastern South America. The African coast is still
further distinguished from that of South America by the presence of
several groups of volcanic islands--Fernando Po and others in the Gulf
of Guinea, and Cape Verde and Canary Islands. The last-named group,
however, notwithstanding its geographical position, is probably related
rather to the Mediterranean depression than to the Atlantic trough.
The geological structure of the African coast-lands shows that the
earliest to come into existence were those that extend between Cape
Nun and the Cape of Good Hope. The coastal ranges of that section are
much denuded, for they are of very great antiquity, having been ridged
up in Palæozoic times. The later uplifts (negative or positive) of
the same region were not attended by tilting and folding of strata,
for the Mesozoic and Tertiary deposits, like those of South America,
lie in comparatively horizontal positions. Between Cape Nun and Cape
Spartel the rocks of the maritime tracts range in age from Palæozoic to
Cainozoic, and have been traced across Morocco into Algeria and Tunis.
They all belong to the Mediterranean region, and were deposited at a
time when the southern shores of that inland sea extended from a point
opposite the Canary Islands along what is now the southern margin of
Morocco, Algeria, and Tunis. Towards the close of the Tertiary period
the final upheaval of the Atlas took place, and the Mediterranean,
retreating northwards, became an almost land-locked sea.
I need hardly stop to point out how the African coast-lines have been
modified by marine erosion and the accumulation of sediment upon the
continental shelf. The extreme regularity of the coasts is due partly
to the fact that the land is nearly co-extensive with the continental
plateau, but it also results in large measure from the extreme
antiquity of the land itself. This has allowed of the cutting-back of
headlands and the rilling up of bays and inlets, a process which has
been going on between Morocco and Cape Colony with probably little
interruption for a very prolonged period of time. We may note also the
effect of the heavy rains of the equatorial region in washing down
detritus to the shores, and in this way protecting the land to some
extent from the erosive action of the sea.
What now, let us ask, are the outstanding features of the coast-lines
of the Atlantic Ocean? We have seen that along the margins of each of
the bordering continents the last series of great mountain-uplifts
took place in Palæozoic times. This is true alike for North and South
America, for Europe and Africa. Later movements which have added to the
extent of land were not marked by the extreme folding of strata which
attended the early upheavals. The Mesozoic and Cainozoic rocks, which
now and again form the shore-lands, occur in more or less undisturbed
condition. The only great linear uplifts or true mountains of elevation
which have come into existence in western Europe and northern Africa
since the Palæozoic period trend approximately at right angles to the
direction of the Atlantic trough, and are obviously related to the
primitive depression of the Mediterranean. The Pyrenees and the Atlas,
therefore, although their latest elevation took place in Tertiary
times, form no exceptions to the rule that the extreme flexing and
folding of strata which is so conspicuous a feature in the geological
structure of the Atlantic sea-board dates back to the Palæozoic era.
And the same holds true of North and South America. There all the
coastal ranges of highly flexed and folded strata are of Palæozoic
age. The Cordilleras of Venezuela are no doubt a Tertiary uplift, but
they are as obviously related to the Caribbean depression as the Atlas
ranges are to that of the Mediterranean. Again, we note that volcanic
activity along the borders of the Atlantic was much less pronounced
during the Mesozoic period than it appears to have been in the earlier
ages. Indeed, if we except the great Tertiary basalt-flows of the
Icelandic ridge and the Arctic regions, we may say that volcanic action
almost ceased after the Palæozoic era to manifest itself upon the
Atlantic coast-lands of North America and Europe. But while volcanic
action has died out upon the Atlantic margins of both continents, it
has continued during a prolonged geological period within the area of
the Mediterranean depression. And in like manner the corresponding
depression between North and South America has been the scene of
volcanic disturbances from Mesozoic down to recent times. Along the
African coasts the only displays of recent volcanic action that
appertain to the continental margin are those of the Gulf of Guinea
and the Cape de Verde Islands. The Canary Islands and Madeira may come
under the same category, but, as we have seen, they appear to stand in
relationship to the Mediterranean depression and the Tertiary uplift
of North Africa. Of Iceland and the Azores I have already spoken, and
of Ascension and the other volcanic islets of the South Atlantic it is
needless to say that they are related to wrinkles in the trough of the
ocean, and therefore have no immediate connection with the continental
plateau.
Thus in the geographical development of the Atlantic coast-lines we may
note the following stages:--_First_, in Palæozoic times the formation
of great mountain-uplifts, frequently accompanied by volcanic action.
_Second_, a prolonged stage of comparative coastal tranquillity, during
which the maritime ranges referred to were subject to such excessive
erosion that they were planed down to low levels, and in certain areas
even submerged. _Third_, renewed elevation (negative or positive)
whereby considerable portions of the much-denuded Archæan and Palæozoic
rocks, now largely covered by younger deposits, were converted into
high-lands. During this stage not much rock-folding took place, nor
were any true mountains of elevation formed parallel to the Atlantic
margins. It was otherwise, however, in the Mediterranean and Caribbean
depressions, where coastal movements resulted in the formation of
enormous linear uplifts. Moreover, volcanic action is now and has for
a long time been more characteristic of these depressions than of the
Atlantic coast-lands.
I must now ask you to take a comprehensive glance at the coast-lines of
the Pacific Ocean. In some important respects these offer a striking
contrast to those we have been considering. Time will not allow me
to enter into detailed description, and I must therefore confine
attention to certain salient features. Examining first the shores
of the Americas, we find that there are two well-marked regions of
fiords and fringing islands--namely, the coasts of Alaska and British
Columbia, and of South America from 40° S.L. to Cape Horn. Although
these regions may be now extending seawards in places, it is obvious
that they have recently been subject to submergence. When the fiords of
Alaska and British Columbia existed as land-valleys it is probable that
a broad land-connection obtained between North America and Asia. The
whole Pacific coast is margined by mountain-ranges, which in elevation
and boldness far exceed those of the Atlantic sea-board. The rocks
entering into their formation range in age from Archæan and Palæozoic
down to Cainozoic, and they are almost everywhere highly disturbed and
flexed. It is not necessary, even if it were possible, to consider the
geological history of all those uplifted masses. It is enough for my
purpose to note the fact that the coastal ranges of North America and
the principal chain of the Andes were all elevated in Tertiary times.
It may be remarked further that, from the Mesozoic period down to the
present, the Pacific borders of America have been the scene of volcanic
activity far in excess of what has been experienced on the Atlantic
sea-board.
Geographically the Asiatic coasts of the Pacific offer a strong
contrast to those of the American borders. The latter, as we have
seen, are for the most part not far removed from the edge of the
continental plateau. The coasts of the mainland of Asia, on the other
hand, retire to a great distance, the true margin of the plateau being
marked out by that great chain of islands which extends from Kamchatka
south to the Philippines and New Guinea. The seas lying between those
islands and the mainland occupy depressions in the continental plateau.
Were that plateau to be lifted up for 6000 or 7000 feet the seas
referred to would be enclosed by continuous land, and all the principal
islands of the East Indian Archipelago--Sumatra, Java, Celebes, and New
Guinea, would become united to themselves as well as to Australia and
New Zealand. In short, it is the relatively depressed condition of the
continental plateau along the western borders of the Pacific basin that
causes the Asiatic coast-lines to differ so strikingly from those of
America.
From a geological point of view the differences are less striking than
the resemblances. It is true that we have as yet a very imperfect
knowledge of the geological structure of eastern Asia, but we know
enough to justify the conclusion that in its main features that region
does not differ essentially from western North America. During Mesozoic
and Cainozoic times the sea appears to have overflowed vast tracts of
Manchooria and China, and even to have penetrated into what is now the
great Desert of Gobi. Subsequent crustal movements revolutionised the
geography of all those regions. Great ranges of linear uplifts came
into existence, and in these the younger formations, together with
the foundations on which they rested, were squeezed into folds and
ridged up against the nuclei of Palæozoic and Archæan rocks which had
hitherto formed the only dry land. The latest of these grand upheavals
are of Tertiary age, and, like those of the Pacific slope of America,
they were accompanied by excessive volcanic action. The long chains of
islands that flank the shores of Asia we must look upon as a series of
partially submerged or partially emerged mountain-ranges, analogous
geographically to the coast-ranges of North and Central America, and
to the youngest Cordilleras of South America. The presence of numerous
active and recently extinct volcanoes, taken in connection with the
occurrence of many great depressions which furrow the floor of the sea
in the East Indian Archipelago, and the profound depths attained by the
Pacific trough along the borders of Japan and the Kurile and Aleutian
Islands--all indicate conditions of very considerable instability of
the lithosphere. We are not surprised, therefore, to meet with much
apparently conflicting evidence of elevation and depression in the
coast-lands of eastern Asia, where in some places the sea would seem
to be encroaching, while in other regions it is retreating. In all
earthquake-ridden and volcanic areas such irregular coastal changes
may be looked for. So extreme are the irregularities of the sea-floor
in the area lying between Australia, the Solomon Islands, the New
Hebrides, and New Zealand, and so great are the depths attained by
many of the depressions, that the margins of the continental plateau
are harder to trace here than anywhere else in the world. The bottom
of the oceanic trough throughout a large portion of the southern and
western Pacific is, in fact, traversed by many great mountain-ridges,
the summits of which approach the surface again and again to form the
numerous islets of Polynesia. But notwithstanding the considerable
depths that separate Australia from New Zealand there is geological
evidence to show that a land-connection formerly linked both to Asia.
The continental plateau, therefore, must be held to include New
Caledonia and New Zealand. Hence the volcanic islets of the Solomon and
New Hebrides groups are related to Australia in the same way as the
Liu-kiu, Japanese, and Kurile Islands are to Asia.
Having rapidly sketched the more prominent features of the Pacific
coast-lines, we are in a position to realise the remarkable contrast
they present to the coast-lines of the Atlantic. The highly-folded
strata of the Atlantic sea-board are the relics of great mountains of
upheaval, the origin of which cannot be assigned to a more recent date
than Palæozoic times. During subsequent crustal movements no mountains
of corrugated strata were uplifted along the Atlantic margins, the
Mesozoic and Cainozoic strata of the coastal regions showing little
or no disturbance. It is quite in keeping with all this that volcanic
action appears to have been most strongly manifested in Palæozoic
times. So many long ages have passed since the upheaval of the Archæan
and Palæozoic mountains of the Atlantic sea-board that these heights
have everywhere lost the character of true mountains of elevation.
Planed down to low levels, partially submerged and covered to some
extent by newer formations, they have in many places been again
converted into dry lands, forming plateaux--now sorely denuded and
cut up into mountains and valleys of erosion. Why the later movements
along the borders of the Atlantic basin should not have resulted in
the wholesale plication of the younger sedimentary rocks is a question
for geologists. It would seem as if the Atlantic margins had reached a
stage of comparative stability long before the grand Tertiary uplifts
of the Pacific borders had taken place; for, as we have seen, the
Mesozoic and the Cainozoic strata of the Atlantic coast-lands show
little or no trace of having been subjected to tangential thrusting
and crushing. Hence one cannot help suspecting that the retreat of
the sea during Mesozoic and Cainozoic ages may have been due rather
to subsidence of the oceanic trough and to sedimentation within the
continental area than to positive elevation of the land.
Over the Pacific trough, likewise, depression has probably been in
progress more or less continuously since Palæozoic times, and this
movement alone must have tended to withdraw the sea from the surface
of the continental plateau in Asia and America. But by far the most
important coastal changes in those regions have been brought about
by the crumpling-up of the plateau, and the formation of gigantic
mountains of upheaval along its margins. From remotest geological
periods down almost to the present, the land-area has been increased
from time to time by the doubling-up and consequent elevation of
coastal accumulations, and by the eruption of vast masses of volcanic
materials. It is this long-continued activity of the plutonic forces
within the Pacific area which has caused the coast-lands of that basin
to contrast so strongly with those of the Atlantic. The latter are
incomparably older than the former--the heights of the Atlantic borders
being mountains of denudation of vast geological antiquity, while the
coastal ranges of the Pacific slope are creations but of yesterday as
it were. It may well be that those Cordilleras and mountain-chains
reach a greater height than was ever attained by any Palæozoic uplifts
of the Atlantic borders. But the marked disparity in elevation between
the coast-lands of the Pacific and the Atlantic is due chiefly to a
profound difference in age. Had the Pacific coast-lands existed for as
long a period and suffered as much erosion as the ancient rocks of the
Atlantic sea-board, they would now have little elevation to boast of.
The coast-lines of the Indian Ocean are not, upon the whole, far
removed from the margin of the continental plateau. The elevation of
East Africa for 6000 feet would add only a narrow belt to the land.
This would still leave Madagascar an island, but there are geological
reasons for concluding that this island was at a far distant period
united to Africa, and it must therefore be considered as forming
a portion of the continental plateau. The great depths which now
separate it from the mainland are probably due to local subsidence,
connected with volcanic action in Madagascar itself and in the Comoro
Islands. The southern coasts of Asia, like those of East Africa,
approach the edge of the continental plateau, so that an elevation
of 6000 feet would make little addition to the land-area. With the
same amount of upheaval, however, the Malay Peninsula, Sumatra, Java,
and West Australia would become united, but without extending much
further seawards. Land-connection, as we know, existed in Mesozoic
times between Asia, Australia, and New Zealand, but the coast-lines
of that distant period must have differed considerably from those
that would appear were the regions in question to experience now a
general elevation. The Archæan and the Palæozoic rocks of the Malay
Peninsula and Sumatra are flanked on the side of the Indian Ocean
by great volcanic ridges, and by uplifts of Tertiary strata, which
continue along the line of the Nicobar and the Andaman Islands into
Burma. Thus the coast-lines of that section of the Indian Ocean exhibit
a geographical development similar to that of the Pacific sea-board.
Elsewhere, as in Hindustan, Arabia, and East Africa, the coast-lines
appear to have been determined chiefly by regional elevations of the
land or subsidence of the oceanic trough in Mesozoic and Cainozoic
times, accompanied by the out-welling of enormous floods of lava.
Seeing, then, that the Pacific and the Indian Oceans are pre-eminently
regions which, down to a recent date, have been subject to great
crustal movements and to excessive volcanic action, we may infer
that in the development of their coast-lines the sea played a very
subordinate part. The shores, indeed, are largely protected from marine
erosion by partially emerged volcanic ridges and by coral islands
and reefs, and to a considerable extent also by the sediment which
in tropical regions especially is swept down to the coast in great
abundance by rains and rivers. Moreover, as the geological structure of
these regions assures us, the land would appear seldom to have remained
sufficiently long at one level to permit of much destruction by waves
and tidal currents.
In fine, then, we arrive at the general conclusion that the coast-lines
of the globe are of very unequal age. Those of the Atlantic were
determined as far back as Palæozoic times by great mountain-up lifts
along the margin of the continental plateau. Since the close of that
period many crustal oscillations have taken place, but no grand
mountain-ranges have again been ridged up on the Atlantic sea-board.
Meanwhile the Palæozoic mountain-chains, as we have seen, have suffered
extensive denudation, have been planed down to sea-level, and even
submerged. Subsequently converted into land, wholly or partially as
the case may have been, they now present the appearance of plains
and plateaux of erosion, often deeply indented by the sea. No true
mountains of elevation are met with anywhere in the coast-lands of the
Atlantic, while volcanic action has well-nigh ceased. In short, the
Atlantic margins have reached a stage of comparative stability. The
trough itself, however, is traversed by at least two well-marked banks
of upheaval--the great meridional Dolphin Ridge, and the approximately
transmeridional Faröe-Icelandic belt--both of them bearing volcanic
islands.
But while all the coast-lands of the Atlantic proper attained relative
stability at an early period, those of the Mediterranean and Caribbean
depressions have up to recent times been the scenes of great crustal
disturbance. Gigantic mountain-chains were uplifted along their margins
at so late a period as the Tertiary, and their shores still witness
volcanic activity.
It is upon the margins and within the trough of the Pacific Ocean,
however, that subterranean action is now most remarkably developed.
The coast-lines of that great basin are everywhere formed of grand
uplifts and volcanic ranges, which, broadly speaking, are comparable
in age to those of the Mediterranean and Caribbean depressions. Along
the north-eastern margin of the Indian Ocean the coast-lines resemble
those of the Pacific, being of like recent age, and similarly marked
by the presence of numerous volcanoes. The northern and western
shores, however (as in Hindustan, Arabia, and East Africa), have
been determined rather by regional elevation or by subsidence of the
ocean-floor than by axial uplifts--the chief crustal disturbances
dating back to an earlier period than those of the East Indian
Archipelago. It is in keeping with this greater age of the western and
northern coast-lands of the Indian Ocean that volcanic action is now
less strongly manifested in their vicinity.
I have spoken of the comparative stability of the earth's crust within
the Atlantic area as being evidenced by the greater age of its coastal
ranges and the declining importance of its volcanic phenomena. This
relative stability is further shown by the fact that the Atlantic
sea-board is not much disturbed by earthquakes. This, of course, is
what might have been expected, for earthquakes are most characteristic
of volcanic regions and of those areas in which mountain-uplifts of
recent geological age occur. Hence the coast-lands of the Pacific and
the East Indies, the borders of the Caribbean Sea, the volcanic ridges
of the Atlantic basin, the lands of the Mediterranean, the Black Sea,
and the Aralo-Caspian depressions, the shores of the Red Sea, and vast
tracts of southern Asia, are the chief earthquake regions of the globe.
It may be noted, further, that shocks are not only most frequent but
most intense in the neighbourhood of the sea. They appear to originate
sometimes in the volcanic ridges and coastal ranges, sometimes under
the floor of the sea itself. Now earthquakes, volcanoes, and uplifts
are all expressions of the one great fundamental fact that the earth
is a cooling and contracting body, and they indicate the lines of
weakness along which the enormous pressures and strains induced by the
subsidence of the crust upon its nucleus find relief. We cannot tell
why the coast-lands of the Atlantic should have attained at so early
a period a stage of relative stability--why no axial uplifts should
have been developed along their margins since Palæozoic times. It may
be that relief has been found in the wrinkling-up of the floor of the
oceanic trough, and consequent formation of the Dolphin Ridge and other
great submarine foldings of the crust; and it is possible that the
growth of similar great ridges and wrinkles upon the bed of the Pacific
may in like manner relieve the coast-lands of that vast ocean, and
prevent the formation of younger uplifts along their borders.
I have already remarked that two kinds of elevatory movements of
the crust are recognised by geologists--namely, axial and regional
uplifts. Some, however, are beginning to doubt, with Professor
Suess, whether any vast regional uplifts are possible. Yet the view
that would attribute all such apparent elevations of the land to
subsidence of the crust under the great oceanic troughs is not without
its difficulties. Former sea-margins of very recent geological age
occur in all latitudes, and if we are to explain these by sub-oceanic
depression, this will compel us to admit, as Suess has remarked, a
general lowering of the sea-level of upwards of 1000 feet. But it is
difficult to believe that the sea-floor could have subsided to such an
extent in recent times. Suess thinks it is much more probable that the
high-level beaches of tropical regions are not contemporaneous with
those of higher latitudes, and that the phenomena are best explained
by his hypothesis of a secular movement of the ocean--the water being,
as he contends, alternately heaped up at the equator and the poles.
The strand-lines in high latitudes, however, are certainly connected
with glaciation in some way not yet understood; and if it cannot be
confidently affirmed that they indicate regional movements of the land,
the evidence, nevertheless, seems to point in that direction.
In concluding this imperfect outline-sketch of a large subject, I
ought perhaps to apologise for having trespassed so much upon the
domains of geology. But in doing so I have only followed the example
of geologists themselves, whose divagations in territories adjoining
their own are naturally not infrequent. From much that I have said, it
will be gathered that with regard to the causes of many coastal changes
we are still groping in the dark. It seems not unlikely, however, that
as light increases we may be compelled to modify the view that all
oscillations of the sea-level are due to movements of the lithosphere
alone. That is a very heretical suggestion; but that a great deal can
be said for it any one will admit after a candid perusal of Suess's
monumental work, _Das Antlitz der Erde_.
[Illustration:
PLATE VI
BATHY-HYPSOMETRICAL MAP OF THE WORLD
NOTE
_The map is coloured to show the surface relief of the Globe
without water in the Ocean Basins._
EXPLANATION OF COLOURING
_The Colouring is graded from the Darkest Tint at the Highest Level
to the Lightest Tint on the Lowest._
The Edinburgh Geographical Institute J. G. Bartholomew F.R.G.S.
]
EDINBURGH
PRINTED BY ST GILES' PRINTING COMPANY,
32 YORK PLACE.
THE END.
Other Works by Professor James Geikie.
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Transcriber's Note
Hyphenation was not standardized. The section labeled "Explanation of
Plate III." (p. 325) in the original printed version appears to describe
Plate IV and changed accordingly. Paragraphs split by illustrations were
rejoined. Some plates were moved to end of the chapter.
*** End of this LibraryBlog Digital Book "Fragments of Earth Lore - Sketches & Addresses Geological and Geographical" ***
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