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Title: The evolution of climate
Author: Brooks, Charles Ernest Pelham
Language: English
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THE EVOLUTION OF CLIMATE
THE EVOLUTION
OF CLIMATE
BY
C. E. P. BROOKS,
M.Sc., F.R.A.I., F.R.Met.Soc.
WITH A PREFACE BY
G. C. SIMPSON, D.Sc., F.R.S.,
DIRECTOR OF THE METEOROLOGICAL OFFICE
LONDON: BENN BROTHERS, LIMITED
8 BOUVERIE STREET, E.C.4
1922
PREFACE
Geologists very early in the history of their science, in fact as
soon as fossils began to be examined, found indisputable evidence
of great variations in climate. The vegetation which resulted in
the coal measures could have grown only in a sub-tropical climate,
while over these are vast remains of ice-worn boulders and scratched
rocks which obviously have been left by ice existing under polar
conditions. Such records were not found only in one region, but
cropped up in juxtaposition in many parts of the world. Remains of
sub-tropical vegetation were found in Spitzbergen, and remains of an
extensive ice-sheet moving at sea-level from the south were clearly
recognized in central and northern India. At first it was simply
noticed that the older fossils generally indicated a warmer climate,
and it was considered that the early climate of a globe cooling from
the molten state would be warm and moist, and so account for the
observed conditions. It was recognized that the ice remains were
relatively recent, and so far as a cause for the Ice Age was sought
it was considered that astronomical changes would be sufficient.
It was only when geologists began to find records of ice ages
far anterior to the Carboniferous Age, and astronomers proved by
incontrovertible observations and calculations that changes in the
earth’s orbit, or its inclination to that orbit, could not account
for the ice ages, that the importance and inexplicability of the
geological evidence for changes of climate came to be clearly
recognized.
During the last few years much study has been given to
“palæoclimatology,” but such a study is extremely difficult.
Only a very small fraction of the total surface of the earth can
be geologically examined, and of that fraction a still smaller
proportion has up to the present been studied in detail. There has
been a great tendency to study intently a small region and then to
generalize. The method of study which has to be employed is extremely
dangerous. A geological horizon is determined by the fossils it
contains. Wherever fossils of a certain type are found the strata are
given the same label. Isolated patches correlated by their fossils
are found in different parts of the world, and it is frequently
assumed not only that these rocks were laid down at the same time,
but that the conditions which they indicate existed over the whole of
the earth’s surface simultaneously. Thus geologists tell us that the
climate of the Carboniferous Age was warm and damp; of the Devonian
Age cool and dry; of the Eocene Age very warm; of the Ice Age very
cold.
But has the geologist given sufficient attention to the climatic
zones during the various geological climates? It is true that the
geologist has definitely expressed the view that in certain ages
climatic zones did not exist; but from a meteorological point of
view it is difficult to see how the climate could have been even
approximately the same in all parts of the world if solar radiation
determined in the past as in the present the temperature of the
surface of the earth.
The climatic zones of the various geological periods will need much
closer study in the future; the data hardly exist at present, and the
great area covered by the ocean will always make the study difficult
and the conclusions doubtful. Admitting, for the sake of argument
only, large changes in average conditions, but with zonal variations
of the same order of magnitude as those existing to-day, the slow
changes from period to period will cause any given climatic state
to travel slowly over the surface of the earth, and this will so
complicate the problem as to make it doubtful whether any conclusions
can be reached so long as the same criteria are used to determine
both the geological epoch and the climatic conditions.
These considerations apply more particularly to the earlier records,
while Mr. Brooks has confined his work chiefly to the later records,
beginning with those of the Great Ice Age, in which climatic zones
are clearly indicated by the limits of the ice; but in this problem
one cannot confine one’s attention to a portion of the record, for
the test of any explanation must be its sufficiency to explain all
the past changes of climate. One will not be satisfied with an
explanation of the Great Ice Age which does not explain at the same
time the records of earlier ice ages, of which there is indubitable
evidence in the Permo-Carboniferous and Pre-Cambrian periods, and
the records of widespread tropical or sub-tropical conditions in the
Carboniferous and Eocene Ages. Whether Mr. Brooks’ theory for the
cause of the recent changes of climate satisfies this criterion must
be left to each reader to decide.
As Mr. Brooks says, the literature on this subject is now immense,
and it is most unsatisfactory literature to digest and summarize. In
the first place, many of the original observations which can be used
in the study of past climates are hidden away in masses of purely
geological descriptions, and a great deal of mining has to be done
to extract the climatic ore. Then, again, most of the writers who
have made a special study of climatic changes have had their own
theoretical ideas and most of their evidence has been _ex parte_. To
take a single example, for one paper discussing dispassionately the
evidence for changes in climate during the historical period, there
have been ten to prove either that the climate has steadily improved,
steadily deteriorated, changed in cycles or remained unchanged. It is
extremely difficult to arrive at the truth from such material, and
still more difficult to summarize the present state of opinion on the
subject.
It may be complained that Mr. Brooks has himself adopted this same
method and has written his book around his own theory. But was
there any alternative? There are so many theories and radically
different points of view that no writer could confine himself to the
observations and say what these indicate, for the indications are so
very different according to each theory in turn. And new theories are
always being propounded; since Mr. Brooks commenced to write this
book, Wegener has put forward his revolutionary theory according
to which the polar axis has no stability, and the continents are
travelling over the face of the globe like debris on a flood. Where
is there solid ground from which to discuss climatic changes if the
continents themselves can travel from the equator to the pole and
back again in the short period of one or two geological epochs?
Mr. Brooks has studied deeply geology, anthropology, and meteorology,
and he has considerable mathematical ability. By applying the latter
to the results of his studies he has developed a theory for the cause
of climatic changes based on changes of land and sea area, and on
changes of elevation of land surfaces, and naturally he has made this
theory the basis of his work.
That there will be some who are not able to agree with him as to
the sufficiency of the causes he invokes, or who may even question
whether he also has not taken for granted what others dispute, goes
without saying; but all will agree that he has presented a difficult
subject in a clear and concise way, and that meteorologists (and may
I add geologists?) owe to him a deep debt of gratitude.
G. C. SIMPSON.
CONTENTS
PAGE
PREFACE 5
INTRODUCTION 11
I. FACTORS OF CLIMATE AND THE CAUSES OF CLIMATIC FLUCTUATIONS 15
II. THE CLIMATIC RECORD AS A WHOLE 32
III. CONDITIONS BEFORE THE QUATERNARY ICE AGE 42
IV. THE GREAT ICE AGE 47
V. THE GLACIAL HISTORY OF NORTHERN AND CENTRAL EUROPE 55
VI. THE MEDITERRANEAN REGIONS DURING THE GLACIAL PERIOD 68
VII. ASIA DURING THE GLACIAL PERIOD 76
VIII. THE GLACIAL HISTORY OF NORTH AMERICA 86
IX. CENTRAL AND SOUTH AMERICA 97
X. AFRICA 103
XI. AUSTRALIA AND NEW ZEALAND 109
XII. THE GLACIATION OF ANTARCTICA 114
XIII. THE CLOSE OF THE ICE AGE—THE CONTINENTAL PHASE 118
XIV. THE POST-GLACIAL OPTIMUM OF CLIMATE 127
XV. THE FOREST PERIOD OF WESTERN EUROPE 136
XVI. THE “CLASSICAL” RAINFALL MAXIMUM, 1800 B.C. TO A.D. 500 140
XVII. THE CLIMATIC FLUCTUATIONS SINCE A.D. 500 149
XVIII. CLIMATIC FLUCTUATIONS AND THE EVOLUTION OF MAN 159
XIX. CLIMATE AND HISTORY 162
APPENDIX—THE FACTORS OF TEMPERATURE 166
INDEX 169
INTRODUCTION
The following study is an attempt to reconstruct in some detail
the sequence of climatic changes through which the world passed
during that important stage of its geological history which is
variously known as the Ice Age or Glacial Period, the Pleistocene,
the Quaternary, or the Human Period. That time saw the growth of
humanity from a primitive stage but little removed from the higher
animals to the beginnings of a complicated civilization, and it saw
that human life spread from its cradle or cradles to the ends of the
earth; it saw the configuration of the globe passing through a series
of modifications which ended by establishing the physical geography
of the present day. Finally, it saw a series of startling changes
of climate which almost merit the term “Revolutions” of the old
catastrophic geologists, at the conclusion of which we can trace the
gradual development of the climatic conditions of the present day.
In short, it is a period of immense interest which has a personal
application lacking in the remoter parts of geological time, and for
that reason it is worthy of the fullest study.
On the geological side the literature of the Ice Age is immense, and
is beyond the power of any one man to master. Volumes might be, and
not infrequently have been, written on the glacial geology of areas
limited to a few square miles, or even on the deposits of a single
section. On the archæological side the literature is not yet so
voluminous, but is technical and conflicting in a high degree. It is
only when we seek the contributions of competent meteorologists that
we find a serious gap in the literature. Nor is this surprising, for
meteorologists are still so much occupied with the present vagaries
of the weather, that few of them have time to extend their researches
into the geological past. Yet this is eminently a case where the past
is the key to the present, and it may be that the solution of many
problems which meteorologists have hitherto faced in vain will yet be
suggested by studies of the climatic changes of the Ice Age.
The writer’s excuse for setting down his views is that he is
intensely interested in all three sciences—geology, anthropology,
and meteorology. The combination of these three subjects naturally
ended in specialization on their common meeting place, and led him to
hope that he could assist his fellow geologists and anthropologists
by acquainting them with some of the bearings of meteorology on
their subject, and could open out to his fellow meteorologists a
fascinating branch of their science.
The Quaternary, however, was not the only geological period to
exhibit the phenomena of an Ice Age, and in order that we may more
fully understand the status of the Quaternary Ice Age in the long
succession of geological climates, and also to avoid the charge of
presenting part of the evidence only, a brief discussion of the
climates of the earlier periods has been attempted. The plan of
the work is as follows: Chapter I deals generally with the causes
of climatic fluctuations and with the meteorology of an Ice Age.
Chapter II gives a brief account of the climatic record as a whole,
and Chapter III deals with the Tertiary period considered as leading
up to the Quaternary Ice Age. Chapter IV discusses the subdivisions
of the Glacial period, and the conflict between advocates of one
and of repeated glaciations. Chapters V to XII give brief accounts,
_from the standpoint of a meteorologist_, of the glacial history of
Northern Europe, the Mediterranean Region, Asia, North and South
America, Africa, Australia, and the South Polar regions. In Chapters
XIII to XV post-Glacial climatology is considered. Chapters XVI and
XVII deal with the major climatic fluctuations of the “historic”
period, and finally, in Chapters XVIII and XIX, is a short discussion
of the influence of climate on the evolution and history of man. A
brief bibliography concludes each chapter.
THE EVOLUTION OF CLIMATE
CHAPTER I
FACTORS OF CLIMATE AND THE CAUSES OF CLIMATIC FLUCTUATIONS
The climate of any point on the earth’s surface depends on a complex
of factors, some of them due to influences arriving from outside the
earth, and others purely terrestrial. Since any variations of climate
must be due to a change in one or more of these, it is necessary,
before we can discuss changes of climate, to consider briefly what
the factors are.
The only important extra-terrestrial factor of climate is the amount
of radiant energy which reaches the borders of the earth’s atmosphere
from the heavenly bodies—that is, from the sun, for the moon and
stars can be ignored in this connexion. The only other conceivable
factor is the arrival of meteorites, bringing kinetic energy which is
converted into heat, and introducing cosmic dust into the atmosphere;
but it is highly improbable that this is of appreciable effect.
The amount of solar radiation[1] which reaches the earth depends in
the first place on the total radiation emitted by the sun, and in
the second place on the distance of the earth from the sun, both of
which quantities are variable. It has been calculated that if other
factors remained unchanged an increase of ten per cent. in the solar
radiation would raise the mean temperature of the earth’s surface
by about 7° C., or between 12° and 13° F., with, of course, a
corresponding fall for a decrease.
After the sun’s radiation reaches the outer limits of the earth’s
atmosphere its nature and intensity are modified by the composition
of the air through which it passes. In general the air itself is
very transparent to the small wave-lengths which make up the solar
rays, but the presence of fine dust, whether of volcanic or of cosmic
origin, has been shown by Humphreys to be a distinct hindrance to
their passage, so that volcanic eruptions of an explosive nature,
such as that of Krakatoa in 1883, La Soufrière (St. Vincent) in 1902,
or Katmai (Alaska) in 1912, may result in a fall of temperature over
the world as a whole.
The temperature of the earth is determined by the balance between
the radiation received from the sun and the terrestrial radiation to
space, and a decrease in the latter would be as effective in raising
the mean temperature as an increase in the former. The use of glass
for greenhouses depends on this principle; for glass is transparent
to heat rays of small wave-length, but is largely opaque to the rays
of greater wave-length which make up terrestrial radiation. Certain
constituents of the atmosphere, especially water-vapour, carbon
dioxide and ozone, are effective in this way, and variations in the
amount of these gases present may affect the temperature.
The angle at which the sun’s rays strike the earth’s surface is a
highly important factor. Within the Tropics the sun at midday is
nearly vertical throughout the year, and the mean temperature in
these regions is correspondingly high; on the other hand, during
the long polar night the sun is not seen for half the year, and
very low temperatures prevail. There is thus a seasonal variation
of the heat received from the sun in middle and high latitudes, the
extent of which depends on the “obliquity of the ecliptic,” i.e. the
inclination of the earth’s axis to the plane of its orbit round the
sun, and any changes in this factor must alter the seasonal variation
of climate.
Further, since the climate of any place depends so closely on its
latitude, it follows that if the latitude changes the climate will
change. A ship can change its latitude at will, but we are accustomed
to regard the position of the “firm ground beneath our feet”
relatively to the poles as fixed within narrow limits. This stability
has, however, been questioned from time to time, mainly on evidence
derived from palæoclimatology, and theories of climatic change have
been based on the wanderings of continents and oceans. Finally, local
climate is intimately bound up with the distribution of land and sea,
and the marine and atmospheric currents resulting therefrom, and on
elevation above sea-level, both of which factors, as we shall see,
have suffered very wide variations in the geological past.
Nearly all the theories which have been put forward to account for
geological changes of climate, and especially the occurrence of the
last or Quaternary Ice Age, are based on the abnormal variation of
one or other of the above factors, and we may consider them briefly
in turn. Very few have ever been taken seriously. In the first place,
we can at once dismiss fluctuations in the radiation emitted by
the sun as a cause of _great_ changes of climate. It is true that
many small fluctuations are traceable directly to this cause, such
as the eleven-year periodicity of temperature and rainfall; but
these fluctuations are, and must be, greater at the equator than at
the poles, while the fall of temperature during the Glacial period
reached its maximum near the poles and was least at the equator.
Moreover, there is not the slightest direct evidence in support of
such a theory, and it can only be admitted when all other hypotheses
have failed.
The “astronomical” theory of the cause of climatic fluctuations is
associated chiefly with the name of James Croll. Croll’s theory
connects abnormal variations of climate with variations, firstly of
the eccentricity of the earth’s orbit, and secondly of the ecliptic.
In periods of high eccentricity the hemisphere with winter in
aphelion is cold because the long severe winter is far from being
balanced by the short hot summer; at the same time the opposite
hemisphere enjoys a mild equable climate. This theory commanded
instant respect, and still finds a place in the text-books, but
difficulties soon began to appear. The evidence strongly suggests
that glacial periods did not alternate in the two hemispheres,
but were simultaneous over the whole earth; even on the equator
the snow-line was brought low down. Moreover, on Mars the largest
snow-cap appears on the hemisphere with its winter in perihelion.
Although Croll’s reasoning was beautifully ingenious he gave very few
figures; while the date which he gives for the conclusion of the Ice
Age, 80,000 years ago, has been shown by recent research to be far
too remote, 15,000 years being nearer the mark.
Croll’s theory has recently been revived in an altered form by R.
Spitaler, a Czecho-Slovakian meteorologist, who calculated the
probable alteration in the mean temperature of each latitude under
maximum eccentricity (0.7775) and maximum obliquity (27° 48′), the
distribution of land and water remaining unchanged. The results are
shown in the attached table, where - means that the temperature was
so much below the present mean, and + that it was so much above.
-------+-------------------------+--------------------------
| Aphelion December. | Aphelion June.
+-------------------------+--------------------------
| Winter. Summer. Year. | Winter. Summer. Year.
+-------------------------+--------------------------
| °F. °F. °F. | °F. °F. °F.
N. 60° | -9 +15 -1 | -5 -4 -1
30° | -13 +13 -2 | +1 -8 -2
Equator| -8 +4 -2 | +1 -6 -2
S. 30° | -6 +1 -2 | +3 -5 -2
60° | -2 -1 -1 | +1 -2 -1
-------+-------------------------+--------------------------
Spitaler claims that these differences are sufficient to cause
a glacial period in the hemisphere with winter in aphelion, but
from this point his theory departs widely from Croll’s. During the
long severe winter great volumes of sea water are brought to a low
temperature, and, owing to their greater weight, sink to the bottom
of the ocean, where they remain cold and accumulate from year to
year. But the water warmed during the short hot summer remains on the
surface, where its heat is dissipated by evaporation and radiation.
Thus, throughout the cold period, lasting about 10,000 years, the
ocean in that hemisphere is steadily growing colder, and this mass
of cold water is sufficient to maintain a low temperature through
the whole of the following period of 10,000 years with winter in
perihelion, which would otherwise be a genial interval. In this
way a period of great eccentricity becomes a glacial period over
the whole earth, but with crests of maximum intensity alternating
in the two hemispheres. Unfortunately the numerical basis of this
theory is not presented, and it seems incredible that a deficiency
of temperature could be thus maintained through so long a period.
Further, the difficulty about chronology remains, and the work brings
the astronomical theory no nearer to being a solution of the Ice Age
problem than was Croll’s.
The theory which connects fluctuations of climate on a geological
scale with changes in the composition of the earth’s atmosphere is
due to Tyndall and Arrhenius, and was elaborated by Chamberlin. The
theory supposed that the earth’s temperature is maintained by the
“blanketing” effect of the carbon dioxide in the atmosphere. This
acts like the glass of a greenhouse, allowing the sun’s rays to
enter unhindered, but absorbing the heat radiated from the earth’s
surface and returning some of it to the earth instead of letting it
pass through to be lost in space. Consequently, any diminution in the
amount of carbon dioxide present would cause the earth to radiate
away its heat more freely, so reducing its temperature. But it is
now known that the terrestrial radiation which this gas is capable of
absorbing is taken up equally readily by water-vapour, of which there
is always sufficient present, and variations of carbon dioxide cannot
have any appreciable effect.
Brief mention may be made here of a theory put forward by Humphreys,
who attributed glaciation to the presence of great quantities of
volcanic dust in the atmosphere. It would require an enormous output
of volcanic dust to reduce the temperature sufficiently; but in
any case the relation, if any, between vulcanicity and temperature
during the geological ages is rather the reverse of that supposed
by Humphreys, periods of maximum volcanic action coinciding more
frequently with high temperatures than with low. Perhaps the best
comment on Humphreys’ theory is that in 1902 F. Frech produced its
exact opposite, warm periods being associated with an excess of
vulcanicity and cold periods with a diminution.
The theory which attributes climatic changes in various countries
to variations in the position of the poles has been adduced in
two main forms. The first is known as the Pendulation Theory, and
supposes the existence of two “oscillation poles” in Ecuador and
Sumatra. The latitude of these points remains unchanged, and the
geographical poles swing backwards and forwards along the meridian
of 10 E. midway between them. Varying distances from the pole cause
changes of climate, and the movements of the ocean, which adjusts
itself to the change of pole more rapidly than the land, causes the
great transgressions and regressions of the sea and the elevation and
subsidence of the land.
An alternative form put forward by P. Kreichgauer, and recently
brought up again by Wegener, explains the apparent variations in
the position of the pole, not through a motion of the earth’s
axis, but by the assumption that the firm crust has moved over the
earth’s core so that the axis, remaining firm in its position,
passes through different points of the earth’s crust. The cause of
these movements is the centrifugal force of the great masses of the
continents, which are distributed symmetrically about the earth.
Imagine a single large continent resting on a sub-fluid magma in
temperate latitudes. Centrifugal force acting on this continent
tends to drive it towards the equator. There is thus a tendency for
the latitude of Europe to decrease. Similar forces acting through
geological ages have caused the poles and equator to wander at large
over the earth’s surface, and also caused the continents to shift
their positions relatively to one another. According to Wegener, in
the Oligocene there was only a single enormous continent, America
being united to Europe and Africa on the one hand, and through
Antarctica to Australia on the other; while the Deccan stretched
south-westwards nearly to Africa. The poles were in Alaska and north
of the Falkland Islands. The treatment in Kreichgauer’s original book
is speculative and at times fanciful; Wegener’s treatise appears
to demand more respectful attention, but is open to some vital
objections. In the first place, theories of this class demand that
the glaciation occurred in different regions at widely different
times, whereas we shall see in the following pages that the evidence
points very strongly to a double glaciation during the Quaternary
occurring simultaneously over the whole earth. This objection, which
was fatal to Croll’s theory in its original form, is equally fatal
to theories of pole-wandering as an explanation of the Quaternary
Ice Age. Secondly, we know that the last phase of this glaciation,
known as the Wisconsin stage in America and the Wurmian in Europe,
was highly developed only 20,000 years ago, and probably reached its
maximum not more than 30,000 years ago. In the last 5000 years there
has been no appreciable change of latitude, at least in Eurasia; and
it seems impossible for the extensive alterations required in the
geography of the world by Wegener’s theory to have taken place in so
short a time.
The great glaciation of the Permian period, referred to in the
next chapter, is a totally different matter. During this time the
ice-sheets appear to have reached their maximum area, and to have
extended to sea-level, in countries which are at present close to
the equator, while lands now in high latitudes remained unglaciated.
It is true that at the present day glaciers exist at high latitudes
under the equator itself, and given a ridge sufficiently steep and
a snowfall sufficiently heavy such glaciers would possibly extend
to sea-level; but even these conditions would not give rise to the
enormous deposits of true boulder-clay which have been discovered,
and there seems no way of avoiding the supposition of an enormous
difference in the position of the pole relatively to the continents
at this time.
Wegener’s theory alone, however, requires that glaciation should
always have been proceeding in some part of the globe (unless both
poles were surrounded by wide expanses of ocean), which is hard to
reconcile with the extremely definite and limited glaciations which
geological research has demonstrated. In these circumstances we may
tentatively explain the pre-Tertiary glacial periods by combining
Wegener’s theory of the movements of continents and oceans as a
whole with the theory of changes of elevation and of land and sea
distribution which is outlined below. That is to say, we may suppose
that the positions of the continents and oceans have changed,
relatively both to each other and to the poles, slowly but more
or less continuously throughout geological time; while at certain
periods the land and sea distribution became favourable for extensive
glaciation of the regions which at that time were in high latitudes.
The geographical theory, which states that the Ice Age was brought
about by elevation in high latitudes, and by changes in the land and
sea distribution, though never seriously challenged, has suffered
until recently from a lack of precision. The present author attempted
to remedy this by a close mathematical study of the relation of
temperature to land and sea distribution at the present day. The
method at attack was as follows: from the best available isothermal
charts of all countries the mean temperature reduced to sea-level was
read off for each intersection of a ten-degree square of latitude and
longitude, for January and July, from 70° N. to 60° S. latitude; this
gave 504 values of temperature for each of these months. Round each
point was next drawn a circle with an angular radius of ten degrees,
divided into east and west semicircles. The area of each semicircle
was taken as 100, and by means of squared paper the percentage of
land to the east and land to the west were calculated; finally, in
each month the percentage of the whole circle occupied by land, ice,
or frozen sea was calculated, this figure naturally being greater in
winter than in summer. The projection used was that of the “octagonal
globe,” published by the Meteorological Office, which shows the world
in five sections, the error nowhere exceeding six per cent.
These figures were then analysed mathematically, and from them the
effects on temperature of land to the east, land to the west, and
ice were calculated. The detailed numerical results are set out in
an Appendix; it is sufficient here to give the following general
conclusions:
(1) In winter the effect of land to the west is always to lower
temperature.
(2) In winter the effect of land to the east is almost negligible,
that is to say, the eastern shore of a continent is almost as cold
as the centre of the continent. The only important exception to this
rule is 70° N., which may be considered as coming within a belt of
polar east winds.
(3) In summer the general effect of land, whether to the east or
west, is to raise temperature, but the effect is nowhere anything
like so marked as the opposite effect in winter.
(4) The effect of ice is always to lower temperature.
(5) For every latitude a “basal temperature” can be found. This is
the temperature found near the centre of an ocean in that latitude.
This “basal temperature” is a function of the amount of land in the
belt of latitude. Poleward of latitude 20° an increase of land in the
belt lowers the winter basal temperatures very rapidly and raises the
summer basal temperature to a less extent. The “basal temperature”
is important, since it is the datum line from which we set out to
calculate the winter and summer temperatures of any point, by the
addition or subtraction of figures representing the local effect of
land in a neighbouring 10° circle.
As an illustration of the scale of the temperature variations which
may be due to geographical changes, suppose that the belt between
50° and 70° N. is entirely above the sea. Then we have the following
theoretical temperatures; for a point on 60° N. at sea-level:
January -30° F.; July 72° F.
Data for calculating the effect of ice are rather scanty, but the
following probable figures can be given, supposing that the belt in
question were entirely ice-covered:
January -30° F. (as for land); July 23° F.
Supposing that the belt were entirely oceanic, the mean temperature
in 60° N. would be:
January 29° F.; July 41° F.
These figures show how enormously effective the land and sea
distribution really is. From Appendix it is easy to calculate the
probable temperature distribution resulting from any arrangement
of land and water masses. Since the geography of the more recent
geological periods is now known in some detail, we have thus a means
of restoring past climates and discussing the distribution of animals
and plants in the light of this knowledge. Of course it is not
pretended that no other possible causes of great climatic variation
exist, but no others capable of seriously modifying temperature
over long periods are known to have been in operation. As we shall
see later, there are solar and other astronomical causes capable of
modifying climate slightly for a few decades or even centuries, but
these are insignificant compared with the mighty fluctuations of
geological time.
In applying the results of this “continentality” study to former
geological periods the method adopted is that of differences. The
present climate is taken as a standard, and the temperatures of,
for instance, the Glacial period are calculated by adding to or
subtracting from the present temperatures amounts calculated from
the change in the land and sea distribution. This has the advantage
of conserving the present local peculiarities, such as those due
to the presence of the Gulf Drift, but such a procedure would be
inapplicable for a totally different land and sea distribution, such
as prevailed during the Carboniferous period. That it is applicable
for the Quaternary is perhaps best shown by the following comparison
of temperatures calculated from the distribution of land, sea and ice
with the actual temperatures of the Ice Age as estimated by various
authorities (inferred fall):
------------+--------------------+--------------+-----------------
| | | Calculated Fall.
Locality. | Author. |Inferred Fall.+-----+-----+-----
| | | Jan.|July.|Mean.
------------+--------------------+--------------+-----+-----+-----
| | °F. | °F. | °F. | °F.
Scandinavia | J. Geikie | More than 20 | 36 | 18 | 27
East Anglia | C. Reid | 20 | 18 | 13 | 15
Alps | Penck and Brückner | 11 | 13 | 9 | 11
Japan | Simotomai | 7 | 9 | 5 | 7
------------+--------------------+--------------+-----+-----+-----
It is seen that the agreement is quite good.
There is one other point to consider, the effect of height. The
existence of a great land-mass generally implies that part of it
at least has a considerable elevation, perhaps 10,000 or 20,000
feet, and these high lands lave a very different climate to the
neighbouring lowlands. Meteorologists have measured this difference
in the case of temperature and found that the average fall with
height is at the rate of 1° F. in 300 feet. In the lower levels the
fall is usually greater in summer than in winter, but at 3000 feet it
is fairly uniform throughout the year. Consequently, quite apart from
any change in climate due to the increased land area, an elevation of
3000 feet would result in a fall of temperature of 10° F., winter and
summer alike. This reinforces the effect of increased land area and
aids in the development of ice-sheets or glaciers.
The effect of geographical changes on the distribution of rainfall
are much more complicated. The open sea is the great source of
the water-vapour in the atmosphere, and since evaporation is very
much greater in the hot than in the cold parts of the globe, for
considerable precipitation over the world as a whole there must
be large water areas in the Tropics. In temperate latitudes the
water-vapour is carried over the land by onshore winds, and some of
it is precipitated where the air is forced to rise along the slopes
of hills or mountains. Some rain falls in thunderstorms and similar
local showers, but the greater part of the rain in most temperate
countries is associated with the passage of “depressions.” These are
our familiar wind- and rain-storms; a depression consists essentially
of winds blowing in an anti-clockwise direction round an area of low
pressure.
These centres of low pressure move about more or less irregularly,
but almost invariably from west to east in the temperate regions.
They are usually generated over seas or oceans, and, since a supply
of moist air is essential for their continued existence, they tend to
keep to the neighbourhood of water masses or, failing that, of large
river valleys. In a large dry area depressions weaken or disappear.
Their tracks are also very largely governed by the positions of areas
of high pressure or anticyclones, which they tend to avoid, moving
from west to east on the polar side of a large anticyclone and from
east to west on the equatorial side. Since anticyclones are developed
over the great land areas in winter, this further restricts the paths
of depressions to the neighbourhood of the oceans at that season.
For all these reasons the tracks of depressions, and therefore the
rainfall, are intimately connected with the distribution of land and
sea. In winter there is little rainfall in the interior of a great
land-mass, except where it is penetrated by an arm of the sea like
the Mediterranean; on the other hand, the coasts receive a great deal
of rain or snow. The interior receives its rain mostly in spring or
summer; if the coastal lands are of no great elevation this will be
plentiful, but if the coasts are mountainous the interior will be
arid, like the central basins of Asia.
The development of an ice-sheet is equivalent to introducing
perpetual winter in the area occupied by the ice. The low temperature
maintains high pressure, and storm-tracks are unable to cross the
ice. At the present day depressions rarely penetrate beyond the
outer fringe of the Antarctic continent, and only the southern
extremity of Greenland is affected by them. Since the total energy
in the atmosphere is increased by the presence of an ice-sheet,
which affords a greater contrast of temperature between cold pole
and equator, storms will increase in frequency and their tracks
must be crowded together on the equatorial side of the ice-sheet.
In the southern hemisphere we have great storminess in the “roaring
forties”; south of Greenland the Newfoundland banks are a region
of great storminess. Hence, when an ice-sheet covered northern and
central Europe the Mediterranean region must have had a marked
increase of storminess with probably rain in summer as well as in
winter.
But if snow-bearing depressions cannot penetrate an ice-sheet, it may
be asked how the ice-sheet can live. The answer depends on the nature
of the underlying country. A land of high relief such as Antarctica
is, and as Greenland probably is, rising to a maximum elevation of
many thousand feet near its centre, draws its nourishment chiefly
from the upper currents which flow inward on all sides to replace
the cooled air which flows outwards near the surface. These upper
currents carry a certain amount of moisture, partly in the form of
vapour, but partly condensed as cirrus and even cumulus cloud.
At low temperatures air is able to hold only a negligible amount of
water-vapour, and this current, coming in contact with the extremely
cold surface of the ice, is sucked dry, and its moisture added to
the ice-sheet. Probably there is little true snowfall, but the
condensation takes place chiefly close to the surface, forming a
frozen mist resembling the “ice-mist” of Siberia. Even if the central
land is not high enough to reach into the upper current at its normal
level, the surface outflow of cold air will draw the current down
to the level of the ice. This will warm it by compression, but the
ice-surface is so cold that such warming makes little difference in
the end. This process of condensation ensures that after the ice
reaches a certain thickness it becomes independent of topography, and
in fact the centre of the Scandinavian ice-sheet lay not along the
mountain axis, but some distance to the east of it.
It is probably only on the edges of the ice-sheet, and especially in
areas of considerable local relief, that snowfall of the ordinary
type takes place, associated with moist winds blowing in the front
section of depressions which skirt the ice-edge. But when conditions
are favourable this source of supply is sufficient to enable these
local ice-sheets to maintain an independent life, merely fusing with
the edges of the larger sheet where they meet. Examples of such
local centres in Europe were the Irish and Scottish glaciers, and
at a later stage the Lofoten glaciers of the west of Norway, and in
America the Cordilleran glaciers of Columbia.
Penck and Brückner have demonstrated that in the Alps the increase of
glaciation was due to a fall of temperature and not to an increase
of snowfall. The argument is threefold: firstly, the lowering of the
snow-line was uniform over the whole Alpine area, instead of being
irregular as it would be if it depended on variations of snowfall;
secondly, the area and depth of the parent snow-fields which fed
the glaciers remained unchanged, hence the increased length of the
glaciers must have been due to decreased melting below the snow-line,
i.e. to lower temperatures; thirdly, the upper limit of tree-growth
in Europe sank by about the same amount as the snow-line. The same
conclusion holds for the great Scandinavian and North American
ice-sheets, the extension of which was undoubtedly due to a great
fall of temperature. In the case of the Alps the interesting point
has come to light that the fall of temperature, though due in part
to increased elevation, is mainly accounted for by the presence of
the Scandinavian ice-sheet, which extended its influence for many
miles beyond the actual limits of glaciation, so that its waxings and
wanings are faithfully reproduced in those of the Alpine glaciers,
even to the details of the final retreat after the last maximum.
It is only when we turn to tropical and sub-tropical regions that
we find variations of temperature unable to account for increased
glaciation. Not only were the changes of land and sea distribution
on a very much smaller scale than further north, but the Appendix
shows that the temperature value of a corresponding change of land
area is also very much less. But the high intertropical mountains—the
Andes and Kenya and Kilimanjaro in central Africa—which to-day bear
glaciers, in Quaternary times carried much greater ones. We cannot
call in a fall of temperature, for the reason above stated, and also
because at lower levels there is no evidence of colder conditions.
In the Glacial period the marine fauna was the same as to-day, and
mountains which now fall short of the snow-line by a few hundred
feet were still unglaciated even then. The only alternative is
increased snowfall on the higher mountains. Fortunately this fits in
well with meteorological theory. The rain and snowfall of tropical
regions depends, first of all, on the evaporation over the oceans.
But evaporation is profoundly influenced by the velocity of the wind;
and the wind, which in the Tropics represents the strength of the
atmospheric circulation, depends on the thermal gradient between the
equator and the poles; since there is no evidence of any appreciable
change of temperature over the Tropics as a whole, while there was
a very great fall in cold temperate and polar regions, the thermal
gradient, and therefore, ultimately, the tropical and sub-tropical,
rain and snowfall must have been very greatly increased. Hence the
increased glaciation of high mountains near the equator, and hence
also the evidence of “Pluvial periods” in the sub-tropical arid
regions on either side of the equator.
Thus during Glacial periods we have:
(1) Elevation in high latitudes caused a great increase of land areas
there.
(2) Both elevation and increase of land area resulted in a lowering
of temperature, materially increased by the gradual development of
great ice-sheets.
(3) These ice-sheets caused the development of subsidiary ice-sheets
on their southern and western borders.
(4) The lowering of temperature in high latitudes increased the
thermal gradient between equator and poles, resulting in:
(_a_) Increased snowfall, and hence increased glaciation on
high mountains near the equator.
(_b_) Pluvial periods in the sub-tropical arid regions.
BIBLIOGRAPHY
Humphreys, W. J. “Physics of the air.” Philadelphia, 1920. [Pt. 4,
pp. 556-629.]
Chamberlin, T. C. “An attempt to frame a working hypothesis of
the cause of glacial periods on an atmospheric basis.” _Journal
of Geology_ (American), Vol. 7, 1899, pp. 545-84, 667-85, 751-87.
[Carbon dioxide theory.]
Croll, J. “Climate and time in their geological relations.” London,
1875. “Discussions on climate and cosmology.” London, 1889.
[Eccentricity of earth’s orbit.]
Spitaler, R. “Das Klima des Eiszeitalters.” Prag, 1921.
Lithographed. [Eccentricity of earth’s orbit. Reviewed in the
_Meteorological Magazine_, London, September, 1921.]
Simroth, H. “Die Pendulationstheorie.” Leipzig, 1908.
Kreichgauer, P. “Die Aequatorfrage in der Geologie.” Steyr, 1902.
Wegener, A. “Die Entstehung der Continente und Ozeane.” _Die
Wissenschaft_, Bd. 66, Braunschweig, 1920.
Köppen, W. “Ueber Aenderungen der geographischen Breiten und des
Klimas in geologischer Zeit.” Stockholm, _Geografiska Annaler_, 2,
1920, pp. 285-99.
Brooks, C. E. P. “Continentality and temperature.” _Quarterly
Journal of Royal Meteorological Society_, Vol. 43, 1917, p. 169;
and 44, 1918, p. 253. [Influence of land and sea distribution.]
Enquist, F. “Eine Theorie über die Ursache der Eiszeit und die
geographischen Konsequenzen derselbe.” _Bull. Geol. Inst._, Upsala,
13, 1915, No. 2. [Influence of land and sea distribution.]
Hobbs, W. H. “Characteristics of existing glaciers.” New York,
1911. [Glacial anticyclone.]
TABLE OF GEOLOGICAL FORMATIONS
{ Recent
QUATERNARY { Pleistocene
{ Pliocene
TERTIARY or { Miocene
CAINOZOIC { Oligocene
{ Eocene
{ Cretaceous
MESOZOIC or { Jurassic
SECONDARY { Triassic
{ Permian
{ Carboniferous
{ Devonian
PALÆOZOIC { Silurian
{ Ordovician
{ Cambrian
PROTEROZOIC Pre-Cambrian
CHAPTER II
THE CLIMATIC RECORD AS A WHOLE
It is a remarkable fact that one of the oldest known sedimentary
rocks is glacial in origin, and indicated the presence of an
ice-sheet at a very early stage in the earth’s history. This is a
“tillite,” or boulder-clay, discovered by Prof. Coleman at the base
of the Lower Huronian (Early Proterozoic) of Canada. It extends in
an east and west direction for 1000 miles across northern Ontario,
and northward from the northern shore of Lake Huron for 750 miles. It
rests on a scratched or polished surface of various rocks, and the
included boulders are not always local, but some have been brought
from a considerable distance. All these characters point to a large
ice-sheet.
Traces of Proterozoic glaciations have been discovered in various
other parts of the world, and some of these may be of the same age
as the Canadian ice-sheet, but they cannot yet be dated exactly. An
interesting example is western Scotland, which J. Geikie considered
to have been glaciated by ice from the north-west which has since
sunk into the North Atlantic. Other glacial remains have no doubt
been destroyed or deeply buried, while some may still await
discovery, and at present we must be content to note the occurrence
of a glacial period at this time without attempting any description
of the distribution of climates over the globe.
Followed a long period of milder climate indicated in America by
thick deposits of limestone with the remains of reef-building
organisms and other marine life. This period may have been
interrupted at least once by the recurrence of glacial conditions,
but the evidence for this is doubtful. It must be remembered that the
duration of the Proterozoic was very great, at least as long as all
subsequent time, while the relics of it which are now known to us are
few and scattered, so that much which happened during that time is a
closed book. It is not until the very close of the Proterozoic that
we again find indisputable evidence of widespread glacial action.
This second great glaciation was placed originally in the earliest
Cambrian (see table of geological formations at the end of Chapter
I), but later evidence shows that it is slightly older than the
oldest deposit which can be referred to this series, and it may
be designated the Pre-Cambrian glaciation. Tillites of this age
have been found in the middle Yangtse region of China and in South
Australia (where they extend from 20 miles south of Adelaide to 440
miles north, with an east-west extension of 200 miles). Glacial
deposits which probably refer to this period have been found also
in India, both in the Deccan and near Simla, over a wide area in
South Africa, and in the extreme north of Norway. This distribution
suggests the presence of two centres of glaciation, one between
China, India and Australia, and the other north-west of Europe. The
south-eastern of these was the most extensive, and probably indicates
a ring of glaciated continents surrounding the pole, rather than a
single enormous ice-sheet.
During the Cambrian all evidence of glacial action ceases, and we
have, instead, evidence of a warm, fairly uniform climate in the
abundant marine life. This continued during the Ordovician and became
accentuated during the Silurian period, when reef corals lived in the
seas of all parts of the world. Terrestrial deposits are curiously
lacking in all this series, and this suggests that in the absence
of any great mountain-building and elevation shallow seas extended
over almost the whole of the surface, accompanied by mild oceanic
climates extending to high latitudes.
At the close of the Silurian there was a period of mountain-building
and the formation of continents. The extinction of numerous species
of marine organisms and the rapid evolution of others point to the
seas becoming cooler and the stress of life more acute. In the
succeeding Devonian period there is evidence of glacial conditions
in South Africa in the form of a thick series of quartzites
with striated pebbles up to fifteen inches long, but no typical
boulder-clay has been discovered. There are also some doubtful traces
from England. The most noteworthy development of the Devonian in the
British Isles is, however, a thick deposit of red sandstone (Old Red
Sandstone) of the type that is formed in shallow lagoons or inclosed
basins, and suggesting desert conditions, so that the rainfall of the
British Isles was probably slight.
These continental conditions passed away towards the close of the
Devonian period, and once again extensive warm oceans appear to
have spread over a large part of the globe, associated with the
development of reef-building corals. Climate continued warm and
equable throughout the greater part of the Carboniferous. The
important feature of this period is the great system of coal-beds
which extends through North America and Europe to China, with
northern and southern limits in 80° N. (north-east Greenland and
Spitzbergen) and 15° S. (Zambesi River). Wegener, summing up the
evidence, and considering especially the absence of annual rings
in the trees, concludes that the coal-beds are the remains of the
tropical rain-forest when the equator lay across Europe some 30
degrees north of its present position.
Towards the close of the Carboniferous period great mountain-building
set in, resulting in the formation of the famous Gondwanaland,
including south and central Africa, southern Asia, part of Australia
and possibly Brazil. From a consideration of the glacial evidence,
however, it appears, as will be seen later, that this was probably
a ring of neighbouring and partly adjoining land areas rather than
a single enormous continent. At the same time the climate became
cooler, and a hardier vegetation, known as the _Glossopteris_ flora,
developed in the southern hemisphere, including woody trees with
annual rings indicating seasons. The large insects of the coal
forests which did not undergo a metamorphosis were replaced by
smaller types which did pass through such a stage; this change of
habit is considered to be due to the winters having become severe,
so that the insects learnt to hibernate through them. In the early
Permian, Gondwanaland was occupied by great ice-sheets, remains of
which in the form of tillites of great thickness, ice-worn surfaces
and striated boulders have been found in South Africa, Belgian Congo,
and Togoland, Tasmania and widely separated parts of Australia,
peninsular and north-western India, and probably also Afghanistan.
In India the glacial striæ show that the ice-sheet was moving north,
while in South Africa it was moving south, i.e. away from the present
equator in both cases. Widespread glacial remains have been found
also in Brazil, northern Argentine and the Falkland Islands, and
there are probable traces near Boston in North America, in Armenia,
the Urals and the Alps, and possibly also in England.
Wegener’s reconstruction of the geography of this period places the
south pole a little to the south-east of South Africa, surrounded by
a great continent composed of the junction of Africa, South America,
Antarctica, Australia, and an extended Deccan added to by smoothing
out the folds of the Himalayas. This great circumpolar continent he
considers to have been the site of an immense ice-cap. The North Pole
lay in the Pacific Ocean, so that almost all the remaining land areas
enjoyed temperate or tropical climates.
It is admitted that this peculiar distribution of glacial
remains apparently necessitates a position of the pole somewhere
near that described by Wegener, but the theory of a single polar
ice-cap extending beyond 50° latitude on nearly all sides presents
difficulties. From the mechanism of the supply of snow to an
ice-sheet described in the preceding chapter it follows that, except
close to the edges, all the moisture precipitated must be brought in
by upper currents. But even if we take into account the increase in
the strength of the atmospheric circulation due to the introduction
of an ice-cap, there is a limit to the supply of moisture by this
process. All such moisture has to cross the periphery, and with
increasing radius; the number of square miles of area to each mile
of periphery becomes greater, slowly at first, then more and more
rapidly. We shall see in Chapter VIII that even the North American
Quaternary ice-sheet became unwieldy from this cause, and suffered
several changes of centre.
Hence it seems that the _rapprochement_ of the continents in
Permo-Carboniferous times need not have been so complete as Wegener
supposes, the glacial phenomena being more readily explicable by a
ring of continents surrounding a polar sea, as in the case of the
Quaternary glaciation of the northern hemisphere. The local Permian
glaciations of Europe and North America, some of which fell close to
Wegener’s equator, are easily explicable as due to mountain glaciers
similar to those of Ruwenzori and other tropical mountains during the
Quaternary. There were interglacial periods in South Africa, Brazil
and New South Wales, which increase the resemblance between the
Permian and Quaternary Ice Ages.
In Upper Permian times there was a widespread development of arid
climates, especially in the present temperate parts of North America
and Europe. Wegener attributes this to the northerly position of the
equator bringing the sub-tropical desert belt (Sahara, Arizona) to
these regions. In the Trias these conditions gradually gave place
to another period of widespread warm shallow seas, with abundant
marine life and corals extending over a large part of the world, even
to Arctic Alaska. In the United States there are the remains of the
forests of this period, in which the tree-trunks show very little
evidence of annual rings, indicating that the seasonal changes were
slight, so that the climate had again become mild and oceanic.
In the Lias (Lower Jurassic), there was crustal movement and
volcanic action accompanied by land-formation and a gradual lowering
of temperature. There was a great reduction in the abundance and
geographical range of corals, and most of the species of insects are
of dwarf types. There is, however, no evidence of glacial action.
The Upper Jurassic period appears to have been warmer than the
Lias. Insects of a large size and corals again attained a very wide
distribution, but there is enough difference in the marine faunas of
different regions to indicate a greater development of climatic zones
than in the extremely oceanic periods such as the late Triassic.
Schuchert points out that the plants of Louis Philippe Land in 63° S.
are the same, even to species, as those of Yorkshire.
In the Cretaceous period the climate was at first similar to that of
the Jurassic, and trees grew in Alaska, Greenland and Spitzbergen.
These trees, however, show marked annual rings, indicating a
considerable differentiation of seasons, while trees of this age
found in Egypt are devoid of rings. Towards the close of the
Cretaceous there were many crustal movements and great volcanic
outbursts, accompanied by a considerable reduction of temperature,
which led to the extinction of many forms of life and the rapid
evolution of others. There is no evidence of glacial action during
the Cretaceous, however, though at the beginning of the Eocene
there was a local glaciation of the San Juan Mountains of Colorado.
According to W. W. Atwood this glaciation was double, the first
stage being of the Alpine mountain glacier type, separated by an
interglacial from the second stage, which was of the Piedmont type
(mountain glaciers spreading out on the plain at the foot of the
mountain). This Eocene glaciation has been found nowhere else,
however, and the climate of the Tertiary, which is discussed more
fully in the next chapter, was in general warm and oceanic, becoming
rigorous towards its close.
Summing up, we find that in the geological history of the earth two
main types of climate seem to have alternated. Following on periods
of great crustal movement, and the formation of large land areas,
the general climate was cool, with a marked zonal distribution
of temperature, culminating during at least four periods in the
development of great sheets of inland ice. It is in such a period,
though, fortunately, not at its worst, that we are living at present.
During quiescent periods, on the other hand, when these continents
largely disappeared beneath the sea, climate became mild and equable,
and approached uniformity over a great part of the world. At these
times, as soon as the surface water of the sea in high latitudes
began to cool, it sank to the bottom, and its place was taken by
warmer water from lower latitudes. The oceanic circulation was very
complete, but there were practically no cold surface currents.
Instead, there was probably a general drift of the surface waters
from low to high latitudes (with an easterly trend owing to rotation
of the earth), and a return drift of cooled water along the floor of
the ocean. The formation of sea-ice near the poles became impossible,
while the widespread distribution of marine life was facilitated.
The alternation of periods of crustal deformation with periods of
quiescence has frequently been noticed, and has been termed the
“geological rhythm.” It may be attributed to the gradual accumulation
of small strains during a quiescent period until the breaking point
is reached, when earth-movements take place until equilibrium is
restored, when the process is repeated.
The gradual erosion of the land by river and wave-action and the
consequent shifting of the load provides a certain amount of stress;
but this is local, and calls for local readjustments only. A more
generally effective agency may be the gradual slowing down of the
earth’s rotation under the influence of tidal friction. The mechanism
of this process was described by A. E. H. Love (“Nature,” 94, 1914,
p. 254): “The surface of the ocean, apart from waves and tides, is at
any time a figure of equilibrium answering to the speed of rotation
at the time, more oblate when the speed is greater, less oblate when
it is slower. Let us imagine that the lithosphere also is at some
time a figure of equilibrium answering to the speed of rotation at
that time. If the speed remained constant, the lithosphere would
retain this figure, and the matter within it would remain always
in the same configuration without having to support any internal
tangential stress. Now suppose that the speed of rotation gradually
diminishes. The surface of the ocean will gradually become less and
less oblate. The lithosphere also will gradually become less oblate,
but not to such an extent as to make it a figure of equilibrium
answering to the diminished speed of rotation, while the matter
within it will get into a state of gradually increasing internal
tangential stress. The effect on the distribution of land and water
will be that the depth of the ocean will gradually diminish in lower
latitudes and increase in higher latitudes, the latitudes of no
change being 35° 16′ N. and S.
“The internal tangential stress in the matter within the lithosphere
may increase so much that it can no longer be supported. If this
happens a series of local fractures will take place, continuing until
the lithosphere is again adjusted much more nearly to a figure of
equilibrium, which will be less oblate than the original figure. The
effect on the distribution of land and water will be that the depth
of the ocean will increase rather rapidly and spasmodically in lower
latitudes and diminish in higher latitudes.
“Accordingly, the kind of geological change which the theory of tidal
friction would lead us to expect is a sort of rhythmic sequence,
involving long periods of comparative quiescence, marked by what
Suess calls ‘positive movements of the strand,’ in the higher
latitudes, and ‘negative movements’ in the lower, alternating with
comparatively short periods of greater activity, marked by rise of
the land around the poles and subsidences in the equatorial regions.”
The main periods of adjustment under this scheme fall at the
beginning and end of the Proterozoic, in the Permo-Carboniferous
and in the Quaternary. The two latter at least were periods of
great earth-movement, while the two former were also continental
periods, since the land-masses were large and high enough to develop
ice-sheets.
The difficult question raised by the low latitudes in which the
Pre-Cambrian and Permo-Carboniferous glaciations were chiefly
developed cannot yet be regarded as solved, but the geological facts
speak strongly in favour of ice-sheets rather than mountain glaciers,
and practically speaking it is meteorologically impossible for large
ice-sheets to extend to sea-level in the Tropics while the rest of
the world enjoys a temperate climate. The only escape seems to be to
assume a position of the South Pole somewhere between Africa, India
and Australia throughout the whole of the Proterozoic and Palæozoic
periods. On the other hand, from the Jurassic onwards, there is no
real support to the hypothesis that the positions of the poles were
other than they are now. Wegener’s explanation of the Quaternary Ice
Age we have seen to be untenable. The period of transition appears
to lie in the later Permian and Triassic. The Proterozoic and
Permo-Carboniferous glacial periods were much less definite in the
north than in the south-east; but such as they were they appear to
have been most severe in the east of North America, where the ice was
coming from the north; there are also some glacial traces in Europe.
This indicates that the position of the North Pole cannot have been
in the North Pacific Ocean, which is antipodal to the South Indian
Ocean. Hence it seems that what we have to consider is not so much
the wanderings of the poles at large among the continents as the
break-off at the close of the Palæozoic period of portions of the
Antarctic continent and their drift northwards towards the equator.
Without going into the mathematics of the question, it seems just
possible that the periodic overloading of circumpolar continents by
large ice-masses could have this effect in the course of time,[2] but
the suggestion is put forward tentatively for consideration rather
than as a definite hypothesis. We must be thankful that in the next
chapter we are on safer ground.
BIBLIOGRAPHY
Coleman, A. P. “Climates and physical conditions of the early
Pre-Cambrian.” _Geol. Mag._ (6), Vol. 1, 1914, p. 466.
Eckardt, W. R. “Paläoklimatologie.” Sammlung Goschen, Leipzig, 1910.
Geikie, J. “The Evolution of Climate.” Edinburgh, _Scot. Geogr.
Mag._ (6), 1890, p. 57.
Grabau, A. W. “Principles of Stratigraphy.” New York, 1913, pp. 74,
_et seq._
Neumayr, M. “Ueber klimatischen Zonen während der Jura- und
Kreidezeit.” Wien, _Denkschr. Ak. Sci._, 47, 1883, p. 211.
Ramsay, W. “Orogenesis und Klima.” _Ofvers af Finska Vetenskaps.
Soc. Forb._, 52, 1909-10, A, No. 11. Helsingfors, 1910.
Schuchert, C. “Climates of Geologic Time, In: The climatic
factor as illustrated in arid America,” by Ellsworth Huntington.
Washington, 1914, Pt. 2.
CHAPTER III
CONDITIONS BEFORE THE QUATERNARY ICE AGE
The third of the great periods into which the geological record is
divided is known as the Tertiary. Throughout most of its length it
appears to have been characterized by remarkably mild and equable
climatic conditions extending into comparatively high latitudes,
so that the west coast of Greenland, for instance, had a flora of
almost sub-tropical aspect. Since the plants in question—chiefly
palms and cycads—are not of identical species with their present-day
representatives, it is unsafe to base numerical estimates of the
temperature upon them, but it is at least obvious that these regions
were warmer than they are at present.
Let us glance for a moment at the geography of the Tertiary
period. The most noticeable point is a great expanse of sea over
south-eastern Europe, including the Mediterranean countries,
extending away over the Black Sea and Caspian, and stretching in a
great arm to the Arctic Ocean, south of Novaya Zemlya. The geology of
the archipelago north of Canada is not yet well known, but it seems
probable that there was a considerable area of Tertiary ocean there
also. The sea further encroached on the present boundaries of North
America, both east and west, and on the north-eastern coast of Asia.
Bearing in mind the principles set out in the first chapter, we can
infer from these changes a great increase in the winter temperature
of the regions along the Arctic circle. The increase reached a
maximum on the west of Greenland and in western Siberia, but the
west coast of Alaska also had a decidedly warm climate in the late
Miocene and Pliocene.
The basin of the Arctic Ocean, which already existed at that stage,
was raised to a temperature considerably higher than the present
by three great streams of warm water flowing into it. If, as seems
probable, the Bering Strait was deeper, and the submarine ridge
across the North Atlantic less pronounced, the obstacles to the
outflow of cold water along the ocean floor were much less than now.
Finally, the winter temperatures of the land-masses to the south,
and especially Siberia, being already very much less severe owing
to the sea over Europe, the temperature of the water of the great
rivers flowing into the Arctic was not so low. For these reasons the
development of ice in the Arctic Ocean was very much diminished,
and possibly entirely absent, allowing a great amelioration of the
climate of Greenland, the rigor of which is at present much enhanced
by the ice which flows down the Greenland Sea and round Cape Farewell.
The cumulative effect of all these changes—greater water area,
greater inflow of warm surface water, less inflow of cold river
water, less ice-development—must have been a mild equable rainy
climate, entirely suitable to a rich vegetation. The sub-tropical
aspect of that vegetation should not be stressed, for it was probably
as much an expression of the geological age of the period in question
as of its climate.
The objection may be raised that at the present time the
sub-antarctic islands in the great Southern Ocean have the most
maritime climate in the world, but are not by any means places of
opulent vegetation. The difference is entirely accounted for by the
presence of the great ice-bearing Antarctic continent. Its effect
is twofold. Firstly, the glaciers shed into the Southern Ocean an
immense quantity of ice and ice-cold water annually, which must have
an appreciable effect on temperature. Secondly, the presence of this
ice-covered continent and the floating ice in its neighbourhood
extending as far as the sixtieth parallel, by forming a marked
contrast with the warmer waters further north, greatly intensifies
the strength of the atmospheric circulation in these regions,
resulting in the development of a great succession of severe storms
which sweep the sub-antarctic islands. There are no great land-masses
to break the force of the wind, and these latitudes are among the
stormiest, windiest regions of the earth—gale succeeding gale, winter
and summer alike; and it is largely to the extraordinary power of the
wind that we must attribute the desolate appearance of the islands.
The picture we have drawn of the high northern latitudes in early
Tertiary times is vastly different. A great warm ocean occupied the
Arctic regions, fed by three ocean currents analogous to the Gulf
Drift, and the fall of temperature was gradual from the tropic to the
pole. The return colder currents were mainly along the ocean floor
and with little ice-formation the storms were few and not severe. On
the western shores of the continents mild rain-bearing south-west
winds prevailed, and a quiet moist warm atmosphere existed which was
especially favourable to plant life. This favourable state of affairs
lasted until well on in the Miocene, and then changes set in. The
land and sea distribution underwent essential modifications. The
great Tertiary continent or archipelago which is believed to have
existed in the western Pacific, and whose last remaining summits
now form the scattered islands of that ocean, gradually subsided,
and in its place elevation began in higher latitudes. Bering Strait
became narrow and shallow, and was probably for a time entirely
closed, while the connexion between the Arctic and Indian Oceans was
closed permanently, leaving in its lowest areas a chain of great
inland seas and lakes, of which the Caspian and Aral Seas are now
the greatest representatives. The Canadian Archipelago was probably
raised above its present level, and formed a great northern extension
of the American continental area. The changes in the Atlantic also
were very extensive. The West Indies were the site of a large and
lofty Antillean continent; further north a considerable land-mass
existed east of Newfoundland; Greenland was joined on the west to
the extended American continent, and considerably enlarged to the
south-east. Iceland, though it remained an island, was elevated and
probably nearly doubled in area, and between Iceland and the north of
Scotland was developed a great submarine ridge, which may or may not
have risen above the sea in places. The British Isles became a solid
block of land, united with continental Europe across the English
Channel and the great plain which is now the North Sea. Scandinavia
was elevated by more than a thousand feet, and the elevation extended
at least as far as Spitzbergen. The Murman area had a considerable
extension. In eastern Asia the Sea of Okhotsk was land and Japan was
united to the mainland.
In the southern hemisphere our knowledge is not nearly so detailed.
The presence of marine Middle-Tertiary beds with temperate mollusca
in Graham Land and of plant-bearing beds in Seymour Island point to
a smaller Antarctic continent and very much warmer conditions at
this time in the South as well as in the North Polar regions. For
the close of the Tertiary, however, we have strong grounds in the
distribution of animals and plants for assuming that the Antarctic
continent was greatly increased in size, with promontories uniting
it to Australia on the one hand and to South America on the other.
New Zealand was largely increased in area, and South Africa probably
extended further polewards. The sub-antarctic islands attained a much
greater area. Conditions were ripe for the Ice Age in the southern as
well as the northern hemisphere.
BIBLIOGRAPHY
Kerner von Marilaun, F. “Synthese der morphogenen Winterklimate
Europas zur Tertiärzeit.” Wien, _SitzBer. K. Akad. Wiss_, 122,
1913, pp. 233-98.
Osborn, H. F. “The age of mammals in Europe, Asia and North
America.” 8vo. New York, 1910.
Nathorst, A. G. “On the fossil floras of the Arctic regions as
evidence of geological climates.” London, _Bot. J._ 2, 1913, pp.
197-202; and Washington, _Report Smithsonian Inst._, 1911.
Dall, W. H. “On climatic conditions at Nome, Alaska, during the
Pliocene.” _Amer. J. Sci._, ser. 4, Vol. 23, 1907, p. 457.
Nansen, F. “The bathymetrical features of the North Polar seas,
with a discussion of the continental shelves and previous
oscillations of the shore-line.” _Norwegian N. Polar Exped._,
1893-96. _Scientific Results_, Vol. 4.
Spencer, J. W. “Reconstruction of the Antillean continent.” _Bull.
Amer. Geol. Soc._, 6, 1895, pp. 103-40.
Wilckens, D. “Die Mollusken der antarktischen Tertiärformation.”
_Wiss. Ergebn. der Schwed. Sudpolar Exped._, 1901-3, Bd. 3, 1911.
Hedley, C. “The palæographical relations of Antarctica.” London,
_Proc. Linnæan Soc._, 124, 1911-2, p. 80.
CHAPTER IV
THE GREAT ICE AGE
As the land began to rise, the first effect was an increased
snowfall on the higher summits, and increased rainfall on the
rising coast lands. The rivers had an increasing fall towards the
sea, and rapidly carved out deep narrow valleys, which were later
developed by the ice into the great fiords of Norway and other
heavily glaciated regions. But on the whole the first beginnings of
the Ice Age occurring towards the close of the Pliocene period are
obscure, and are likely always to remain so, for the simple reason
that the advancing and retreating ice-sheets have wiped out most
of the evidence of the conditions which immediately preceded their
advent. The deteriorations of the climate had begun long before the
geographical changes outlined at the close of the last chapter were
complete, for mollusca of the Pliocene beds in East Anglia indicate
progressive refrigeration of the North Sea at the same time as it
became increasingly shallow. At the close we have great shell-banks
with northern species which must have been piled up by powerful
easterly winds; these easterly winds show that the storm-tracks
had been driven south of their present course and suggest that the
glacial anticyclone already existed over Scandinavia. At the present
day similar shell-banks are forming on the coast of Holland under
the influence of the prevailing westerly winds. The next series of
deposits in this region are fresh-water “forest beds,” attributed to
a greatly extended Rhine, and belong to the period when the North
Sea had become a plain.
It is no part of the plan of this work to go over the geological
ground of the Quaternary Ice Age, which has already been so
frequently and so efficiently covered. All I can hope to do is to
give a brief general account of the succession of climatic changes
involved, necessarily incomplete since so much of the world is at
present insufficiently explored for glacial traces. But a certain
amount of explanatory introduction is necessary.
In Europe and North America there are distinct traces of several
separate glaciations with “interglacial” periods when the climate
approached or became even warmer than the present. The time-relations
of these glaciations are not yet fully worked out, but there seems
little doubt that they were contemporaneous in the two continents.
The correlation is not perfect, however, since the United States
geologists recognized altogether five glaciations. The explanation
appears to be that the equivalent of the Rissian glaciation in
America is double; two stages, the Illinoian and Iowan, being
recognizable, separated by a retreat of the ice. The series is as
follows:
-----------+----------------+--------------------+----------------
Alps. |Northern Europe.| North America. | Date. B.C.
-----------+----------------+--------------------+----------------
I Gunz | ? | Jerseyan or | ?
| | Nebraskan |
II Mindel | Lower Diluvium | Kansan | 430,000-370,000
III Riss | Middle Diluvium| { Illinoian } | 130,000-100,000
| | { Iowan } |
IV Wurm | Upper Diluvium | Wisconsin | 40,000- 18,000
-----------+----------------+--------------------+----------------
The correlation is based on the amount of weathering and erosion
which the various deposits have undergone. The time which has
elapsed since the glaciers of the last or Wurm stage were in active
retreat has been estimated by comparing the growth of peat-bogs,
river-deltas, etc., during historical times with that since the last
retreat of the ice. But the most conclusive method is due to the
Swedish geologist Baron de Geer, who has actually counted the years
since the ice in its final retreat left any particular point between
Ragunda and the south of Sweden. The work is based on the idea that
the lamination observed in certain marine and lacustrine clays in
Sweden is seasonal, the thick coarse layers being due to the floods
produced by the rapid melting of the ice in summer, and the thinner
and finer layers being due to the partial cessation of melting in
winter. By correlating one section with another it is possible to
date any particular layer with great exactness, and further to prove
the existence of several great climatic fluctuations during the
retreat. The topmost limit of the section is given by the surface of
the old floor of Lake Ragunda in Jemtland, which received its waters
from one of the permanent glaciers and was accidentally drained in
1796. De Geer finds that the edge of the last great ice-sheets lay
over southern Scania about 12,000 years ago, and further estimates
8000 years for the retreat across the Baltic. These results are in
general agreement with those obtained by other methods, and we may
accordingly, with some confidence, put the date when the ice-sheet of
the Wurm glaciation finally left the coast of Germany at about 18000
B.C.
This period fixed, we have a datum for estimating the duration of the
interglacial periods. The moraines of the Wurm glaciation present
everywhere a very fresh appearance, and the chemical change which
the boulders they contain have undergone is slight, while weathering
extends to a depth of scarcely a foot. The moraines of the Riss
glaciation are weathered somewhat more deeply, and those of the
Mindel glaciation very much more. Assuming that chemical weathering
has proceeded uniformly during the interglacial periods and ceased
during the glaciations, Penck and Brückner, who have studied
exhaustively the glaciation of the Alps, find that the Riss-Wurm
interglacial lasted about three times as long as the interval between
the Wurm glaciation and the present day, or 60,000 years, and the
Mindel-Riss interglacial about twelve times as long, or 240,000
years. No data are available for the Gunz-Mindel interglacial, but it
is provisionally made equal to the Riss-Wurm, another 60,000 years.
No possibility of such direct measurement of the duration of the
glacial periods themselves has yet been found. Penck and Brückner
assume that the duration equalled that of the Riss-Wurm interglacial,
or 60,000 years in each case. This seems unnecessarily long. The
Yoldia Sea, the deepest part of which coincided with the centre of
the Scandinavian glaciation, appears to have reached its greatest
depth not more than 6000 years after the maximum of glaciation,
indicating a lag of this period. The subsidence of the land due to
the weight of the ice-sheet may have commenced some time before the
maximum of glaciation, but the duration of the subsidence can hardly
have been more than 10,000 years, and this is the limit for the
second half of the Wurm glacial period. Further, we know that during
the growth of the ice-sheets there was comparatively little melting,
for the rivers then had little power of carrying debris. Recent
measurements in Greenland give the rate of ice-growth on the surface
of the ice-sheet as 40 cm., or 15 inches a year; let us say a foot,
and assume a marginal loss equivalent to half this amount over the
whole ice-sheet. This gives us an average increase of six inches a
year, or 10,000 years for growth to a maximum thickness of 5000 feet.
On these grounds the estimated duration of the Rissian glacial period
has been reduced to 30,000 years, and that of the Wurm period to
22,000 years. Only in the case of the long and complicated Mindelian
period, which, as will be seen later, was virtually a series of
overlapping glaciations from various centres, has the figure of
60,000 years been accepted. In the present state of our knowledge no
estimate of the duration of the Gunz-Mindel interglacial can have
any value, and the dates are accordingly carried back only to the
Mindelian. In this way we obtain the time-scale given on page 48.
The fourfold glaciation has been recognized with certainty only
in Europe and North America, and even in these countries there is
considerable doubt whether the northern ice-sheets shrank back as
far as their present narrow limits during the interglacial periods.
The long Mindel-Riss interglacial, which was probably the Chellean
stage of flint industry,[3] was characterized by a very warmth-loving
fauna, and it is possible, even probable, that during this period the
glaciers melted completely away, except on the very highest summits.
Of the climate of the Gunz-Mindel interglacial (termed by J. Geikie
the “Norfolkian,” from the Cromer Forest Bed), we have comparatively
little evidence. If the suggestion put forward in the following
chapter is correct, the Gunz-Mindel interglacial was merely a local
incident in the glaciation of the Alps, and not a true interglacial
at all. Even the Cromer Forest Bed itself is not conclusive, since it
is a river deposit largely composed of material drifted from lower
latitudes. The Riss-Wurm interglacial (J. Geikie’s “Neudeckian”)
nowhere gives us evidence of a climate as warm as the present, and as
regards the Scandinavian and Canadian ice-sheets may have been merely
an extensive and prolonged oscillation of the ice-edge.
As regards the glaciation of Norway, the question has been
investigated recently by H. W. son Ahlmann, who has published a long
and detailed paper in English in volumes 1 and 2 of the Swedish
_Geographiska Annalen_. He concludes that the morphology of Norway,
without reference to stratigraphical or biological data, gives
conclusive evidence of two glaciations. The first of these was the
greater, and between that time and the second smaller glaciations
there occurred an interglacial period of such considerable length
that the greater part of the present gorges was then formed by
fluvial erosion.
We may, accordingly, consider the Ice Age as fourfold or double,
according to the point of view from which we regard it. In the Alps
and other mountain ranges on the borders of the great northern
ice-sheets, which respond very readily to small changes, it was
fourfold. In the peripheral regions of the northern ice-sheets
themselves it has an appearance of being threefold or fourfold. In
the more central regions of these great ice-sheets, where response
to climatic change is very slow, there is no evidence of more than
two glaciations; but in these regions, where the destructive effect
of the ice reached its maximum, it is only by the merest chance that
evidence of interglacial periods is preserved at all. And finally, in
all other parts of the world we have evidence of only two glaciations
at most.
There is one deposit which is of considerable importance in the
study of interglacial climates, and that is the loess. Loess is an
exceedingly fine-grained homogeneous deposit resulting from the
gradual accumulation of wind-blown dust on a surface sparsely covered
with vegetation. It is to be seen accumulating at the present day in
parts of south-east Russia and central Asia. Its formation, except in
closed basins, needs a climate of the steppe character, with not much
rainfall, and especially with a long dry season. Now loess was very
extensively developed in Europe during the Quaternary. Its occurrence
is peculiar, since it is found most widely developed resting on the
deposits of the Rissian glaciation, and is never found resting on
the moraines of the Wurm glaciation. A little loess is found below
the Riss moraines, and it has also been found between the Riss and
Wurm moraines. In the pre-Rissian loess an implement of Acheulian
age was discovered in 1910 at Achenheim (Alsace), by R. R. Schmidt
and P. Wernert, indicating that the deposit was formed towards the
close of the Chellean industry, when the climate was already cold and
dry. In the same section the younger loess seems to fill completely
the Riss-Wurm interglacial, since Mousterian implements were found
at the base and Aurignacian implements in the middle. The younger
loess contains remains of the jerboa and other rodents at present
inhabiting the Siberian steppes. It is therefore reasonable to
conclude that steppe conditions prevailed in central Europe through
practically the whole of the Riss-Wurm interglacial, and the same
probably applies to the corresponding pre-Wisconsin interglacial in
America. But if a steppe climate prevailed in central Germany there
must have been very severe conditions in Scandinavia, and probably
the ice-sheet maintained a quite considerable area there throughout
the whole period, though without encroaching on the Baltic basin. In
North America the loess was deposited by westerly winds, indicating
that the ice-development was not sufficient to impose anticyclonic
conditions in place of the present prevailing westerly winds, and the
same appears to be true of Europe. Similar climatic conditions were
developed for a short time at the close of the Wurm glaciation, but
without any appreciable development of loess. (See Chapter XIII.)
BIBLIOGRAPHY
Wright, W. B. “The Quaternary Ice-age.” London, Macmillan, 1914.
Brooks, C. E. P. “The correlation of the Quaternary deposits of the
British Isles with those of the continent of Europe.” _Ann. Rep.
Smithsonian Inst._, 1917, pp. 277-375.
de Geer, G. “A thermographical record of the late Quaternary
climate.” _Ber. Internat. Geologenkongr._, Stockholm, 1910. “_Die
Veränderungen des Klimas_,” p. 303.
——. “A geochronology of the last 12,000 years.” _Ber. Internat.
Geologenkongr._, Stockholm, 1910, Vol. 1, p. 241.
Penck, A., and Brückner, E. “Die Alpen in Eiszeitalter.” Leipzig, 3
Vols., 1901-9.
Ahlmann, H. W., son. “Geomorphological studies in Norway.”
Stockholm, _Geografiska Annaler_, 1, 1919, pp. 1-157, 193-252.
Richthofen, F. “On the mode of origin of the loess.” _Geol. Mag._,
1882, p. 293.
CHAPTER V
THE GLACIAL HISTORY OF NORTHERN AND CENTRAL EUROPE
The literature of the glacial period in Europe is stupendous and is,
further, of a highly contradictory nature. Space does not permit of
any summary of the great conflict between the monoglacialists and
the polyglacialists; it is sufficient to say that the latter often
went to extremes and so laid themselves open to defeat, but the
twofold nature of the glaciation is now widely accepted. It must
be understood, however, that the following summary represents the
views of a certain section of geologists only, views which are not
universally held. In the British Isles especially, where the remains
of the maximum glaciation completely dominate those of all the
others, the theory of a single glaciation still largely prevails.
When ice began to accumulate on the rising Scandinavian plateau
it naturally formed at first on the Norwegian mountains near the
Atlantic, which was the chief source of snowfall. These mountain
glaciers spread rapidly down the steep seaward slopes to the west and
more slowly down the gentler landward slopes to the east. At this
stage the centre of the ice-sheet, and consequently the centre of
the glacial anticyclone, as soon as the latter developed a definite
existence, lay quite near the Norwegian coast. Under anticyclonic
conditions the circulation of the winds round the centre is in the
same direction as the motion of the hands of a watch, combined with
an outward inclination at an angle of about thirty to forty-five
degrees. Consequently, while the centre lay in Norway, due north of
the Alps, the prevailing winds in the latter must have been from
north-east, and therefore very cold. Accordingly, this stage is
probably contemporaneous with the Gunz glaciation of the Alps. In the
same way, over the North Sea area the winds must have been easterly,
causing the currents which piled up the great shell-banks of the East
Anglian coast, already referred to as marking the end of the Tertiary
and beginning of the Quaternary period.
But the ice which reached the northern North Sea broke up into
icebergs not far from the coast, and floated away, while that which
moved east into the north of Sweden could only be dissipated by
melting and ablation, processes which we have reason to believe went
on very slowly. Hence ice began to accumulate and spread over a wide
area east of the main Scandinavian mountain chain. Fresh snow was
deposited directly on this ice-surface, until it gradually overtopped
the mountains which originally gave rise to it, and reversed the
flow, so that the ice actually moved uphill across the mountain
chain. As the center of the ice-sheet moved eastward the glacial
anticyclone moved with it, and this new position to the eastward
caused an alteration in the direction of the prevailing winds over
the rest of Europe. The Alps were now south-west of the anticyclonic
centre, and the winds in that district accordingly became easterly
instead of north-easterly. Of course, the glacial anticyclone was
now more intense, but in summer in central Europe easterly winds are
naturally so much warmer than north-easterly winds that at first this
increase in intensity was not enough to counterbalance the change in
direction, and there was a slight improvement in the Alpine climate.
In the same way, over the North Sea district the prevailing winds had
now become south-easterly instead of easterly, which would make for a
slight rise of temperature, as also would the occasional depressions
which would be able to make their way in from the westward, bringing
warm moist air from the Atlantic and occasional rainfall. By this
time the process of elevation had converted the North Sea floor into
an extensive plain.
From Sweden and the Gulf of Bothnia the ice spread out in all
directions, extending in the east to the foot of the Ural Mountains,
which formed an independent centre of glaciation; in the south-east
over a large part of European Russia, where it reached as far south
as latitude 40° in the Dnieper valley; in the south over almost the
whole of Germany as far as the Riesengebirge and Harz Mountains;
and in the south-east over the whole of Holland and the North Sea
basin. It should be noted that Holland and Denmark were glaciated,
not by Norwegian ice, but by ice from the Baltic sheet which had
crossed southern Sweden. The North Sea glacier extended across East
Anglia as far as Cambridge, while a northern branch of it swept
across Caithness and the Orkney and Shetland Islands, but most of the
British Isles were glaciated from independent centres—the Scottish
Highlands, the Pennines, Cumberland, Wales and northern Ireland.
With the growth of the glaciated area, and particularly with its
extension south-westward across the North Sea, the Alpine climate
again became very severe, and the local glaciers and Piedmont
ice-sheets of the Alps reached their maximum development in the
Mindelian. At the same time the central plateau of France developed
a local plateau glacier of its own, and the Pyrenees underwent their
first and greatest glaciation, no traces of the Gunzian having been
found in this range.
The British Isles show an interesting outward migration of the local
centres of maximum ice-development. The Scandinavian glacier which
invaded East Anglia extended arctic anticyclonic conditions across
the North Sea, and induced a heavy snowfall over the high lands of
Great Britain. These, in consequence, developed independent glaciers,
which on their eastern sides fused with the Scandinavian glacier
and, partly by deflecting its flow, partly by intercepting some of
its snowfall, pushed it back into the North Sea plain. The Scottish
glaciers became strong enough to encroach on Ireland, partly in the
north-east, and partly by way of the Irish Sea and St. George’s
Channel (then a valley) on to the south-east. This further extension
of the cold area enabled the Irish glaciers to develop, and these
in turn pushed back the Scottish glaciers until Ireland was solely
glaciated by Irish ice.
The southern margin of the ice-sheet did not extend beyond the Thames
valley, but at some stage the English Channel carried floating ice,
which formed the deposits of ice-borne boulders, of which that at
Selsey is a well-known example.
This great ice-sheet nowhere formed marked terminal moraines, but its
deposits fade away in thin beds of stiff boulder-clay. This absence
of moraines is probably connected with the great thickness of the
ice-sheets, which did not leave any appreciable nunataks or rocky
“islands” exposed in its path, so that there was nothing to give
rise to detritus on the surface of the ice. All the transportation
had to be carried on beneath the ice-sheets, and these, penetrating
into comparatively low latitudes where the sun is powerful in summer,
would suffer gradual melting and ablation for some distance from
their margins. Near the actual ice-limit the motion must have been
slow and the thickness of the ice small, so that conditions were
against the accumulation of thick beds of detritus.
On the borders of the ice-sheet the climate cannot have been
over-rigorous, for pre-Chellean man was able to live almost up to the
ice-edge. The air must have been extremely cold, and there was a belt
of high arctic climate round the ice, but in the south and south-west
this appears to have been very narrow, and sub-arctic conditions,
no worse than those in which many races live to-day, prevailed not
very far from the ice. The configuration of the ice-surface largely
explains this. A high steeply sloping wall of ice causes intensely
violent winds, carrying dense clouds of drift-snow—blizzards, in
fact, similar to those now experienced in parts of Antarctica under
similar circumstances, which sweep the land bare of all life for a
considerable distance. But a low and gradually sloping surface, such
as seems to have existed near the borders of the maximum glaciation,
favours instead comparatively gentle winds without much drift snow.
It is only on the north-west ice-ridge, where ice-cliffs fronted
the sea and where severe storms from the Atlantic were frequent in
winter, that blizzards occurred.
When the land in Scandinavia began to sink under the ice-load more
rapidly than the supply of snow could build up the surface of the
ice-sheet the force which pushed out the ice in all directions from
the centre gradually died away, and the ice-masses over the North
Sea area—now probably again below sea-level—and the low grounds of
Europe were left derelict, with no resources but the snowfall on
their own surfaces. Under these conditions they melted away more
or less rapidly. While these derelict ice-masses were still large,
the auxiliary peripheral centres in the Alps, Pyrenees and British
Isles maintained an independent existence for a while, probably with
fluctuations similar to those which marked the close of the last
glaciation in the Alps, though the evidence of these has now been
wiped away. It is even likely that the beginnings of the weakening
of the central source of supply helped the British ice to divert the
Scandinavian ice into the North Sea. Had there been any powerful
rivers bearing great masses of detritus from the south, as there
are in Siberia, some of these derelict ice-sheets might have been
preserved for a time, at least, as “fossil ice,” but in western
Europe conditions were not favourable for this.
With the disappearance of the ice-sheets the general climate of
Europe must have passed through a series of stages of amelioration,
of which traces can be found here and there, though the details
are lost to us. Ultimately temperate conditions again prevailed;
and for a very long time, approaching a quarter of a million years,
Europe cannot have differed greatly from present climatic conditions.
In Scandinavia the mammoth roamed in forests of birch, pine and
spruce; further south the mammoth is absent, and we find instead more
southern forms—_Elephas antiquus_, resembling the Indian elephant,
_Rhinoceros merckii_, a southern form, the sabre-toothed tiger,
cave-lion, cave-bear and cave-hyæna, wolf, beaver, horse and various
forms of deer, while the flora included even such warmth-loving trees
as the fig. Obviously, during part of this interglacial period, the
climate must have been even warmer than the present.
Let us glance for a moment at the probable conditions. One of
the dominant features in the present weather of Europe is the
accumulation of floating ice in the Arctic basin. This keeps the
temperature low and the pressure high—forms in fact during the spring
and summer months a temporary glacial anticyclone similar in kind
to, though of less intensity than, that which has been described as
covering the Scandinavian ice-sheet. This anticyclone maintains on
its southern edges a belt of easterly winds, and these winds enter
into the general circulation of the earth. Their effect is to push
southward the permanent storm-centres normally situated near Iceland
and the Aleutian Islands, and it is these storm-centres which play
a large part in causing the rainy weather of northern and central
Europe. But occasionally—as in the remarkable spring and summer of
1921—these conditions break down. The Arctic Ocean becomes unusually
ice-free and warm, the pressure falls, and in consequence the
storm-centres move northward. Europe comes under the influence of
the permanent anticyclones of sub-tropical latitudes, rain-bearing
storms pass far to the northward, and we have a dry warm summer of
the Mediterranean type.
This is presumably what happened during the long warm Mindel-Riss
interglacial. For some reason, possibly connected with a temporary
widening and deepening of the Bering Strait, the waters of the Arctic
Ocean became warmer and the amount of floating ice less. Pressure
became lower in the polar basin and therefore higher over the
Atlantic and Europe, and fine warm conditions prevailed in Europe as
the normal climate instead of only as an occasional event.
This warm interval was finally brought to a close by the renewed
elevation of Scandinavia, and the ice-sheets began to develop again,
heralded by a period of dry steppe climate. This time, however, the
conditions were different; the elevation was not so great, and was
more local. Hence the resulting glaciation was less intense; it
filled the Baltic basin and extended some distance on to the North
German plain and into Holland. It failed to reach the coast of
Britain, but that it extended some way across the North Sea plain is
indicated by the peculiar distribution of the Newer Drift of Britain,
to be referred to later. In the north of Norway the slope of the ice
towards the sea was very steep, so that many of the coastal hills
extended above it as nunataks. The ice extended into the channel
between the mainland and the Lofoten Islands (then a peninsula), but
according to Ahlmann these islands were an independent centre of
local glaciation, as the British Isles had been during the preceding
period, and the local ice met the main ice-sheet in the fiords. On
the coast of Nordland sufficient land lay bare to harbour a small
Arctic flora, and Vaero, the southernmost island of Lofoten, had only
small hanging snow-banks.
The interpretation of the British glacial deposits is still very
much under discussion, but it seems probable that the Scottish
highlands formed a subsidiary centre which glaciated the whole of
Scotland and north-east England, sending a stream south-eastward,
which was prevented from spreading across the North Sea plain by
the presence of Scandinavian ice to the east and impinged on the
coast of Yorkshire and Lincolnshire, just reaching the northern
extremity of Norfolk. Many British geologists regard this development
as the concluding phase of a single glaciation of Britain, but the
differences in the amount of weathering undergone are against such
an interpretation. At the same time there were local glaciers in
Cumberland, Wales and Ireland.
In England limits of this glaciation are characterized by a
well-marked series of end-moraines, which indicate that the ice
carried much surface detritus, and probably ended in a steep cliff.
In Scandinavia, on the other hand, the centre of glaciation again
lay over the low ground well to the east of the mountains, and the
ice which reached Germany and Denmark was still largely free of
surface detritus, and so did not form marked end-moraines. There
was a difference, however. On this occasion, owing to the local
nature of the elevation in Scandinavia, the ice-sheet did not extend
its borders so far to the eastward, and the glaciation of Asia, as
described in Chapter VII, was slight. Europe came more under the
influence of cold north-easterly and northerly winds, and life on
the ice-borders was not so easy as during the preceding glaciation.
Man could still live near the ice, but he took to making his home in
caves, and to clothing himself in skins for warmth.
After the ice had reached its Rissian maximum of glaciation the
climate improved somewhat. The ice-edge retreated, leaving Denmark
and the German coast, and vacating the Baltic basin, but not
disappearing altogether from Scandinavia. At Rixdorf, near Berlin,
there is a bed of gravel deposited in this “interglacial,” containing
numerous and well-preserved bones of the mammoth, woolly rhinoceros,
aurochs, bison, horse, reindeer, red deer and other species of
_Cervus_, musk ox and wolf—a cold temperate to sub-arctic fauna.
In south Germany fresh-water mollusca indicate that the summers in
that district were almost as warm as at present, but the winters
were probably severe. As described in the preceding chapter this
“interglacial” was the time of loess formation _par excellence_, with
a continental climate and steppe conditions over much of central
Europe.
Investigations at Skærumhede in Denmark show that this recession of
the ice was accompanied by, and presumably due to, a fall in the
level of the land relatively to that of the sea, for at the beginning
of the oscillation the land lay about 350 feet above its present
level, sinking gradually to only 30 feet above present. Even at its
best during this interglacial the climate was almost sub-arctic in
Denmark. In northern Finland, on the eastern edge of the ice-sheet,
there was also an “interglacial,” with a slight improvement in the
climate accompanying a temporary submergence. But in Scandinavia
there are no traces of any interglacial deposits of this period, and
considering the cold climates which prevailed in Denmark and North
Germany, it seems probable that Scandinavia continued to be glaciated
during the whole period.
The mode of life among Mousterian men, who lived during this
“interglacial,” also points to a severe climate. For at this time man
did not live in the open, but in caves and rock-shelters, and the
practice of wearing the fur skins of animals as a protection against
the cold, begun in the preceding Rissian glacial period, was not
discontinued.
After the temporary subsidence had ceased, elevation again set in,
causing a readvance of the ice-sheets and glaciers. The limits fell
short of those of the preceding maximum, and the climate was not
so severe, but in its general character it resembled that of the
preceding maximum, but was much stormier, and there were probably
frequent blizzards of the Antarctic type, carrying drift-snow. The
new ice-sheet carried more surface detritus than its predecessors,
presumably because all the high ground was not covered, and it
formed high terminal moraines. The close association of cold ice
and irregular masses of bare sand and stones, strongly heated by the
summer sun, set up a belt of powerful convection very favourable for
the development of blizzards; possibly there was something in the
nature of an ice-cliff down which the cold winds could blow with
great strength. At any rate, man found the near neighbourhood of
the ice unpleasant, and went, so that there are no contemporaneous
human implements near the moraines. The limits of the Scandinavian
ice-sheet ran from the Norwegian coast across Denmark from north
to south, through North Germany and northern Russia, and included
Finland. The ice probably did not extend far across the North Sea
plain, and in the British Isles there was no ice-sheet, but the high
mountains of Scotland, Ireland, Wales and Cumberland bore small
local glaciers, which were long enough to reach the sea in the
Scottish highlands. The Alps bore considerable glaciers, indicating
a depression of the snow-line of about 3500 feet, corresponding to a
temperature 11° F. lower than the present.
After this ice-development had reached its maximum limits and
remained there for perhaps a thousand years, retreat set in, and
the Scandinavian ice once more withdrew from Germany and Denmark to
the Baltic basin. But its edge was never far from the German coast,
and occasionally readvanced across it, for numerous fossiliferous
deposits are intercalated in boulder-clay. The fauna and flora, which
are well known, point to an arctic climate. At its best the mean
temperature of July rose to about 50° F., and there was a vegetation
period of three or four months with an average temperature of about
40° F., but these relatively mild conditions lasted at most for a few
decades or perhaps a century at a time, and the winters were severe
throughout. The duration of the whole of this “Baltic Interstadial”
was from one to two thousand years.
Next followed the final readvance of the ice forming the great
“Baltic” moraines which fringe the Baltic coast of Germany, turning
northward in the west into Denmark and in the east into Finland.
There was a corresponding re-development of glaciers in the Alps
(Bühl stage) and in the mountains of Ireland and Scotland, though
these probably failed to reach the sea even in Scotland. This period
gave us a repetition of the climate of the preceding maxima. In this
case we have definite evidence of the presence of a belt of easterly
winds on the southern side of the ice-sheet, in a series of “barkans”
or fossil dunes in Holland, Germany and Galicia. These dunes were
formed of fine ice-deposited material, and they are crescent-shaped,
with their convexities to the east, indicating that they were built
by strong easterly winds. A moment’s consideration will show the
truth of the latter statement. Suppose there is an isolated round
hillock of sand exposed to strong easterly winds. The sand grains
will travel up the easterly windward slope of the hillock and roll
down the westerly leeward side. In this way the whole hillock will
advance very slowly westwards. But in the centre, where the hillock
reaches its greatest height, the grains will take longer to reach
the highest point than near the edges, where they have not to rise
so high. At the edges a strong gust will carry some of the heavier
grains right over the dune, while nearer the centre they will be left
half-way, and when the gust ceases will perhaps roll back to their
original position. In this way the margins of the dune will advance
westward more rapidly than the centre, producing the crescent shape
with the convex side to the east. At the time of their formation
these dunes must have had their steepest side to the westward, but
the westerly winds which have prevailed during the last few thousand
years have succeeded in modifying that detail, without destroying
the general shape of the dunes, and the steepest slopes are now on
the eastern side. The preservation of the original shape, in spite
of the subsequent development of westerly winds, is due in part to
the coating of vegetation, which protected the dunes as soon as more
favourable conditions occurred, and probably in part to the lesser
velocity of the westerlies. If the period of east winds and dune
formation had been long enough, we might have had another deposit of
loess, but it was short, and vegetation, which is necessary to the
genesis of true loess, had no time to establish itself before the
climate changed again with the final retreat of the ice. The climate
of this period in Rumania has been ably described by G. Murgoci:
“In general the prevailing climate of the time of the formation of
loessoid soils and blown sands must have been that which is named by
E. de Martonne the aralian climate, a dry climate with some rain in
spring to call forth a poor and transient vegetation and to maintain
the flowing water in rivers and lakes. The temperature with great
extremes, in summer up to 120° F. and in winter below 20° F., was the
characteristic of this climate; the atmosphere was very dry in the
hot season, but in the rest of the year there was some humidity in
the air and moisture in the soil, the water of the subsoil being not
very deep. The atmospheric precipitation in this region was caused
by the south-west wind just as at present; but the dominant wind
giving the character of a dry continental climate was the north-east
wind (Crivat) which has left its traces in the fossil dunes of the
Baragan.”
A history of the changes of climate in Europe which followed the
maximum of the last readvance of the ice-sheet must be left to later
chapters.
BIBLIOGRAPHY
Brooks, C. E. P. “The correlation of the Quaternary deposits of the
British Isles with those of the continent of Europe.” _Ann. Rep.
Smithsonian Inst._, 1917, pp. 277-375. [Full list of references.]
Penck, A., and Brückner, E. “Die Alpen in Eiszeitalter.” 3 Vols.
_Leipzig_, 1901-9.
Gagel, C. “Die Beweise für eine mehrfache Vereisung
Norddeutschlands in diluvialer Zeit.” _Geol. Rundschau_, 4, 1913,
p. 39.
Wahnschaffe, F. “Die Oberflächengestaltung des norddeutschen
Flachlandes.” Stuttgart, 1910.
Svastos, R. “Le postglaciare dans l’Europe centrale du nord et
orientale.” _Ann. Sci. Univ. Jassy_, 4, 1908, p. 48.
CHAPTER VI
THE MEDITERRANEAN REGIONS DURING THE GLACIAL PERIOD
Our knowledge of the history of the Mediterranean basin during the
Glacial period is not nearly so complete as is that of the more
northern regions, chiefly for the reason that during most of the
period the land lay above its present level, and except for local
glaciers in the mountain regions there was no ice to leave us a
record of the changing climates. Most of what we do know relates to
the relatively brief periods of submergence.
At the beginning of the Glacial period the sea lay some 500 feet
above its present level, and we can trace the first appearance of a
northern marine fauna. This stage is known as the Calabrian; it is
divided into two horizons—a lower, in which northern forms are still
rare, and an upper, in which they are becoming abundant. The most
typical species are two mollusca whose present habitat is the coast
of Iceland—_Chlamys (Pecten) islandicus_ and _Cyprina islandica_.
The Calabrian beach is not found on the coast of Spain or at
Gibraltar, and in Algeria it probably occurs at a lower level. This
suggests that the subsidence at this period was local, and the
western lands stood up as a barrier against the Atlantic. There
must have been a channel of some sort, however, on the site of the
present Straits of Gibraltar, to provide an inlet for the immigrating
northern mollusca. In the Maritime Alps, and again in the eastern
Mediterranean, the Calabrian beaches are at a much greater height
owing to local elevation.
After the formation of the Calabrian beach the whole Mediterranean
region was elevated above its present level. This elevation must be
contemporaneous with the period of maximum elevation in north-west
Europe associated with the great Mindelian glaciation. It is
suggested that the “sill” of the outlet channel at Gibraltar was
raised above the level of the Atlantic, and the Mediterranean became,
first a closed salt lake, and then a pair of lakes, the eastern fresh
draining into the western, which was salt, the two being separated
by a ridge of land between Italy and Tunis. This period of elevation
was long enough for a great deal of denudation to take place. Even in
the Mediterranean this was a time of severe climate. On the eastern
side of Gibraltar there are breccias, known as the “Older Limestone
Agglomerate,” which reach a thickness of 100 feet in places, and are
now much weathered. Similar agglomerates are found in Malta. These
resemble the “head” of the south of England, and appear to be due
to frost action in a severe climate. In Corsica there are traces of
four periods of mountain glaciation, and the two oldest of these are
provisionally correlated with the Gunzian and Mindelian of the Alps.
In the Balkan highlands there are traces of two distinct glaciations:
the older, which was the more general and reached the greater
intensity, probably corresponding to the Mindelian. In the Atlas
Mountains there are great boulder moraines which seem to belong to
three distinct glaciations, the oldest extending to about 2000 feet
above sea-level, and the second terminating at about 4000 feet, while
the third glaciation consisted of small valley glaciers only.
Towards the close of the Mindelian glacial period the land sank or
the ocean rose again, and the waters of the Atlantic poured in,
bringing with them a great number of high northern and Arctic
mollusca. The theory has been put forward that this influx was in the
nature of a debacle and carved out a deep gorge through the present
Straits of Gibraltar. The beaches deposited by this sea lie at a
height of 250 to 330 feet above the present sea-level. The fauna
resembles that inhabiting the northernmost parts of Europe at the
present day, and the waters must have been several degrees colder
than at present. This stage is termed the Sicilian.
As the climate improved the land gradually rose again, and the
next general raised beach lies at a height of only about 100 feet
in southern Italy (except where it has been elevated by local
earth-movements). Further west it lies still nearer the present
sea-level—twenty feet in the Balearic Islands and only seven feet on
the coast of Spain. On the coast of Algeria and Tunis this beach is
found at a height of about forty-five feet.
The beach contains no trace of the northern fauna found in the
Sicilian stage; instead it is marked by an assemblage of mollusca
of a sub-tropical aspect, including _Strombus bubonius_, _Mytilus
senegalensis_ and _Cardita senegalensis_. The bones of large mammals
are also found, including the hippopotamus and southern forms of
elephant (_E. antiquus_) and rhinoceros (_Rh. merckii_). This warm
stage corresponds to the Chellean interglacial fauna of northern
Europe, though so far as I am aware no Chellean implements have been
found associated with it.
About this time the Older Limestone Agglomerate of Gibraltar had
been worn into caves, in which are found the bones of ibex, wild
boar, leopard, spotted hyena, _Rhinocerus leptorhinus_, _Elephas
meridionalis_, lion, southern lynx, bear, wolf, stag, horse, etc.,
so that the rock must have been covered by a rich vegetation, and
must have had a greater extent than now, and a connexion with
the continent of Africa. This is said to have been followed by a
submergence of about 700 feet with numerous oscillations. This
submergence, if it is really attributable to the interglacial, must
have been extremely local, and possibly it is much older.
After the warm Chellean period the Mediterranean region rose again,
probably contemporaneously with the rise which caused the Rissian
glaciation of northern Europe. But the climate was nothing like
so severe as in the Sicilian. We have no old beaches containing a
molluscan fauna of this period, but at the Grotte au Prince near
Mentone, investigated by M. Boule, the _Strombus_ beach is overlain
by a bed of cemented pebbles and “hearths” containing Mousterian
implements and bones of a temperate fauna. The Newer Limestone
Agglomerate on the east of Gibraltar may have been formed during this
period. The Mediterranean lands remained above their present level
until the close of the Glacial period.
Each glaciation of northern Europe must have been a time of greater
rainfall as well as of lower temperature in the Mediterranean. The
glacial anticyclone in the north displaced the storms from the
Atlantic, which now mostly either skirt the north-west coast of
Norway or pass across Denmark into the Baltic. These storms had
to take a more southerly course, and entered the Mediterranean
basin either across the south of France or in the neighbourhood of
Gibraltar: tracks which are still occasionally followed in winter.
These storms brought a rainfall much heavier than the present, and
of a different character. The Mediterranean is now a “winter rain
region,” and the north of Africa is entirely rainless for several
months in the summer. But during the “Pluvial periods” it is probable
that rain fell throughout the year, though the winter still had more
than the summer. The winter rains were in the form of steady falls
of long duration, such as we experience now in England, while the
summer rains fell in short, heavy showers, perhaps accompanied by
thunder. The Older Pluvial period, which corresponds to the Mindelian
glaciation, had these conditions in their greatest development.
Depressions cannot live long without a supply of moisture, either
from the sea or from transpiring vegetation, and at present such
winter storms as enter the Mediterranean are almost confined to its
surface, and on the African side rarely penetrate more than one
hundred miles inland. But at the period of greatest elevation the
shrunken Mediterranean offered no such great attraction, and with a
comparatively well-watered Sahara the storms were able to pass much
further south. Consequently, northern Africa possessed a number of
large and permanent rivers which reached the sea. It was along these
rivers and their banks that the fauna still inhabiting the Saharan
oases made its way, to be isolated there by the decrease of the
rainfall, so that crocodiles and many species of fish now live in
isolated pools and in rivers which lose themselves in the sand.
In Egypt and Syria the first Pluvial period is double, corresponding
to the Gunz and Mindel glaciations, with an intervening phase of
feeble desert conditions, during which, however, the rainfall
remained greater than the present. The second stage, corresponding to
the Mindelian, indicates very great activity; at this time the Jordan
Sea (Dead Sea) reached its greatest area, extending to the northern
end of the Sea of Tiberias.
Conditions in Egypt at this time are very interesting. South of Cairo
the alluvial Nile muds are at most thirty to thirty-five feet thick,
and ten feet of this thickness has been deposited since the time of
Ramesis II. If the rate of deposition has been uniform, this gives a
period of only 14,000 years for the deposition of the whole thickness
of the muds. The theory put forward by Hume and Craig (British
Association Report, 1911, p. 382) is briefly as follows: The mud
deposits of the Nile valley are carried down with the flood waters
of the Blue Nile, Atbara, etc. These rivers rise in the highlands
of Abyssinia, where they are fed by the rains of the south-west
monsoon. The incidence of the monsoon is determined by a number of
factors, prominent among which is the temperature of southern Asia.
During the winter, at present, the low temperature of the Himalayan
and Tibetan region results in a great outflow of cold air, which
strikes the coast of Africa as the cool dry north-east monsoon.
During this time there is very little rain in Abyssinia. It is only
when the Asiatic land-mass warms up in summer that the south-west
monsoon is established.
But during the Glacial period, as we shall see, there was a great
development of snow and ice on the Himalayas. The result was that
winter conditions, i.e. the north-east monsoon, prevailed more
or less throughout the year, and the rivers which feed the Nile
contained only a small volume of water. Hence they lost themselves in
the desert before reaching Cairo, and the Nile in its present form
did not exist. On the other hand, the westerly winds which at present
bring a moderate winter rainfall to the coast of Syria were greatly
increased in intensity and extended further south, replacing the dry
north and south winds now occupying the Nile valley. The northerly
winds prevailing in the Nile valley in summer are associated with
the low pressure area over the neighbourhood of the Persian Gulf,
which in turn is due to the extremely high temperature experienced
there. Even at the present day the highest hills of Sinai penetrate
above the north winds into a westerly current, and a moderate fall
of temperature over the Persian Gulf would inhibit the north winds
in the Nile valley altogether and allow the westerly winds to reach
the surface. These strong westerly winds brought a heavy rainfall
to the hills, now almost rainless, between the Nile and the Red
Sea. Powerful streams descended the western slopes of these hills,
bringing great quantities of debris, which formed delta-terraces
forty of fifty feet thick where the streams debouched on to the
Egyptian plain. These are especially well developed at Oina, the
meeting place of several dry valleys from the hills, and it is
remarkable that they actually cross the present site of the Nile
valley and reach the desert on its western side, additional evidence
that the Nile was not then in existence.
These gravel terraces contain numerous stone implements of early
(pre-Chellean) types, showing that at this time Egypt had sufficient
rainfall of its own to support human life.
The moist westerly winds carried the climate of the Mediterranean
coast far into the desert. For instance, in the oasis of Khargeh, in
latitude 25°, grew the evergreen oak and other plants not now found
south of Corsica and southern France.
The Mindelian Pluvial period was followed by a long dry period
corresponding to the Chellean, when desert conditions supervened. The
Nile as we know it first appeared during this period. Terraces were
formed on the sides of the valley, probably during the submergence
which produced the _Strombus_ beaches of the western Mediterranean;
these contain Chellean implements. During the succeeding elevation
the Nile cut its bed below the present level.
The Rissian glaciation of northern Europe is represented in Egypt by
a second rainy period, the Lesser Pluvial period. Rain again fell
on the Red Sea hills, forming a newer set of gravel terraces, but
these are much smaller than the great Mindelian terraces. No terraces
are known representing the Wurmian period, and the country does not
seem to have been inhabited at this time. Probably the climate was
semi-desert, with not enough rainfall of its own to support human
life, and yet without the fertilizing Nile floods to enable human
life to exist without rainfall. As has been said, the present regime
did not begin until the last glaciation was nearly over, about 12,000
B.C.
BIBLIOGRAPHY
Gignoux, M. “Les formations marines pliocènes et quaternaires de
l’Italie du Sud et de la Sicilie.” _Ann. l’Univ. Lyon_, n.a., Vol.
1, fasc. 36, Paris, 1911, pp. 693.
Depéret, C. “Les anciennes lignes de rivage de la côte française de
la Mediterranée.” _Bull. Soc. Geol. de France_, ser. 4, Vol. 6, pp.
207-30.
Douvillé, R. “Espagne,” _Handbuch regional Geol., H. 7_, 1911.
(Includes Gibraltar and Balearic Is.)
Hume, W. F., and Craig, J. I. “The Glacial period and climatic
change in North-east Africa.” _Rep. Brit. Assoc._, 1911, p. 382.
CHAPTER VII
ASIA DURING THE GLACIAL PERIOD
The great area of Asia is at present but little explored for glacial
traces, but a certain amount of evidence has been collected, and the
data from the various mountain districts are consistent enough to map
out the general trend of the history of the continent during the Ice
Age.
The earth-movements which brought about the present configuration of
Asia were completed as regards their major details by the close of
the Tertiary period. These movements left a number of great basins
closed in on all sides by enormous mountain walls; at first all these
basins contained lakes, and the subsequent geographical history has
consisted largely in the gradual silting up of the lakes and the
development of more and more arid conditions. The fluctuations of the
Ice Age were superposed on this secular desiccation, but except in
northern Siberia the part played by glaciation in the history of the
country has been relatively small.
Consider for a moment the relief of Asia. The orographic centre may
be taken as the great Pamir plateau, the “Roof of the World,” with
an average elevation exceeding the height of Mont Blanc, diversified
by ranges of mountains exceeding 25,000 feet in places. East of this
is the great plateau of Tibet, 10,000 to 17,000 feet, bounded on the
south by the mighty Himalayas, and on the north by the mountains
of Kuen Lun. On the north the Pamir plateau is bounded by the Alai
range, passing north-east into the Tian-Shan mountains, rising to
24,000 feet in Khan-tengri. Still further north-east comes the Altai
range, with an elevation of 9000 feet. East of Lake Baikal lie a
series of ranges averaging 8000 feet in height, and passing into the
Stanovoi range of eastern Siberia and the mountains of Kamchatka.
The Himalayas, owing to their heavy snowfall derived from the
south-west monsoon, bear numerous great glaciers, but with the series
of ranges extending from the Pamirs to north-east Siberia the case is
different. These ranges all rise above the snow-line in places, but
owing to the scanty snowfall they bear at most a few small glaciers
on their northern sides, and none at all on the slopes which face
towards the deserts of western China, and in all cases the glaciation
is very slight in comparison with their elevation.
This distribution was characteristic also of the Ice Age. In the
Pamirs there is evidence of two periods when the glaciers had a
greater extent; in the first they extended to a level of 5000 feet,
in the second to 7000 feet. The present limit of the glaciers lies at
about 10,000 feet. The first glaciation was remote, for the moraines
are worn and weathered, but the second was much more recent, for
the moraines are fresh, and in some cases there are still masses
of “dead” ice buried beneath great accumulations of debris and
occasionally exposed by slips.
In the Tian-Shan mountains there are remains of two glaciations.
The earlier was the greater, and the glaciers descended well below
10,000 feet. This glaciation was followed by a long interval, when
the erosion of the rivers converted the U-shaped glacial valleys into
V-shaped gorges. A second glaciation descended to a level of 10,000
feet, and again developed U-valleys to this level; the end-moraines
of these glaciers are young and fresh-looking. In the Altai range
there were also two glacial periods. In the older and greater the
snow-line was depressed by 3000 feet. The glaciers attained a length
of twelve miles and descended to a level of only 3000 feet above the
sea. The second glaciation was less extensive.
So far we have been dealing with small mountain glaciers only. But
in north-eastern Siberia we find a different state of affairs. The
Stanovoi and Verkhoiansk mountains were heavily glaciated, and during
the first glaciation were probably the centre of an actual ice-sheet
similar to that of Scandinavia. The ice descended the valleys of
the rivers Yana, Indijirka and Kolyma and covered the New Siberian
Islands, which were at that time connected with the mainland. The
upper valley of the Lena was also heavily glaciated by an ice-sheet
moving southward, probably from the Patom highlands. When this
glaciation drew to a close the source of supply among the mountains
ceased, and the ice on the lowlands and in the lower parts of the
river valleys was left stranded as “dead” ice. When the mountains
became free of ice, the re-born rivers carried great quantities of
moraine-clay and other debris with them, and flooding the ice-surface
over wide areas deposited their load above the ice. In course of
time the remains of the ice-sheet were deeply covered by a layer of
earth and stones, which prevented the ice from melting and preserved
it to the present day. This is the probable origin of the well-known
“fossil ice” of Siberia. Other theories have been put forward, such
as the freezing of ground water during the winter, but none are
satisfactory, and that given here was generally adopted by Russian
geologists.
During the long warm interglacial which followed, the surface of the
thick earth-layer covering the ice bore low-growing herbage in the
same way as any other earth-surface. (A parallel to this is found
in Alaska, where the glaciers terminate among the forests, which
actually grow over the moraines covering their snouts.) The rivers
cut their way down through the earth and ice, exposing ice-cliffs,
which were quickly buried by talus from above. The mammoth and
woolly rhinoceros roamed the land, and their tusks remain in great
numbers as the “fossil ivory” of Siberia and the Arctic Ocean. Still
more remarkable is the fact that mammoths have been found buried
entire, and preserved by the frozen ground to the present day. It is
difficult to say how the animals reached such a position, but most
probably they sank into swamps formed during the summer and were
quickly frozen.
In western Europe the mammoth and woolly rhinoceros are regarded as
indications of severe climate, but their presence in north-eastern
Siberia in large numbers is evidence of a climate probably somewhat
warmer than that of the present day, especially as regards the length
of the vegetation period. Probably the winter snowfall also was less
than now. It is difficult to see how the fauna could have moved from,
say, the New Siberian islands into a warmer climate each winter, for
the winter climate becomes markedly more severe as one penetrates
south from the Arctic coast into the interior. It is possible that
the mammoth and woolly rhinoceros hibernated during the winter.
After this interglacial there came a recrudescence of glacial
conditions. In this case, however, the Stanovoi and Verkhoiansk
mountains and the Patom highlands were not buried in an ice-sheet,
but became the centre of great valley glaciers, which reproduced the
well-known glacial phenomena—corries, glacial terraces, U-valleys,
etc. The ice extended down the great river valleys, leaving a typical
moraine landscape on either side, and again reached the New Siberian
islands. In course of time the climate ameliorated, again commencing
in the south, and again the ice of the glaciers was buried. In the
New Siberian islands the happenings are summarized very expressively
by a rock-section described by Vollossovitsch. The bottom of the
section is formed by the older layer of “fossil ice.” Above this is
a sandy clay with remains of meadow vegetation and shrubs, followed
by a fine clay with remains of alder and white birch, and the bones
of mammoth and rhinoceros. Above this comes another layer of “fossil
ice,” followed by clay with the dwarf birch, Arctic willow, and bones
of musk ox, horse and later mammoth. After this the coastal regions
sank beneath the sea for a time and marine clays were formed in a
climate somewhat warmer than the present. When the land rose again
the conditions resembled those now prevailing.
Though not part of Asia, reference may be made here to the
glaciation of Spitzbergen, which runs strictly parallel with that of
northern Siberia. The first glaciation was of the “ice-sheet” type,
originating in the region north of Storfjiord, filling the whole
of that fiord and extending south of South Cape. Barentz Land and
Stans Foreland were at least partially ice-covered. The ice-floor of
Spitzbergen, which resembles that of Siberia, may have originated
during this glaciation. This was followed by subsidence to 230
feet below present level, and the ice retreated, giving place to
an “interglacial,” during which frost was very active and largely
obliterated the traces of the ice-sheet. This “interglacial” was
followed by a second extension of the ice, which affected the valleys
and fiords only, leaving the plateaux free. This again was followed
by subsidence and a warm period.
In southern Kamchatka there was a great development of ice, but
in the form of a network of glaciers rather than of an inland
ice-sheet. In the east the ice reached the sea, but on the west it
left a zone forty to sixty miles broad, and up to a thousand feet
high unglaciated, so that there was the same difference then as now
between the rainy east side and the drier west side of the peninsula.
The present snow-line in the centre of southern Kamchatka is about
5500 feet, and at the maximum of the glaciation it must have been
fully 3000 feet lower.
This glaciation was followed after an interval by a second, which was
confined to the mountains. The moraines of this glaciation are much
fresher than are those of the earlier one.
In Japan the mountains were only just high enough for glaciers to
develop in the north. The moraines are old and weathered, and their
meaning has been disputed; but recent work by Simotomai and Oseki
seems to have established their glacial origin. The depression of the
snow-line necessary to produce them—about 3000 feet—fits in very well
with that observed in adjoining parts of the continent. The phenomena
were confined to small hanging glaciers in the Hida mountains which
cut out corries and descended to a level of about 8000 feet, leaving
small morainic ridges. This glaciation was probably contemporaneous
with the earlier and greater glaciation of Siberia. To the succeeding
interglacial may be attributed the marine deposits found near Tokio
containing corals, at present living some distance further south. No
trace of any subsequent glaciation of Japan has yet been found.
J. S. Lee has recently called attention to the existence of a
glaciated area in northern China, the evidence for which consists of
moraines and striated slabs found in southern Chi-li, and a glaciated
valley with travelled boulders in the north of Shan-si. The glacial
deposits in Chi-li are closely associated with a layer of quartzite
pebbles which continues southward beneath the loess on the eastern
side of the Tai-hang range, and is attributed to either torrential
rain or the melting of glaciers. J. Geikie had long ago stated that
there once existed ice-masses all over northern China, and considered
that the ice came from the Himalayas. This origin is impossible, the
probable source of the ice being the Yablonoi mountains in southern
Mongolia.
In the Himalayas the glaciers formerly had a much greater extension.
The glaciers at present extend downwards to 11-13,000 feet, but old
moraines are found at 7000 feet, and near Dalhousie on the southern
slopes of the Dholadar range to 4740 feet.
On the northern side of the Himalayas there was a great development
of ice over Tibet, but there was not a real ice-sheet such as
occurred further north. Oldham records three separate periods of
glaciation in Kashmir, but it is not yet possible to discuss the
glacial history of the Himalayas in detail. The latter is likely to
prove complicated, since the range is still rising, and has probably
been doing so either continually or intermittently throughout the
Quaternary.
The great development of ice in Tibet, which is now semi-arid,
owing to interception of the rain-bearing winds by the Himalayan
range, suggests a considerable alteration in the present
meteorological conditions. The Tibetan snowfall was probably due to
the Mediterranean storms, which now give a small winter rainfall
in north-west India, and which during the Glacial period greatly
increased in strength and frequency and occurred throughout the year
(Chapters IV and VI), giving the Pluvial period of North Africa.
These storms would pass across Persia and continue to the north of
the Himalayas, probably breaking up over the Tibetan plateau.
It is evident that, taking northern Asia as a whole, there have
been two general glaciations, of which the first was the more
severe, separated by a long interglacial, during which, in Japan
at least, the climate became appreciably warmer than the present.
The first glaciation is related to elevation in the Arctic basin,
which closed Bering Strait and united the New Siberian islands to
the mainland. It was almost certainly contemporaneous with the first
glaciation (Gunz-Mindel) of Europe. The ice began as glaciers on the
mountains as in Scandinavia, but, owing to the scanty supply of snow,
developed more slowly and only reached the dignity of ice-sheets
in north-east Siberia. Then followed subsidence below the present
level, wider opening of the Bering Strait, warm ocean currents and
a long interglacial. After this there was again elevation and a
re-development of ice-sheets, but apparently once only, and not twice
as in Europe. This glaciation probably corresponded in point of time
more or less with the Rissian, for the post-glacial dry of central
Asia appears to have been of enormous period length.
There is one other phenomenon which must be considered in connexion
with the glacial history of Asia, and that is the loess. Loess has
already been referred to in connexion with the glaciation of Europe,
but in China its development is much greater. Richthofen, who first
studied this deposit attentively, and to whom we owe the æolian
theory of its origin, found that it was formerly deposited in China
over a much greater area than that over which it is accumulating
at present, and attributes this cessation of growth to the heavier
rainfall brought by the Glacial period, which enabled the rivers
to cut back their valleys and drain some of the mountain basins,
formerly enclosed. He considered that loess can accumulate more
rapidly in a closed basin, where occasional floods leave behind
them layers of bare sand and mud, easily dried to dust, than in a
well-drained river valley where floods are rare.
In western Asia outside the limits of glaciation we have further
evidence of at least one Pluvial period in the former far greater
extent of all the enclosed lakes, due partly to greater precipitation
and partly to decreased evaporation. The Caspian Sea and Aral Sea
were extended to several times their present size and united into a
single sheet of inland water. Lake Lop-Nor was greatly increased in
size, and many of the desert basins, at present dry, were the sites
of salt lakes. This is especially the case in central Persia, where
there were large salt or brackish lakes.
These Pluvial conditions have not yet been correlated with the
glaciations of Asia, but, by analogy with the conditions in America
discussed in the next chapter, there is little doubt that they were
contemporaneous with one at least of the glaciations, and probably
there were two main Pluvial periods coinciding with the two Glacial
periods. At Baku, on the shores of the Caspian river, Pumpelly has
found old shore lines at heights of 600, 500, and 300 feet above the
present level of the water. Still more interesting are the conditions
found by Sven Hedin in the Kavir basin of Persia. Here there are
lacustrine clays and silts referable to a Pluvial period covered by
beds of almost pure salt, suggesting a rapid and complete drying up
of the lake. Above this again are further silts indicating a return
of Pluvial conditions. In addition to this the succession of silts
and clays show that there were several minor fluctuations superposed
on the main wet periods, giving ten moist phases altogether.
BIBLIOGRAPHY
Many of the more important references are in Russian, and for these
reference is made to summaries in other languages.
Sevastianov, D. P. “On the glaciation of the extreme north-east of
Siberia.” _J. 12 Congr. Russ. Nat._, Moscow, 1910, No. 10, p. 491.
(Russian, see _Geol. Centralblatt_, 15, p. 205.)
Riesnitschanko, W. “Ancient and modern glaciers of the
south-western Altai.” _Mem. Russ. Geogr. Soc._, 48, 1912, p. 357.
(Russian, see _Geol. Centralblatt_, 19, p. 131.)
Komarov, W. “On the Quaternary glaciation of Kamchatka—Travels in
Kamchatka in 1908-9,” Vol. 1 (Russian, see _N. J. Min._, 1915, Pt.
2, P. 117).
Merzbacher, G. “Zur Eiszeitfrage in der nordwestlichen Mongolei.”
_Peterm. Mitt._, Gotha, 57, 1911, p. 18.
Prinz, Gyula. “Die Vergletscherung des nördlichen Teiles des
zentralen Tien-schan-Gebirges.” Wien, _Mitt. K. K. geogr.
Gesellsch_, 52, 1909, p. 10.
Obrutschev, W. A. “Geological map of Lena gold-bearing region.” St.
Petersburg, 1907. [Text in Russian; see _Geol. Centralblatt_, 12,
pp. 507-9.]
Simotomai, H. “Die diluviale Eiszeit in Japan.” Berlin, _Zs. Ges.
Erdkunde_, 1914, p. 56.
Oseki, K. “Some notes on the glacial phenomena in the North
Japanese Alps.” Edinburgh, _Scot. Geogr. Mag._, 31, 1915, p. 113.
Lee, J. S. “Note on traces of recent ice-action in North China.”
_Geol. Mag._, 59, 1922, p. 14.
Burrard, S. G., and Hayden, H. H. “A sketch of the geography and
geology of the Himalaya Mountains and Tibet.” Calcutta, 1907-8.
Hogböm, G. “Bidrag till Isfjordsomradets kvartargeologi.” _Geol.
Foren. Stockholm Forb._, 1911. (Spitsbergen; résumé in German.)
Richthofen, F. Freih. von. “China.” 5 Vols., 1907-12. (Loess, see
Vol. 1, p. 74 ff.)
Hedin, Sven. “Some physico-geographical indications of post-Pluvial
climatic changes in Persia.” _Internat. Geol. Congr._, Stockholm,
1911. “_Die Veränderung des Klimas._”
CHAPTER VIII
THE GLACIAL HISTORY OF NORTH AMERICA
The glaciation of North America was even greater and more complicated
than was that of Europe. It spread from three main centres, the
Cordilleran or Rocky Mountain centre, the Keewatin centre west of
Hudson Bay, and the Labradorean centre. Vancouver Island in the west
and New Brunswick and Newfoundland in the east, were also independent
centres of glaciation, and ice from the latter may have reached the
coast of the United States in places. The ice covered an area of
about 4,000,000 square miles, and the main ice-sheet extended to 38°
N., or twelve degrees further south than the Scandinavian ice-sheet.
Nine stages are recognized by American geologists, though opinion is
divided as to whether all the stages of “deglaciation” represent real
interglacial periods. The sequence is as follows:
1. Nebraskan, Jerseyan or pre-Kansan glaciation.
2. Aftonian deglaciation.
3. Kansan glaciation.
4. Yarmouth deglaciation.
5. Illinoian glaciation.
6. Sangamon deglaciation.
7. Iowan glaciation.
8. Peorian deglaciation.
9. Wisconsin glaciation.
On the other hand, in the northern part of the Rocky Mountains there
is evidence of only two Glacial periods, separated by a single long
interglacial, though perhaps the second glaciation was double.
Further south, out of reach of the main ice-sheets, there are traces
of two and in places three separate developments of valley glaciers
resembling those of the Alps.
As in the case of Europe, the literature of the subject is extensive
and conflicting, but the following summary of the course of events
represents the views of most moderate American geologists.
The Quaternary period opened with extensive elevation of the whole
North American continent, which raised the Rocky Mountains several
thousand feet above their present level and extended the continental
area over much of the northern archipelago. In the east Newfoundland
is considered to have been raised at least 1000 feet, a movement
which converted the banks into dry land and interposed a large
cold area in the path of the moisture-bearing southerly winds. As
in northern Europe the high mountains of the west were the first
to develop large glaciers, which coalesced into an ice-sheet,
filling the valleys and rising up the slopes of the mountains until
it reached a thickness of 5000 feet. In Puget Sound the ice was
4000 feet thick, but seawards the slope is very rapid and the ice
was unable to extend far from the shore. This ice-sheet extended
south-eastwards some distance into the United States, forming
the first ground-moraine of that district. Probably while this
Cordilleran glaciation was still in progress ice began to spread
outwards also from the Labradorean centre, forming the oldest drift
of that region. These oldest deposits are, however, not yet well
understood.
This oldest boulder-clay is separated from the moraines of the main
glaciation near its southern limit by river gravels containing the
remains of mollusca and large herbivorous mammals—extinct species of
horse, the hairy mammoth of the old world (_Elephas primigenius_),
and two other extinct species of elephant, and also the true American
mammoth. This is the Aftonian fauna, which has been claimed as
evidence of an Interglacial period. That it evidences a retreat of
the ice-edge in that particular region is certain, but that the
climate became really temperate is very doubtful. More probably it
corresponds to the Gunz-Mindel “interglacial” of the Alps, and was
formed when the Cordilleran ice-sheet was retreating, but before the
Keewatin sheet had reached its maximum.
The Aftonian stage was followed by the Kansan glaciation, when the
ice-sheets reached their maximum area over the greater part of
North America. The chief centre of glaciation at this stage was the
Keewatin, west of Hudson Bay. While it is certain that the Keewatin
centre reached its maximum later than the Cordilleran, geological
opinion in America is divided as to whether or no the two ice-sheets
ever coalesced, but it is difficult to understand how an independent
ice-sheet could have grown up on the comparatively low ground of
the Keewatin centre. Most probably the course of events here was an
exact parallel of that in the better-known Scandinavian region—the
Cordilleran ice-sheet extended eastwards over the lower ground until
a glacial anticyclone developed east of the Rockies. When this
happened the supply of moisture to the western part of the ice-sheet
fell off somewhat, and the eastern part took on an independent life,
ultimately becoming the main centre of glaciation. It was while these
changes were in progress that the southern limit of the ice retreated
northwards and the “Aftonian” deposits were formed.
The next stage (Kansan) occurred when the ice from the Keewatin
centre spread outwards in all directions, and in the south reached
the maximum limits of glaciation in America. In the west this sheet
overlapped on to the ground-moraine of the former Cordilleran ice,
but the Rocky Mountains were too far away and too high for Keewatin
ice to dominate them and overflow them from east to west. Instead
these mountains must have maintained an extensive glaciation of
their own.
With the growth of the Keewatin centre the Labradorean also
decreased, but more slowly, and this change was not associated with a
retreat of the southern ice-edge, so that there was no corresponding
“interglacial” in the east of the United States. The moraines of
these older glaciations resemble those of the early ice-sheets of
Europe in presenting only featureless level surfaces of boulder-clay
without morainic ridges, lakes and the other characteristics of
ice-bearing surface detritus, and there is no doubt that conditions
at the southern edge were similar—the climate was severe in winter,
but not insupportable in summer. At the same time it was decidedly
more severe than the present, even as far south as Florida, where
there are colonies of northern plants, which migrated southwards
during the Ice Age, still living on local cold slopes with a
northerly aspect. After the maximum of glaciation the disappearance
of the ice took place gradually and chiefly by ablation, for there
are none of the extensive river gravels and flood terraces which we
should find had the melting been rapid. It is only in the valleys
of the Rocky Mountains that such deposits occur, testifying to
conditions such as obtained in the Alps.
The succeeding Yarmouth stage of deglaciation was very long,
corresponding in this respect to the Mindel-Riss interglacial of
Europe. The Kansan moraine was weathered to a depth of ten or twenty
feet, and four-fifths of its surface was removed by the erosion of
streams and rivers. In the mountain districts the side streams which
had been left occupying “hanging valleys” by the over-deepening of
the heavily glaciated main valleys, had time to cut out uniformly
graded broad V-shaped valleys descending to the level of the
main stream. In the Great Basin also, where the periods of high
water-level are considered to correspond to the main glaciations,
the interval of low water corresponding to the Yarmouth stage
was very long. A rough estimate of its length is about 200,000
years—somewhat shorter than the Mindel-Riss. Actually, though the
Kansan and Mindelian glaciations were approximately contemporaneous,
the subsequent recurrence of glaciation in America appears to have
preceded slightly that in Europe.
Of the climate of this stage we have unfortunately little evidence.
Old land surfaces of this age are known, containing deposits of peat
and bones of the wood rabbit and common skunk, but both of these
animals have a wide range. Perhaps the climate resembled the present
during most of the period; there is no evidence that it was ever
warmer, and it appears quite likely that ice-sheets maintained their
existence in the far north through the whole of this stage.
After this interglacial there set in a period of renewed elevation
in the Rocky Mountains and in the Labrador-Newfoundland centres,
which brought about a recurrence of the glaciation. In the Rocky
Mountains the ice was not so thick as in the preceding stage, but all
the valleys were occupied to a considerable depth and the ice spread
out to the eastward. The Labrador ice-sheets also developed again,
forming the Illinoian glaciation, the moraines of which are found as
far west as Illinois, but no moraines are known of this age due to
the Keewatin ice-sheet. The latter developed later, and is classed by
some American geologists as a separate glaciation, the Iowan, which
is only certainly found in northern Iowa, but may be represented
further east by a thin sheet of boulder-clay overlapping the
Illinoian moraine. The supposed interglacial between the Illinoian
and Iowan, the “Sangamon Stage,” is represented only by land surfaces
formed of the Illinoian moraine and covered by the loess or locally
by the equivalent of the Iowan moraine, and there is no evidence that
the ice-edge retreated far. Other American geologists, including
F. Leverett, do not recognize the existence of a separate Iowan
glaciation, and as the amount of weathering and denudation undergone
by the two moraines differs very little, this seems the more natural
view. The natural explanation seems to be that this was another case
of “glacial piracy,” the Keewatin ice-sheet, owing to its lesser
snowfall, developing more gradually, and finally diverting the supply
of moisture from the Labradorean ice-sheet, until it reached a
maximum after the latter was already on the wane. Both these sheets
of drift present similar flat features to the Kansan sheet, without
morainic ridges.
Leverett’s interpretation of the succession is as follows: The third
(Illinoian-Iowan) glaciation was followed by a period of moist
climate, when peat-bogs were formed on level poorly-drained surfaces,
while elsewhere coniferous forests developed. This was followed by a
period of dry steppe-like conditions with a cold temperate climate,
when the great American loess sheet was deposited. This loess sheet
extends northwards, overlapping the Iowan moraine, and in places
passing under the Wisconsin drift. The material has come from the
west, and probably most largely from the dry plains east of the
Rocky Mountains, from which it diminishes in thickness eastwards.
But unlike Europe this phase of steppe conditions was followed in
America by a definite interglacial, when the climate seems to have
become rather warmer than the present. In the northern States an old
land-surface formed on the loess, and, termed the Peorian stage, is
overlain by the Wisconsin drift; but near Toronto, on the shores
of Lake Ontario and in the Don valley, the gap represented by this
land-surface is partly filled by a remarkable series of lacustrine
deposits known as the Toronto stage. The Lake Ontario beds indicate
a climate slightly colder than the present, but the Don valley beds
contain plants and animals living in the central States, and refer to
conditions more favourable than those now found in the district.
The duration of this interglacial has been worked out in a
remarkable way by A. P. Coleman, who on the basis of wave-action
estimated it as 62,000 years, which agrees very closely with the
60,000 years found by Penck and Brückner in the Alps. This period
was not long enough for streams in the “hanging valleys” to cut
out uniformly graded valleys down to the main rivers, and was
consequently much shorter than the preceding interglacial.
The last glaciation of North America was the Wisconsin, which closely
resembles the Wurmian of Europe both in its relations to the older
glaciations and in the rough topography and unworn character of its
moraines. It extended within the limits of the Kansan drift across
fully two-thirds of the continent, from Nantucket and Cape Cod
through Long Island, northern New Jersey, Pennsylvania, southern New
York, Ohio, Indiana, Illinois, Michigan, Wisconsin, Minnesota, Iowa
and the Dakotas, Manitoba, Saskatchewan and Alberta. At the same time
the Cordilleran centre probably bore increased local valley glaciers.
Like the Wurm glaciation, the Wisconsin was double. The older
moraines are well-marked, and in places are covered by a foot or two
of loess, though this deposit reaches nothing like the thickness of
that overlying the moraines of the earlier glaciations. The moraine
under this loess is very little weathered, so that the time interval
was very short; possibly this loess is redistributed older loess
associated with glacial east winds. The ice of the first glaciation
melted very slowly and there is very little gravel outwash to the
moraines. But “after the Wisconsin ice-sheet had reached a position a
little outside the limits of the Great Lakes the retreat became much
more rapid, and large outwash aprons were formed from which valley
trains of gravel led far down the drainage lines. From this position
... the moraines are practically free from loess-like silts.”[4]
From this point onwards the glacial history of America is one
of irregular retreat, with occasional halts or even readvances
resembling those of the Scandinavian ice. Banded clays are found
similar to those used so successfully by Baron de Geer in dating
the retreat stages of Scandinavia, and this geologist has recently
been investigating them, but until his results are worked out no
correlation with Europe can be attempted.
A natural clock of another type is provided by Niagara Falls, which
are cutting their way back up the gorge at a rate which has been
definitely ascertained. Taking into account the varying amounts
of water which have passed over the falls at different stages of
post-glacial geography, the duration since the region became free of
ice has been calculated at about 20,000 years, which agrees closely
with the time elapsed since the Scandinavian ice-sheet left the North
German coast.
Before leaving North America it is necessary to give a brief
account of the phenomena outside the main centres of glaciation,
and especially of the history of the Great Basin between the Sierra
Nevada and Wasatch Mountains. The lowest levels of this basin are
at present occupied by several salt lakes without outflow, of
which the largest is the Great Salt Lake, the level of the water
being determined by the balance between inflow of the rivers and
evaporation from the surface. Twice in the past this balance has
been decidedly more favourable, and then the lakes grew to many
times their present size. The two greatest of these old lakes have
been fully described under the names of Lake Bonneville (of which
the Great Salt Lake is a vestige) and Lake Lahontan, further to
the west. The investigations have shown that before the Glacial
period, and extending back into an unknown past, there was a period
of great aridity. To this succeeded a long period of high water,
during which, however, neither of the lakes overflowed. This stage
was followed by a very long period of great aridity, during which
the lakes dried up completely, and all their soluble matter was
deposited and buried by alluvial material. This period was followed
by a return of moist conditions, during which the water reached a
higher level than before, and in the case of Lake Bonneville actually
overflowed into the Snake river, cutting a deep gorge. This period,
however, was shorter than the preceding moist period. It was followed
by an irregular fall interspersed with occasional slight rises,
but ultimately both lakes descended below their present level and
probably again dried up completely. Both lakes suggest that this low
level was followed by a third rise to a height very slightly above
the present level, followed by a slow fall in recent years.
The relations of the periods of high water to the glaciations are
not clear in these large lakes, but in the Mono Basin, a small
basin further west, there is no doubt that the two were almost
contemporaneous, high water accompanying the maxima of glaciation and
extending some way into the retreat phase. The very long interval
between the first and second period of high water, several times that
since the second period, agrees with this correlation. We find then
that south-west of the main glaciated area there was a district of
greater precipitation or less evaporation, or more probably both.
This is confirmed by the valley moraines of all this region—Sierra
Nevada, Uinta and Wasatch mountains, Medicine Bow Range of northern
Colorado, etc., all of which indicate two glaciations, of which the
first was the greater, separated by a very long interval. In several
ranges the moraines of the second glaciation are double, and some
geologists consider that there were three Glacial periods in these
regions.
In the extremely arid region of Arizona, on the other hand, which is
considerably further south, the evidence of the Gila conglomerates
indicates that while frost was very active, the increase of
precipitation, though undoubtedly present, was comparatively slight.
This shows that the climatic balance was not greatly disturbed, the
chief effect being an important lowering of temperature, probably
due to cold northerly winds. The Gila conglomerates are double,
separated by a period representing present-day conditions.
Summing up the evidences of glacial climate in North America, we find
a striking similarity to Europe. In the north elevation and increased
land area caused the development of large ice-sheets, which appeared
first in the mountainous regions with a heavy snowfall, and later
spread over the drier plains and plateaux of the interior. This first
glaciation was long and complex. Owing to the anticyclonic conditions
which formed over the ice, the rain- and snow-bearing depressions
were forced to pass further southward, causing greater snowfall on
the mountains and high water-level in the lake basins. This greater
snowfall, together with the cold conditions due to the existence
of the ice-sheets to the north, caused the development of mountain
glaciers south of the main glaciated region. In the east there were
cold northerly winds which carried a severe climate as far south
as Florida. This Glacial period was followed by subsidence, and a
long spell of dry, moderately warm climate lasting perhaps 200,000
years, after which elevation and glacial conditions again set in.
These conditions were not so severe as the first, and their duration
was much less, while they were broken up by several intervals of
temporary recession of the ice, one of which, corresponding to
the Riss-Wurm period, lasted for 60,000 years, and perhaps should
be considered as an “interglacial.” This period was marked in its
early stages by the deposition of the curious æolian deposit known
as “loess,” indicating steppe conditions. After the last glaciation
there set in a stage of irregular retreat.
BIBLIOGRAPHY
Leverett, F. “Comparison of North American and European glacial
deposits.” _Zs. f. Gletscherkünde_ 4, 1910, pp. 280, 323.
Wright, W. B. “The Quaternary Ice Age.” London, 1914, Chs. 8-9.
Attwood, W. W. “The glaciation of the Uinta Mountains.” _J. Geol._,
15, 1907, p. 790.
Henderson, J. “Extinct glaciers of Colorado.” _Colo. Univ.
Studies_, 3, 1905, p. 39.
Gilbert, G. K. “Lake Bonneville.” Washington, _U.S. Geol. Survey
Monograph I_, 1890.
Russell, I. C. “The geological history of Lake Lahontan.”
Washington, _U.S. Geol. Survey Monograph XI_, 1885.
Coleman, A. P. “An estimate of post-Glacial and interglacial time
in North America.” _Rep. 12 Internat. Congr. Geol._, 1913, p. 435.
CHAPTER IX
CENTRAL AND SOUTH AMERICA
The scarcity of data which was bewailed in dealing with Asia is still
more marked in the case of South America, and it will be necessary
to present the glacial history of that continent in the barest
outline only. This is the more unfortunate as the chain of the Andes,
extending from north of the equator to high southern latitudes, is of
enormous importance in glacial theory, and especially in the question
of simultaneity of glaciation in the two hemispheres.
The beginnings of glaciation in South America are obscure. The
distribution of animals shows that towards the close of the Tertiary
the Falkland Islands were greatly elevated and were united to Tierra
del Fuego and Patagonia, and this enlarged land area was connected
in some way with Australia and Tasmania, but the mode of this latter
connexion is not definitely known. This question will be discussed
more fully in Chapter XI; it is sufficient to say here that the
amount of elevation may have reached 12,000 feet in Tierra del Fuego.
Equatorwards the elevation diminished, and near the equator the land
probably lay somewhat lower than now.
In South Georgia the present glaciers greatly expanded, until
practically the whole island was buried in ice, and the same is
true of the Falkland Islands and Tierra del Fuego, only the highest
peaks remaining above the ice. In the latter district there is
some evidence of two glaciations separated by an interglacial, the
earlier glaciation being due to a regional ice-sheet and the later
to smaller valley glaciers. The intricate coast-line of the Falkland
Islands and Tierra del Fuego points to fiord erosion by ice which
extended well beyond the present limits of the land, and can only
have occurred during considerable elevation. As to the character of
the interglacial, little is known. In the Falklands there is a bed of
black vegetable soil full of tree-trunks, indicating the existence of
luxuriant forests and a temperate climate. This deposit is overlain
by boulder-clay, and may be either interglacial or pre-glacial, but
since it was formed when the land stood at a comparatively low level,
while we have reason to believe (see Chapter XII) that during the
close of the Tertiary period these islands were greatly elevated,
it is probably an interglacial formation, and indicates a great
amelioration of climate. In Gable Island, Tierra del Fuego, Halle
found beneath boulder-clay a Quaternary fauna of barnacles and marine
mollusca indicating a climate slightly warmer than the present, and
this probably belongs to the same period. To the concluding stages
of the Glacial period in the Falklands belong the curious “stone
rivers,” great streams of moss-grown boulders which fill the valleys,
and under the influence of temperature changes are probably still
slowly advancing.
Passing further north to the Andes, between 39° and 44° south
latitude, the glaciation was not so severe, and its records are
therefore clearer. The first result of elevation was the cutting of
deep canyons by the rivers. This was followed, possibly without much
further elevation, by a fall of temperature, which in this connexion
may be attributed to the extension of the Antarctic and Tierra del
Fuego ice-sheets. Glaciers now developed and spread down the canyons,
leaving moraines of great volume and height, associated with all
the other criteria of glaciation. The snow-fields from which these
glaciers originated lay between 5000 and 6000 feet above the sea,
and the snow-line lay at about 3000 feet instead of above 6000 as at
present.
This glaciation was followed by a long interglacial, during which the
glaciers retreated to the highest summits of the Andes. The length of
this period is indicated by the fact that the earlier moraines have
been eroded to such an extent that they no longer present distinctly
the typical features of glacial topography, while the materials of
which they are composed are decayed to somewhat the same extent as
the older moraines of North America, the granite boulders especially
being rotten and friable. This interglacial was followed by a
re-development of the glaciers, but to nothing like the same extent
as formerly; their moraines are smaller and fresh-looking, indicating
that this glaciation was comparatively recent.
Still further north, in latitude 20°-25° S., we come to a region of
very slight snowfall, where the snow-line lies higher than anywhere
else on the face of the earth. The glaciation here was comparatively
unimportant, the snow-line descending only 1600 to 2500 feet. Here
Keidel found moraines of three glacial advances, and from his
description it appears probable that the earliest and greatest was
separated by a considerable interval from the two younger, the
interglacial between which was short and not characterized by a
return to present-day climatic conditions, since during this interval
there was very little weathering. Probably we have here to do with
two glaciations, of which the second was double. In fact, some
writers have described no less than five glacial advances in the
Argentine Andes, but most of these are probably merely retreat stadia.
In Peru, W. Sievers reports the existence of two glaciations
separated by a considerable interval. The present limit of the
glaciers is about 15,200 feet; during the first glaciation they
descended to about 11,000 feet, and during the second to 12,800
feet. The evidence is very complete. In Ecuador, H. Meyer records
a similar bipartition. The oldest glaciation is represented by
trough-like valleys, enormous gravel terraces, and old moraines
much weathered; the limits are far below the present limits of
glaciers, but have been much obscured by subsequent erosion. This
glaciation was followed by a long period of steppe climate resembling
the present, during which the loess-like Cangagua formation was
deposited. This in turn was followed by a readvance of the glaciers
to a level about 2700 feet below the present limit. This glaciation
is associated with crescent-shaped moraines, corrie lakes,
hanging valleys and gravel terraces, covered with vegetation, but
otherwise fresh-looking. The snow-line lay about 1600 feet below
the present. Probably during the first glaciation the Andes were
invaded by numerous mountain plants and animals related to North
American forms—a valuable piece of evidence which indicates that
the glaciation was contemporaneous with that in North America. In
Columbia and Venezuela there are traces of Glacial periods, but these
have not yet been studied in detail. The most northerly evidence of a
Glacial period comes from the Sierra Nevada de Santa Maria, near the
north coast of Venezuela in 11° N.
Except in Tierra del Fuego and Patagonia the ice did not extend far
from the mountains. But in the eastern Argentine there is a great
series of Quaternary deposits known as the Pampean. This formation
covers 200,000 square miles, and consists of at least ninety feet of
fine loam without a single pebble (except for a few thin calcareous
layers), but containing large numbers of complete skeletons of
mammals. It raises several interesting problems. Apparently it
represents the whole course of the Glacial period. By some geologists
it is considered to be a delta deposit of the combined Parana and
Paraguay rivers, but the absence of mollusca, except in a marine
intercalation near its summit, is against this view, and Steinmann
attributes it to æolian agencies and compares it to the loess of
Europe and North America. If this view is correct the Pampean
represents steppe conditions prevailing on the equatorial side of the
Patagonia-Falkland Islands ice-sheet. Apparently before the incoming
of the greatest cold the Pampas were in part at least forest-clad,
for in the older beds are found peculiar forms of ground-sloths
which were adapted for forest life and have been found also in
cave-deposits of Brazil. At the maximum of glacial conditions the
Pampas probably had a steppe climate, but the disappearance of the
forests is to be attributed rather to drought than to cold. Elevated
glacier-bearing Andes to the west and ice-sheets to the south would
render the Argentine extremely arid, and this accounts for the
gradual extinction of so many giant forms whose remains are found
in the Pampean deposits. Conditions ultimately became too severe
even for the horse, which died out in South America. The marine
transgression which left its mark near the top of the Pampean is
probably post-glacial.
In Brazil, on the other hand, there is no evidence that the climate
has ever been drier than the present, and in the semi-arid regions of
the north-east it is even probable that during the Glacial period the
climate was moister, presumably owing to the greater strength of the
rain-bearing east and north-east winds. Further west in the Andes the
existence of this wet period is borne out by the former greater size
of Lake Titicaca, and there seems to be additional evidence to the
same effect in the Chilian deserts.
BIBLIOGRAPHY
Steinmann, J. “Diluvium in Südamerika.” _Zs. d. D. Geol.
Gesellsch._, 58, 1906, p. 215.
Meyer, H. “In den Hochanden von Ekuador.” Berlin, 1907.
Sievers, W. “Reise in Peru und Ecuador, ausgeführt 1909.” München
und Leipzig, 1914.
Keidel, H. “Ueber den Anteil der Quartaren Klimaschwankungen an
der Gestaltung der Gebirgsoberfläche in dem Trockengebiete der
mittleren und nördlichen Argentinischen Anden.” _Congr. Geol.
Internat._, 12, Canada, 1913, p. 757.
Willis, Bailey. “Physiography of the Cordillera de los Andes
between latitudes 39° and 44° S.” _Congr. Geol. Internat._, 12,
Canada, 1913, P. 733.
Halle, T. G. “On Quaternary deposits and changes of level in
Patagonia and Tierra del Fuego.” _Bull. Geol. Inst._, Upsala, 9,
1908-9, p. 93.
CHAPTER X
AFRICA
The Quaternary history of Africa can unfortunately be dismissed in
a very few words. The glaciation of the Atlas Mountains has already
been referred to in connexion with the Mediterranean region. Further
south we have no great mountain chain such as the Andes extending
above the snow-line over the whole extent of the country, but merely
a few isolated peaks. Three of these, all close to the equator, are
known to show traces of a greatly extended glaciation in the past:
Ruwenzori, just north of the equator, on the borders of Uganda and
the Congo, reaching an elevation of 16,794 feet, with the present
snow-line at 15,000 feet, and glaciers extending to 10,000 feet,
formerly bore glaciers extending down as far as 5200 feet; Kenya, on
the equator in Kenya Colony, height 17,040 feet, present snow-line
about 15,000 feet, past snow-line 12,000 feet, and old moraines
at 10,000 feet; finally, Kilimanjaro, 3° S., on the borders of
Tanganyika territory, height 19,320 feet, present limit of glaciers
13,650 feet, past limit 4870 feet. Further south, the Drakensberg
Mountains, between Basutoland and Natal, were glaciated on their
higher summits. In none of these cases have the remains of more than
one glaciation been described, but the mountains are still very
little known and this negative evidence is not conclusive. In the
neighbourhood of Ruwenzori there are several peaks, which approach
12,000 feet, but these were not glaciated, pointing to a snow-line
above this level. Unfortunately the latter piece of evidence is of
doubtful validity since these mountains are volcanic and possibly of
post-glacial age; we may consider, however, that the glaciation of
the central African mountains was characterized by a great increase
in the length of the glaciers with only a slight depression of the
snow-line, conditions showing that the glaciation was due chiefly
to an increase of snowfall, and only in a minor degree to a fall of
temperature. This conclusion is borne out by the low-level beds,
which nowhere show an appreciably lower temperature, but abound in
indications of a former greatly increased rainfall. The first of
these is the former greater size of the African great lakes.
Abyssinia, as we have seen, was probably drier than at present, but
further south the rainfall must have been considerably greater. Lake
Kioga stood 600 feet above its present level and was connected with
Lake Victoria. Lake Victoria and the smaller lakes were twice their
present size, and most of the broad valleys were filled with water.
Lake Magadi is the attenuated relic of a vast sheet of water, and
other great lakes have disappeared entirely. One of these, in the
Rift valley, south of Lake Naivasha, has been mapped by Professor
Gregory and named after Professor Suess. Part of this decrease of the
lakes has undoubtedly taken place within historic times, and part may
be attributed to changes in the drainage, but there remains enough
evidence to show that some time in the Ice Age the great lakes were
very much larger than the present.
Mr. E. J. Wayland, the Government Geologist of Uganda, informs me
that in the old basin of Victoria Nyanza there are masses of gravel
which may be two or three miles in breadth, the surface of which
forms two terraces at different levels. Above the level of these is
an old peneplain with ancient beach gravels. Mr. Wayland considers
that this peneplain was formed probably during the Pliocene by the
first Victoria Nyanza occupying a basin between folds. The initial
high level was due to the want of an outlet, but may have been
amplified by other causes. The level of the lake then sank gradually
to a considerably lower level, after which it rose again nearly to
its old level and remained there for a considerable time. During this
period the great gravel deposits were formed; they contain flood
deposits, especially near their base. The level of the lake then
sank again and this part of the basin was converted into a valley
occupied by a river. Subsequently the level rose again sufficiently
to carve out the lower terrace in the gravels. Mr. Wayland considers
that the upper terrace may also represent a stage distinct from that
in which the gravels were actually deposited, but the upper terrace
may be the original surface of the gravels. Thus there is evidence of
two Pluvial periods in central Africa, of which the first, probably
corresponding with the great extension of the mountain glaciers, was
the greater. From the archæological evidence it appears to correspond
with the Mindelian glaciation of Europe.
A second line of evidence has been pointed out by C. W. Hobley. At
the entrance to Kilindi Harbour, Mombasa, there is a gap in the coral
barrier through which the fresh water from the river finds its way.
These gaps are always found opposite the mouths of rivers, and are
due to the inability of the coral polyp to live in fresh or brackish
water. In Pleistocene times the land stood some seventy feet lower
relatively to the sea, and the old channel through the reef at this
height is almost double the width of the present channel, showing
that the river then had a greater volume, i.e. the rainfall in its
basin was greater.
But Africa is noteworthy chiefly for its deserts, and the most
important evidence of climatic change is found in the deserts of
Sahara and Kalahari. From the time of the ancient Greeks it had been
believed that the Sahara was formerly the site of a great inland sea,
and the presence of this sea had even been suggested as the cause
of the Ice Age in Europe, but recent investigations have shown that
this is not so; the Sahara has been land at least throughout the
Tertiary period. There is, however, abundant evidence that during the
Quaternary the rainfall was considerably greater than the present.
The presence of numerous animals closely associated with water, such
as the hippopotamus and even the crocodile, in oases now entirely
isolated, shows that these oases were formerly connected with the big
rivers. The most definite evidence, however, comes from Lake Tchad.
This was formerly of much greater area, but Chudeau and Freydenberg
have made out a whole series of changes from desert conditions in the
Tertiary through pluvial conditions in the Quaternary back to desert
conditions of the present. The sequence is as follows:
1. A regime of dunes.
2. A slow transgression causing a long marshy period, during which
numerous plants whose remains are found lived in the period.
3. A slow regression.
4. A rapid transgression (grey loam).
5. A slow regression (clayey white loam with traces of roots).
6. A transgression (white loam).
7. Establishment of a new dune regime.
In the Chari basin east of Lake Tchad are the remains of fish and
shells, and also small pebbles of sandstone and chalcedony, which are
not local, but must have been brought from the mountains of Tibesti
by the rivers Egnei and Toro when their current was much stronger
than at present. In Senegal, south of the 15th parallel, the present
dune sands are underlain by an alluvial soil, indicating moister
conditions preceding the present climate. There is no means of dating
the moist periods indicated by these phenomena, but it is reasonable
to correlate them with the former extension of the central African
lakes.
Passing south to the Kalahari, we find evidence of a number of
moist stages separated by drier intervals, but they can apparently
be grouped into two main Pluvial periods, separated by a long
interpluvial with steppe-like conditions. One at least of these
Pluvial periods must be correlated with the former immense extension
of Lake Ngami and the Etosha Pan.
From Cape Colony, there is some evidence of moister conditions in the
past, but the Quaternary variations cannot be separated from those of
historic times.
Before leaving Africa some reference must be made to an interesting
suggestion by C. W. Hobley, as to the mechanism of climatic change
in tropical countries. He notes that the north-east and south-west
monsoons extend to a height of only a few thousand feet. Above
them are the very steady “trade winds” connected with the general
circulation of the atmosphere. In Kenya Colony these blow from east
or a little south of east. “Their effect is very marked on the high
mountains of the interior, such as Kenya, Kilimanjaro and Elgon; in
the early morning they are generally quite clear, but about 10 a.m.
the clouds sweep up from the S.S.E. and collect on the mountains and
blot them out from view for the rest of the day. These are believed
to be clouds borne inland by the trade winds, and the moisture they
carry is precipitated mainly on the south and south-east sides of
the mountains.” Hobley suggests that there was formerly a nearly
continuous ridge of high land extending north and south, and this
caught the moisture from the trade winds, so causing the Pluvial
period, the evidence for this ridge being the distribution of alpine
plants on the now isolated high mountains. An alternative explanation
is that the greater strength of the earth’s circulation during
glacial times caused the trade winds to be much stronger and also
to extend to a lower level at the expense of the monsoons, just as
the west winds extended to a lower level in northern Egypt. This
would bring a great deal more moisture to be precipitated on the
mountains, increasing the length of the glaciers and also the volume
of the rivers.
BIBLIOGRAPHY
Scott Elliott, G. F. “The geology of Mount Ruwenzori and some
adjoining regions of tropical Africa.” _Q.J.G.S._, 51, 1895, p. 669.
Hobley, C. W. “The alleged desiccation of East Africa.” _Geogr.
Journ._, 44, 1914, p. 467.
Freydenberg, H. “Le Tchad et le Bassin du Chari.” Diss. Paris, 1908.
Passarge, H. “Die Kalahari.”
CHAPTER XI
AUSTRALIA AND NEW ZEALAND
The continent of Australia has a relatively low relief, only rising
above the snow-line in Mount Kosciusko, and glacial traces have a
relatively unimportant development. The history of the region appears
to be as follows:
In late Tertiary times the shore-line lay some distance to the
east towards New Zealand, this being a relic of a much earlier
connexion between the two lands. Towards the close of the Tertiary
earth-movements set in, which elevated the mountain belt of eastern
Australia and formed a land connexion with Tasmania and the Antarctic
continent. At the same time the land to the east and the closed
basins of central Australia were also probably developed about this
time. The climate was then somewhat warmer than the present, at least
on the east coast, for the Australian barrier reef extended further
south. Probably at this time the Antarctic ice-sheet did not reach
the sea, and there was none of the floating ice which is such an
important factor in cooling the Southern Ocean.
The next stage was the lowering snow-line on Kosciusko to about 3000
feet below the present and the development of extensive glaciers,
which descended to 5500 feet above the sea, and attained an area
of 80 to 100 square miles and a thickness of at least 1000 feet.
Tasmania was also extensively ice-covered, probably by glaciers
which coalesced at low levels, forming what is known as a “piedmont”
ice-sheet, which possibly reached the sea. The lowering of the
snow-line in Tasmania is estimated as 6000 feet, corresponding to
a fall in temperature of 18° F. Probably a large part of this fall
is accounted for by the increased elevation, which may have been
several thousand feet in Tasmania and more than a thousand feet even
in New South Wales. This glaciation, which was probably dependent on
the growth of the Antarctic ice-sheet, was followed by a very long
interglacial, the duration of which has been estimated by Professor
David as 100,000 to 200,000 years. The old moraines are much
weathered and denuded, resembling in this respect the older moraines
of Europe. No information is available as to the climate of this
interglacial period. Possibly some of the Quaternary raised beaches
with warmth-loving mollusca found in unglaciated parts of Australia
belong to this period, and if so the climate was warmer than the
present for at least part of the time.
The interglacial was followed by uplift and a second much less severe
Glacial period, characterized by valley glaciers on Kosciusko and in
Tasmania, reaching the sea in places on the latter island. It was
at the close of this Glacial period that man reached Tasmania; its
conclusion is dated by Prof. David at about 10,000 years ago. It was
terminated by a period of depression below the present level with a
warm climate.
In the dry interior of Australia there is evidence that at one time,
probably during the maximum glaciation, the rainfall was heavier than
the present, and numerous lakes were developed which have now been
dry for a very long time. It is possible that the artesian water
supply of Australia, which Gregory considers to be “fossil water”
accumulated under different conditions from the present, is a vestige
of the rainfall of this period. Further north, in Java, the beds in
which the famous _Pithecanthropus_ skeleton was found, believed to be
lower glacial, contain also plant remains similar to those now found
in the Khassian mountains of Assam, one of the rainiest climates in
the world. The climate of Java during the maximum glaciation was
thus decidedly rainier, and probably somewhat cooler than the present.
An extraordinary find which may be referred to here is that of
Professor Neuhauss, who discovered giant erratics, scratched and
polished, and moraines at _sea-level_ at the western end of Huon
Gulf, New Guinea. The region is very unstable, and is known to have
stood at a very much higher level, perhaps 10,000 feet or more, in
Quaternary times, and if the moraines indicate glaciers terminating
at 10,000 feet above the sea they are explicable by a slight fall of
temperature and increase of snowfall.
Turning now to New Zealand, we find extensive glacial remains on
South Island, though not on North Island. As in so many other
countries, the Quaternary opened with great elevation, which reached
at least 1500 feet over the whole group. North and South Islands
were united with each other, with Stewart Island and probably also
the outlying islands, even including the Chatham Islands, forming a
great land-mass several times the present area of New Zealand. On the
southern part of this land-mass extensive glaciers were formed; on
the east these did not reach the present sea-level, but on the snowy
south-west they extended far below it, so that the terminal moraines
are now completely submerged; possibly they were never formed, but
the debris was floated away seaward on icebergs. Further north
moraines are found near the present shore-line at many places between
Milford Sound and Hokitika, and morainic mounds cover a large part of
the low ground. Still further north they retreat inland, and in the
Nelson Province are not found below a level of 2000 feet at the foot
of Lake Rotoiti.
In the south-east a great moraine has been described at the south end
of Lake Wakatipu and others at the north-east ends of Lakes Manapouri
and Te Anau, but none are found nearer the sea-coast. The glaciated
area of New Zealand was at least ten times the present ice-covered
area, and the Tasman, the longest glacier in New Zealand, was
expanded from its present length of 16 miles to at least 30 miles.
Much of the apparent fall of temperature shown by this glaciation
was probably due to the great elevation, but apart from this the ice
had a marked influence on climate. Outside the limits of glaciation
on the east is a thick deposit of typical loess, which extends up
to a level of 1000 feet on the flanks of the hills. The occurrence
of this loess points to a steppe climate with dry, cold, southerly
winds on the lee side of the glaciated mountains, and is probably
also connected with the increase of land area. Further north, north
of Auckland in North Island, the present treeless plains were covered
by forests; for Kauri gum, apparently very old, has been found. The
sub-antarctic islands—Campbell, Antipodes, etc.—were not covered by
either New Zealand or Antarctic ice, but were the centres of local
severe glaciations of their own.
The next stage was a great subsidence, during which the glaciers
retreated. The land sank below its present level, raised beaches
probably of interglacial age being found at various heights ranging
from 10 feet above the sea at Manukau in the centre of North Island
to 150 feet at Taranaki, 200 feet at Cape Palliser, 400 feet on
the west coast of South Island, 500 feet at Amuri Bluff, and even
800 feet in the entrances to the south-western sounds. This great
submergence was associated with the deposition of extensive gravel
deposits by the rivers.
The interglacial was followed by a second period of elevation. It is
not certain how far this went. Submerged peat-bogs have been found at
a depth of nearly 600 feet below sea-level near Canterbury, but these
may belong to the early stages of the interglacial and not to the
post-Glacial period. On the other hand, the submerged forests which
are found at many points on the coast of New Zealand are evidently
post-glacial and indicate a slight rise above present level. At the
same time there was a renewal of the glacial conditions, but the ice
was confined to the valleys and had a much less extent than in the
first glaciation. This period seems to have been followed by a slight
submergence and a temporary warm period.
BIBLIOGRAPHY
David, T. W. E. “Australasie. Les conditions du climat aux époques
géologiques.” _Rep. Congr. Geol. Internat._, 10, 1906, pp. 275-98.
Süssmilch, C. A. “An introduction to the geology of New South
Wales.” Sydney, 1914.
Gagel, C. “Beiträge zur Geologie von Kaiser-Wilhelms Land.” _Beitr.
geol. Erforsch. deutsch. Schutzgebieten_, Berlin, 1913, H. 4.
Marshall, P. “The glaciation of New Zealand.” _Trans. New Zealand
Inst._, 42, 1909, p. 334.
Salenka, M. L. et al. “Die Pithecanthropusschichten auf Java.”
_Geol. und Palæol. Ergebnisse der Trinilexpedition (1907 und
1908)._ Leipzig, 1911.
CHAPTER XII
THE GLACIATION OF ANTARCTICA
The great Antarctic continent offers a unique problem to the glacial
climatologist, for here we have a land area with the theoretical
snow-line already at sea-level, and accordingly covered with a thick
ice-sheet that leaves only a few mountain ranges and nunataks exposed
above its surface, and yet in the past these ice-sheets and glaciers
have attained a thickness several thousand feet greater, and have
extended further north. Various suggestions have been made to account
for this former extension, perhaps the most remarkable being that it
coincided with a milder and therefore snowier climate. This, however,
is untenable, for the Glacial period of Graham Land and the South
Orkneys is obviously a southward extension of the Glacial period of
Tierra del Fuego, which was obviously due to a colder climate, and
can be traced northward along the Andes into tropical regions. A more
fruitful suggestion is that as one of the most potent factors in
preventing the accumulation of snow is at present the wind, it was a
decrease in the strength of the wind which enabled the ice to reach a
greater thickness. This is probably true in a sense, the decrease of
wind force being due to a great increase in the area of the Antarctic
continent during the Quaternary.
We have seen that in the early Quaternary there was great elevation
in the south of South America and also in Australia and New Zealand.
The amount of this elevation increased southward and was very great
near the polar circle. This is borne out by considerations based
on the distribution of living and fossil animals, which point very
definitely to a land connexion between Australia and South America
in Tertiary and early Quaternary times, most probably by way of
Antarctica.
The first line of evidence is the distribution of the marsupials,
living and extinct. As is well known the chief home of this type of
mammal is now in Australia and New Guinea, but in Tertiary deposits
in Patagonia remains of extinct forms known as Dasyurids have been
found, which are allied to Australian forms, and can only have
come from Australia, probably via Tasmania. Secondly, there are
two peculiar families of fresh-water fishes, the _Haplochitonidæ_
and _Galaxiidæ_, the first common to Australia and South America,
while one species of the second is found in New Zealand, Tasmania,
the Falkland Islands and Patagonia. Thirdly, Beddard has found an
intimate relation between the earthworms of New Zealand, Eastern
Australia and Patagonia. Finally there is a curious similarity
between the slugs of Patagonia and those of Polynesia.
What is the explanation of these relationships? Assuming that
there has been a land connexion, it can have been either by way
of Antarctica or Polynesia. The earthworms cannot endure a very
severe climate, but on the other hand there is a total absence of
any tropical forms common to Australia and South America, and the
general dissimilarity of the faunas shows that the connexion cannot
have been available for a very long period. A study of the oceanic
depths suggests that the Antarctic connexion is the more probable.
A comparatively slight elevation would connect Patagonia and the
Falkland Islands with the South Shetlands and Graham Land, and an
elevation of 12,000 feet would give a large land connexion between
Australia and the opposite coast of Antarctica via Kerguelen. Forbes
even postulates an immense Tertiary Antarctica in which several forms
of animals and plants were able to evolve, but except possibly in
the case of the edentates this supposition is not necessary.
The course of events may provisionally be taken as follows: In late
Tertiary times an elevation of at least 12,000 feet in the South
Polar regions caused a great increase in the area of Antarctica,
which was united to South America on the one hand and Australia on
the other. The northern shores of this continent were far to the
north of their present position, and though the interior was very
cold the coast lands had at first a moderate temperature, and for a
short time allowed animals to migrate from Australia to South America
or vice versa. But the high mountains of the interior were already
glaciated, and ice-sheets gradually crept down their slopes. Owing
to the small precipitation the advance of the ice-sheets was slow,
but ultimately, probably in late Tertiary times, they approached
the coast, and the track along which migrations had taken place
was closed. The distribution of animals and plants shows quite
clearly that the land connexion was maintained into the period of
refrigeration. The shores of the continent being further north, the
pressure gradient between the pole and the present coast was less,
and consequently the winds were lighter. This and the diminished loss
by calving into glaciers allowed the ice to become thicker than it is
now.
Hedley apparently considers that the migrations referred to
above took place in an interglacial period, but the Patagonian
beds in which the fossil marsupials are found are Tertiary and
not Quaternary. No direct evidence of an interglacial period has
been found in Antarctica, nor, considering the intensity of the
glaciation which the country is even now undergoing, is any such
to be expected, and we can only infer from the bipartition of the
Glacial period in Australia, New Zealand and South America—which,
in New Zealand at least, was associated with submergence—that there
was probably a similar bipartition in Antarctica. Nordenskjold
states that the submarine relief showing river erosion which, in
Tierra del Fuego, was developed partly at least in the interglacial
period, is also developed in West Antarctica. It is improbable that
the ice ever entirely vanished from the continent. We shall see in
Chapter XIV that even the comparatively brief warm period known as
the post-glacial climatic optimum extended to the Antarctic coast,
and this is additional argument for extending the much greater
interglacial oscillation southward beyond its known limits in Tierra
del Fuego, Australia and New Zealand, but here the matter must be
left.
BIBLIOGRAPHY
Hedley, C. “The palæographical relations of Antarctica.” _London,
Proc. Linnæan Soc._, 124, 1911-2, p. 80.
Lydekker, R. “A geographical history of mammals.” Cambridge
University Press, 1896, pp. 125 ff.
David, T. W. E. “Antarctica and some of its problems.” London,
_Geogr. J._ 43, 1914, pp. 605-30.
Nordenskjold, O. “Antarktis.” _Handbuch Regional Geologie_, Heft
15, 1913.
CHAPTER XIII
THE CLOSE OF THE ICE AGE—THE CONTINENTAL PHASE
In Chapter V we left the climatic history of northern Europe at the
point where the ice in its final readvance had once more reached
the German coast. But Scandinavia was now sinking, and the margin
of the ice soon began to retreat again. At the same time the Alpine
glaciers diminished in size, while the Irish and Scottish glaciers
disappeared. This is the critical period in the change from glacial
to temperate conditions, and, thanks to the researches of the
Swedish geologists, and especially G. de Geer, H. Munthe and Gunnar
Andersson, we are very well acquainted with it. The change was
not uniform; at first the recession was very slow, and there were
periods when for scores of years the ice-edge remained stationary
or even readvanced, but on the whole the time was one of persistent
amelioration. The following description is based chiefly on W. B.
Wright’s summary of de Geer’s work.
After leaving the coast of Germany the ice-edge appears to have
remained in the western Baltic, retreating slowly for some 8000
years. About 10,000 B.C. it lay along the southern coast of Sweden,
and during the next 2000 years it withdrew to about 59° N. This was
the Gotiglacial stage. Here came a pause, when, for 200 years, about
8000 B.C., owing presumably to a change for the worse in the climate,
the ice-edge remained in one position, forming a great moraine. Then
came another period of very rapid retreat, the Finiglacial occupying
nearly 3000 years, followed by a further halt of some duration near
Ragunda, about 5000 B.C. After this the ice-sheet split into two
portions, and the Glacial period is regarded as over.
In the Alps there were similar periods of regression and of halting
or readvance. The first, known as the Bühlstadium, corresponded to
the Baltic readvance (Chapter V). The second, the Gschnitz-stadium,
with a snow-line 2000 feet below the present (i.e. mean temperature
about 6° F. lower than now), has not been dated, but probably
occurred about 8000 B.C. This was followed by a warmer period,
probably as warm as and drier than the present, after which the
glaciers readvanced about 5000 B.C. in the third or Daun-stadium,
when the snow-line was depressed 1000 feet (temperature 3° F. lower
than now).
In the lower Nile valley the deposition of gravel ceased, and that of
mud began about 8000 B.C., indicating that at this time the climate
of north-east Africa reached its present state of dryness.
It is at present difficult to give more than a tentative explanation
of these oscillations of climate during the Retreat Phase. Northern
Europe was at the time passing through a complicated series of
geographical changes. As the ice left the Baltic basin the latter
became the site of a cold ice-lake, with narrow outlets to the
Atlantic by way of the Sound and the Belts. At this time the
recession was slow. Then the retreat of the ice opened a connexion
with the White Sea, and elevation closed the outlet to the west.
This probably made the waters still colder, and the Fennoscandian
pause occurred. Elevation now closed the connexion with the White
Sea, and an entirely closed-in ice-lake resulted. During this stage
the retreat was slow, until between 7000 and 6000 B.C., when the
ice-sheet vacated Scania, and direct communication between the Baltic
and the Atlantic was opened across Lakes Wener and Wetter, and the
climate, though still arctic at first, became appreciably warmer by
6000 B.C.
For more than 10,000 years of the retreat, or until 6000 B.C., the
ice-sheet was still sufficiently large and powerful to maintain a
border of Arctic anticyclonic conditions on its southern edge. During
the retreat the mean annual temperature of southern Sweden increased
from 17° F. to 35° F., equivalent to a change from North-east
Greenland to South Greenland. The July temperature rose to about 43°
F. On the North German Plain still lived the reindeer and the fauna
and flora of the sub-Arctic tundras; the mean annual temperature rose
to 45° F. by the close of the period. The land flora in Sweden was
entirely xerophilous, indicating a slight rainfall. There is also
geological evidence of a small annual rainfall on the south-west
coast of Norway. This period covers the transition from Palæolithic
to Neolithic culture.
It seems probable that the continental character of the climate of
the final stages of the retreat phase was slightly increased by
astronomical causes, the obliquity of the ecliptic being probably
nearly one degree greater about 7500 B.C. than it is now. In Germany
and Sweden this would have the effect of lowering the winter
temperature and raising the summer temperature by rather more than 1°
F.
While the land was still falling rapidly in the north of Scandinavia
and the Gulf of Bothnia, the coasts of Germany and Denmark began to
rise, and about 6000 B.C. again closed[5] the outlet of the Baltic,
converting it into a large fresh-water lake, the _Ancylus_ lake. A
similar lake was formed farther east in central Finland. At this time
the south-west Baltic lands stood more than 100 feet higher than at
present. The land was probably still largely under the influence of
dry easterly winds, and the shutting out of the Atlantic accentuated
the continental conditions, and this stage in the climatic history
of Europe is known as the “Continental Phase.” The winter climate
was severe; at first the summers were not especially warm (July
temperature about 54° F. in southern Sweden). This is probably the
period of formation of the Ragunda moraines, and of a readvance of
the glaciers on the Norwegian side of the divide, when the snow-line
lay 200-300 metres lower than at present; it was also the time of
the Daun readvance in the Alps. But as the land sank in the north
and rose in the south, the waters of the _Ancylus_ lake retreated
farther and farther north, and the summers became hot and dusty, with
a mean July temperature of about 60° F. Everywhere in the Baltic
regions the older _Ancylus_ beds show a monotonous pine-wood, but
in the upper _Ancylus_ these are followed closely by a number of
plants and shrubs of southern type—black alder, curled birch, linden,
etc. The temperature continued to rise, and oak, Norway maple, ash,
and finally, in the southernmost parts of Sweden, the common maple
appeared. The last-named plant has been found below the present level
of the sea in Ystad Harbour.
Under the influence of these conditions the remnant of the
Scandinavian ice-sheet again decreased in size, until it split into
two portions, the break occurring at Ragunda, and this is considered
by Scandinavian geologists to mark the end of the Ice Age in Europe.
Gunnar Andersson compares the climate of southern Sweden at this time
to the Baraba Steppes in western Siberia, with an annual rainfall
of 12 to 16 inches, but this seems an extreme estimate. The “Karst”
flora of the limestone areas of south-east Europe immigrated into
eastern Sweden during this period, and south-east Europe probably
gives a better idea of the climate of Sweden during the continental
phase. Farther east, in Finland, Kupffer describes the climate as
resembling that of central Russia. In central Germany the climate was
dry, with a mean temperature in the four summer months of 63° F.;
it resembled that of south-west Russia. This period of warm summers
began earlier in Germany than in Sweden, and throughout this phase
Scandinavia was occupied by a rich forest flora. The hazel extended
several degrees north of its present position, and to higher levels,
indicating a July temperature about 7° F. higher than the present. In
southern Norway the pine extended to much greater heights. But the
ivy and yew, whose limits depend on the winter rather than on the
summer temperature, showed no such extension, indicating that the
winters remained severe. In Denmark there was a dry climate, fairly
warm at the close, with fir forests, though western Denmark is now
too wet for this tree. On the coast of Norway the seas were still
cold, so that there is a contrast between the animal life of the sea
and the plant life of the land. The Alps also became warm and dry,
and were occupied by a xerophilous flora.
As the glacial anticyclone decreased in intensity, depressions from
the Atlantic began to take a more northerly course, but were held up
near the British Isles and materially increased the rainfall. This is
the first peat-bog period of these islands, when the birch and pine
forests which had covered the non-glaciated lands during the cold
dry period gave way to extensive growths of peat-bogs. Southern and
eastern England, however, largely escaped this damp period, sharing
in the dry climate of the Continent.
The absence of storms off the north-west coast of Norway is shown by
the forests which at this period covered all the outermost islands
of Norway as far as Ingo Island, off North Cape. These islands are
now barren, and their afforestation indicates a drier and especially
a less stormy climate than the present, with a decreased frequency
of winds from the sea. These conditions were well developed about
5000 B.C. This is the Early Neolithic period. Owing to the great
development of forests, this period is sometimes called the _Early
Forest period_.
The late-glacial history of North America was equally complicated.
Consider first the region of the St. Lawrence Estuary and the Great
Lakes. As the Wisconsin ice-sheet retreated across the present
site of the Lakes, the latter underwent a remarkable series of
fluctuations of area and outflow, which have been made the subject
of brilliant studies by several American geologists. The opening
stage began when the ice abandoned the high ground south of the
lakes, leaving depressions bounded on the south by the hills and
on the north by the ice. The earliest of these in the basins of
Lakes Erie and Huron are known as the first and second Lake Maumee.
These gradually grew in size and coalesced, forming several series
of connected lakes, to which various names have been given; thus
Lake Warren extended well outside the present limits of Lake Erie
and southern Huron, and was held up by ice over Lake Ontario and
northern Huron. At a later stage an enormous Lake Algonquin extended
beyond the combined limits of Lakes Superior, Michigan and Huron, and
communicated by broad channels with an enlarged Lake Ontario known
as Lake Iroquois, and with Lake Erie. But even before this time the
northern shores of the lakes, relieved of the major portion of their
ice-load, had begun to rise rapidly, and ultimately reduced the lakes
to their present size.
These great areas of ice-cold water, bathing the southern edges
of the ice-sheet, must have had an unfavourable influence on the
climate, keeping it cold and damp, and preventing dry continental
conditions from becoming established. They probably retarded the
ice-retreat in these regions quite considerably, so that a lobe of
ice was left here long after the edge had retreated northwards on
either side. At the same time the climate further south was dry, with
æolian deposits; but as the anticyclonic winds blew off the Atlantic
the evidence of drought is not so marked as in Europe.
After this slow retreat had been in progress for a considerable time
a submergence, known as the “_Champlain stage_” set in, reaching a
depth of at least 600 feet and opening the St. Lawrence regions
wide to the Atlantic, which penetrated into Lake Ontario. The ice
now retreated rapidly under the influence of a maritime climate
little colder than the present. In phase this period corresponds
to the second _Yoldia Sea_ stage of Scandinavia; in point of time
it was probably somewhat earlier. This was followed by elevation,
the first result of which was to cut off the warm water and cause a
sharp fall of temperature exactly analogous to that of the Ragunda
moraines, but a few thousand years earlier and probably more marked.
The continuance of elevation brought on a long continental period
of extreme aridity, when trees grew on the peat-bogs of the eastern
States, while the lakes of the Great Basin further west were almost
or wholly dried up. At the maximum of the continental conditions the
summers at least were warmer than at present, as indicated by the
northward extension of various species of plants and fresh-water
mollusca. The winters were probably more severe. Possibly the great
aridity of this period was partly due to a sub-glacial continental
anticyclone obstructing the path of depressions across America from
west to east. The drainage area of the Great Basin received hardly
any rainfall and was a hopeless desert, but the Atlantic States were
able to grow trees on the old peat-bogs, probably with rainfall
derived from the Atlantic. By reference to the cutting of Niagara
gorge, we can infer that the warm dry period began about 6000 B.C.,
so that it corresponds exactly with the continental phase (_Ancylus_
stage) of Europe. This period of aridity was finally ended by a fresh
submergence, the “_Micmac_,” which carried the land about twenty feet
below its present level.
In Yukon and Alaska, where the glaciation was not nearly so severe as
further to the south-east, the depression of the land by the ice-load
and consequently the subsequent rise on its removal were not great.
There were no complicated geographical changes, and correspondingly
there appear to have been no fluctuations of climate, but only a
gradual passage to present conditions.
Even in Iceland there are indications of a dry period following the
last glacial maximum, for tree-trunks, buried in the peat-bogs, show
that the birch formerly had a much greater extension. It is also
quite possible that there was an accentuation of desert conditions in
Asia during the retreat of the glaciers in Europe and North America,
which may have played a part in the wave of Neolithic migration that
appears to have overwhelmed the artistic Palæolithic races of western
Europe; but of this we have as yet no direct evidence. The Neolithic
invasion of Europe took place along two main routes, the Nordics
passing from the centre of Asia north of the Caspian, across Russia
to the Baltic shores, where they became the Kitchen-midden people;
and the Alpine race passing from Transbaikalia, south of the Caspian
and Black Sea, into southern Europe. The Nordics drove before them
an older race, characterized by the transitional Maglemose culture,
which passed from east of Russia to the shores of the Baltic and
ultimately to England, where harpoons of Maglemose type have been
found beneath the peat of Holderness.
In the southern hemisphere the continental phase does not appear
to have been so well developed. The uppermost part of the Pampean
loess is possibly post-glacial; more certainly so are the sand-dunes
on the coast near Buenos Aires, in which human remains have been
found in association with the bones of some extinct animals. In New
South Wales, after the retreat of the glaciers, there was a period
with land a little above its present level, so that the stools of
Eucalyptus trees are now found ten feet below sea-level; but there is
no evidence as to the climate of this stage. In New Zealand we have
no definite post-glacial beds of continental type. The occurrence of
xerophilous plants, such as _Aciphylla_, still living in a climate
which is now decidedly moist, may be a remnant of a continental phase
in New Zealand, or may date back to the steppe conditions of the
loess. As to Antarctica, we have, of course, no evidence.
BIBLIOGRAPHY
Brooks, C. E. P. “The evolution of climate in north-west Europe.”
London, _Q. J. R. Meteor. Soc._, 47, 1921, p. 173.
Wright, W. B. “The Quaternary Ice Age.” London, 1914. (Ch. 17,
Late-glacial changes of level in North America.)
_Bericht Internat. Geologenkongress_, Stockholm, 1910. “Die
Veränderungen des Klimas seit der Maximum des letzten Eiszeits.”
Numerous papers, dealing with Europe and North America.
Munthe, H. “Studies in the Late-Quaternary history of southern
Sweden.” Stockholm, _Geol. Foren. i Forh._, 22, 1910, pp. 1197-1292.
Antevs, E. On the late-glacial and post-glacial history of the
Baltic. _Geogr. Rev._, New York, 12, 1922, p. 521.
CHAPTER XIV
THE POST-GLACIAL OPTIMUM OF CLIMATE
In most of the polar and temperate regions of the world the Glacial
period seems to have been separated from the present by a short
interval of slightly more maritime climate. The existence of this
phase was the chief point brought out in the great collection of
papers communicated to the Stockholm meeting of the International
Geological Congress, which has frequently been referred to in
this volume. The pioneer work on the subject has been done by the
Scandinavian geologists, and we may commence with a discussion of
this period in the countries bordering on the Baltic.
About 4000 B.C., at the conclusion of the continental phase referred
to in the preceding chapter, a rapid movement of submergence set
in over the whole of the southern Baltic, and shortly afterwards
the land-bar which had formerly separated the fresh waters of the
_Ancylus_ lake from the Atlantic gave place to a wide strait, through
which the waters of the ocean flowed into the Baltic across southern
Sweden. Ultimately this channel became wider than the present outlet
between Sweden and Denmark, and maritime influences penetrated to
all parts of the Baltic. In recognition of this influence the period
was termed by Blytt the “Atlantic stage.” The much greater freedom
with which the waters of the Atlantic were able to enter is shown by
a comparison of the “isohalines” of this period with those of the
present day. Isohalines indicate the degree of saltness of the water;
those of to-day can, of course, be measured directly, and show that
in the Gulf of Bothnia the water becomes continually less salt as we
go northward, for which reason many species of marine mollusca are
unable to live. By studying the distribution of the mollusca in the
_Littorina_ Sea the isohalines of that period have been reconstructed
also, and show that the salt content was much greater than at the
present day, indicating a greater influx of oceanic waters.
If we take a map showing a reconstruction of the geography of
_Littorina_ time, and apply to it the formulæ given in the Appendix,
comparing our results with the inferences of Scandinavian and north
German geologists as to the temperature, we find that there is a
remarkably good agreement. Many of the palæo-botanists comment on
the prolongation of the autumn into the present winter, which is
especially characteristic of a more insular climate. The amounts of
change in each case are also in good agreement, except perhaps in
the Christiania region and in north Denmark, where the geologists
require a greater change than that calculated from the land and sea
distribution; this is probably accounted for by a higher temperature
in the waters of the Atlantic. The maximum change as calculated
is shown in south-west Finland (winter 6° F. warmer, summer 2° F.
cooler). Finland is described as having at that time the climate of
western Europe, which we may take as meaning winter 8° warmer, summer
3-4° cooler. There was thus a great change from the extreme climate
of the continental phase with its hot summers and severe winters and
little rain, to an extremely temperate climate with cool summers,
mild winters and a heavy rainfall. The warmth-loving plants which had
begun to immigrate during the later part of the continental phase
continued to spread, and probably the highest average temperatures
were reached at the time of maximum submergence, but now they were
accompanied by plants for which a large rainfall is necessary, and it
seems that the average rainfall of southern Sweden must have been
about 40 inches a year. The oak began to dominate the forests in
place of the hazel, and the peat-bogs, which during the preceding dry
period had hardened into a firm surface on which birch and pine were
able to take root, again became moist, so that the trees were choked
by growths of bog-plants. On the shores lived men of the Transition
and Early Neolithic. As the land rose again and the _Littorina_ Sea
decreased in area the climate again became drier and more rigorous.
In Denmark the forests of the _Ancylus_ period gave place to oak as
the land sank, and there are also remains of two water plants, the
water-nut (_Trapa natans_), which is no longer found in Denmark,
and _Najas marina_, still living in one isolated locality. Northern
Denmark was broken up into islands, among which marine deposits were
formed, containing the remains of southern mollusca, many of which
are found in the kitchen-middings. Most of the wood used by Neolithic
man was oak; there is little fir and no beech.
In Norway the work of C. Brögger has made us familiar with the
_Tapes_ beds, which correspond in point of time to the _Littorina_
stage of the Baltic. _Tapes decussatus_ is itself a southern species
of mollusc, and it is associated with a very rich warmth-loving
fauna. In southern Norway the geographical conditions were different
from those in Sweden, for the land reached its lowest level
relatively to the sea about the close of the Glacial period, and has
been rising throughout the post-glacial. The seas show a progressive
rise of temperature from 8° F. below present at the close of the
Glacial period to 4° F. above the present in the older _Tapes_ beds.
The littoral climate at this stage resembled that prevailing at
present on the coast of northern England. After this, as the land
approached its present level, the temperature fell again, and in the
upper _Tapes_ stage was only 2° F. above the present.
The warm period represented by the _Tapes_ beds is found at intervals
along the west coast of Norway, and we again find evidence of a
submergence of the land contemporary with the maximum temperature.
These conditions extend even as far north as Tromsö, within the
Arctic circle. In Spitzbergen there are raised beaches 30 to 80
feet above the sea, containing remains of molluscs and a species of
_Fucus_, none of which are now living so far north. On the land there
are old peat-bogs of great thickness, though peat-mosses cannot now
grow, since the ground never thaws below a depth of 6-10 inches. It
has been pointed out that a great number of the plants now living in
Spitzbergen are unable to ripen their seeds under present climatic
conditions, though they must have done so in the past. Ripe seeds
of some species, in fact, have been found in the peat-bogs, which
are contemporaneous with the raised beach. There is thus evidence
of a very well-marked warm period associated with submergence in
Spitsbergen.
In Franz Josef Land, Nansen found raised beaches with mussels 10 to
20 feet above the present level; this shell does not now live so far
north. In the White Sea and on the Murman coast there are also raised
beaches with a southern fauna. The warm period shown in the beds of
the New Siberian Islands has already been referred to (p. 79).
Returning to the British Isles we find that in the south the land
was above its present level throughout the whole of the post-glacial
period. On the other hand, a 25-foot beach is found in north-west
England (Formby and Leasowe marine beds), but without any evidence
as to climate; the same applies to the 25-foot beach of Scotland.
It is only when we come to the north-east of Ireland that we find
evidence of conditions appreciably warmer than the present, in the
section of the Alexandra Dock, Belfast, where marine clays overlie
beds of grey sand and peat. The lower estuarine clay is essentially a
littoral clay, known as the _Scrobicularia_ zone. It is brownish-blue
and sandy, and contains in abundance the roots and leaves of the
grass-wrack (_Zostera marina_), and a vast number of shells of a few
species which live between tide-marks, indicating that the land stood
10 feet or so above its present level at first, while the climate
cannot have differed greatly from that prevailing at the present day.
It must have been formed during a period of gradual depression, for
throughout its six feet or more of thickness it preserves identical
littoral characters. After a time this depression became more rapid,
and the upper estuarine clay began to form—a light blue clay, very
pure and unctuous, with a very rich and well-preserved fauna, known
as the _Thracia_ zone. The fauna has a decidedly southern aspect,
and indicates that the coasts of north-east Ireland had the present
temperature of Bantry Bay—an increase of at least 3° F. in the mean
annual temperature. The _Thracia_ zone is followed by a bed of yellow
shore sand, indicating re-elevation to about seven feet above the
present level.
Corresponding to the upper estuarine clay are raised beaches at
a height of 25 feet in north-east Ireland, falling to 15 feet at
Dublin, and to only 6 or 7 feet in western Donegal and Sligo. The
mollusca indicate a somewhat higher temperature than the present. In
the beaches have been found flint scrapers and arrowheads of early
Neolithic type.
Looking further westwards, we find that Iceland, which had undergone
a slight elevation during the continental phase, so that peat was
formed below present sea-level, again subsided, falling to 10 or 12
feet below its present level. During this subsidence the temperature
rose, the greatest warmth coinciding with the lowest level of the
land. Species from the south-west shores, where the temperature of
the water is directly influenced by the Gulf Drift, extended to the
cold northern coast. In some places the marine clays of this period
have been ploughed up by a subsequent readvance of the glaciers.
From Greenland comes abundant evidence of a post-glacial warm period
coincident with a subsidence of about 80 feet. Raised beaches all
along the west coast contain mollusca, some species of which are not
now living north of the St. Lawrence estuary. On the other hand,
some northern species which lived off the west coast during the
glacial maximum retreated northwards during this period, and have
not re-established themselves, though the climate is now suitable.
Further, K. Steenstrup describes the occurrence of “dead ice” at
several places in North Greenland—masses of ice which have become
separated from their parent glaciers owing to rapid recession,
and are now buried in morainic matter. Subsequently the ice again
advanced, and in some cases a new glacier has advanced over these
masses of “dead ice.”
Passing to the mainland of North America, we find in eastern Canada
colonies of southern mollusca, especially oysters and quohogs,
separated from their main area of distribution south of Cape Cod
by a wide area of cold seas—the Gulf of Maine and Bay of Fundy. At
the beginning of the warm phase the land lay slightly below its
present level, but subsequently rose above it. The climate became
still warmer, until its temperature resembled that of the middle New
England States. At the same time the rainfall diminished and the
peat-bogs were replaced by forests of hardwood trees. In the basin of
the Great Lakes the warm period is represented by gravel beds in the
Niagara gorge, which from their position must, according to the most
recent determinations, have been formed about 4000 to 3000 B.C. These
gravels contain shells of fresh-water mollusca, especially species of
_Unio_, which are not now living in the St. Lawrence system, but are
found in tributaries of the Mississippi further south. Further south
on the eastern coast of the United States there are marine deposits
indicating a slight submergence, with a climate somewhat warmer than
the present.
Passing to South America, we find in southern Patagonia and Tierra
del Fuego exactly similar evidence of a post-glacial subsidence with
a warmer climate than the present. Raised beaches at a height of 50
feet contain mollusca, some of which are now rare or extinct in that
locality, and in sheltered situations plants are found still living
whose nearest neighbours are some way to the north.
In the same way, in southern and eastern Australia there are beaches
a few feet above present level, containing warmth-loving species of
mollusca and indicating a post-glacial warm period. There is some
evidence in the distribution of plants and marine mollusca that this
warm period extended to New Zealand. Raised beaches at a height of
50 to 180 feet are also known from many places in Antarctica, and
these contain mollusca, some of which are not now living south of
the sub-antarctic islands. An interesting confirmation of this has
been given by E. Philippi, from the results of an examination of the
sea-floor at four points in about 63° S., 75-95° E., all within the
present limit of pack-ice. The deposit at present forming is poor
in pelagic foraminifera, and consequently contains little lime, but
this deposit is very thin, and beneath it is a much more calcareous
clay especially rich in _Globigerina_. The latter deposit is still
forming north of the limit of pack-ice, and Philippi concludes
that at no very distant date the limits of ice were further south,
indicating warmer conditions. It is interesting to note that a
similar sequence has been found in the Norwegian North Sea, the brown
foraminiferous deposit (in this case containing _Biloculina_) being
known to be underlain as well as overlain by an unfossiliferous
grey clay attributed to the Glacial period. Finally, with regard to
Cape Colony, A. W. Rodgers says: “It is possible that the presence
of marine mollusca belonging to species that are only known in the
living state from the coast north of Pondoland, in the raised beaches
of Mossel and Algoa Bays, indicates that the sea on the south coast
was formerly warmer than now.”
Thus we have evidence of a period of submergence and climates warmer
than the present from a large number of places, including the Arctic
Ocean and Greenland, the temperate coasts of North America and
Europe, the Southern Ocean and Antarctica. The stage appears to be
missing on the temperate coasts of the Pacific, on both the Asiatic
and North American sides, and from the whole of the Tropics. It is
fairly certain that the warm period occurred at the same time in
eastern North America and western Europe; in the case of the southern
hemisphere we have no direct proof of this, but in all cases the
deposits are comparatively recent, and since they obviously refer to
a similar state of affairs we may assume that they are of the same
date.
In the Baltic area we know that the great change of level was due
largely to a subsidence of the land and only to a small extent to
a rise of the sea. But in other parts of the world the amount of
submergence was remarkably uniform at places in the same latitude,
and decreased steadily from the polar regions to about latitude
40-50°, where it became zero. Now such a general change suggests that
it was the sea which rose rather than the land which sank, and points
to some general cause which piled up the waters of the oceans in the
higher latitudes. A possible cause of this nature has been adduced
by O. Pettersson, which he terms the “tide-generating force,” which
reached one of its maxima in an 1800-year cycle about 3500 B.C. This
possibility will be dealt with more fully in Chapter XVII.
BIBLIOGRAPHY
_Bericht Internat. Geologenkongress_, Stockholm, 1910. “Die
Veränderungen des Klimas seit der Maximum des letzten Eiszeits.”
Numerous papers ranging from the Arctic to the Antarctic.
Brooks, C. E. P. “The evolution of climate in north-west Europe.”
London, _Q. J. R. Meteor. Soc._, 47, 1921, p. 173.
Praeger, R. Ll. “Report on the estuarine clays of the north-east of
Ireland.” _Proc. R. Irish Acad._, ser. 3, Vol. 2, 1892, pp. 212-89.
Goldthwait, J. W. “The twenty-foot terrace and sea-cliff of the
lower St. Lawrence.” _Amer. J. Science_, ser. 4, Vol. 32, 1911, pp.
291-317.
Cowles, H. C. “A remarkable colony of northern plants along the
Apalachicola River, Florida, and its significance.” _Rep. 8
Internal. Geogr. Congress_, 1904, p. 599.
Shimer, H. W. “Post-glacial history of Boston.” _Amer. J. Science_,
ser. 4, Vol. 40, 1915, pp. 437-42.
Halle, T. G. “On Quaternary deposits and changes of level in
Patagonia and Tierra del Fuego.” _Bull. Geol. Inst._, Upsala, 9,
1908-9, pp. 93-117.
Süssmilch, C. A. “An introduction to the geology of New South
Wales,” Sydney, 1914.
Marshall, P. “New Zealand.” _Handbuch regional Geologie_, Heft 5,
1911.
CHAPTER XV
THE FOREST PERIOD OF WESTERN EUROPE
Hitherto we have been dealing with climatic changes which can be
recognized with more or less certainty over most of the polar and
temperate regions of the world, but we have now to describe a stage
which appears to have been peculiar to Europe and possibly Asia—the
Forest period. By 3000 B.C., or towards the close of the Neolithic
period, considerable elevation had again taken place over the central
latitudes of western Europe (the northern parts of Norway and Sweden
were still several hundred feet below their present level). The
southern part of the British Isles, which had remained slightly
elevated since the last Glacial period, had now emerged to a height
of nearly ninety feet above its present level; the area of Ireland
had increased appreciably and part of the North Sea was land. The
geographical changes were not great, but they were sufficient to turn
the scale in the direction of a continental climate in the British
Isles. The more or less complete closing of the Straits of Dover, and
the consequent bar to the free circulation of the Gulf Drift, must
have had an appreciable effect on the climate in the direction of
continentality. At the same time the low level of northern Norway,
and possibly the persistence of warm conditions in the Arctic basin,
more and more attracted depressions to the northernmost track, so
that the British Isles especially, and to a lesser extent Holland,
Germany, southern Scandinavia and Russia, came more persistently
under the influence of anticyclonic conditions. The rainfall of
these countries diminished, and the surface of the bogs dried
sufficiently to enable forests to grow in the western countries; in
Germany heath-plants took the place of bog-plants, while in Russia
steppe conditions supervened. The normal meteorological conditions at
this time in fact resembled those of the memorable drought of 1921,
which was characterized by low pressure and stormy conditions in
the Arctic Ocean and a belt of high pressure and persistently fine
weather across central Europe.
During this phase the winters may have been severe, but the summers
were warmer than the present, for in the peat-bogs of Ireland and
Scotland are the remains of trees larger than any now found in the
neighbourhood. The Irish bogs dried so completely that they were
extensively inhabited; corded oak roads have been found at this
horizon, while in 1883 a two-story log house, surrounded by an
enclosure, was found in Drumkelin Bog, Co. Donegal; it was twelve
feet square and nine feet in height, and a roadway led to it across
the bog. Both house and roadway were entirely constructed of oak.
With the hut were found a stone chisel and a flint arrowhead. Beneath
the floor were fourteen feet of bog, and above the floor twenty-six
feet. This time was also one of relatively little wind movement,
for stools occur even in exposed positions on the mountain slopes
of western Ireland, where trees will only grow now in sheltered
positions near sea-level.
Further evidence of the very dry climate of this phase is the
frequent occurrence of trees apparently _in situ_ beneath the surface
of fresh-water lakes, both in Ireland and Scandinavia. I was able to
examine one very good example near Lough Toome in north-west Ireland.
An unusually dry spring had lowered the surface of the water and
a large number of tree-stools were exposed; when these trees were
growing the water-surface must have been at least two feet below
the level of the present outlet. Most of the lakes in which these
stools are found are shallow upland basins with a small drainage
area, and if the present climate became drier they would more or less
completely disappear.
Mr. Fairgrieve has noted the action of blown sand on the westward
side of broken-off tree-stumps in a submerged forest on the shore in
south Wales, which, though not conclusive, suggests dry conditions.
Mr. Fairgrieve also noted the direction of fall of twenty-one trees,
and found that in the great majority of cases they were blown down by
westerly winds.
The forest phase was short; according to the late C. Reid the land
again began to subside shortly after 3000 B.C., and by 1600 B.C.,
in Britain at least, had reached its present level; this carries
us to the beginning of the Bronze Age. In connexion with Ellsworth
Huntington’s theory that the dampness of Ireland lowers the energy
of its inhabitants, it is interesting to note that this dry period
apparently corresponds to the legendary Heroic Age, when the vigour
of the Irish reached a level never since attained. Civilization in
Scandinavia also seems to have benefited by the drier conditions,
for Scandinavian technique advanced rapidly to a high level about
1800 B.C. But though there is evidence of a considerable sea-borne
commerce with Britain and Ireland, there appears to have been
comparatively little land traffic between different parts of
Scandinavia at this time. In fact, to primitive man dense forest with
thick undergrowth was almost impenetrable. But at the close of the
forest phase and the beginning of the peat-bog phase the trees were
weakening under conditions becoming unfavourable. Such dying forests
are marked by the absence of undergrowth and young trees, and afford
safe and easy land communication. Accordingly we find that by 1500
B.C. a considerable traffic had developed across Scandinavia by land.
Although we have no direct evidence, the meteorological conditions
suggest very strongly that the dry belt extended across Russia into
Siberia as a marked period of desiccation, possibly worse than any
droughts of the historic period. At present Siberia receives its
rainfall mainly from depressions which cross Russia from the Baltic
or Black Seas, and follow a well-marked track north of the central
Asiatic mountains. But during the forest period these tracks were
abandoned, and the majority of the depressions passed north-eastward
off the coast of Norway into the Arctic Ocean. The result must
have been a great diminution of rainfall over the continent. We
shall see later (Chapter XIX) that this period of drought was of
extraordinary importance in human history. For during the moist
maritime phase central and eastern Europe, and probably also Asia,
had become extensively peopled by neolithic nomads of Aryan and
Semitic races, while the great river valleys of the south were in
the possession of dense agricultural populations in a more advanced
state of civilization. As the climate became progressively drier and
the pasture diminished, the land was unable to support such a large
nomadic population, and there was a great outburst of raiding and
conquering expeditions directed southwards and westwards, resulting
in a succession of empires in the rich Mesopotamian regions and
neighbouring countries, which form the beginnings of our history. The
beginnings of history in China also, about 2500 B.C., show that at
this time the settled peoples of that country were in trouble with
the nomads of the interior.
BIBLIOGRAPHY
Reid, C. “Submerged forests.” Cambridge University Press, 1913.
Lewis, F. J. “The history of the Scottish peat-mosses and their
relation to the Glacial period.” Edinburgh, _Scot. Geogr. Mag._,
22, 1906, p. 241.
CHAPTER XVI
THE “CLASSICAL” RAINFALL MAXIMUM, 1800 B.C. TO A.D. 500
About 1800 B.C., or the beginning of the Bronze Age in Britain, the
subsiding land finally attained approximately its present level. At
the same time the climate of western Europe deteriorated, becoming
much more humid and rainy, and there set in a period of intense
peat-formation in Ireland, Scotland and northern England, Scandinavia
and North Germany, known as the Peat-Bog Period or Upper Turbarian.
The peat-beds choked and killed the forests which had developed on
the older peat-bogs, and grew up above the stools and fallen trunks,
so that we have two layers of peat separated by an old forest. The
forest level contains neolithic articles, the peat contains gold
collars, bronze swords and pins, and other objects of the Bronze
Age. This growth also went on even over high ground, which had not
previously been covered by peat, for Professor Henry informs us that
on Copped Mountain, near Enniskillen, and at other places in Ireland,
Bronze Age cairns and tumuli are found resting on rock and covered
by several feet of bog. Peat-beds on the Frisian dunes between two
layers of blown sand are dated about 100 B.C., and some bogs in
northern France were formed during the Roman period. There is also
some much-disputed contemporary Latin evidence that at the time of
the Roman occupation the climate of Britain was damp and boggy, while
Gibbon (“Decline and Fall of the Roman Empire”), referring to the
climate of central Europe at the beginning of the Christian era,
points to some evidence that the climate was colder. This is, that
the Rhine and the Danube were frequently frozen over, so that the
natives crossed them with cavalry and wagons without difficulty,
although at the present time this never happens. It is possible
that this severe climate is referred to in the Germanic legend of
the “Twilight of the Gods,” when frost and snow ruled the world
for generations. The Norse sagas point to a similar cold period in
Scandinavia. This lapse of climate occurred in the Early Iron Age,
about 650 to 400 B.C., when there was a rapid deterioration from the
high Scandinavian civilization of the Bronze Age. This deterioration
of culture was probably the direct result of the increased severity
of the climate.
This Pluvial period has been made the subject of special studies
by Ellsworth Huntington in several important books and papers; he
finds evidence of a distinctly Pluvial period in three regions—the
Mediterranean, central and south-western Asia, and an area including
the southern United States and northern Mexico. In the first of
these, the Mediterranean, Huntington considers that the Græco-Roman
civilizations grew up in a period of increased rainfall which lasted
from about 500 B.C. to A.D. 200. These states were able to develop
in comparative peace because during this time there were no great
invasions of nomadic peoples from eastern Europe or central Asia,
a fact which points to good rainfall in these comparatively dry
regions, so that their inhabitants had no need to emigrate in quest
of a living. In the Mediterranean itself the heavier rainfall allowed
a solid agricultural basis which produced a sturdy race of peasants
who made good soldiers. Owing to the greater cyclonic control of
climate and consequent changeable weather, these inhabitants were
more vigorous in mind and body, for Huntington’s researches have
demonstrated that long spells of monotonous weather, either fine
or rainy, are unfavourable for human energy. Finally the heavier
rainfall maintained a perennial flow in the rivers, giving plentiful
supplies of good drinking water. These conditions broke down earlier
in Greece than in Italy, as the latter naturally has a heavier
rainfall. Huntington considers that the decline of Greece was largely
due to malarial poisoning, the decreasing rainfall causing the
river-flow to break down in summer, leaving isolated pools forming a
breeding ground for mosquitoes.
After A.D. 200 the climate of Italy also deteriorated. The decrease
of rainfall, combined with gradual exhaustion of the soil, made
wheat-growing more and more difficult for the small agriculturalist,
and the farms came into the hands of large landowners, who worked
them by slave labour, and in place of wheat either grew vines or
olives or raised flocks and herds. The agricultural population
gravitated to Rome and a few other large cities, and had to be fed by
imported wheat. The decline was probably aided by the introduction of
malaria, as in Greece.
In north Africa and Palestine the question is more debatable. C.
Negro, who has investigated the supposed desiccation of Cyrenaica,
concludes that there has been no change of climate since Roman times,
but a careful study of his evidence suggests that his conclusions are
open to criticism. All that he has proved is that there has been no
marked _progressive_ decrease of rainfall since about A.D. 200; he
has ignored the possibility of great fluctuations before and since
that date. In north Africa it seems difficult to believe that the
great cities of antiquity could have existed under present climatic
conditions, but when we turn to Palmyra in the Syrian desert we have
practically incontrovertible proof in the great aqueducts, built to
carry from the hill-springs to the city large volumes of water which
these springs no longer deliver, so that even where they are intact
the aqueducts now carry only the merest trickle.
In Persia we find numerous ruins, which point to a much greater
population two thousand or more years ago. This population lived
by agriculture, and the remains of their irrigation works are now
found in regions where running water never comes. Even the scanty
population of to-day can hardly live on the present rainfall of the
country, and it is unbelievable that the much greater population
indicated by these ruined cities could have existed without a very
much greater supply of water. The same condition is indicated by the
ruined cities of the great deserts of central Asia. These cities were
inhabited by agriculturalists, and the remains of tilled fields,
terraces and irrigation works abound in places where the supply of
brackish water would now be barely sufficient for drinking purposes
for such a large population. Huntington has also made a careful study
of the water-level of the Caspian Sea in classical times, and finds
that there was a great period of high water extending from unknown
antiquity to about A.D. 400.
There is only one region in central Asia where the population appears
to have been less in classical times than now, and that is the high
basin of Kashmir. Huntington points out that this basin is at present
near the upward limit of agriculture, and any fall of temperature
and increase of snowfall would drive out the inhabitants. But local
legends point to such a period in the remote past, corresponding to
the period of increased habitability of the central Asian deserts; at
its close there were extensive migrations from Turkestan into Kashmir.
Passing to America, we come to interesting evidence of a very
different class—I refer to the “big trees” (_Sequoia_) of California.
Since these trees live in a semi-arid climate, the amount of rainfall
is the chief factor in their growth, which finds an expression in
the breadth of the annual rings measured on the stump of the tree
when it is cut down. The method of utilizing the data was due to
A. E. Douglass. A careful comparison was first made between the
measurements of rings and the rainfall measured at neighbouring
stations, and a formula was developed by which the rainfall of each
year could be reconstructed from the tree-growth with a high degree
of accuracy. In extrapolating to find the rainfall for earlier years
before rainfall measurements began, various corrections had to be
applied, for instance trees grow more rapidly when young than when
they are old, while trees which are likely to live to a great age
grow more slowly at first than trees which die younger. These methods
were applied to nearly two thousand “big trees,” some of which were
found to be four thousand years old, but it is pointed out that the
corrections eliminate any progressive variation of climate which may
have occurred, so that the results show only “cycles” of greater or
lesser length. Summing up, Huntington says: “Judging from what we
have seen of the rainfall of to-day and its relation to the growth
of the Sequoias, high portions of their curve (of growth) seem to
indicate periods when the winters were longer than now, when storms
began earlier in the fall and lasted later into the spring, and
when mid-winter was characterized by the great development of a
cold continental high-pressure area, which pushed the storms of the
prevailing zone of westerly winds far down into sub-tropical regions
and thus caused sub-tropical conditions to invade what is now the
zone of equatorial rains.” Neglecting later favourable periods,
which are relatively short and unimportant, it is found that these
conditions prevailed very markedly between 1200 B.C. and A.D. 200,
with maxima about 1150 B.C., 700 B.C., and from 450 B.C. to 250 B.C.
Thus over the greater part of the temperate regions of the northern
hemisphere we have evidence of an important rainy period between the
extreme limits of 1800 B.C. and A.D. 400 or 500. This period was best
developed from 1200 B.C. to A.D. 200, and reached its maximum about
400 B.C. It constitutes a remarkable wave of climatic variation,
which is hitherto without adequate explanation. A somewhat similar,
though less intense, wave which occurred about A.D. 1200-1300, and
which is described in the following chapter, was associated by Wolf
to a great outburst of sunspots which took place about A.D. 1200. It
is well known that sunspots are an index of solar activity, the sun’s
radiation being greater at times of spot maximum than at times of
spot minimum. Greater solar radiation increases the evaporation over
the oceans, so that the air becomes more humid. This moist air is
carried by atmospheric currents over the land, where the moisture is
condensed into clouds and greatly increases the rainfall. At the same
time the cloud canopy shuts off some of the direct heat of the sun,
and we have the curious paradox that at times of sunspot maximum, or
greatest solar radiation, the temperature of the earth’s surface is
lowest.
The connexion outlined above is, however, extremely problematical
for temperate regions. Since the absolute sunspot maximum at A.D.
1200 is also very doubtful, it will be realized that the evidence for
the sunspot hypothesis of the mediæval rainfall maximum is extremely
slender. Furthermore, since we know nothing whatever about the solar
activity during the classical rainfall maximum, we are still less in
a position to extend the sunspot hypothesis to that period also.
The interesting theory recently put forward by O. Pettersson, already
alluded to, provides a plausible alternative explanation of the
severe stormy climate of the Peat-bog period, which reached a maximum
near 400 B.C. Without going into details this theory is that the
strength of the tides depends on the relative positions of the sun
and moon, and the tides are greatest when these act in conjunction,
and also when they are nearest to the earth. This fluctuation of
strength passes through various cyclic variations with periods of
nine years, about ninety years and about 1800 years, though the
lengths of the periods are not constant. The latter cycle is most
important to our purposes; according to Pettersson’s calculations
the fluctuations of the “tide-generating force” were as follow:
Maxima 3500 B.C. 2100 B.C. 350 B.C. A.D. 1434
Minima 2800 B.C. 1200 B.C. A.D. 530.
Increased range of the tides means increased circulation in the
waters of the oceans, especially an increased interchange between the
warm North Atlantic and the cold Arctic waters. It also means than an
unusual amount of ice is brought down from high into low latitudes.
Wide local variations of temperature of the surface waters of the
oceans cause increased cyclonic activity, and hence we may expect a
generally increased storminess at times of maximum “tide-generating
force,” and the reverse at times of minimum.
For the last maximum (A.D. 1434) Pettersson is able to adduce a good
deal of historical evidence of increased storminess in north-west
Europe and bad ice-conditions near Iceland and Greenland, while
Huntington has found an increase of rainfall shown by the big trees
of California. The next preceding maximum, that of 360 B.C., marks
the culminating point of the Peat-bog phase. The Norse sagas and
the Germanic myths point to a severe climate about 650 B.C., which
destroyed an early civilization. This was the “Twilight of the Gods,”
when frost and snow ruled the world for generations. The period
was the Early Iron Age, when civilization deteriorated greatly in
north-west Europe.
Of the maximum of 2100 B.C. there is no trace. It is possible that
the great Atlantic submergence of the Maritime phase is connected
with the tidal maximum of 3500 B.C., but the phenomena were on a
scale so much greater than those of the more recent maxima that this
can hardly have been the sole cause.
The minima should have been characterized by periods of relatively
quiet stable climate with little ice near Iceland and Greenland.
That the last minimum, in A.D. 530, was such a period there is
considerable evidence in the high level reached by civilization at
that period in Scandinavia and by the revival in Ireland. Again,
about 1200 B.C., in the early part of the Peat-bog phase, there
is evidence of considerable traffic by sea between Scandinavia
and Ireland. The Irish Museum has lately discovered a hoard of
gold objects dated about 1000 B.C., in which the designs show a
Scandinavian origin. The minimum of 2800 B.C., which occurred in the
Forest phase, may have contributed to the dry climate of that period,
but otherwise has left no trace.
Although at first sight the effect which Pettersson sets out to
explain seems out of all proportion to the smallness of his cause,
the coincidences after 2000 B.C. are extremely interesting, and
suggest that after the land and sea distribution reached its present
form the astronomical cause adduced by Pettersson was possibly
effective, but before that date the astronomical cause, if it
existed, was masked by the much greater climatic variations due to
changes in the land and sea distribution.
The opinion has frequently been expressed that the “Classical” and
“Mediæval” rainfall maxima were phenomena similar to the Glacial
period, but less intensive. This view is often carried to its logical
conclusion, that the thirty-five-year cycle, the eleven-year,
and still smaller cycles of climate, are also part of the same
series, and that the Glacial period and, let us say, the three-year
periodicity of rainfall are therefore due to variations of the same
agent, in this case the sun. This logical extension of the theory
is, however, completely untenable. The eleven-year periodicity is
admittedly connected with variations in the solar activity, but there
are other cycles which are completely independent of such variations,
such as, for instance, the annual variation undergone by all
meteorological elements, which depends entirely on the inclination
of the earth’s axis. There is a well-marked 4.8 year period in the
amount of ice off Iceland, the half-cycle of which is exactly equal
to the distance travelled by the water taking part in the North
Atlantic circulation, divided by the velocity with which it travels.
There is, therefore, no _a priori_ reason for assuming that the cause
of the Glacial period was identical with the cause of the Classical
and Mediæval rainfall maxima. Further, in the latter case, the chief
phenomenon was the increase of rainfall; the decrease of temperature
was merely incidental, but in the Glacial period the outstanding
feature was a great lowering of temperature in the polar and
temperate regions, and in this case it was the increase of rainfall
which was incidental.
BIBLIOGRAPHY
Lewis, F. J. “The history of the Scottish peat-mosses and their
relation to the Glacial period.” Edinburgh, _Scot. Geogr. Mag._,
22, 1916, p. 241.
Brooks, C. E. P. “The correlation of the Quaternary deposits of
Great Britain with those of the Continent of Europe.” _Ann. Rep.
Smithsonian Inst._, 1917, p. 277.
Huntington, Ellsworth. “The pulse of Asia.” Boston and New York,
1907.
——. “The climatic factor as illustrated in arid America.”
Washington, _Carnegie Institution_, 1914.
——. “World power and evolution.” New Haven, 1919, pp. 186-207.
Pettersson, O. “Climatic variations in historic and prehistoric
time.” _Svenska Hydrogr.-Biol. Komm. Skrifter_, Heft 5.
CHAPTER XVII
THE CLIMATIC FLUCTUATIONS SINCE A.D. 500
The question of climatic changes during the historic period has been
the subject of much discussion, and several great meteorologists
and geographers have endeavoured to prove that at least since about
500 B.C. there has been no appreciable variation. It is admitted
that there have been shiftings of the centres of population and
civilization, first from Egypt and Mesopotamia to the Mediterranean
regions, and later to northern and western Europe, but these have
been attributed chiefly to political causes, and especially to the
rise of Islam and the rule of the “accursed Turk.” Recently, however,
there has arisen a class of evidence which cannot be explained away
on political grounds, and which appears to have decided the battle
in favour of the supporters of change; I refer to the evidence of
the trees, explained in the preceding chapter. The conclusions
derived from the big trees of California have fallen admirably into
line with archæological work in central America, in central Asia
and other regions, and have shown that the larger variations even
of comparatively recent times have been very extensive, if not
world-wide, in their development.
Let us consider first the evidence of the trees. These indicate
that after the moist period ending about A.D. 400, described in the
preceding chapter, the rainfall was generally light until about A.D.
1000, when it showed a sharp rise, probably to the level attained in
A.D. 1. (The correction for age renders an exact comparison between
periods a thousand years apart difficult.) This period of abundant
rainfall lasted some fifty years, followed by a gradual decline to a
brief minimum, shortly before A.D. 1200. About 1300 occurred another
rapid rise, reaching a maximum before 1350; the period of heavy rain
continued a short while after 1400, when a decline set in, reaching
a minimum at 1500, after which the rainfall recovered somewhat, and
subsequently maintained approximately its present level, with a
slight maximum about 1600 to 1645.
In the desert of Arizona, in regions at present too dry for
agriculture, there are abundant ruins, which are attributed by
Huntington to three periods:
(_a_) Pueblo ruins, dating back to just before the coming of the
Spaniards (i.e. about A.D. 1600), and indicating merely an increase
of population at the present centres.
(_b_) Ruins of an older civilization, termed by Huntington the
Pajaritan, during which numerous inhabitants lived in places where
at present no crops can be raised. “These people, as appears from
their pottery, their skulls and their methods of agriculture, belong
to a different civilization from that of the modern Pueblos who
inhabited Gran Quivera at the time of the coming of the Spaniards.
They had evidently disappeared long before that date, as is evident
from the present ruins of their villages, and from the absence of any
hint of their existence in the early annals of the country” (_Geogr.
Journal_, 40, 1912, p. 396).
The largest ruins of this type invariably lie near the main lines of
drainage. They consist of villages with houses of several storeys.
But digging down beneath these ruins we find (_c_) traces of an
older occupation, and ruins of a primitive type are also found on
the plateaus remote from any except small valleys. “They are usually
small, and are greatly ruined, and seem to belong to a time long
anterior to the main large ruins.” Huntington terms this type the
Hohokam; unfortunately this and the Pajaritan occupations cannot
be accurately dated, but it is reasonable to connect them with the
rainfall maxima shown by the trees, about the time of Christ, and in
A.D. 1000 to A.D. 1300.
A similar succession has been found in the neighbourhood of Mexico
City. The earliest trace of occupation is a crude “mountain pottery,”
in ordinary river sand and gravel. These deposits are succeeded by
finer sand with better pottery known as the “San Juan” type, above
which comes a culture layer with the remains of houses. This is
covered by a bed of “tepetate,” a white calcareous deposit frequently
found in dry regions where much water evaporates. The gravels suggest
the occasional heavy rains of arid countries. The San Juan pottery
extends throughout the “tepetate,” which probably corresponds to the
dry period of A.D. 400-1000 in California.
Historical records in Mexico date back to the coming of the Aztecs in
A.D. 1325. They show that in 1325 and again in 1446 the level of the
lake of Mexico was high, but towards the end of the fifteenth century
the water was much lower. In 1520 it was high again; in 1600 it was
low, but high from 1629 to 1634. From 1675 to 1755 was a long dry
period. On the whole the climate from 300 to 600 years ago seems to
have been moister than that of to-day.
Still further south in the Peninsula of Yucatan recent explorations
have yielded results of extreme interest. Yucatan lies within the
tropical rain-belt, and is covered by almost impenetrable forests.
The climate is enervating and unhealthy, and the present inhabitants
are greatly lacking in vigour. In the forest, however, have been
found the ruins of ninety-two towns, some of them of great size,
and all remarkable for the beauty as well as the solidity of their
architecture.
These ruins belong to the great Mayan civilization. Mayan history
has been briefly summarized by Huntington as follows: “First we have
a long period of active development, during which the calendar was
evolved and the arts of architecture and sculpture were gradually
developed.... This time of marked growth must have preceded the
Christian era. Then comes ... the building of the great cities of
Copan, Quirigua, Tikal and others. These first great cities were in
the southern part of the Maya area, on the borders of Honduras or in
eastern Guatemala. They lasted perhaps three or four centuries; then
quickly declined. So far as we have any evidence, civilization never
revived in this southern area, for the structures of the great period
have not been rebuilt by later inhabitants. Towards the end of the
period of greatness the centre of Mayan culture moved northward....
The great period, according to Bowditch, lasted from 100 B.C. to
A.D. 350 ... then came a time of very low civilization, lasting for
centuries.... A revival ensued about A.D. 900 or A.D. 1000, and
architecture once more reached a high pitch, but ... only in northern
Yucatan; all the rest of the country seems to have remained in
darkness. Moreover, this mediæval revival was relatively shortlived.
Since that time the condition of the Mayas has fluctuated more or
less, but on the whole there has been a decline.”
Now at the present day the densest and most progressive population in
Yucatan is found in the driest part of the country, where the forest
gives place to jungle. If the line of separation between jungle and
forest were moved southward 300 miles, the former would include
all the districts where ruins are now found. We see from the above
summary that the prosperous periods of Mayan history were just those
periods which in California were moist; in Yucatan they must have
been dry. Huntington’s explanation is the theory of the “shifting of
climatic belts”; during the rainy period in California the temperate
storm-tracks were shifted further southward. At the same time the
sub-tropical high-pressure belt, which at present lies over the
West Indies, was also shifted southwards, and this brought a dry
cool winter to Yucatan, with an increased contrast of seasons, and
consequently a more invigorating climate.
In Asia, Huntington and other explorers have found similar traces of
past variations of climate, a fascinating account of which is given
in “The Pulse of Asia.” Space will not permit of a summary in detail,
but the following general conclusions may be quoted:[6]
“If we omit the Volga and the European portions of the Caspian
drainage area, the limits (of the six basins considered) lie over
sixteen hundred miles apart from north to south and over three
thousand from east to west. All this great area seems to have been
subject to the same great waves of climatic change.
“In the ancient days when the Oxus River entered the Scythian Gulf
of the expanded Caspian Sea, and Lake Gyoljuk discharged permanently
to the Tigris, the lake of Seyistan had not been converted into dry
land by the giants. Kashmir was so cold and snowy that agriculture
was impossible.... In the Lop basin the rivers were full of water;
Lop-Nor was the “Great Salt Lake”; the desert was comparatively small
and the zone of vegetation extensive; and on all sides there was a
density of population and a degree of prosperity far beyond those of
to-day. And in the Turfan basin the same was probably true.
“A great change took place throughout the six basins during the early
centuries of the Christian era. The lakes of Gyoljuk, Seyistan, the
Caspian, Lop-Nor and presumably Turfan were greatly reduced in size.
In the case of the first three, parts of the old lake-beds were used
as sites for villages. Except in Kashmir, the change of climate
appears to have brought disaster....
“Again there came a change (about A.D. 700). The process of
desiccation gave place to a slight but important tendency toward
increased rainfall and lower temperature. Kashmir became colder and
more snowy, and hence more isolated; the rivers of Lop and Turfan
gained greater volume; and the lakes of Lop, the Caspian and Turfan
expanded once more. The habitability of the arid regions began to
increase; migrations came to an end; and central Asia was prosperous
for a time. Finally (about 1350) a latest and slightest change took
place in the other direction, and we seem to-day to be in the midst
of an epoch of comparative equilibrium, with no marked tendency
towards climatic change in either direction.” There was, however, a
period of comparatively high water in the Caspian in the early part
of the seventeenth century.
In Europe the evidence for climatic changes during historical times
is more difficult to follow, since variations of rainfall leave fewer
traces in a moist than in an arid or semi-arid country. A certain
amount of material is given by Brückner in his “Klimaschwankungen.”
He finds that there was a great advance of the Alpine glaciers from
1595 to 1610, while two Italian lakes without outlet, the Lago di
Fucina and Lake Trasimeno, attained a high level about the same
time. Other evidence for western Europe is derived from the date
of the wine-harvest and from the records of severe winters. Like
the growth-curves of the big trees, they need a secular correction
to alter the general slope of the curve, especially in the case of
severe winters, but the larger irregularities probably correspond to
real variations of climate. I have added in column 4 the numbers of
winters with sea-ice on some part of the Danish coast, as tabulated
from the records compiled by Captain C. I. H. Speerschneider. The
results are in general agreement with column 3, particularly as
showing that the period 1401-50 was relatively mild; but the first
half of the seventeenth century is less instead of greater than its
two neighbours in this column.
The figures for the wine-harvest refer to the average for the period
1816-80; - indicates that the harvest was so many days earlier than
normal, corresponding to a high summer temperature (May to August).
The table shows that cold winters were especially numerous in the
first half of the twelfth century and again in the thirteenth. The
end of the fifteenth century was marked by hot summers and mild
winters, or a warmer climate; the beginning of the seventeenth
century by cold (presumably snowy) winters and cool summers. Thus the
periods of increased rainfall in the arid regions of Asia and America
were marked by a colder climate in the rainy regions of western
Europe.
--------+---------+---------+-------------+-------------
1. | 2. | 3. | 4. | 5.
| | | |
Period,| Date of | No. |Winters with |
50 years| Wine |of Severe|Ice on Danish| Remarks.
about. | Harvest.| Winters.| coast. |
--------+---------+---------+-------------+-------------
825 | | 4 | |
875 | | 7 | |
925 | | 5 | |
975 | | 6 | |
1025 | | 6 | |
1075 | | 10 | |
1125 | | 15 | | Cold winters
1175 | | 10 | |
1225 | | 13 | } |
1275 | | 13 | } | Cold winters
1325 | | 13 | 7 } |
1375 | | 11 | 4 |
1425 | +5 | 13 | 7 |
1475 | +1 | 7 | 2 | Warm
1525 | +2.9 | 10 | 5 |
1575 | +2.2 | 14 | 14 } |
1625 | +4.1 | 17 | 11 } | Cold
1675 | +2.7 | 15 | 14 } |
1725 | +0.1 | 10 | 5 |
1775 | -0.2 | -- | 22 |
1825 | -0.9 | -- | 21 |
--------+---------+---------+-------------+-------------
The date of the break up of the River Dwina at Mitau was recorded
intermittently from 1530 to 1709, and regularly since that date, and
the figures have been discussed by Rykatchef. Recasting them in our
unit of fifty years we find the mean dates to be:
----------+----------+-----------+----------+-----------+----------
1551-1600 | 1601-50 | 1651-1700 | 1701-50 | 1751-1800 | 1801-50
March 29 | March 30 | March 5 | March 26 | March 26 | March 28
----------+----------+-----------+----------+-----------+----------
This again points to a cold period about the beginning of the
seventeenth century.
The climate of Iceland and Greenland in the Middle Ages has been
the subject of much controversy, the view that there were extensive
changes during that period being warmly upheld by one party and as
warmly combated by the other party. The case for climatic change has
been well set out by O. Pettersson[7]. The Roman authors (Pliny,
Solinus, etc.) wrote that there was a frozen sea about Thule
(Iceland), but a party of monks who visited the island about A.D.
795 during the months of February to August, in which the ice is
normally most abundant in Icelandic waters, found the coast free,
though they met with a frozen sea a day’s journey to the northward.
In the ninth century the Norsemen visited Iceland regularly, and at
times sailed round it, apparently without interference from ice.
The early settlers practised agriculture with some success. In the
thirteenth century, however, the reports of ice off Iceland became
frequent—apparently the conditions were worse than those of the
present day, and much more so than in the eighth and ninth centuries.
According to Rabot, it appears from ancient records that considerable
areas cultivated in the tenth century are now covered with ice. The
first spread of the glaciers took place in the first half of the
fourteenth century. In the fifteenth and sixteenth centuries the
climate of Iceland ameliorated somewhat, but in the seventeenth there
was a readvance, which destroyed several farms about 1640 or 1650.
Since then there has been a slight retreat.
The ice-conditions of Greenland are closely related to those of
Iceland, and the records of the Norse colonization of Greenland bear
out the conclusions drawn from the latter island. Up to the close
of the twelfth century ice is hardly ever mentioned in the accounts
of voyages, though it is now a great hindrance. Eric, the pioneer
explorer of West Greenland, spent three successive winters on the
islands in Juliaanehaab Bay (latitude 60° 45′ N.), and explored the
country during the summer; “this cannot be explained otherwise than
by assuming that the Polar ice did not reach Cape Farewell and the
west coast of Greenland in those days.” In the thirteenth century ice
is first specifically mentioned as a danger to navigation, and at
the end of the fourteenth century the old Norse sailing route was on
account of ice definitely abandoned in favour of one further south.
Shortly afterwards the Norse colonies were wiped out by a southward
migration of the Eskimos. Even in Norway itself the fourteenth
century was a time of dearth, short harvests and political troubles,
when corn had to be imported from Germany instead of being exported
to Iceland as in former years.
It should be noted that Pettersson’s conclusions are considered
invalid by H. H. Hildebrandsson[8] on the ground of the
incompleteness of the records.
For the southern hemisphere our records are naturally much rarer and
of less antiquity than for the northern hemisphere, and until the
tree-rings are investigated we cannot carry our study back beyond
the sixteenth century. From some researches into the municipal
archives of Santiago de Chile, latitude 33½° S., published by B. V.
Mackenna in 1877, we can infer, however, that the general course of
variation since 1520 was similar to that of corresponding regions in
North America. Santiago lies in a semi-arid region where a temporary
shortage of water is severely felt, the average annual rainfall
being only 364 mm. (14.3 inches). The early travellers, however,
make no specific mention of drought, and in 1540 Pedro de Valdivia
crossed the desert of Atacama with a column of troops and cattle
without inconvenience—a feat which would be difficult nowadays. In
1544 there were heavy rains and great floods in June. The next record
is for the year 1609, recording another heavy flood on the Mapocho,
which was repeated nine years later in 1618. The first recorded
drought occurred in the years 1637 to 1640; there was another flood
in 1647, after which came a series of severe droughts interrupted by
occasional floods, which lasted until the close of the eighteenth
century. The first half of the nineteenth century was again
comparatively rainy. The records thus indicate a wet period centred
about 1600, followed by a dry period during the eighteenth century,
exactly parallel to the records from the United States and Europe.
BIBLIOGRAPHY
Huntington, E. “The climatic factor as illustrated in arid
America.” Washington, Carnegie Institution, 1914.
——. “The fluctuating climate of North America.” _Geogr. J._ 40,
1912, pp. 264, 392.
——. “The pulse of Asia.” Boston and New York, 1907.
——. “World power and evolution.” New Haven, 1919.
Pettersson, O. “Climatic fluctuations in historic and prehistoric
time.” _Svenska Hydrogr.-Biol. Komm. Skrifter_, H. 5.
Rabot, Ch. “Essai de chronologie des variations glaciaires.” _Bull.
géogr. historique et descriptive_, No. 2, 1902.
Brückner, E. “Klimaschwankungen seit 1700” ... Vienna, 1890.
Hildebrandsson, H. H. “Sur le prétendu changement du climat
européen en temps historique.” Upsala, _Nova Actae Regiae Soc.
Sci._ (4), Vol. 4, No. 5, 1915.
Brooks, C. E. P. “An historical notice of the variations of climate
in Chile.” Washington, _Dept. Agric., U.S. Weather Bureau, Monthly
Weather Rev._ 47, 1919, p. 637.
CHAPTER XVIII
CLIMATIC FLUCTUATIONS AND THE EVOLUTION OF MAN
The origin of man from an ape-like ancestor[9] is generally admitted,
but owing to the incompleteness of the palæontological record we
are still in ignorance as to the circumstances, while the place is
generally put vaguely as somewhere in Asia, and the time as the late
Tertiary (Prof. Elliot Smith places it near the Siwalik hills in the
Miocene). For this early period we are reduced to speculation, in
which we may reasonably utilize the facts which we have gained about
climatic variation.
The chief problem to be explained is why man’s arboreal ancestor
left the safe shelter and easy food supply of his primæval forest
and ventured forth into the plains. An article by Professor J.
Barrell,[10] of Yale University, gives a plausible account of the
change, putting it down to necessity, and not to choice. His theory
is that the human ancestor lived in the forests spread over Asia,
then a vast well-watered plain, during the middle-Tertiary period.
Then the gradual uplifting of the Himalayas and other mountain
ranges caused a decrease in the rainfall of central Asia, so that
ultimately the forests were unable to thrive, and gradually gave
place to steppe conditions. The change was slow enough to give the
less specialized inhabitants of the forest time to change their
habits and evolve into forms suitable to a terrestrial life, and the
chief of the animals which took advantage of this period of grace
was the pre-human. Forced to live on the ground, with a diminishing
food supply, only the most progressive individuals were able to
survive, and evolution was rapid. The changing type was saved from
being submerged in the great mass of the original type in the forests
which continued to exist further south by the impassable wall of
mountains. Major Cherry[11] considers that there is sufficient
evidence to prove that a portion of this evolution took place on the
seashore, an environment which would have been much more favourable
to a small ape-like animal than the open steppe would have been. It
is quite likely that the earliest migrations, such as that which
carried _Pithecanthropus_ to Java, took place along the shore. But
after a time, when increasing brainpower and the use of primitive
stone implements enabled man to take the offensive against the
larger animals, the centre of activity changed to the steppes. A
familiar view of the early development of man was advocated by W.
D. Mathew,[12] who writes: “In view of the data obtainable from
historical record, from tradition, from the present geographical
distribution of higher and lower races of men, from the physical
and physiological adaptation of all and especially of the higher
races, it seems fair to conclude that the centre of dispersal of
mankind in prehistoric times was central Asia, north of the great
Himalayan ranges, and that when by progressive aridity that region
became desert it was transferred to the regions bordering it to the
east, south and west. We may further assume that the environment in
which man primarily evolved was not a moist tropical climate, but
a temperate and more or less arid one, progressively cold and dry
during the course of his evolution. In this region and under these
conditions, the race first attained a dominance which enabled it to
spread out in successive waves of migration to the most remote parts
of the earth.”
We do not know anything of the migrations of the Eolithic and
earlier Palæolithic races, except that they spread rapidly
over a considerable portion of the earth. Both migration and
evolution, especially mental evolution, must have been accelerated
by the great changes of climate which were taking place. In
the Mindel-Riss interglacial period we know of two types, the
Piltdown man (_Eoanthropus dawsoni_) and the Heidelberg man (_Homo
heidelbergensis_), the latter a true man, though probably not on
the direct line of evolution of _Homo sapiens_. The stress of the
succeeding second Glacial period was too great for _Eoanthropus_,
which appears to have died out, but _Homo_, probably an Asiatic or
African type similar to _H. Heidelbergensis_, survived. The next
form, associated with Mousterian implements, is Neanderthal man (_H.
neanderthalensis_), who closely resembled modern man, and all the
remains of races which lived subsequently to the last glaciation
are those of modern man (_H. sapiens_), including the magnificent
Cro-Magnards and the negroid Grimaldi race. Thus each glaciation
has been marked by a step upwards in the scale of humanity; does
this mean that the coming of the super-man is contingent on another
glacial epoch?
BIBLIOGRAPHY
Barrell, J. “Probable relations of climatic change to the origin of
the Tertiary Ape-man.” _Scientific Monthly_, New York, 4, 1917, p.
16.
Mathew, W. D. “Climate and evolution.” _Annals New York Acad.
Sci._, 24, 1915, p. 212.
London, British Museum. “A guide to the fossil remains of man....”
London, 1918.
CHAPTER XIX
CLIMATE AND HISTORY
It is a remarkable fact in human history that civilization began in
regions which are at present inhabited chiefly by backward races,
and the centres of progress have shifted from one country to another
with the passage of time. Many accidental factors—position on
trade-routes, possession of special mineral advantages, and so on,
have undoubtedly played a part in this, but it will not be difficult
to show that climatic fluctuations have also had their share.
A brilliant study of Ellsworth Huntington[13] has shown that there
are certain optimum conditions of climate which are most suitable
for efficient work. These conditions, which were determined by an
analysis of the output of work in American factories, were then
found to be just those which prevail in the most progressive regions
of the globe, which are located in the temperate storm-belts, and
it is shown in certain instances that fluctuations in the position
of this storm-belt coincided with fluctuations in the centres of
civilization. A few additional examples of this may be given.
The beginnings of civilization may reasonably be placed with the
transition from the Palæolithic to the Neolithic type, a transition
which involved much more than just the polishing of stone weapons.
It involved also the beginnings of agriculture, crude pottery, and
later, the domestication of animals. One of the earliest Neolithic
cities known is probably that of Anau, near Askabad in Transcaspia,
excavated by Pumpelly in 1904. From the thickness of the accumulated
debris the date of first settlement is placed at or before 8000
B.C., i.e. 10,000 years ago, or during the period which in Europe is
assigned to the concluding stages of the Wurm glaciation. Pumpelly’s
time-estimates are based on careful comparison with accumulations
in Merv and other cities. At present the mean annual rainfall in
that part of Turkestan is below ten inches a year, and the country
is practically desert, and is entirely unfitted for agriculture.
But with the remains of the ice-sheet still over Scandinavia and
depressions following a more southerly course along the Mediterranean
basin and into southern Asia, the rainfall was considerably heavier,
and the climate in general was more suited to a progressive race.
At the outset we find this Neolithic race living in rectangular
houses built of uniform sun-dried bricks; they were skilful potters,
cultivating cereals, but at first without domestic animals.
The beginning of Neolithic civilization in Crete is placed by Evans
at about 12000 B.C., while on the basis of excavations by de Morgan
at Susa in Persia, Montelius places the origin of Neolithic culture
in this part of Asia as early as about 18000 B.C. At Susa the
deposits are 130 feet thick, and of these the upper 40 feet cover a
period of 6000 years.
Thus we see that what may be considered as the great step from
savagery to civilization took place while the present centres of
progress in Europe and America were still in the Ice Age. At this
time the climate of southern Asia must have resembled the present
climate of north-west Europe in heavier rainfall and the day-to-day
fluctuations of weather—in fact, the districts where civilization
began probably had at that time the most stimulating climate in the
northern hemisphere.
With the vanishing of the ice-sheets and the setting in of the mild
climate of the Maritime phase the Neolithic culture spread rapidly to
Europe, and by 2000 B.C. even the Baltic regions were well inhabited,
and it is probable that the Aryan race was developing in the Russian
steppes. About this time Anau was abandoned owing to increasing
aridity.
With the coming of the Bronze Age in western Europe, about 1800
B.C., however, the climate again became colder and rainier,
corresponding to the Peat-bog phase or “Classical” rainfall maximum,
the deterioration culminating in the Early Iron Age. This period
was marked by a great southward spread of the Aryan peoples, and
ushered in the Heroic Age of Greece. The races of the Mediterranean,
as we have seen, continued to thrive throughout this rainy period,
and their power did not diminish until its close, about A.D. 400.
This downfall was accelerated if not caused by the pressure of nomad
peoples driven out of Asia by the increasing drought. These Asiatic
migrations included the great marches of the Tartar hordes and, aided
by religious enthusiasm, the conquests of the Moslems.
The early Middle Ages, after the downfall of Rome, appear to have
been characterized by a dry warm climate. This was the age of the
Vikings, when the Norse races rose to dominance in western Europe,
finally invading and occupying large areas of France and Britain,
and even extending their power to Sicily. With the increasing cold
and wet of the “Mediæval” rainfall maximum came a final burst
of Norse migration, which left the homeland poor and scantily
populated, and the centre of activity and progress lay once again
with the Mediterranean peoples, and especially with Italy and Spain.
The Tartar invasions ceased, and against the increasing power of
Europe the Moslem wave broke and receded. At the close of this
rainy period political dominance again moved north. From that time
the fluctuations of climate have been of minor importance, and
correspondingly there have been no great shiftings of political
power from latitude to latitude.
BIBLIOGRAPHY.
Tyler, J. M. “The New Stone Age in Northern Europe.” London, 1921.
Huntington, Ellsworth, “World Power and Evolution.” New Haven, 1919.
Haddon, A. C., “The wanderings of peoples.” Cambridge University
Press, 1919.
APPENDIX
THE FACTORS OF TEMPERATURE
To calculate the probable temperature of January or July at any
point, the following procedure should be adopted:
Draw a circle round the point of angular radius ten degrees (i.e. set
the compass to cover ten degrees of latitude) and divide this into
two halves by a line passing from north to south through the centre.
By means of squared tracing paper, or otherwise, measure: (_a_) the
amount of ice in the whole circle; (_b_) the amount of land in the
western half; (_c_) the amount of land in the eastern half. (_a_) is
expressed as a percentage of the area of the whole circle; (_b_) and
(_c_) as percentages of the area of a semicircle.
The term “ice” includes ice-sheets such as that of Greenland or
Antarctica, and also frozen sea or sea closely covered by pack-ice;
the latter figure may vary in different months.
The temperature in January or July is then calculated from the
following formula:
Temperature = basal temperature + ice coeff. x per cent. of ice +
land west coeff. x per cent. of land to west + land east coeff. x per
cent. of land to east.
The basal temperatures and the appropriate coefficients are given in
the following table.
In calculating the effect of a given slight change of land and sea
distribution, it is not necessary to employ the basal temperature.
Instead the equation can be treated as a differential, and the change
of temperature due to the change of land and ice calculated from the
figures in columns 3 to 5. The figures are given in degrees absolute,
273°0 = 32° F. To convert differences to Fahrenheit, multiply by 1°8.
---------+-------------+----------+-----------+-----------
Latitude.| Basal Temp. |Ice Coeff.| Land, | Land,
|(Water Zone).| |West Coeff.|East Coeff.
---------+-------------+----------+-----------+-----------
Jan. | a. | | |
70 N. | 298.8 | -0.49 | -0.43 | -0.20
60 | 277.4 | -0.07 | -0.31 | -0.01
50 | 276.8 | -0.09 | -0.29 | 0.09
40 | 282.5 | -- | -0.17 | 0.04
30 | 289.6 | -- | -0.08 | 0.03
20 | 294.2 | -- | -0.01 | -0.01
10 | 298.6 | -- | -0.01 | 0.03
0 | 299.3 | -- | 0.01 | 0.00
10 S. | 298.2 | -- | 0.04 | -0.01
20 | 296.2 | -- | 0.07 | 0.00
30 | 293.5 | -- | 0.06 | 0.03
40 | 289.3 | -- | 0.09 | -0.03
| | | |
July. | | | |
70 N. | 279.3 | -0.16 | 0.02 | 0.02
60 | 280.7 | -- | -0.01 | 0.11
50 | 285.8 | -- | 0.04 | 0.06
40 | 291.1 | -- | 0.05 | 0.07
30 | 296.8 | -- | 0.08 | -0.01
20 | 297.6 | -- | 0.07 | 0.02
10 | 298.8 | -- | 0.03 | -0.01
0 | 298.6 | -- | 0.02 | -0.01
10 S. | 296.9 | -- | 0.04 | -0.03
20 | 293.1 | -- | 0.02 | -0.02
30 | 288.2 | -- | -0.01 | -0.01
40 | 284.0 | -- | 0.00 | -0.03
---------+-------------+----------+-----------+-----------
In the case of the calculation of the effect of comparatively slight
and irregular changes in land and sea distribution in a limited area,
such as those of the _Littorina_ Sea referred to on p. 128, it may
be found that a ten-degree circle is too wide an area to employ, the
changes from land to sea at one point being nullified by changes
from sea to land at another more distant point. In such a case a
smaller unit such as a circle of five degrees radius can be employed.
As a rough approximation it may be said that the effect of the
conversion of a square mile of land into sea, or _vice versa_, on
the temperature of a neighbouring point is inversely proportional to
its distance. Since the area of a five-degree circle is one-quarter
that of a ten-degree circle, while the average distance of the
land composing it is one-half, we have to divide our regression
coefficients by two in order to fit the new data.
This method was applied to obtain the probable temperature
distribution on the shores of the _Littorina_ Sea at its maximum
extension, and gave results which agreed remarkably well with those
calculated by geologists from the animal and plant life of the time.
See London _Q. F. R. Meteor. Soc._, 43, 1917, pp. 169-171.
INDEX
A.
Acheulian, 52
_Aciphylla_, 125
Africa, 103, 133, 142
Aftonian, 87
AHLMANN, 51, 61
Alaska, 43, 124
Algonquin, Lake, 123
Alps, dry period, 122
glaciation, 29, 52, 56
retreat stadia, 119
Altai Mountains, 77
Anau, ruins, 163
_Ancylus_, 120, 127
ANDERSSON, 118, 121
Andes, 98
Antarctica, 114, 133
Anticyclonic circulation, 55
Antipodes Is., 112
Aral Sea, 83
Argentine, 100
Arizona, 94, 150
ARRHENIUS, 19
Artesian water (Australia), 110
Aryans, 164
Asia, 76, 125, 139, 143, 153
Astronomical theory, 17
Atlantic Stage, 126
Atlas Mountains, 69
Australia, 109, 125, 155
B.
Balearic Is., 70
Balkans, 69
Baltic Interstadial, 64
Banded clays, 49, 93
Baraba steppes, 121
Barkans, 65
BARRELL, 159
BEDDARD, 115
Belfast, 130
_Biloculina_, 133
BLYTT, 127
Bonneville, Lake, 93
Brazil, 101
British Isles, 57, 62, 64, 136
BRÖGGER, 129
Bronze Age, 138
BRÜCKNER, 49, 57, 154
Buenos Aires, sand-dunes, 125
Bühlstadium, 119
C.
Calabrian, 68
Cambrian, 33
Campbell Is., 102
Canada, post-glacial, 132
Cape Colony, raised beaches, 133
Carbon dioxide, 19
Carboniferous, 34
Caspian, 83, 143, 153
CHAMBERLIN, 19
Champlain Stage, 123
Chellean, 51
CHERRY, 160
Chile, rainfall fluctuations, 157
China, 81, 139
Chronology, 48, 92
CHUDEAU, 106
Classical Rainfall Maximum, 140
Climatic Record, 132
COLEMAN, 92
Colorado, 94
Continentality, 25
Continental Phase, 120
Continents, movement of, 21
Cordilleran glaciation, 87
Corsica, 69
CRAIG, 72
Cretaceous, 37
Crete, Neolithic, 163
CROLL, 18
Cro-Magnards, 161
Cyrenaica, desiccation, 142
D.
Daun-stadium, 119
DAVID, 110
Dead ice, 132
Denmark, continental phase, 122
Depressions, path of, 47, 60, 71, 122, 139
Devonian, 34
Diluvium, 48
Don Valley, 91
DOUGLASS, 143
Drakensberg Mountains, 103
Drought in Forest Period, 139
Drumkelin Bog, 137
Dunes, fossil, 65
Frisian, 140
E.
Early Iron Age, 141
Earth’s Orbit, eccentricity of, 18
Earthworms, 115
East Anglia, 47, 57
Eccentricity of Earth’s Orbit, 18
Ecuador, 99
Egypt, 72
_Eoanthropus_, 161
Eocene glaciation, 37
Etosha Pan, 107
Europe, 49, 55, 118, 127, 136, 154
EVANS, 163
Evolution of Man, 155
F.
FAIRGRIEVE, 138
Falkland Is., 97
Fennoscandian Pause, 119
Finiglacial, 118
Finland, post-glacial, 120, 128
Florida, 95
Forest bed, 47, 51
period, 122, 136
Forests, submerged, 137
Formby and Leasowe Beds, 130
Fossil ice, 59, 78
Franz Josef Land, 130
FRECH, 20
FREYDENBERG, 106
Frisian dunes, 140
Fucino, Lago di, 154
_Fucus_ in Spitzbergen, 130
G.
Gable Island, 98
_Galaxiidæ_, 115
GEER, G. DE, 49, 93, 118
GEIKIE, J., 51, 81
Geographical theory, 22
Geological formations, 31
rhythms, 38
GIBBON, 140
Gibraltar, 69, 70
Gila conglomerate, 95
Glacial anticyclone, 55
stages, 48
_Globigerina_, 133
_Glossopteris_, 35
Gondwanaland, 34, 35
Gotiglacial, 118
Graham Land, glaciation, 114
Great Basin, America, 89, 93, 124
Great Lakes, history, 123
Great Salt Lake, 93
Greece, Heroic Age, 164
Greenland, 131, 156
GREGORY, 104
Grimaldi Race, 161
Gschnitz Stadium, 119
Gunz Glaciation, 56
Gunz-Mindel Interglacial, 50, 51, 56
H.
_Haplochitonidæ_, 115
Hazel, post-glacial extension, 122
HEDLEY, 116
Heidelberg Man, 161
Height and temperature, 26
HILDEBRANDSSON, 157
Himalayas, 81
HOBLEY, 105, 107
Hohokam, 150
HUME, 72
HUMPHREYS, 20
HUNTINGTON, 141, 144, 150, 153, 162
I.
Ice on Danish coasts, 155
Iceland, 125, 156
Illinoian glaciation, 90
Ingo Is., forests, 122
Iowan Glaciation, 90
Ireland, glaciation, 57, 62, 64
Heroic Age, 138
Iroquois, Lake, 123
Isohalines, 127
J.
Japan, 81
Jurassic, 37
K.
Kalahari, 107
Kamchatka, 80
Kansan, 88
Karst flora, 121
Kashmir, 143, 153
Keewatin, 88, 91
KEIDEL, 99
Kenya, 103
Kilimanjaro, 103
Kioga, Lake, 104
Kitchen-midden, 125
Kosciusko, 109
KREICHGAUER, 20
KUPFFER, 121
L.
Labradorean Glaciation, 87, 89, 90
Lahontan, Lake, 93
Lena Valley, 78
LEVERETT, 91, 92
Limestone Agglomerate, 70
_Littorina_, 128
Loess, 52, 83, 91, 112
Lofoten Islands, 61
Lop-Nor, 83, 153
M.
MACKENNA, 157
Maglemose culture, 125
Malta, 69
Mammoths, frozen, 79
Marsupials, 115
MATHEW, 160
Maumee, Lake, 123
Maya ruins, 151
Mediæval Rainfall Maximum, 164
Medicine Bow Range, 94
Mediterranean, 68, 142
Mesopotamia, Empires, 139
Mexico, culture, 151
MEYER, 99
Micmac Stage, 124
Mindelian Glaciation, 49, 69
Mindel-Riss Interglacial, 50
Miocene, 44
Mombasa, 105
Mono Basin, 94
MONTELIUS, 163
Mousterian Man, 63
MUNTHE, 118
MURGOCI, 66
Murman coast, 130
N.
_Najas_, 129
Neanderthal Man, 161
NEGRO, 142
Neolithic, 122, 131, 136, 163
migration, 125, 163
Neudeckian, 51
NEUHAUSS, 111
Newfoundland, 87, 90
New Guinea, 111
New Siberian Islands, 78
New South Wales, 125
New Zealand, 111, 125, 133
Ngami, Lake, 107
Niagara, 93, 132
Nile, 72, 119
NORDENSKJOLD, 117
Nordic Race, 125
Norfolkian, 51
North America, 86, 122, 132, 141, 149
North Sea, 56, 61
Norway, 51, 55, 129
O.
Obliquity of Ecliptic, 16, 120
Old Red Sandstone, 34
Optimum of Climate, 127
Ordovician, 33
P.
Pajaritan, 150
Palmyra, 142
Pamirs, 77
Pampean, 100, 125
Patagonia, post-glacial, 133
Patom Highlands, 78
Peat-bog Period, 140
PENCK, 49, 51
Pendulation Theory, 20
Peorian, 91
Permian, 35
Persia, 84, 142
Peru, 99
PETTERSSON, 134, 145
Piedmont ice-sheets, 57, 109
Piltdown Man, 161
_Pithecanthropus_, 160
Pliocene, 47
Pluvial periods, 71, 140
Poles, motion of, 20, 40
Pre-Cambrian Glaciation, 33
Proterozoic Glaciation, 32
Pueblo ruins, 150
Pulse of Asia, 153
PUMPELLY, 84, 163
Pyrenees, 57
Q.
Quaternary Ice Age, 47
R.
Ragunda, Lake, 49
moraines, 121
REID, 138
Retreat of the Ice, 49
Riss Glaciation, 49, 61
Riss-Wurm Interglacial, 50, 53
Rixdorf, 62
RODGERS, 133
Romania, 66
Ruwenzori, 103
S.
Sagas, 141, 146
Sahara, 74, 105
Sangamon, 90
Scania, 49
SCHMIDT, 53
Scotland, 57, 61, 64
_Scrobicularia_ Zone, 130
Selsey, 58
_Sequoia_, 143
Shell-banks, 47, 56
Siberia, 78
Sicilian, 70
Sierra Nevada, 93, 94
SIEVERS, 99
Silurian, 33
Skærumhede, 63
Slugs, 115
SMITH, ELLIOTT, 159
Solar radiation, 15
South America, 97, 125, 132, 157
South Georgia, 97
South Orkneys, 114
SPITALER, 18
Spitzbergen, 80, 130
Stanovoi Mountains, 79
Steppe climate, 53
Stone rivers, 98
Submerged forests, 137
Suess, Lake, 104
Sunspots, 145
Susa, Neolithic, 163
SVEN HEDIN, 84
Sweden, 49, 56, 118
Syria, 72
T.
_Tapes_, 129
Tasmania, 109
Tchad, 106
Tertiary, 42, 116
_Thracia_ Zone, 131
Tian-Shan Mountains, 77
Tibet, 82
Tidal friction, 39
Tide-generating force, 134, 145
Tierra del Fuego, 97, 133
Tillite, 32
Titicaca, Lake, 101
Toronto Stage, 91
_Trapa_, 129
Trasimeno, Lake, 154
Tree-rings and rainfall, 143
Turbarian, 140
Triassic, 37
TYNDALL, 19
U.
Uinta Mountains, 94
_Unio_ in Niagara, 132
Ural Mountains, 57
V.
Venezuela, 100
Verkhoiansk Mountains, 79
Victoria Nyanza, 104
Vikings, 164
Volcanic dust, 16, 20
VOLLOSSOVITSCH, 79
W.
Wales, 57, 64
Warren, Lake, 123
Wasatch Mountains, 93, 94
WAYLAND, 104
WEGENER, 20, 34
WERNERT, 53
White Sea, 130
Wine harvest, 155
Winters, severe, 155
Wisconsin Glaciation, 91, 92
WOLF, 145
Wurm Glaciation, 48
Y.
Yarmouth Stage, 88
_Yoldia_ Sea, 50, 124
Yucatan, 151
Yukon, 124
_Printed in Great Britain by Jarrold & Sons, Ltd., Norwich._
FOOTNOTES:
[1] By this term we shall in future understand only that part of it
which is responsible for thermal effects.
[2] If the figure of the earth is adjusted to its speed of rotation
before the development of ice-sheets, the latter renders it too
prolate, and there will be a tendency for readjustment by the
transference of mass towards the equator.
[3] This has been the subject of much discussion recently. For a
summary see _Science Progress_, 17, 1922, October, p. 233.
[4] Leverett, F. (see Bibliography).
[5] See reference to Antevs in this connexion.
[6] “The pulse of Asia,” p. 356. See also a new work by E.
Huntington, entitled: “Climatic changes.”
[7] “Climatic variations in historic and prehistoric time.”
[8] “Sur le prétendu changement du climat européen en temps
historique.”
[9] Or lemur-like ancestor. There is evidence to show that man’s
ancestor was a nocturnal animal, whose food supply was governed by
the phases of the moon.
[10] “Scientific monthly,” New York, 4, 1917, pp. 16-26.
[11] “Science progress,” 15, 1920, p. 74.
[12] “Climate and evolution.”
[13] “Civilization and climate.”
TRANSCRIBER’S NOTE
Obvious typographical errors and punctuation errors have been
corrected after careful comparison with other occurrences within
the text and consultation of external sources.
Some hyphens in words have been silently removed, some added,
when a predominant preference was found in the original book.
Except for those changes noted below, all misspellings in the text,
and inconsistent or archaic usage, have been retained.
Pg 64: ‘powerful conviction’ replaced by ‘powerful convection.
Pg 97: ‘and Tierra del Fuega’ replaced by ‘and Tierra del Fuego’.
Pg 103: ‘Drakenberge Mountains’ replaced by ‘Drakensberg Mountains’.
Pg 150: ‘modern Pueblas who’ replaced by ‘modern Pueblos who’.
Pg 166: ‘coffiecients are’ replaced by ‘coefficients are’.
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