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Title: Submerged forests
Author: Reid, Clement
Language: English
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The Cambridge Manuals of Science and
Literature
SUBMERGED FORESTS
CAMBRIDGE UNIVERSITY PRESS
London: FETTER LANE, E.C.
C. F. CLAY, MANAGER
[Illustration: Decoration]
Edinburgh: 100, PRINCES STREET
London: H. K. LEWIS, 136, GOWER STREET, W.C.
WILLIAM WESLEY & SON, 28, ESSEX STREET, STRAND
Berlin: A. ASHER AND CO.
Leipzig: F. A. BROCKHAUS
New York: G. P. PUTNAM’S SONS
Bombay and Calcutta: MACMILLAN AND CO., LTD.
_All rights reserved_
[Illustration: Buried Forest seen at low water at Dove Point, on the
Cheshire coast]
[Illustration:
SUBMERGED
FORESTS
BY
CLEMENT REID, F.R.S.
Cambridge:
at the University Press
1913]
Cambridge:
PRINTED BY JOHN CLAY, M.A.
AT THE UNIVERSITY PRESS
_With the exception of the coat of arms at
the foot, the design on the title page is a
reproduction of one used by the earliest known
Cambridge printer, John Siberch, 1521_
PREFACE
Knowledge cannot be divided into compartments, each given a definite
name and allotted to a different student. There are, and always must
be, branches of knowledge in which several sciences meet or have
an interest, and these are somewhat liable to be neglected. If the
following pages arouse an interest in one of the by-ways of science
their purpose has been fulfilled.
C. R.
_February 17, 1913._
CONTENTS
CHAP. PAGE
Preface v
I. Introductory 1
II. The Thames Valley 11
III. The East Coast 19
IV. The Dogger Bank 39
V. The Irish Sea and the Bristol Channel 50
VI. The English Channel 64
VII. Cornwall and the Atlantic Coast 80
VIII. Summary 105
Bibliography 122
Index 125
LIST OF ILLUSTRATIONS
FIG. PAGE
Buried Forest seen at low-water at Dove Point,
on the Cheshire coast. (From the _Cambridge
County Geography of Cheshire_) _frontispiece_
1. Diagram to show the relations of the Submerged
Forests to the sea-level 7
2. Section at Tilbury Docks 14
3. Section across the Humber between Hessle and
Barton 36
4. Approximate Coast-line at the period of the
lowest Submerged Forest 40
CHAPTER I
INTRODUCTORY
Most of our sea-side places of resort lie at the mouths of small
valleys, which originally gave the fishermen easy access to the
shore, and later on provided fairly level sites for building. At
such places the fishermen will tell you of black peaty earth,
with hazel-nuts, and often with tree-stumps still rooted in the
soil, seen between tide-marks when the overlying sea-sand has been
cleared away by some storm or unusually persistent wind. If one is
fortunate enough to be on the spot when such a patch is uncovered
this “submerged forest” is found to extend right down to the level
of the lowest tides. The trees are often well-grown oaks, though
more commonly they turn out to be merely brushwood of hazel, sallow,
and alder, mingled with other swamp-plants, such as the rhizomes of
_Osmunda_.
These submerged forests or “Noah’s Woods” as they are called locally,
have attracted attention from early times, all the more so owing to
the existence of an uneasy feeling that, though like most other
geological phenomena they were popularly explained by Noah’s deluge,
it was difficult thus to account for trees rooted in their original
soil, and yet now found well below the level of high tide.
It may be thought that these flats of black peaty soil though curious
have no particular bearing on scientific questions. They show that
certain plants and trees then lived in this country, as they do now;
and that certain animals now extinct in Britain once flourished here,
for bones and teeth of wild-boar, wolf, bear, and beaver are often
found. Beyond this, however, the submerged forests seem to be of
little interest. They are particularly dirty to handle or walk upon;
so that the archaeologist is inclined to say that they belong to the
province of geology, and the geologist remarks that they are too
modern to be worth his attention; and both pass on.
Should we conquer our natural repugnance for such soft and messy
deposits, and examine more closely into these submerged forests,
they turn out to be full of interest. It is largely their extremely
inconvenient position, always either wet or submerged, that has
made them so little studied. It is necessary to get at things more
satisfactorily than can be done by kneeling down on a wet muddy
foreshore, with the feeling that one may be caught at any time by the
advancing tide, if the study is allowed to become too engrossing. But
before leaving for a time the old land-surface exposed between tide
marks, it will be well to note that we have already gained one piece
of valuable information from this hasty traverse. We have learnt that
the relative level of land and sea has changed somewhat, even since
this geologically modern deposit was formed.
Geologists, however, sometimes speak of the submerged forests
as owing their present position to various accidental causes.
Landslips, compression of the underlying strata, or the removal of
some protecting shingle-beach or chain of sand-dunes are all called
into play, in order to avoid the conclusion that the sea-level
has in truth changed so recently. The causes above mentioned have
undoubtedly all of them affected certain localities, and it behoves
us to be extremely careful not to be misled. Landslips cannot happen
without causing some disturbance, and a careful examination commonly
shows no sign of disturbance, the roots descending unbroken into
the rock below. It is also evident in most cases that no landslip
is possible, for the “forest” occupies a large area and lies nearly
level.
Compression of the underlying strata, and consequent sinking of the
land-surface above, is however a more difficult matter to deal with.
Such compression undoubtedly takes place, and some of the appearances
of subsidence since the Roman invasion are really cases of this sort.
Where the trees of the submerged forest can be seen rooted into
hard rock, or into firm undisturbed strata of ancient date, there
can, however, be no question that their position below sea-level is
due to subsidence of the land or to a rise of the sea, and not to
compression. But in certain cases it is found that our submerged
land-surface rests on a considerable thickness of soft alluvial
strata, consisting of alternate beds of silt and vegetable matter.
Here it is perfectly obvious that in course of time the vegetable
matter will decay, and the silt will pack more closely, thus causing
the land-surface above slowly to sink. Subsidence of this character
is well known in the Fenland and in Holland, and we must be careful
not to be misled by it into thinking that a change of sea-level has
happened within the last few centuries. The sinking of the Fenland
due to this cause amounts to several feet.
The third cause of uncertainty above mentioned, destruction of some
bank which formerly protected the forest, needs a few words. It is
a real difficulty in some cases, and is very liable to mislead the
archaeologist. We shall see, however, that it can apply only to a
very limited range of level.
Extensive areas of marsh or meadow, protected by a high shingle-beach
or chain of sand-dunes, are not uncommon, especially along our
eastern coast. These marshes may be quite fresh, and even have trees
growing on them, below the level of high tide, as long as the
barrier remains unbroken. The reason of this is obvious. The rise
and fall of the tide allows sea water to percolate landward and the
fresh water to percolate seaward; but the friction is so great as to
obliterate most of the tidal wave. Thus the sea at high tide is kept
out, the fresh water behind the barrier remaining at a level slightly
above that of mean tide, and just above that level we may find a wet
soil on which trees can grow. But, and here is the important point, a
protected land-surface behind such a barrier can never lie below the
level of mean tide; if it sinks below that level it must immediately
be flooded, either by fresh water or by sea water. This rule
applies everywhere, except to countries where evaporation exceeds
precipitation; only in such countries, Palestine for instance, can
one find sunk or Dead Sea depressions below mean-tide level of the
open sea.
The submerged forest that we have already examined stretched far
below the level of mean tide, in fact we followed it down to the
level of the lowest spring tides. Nothing but a change of sea-level
will account for its present position. In short, the three objections
above referred to, while teaching us to be careful to examine the
evidence in doubtful cases, cannot be accepted as any explanation
of the constant and widespread occurrence of ancient land-surfaces
passing beneath the sea.
We have thus traced the submerged forest down to low-water mark, and
have seen it pass out of our reach below the sea. We naturally ask
next, what happens at still lower levels? It is usually difficult
to examine deposits below the sea-level; but fortunately most of
our docks are excavated just in such places as those in which the
submerged forests are likely to occur. Docks are usually placed in
the wide, open, estuaries, and it is often necessary nowadays to
carry the excavations fully fifty feet below the marsh-level. Such
excavations should be carefully watched, for they throw a flood of
light on the deposits we wish to examine.
Every dock excavation, however, does not necessarily cut through
the submerged forests, for channels in an estuary are constantly
shifting, and many of our docks happen to be so placed as to coincide
with comparatively modern silted-up channels. Thus at King’s Lynn
they hit on an old and forgotten channel of the Ouse, and the bottom
of the dock showed a layer of ancient shoes, mediaeval pottery, and
such-like—interesting to the archaeologist, but not what we are now
in search of. At Devonport also the recent dock extension coincided
with a modern silted-up channel. In various other cases, however, the
excavations have cut through a most curious alternation of deposits,
though the details vary from place to place.
The diagram (fig. 1) shows roughly what is found. We will suppose
that the docks are placed, as is usually the case, on the salt
marshes, but with their landward edge reaching the more solid rising
ground, on which the warehouses, etc., are to be built. Beginning at
or just above the level of ordinary high-water of spring tides, the
first deposit to be cut through is commonly a bed (_A_) of estuarine
silt or warp with remains of cockles, _Scrobicularia_, and salt-marsh
vegetation. Mingled with these we find drifted wreckage, sunk boats,
and miscellaneous rubbish, all belonging to the historic period.
The deposits suggest no change of sea-level, and are merely the
accumulated mud which has gradually blocked and silted up great part
of our estuaries and harbours during the last 3500 years.
[Illustration: Fig. 1.]
This estuarine silt may continue downward to a level below mean tide,
or perhaps even to low-water level; but if the sequence is complete
we notice below it a sudden change to a black peaty soil (_B_),
full of vegetable matter, showing sallows, alder, and hazel rooted
in their position of growth. In this soil we may also find seams of
shell-marl, or chara-marl, such as would form in shallow pools or
channels in a freshwater marsh. This black peaty soil is obviously
the same “submerged forest” that we have already examined on the
foreshore at the mouth of the estuary; the only difference being that
in the more exposed situation the waves of the sea have cleared away
the overlying silt, thus laying bare the land surface beneath. In
the dock excavations, therefore, the submerged forest can be seen in
section and examined at leisure.
The next deposit (_C_), lying beneath the submerged forest, is
commonly another bed of estuarine silt, extending to a depth of
several feet and carrying our observations well below the level of
low-water. Then comes a second land-surface (_D_), perhaps with trees
differing from those of the one above; or it may be a thick layer of
marsh peat. More silt (_E_) follows; another submerged forest (_F_);
then more estuarine deposits (_G_); and finally at the base of the
channel, fully 50 feet below the level of high-water, we may find
stools of oak (_H_) still rooted in the undisturbed rock below.
As each of these deposits commonly extends continuously across the
dock, except where it happens to abut against the rising ground, it
is obvious that it is absolutely cut off from each of the others.
The lowest land-surface is covered by laminated silts, and that
again is sealed up by the matted vegetation of the next growth. Thus
nothing can work its way down from layer to layer, unless it be a
pile forcibly driven down by repeated blows. Materials from the older
deposits in other parts of the estuary may occasionally be scoured
out and re-deposited in a newer layer; but no object of a later
period will find its way into older beds.
Thus we have in these strongly marked alternations of peat and warp
an ideal series of deposits for the study of successive stages.
In them the geologist should be able to study ancient changes of
sea-level, under such favourable conditions as to leave no doubt
as to the reality and exact amount of these changes. The antiquary
should find the remains of ancient races of man, sealed up with his
weapons and tools. Here he will be troubled by no complications
from rifled tombs, burials in older graves, false inscriptions, or
accidental mixture. He ought here to find also implements of wood,
basket-work, or objects in leather, such as are so rarely preserved
in deposits above the water-level, except in a very dry country.
To the zoologist and botanist the study of each successive layer
should yield evidence of the gradual changes and fluctuations in our
fauna and flora, during early periods when man, except as hunter,
had little influence on the face of nature. If I can persuade
observers to pay more attention to these modern deposits my object is
secured, and we shall soon know more about some very obscure branches
of geology and archaeology.
I do not wish to imply that excellent work has not already been done
in the examination of these deposits. Much has been done; but it
has usually been done unsystematically, or else from the point of
view of the geologist alone. What is wanted is something more than
this—the deposits should be examined bed by bed, and nothing should
be overlooked, whether it belong to geology, archaeology, or natural
history. We desire to know not merely what was the sea-level at each
successive stage, but what were the climatic conditions. We must
enquire also what the fauna and flora were like, what race of man
then inhabited the country, how he lived, what weapons and boats he
used, and how he and all these animals and plants were able to cross
to this country after the passing away of the cold of the Glacial
period.
To certain of the above questions we can already make some answer;
but before dealing with conclusions, it will be advisable to give
some account of the submerged land-surfaces known in various parts of
Britain. This we will do in the next chapters.
Before going further it will be well to explain and limit more
definitely the field of our present enquiry. It may be said that
there are “submerged forests” of various geological dates, and this
is perfectly true. The “dirt-bed” of the Isle of Purbeck, with its
upright cycad-stems, was at one time a true submerged forest, for
it is overlain by various marine strata, and during the succeeding
Cretaceous period it was probably submerged thousands of feet. Every
coal seam with its underlying soil or “underclay” penetrated by
stigmarian roots was also once a submerged forest. Usage, however,
limits the term to the more recent strata of this nature, and to
these we will for the present confine our attention. We do not
undertake a description of the earlier Cromer Forest-bed, or even of
the Pleistocene submerged forests containing bones of elephant and
rhinoceros and shells of _Corbicula fluminalis_. These deposits will,
however, be referred to where from their position they are liable to
be confounded with others of later date.
CHAPTER II
THE THAMES VALLEY
In the last chapter an attempt was made to give a general idea of
the nature of the deposits; we will now give actual examples of what
has been seen. Unfortunately we cannot say “what can be seen,” for
the lower submerged forests are only visible in dock excavations.
As these works are carried well below the sea-level and have to
be kept dry by pumping, it is impossible for them to remain open
long, and though new excavations are constantly being made, the old
ones are nearly always hidden within a few weeks of their becoming
visible. Of course these remarks do not apply to the highest of
these submerged land-surfaces, which can be examined again and again
between tide-marks, whenever the tide is favourable and the sand of
the foreshore has been swept away.
The most convenient way of dealing with the evidence will perhaps be
to describe first what has been seen in the estuary of the Thames.
Then in later chapters we will take the localities on our east coast
and connected with the North Sea basin. Next we will speak of those
on the Irish Sea and English Channel. Lastly, the numerous exposures
on the west or Atlantic coast will require notice, and with them may
be taken the corresponding deposits on the French coast. Each of
these groups will require a separate chapter.
The Thames near London forms a convenient starting point, for the
numerous dock-excavations, tunnels, deep drains and dredgings have
laid open the structure of this valley and its deposits in an
exceptionally complete way. The published accounts of the excavations
in the Thames Valley are so voluminous that it is impossible here to
deal with them in any detail; we must therefore confine ourselves
to those which best illustrate the points we have in view, choosing
modern excavations which have been carefully watched, noted, and
collected from rather than ancient ones.
We cannot do better than take as an illustration of the mode of
occurrence and levels of the submerged land-surfaces the section
seen in the excavation of Tilbury Docks, for this was most carefully
noted by the engineers, and was visited by two competent observers,
Messrs. W. Whitaker and F. C. J. Spurrell. This excavation is of
great scientific importance, for it led to the discovery of a human
skeleton beneath three distinct layers of submerged peat, and these
remains have been most carefully studied by Owen and Huxley, and more
recently by Professor Keith.
The section communicated to Sir Richard Owen by Mr Donald Baynes, the
engineer superintending the excavation at the time of the discovery,
is shown in the diagram on p. 14. As Mr Baynes himself saw part of
the skeleton in the deposit, his measured section is most important
as showing its exact relation to the submerged forests. It is also
well supplemented by the careful study of the different
[Illustration:
———————————————————————————————————————Trinity High Water Level
Water 6’·77
+————+————————————————————————————+— Marsh Level
| | | ^
| 1 | Clay 6’·04 | |
| | | |
|————+————————————————————————————+ |
| | | |
| | | |
| 2 | Mud 10’·76 | |
| | | |
| | | |
|————+————————————————————————————+ |
| 3 | Mud and Peat 1’·70 | |
|————+————————————————————————————+ |
| 4 | Peat 1’·08 | |
|————+————————————————————————————+ |
| | | |
| 5 | Mud 3’·88 | | } 34’·54
| | | |
|————+————————————————————————————+ |
| 6 | Peat 3’·58 | |
|————+————————————————————————————+ |
| 7 | Mud 1’·76 | |
|————+————————————————————————————+ |
| | | |
| 8 | Mud and Peat 3’·24 | |
| | | |
|————+————————————————————————————+ |
| 9 | Sand & Decomposed Wood ·62 | |
|- - - - - - - - - - - - - - - - -|-v Level at which human
| | | remains were found
| | |
| | |
| 9*| Sand 12’·47 |
| | |
| | |
| | |
|————+————————————————————————————+
| | |
| | |
| 10 | Ballast Gravel |
| | |
Fig. 2. Section at Tilbury Docks.] layers made by Mr Spurrell, for
though his specimens did not come from exactly this part of the
docks, the various beds are traceable over so large an area that
there is no doubt as to their continuity.
Owen thought that this skeleton belonged to a man of the Palaeolithic
period, considering it contemporaneous with the mammoth and
rhinoceros found elsewhere in the neighbourhood. Other geological
writers showed however that these deposits were much more modern,
and some of them spoke somewhat contemptuously of their extremely
recent date. But Huxley saw the importance of this “river-drift
man” as an ancient and peculiar race, and Professor Keith has more
recently drawn especial attention to the well-marked characteristics
of the type. The skeleton is not of Palaeolithic date, but neither is
it truly modern; other examples have turned up in similar deposits
elsewhere.
We will now describe more fully the successive layers met with in
Tilbury Docks, condensing the account from that given by Messrs.
Spurrell and Whitaker, and using where possible the numbers attached
by the engineer to the successive beds.
It will be noticed that the marsh-level lies several feet below
Trinity high water. Below the sod of the marsh came a bed of fine
grey tidal clay (1), in which at a depth of seven feet below
the surface Mr Spurrell noted, in one part of the docks, an old
grass-grown surface strewn with Roman refuse, such as tiles,
pottery, and oyster shells. This fixes the date of the layer above as
post-Roman; but the low position of the Roman land-surface, now at
about mean-tide level, is due in great part to shrinkage since the
marsh was embanked and drained—it is unconnected with any general
post-Roman subsidence of the land.
Beneath the Roman layer occurs more marsh-clay and silt (2), resting
on a thin peat (4) which according to Mr Whitaker is sometimes
absent. Then follows another bed of marsh-clay (5), shown by the
engineer as four feet thick, but which in places thickens to six
or seven feet. Below this is a thick mass (6) of reedy peat (the
“main peat” of Mr Spurrell), which is described as consisting mainly
of _Phragmites_ and _Sparganium_, with layers of moss and fronds
of fern. The other plants observed in this peat were the elder,
white-birch, alder and oak. Associated with them were found several
species of freshwater snails and a few land forms; but the only
animal or plant showing any trace of the influence of salt water was
_Hydrobia ventrosa_, a shell that requires slightly brackish water.
The main peat rests on another bed of estuarine silt (7 and 8), which
seems to vary considerably in thickness, from 5 to 12 feet. It is
not quite clear from the descriptions whether the “thin woody peat”
of Mr Whitaker and the “sand with decayed wood” (9) of the engineer
represent a true growth in place, like the main peat; it is somewhat
irregular and tends to abut against banks of sand (9*) rising from
below. In one of these banks, according to Mr Spurrell, the human
skeleton was found.
The contents of the sand (other than the skeleton) included
_Bythinia_ and _Succinea_; and as Mr Spurrell calls it a “river
deposit,” it apparently did not yield estuarine shells, like the
silts above. The sub-angular flint gravel (10) below has all the
appearance of a river gravel; it may be from 10 to 20 feet thick, and
rests on chalk only reached in borings.
The floor of chalk beneath these alluvial deposits lies about 60 or
70 feet below the Ordnance datum in the neighbourhood of Tilbury and
Gravesend, and in the middle of the ancient channel of the Thames
it may be 10 feet lower; but there is no evidence of a greater
depth than this. We may take it therefore that here the Thames once
cut a channel about 60 feet below its modern bed. We cannot say,
however, from this evidence alone that the sea-level then was only 60
feet below Ordnance datum, for it is obvious that it may have been
considerably lower. If, as we believe, the southern part of the North
Sea was then a wide marsh, the Thames may have followed a winding
course of many miles before reaching the sea, then probably far away,
in the latitude of the Dogger Bank. This must be borne in mind: we
know the minimum extent of the change of level; but its full amount
has to be ascertained from other localities.
This difficulty has seemed of far greater importance than it really
is, and some geologists have suggested that at this period of maximum
elevation, England stood several hundred feet higher above the sea
than it does now. I doubt if such can have been the case. Granting
that the sea may have been some 300 miles away from Tilbury, measured
along the course of the winding river, this 300 miles would need a
very small fall per mile, probably not more than an inch or two. The
Thames was rapidly growing in volume, from the access of tributaries,
and was therefore flowing in a deeper and wider channel, which was
cut through soft alluvial strata; it therefore required less and less
fall per mile. Long before it reached the Dogger it probably flowed
into the Rhine, then containing an enormous volume of water and
draining twice its present catchment area.
The clean gravel and sand which occupy the lower part of the ancient
channel at Tilbury require to be more closely examined, for it is
not clear that they are, as supposed, of fluviatile origin; they may
quite well be estuarine. In the sand Mr Spurrell found the freshwater
shells _Bythinia_ and _Succinea_, and in it was also found the
human skeleton described by Owen; but, according to Mr Spurrell, on
the surface of this sand lay a few stray valves of the estuarine
_Scrobicularia_ and of _Tellina_. The bottom deposits were probably
laid down in a tidal river; but whether within the influence of the
salt water is doubtful.
As far as the Tilbury evidence goes it suggests a maximum elevation
of the land of about 80 feet above its present level; but we will
return to this question when we have dealt with the other rivers
flowing more directly into deep sea. The animals and plants found at
Tilbury were all living species.
It is unnecessary here to discuss more fully the submerged forests
seen in dock and other excavations in the Thames flats, for they
occupy a good many pages in the _Geology of London_ published by the
Geological Survey. Even 250 years ago, the hazel trees were noticed
by the inquisitive Pepys during one of his official visits to the
dockyards, and later writers are full of remarks on the ancient yew
trees and oaks found well below the sea-level. Most of these early
accounts are, however, of little scientific value.
CHAPTER III
THE EAST COAST
It is not our purpose to describe in detail the many exposures of
submerged land-surfaces which have been seen on the shores of the
North Sea. This would serve no useful purpose and would be merely
tedious. We need only say that the floor of Eocene or Cretaceous
strata on which these ancient subaerial deposits rest is constantly
found at depths of 50 or 60 feet below the level of the existing
salt-marsh. But where, as in the estuary of the Thames and Humber, an
older channel underlies a modern channel, the floor sinks about 30
feet lower. From present marsh-level to ancient marsh-level is about
60 feet; from present river-bottom to old river-bottom is also about
60 feet. This, therefore, is the extent of the former elevation,
unless we can prove that the sea was then so far away that the river
once had many miles to flow before reaching it. This is the point we
have now to consider as we trace the submerged forests northward and
towards the deeper seas.
Before we leave the southern part of the North Sea basin it will be
well to draw attention to a few of the half-tide exposures which for
one reason or another may tend to mislead the observer. The mere
occurrence of roots below tide marks is not sufficient to prove that
the land-surfaces seen are all of one date.
Not far from Tilbury is found the well-known geological hunting
ground of Grays, where the brick-yards have yielded numerous extinct
mammalia and several land and freshwater shells now extinct in
Britain. These deposits lie in an old channel of the Thames, cut to
below mean-tide level, but here not coinciding exactly in position
either with the channel of the existing river, or with the channel in
which the submerged forests lie.
It is fortunate that the channels do not coincide, for this enables
us to distinguish the more ancient deposits. A glance at a geological
map shows, however, that they must coincide elsewhere, and where
the Thames has re-occupied its old channel it is clear that the
destruction of the earlier deposits may have led to a mixture of
fossils and implements belonging to three different dates. Mammoth
teeth and Palaeolithic implements, Irish elk and polished stone
implements, may all be dredged up in the modern river gravel,
associated with bits of iron chain, old shoes, and pottery. Such
a mixture does actually occur in the Thames estuary, and it makes
us hesitate to accept the teeth of mammoth which were dredged in
the Thames as really belonging to so late a period as that of the
submerged forests.
At Clacton a similar difficulty is met with, for there again an
ancient channel contains alternating estuarine and freshwater
deposits with layers of peat, and is full of bones belonging to
rhinoceros, hippopotamus, elephant and other extinct mammalia. Of
course the peat-beds in this channel are just as much entitled to
the name “submerged forest” as the more modern deposits to which
recent usage restricts it. They belong, however, to another and more
ancient chapter of the geological record than that with which we are
now dealing. I do not say a less interesting one, for they are of the
greatest importance when we study the times when Palaeolithic man
flourished; but at present we have as much as we can do to understand
the later deposits and to realize the great changes to which they
point. We must not turn aside for everything of interest that we come
across in this study; these earlier strata are worthy of a book to
themselves.
As we travel northward along the coast, again and again we meet with
evidence of a submerged nearly level platform, “basal plane,” or
ancient “plane of marine denudation,” lying about 50 feet below the
sea. We find it at Langer Fort, which lies opposite to Harwich on a
spit of sand and shingle stretching across Harwich Harbour. Here the
floor of London Clay was met with in a boring at 54 feet below the
surface.
The Suffolk coast north of Southwold yields yet another complication,
for between Southwold and Sherringham in Norfolk there appears at
the sea-level a land-surface considerably more ancient than anything
we have yet been dealing with. This is the so-called “Cromer
Forest-bed,” which consists of alternating freshwater and estuarine
beds, with ancient land-surfaces and masses of peat. It contains
numerous extinct mammals, mainly of species older than and different
from those of Clacton and Grays.
The mammalian remains differentiate these deposits at once; but if
no determinable mammals are found, the crushing of the bones and
the greater compression and alteration of the peaty beds serves to
distinguish them, for this Forest-bed dates back to Pliocene times,
passes under a considerable thickness of glacial beds, and has been
over-ridden by the ice-sheet during the Glacial epoch.
The Cromer Forest-bed has been exposed particularly well of late
years at Kessingland, near Lowestoft, where the sea has encroached
greatly. It is well worth while to make a comparative study of this
deposit, of the Grays and Clacton _Cyrena_-bed, of the submerged
forests of the Thames docks, and of the strata now being formed in
and around the Norfolk Broads. By such a comparison we can trace the
effects of similar conditions occurring again and again. The fauna
and flora slowly change, species come and go, man appears and races
change: though the same physical conditions may recur life ever
changes.
The Norfolk Broads, just referred to, deserve study from another
point of view: their origin is directly connected with the
submergence which forms the theme of this book. These broads are
shallow lakes, always occupying part of the widest alluvial flats
which border the rivers; but they are usually out of the direct
course of the present river; they therefore receive little of the
sediment brought down in flood-time. On the other hand they are
steadily being filled up with growing vegetation and turned into peat
mosses.
The origin of these shallow freshwater lakes, which form a
characteristic feature in the scenery of East Anglia, has been
much debated; but with the knowledge obtained from a study of the
submerged forests the explanation is perfectly simple. During this
period of slow submergence each of the shallow valleys in which the
broads now lie was turned into a wide and deep navigable estuary,
which extended inland for many miles. When the subsidence stopped the
sea and tides soon formed bars and sand-banks at the mouths of the
estuaries, and lateral tributaries pushed their deltas across. The
Norfolk rivers, being small and sluggish, were driven to one side,
and could neither cut away the sand-banks nor fill up with sediment
such wide expanses. These estuaries therefore were silted up with
tidal mud and turned into irregular chains of lakes, separated by
irregular bars and sand-banks. The lakes, instead of becoming rapidly
obliterated and filled up by deltas which crept gradually seaward,
remained as freshwater broads; for as soon as a bank became high
enough for the growth of reeds and sedges the river mud was strained
out and only nearly clean water reached the lagoon behind. Thus a
depression once left, provided it was out of the direct course of the
river, tended to remain as a freshwater lake until vegetable growth
could fill it, and the river mud was spread out over the salt-marshes
or went to raise the sand-banks till they became alluvial flats, and
thus still more thoroughly isolated the broad.
A few centuries will see the disappearance of the last of the broads,
which have silted up to an enormous extent within historic times;
but the fact that so many of these broads still exist may be taken
as clear evidence of the recent date of the depression which led to
their formation.
When we look at ancient records, and notice the rapidity with which
the broads and navigable estuaries are becoming obliterated, we
cannot help wondering whether the measure of this silting up may not
give us the date of the last change of sea-level. It should do so
if we could obtain accurate measurements of the amount of sediment
deposited annually, of the rate at which the sea is now washing
it in, and of the rate at which the rivers are bringing it down.
All these factors, however, are uncertain, and it is particularly
difficult to ascertain the part played by the muddy tidal stream
which flows in after storms and spreads far and wide over the marsh.
Though all the factors are so uncertain, we can form some idea of
the date of the submergence. Many years ago I made a series of
calculations, founded on the silting up of our east coast estuaries,
the growth of the shingle-spits, and the accumulation of sand-dunes.
The results were only roughly concordant, but they seemed to show
that the subsidence stopped about 2500 years ago and was probably
still in progress at a date 500 years earlier. This question of dates
will be again referred to in a later chapter.
Before leaving the Broad district we must refer to a boring made at
Yarmouth, which, according to Prof. Prestwich, showed that the recent
estuarine deposits are there 120 feet thick, and consequently that
the ancient valley was far deeper than any recorded in the foregoing
pages. There is no doubt, however, that this interpretation is
founded on a mistake, for other borings at Yarmouth, Lowestoft, and
Beccles came to muddy sands and clays belonging to the upper part of
the Crag, now known to thicken greatly eastward. The recent deposits
descend only to a depth of about 50 feet at Yarmouth, and consist
of sand and shingle; the beds below contain Pliocene mollusca. This
emendation is also borne out by the entirely different character
of the recent estuarine deposits at Potter Heigham, where we again
find a submerged forest at about 56 feet below the marsh-level. The
section recorded by Mr Blake is as follows:—
feet
Bluish-grey loam 24
Grey silty sand ½ to 2
Stiff bluish-grey loam, clay, and silt
full of cockles, &c. 13
Black peat, hard, and much compressed 17
White and buff sand 2
—-
58
It will be noticed that here only one peat bed was found, and was at
the usual depth of the lowest submerged forest. Possibly the white
sand below was the bleached top of the Crag; but this point was not
cleared up.
If we resume our journey northward along the Norfolk coast we come
to the well-known locality of Eccles, where the old church tower
described and figured by Lyell in his _Principles of Geology_ long
stood on the foreshore, washed by every spring tide. The position of
this church formed a striking illustration of the protection afforded
by a chain of sand-dunes. The church was originally built on the
marshes inside these dunes, at a level just below that of high-water
spring tides. But as the dunes were driven inland they gradually
overwhelmed the church, till only the top of its tower appeared above
the sand. In this state it was pictured by Lyell in the year 1839.
Later on (in 1862) it was again sketched by the Rev. S. W. King, and
stood on the seaward side of the dune and almost free from sand. For
a series of years, from 1877 onward, I watched the advance of the
sea, and as the church tower was more and more often reached by the
tides, its foundations were laid bare and attacked by the waves, till
at last the tower fell.
Not only were the foundations of Eccles church exposed on the
foreshore, but an old road across the marshes also appeared on
the seaward side of the dunes, giving a still more exact idea of
the former great influence of the chain of dunes in damping the
oscillations of the tidal wave. The tide outside now rises and falls
some 12 or 15 feet; on the marsh within its influence is only felt
under exceptional circumstances. A road across the marsh at a level
four or five feet below high-water, as this one stood, would still be
passable, except during unusual floods.
Eccles Church is an excellent example of the way in which an ancient
land-surface may now be found below the level of high-water, and yet
no subsidence of the land has taken place. But this coast can give
even more curious examples. It does not need a sand-dune to deaden
the rise and fall of the tides; even a submerged bank will have much
the same effect. Extensive submerged sand-banks extend parallel with
the coast, protecting the anchorage known as Yarmouth Roads. These
banks rise so nearly to the surface of the sea that not only do they
protect the town and anchorage against the waves, they deaden the
tidal oscillation to such an extent that its range is much greater
outside the bank than within.
If these submerged outer banks were to be swept away by some change
in the set of the currents, large areas now cultivated and inhabited
would be flooded by salt water at every spring tide, and the turf
of the meadows would be covered by a layer of marine silt, such
as we see alternating with the submerged forests in the docks of
the Thames. Such alternations, if thin, do not necessarily prove a
change in the level of the sea; they may only point to the alternate
accumulation and removal of sand-banks in a distant part of the
estuary.
The Norfolk coast trends westward soon after leaving Cromer, and
where the cliff seems to pass inland at Weybourn we enter an ancient
valley, one side of which has been entirely cut away by the sea,
except for a few relics of the further bank, now included in the
shingle beach which runs out to sea nearly parallel with the coast
and protects Blakeney Harbour. Here again we find that in the bottom
of the valley there must be a submerged forest, for slabs of peat are
often thrown up at Weybourn, and by the use of a grapnel the peat was
found in place off Weybourn at a depth of several fathoms.
When the coast turns southward again, and the wide bay of the Wash is
entered, we find an extensive development of submerged land-surfaces
and peat beds, extending over great part of the Fenland. In fact the
whole Fenland and Wash was once a slightly undulating plain, cut into
by numerous shallow open valleys. The effect of the submergence of
this area has been to cause the greater part of it to silt up to a
uniform level, through the accumulation of warp and growth of peat;
so that now the Fenland has become a dead level, out of which a few
low hills rise abruptly. The islands of the Fenland, such as those on
which Ely and March are built, are merely almost submerged hill-tops;
they were not isolated by marine action.
It is obvious that a wide sheltered bay of this sort forms an ideal
area in which to study the gradual filling up and obliteration of the
valleys, as the land sank; and it may enable us to learn the maximum
amount of the change of sea-level. The Fenland unfortunately does
not contain very deep dock excavations, and we have only various
shallower engineering works to depend on, though numerous borings
reach the old floor.
A preliminary difficulty, however, meets us in the study of the Fen
deposits; it is the same difficulty that we have already referred
to when describing Clacton and Grays, and we shall meet with it
again. In certain parts of the Fenland, particularly about March and
Chatteris, a sheet of shoal-water marine gravelly sand caps some of
the low hills, which rise a few feet above the fen-level. The gravel
for long was taken to be the same bed that passes under the marshes.
Later work showed however that these gravels, with their sub-arctic
marine fauna and containing also _Corbicula fluminalis_, were of
much earlier date than the true fen-deposits. Just as we saw happen
in the Thames Valley, a wide plain and estuary existed long before
the deeper channels containing the submerged forests were cut; and
the deposits of this older estuary and its tributaries are still to
be found in patches here and there. Sometimes, as at March, they cap
hills a few feet above the fen-level; but as often they fill channels
not quite coinciding with the later channels; just as they do at
Grays. Or two deposits of quite different date may lie side by side,
as they do in the Nar Valley, or at Clacton, or on the Sussex coast.
The true fen-deposits were carefully examined by Messrs. Marshall,
Fisher, and Skertchly, as far as the shallow sections would allow,
and the following account is mainly condensed from that given by Mr
Skertchly in his _Geology of the Fenland_.
During the excavation of certain deep dykes for the purpose of
draining the fens there was discovered at a depth of about 10 feet
below the surface a forest of oaks, with their roots imbedded in the
underlying Kimmeridge Clay. The trunks were broken off at a height of
about three feet. Some of the fallen trees were of fine proportion,
measuring three feet in diameter, quite straight and seldom forked.
At an average height of two feet above this “forest No. 1” the
remains of another were found (in the peat) consisting of oaks and
yews. Three feet above “forest No. 2” lay the remains of another, in
which the trees are all Scotch firs, some of which were three feet in
diameter. Above this and near to the surface was seen a still newer
forest of small firs. The peat close to the surface contains remains
of sallow and alder, and was formed with the sea at its present level.
It will be noticed that the greatest depth at which these rooted
trees were found was only about ten feet below the sea-level. At
this high level we must expect to find that the growth of the peat
was practically continuous, and that the different submerged forests
run together. In adjoining depressions the different forests would
occur at lower levels and would be separated by beds of marine silt.
It does not follow from the position that a low-level submerged
land-surface is older than one at a higher elevation, for above the
present sea-level all these stages are represented by a few inches
of soil, on which forest after forest has grown and decayed. Anyone
who has collected antiquities on fields knows what a curious jumble
of Palaeolithic, Neolithic, bronze age, Roman, mediaeval and recent
things may be found mixed in these few inches of soil, or may be
thrown up by an uprooted tree. The great advantage of studying the
deeply submerged forests is that in them the successive stages are
separated and isolated, instead of being mingled in so confusing a
fashion.
For further information as to the more deeply submerged land-surfaces
we may turn to the numerous records of borings made in the Fenland
and collected by the Geological Survey. These show that the thickness
of the fen-deposits varies considerably from place to place, that
the floor below undulates and is by no means so flat as the surface
of the fen above. Most of these borings, however, were not continued
through the gravels which lie at the base of the deposit, and thus
we can only be certain of the total depth to the Jurassic clay or
boulder clay in a few places. The maximum thickness of the fen-beds
yet penetrated is less than 60 feet, and a submerged forest was found
at Eaubrink at about 40 feet. It is possible however that none of
these scattered borings has happened to hit upon one of the buried
river-channels, which formerly wandered through this clayey lowland;
if one were found it would probably show that the alluvial deposits
are somewhat thicker than these measurements, and that they descend
to a depth about equal to that reached in the valleys of the Thames
or Humber.
It is useless to discuss in more detail the lower submerged forests
of the Fenland, for we cannot get at them to examine them properly.
They have been as effectually overwhelmed and hidden as the remains
of King John’s baggage train, which has never been seen again
since it wandered off the flooded causeway during the disastrous
spring-tide of October 11, 1216, and sank into the soft clay and
quicksands.
The higher submerged forests of the Fenland are however of great
interest, and as already pointed out they have been exposed to view
in cutting the fen dykes, especially near Ely. Perhaps a closer
study of these might enable us to arrive at some idea of the time
taken for the growth of a series of forests of this sort, and for
the accompanying mass of peat. The variations in the flora also
need more exact analysis before we can say what they betoken. The
oak-forest at the bottom is what we should expect on a clay soil; but
the reason for the succession of trees above is not obvious. It need
not necessarily point to climatic change, though it may do so; but
it certainly looks as if the peaty bogs were alternately wetter and
drier, so that sometimes moss grew, and sometimes fir-trees. Neither
need this change imply an up-and-down movement of the land, though it
may be due to such a cause.
Subsidence would destroy the oaks and allow a peat-moss to form;
but if the subsidence were intermittent the moss would increase in
thickness, become more compact, and its surface rise, till it was
dry enough for pines. Another subsidence would cause spongy peat
again to spread and kill the pine, and so on. Intermittent subsidence
seems sufficient to account for all the changes of vegetation we have
yet noticed in connexion with these submerged land-surfaces.
Of the fauna of the fen-silts and peats it is very difficult to give
any satisfactory account. If we put aside the March and Chatteris
marine gravels with _Corbicula fluminalis_, and the Nar Valley Clay
with its northern marine mollusca as being of older date; and if
we also reject the marginal gravels with hippopotamus and mammoth
as being more ancient, there only remain a few mammals such as the
beaver, wolf, wild boar, and certain cetacea, which we can be sure
came out of the true fen-deposits. Implements made by man have only
been found in the higher layers, and there seems to be no record in
this area of a stone implement found below a submerged forest.
Submerged forests of the ordinary type are often to be seen between
tide-marks on the flat shores of Lincolnshire; but as they still
await proper study they need not here detain us, and we will pass on
to the next large indentation of the coast-line, the estuary of the
Humber.
Here, owing to the excavation of extensive docks, and to a series of
trial borings for a tunnel beneath the Humber, the structure of the
valley has been clearly laid open. It is much the same as that of the
Thames; but as we are in a glaciated area we find, as in the Fenland,
that much of the erosion had taken place before or during the Glacial
Epoch, for boulder clay occupies part of the valley.
[Illustration: Fig. 3.]
Boulder clay or till not only occupies part of the valley, it
descends far below the present river bottom and even below the
lowest submerged forest. This we find always to be the case in the
glaciated parts of Britain; but whether the deep trenching is due
to the ploughing out of a trough by a tongue of the ice-sheet,
to sub-glacial streams below sea-level, or to erosion by a true
subaerial river is still a doubtful point. However, this question
must not detain us; we are not now dealing with elevations and
depressions of so ancient a date, and must confine our attention to
post-glacial movements.
The section shown in fig. 3 will explain better than any words the
structure of the Humber Valley. It is drawn to scale from the
engineer’s section, and shows at a glance the three channels. The
deepest and widest channel is that occupied by glacial deposits;
an intermediate channel (shown in black) is occupied by silt and
submerged forests; and a shallower channel is occupied by the present
Humber and its alluvium. One interesting point, however, this section
does not happen to illustrate. Somewhat lower down the Humber we
come to gravels and silts full of sub-arctic marine mollusca and
_Corbicula fluminalis_, exactly as in regions further south, and
presumably of the same age as the deposits we have already mentioned
as found at March in the Fenland and at Grays in Essex. The exact
relation of these _Corbicula_-beds to the deep channel filled with
glacial drift, below the marshes of the present Humber, is still
somewhat uncertain, but the marine beds clearly rest on boulder clay,
and seem also to be overlain by another glacial deposit.
The section leaves no doubt that in post-glacial times the Humber
cut a channel about 60 feet below its present bed, or to just the
same depth as did the Thames. This may possibly be an accidental
coincidence; but it is very suggestive that both these rivers should
have cut their beds to the same depth. Such coincidences suggest that
we are dealing with a period when each of our great rivers was able
to cut to a definite base-level, below which it could not go. This
base-level must either have been the sea, or some vast alluvial plain
then occupying the bed of the North Sea. In either case the plain
must then have been fully 60 feet lower than the present sea-level.
Not only did the ancient Humber cut to the same depth as the ancient
Thames, but in each area the ancient river was flanked by a wide
alluvial flat which now lies from 40 to 60 feet below the modern
marsh level.
The flat coast of Holderness, which stretches from the Humber
northward to Flamborough Head, shows also occasional submerged
forests; but the want of excavations beneath the sea-level makes
it impossible to say much about them. North of Flamborough Head it
seems as though depression gave place to elevation, and when we
pass into Scotland the Neolithic deposits seem to be raised beaches
instead of submerged forests. We need not therefore devote more
time to a consideration of the details connected with the submerged
land-surfaces which border the lands facing the North Sea. They
evidently once formed part of a wide alluvial flat stretching seaward
and running up all our larger valleys. We must now consider how far
seaward this plain formerly extended.
Here, fortunately, we meet with a most surprising piece of evidence,
which adds enormously to the importance of this plain, and shows that
the submergence is no local phenomenon, but a widespread movement
of depression which must greatly have altered the physical geography
of north-western Europe during times within the memory of man. This
evidence deserves a separate chapter.
CHAPTER IV
THE DOGGER BANK
For the last 50 years it has been known to geologists that the
bed of the North Sea yields numerous bones of large land animals,
belonging in great part to extinct species. These were first obtained
by oyster-dredgers, and later by trawlers. Fortunately a good
collection of them was secured by the British Museum, where it has
been carefully studied by William Davies. The bones came from two
localities. One of them, close to the Norfolk coast off Happisburgh,
yielded mainly teeth of _Elephas meridionalis_, and its fossils were
evidently derived from the Pliocene Cromer Forest-bed, which in that
neighbourhood is rapidly being destroyed by the sea. This need not
now detain us.
[Illustration: Fig. 4.—Showing approximate Coast-line at the period
of the lowest Submerged Forest.]
The other locality is far more extraordinary. In the middle of the
North Sea lies the extensive shoal known as the Dogger Bank, about
60 or 70 miles from the nearest land. This shoal forms a wide
irregular plateau having an area nearly as big as Denmark. Over
it for the most part the sea has a depth of only 50 or 60 feet;
all round its edge it slopes somewhat abruptly into deeper water,
about 150 feet in the south, east, and west, but much deeper on
the north. This peculiar bank has been explained as an eastward
submerged continuation of the Oolite escarpment of Yorkshire; or,
alternatively, as a mere shoal accumulated through the effects of
some tidal eddy; but neither of these explanations will hold, for
Oolitic rocks do not occur there, and the bank has a core quite
unlike the sand of the North Sea.
When trawlers first visited the Dogger Bank its surface seems to
have been strewn with large bones of land animals and loose masses
of peat, known to the fishermen as “moorlog,” and there were also
many erratic blocks in the neighbourhood. As all this refuse did much
damage to the trawls, and bruised the fish, the erratics and bones
were thrown into deeper water, and the large cakes of moorlog were
broken in pieces. A few of the erratics and some of the bones were
however brought to Yarmouth as curiosities. Now the whole surface of
the Dogger Bank has been gone over again and again by the trawlers,
and very few of the fossil bones are found; unfortunately no record
seems to have been kept as to the exact place where these bones were
trawled.
The species found were:—
Ursus (bear)
Canis lupus (wolf)
Hyaena spelaea (hyaena)
Cervus megaceros (Irish elk)
” tarandus (reindeer)
” elaphus (red-deer)
” a fourth species
Bos primigenius (wild ox)
Bison priscus (bison)
Equus caballus (horse)
Rhinoceros tichorhinus (woolly rhinoceros)
Elephas primigenius (mammoth)
Castor fiber (beaver)
Trichechus rosmarus (walrus)
Though mammalian bones are now so seldom found, whenever the
sand-banks shift slightly, as they tend to do under the influence
of tides and currents, the edges of the submerged plateau are laid
bare, exposing submarine ledges of moorlog, which still yield a
continuous supply of this material. Messrs. Whitehead and Goodchild
have recently published an excellent account of it, having obtained
from the trawlers numerous slabs of the peculiar peaty deposit, with
particulars as to the latitude and longitude in which the specimens
were dredged. Mrs Reid and I have to thank the authors for an
opportunity of examining samples of the material, which has yielded
most interesting evidence as to the physical history, botany, and
climatic conditions of this sunken land. The following account is
mainly taken from their paper and our appendix to it.
We are still without information as to the exact positions of the
submarine ledges and cliffs of peat from which the masses have been
torn; but there seems little doubt that some of them were actually
torn off by the trawl. One block sent to me was full of recently dead
half-grown _Pholas parva_, all of one age, and must evidently have
been torn off the solid ledge. _Pholas_ never makes its home in loose
blocks. We unfortunately know very little about the natural history
of the boring mollusca and their length of life. If, as I think, this
species takes two years to reach full growth, then it is evident that
the ledge of moorlog full of half-grown specimens must have been
exposed to the sea continuously for one year, but not for longer. It
ought also perhaps to tell us the depth of water from which the mass
was torn; but nothing is known as to the depth to which _Pholas_ may
extend—it has the reputation of occurring between tide-marks or just
below, but it may extend downwards wherever there is a submarine
cliff.
Though we are still unable to locate exactly these submarine ledges
or fix their depth below the sea, the blocks of moorlog are so widely
distributed around the Dogger Bank, and have been dredged in such
large masses, that it seems clear that a “submerged forest” forms
part of the core of the bank. As nothing else approaching to a solid
stratum appears to be dredged over this shoal, we may assume that
the moorlog forms a sort of cap or cornice at a depth of about 10
fathoms, overlying loose sandy strata, and perhaps boulder clay,
which extend downward to another 10 fathoms, or 120 feet altogether.
Unfortunately we cannot say from what deposit the large bones of
extinct animals were washed; they may come from the sands below the
moorlog, but it is quite as probable that the Pleistocene deposits
formed islands in the ancient fen—as they do now in East Anglia,
Holderness, and Holland.
More than one submerged forest may be present on the Dogger Bank. The
masses of moorlog are usually dredged on the slopes at a depth of
22 or 23 fathoms; but at the south-west end it occurs on the top as
well as on the slope, the sea-bottom on which the moorlog is found
consisting of fine grey sand, probably an estuarine silt connected
with the submerged forest, for the North Sea sand is commonly coarse
and gritty.
With regard to the moorlog itself and its contents, it is possible
that some of the mammals in the list, such as the reindeer, beaver,
and walrus, may belong to this upper deposit; but we have no means of
distinguishing them, as the bones were all found loose and free from
the matrix. The insects and plants were all obtained from slabs of
this peat.
The dredged cakes of peat handed to us for examination came from
different parts of the Bank; but they were all very similar in
character, and showed only the slight differences found in different
parts of the same fen. The bed is essentially a fen-deposit of purely
organic origin, with little trace of inorganic mud. It is fissile
and very hard when dry, and in it are scattered a certain number of
fairly well-preserved seeds, principally belonging to the bog-bean.
Other recognisable plant-remains are not abundant. They consist of
rare willow-leaves, fragments of birch-wood and bark, pieces of the
scalariform tissue and sporangia of a fern, and moss, and, curiously
enough, of groups of stamens of willow-herb with well-preserved
pollen-grains, though the whole of the rest of the plant to which
they belonged had decayed.
The material is exceptionally tough, and is very difficult to
disintegrate. In order to remove the structureless humus which
composed the greater part of the peat, we found it necessary to break
it into thin flakes and boil it in a strong soda solution for three
or four days. Afterwards the material was passed through a sieve,
the fine flocculent parts being washed away by a stream of water,
the undecomposed plant remains being left behind in a state for
examination. These remains were mixed with a large amount of shreds
of cuticle, etc., but recognisable leaves were not found in the
washed material.
The general result of our examination is to suggest that the deposit
comes from the middle of some vast fen, so far from rising land that
all terrigenous material has been strained out of the peaty water.
The vegetation, as far as we have yet seen, consists exclusively
of swamp species, with no admixture of hard-seeded edible fruits,
usually so widely distributed by birds, and no wind-borne composites.
The sea was probably some distance away, as there is little sign of
brackish-water plants, or even of plants which usually occur within
reach of an occasional tide; one piece however yielded seeds of
_Ruppia_. The climate to which the plants point may be described as
northern. The white-birch, sallow and hazel were the only trees; the
alder is absent. All the plants have a high northern range, and one,
the dwarf Arctic-birch, is never found at sea-level in latitudes
as far south as the Dogger Bank (except very rarely in the Baltic
provinces of Germany).
The plants already found are:—
Ranunculus Lingua
Castalia alba
Cochlearia sp.
Lychnis Flos-cuculi
Arenaria trinervia
Spiraea Ulmaria
Rubus fruticosus
Epilobium sp.
Galium sp.
Valeriana officinalis
Menyanthes trifoliata
Lycopus europaeus
Atriplex patula
Betula alba
” nana
Corylus Avellana
Salix repens
” aurita
Sparganium simplex
Alisma Plantago
Potamogeton natans
Ruppia rostellata
Scirpus sp.
Carex sp.
Phragmites communis
Among the nine species of beetle determined by Mr G. C. Champion it
is noticeable that two belong to sandy places. This suggests that the
fen may have had its seaward edge protected by a belt of sand-dunes,
just as the coast of Holland is at the present day.
This submerged forest in the middle of the North Sea has been
described fully, for it raises a host of interesting questions,
that require much more research before we can answer them. A sunken
land-surface 60 feet and more below the sea at high-tide corresponds
very closely with the lowest of the submerged forests met with in our
dock-excavations. But if another bed of peat occurs at a depth of 130
or 140 feet at the Dogger Bank, this would be far below the level
of any recently sunk land-surface yet recognised in Britain. Also,
if the slabs of very modern-looking peat, containing only plants
and insects still living in Britain, come from such a depth, out of
what older deposit can the Pleistocene mammals, such as elephant,
rhinoceros, and hyaena, have been washed?
These questions cannot be answered conclusively without scientific
dredging, to fix the exact positions and depths of the outcrops of
moorlog. When we remember also that beneath a submerged forest at
about the depth of the Dogger Bank there was found at Tilbury, in
the Thames Valley, a human skeleton; and that both human remains and
stone implements have been discovered in similar deposits elsewhere,
we can point to the Dogger Bank as an excellent field for scientific
exploration.
The Dogger Bank once formed the northern edge of a great alluvial
plain, occupying what is now the southern half of the North Sea, and
stretching across to Holland and Denmark. If we go beyond the Dogger
Bank and seek for answers to these questions on the further shore,
we find moorlog washed up abundantly on the coasts of both Holland
and Denmark, and it has evidently been torn off submerged ledges like
those of the Bank. Numerous borings in Holland give us still further
information, for they show that beneath the wide alluvial plain,
which lies close to the level of the sea, there exists a considerable
thickness of modern strata. At Amsterdam, for instance, two beds
of peat are met with well below the sea-level, the upper occurring
at about the level of low-tide, the lower at a depth of about 50
or 60 feet below mean-tide. That is to say, the lowest submerged
land-surface is found in Holland at just about the same depth as it
occurs in England, and probably on the Dogger Bank also.
Below this submerged land-surface at Amsterdam are found marine
clays and sands, which seem to show that the lowest “continental
deposit,” as it is called by Dutch geologists, spread seaward over
the silted-up bed of the North Sea; but no buried land-surfaces have
yet been found below the 60-foot level anywhere in Holland.
This appearance of two distinct and thick peat-beds, underlain,
separated, and overlaid by marine deposits, seems to characterise
great part of the Dutch plain. It points to a long period of
subsidence, broken by two intervals of stationary sea-level, when
peat-mosses flourished and spread far and wide over the flat,
interspersed with shallow lakes, like the Norfolk broads.
The enclosed and almost tideless Baltic apparently tells the same
story, for at Rostock at its southern end, a submerged peat-bed has
been met with at a depth of 46 feet.
On passing northward into Scandinavia we enter an area in which,
as in Scotland, recent changes in sea-level have been complicated
by tilting, so that ancient beach-lines no longer correspond in
elevation at different places. The deformation has been so great
that it is impossible to trace the submerged forests; they may be
represented in the north by the raised beaches, which in Norway and
Sweden, as in Scotland and the north of Ireland, seem to belong to
a far more recent period than the raised beaches of the south of
England. It seems useless to attempt to continue our researches on
submerged forests further in this direction, especially as during the
latest stages, when we know England was sinking, Gothland appears to
have been slowly rising. Those who wish to learn about the changes
that took place in the south of Sweden should refer to the recent
monograph by Dr Munthe.
CHAPTER V
THE IRISH SEA AND THE BRISTOL CHANNEL
On the west coast of Scotland, as on the east, the succession of
events seems to have been quite different from that which can be
proved further south. It looks as though we must seek for equivalents
of our submerged forests in certain very modern looking raised
beaches and estuarine deposits, such as those of the Clyde. Even when
we move southward to the Isle of Man deeply submerged post-glacial
land-surfaces appear to be unknown, though there is evidence of a
slight sinking, and roots of trees are found a few feet below the
sea-level. In the Isle of Man we still come across the modern-looking
raised beaches so prevalent in Scotland though unknown in England.
The Lancashire and Cheshire coasts, with their numerous deep
estuaries and extensive flats, are noted, however, for their
submerged forests, sometimes seen on the foreshore between
tide-marks, sometimes laid open in the extensive dock or harbour
works. The Heysham Harbour excavations, for instance, were carried
far below sea-level and a thin peat-bed was met with in a boring at
52 feet below Ordnance datum. Mellard Reade considered this peat
once to have been continuous with an ancient land-surface seen
between tide-marks. A boring is not altogether satisfactory evidence
for the occurrence of a land-surface at such a depth; but if it is
trustworthy it points to a subsidence of about 60 feet, an amount
identical with that observed in the Thames Valley.
The estuaries of the Ribble, Mersey, and Dee tell a similar story,
for on their shores and under their marshes are found some of the
most extensive submerged land-surfaces now traceable in Britain. Many
accounts of these have been published; but the alternations of marine
with freshwater strata and with land-surfaces are so like those
already described that a short account will suffice.
Carefully plotted engineer’s sections will be found in Mellard
Reade’s papers, and his account of the succession is so interesting
that it is worth quoting. He postulates two periods of elevation,
alternating with three periods of depression; but in this area, as
in the Thames Valley, it appears as though all the phenomena can be
accounted for by one long period of intermittent depression. His
generalised section of the deposits in these estuaries is as follows:—
3rd period { Blown sand
of depression {
{ Recent silts with beds of peat;
{ _Scrobicularia_, occasional
{ freshwater shells, red-deer, horse,
{ _Bos primigenius_, _Bos longifrons_,
{ and human skull
2nd period } Superior peat- and forest-bed
of elevation }
2nd period { Formby and Leasowe marine beds; human
of depression { skeleton, bones of horse and red-deer,
{ _Scrobicularia_, _Tellina baltica_,
{ _Turritella communis_, etc.
1st period } Inferior peat- and forest-bed
of elevation }
1st period { Washed drift-sand (apparently no
of depression { contemporaneous fossils)
{ Boulder clay
It may be an accidental coincidence; but it is noteworthy that both
the Mersey and Thames show two main peat-beds separated by marine
strata.
The forest exposed on the foreshore at Leasowe (frontispiece) is a
particularly good example of these old land-surfaces, and it is often
visible. It evidently once formed a wet, peaty flat on which grew
swamp plants, brushwood, and some large trees. Parts of it show a
perfect network of the rhizomes of _Osmunda_. This “superior peat-
and forest-bed” was forming when the sea was only a few feet below
its present level. The “inferior peat- and forest-bed” probably
indicates a drier soil; but it is difficult to get at and requires
fuller investigation.
The excavation for an extension of the Barry Docks, in
Glamorganshire, exposed in 1895 an interesting succession of
deposits, and fortunately a particularly competent observer, Dr
Strahan, was on the spot to note them and their exact levels. He
also obtained masses of material from each of the beds, and from an
examination of the contents of these I was able to gather a clear
idea of the changes of sea-level which had affected this part of
South Wales. The following sequence was met with:—
1. Blown sand. }
} Recent subaerial
2. _Scrobicularia_-clay. } and
1. Blown sand. } tidal deposits.
3. Sand and gravel with rolled }
shells (_Scrobicularia_, _Tellina_, }
_Cardium_, _Patella_, _Littorina_).}
Strong line of erosion.
4. Blue silt with many sedges, and at the bottom a few foraminifera.
5. _The Upper Peat Bed_, about four feet below Ordnance datum
and fairly constant in level. It ranges from one to two feet in
thickness, and where fully developed it presents the following
details:—
5 _a._ Laminated peat with logs of willow, fir and oak, passing
down into
5 _b._ Light-coloured flexible marl composed of ostracoda with much
vegetable matter.
5 _c._ Shell-marl composed principally of _Limnaea_, _Bythinia_,
etc., with ostracoda and much vegetable matter. This seam must have
been formed in a nearly freshwater tidal marsh; it yielded _Najas
marina_, a plant now confined to Norfolk.
5 _d._ Peat with logs of oak, etc. A Neolithic worked flint
was found by Mr Storrie in this seam, three inches below the
shell-marl. This implement is a fragment of a polished flint celt,
which seems to have been used subsequently as a strike-a-light. Two
bone needles are said to have been found in this peat-bed during
the construction of the first Barry Dock.
6. Blue silty clay with many sedges. From five to seven feet in
thickness.
7. _The Second Peat_ is an impersistent brown band, a few inches in
thickness, composed mainly of _Scirpus maritimus_. It suggests merely
that for the time plant-remains were accumulating more rapidly than
mud.
8. Blue silty clay, like Nos. 6 and 4. In its upper part, immediately
under the peat bed No. 7, it contains land and salt-marsh shells,
_Helix arbustorum_, _Pupa_, _Melampus myosotis_, _Hydrobia ventrosa_.
Upright stems of a sedge, probably _Scirpus maritimus_, occur
throughout this bed as through all the other silts.
9. _The Third Peat_ occurs at or close to the bottom of the dock,
at 20 feet below Ordnance datum. It rarely exceeds eight inches in
thickness, but is persistent. In several places it is made up almost
entirely of large timber, both trunks and stools of trees, while in
one section roots and rootlets extended downward from the peat into a
soil composed of disintegrated Keuper Marl. Mr Storrie identified oak
and roots of a conifer. On washing a sample collected at a few yards’
distance, I found it to consist of a tough mass of vegetable matter,
principally sallow and reed, both roots and stems. It also contained
seeds of _Valeriana officinalis_ and _Carex_, and elytra of beetles.
There was no evidence of salt water.
At this point it will be observed that the floor of Keuper Marl
rises, and Bed 9 abuts against it. Beds 10, 11 and 12 lay below
the dock bottom, and were exposed only in the excavation for the
foundations of walls, etc. Fortunately, Dr Strahan was able to
examine a good exposure of the important part of them.
10. The section commenced at the dock bottom—that is, at the peat
last described (No. 9); in the upper part it was timbered up, but at
a depth of about nine feet, blue silty clay of the usual character
could be seen and dug out through the timbers. This was followed
by two feet of greenish sandy silt full of reeds, and containing
leaves of willow, and land and freshwater shells, such as _Limnaea
auricularia_, _Planorbis albus_, _P. nautileus_, _Hydrobia ventrosa_,
_Valvata piscinalis_, _V. cristata_. The plants were _Salix caprea_
and _Phragmites_.
11. Peat with much broken oak-wood, mixed with seeds and freshwater
shells. The plants obtained were oak, hazel, cornel, hawthorn,
bur-reed and sallow.
12. Reddish clayey gravel with land shells and penetrated by roots,
passing down into red and green grits, limestone and marls. This
gravel is undoubtedly an old land-surface, lying at a depth of 35
feet below Ordnance datum. This old soil contains:—
Carychium minimum
Helix arbustorum
” rotundata
” hispida
Hyalinia
Succinea
Limnaea truncatula
Pupa
Valvata piscinalis
Cardium edule (two fragments—probably brought by gulls)
Crataegus Oxyacantha (seed)
Cornus sanguinea (seed)
Quercus Robur (wood)
The examination of these deposits made it perfectly clear that the
lowest land-surface represents a true forest-growth, such as could
only live at an elevation clear of the highest tides; one tide of
brackish water in the year would have sufficed to alter markedly the
character of the fauna and flora of the deposit. Dr Strahan, assuming
that the range of the tides was the same as at the present day, and
noting the present highest level to which the salt-marshes reach,
comes to the conclusion that 55 feet at least is the amount of the
subsidence. I should be inclined to add a few feet more, in order to
keep the oak-roots well clear of the highest tide during a westerly
gale. An exceptional gale occurring only once during the lifetime of
an oak might bank up the sea water sufficiently to kill the tree, if
it grew at a lower elevation.
It may be argued that when the land stood at the higher level the
range of the tides was less, and that consequently the amount of
the proved subsidence may not be so great as 55 feet. The old
land-surface on which the oaks grew lies, however, 35 feet below
_mean tide_, so that any supposed lesser tidal range in ancient times
could not make any great difference in the amount of subsidence here
proved—it cannot be less than 45 feet. When, however, we notice
the rapid increase in the range of the tides at the present day as
the channel narrows towards Chepstow, and think what would be the
probable effect of raising the whole country 50 or 60 feet, we are
compelled to think that any narrowing and shoaling of the channel
would have the effect of increasing, not decreasing, the tidal range
at Barry Docks. In short it looks as if when the lowest submerged
forest grew, the abnormal tides of Bristol may have extended further
west, to near Cardiff.
Whatever may have been the exact range of the tides in these early
days, it seems that the Bristol Channel points to a subsidence in
post-glacial times of about 60 feet—or just the same amount as the
Thames, Humber, and Mersey. The amount may have been more; but the
Barry Dock sections show that it cannot have been less; we will
return to the question of its maximum extent later on.
Before leaving this locality it may be well to enquire what further
light it sheds on the movement of submergence, and on its continuous
or intermittent character. The succession of the strata above the
lowest land-surface, and the nature of their enclosed fossils,
suggest long-continued but intermittent subsidence; I can see,
however, no indication of a reversal of the process. The land-surface
is carried beneath the water, the estuary then silts up, becomes
fresh water, marsh-plants grow, and even trees may flourish on this
marsh before it subsides again. But there is no sign that the strata
were ever raised above the level to which ordinary floods could build
up an alluvial flat. The land-surfaces seem always to have been
swampy, and bed succeeds bed in fairly regular sequence, without the
deep channelling we might expect to find when an alluvial flat was
raised to a noticeable extent above the level of high water.
The width of the Bristol Channel makes it clear that this gulf must
occupy a submerged valley of great antiquity. It becomes therefore of
interest to enquire whether the wide valley is correspondingly deep,
or whether its rocky floor is found at the same shallow depth as in
the case of the other river-valleys which we have been considering.
The wide valley may have been formed in either of two ways. It may
have been excavated as a deep valley with its bottom many hundred
feet below the present sea-level. Or it may have commenced as the
shallow valley of a big river with exceptionally powerful tides, and
as this river swung from side to side it greatly widened its valley
without making it any deeper.
Possibly a deep channel may exist towards the Atlantic; but we know
that none extends as far up as Bristol. Near Bristol the Severn
Tunnel was carried through Carboniferous and Triassic rock, and
showed that no buried channel is found much below the present one,
which here happens to be scoured by the tides to an exceptional
depth. The bottom of the old channel cannot be more than 40 feet
below the bottom of the present channel known as The Shoots.
It may be that the Severn was once prolonged seaward as a swift river
falling in a series of rapids over hard ledges of Palaeozoic rocks;
but of this there is no evidence. It also does not seem probable, for
all the geological indications go to suggest that west of Bristol
the Channel coincides in the main with a wide area once occupied by
comparatively soft Secondary or even Tertiary rocks. However this
may be, we can only trace an ancient post-glacial channel cutting to
about the same depth as the channels of the other rivers, and the
lowest submerged land-surface of Barry Docks corresponds quite well
with an alluvial flat formed when the river ran at that level. Here
again we seem to find the river cutting to an ancient base-level
which was about 60 feet below the present sea.
The reader may perhaps think that this point, the limited range of
the upward and downward movements in post-glacial times, is being
insisted on with wearisome iteration. But the insistence is necessary
when we remember how constantly both geologists and naturalists, in
order to account for anomalies in the geographical distribution of
animals and plants, bring into play such movements. The argument
is constantly used, that a certain species cannot cross the sea:
therefore if it is found in an island, that island must once have
been connected with the mainland. Nature is more full of resource
than we imagine, and does not thus neglect her children. The
cumulative effect of rare accidents spread over many thousand years
is also far greater than may be thought by those who only consider
what has been noted since means of dispersal have been studied
scientifically.
An examination of the south side of the Bristol Channel need not
long delay us, except for two pieces of evidence which should not be
passed over. In Somerset there are wide expanses of marsh land known
as the Bridgwater and Glastonbury Levels. These greatly resemble the
Fenland, and like it are underlain by a submerged rock-platform which
has sunk in post-glacial times. But in this case we are able to fix a
definite historical date by which all movement had ceased—it may have
ceased much earlier, but we can prove that at any rate there has been
no change of the sea-level subsequent to a certain date.
The Glastonbury Levels lie at about the height of ordinary high
tides, and the channels through them would still be tidal were it
not for the banks which keep out the sea. Some years ago there
were discovered on the surface of these marshes a number of low
mounds, which on excavation proved to be the remains of a village of
lake-dwellings, approached by a boat-channel, by the side of which
were the remains of a rough landing stage. The dwelling-places rested
on the old salt-marsh vegetation, brushwood and soil being used
to raise their floors above the level of the highest tides. It is
evident that when this village was inhabited the sea-level must have
been the same as now, or within a foot or two of its present height.
If the sea-level was then higher, the village could not be inhabited;
if it were lower the channel would not have been navigable and the
landing stage would have been useless. The archaeological remains
found in this village prove that it belongs to a period dating about
the first century B.C. or the first century A.D.
Another locality on the south side of the Bristol Channel which we
must not pass without notice is Westward Ho, in Bideford Bay. There
is nothing exceptional about the submerged forest at this place, but
it has been carefully studied and collected from by Mr Inkermann
Rogers, and it may be taken as a typical example of such deposits in
the south of England.
The peaty deposits and old land-surface here seen between tide-marks
are rapidly being destroyed by the sea and are now much thinner than
they were a few years since. The soil on which the trees, here mainly
oaks, are rooted consists of a blue clay full of small pebbles and
fragments of the Culm Measure grit. Among these stones are numerous
flint-flakes made by man; but metal implements and pottery, so
common in the later deposits at Glastonbury, have not been found.
This ancient land-surface lies several feet below high water; it
shows therefore that the latest movement of depression dates from a
period between this Neolithic deposit and the Celtic lake-dwelling of
Glastonbury.
The possibility of fixing an approximate date for this submerged
forest, through its numerous flint-flakes and the accompanying bones
of domesticated animals, makes its contents of great interest, for
it shows how recently the movement has ceased—probably not more than
3500 years ago. It will be worth while therefore to give a fuller
account of the contents of this soil and its overlying peat-bed.
As regards articles of human workmanship, I have seen nothing
but waste flakes of flint and perhaps flint knives; and though
good implements may at any time be discovered, neither chipped
nor polished tools seem yet to have been found. Human remains are
represented by a clavicle.
The accompanying mammals are the stag, Celtic shorthorn, horse, dog
(a very slender breed), sheep, goat, and pig, all of which, except
the stag, seem to be domestic animals. Dr Chas. Andrews remarks that
the ox seems to be certainly the Celtic shorthorn (_Bos longifrons_),
while the small sheep is a characteristic Romano-British form, which
has been described from many places, where it has been found with
Roman and earlier remains.
A number of seeds were obtained from the peat which rests on this
old land-surface, and it is noticeable that several of them belong
to brackish water or sea-coast plants. No cultivated species have
yet been found, either here or elsewhere, in even the newest of the
submerged forests. The list of plants is still a small one; but it
may be worth giving, to show what species can be identified. It must
not be forgotten that in such deposits plants which do not possess
either deciduous leaves or hard seeds leave no recognisable traces,
though they may have been quite as abundant as the hazel, of which
everyone notices the nuts. The seeds belong to:—
Ranunculus Flammula
” repens
” sceleratus
Viola
Malachium aquaticum
Stellaria media
Lychnis Flos-cuculi
Rubus fruticosus
Callitriche
Cornus sanguinea
Sambucus nigra
Aster Tripolium
Solanum Dulcamara
Ajuga reptans
Sueda maritima
Atriplex patula
Rumex
Urtica dioica
Alnus glutinosa
Corylus Avellana
Quercus robur
Alisma Plantago
Ruppia maritima
Eleocharis palustris
Scirpus Tabernaemontani
Carex 3 sp.
In this list, as is usually the case with the newest submerged
forest, we find only plants that are still living in the immediate
neighbourhood. Also, only such plants as are widely distributed are
here found as fossils, the characteristic west-country flora being
unrepresented. The reason of this limitation will be discussed later.
For various reasons, which will be explained later, it will be well
before describing the submerged land-surfaces of Cornwall and the
Atlantic coast, to complete the account of those surrounding our
enclosed seas. We will therefore take next those bordering on the
English Channel.
CHAPTER VI
THE ENGLISH CHANNEL
The English Channel, like our other enclosed seas, is bordered on
either side by a fringe of ancient alluvia and submerged forests,
which however are fast disappearing through the attacks of the waves.
The destruction is so rapid, and in many parts has been so complete,
that we are apt to forget how altered is the appearance of the
English coast. Even so recently as the time of Caesar’s invasion flat
muddy shores or low gravelly plains occupied many parts of the coast
where we now see cliffs and rocky ledges.
We will not labour this point, which must be obvious to anyone who
has noticed how little the low terrace which still fringes great part
of the Sussex coast can resist the waves, and how quickly it is eaten
away during storms. Any restoration of our coast-line for the time of
Caesar must take these changes into account.
The material thus being removed by the attacks of the sea is partly
Pleistocene gravel, partly alluvium of later date; and the alluvial
strata with their accompanying buried land-surfaces resemble so
closely those already described that we need not linger long over
their description.
If we commence at the Strait of Dover we are immediately confronted
with clear evidence of the change of sea-level. Submerged forests are
well seen between tide-marks in Pegwell Bay, and valleys with their
seaward ends submerged and forming harbours are conspicuous in Kent.
Owing to local conditions, the valleys are mostly narrow and steep,
and the small harbours therefore soon filled up, or were lost through
the cutting back of the cliffs on either side. Possibly in Caesar’s
day good natural harbours were still in existence here.
Unfortunately on this part of the coast the study of coastal changes
has been involved in a good deal of needless obscurity. Many
writers, even geologists, make no clear distinction between loss by
submergence and loss by marine erosion. We are told, for instance,
that the Goodwin Sands were land about 900 years ago, and that this
land disappeared during an exceptional storm. We are sometimes even
told that here and elsewhere walls are still visible beneath the
sea. Popular writers, to add to the confusion, have some hazy notion
that these changes are connected with the existence of submerged
forests or “Noah’s Woods,” and that these again are evidence of a
universal deluge. The whole of the arguments are strangely tangled,
and we must try and make things a little clearer before passing on.
An understanding of the changes which have taken place on this part
of the coast is needed for historical purposes, and still more needed
if we make a study of the origin of the existing fauna and flora of
Britain.
One of the crucial questions, both for the naturalist and
archaeologist, is the date at which Britain was finally severed from
the Continent. Did this happen within the range of written history,
or tradition? Or if earlier, did it take place after or before
climatic conditions had become such as we now experience? For the
proper understanding of many different problems it is essential to
settle this point.
It is scarcely satisfactory to read history backwards, though
geologists are often compelled thus to work from the known to the
unknown. We will therefore not in this case ask our readers to follow
us through the detailed evidence and arguments which have enabled
geologists stage by stage to reconstruct the physical geography
of this part of Britain as it was in days before written history.
They must take this preliminary work for granted, and allow the
description of the changes to be taken in their correct historical
order.
We need not go back far geologically. In late Tertiary (probably
Newer Pliocene) times there was a ridge of chalk joining the range of
the North Downs to the corresponding hills of France; but the divide
between the North Sea and the English Channel was low at this point.
Afterwards, during the Glacial Epoch, when an ice-sheet accumulated
and blocked the northern outlet of the North Sea, the water was
ponded back in the southern part. There was no easy outlet northward
for the water of the Rhine and other great rivers, so the level of
the North Sea rose slightly till it overflowed this low col and cut
an outlet where lies the present Strait of Dover.
The general sea-level during this period of glaciation seems to have
been a few feet higher than that of the present day, for glacially
transported erratics are found strewn over the flat coastal plain of
Sussex. One erratic block, probably derived from the Channel Islands,
was discovered under the loess as far east as Sangatte cliff, close
to Calais. The icy English Channel must therefore have met the icy
North Sea some time during the Glacial Epoch.
Some time after the cold had passed away there came in the period
with which this book deals—when the lowest submerged forest
flourished, on land now 50 or 60 feet below the sea. This elevation
of the land, as already shown, converted a great part of the North
Sea into a wide alluvial plain. At the same time it raised above the
sea-level and obliterated the newly-formed strait, leaving it in all
probability as a shallow valley sloping both ways and filled up with
alluvium. The Strait of Dover was again a watershed, or perhaps its
position was occupied by a small stream, which may have flowed in
either direction.
Thus the work done during the Glacial Epoch was almost cancelled and
had to be done again; but now there was merely a low narrow divide
of chalk and a strip of marsh between the two basins, and the chalk
ridge was steadily being attacked by the waves of the sea from the
west.
When subsidence again set in the strip of alluvium was soon submerged
and the two seas again met; but in all probability for a long time
the Strait was only a narrow one, over which animals could easily
swim. Then tidal scour, deeper submergence, and the action of the
waves did the rest, so that ever since that time the Strait of Dover
has been getting steadily wider and wider, and also deeper. Its
bottom is to a large extent composed of bare chalk with patches of
gravel; and the movement of this gravel during storms, combined with
the action of boring molluscs must slowly eat away the chalk far
below ordinary wave-action.
The above explanation is needed, for it will not do to take existing
soundings, and say that all the sea-bottom below a certain level,
corresponding with a particular submerged forest, was then sea and
all above was then land. This is an easy way of reconstructing the
physical geography; but it may be a very misleading one. A little
consideration will show that whilst in large areas sand-banks have
accumulated to a great thickness, in other areas, of which we know
the Strait of Dover is one and the Dogger Bank a second, there has
been much submarine erosion, which is still going on. In neither case
is it safe entirely to reconstruct the ancient contours from the
present-day soundings.
Even such a gigantic feature as the continental platform, which
ceases suddenly at a depth of 100 fathoms, is in all probability in
the main a feature formed by the deposition of sediment during long
ages. Its outer edge marks, not the limit of some ancient continent,
but the limiting depth at which gentle wave-action has been felt, and
beyond which the sediment cannot be carried.
After this necessary digression we must return to our study of the
actual evidence for such changes of sea-level in the English Channel.
It has been pointed out already that for this purpose the present
depth below sea-level of the rocky floor of the Strait cannot in
itself be accepted as sufficient evidence. Nor can the depth at which
rock was met with under the Goodwin Sands; though here a cylinder was
sunk 75 feet before it reached the chalk. Unfortunately no record of
the strata passed through seems to have been preserved, though it is
perhaps implied that nothing but sea-sand was penetrated.
Romney Marsh is a wide alluvial flat occupying a silted-up bay, the
floor of which in places lies at least 70 feet below sea-level. There
are here unfortunately no extensive excavations for docks, and all we
can say is that the few borings which have penetrated the alluvial
strata prove the existence of a slightly undulating rock-surface
below. In short Romney Marsh appears to be a submerged flat-bottomed
open valley, like that which we have already seen underlies the marsh
deposits of the Fenland.
In the case of Romney Marsh, however, it is doubtful whether
submerged land-surfaces would be found at any great distance from the
rising ground. There is a striking peculiarity about this marsh; it
only lies partly in a bay, the greater part of the area consisting of
alluvial flats which have accumulated during recent centuries behind
the projecting shingle beaches of Dunge Ness. In short, the marsh
steadily gains on the sea, is advancing into fairly deep water, and
the parts near the Ness may be underlain by marine strata right down
to the Wealden rocks below. The rock floor was met with at 58 feet
below the marsh at Holmston Range, not far from the Ness; but we
have no information as to the character of the strata passed through
before this floor was reached. In all probability this floor at 58
feet would be proved to be part of a true land-surface, could we
examine it.
Near Hastings the submerged forests have long been known, and are
often exposed on the foreshore between tide-marks. They contain
antlers of deer, leaves, hazel nuts, acorns, and oak wood.
Then we come to Pevensey Level, which is another of the submerged and
silted up wide flat-bottomed valleys, such as we have so often met
with. But as we have no details as to strata underlying this marsh we
must pass on.
Along the Sussex coast west of Beachy Head a series of south-flowing
rivers reaches the sea, each cutting through the high chalk-hills
of the South Downs. We need not discuss the origin of these peculiar
courses, which date back to the period when the central axis of the
Weald was uplifted; that discussion would take too much time, and is
here unnecessary. We are now only concerned with the later stages of
the evolution of these river-valleys, each of which yields striking
confirmation of the view that a sinking of the land has taken place
in comparatively modern times.
At the present day the tidal part of each of these rivers extends
right through the Downs into the lower Wealden area, and it is
obvious that their valleys tend to silt up, not to deepen, and
scarcely anywhere to become wider. When we examine further we find
that the true valley-bottom lies far below the present alluvial flat;
though the scarcity of borings and the uncertainty of many of the
records make it difficult to say exactly how deep it lies.
If we follow these rivers upwards we find that in each case the
alluvial flat widens out greatly after we have passed the chalk-hills
and reached the clay lands beyond. These wide flats, according to old
ideas, were formed by the swinging from side to side of the stream,
which thus gradually widened its valley in the softer strata. If
this were the case in these instances, we should find a solid floor
beneath each marsh at a depth not exceeding that of the present
river-channels. The rivers, however, are not now cutting into rocky
banks or flowing over beds of Secondary strata; they are flowing
sluggishly in the middle of alluvial flats, which tend to silt up
with every flood or exceptionally high tide.
Thus all the evidence seems to show that marshes like those near
Lewes and Amberley Wild Brook have originated through the submergence
of flat-bottomed valleys cut in soft strata. The ponding back of the
muddy tidal water would soon lead to the silting-up of any shallow
lakes left after this submergence.
When the land stood 70 or 80 feet higher than it does now, the
country must have looked very different. The rivers then traversed
the chalk downs through V-shaped comparatively narrow valleys; but
these valleys opened out in their upper reaches, where they crossed
the Gault and Weald Clay. If we could lay bare the true floor of
the valley, we should see however that there is always a steady and
fairly regular fall seaward, just as there is in the part of its
course which lies above the influence of the submergence, which is
felt for some 10 or 12 miles from the sea. Except on this theory of
recent submergence it seems impossible to account for these curious
marshes, with tributary valleys obviously plunging sharply beneath
them on either side; they are quite unlike the undulating flats
which occur higher up.
The flat of Selsey Bill yields evidence of submerged land-surfaces
opposite each of the shallow valleys; but here we meet with the same
difficulty which confronted us in the Thames Valley and on the east
coast. Pleistocene land-surfaces and alluvial deposits of early date
are seen on the foreshore side by side with the more modern Neolithic
alluvium and submerged forests. Unless great care is taken it may be
thought that the well-preserved bones of rhinoceros and elephant, and
the shells of _Corbicula fluminalis_, come from the same alluvium
that yields Neolithic flint-flakes, or that plants such as the South
European _Cotoneaster Pyracantha_ flourished in Britain up to this
recent date. Except for the sake of warning against these sources of
error the submerged forests of Selsey Bill need not detain us.
Still travelling westward we next arrive at the series of tidal
harbours opening into Spithead, Southampton Water and the Solent. All
of these are obviously continuations of the valleys which lengthen
them inland; and this is amply proved by dock excavations and borings.
Even Southampton Water and the Solent themselves are nothing but
submerged valleys. A well at the Horse Sand Fort—one of the iron
forts which rises out of the sea at Spithead—showed a band of
compressed vegetable matter, probably an old land-surface, more
than 50 feet below high-water level, the floor of Eocene strata not
being met with till 98 feet below high water was reached. In this
case, however, the strata below 50 feet seem, from the published
description, to be of marine origin.
The well at Norman Fort is stated to have penetrated to a depth of
127 feet below the sea before Eocene strata were reached; but in this
case the lower strata were of marine origin, and the only land animal
recorded was a jaw of red deer, found apparently between 80 and 90
feet down. These deep channels may be relics of the very ancient
(Tertiary) Solent River, and were probably arms of the sea till they
were sufficiently silted up for the lowest submerged forest to grow.
We have not yet sufficient data, nor is it necessary to our purpose,
to give a detailed reconstruction of this interesting area during
the successive stages of elevation and depression. During the time
when the lowest of the submerged forests flourished the Isle of Wight
was connected with the mainland where the Solent now narrows about
Yarmouth, and probably for some distance westward. This connexion was
kept up till comparatively recent times, only breaking down finally a
short time before Caesar’s invasion.
In early Neolithic times the ancient Solent Valley had already been
decapitated by the inroads of the sea west of the Needles, and the
remains of this big valley were occupied by a small river flowing
eastward through the middle of the present Solent. In its course it
received numerous tributaries on either side. It probably opened out
into an estuary where it joined Southampton Water, and so continued
to and beyond Spithead, receiving other tributaries from the valleys
now occupied by Portsmouth Harbour, Langston Harbour and Chichester
Harbour.
In time we may be able to make a more complete reconstruction of the
physical geography of this area for definite dates; but the point now
to be insisted on is that the Isle of Wight was part of the mainland
up to quite recent times, so that its fauna and flora could readily
pass backwards and forwards without crossing the sea.
Perhaps to the geographer or geologist one of the most striking
confirmations of a recent submergence affecting this part of England
will be found in the strange series of enclosed harbours extending
from Chichester westward to Fareham. These harbours are not each
distinct and separate; all of them have cross connexions in the form
of shallower channels some four or five miles inland from Spithead.
I have often been asked what is the meaning and origin of these
peculiar harbours, which are not forming or widening now, but rather
tend to silt up.
The origin of these harbours is quite easy to understand, if we
admit the recent sinking of the land, and for this we will presently
give abundant evidence. On any other hypothesis these inosculating
waterways must seem hopelessly confused and inexplicable. Sea and
waves do not erode enclosed basins such as these.
Granting the submergence, we see that each of these harbours must
once have been a shallow valley; but this does not account for their
basin-like shape and their cross connexions. For the reason of
these peculiar features we must look at the map by the Geological
Survey showing the superficial deposits. It will then be seen that
all this part of Hampshire shows a widespread sheet of gravel and
gravelly loam which slopes gently seaward and passes below the sea
at Spithead. Northward the gravel rises, and the soft Eocene and
Cretaceous strata appear beneath the gravel between tide-marks at
various places toward the northern ends of these harbours.
The waves of the sea can remove loose gravel as readily as clay,
and we see that on this coast wave-action is practically confined
to the low cliff facing the sea and does not affect the interior
of the harbour. But it is well known to geologists that a sheet of
coarse angular gravel such as this, notwithstanding its looseness,
is much less readily attacked by a small stream than is a surface of
hard clay or even chalk. Thus plains of Tertiary deposits capped by
gravel, under the action of rain or rivers develop into gravel-capped
plateaus and hills, which fall abruptly into open flat-bottomed
valleys. The denudation takes place at the edge, where the gravel
rests on the Tertiary strata and numerous springs are given out;
there is scarcely any denudation in the gravel flat, and unless
the height of the land is considerable there is no great amount of
denudation in the flat bottom of the valley.
Thus there is a tendency for the valley to widen out on every side,
wherever the gravel rests on impervious or soft strata. But where
the gravel plunges below the water-level, as it did at the entrance
to each of these harbours, the valley narrowed, for there were no
landslips, the drainage was subterranean, and the stream could not
readily remove the large flints.
The widening of the valleys, where they were cut in soft strata, led
to the development of small lateral valleys to the right and left,
leaving only narrow divides between their head waters and those of
the next valley. When the land sank these divides were flooded, and
so were developed the shallow cross connexions, much as we now see
them.
In order that it may not be imagined that this reconstruction is
merely hypothetical, it will be as well to give some evidence that
such an elevation and submergence did take place in this district
as in others. We cannot in this little book deal with the whole
of the evidence, so we will take the Southampton Dock excavations
as sufficient to prove our point, condensing the account from the
_Geology of Southampton_, published by the Geological Survey.
The general section at Southampton Docks is as follows, though the
thickness varies considerably at different points, and the greatest
depth of the old valley has not yet been proved:—
Feet
Estuarine silt 20
Peat, old vegetable soil, or tufaceous marl;
ox, pig, horse, pine, beech, birch, oak,
and hazel variable up to 17
Gravel, with reindeer 10 or more
The bottom gravel is apparently of Pleistocene date, though it
may include also a basement bed belonging to the newer deposits.
T. W. Shore recorded from the peat above the gravel a fine stone
hammer-head of Neolithic date and worked articles of bone, but no
instruments of metal were found. The associated marl was full of
freshwater shells.
Poole Harbour tells a similar story, and evidence of this submergence
is seen in the various submerged forests found along the Dorset and
Devon coasts, opposite the mouths of the valleys. These rocky coasts
are, however, so different from those we have just been describing,
that they will more conveniently be treated of in a separate chapter.
CHAPTER VII
CORNWALL AND THE ATLANTIC COAST
On travelling westward into Cornwall we enter a region which is
extremely critical in any enquiry as to the amount of change that
the sea-level has undergone. As long as we were dealing with ancient
river-channels opening into enclosed seas, like the North Sea or
Irish Sea, it might be said that the depth to which the channel
was cut was not necessarily governed by the sea-level. It might be
governed by the level of an alluvial plain, which then extended for
hundreds of miles further, and had its upper edge high above the
sea-level.
This cannot be said in Cornwall, for there the sea-bed shelves
rapidly into deep water, and the coast would not be far away, even
were the land raised 200 feet or 300 feet. The rivers then as now
must have flowed almost directly into the Atlantic Ocean, and their
channels must then as now have cut nearly to the sea-level of the
period.
The Cornish rivers yield most valuable information. It so happens
that many of them bring down large quantities of tin ore from the
granitic regions, and this ore being very heavy tends to find its way
to the bottom of the alluvial deposits, out of which it is obtained
in the same way as alluvial gold in other countries. On following
this detrital tin ore downwards towards the estuaries the “tinners”
or alluvial miners found in many cases that a rich layer descended
lower and lower till it passed well below the sea-level in some of
the ancient silted-up valleys. Some of these tin deposits were so
rich that it paid even to divert the rivers, dam out the sea, and
remove the alluvium to considerable depths in search of the ore.
These excavations for tin produced most interesting and continuous
sections of the alluvial deposits, and if only they had been examined
more thoroughly and scientifically they would have thrown much light
on the questions we are here considering. Unfortunately all the
deeper excavations were made in days when all ideas as to the origin
of “diluvial” deposits were so tinged with theories as to the effects
of a universal deluge, that many of the most interesting points
escaped notice. The last of these “stream works” was closed many
years ago.
Notwithstanding the early date of these excavations, some most
interesting observations were recorded; and though they make
us long for fuller details and regret the loss of many of the
objects referred to, we must be grateful that so much was noted,
and by such careful observers. This entire removal of the old
alluvial deposits—for the tin usually occurs concentrated in the
bottom layers—showed clearly that in Cornwall, as elsewhere, old
land-surfaces can be found far below the sea-level. The shape of the
valley-bottom, and the rapid lessening of its fall as the coast is
approached, in several cases point clearly to the proximity of the
sea, and show that its ancient level must have been about 70 feet
below present tides.
Here it may be pointed out that as the sea-level is approached the
steady seaward fall of a rocky =V=-shaped valley quickly lessens,
changes to a gentle slope, and then to a flat, more or less wide
according to the length of time during which the river has been kept
at the same level, and could only swing from side to side, without
deepening its bed. In Cornwall there is a definite limit below which
the erosion of the valleys has not gone, and at this level the valley
widens and flattens as it does elsewhere.
The eastern border of Cornwall is formed by the extensive harbour
which receives the Tamar, Tavy, and Plym, and this harbour is
obviously nothing but a submerged seaward continuation of the
combined valleys eroded by these rivers. The rivers, it must be
remembered, though short, receive great part of the drainage of
Dartmoor, where the rainfall is excessive; they are therefore
very liable to floods. These streams also bring down much coarse
gravel and sharp granitic sand, so that their erosive power must be
exceptionally great during floods. It seems therefore that the scour
would always have been sufficient to keep open a channel well below
low-water level.
No stream tin has been worked in the Plymouth estuary, so that we
cannot point to any continuous sections in the ancient alluvial
deposits, such as are found further west. These tin deposits,
however, date in the main from a period somewhat earlier than that
with which we are now dealing. They were probably swept down from
Dartmoor when floods were far more severe, during the annual spring
melting of the snow during the Glacial epoch. Unfortunately, also,
the lately finished harbour works at Devonport proved the existence
of only modern alluvium, without any submerged forests.
Before dealing with the rivers which flow into Plymouth Sound it
is necessary, however, to say a few words about the harbour itself
and its origin. Plymouth Sound and the various submerged valleys
which open into it illustrate well both the continuity of geological
history, and the great difficulties which await us when we deal with
valley erosion which in part dates far back into Tertiary times.
The Sound is not merely a submerged continuation of the Pleistocene
valleys, and between this wide gulf and the narrow valleys there
is a curious want of continuity. We do not know the true depth to
the rocky floor; but at two places just outside the mouths of the
estuaries deep hollows are scoured through the sands. One of these,
just outside the Hamoaze, or estuary of the Tamar, shows a rocky
bottom at 150 feet, probably the true rock-floor of that part of the
Sound. The other hollow, 132 feet, is just outside the Cattewater;
but does not reach rock.
It is obvious that these depths, both of which are measured from
low water, show a depression of the rocky floor of the Sound far
greater than we meet with in ordinary Pleistocene valleys; but at
present we have no means of proving the true date of this depression.
It represents not improbably a Tertiary basin, like that of Bovey
Tracey, which also descends several hundred feet below sea-level.
In favour of this view we can point to the occurrence of a small
outlier of Trias in Cawsand Bay, which certainly suggests that the
Sound represents an area of depression or synclinal basin, rather
than a mere submerged valley. It has also been stated that relics of
Tertiary material are still to be found in the limestone quarries of
Plymouth; but for this the evidence is not altogether satisfactory.
It may be asked, What practical difference does it make, whether
or no the Plymouth Sound were originally a Tertiary basin, for no
Tertiary gulf could now remain open? If we were dealing with an area
of soft rocks, like the Thames Valley, or with an enclosed sea,
this objection would hold. Around Plymouth, however, the Palaeozoic
rocks are extremely hard, and can resist for ages the attacks of
the sea; but loose Tertiary material, or even Triassic strata, would
readily be swept away by the heavy Atlantic swell and by the scour
of the tides, until they were protected by the building of Plymouth
Breakwater.
There is a general impression that marine action cannot go on much
below low water; but this is altogether a mistake. Tidal scour may
go on at any depth, provided the current is confined to a narrow
channel, so as to obtain the requisite velocity. If in addition there
is a to-and-fro motion, such as that caused by the Atlantic swell
at depths of at least 50 fathoms, the actual current required to
remove even coarse sand need only be very gentle. The oscillation
in one direction may not reach the critical velocity; in the other
this velocity may just be exceeded; the movement, therefore, of the
sand grains may always be in one direction, especially if the courses
taken by the ebb and flood tides do not coincide, or their velocities
differ.
How does this apply to the origin of Plymouth Sound? The mere fact
that opposite the mouth of the Tamar a pit has been scoured to a
depth of 150 feet, and opposite the Hamoaze another to 132 feet below
low water, and that these pits are kept open, notwithstanding the
enormous amount of sediment brought down by these rivers, proves that
tidal scour is now going on, or was recently going on, at depths
of 25 fathoms at least in confined parts of Plymouth Sound. Similar
troughs occur at even greater depths near the Channel Islands, where
the tidal scour is very great, and in the Bay of Biscay coarse sand
is moved at depths of at least 100 metres.
It is necessary to make this digression as to the effects of tidal
scour, for we are sometimes told that the various basins, troughs,
and channels shown on the charts represent submerged land-valleys,
and thus prove enormous changes of sea-level in modern times. How
a submerged valley in a narrow sea with sandy bottom, like the
English Channel, could remain long without silting up is not clear;
the sand-banks on either side should tend to wash into and fill up
the hollows. The troughs, however, all coincide with lines of tidal
scour; they do not continue the lines of existing valleys, unless
these valleys are so large as to produce a great scour, and unless
this scour is aided by the oscillation of the waves. A glance at
the Admiralty chart will show that no submerged channel crosses the
direction of the tidal scour or of the Atlantic swell; the channels
are scoured where tide and swell act together.
We conclude therefore that Plymouth Sound probably represents a basin
once filled with soft Tertiary and Secondary deposits, and that these
soft deposits were cleared out by the sea, leaving the rocky floor
of the basin bare at a considerable depth below sea-level. In part
the basin has now silted up again; but we may fairly consider that at
the time of greatest elevation, when the submerged valleys were being
eroded, the depth of water in the Sound was much the same as it is
now. Then as now the rivers seem to have discharged into a wide open
gulf occupied by the sea.
However this may be, we see now a series of deeply trenched valleys,
partly submerged and all opening into a wide and deep bay. These
valleys do not now show rocky bottoms gradually sloping into the open
harbour. The rock floor ceases several miles up and gives place first
to an alluvial flat and then to an arm of the harbour. Like all the
other valleys with which we have been dealing they cut to a definite
base-level, approximately that of the sea, and the parts below that
level are rapidly silting up.
Fortunately a large series of bridge-foundations has shown well the
character of these valleys, where the rocky floor passes beneath
the sea-level, and the late R. H. Worth gave an excellent series
of sections across them. He took their contours to be evidence of
glaciations. In this I cannot agree with him; but think rather that
the extraordinary flatness of the valley-bottoms, and especially the
uniform depth to which they were excavated, point to the attainment
of a definite base-level.
Commencing with the most easterly of the rivers which enter the
Sound, we find that the Laira Railway Viaduct, across the Cattewater,
proved a breadth of 212 feet at the centre of the channel, with
the rock-floor practically level at 87 feet below low water; no
V-shaped valley or gorge was met with. At Saltash the foundations
of the bridge show the depth to the rock-bottom to be 75 feet; but
the viaduct across the Hamoaze is about three miles higher up the
river than the Laira Viaduct. The Tavy Viaduct, nearly two miles
further from the sea, shows a width of 240 feet of practically level
rocky floor at 67 feet below sea-level. Thus all this evidence is
consistent with the existence of a series of wide open flat-bottomed
valleys, now partly submerged, with a fall of about five feet in
the mile. This is about the fall necessary for even a rapid river
flowing through a flat so full of boulders and coarse gravel as this
must have been. It must not be forgotten also that this five feet in
the mile is the general fall of the valley-bottom, not of the water,
and that a river winding from side to side would have about half or
one-third of this fall. The slope was probably just sufficient to
keep the channel clear and let the water escape.
We may take it, therefore, that the ancient valleys opening into
Plymouth Harbour cut to about 100 feet below mean tide, as do the
Thames and Humber, and that this was the measure of the greatest
elevation of the land in Pleistocene times, for these valleys
opened suddenly into a sea of considerably greater depth. A word of
explanation is still required as to the meaning of the extremely
flat rock-bottom, for one might have expected more of a =U=-shaped
or =V=-shaped valley, unless the period of stationary sea-level were
very long.
Owing to the great rush of water from Dartmoor during floods, and
the enormous amount of coarse gravel swept down, the erosive power
of these streams is very great. This was greatly exaggerated during
the Glacial Epoch, to which the formation of the tin-ground and of
the flat bottom belong. The melting of the snow in spring must have
caused far more severe floods than we now see, and these floods must
have brought down large quantities of river-ice heavily charged
with boulders of hard and angular metamorphic rocks, such as would
erode and trench in a way that does not now happen. Thus as the
river changed its course or swung from side to side according to the
varying amount of water, the ice-laden water must have had an erosive
power more like that of a Canadian river in spring than like anything
we now see in Britain. The wide and deep flat-bottomed trench need
not have taken any enormous length of time to form, for river-ice and
anchor-ice were constantly at work removing the loose material and
laying bare the rock-face so that it could be again attacked.
The period of exceptionally rapid erosion and of low sea-level above
postulated must be our starting point in Devonshire and Cornwall as
elsewhere, for it fixed the shape and depth of the submerged valleys
over wide areas. This erosion came somewhat earlier than the growth
of the submerged forests; but it is impossible to treat of any
particular period of history without some mention of what has gone
before and led up to it. I may say also that I doubt whether there is
any such great gap as is commonly supposed between the Glacial period
and later times.
Unfortunately the succession of the newer deposits in the submerged
valleys near Plymouth appears never to have been worked out,
attention having been concentrated on the contour of the rocky floor.
The recently completed Devonport dock excavations, which I examined,
showed only very modern alluvium and silted-up channels with logs of
wood cut by metal tools. Submerged forests do not appear to have been
met with.
Though Plymouth Harbour has not yielded much information concerning
the particular period with which we are dealing, it is important
as fixing the maximum amount of elevation to which the land was
subjected in Pleistocene or more recent times. We will now turn to
the Cornish stream-tin works, which give more detail as to the later
changes; we regret however that these most interesting excavations
were closed so long ago, for various points were noted about which
we should like further information, and this is not now obtainable.
The old diluvian hypothesis has much to answer for in the long
neglect of those modern strata which help to tie on geology to
archaeology and history.
By far the best account that has come down to us of a Cornish tin
stream-work carried below the sea-level, is that written by J. W.
Colenso in 1829. Colenso had unusual opportunities for watching the
works—apparently either as manager or owner—and he showed a most
exceptional ability to note scientific points, such as were generally
overlooked 90 years ago. It should be remembered that even in days
before Lyell wrote we had in the Cornish tinners a class of men
whose everyday occupations led them thoroughly to understand the
action of running water. Their daily bread depended on their power to
calculate where the ancient flood must have left the heavy tin-ore,
where the barren ground would be found, or where old silted-up
channels might be sought for. In their arrangements for diverting the
streams in order to work the alluvial deposits, and for washing and
concentrating the tin-ore, they were constantly brought face to face
with the action of running water. When the buried tin-ground yielded
anything abnormal the tinner recognised the effects of exceptional
floods, of eddies behind boulders, or of obstructing ledges. Where
he thought he saw the action of the deluge we may be pretty certain
that he was dealing with something truly exceptional and outside his
experience of the effects of a mountain torrent. He was not using the
word as a cloak for ignorance or excuse for indifference, as was so
often the case with the geologist of that day. Unfortunately most of
the tinners could not write.
Colenso’s account is entitled _A Description of Happy Union Tin
Stream-work at Pentuan_. Pentuan lies at the mouth of the St Austell
River, a rapid stream, much liable to sudden floods, which drains
part of the granite and metalliferous region of St Austell Moor.
The conditions are ideal for bringing down large quantities of
the decayed granite which contains the tin-ore. This material was
alternately weathered and broken up, and so sluiced with flood-water
as to wash away the lighter quartz and felspar, thus concentrating
the tin-ore, with a small amount of gold-dust and small gold nuggets,
in the bottom layer.
The alluvium of the St Austell River was therefore so profitable to
work that every channel was followed upwards into the Moor, and the
main valley was followed downward towards the sea. But as the coast
was approached the rocky floor sank below the sea-level, so that this
part was left till last, for it needed the diversion of the river
and much pumping to get rid of the water. This, scientifically, is
a fortunate circumstance, for of the earlier workings in the higher
part of the valley no good accounts have come down to us.
The river is only a small one and its catchment area is very limited;
it has therefore a rapid fall, amounting to 30 feet in the mile
between St Austell and the sea. With this fall the valley is still
silting up and its alluvium rising, principally through the abnormal
amount of sediment and granitic sand sent down by the china-clay
works. If we take the fall of the buried channel, this amounts to
about 45 feet to the mile, for the rock-floor at Pentuan lies about
60 feet below the sea-level. This rock-floor is composed of hard
slates.
The successive deposits met with above the slate were as follows,
commencing with the lowest:—
(_a_) _The tin ground_, or stratum in which the whole of the
stream-tin is found. It lies on the solid rock and is generally from
three to six feet thick, sometimes even ten feet. It extends across
the valley, except where turned by a projecting hill or rock, when
it is found to take the supposed ancient course of the river, which
is generally under the steep bank opposite. This last observation
(often made by tinners) is important, for it suggests that the
heavy tin-ore was brought down by exceptional floods, such as would
swing violently to the outer side of the curve, and there cut a
steep bluff, under which would be left the heaviest gravel. This
observation and the noteworthy absence of any contemporaneous animal
remains in the tin-ground, suggest that the bottom layer may date
back to Pleistocene times, when the climate was colder and floods
more violent.
It is not clear how far seaward the valley may then have extended;
probably not more than half a mile at most. The tin ground was
worked near Pentuan for 1400 yards along the valley, and averaged
about 52 yards in breadth. So here again we meet with a fairly wide
flat-bottomed valley, not a narrow =V=-shaped gorge; we may therefore
take it that the base-level had been reached and that this base-level
was identical with that met with in the rivers which open into
Plymouth Sound.
(_b_) On the tin ground were rooted numerous oaks, which had grown
and fallen on the spot. Their timber was so sound that Colenso
applied one of the trees to make the axle of a water-wheel, and his
comment on this is excellent. “It appears to me likely that at this
period, the rising of the sea had so far checked the current of the
river as to prevent its discharging the mud and sand brought down
with it; thus the roots were buried [submerged?] to a considerable
depth, and the trees killed, before the timber underwent its natural
process of decay.” At one spot he records finding oysters still
remaining fastened to some of the larger stones at the top of the
tin ground and to the stumps of the oaks.
Then comes a stratum of dark silt, about 12 inches thick, with
decomposed vegetable matter, and on this a layer of leaves of trees,
hazel nuts, sticks and moss for 6 or 12 inches more. This layer of
vegetable matter is about 30 feet below the level of the sea at
low-water and about 48 feet at spring tides. It extends with some
interruption across the valley.
The point is not made quite clear in Colenso’s account, but
apparently there is no marine deposit between the “tin ground” and
the peat, the oyster-bed above mentioned representing the base of bed
_c_, which at that point has cut through the peat, so as to lay bare
part of the gravel and some of the oak-stumps rooted in it. So far,
wherever we have a carefully noted section of the lowest deposits in
these valleys, the tin ground or the gravels are directly succeeded
by a growth of oak trees. It looks as though the climate ameliorated,
the more violent floods ceased, and an oak forest grew across the
alluvial flats, without there being any, or much, change of sea-level.
(_c_) Above the vegetable matter and leaves (_b_) was found a
“stratum of sludge or silt” 10 feet in thickness. It showed little
variation except from a brownish to a lead colour. “The whole is
sprinkled with recent shells, together with wood, hazel nuts,
and sometimes the bones and horns of deer, oxen, etc. The shells,
particularly the flat ones, are frequently found in rows or layers;
they are often double or closed, with their opening part upwards.”
From Colenso’s account it seems probable that this bed was a marine
silt with _Scrobicularia_ and cockles in the position of life. He
goes on to say that “There has been recently found imbedded in the
silt, about two feet from the top, a piece of oak, that had been
brought into form by the hand of man; it is about six feet long, one
inch and a half broad, and less than half an inch thick; this is the
greatest depth at which I have ever seen any converted substance.
It appears to have floated in the sea, as at one end, which is much
decayed, a small barnacle has fixed its habitation.”
(_d_) _A stratum of sea-sand_, about four inches in thickness; this
is easily distinguished from the river-sand, being much finer, and
having always more or less shells mixed with it.
(_e_) _Silt_ two feet, with concretions containing wood and bones.
(_f_) _Another stratum of sea-sand_, 20 feet in thickness. In all
parts of this sand there are timber trees, chiefly oaks, lying in all
directions; also remains of animals such as red deer, “heads of oxen
of a different description from any now known in Britain, the horns
of which all turn downwards.” Human skulls were also found near the
bottom of the sand, and one of these with other fossils was presented
by Colenso to the Royal Geological Society of Cornwall. In the upper
part of this sand nearer the mouth of the harbour, the bones of a
large whale were found. The sea at this time seems to have extended
about a mile up the valley.
(_g_) _A bed of rough river-sand and gravel, here and there mixed
with sea-sand and silt._ About 20 feet in thickness. In this sand
was found “the remains of a row of wooden piles, sharpened for the
purpose of driving, which appear to have been used for forming a
wooden bridge for foot passengers: they crossed the valley, and were
about six feet long; their tops being about 24 feet from the present
surface—just on a level with the present low water at spring tides.
Had the sea-level been then as now, such a bridge would have been
nearly useless.”
At Wheal Virgin, which was the upward extension of the Happy Union
works, about a mile higher up the valley than the bridge just
mentioned, the tin ground was only 32 feet from the surface. Here
Colenso mentions seeing “on the surface of the tin-ground two small
pieces of oak, with artificial holes in them: and there were near
them several oak stakes, sharpened and driven into the ground, and
supported by large stones. Near the same spot has been found a
substance resembling the ashes of charcoal.” This account suggests a
fish or otter trap of some sort and the charcoal below the sea-level
suggests that it must date back to at least as early a period as the
submerged bridge. It is a great pity that antiquaries were not at
that period more alive to the great interest of these finds.
Carnon stream-works, on a navigable branch of the Fal, showed a very
similar section, for below about 54 feet of alternating sand and silt
was found, according to Henwood, a bed one and a half feet thick of
wood, moss, leaves, nuts, etc., a few oyster shells, remains of deer
and other mammals, and some human skulls. Below this came the tin
ground varying in thickness from a few inches to 12 feet. Here also
no organic remains were found in the tin ground itself.
The above records may be accepted as giving fair samples of the
deposits which now fill the lower parts of the submerged valleys
of Devon and Cornwall. These valleys were all at one time long
creeks or arms of the sea, navigable for a considerable distance
inland and affording a fine series of sheltered harbours at short
distances apart. A few of these harbours were so deep and large that
they have not yet been obliterated, as is seen in the case of the
Dart, the branches of Plymouth Harbour, the Fal, the Gannel, etc.
A rapid silting-up is, however, now going on, greatly aided by the
refuse from the mines and china-clay works. In the days whilst the
subsidence was in progress Cornwall was essentially a country of
fjords, though now good harbours are few or blocked with sand-banks.
The abundance of sheltered creeks must have had considerable
influence on the manner of living of the inhabitants; but it is
noticeable that though many acres of the silts have been removed in
tinning, and a good many human remains have been found, there is no
mention of boats. This absence of any record of boats in any of the
marine silts associated with or below submerged forests cannot be
an accident; for old boats and dug-out canoes are constantly being
discovered in later alluvial and fen deposits. It looks as if in
those early days man had either no boats, or only used coracles of
skin and wicker, such as would entirely decay and leave no trace.
It may be remarked that the higher submerged forest, that lying just
about low-water level, is not recorded in the deep excavations at
Pentuan and Carnon, though these old land-surfaces are so conspicuous
on the foreshore opposite every smaller creek, when the sea happens
to scour away the sand and beach. A little consideration will show
the reason of this difference. The extensive stream-works of Pentuan
and Carnon happen to lie at the mouths of two of the larger and
deeper creeks, in which silting-up could not keep pace with the
subsidence. Thus the seaward ends were continuously occupied by
sea, from the time when the oak-forest sank right on into historic
times, and over this deeply buried oak-forest we only find alternate
layers of silt and sea-sand. Evidence of the later submerged forest,
however, is not entirely wanting, for the submerged wooden bridge or
causeway of Pentuan must belong to the period when the trees seen on
the foreshore elsewhere were flourishing well above high-water mark.
The submerged forests seen on the foreshore in western Cornwall are
so like those exposed elsewhere that there is no need for a full
description, were it not that they have become so connected with
ancient legends of Lost Lyonesse, a country which is supposed to have
joined the Land’s End to the Isles of Scilly somewhere about the date
of King Arthur and Merlin. To what extent these stories are due to
observation of the submerged forests and of the rapid waste of land
in Mount’s Bay, supplemented by a vivid Celtic imagination, which saw
“the tops of houses through the clear water,” is doubtful. Legend may
assist, as is shown in a later chapter (p. 120). One thing is clear,
the alluvial flat of Mount’s Bay, under which the submerged forest
lies, formerly extended much further seaward; and old writers mention
the tradition that St Michael’s Mount formerly rose as an isolated
rock in a wood. As far as can be calculated from its known rate
of encroachment, the sea cannot have reached the Mount till long
after the Roman period, and the legend is probably quite accurate.
The Mount was surrounded by a wide marshy flat covered with alders
and willows till well within the historic period; the contradictory
story, that the Phoenician traded to St Michael’s Mount for tin seems
to be the invention of a sixteenth-century antiquary.
In Mount’s Bay there has been subsidence as well as loss of land
through the attacks of the sea, for beneath the alluvial plain, part
of which is still seen in Marazion Marsh, is buried a submerged
forest. Stumps of large oaks, as well as roots of hazel and sallow,
are to be seen at various points on the foreshore, where the
overlying alluvium and peat have been cleared away by the sea. But
the oak-stumps seem to be rooted on a soil resting directly on solid
rock; they do not appear to be underlain by estuarine deposits, or by
lower submerged forests. This particular land-surface may therefore
represent a long period of gradual sinking, during which the trees
flourished continuously, and first at a considerable elevation above
the sea.
The deposit would repay closer examination, for it was not well
exposed while I was staying in Cornwall. I could find no trace of man
in it at Penzance, and the contained flora was principally noticeable
for its poverty and the entire absence of any of the characteristic
west-country plants. The trees were the oak, hazel, and sallow,
the seeds obtained belong to the lesser spearwort, blackberry, a
potentil, self-heal, and some sedges.
Carne, however, in 1846 was more successful at the eastern end of the
Bay, for he has handed down to us an account of the strata met with
in a mine-shaft on Marazion Marsh. The height of the ground at this
spot is only about 12 feet above mean-tide level, and as the deposits
penetrated are 32 feet thick, it is clear that both the rocky floor
and the lower peat must lie beneath the level of the lowest spring
tide. The position of the shaft was close to the Marazion River,
where we would expect also to find an ancient buried channel. The
upper deposits may be of very modern date. Commencing at the top the
succession met with was:—
Feet
Slime, gravel and loose ground 8
Recent estuarine { Rather soft peat 4
deposits { White sand with cockles 12
Recent { Layer of trees, principally
or { oak and hazel, all prostrate.
Neolithic { One piece of oak, about 14 feet
{ long, appears to have been wrought,
{ as if it had been intended for the
{ keel of a boat 1 to 2
“Submerged { Hard solid peat, of closer texture
forest” { than the upper bed 3
Alluvial gravel with tin-ore 4
—
Slaty floor at 32
It will be observed that the supposed keel of a boat occurs above
the old land-surface, among driftwood which probably belongs to the
first infilling of the estuary after the submergence took place. The
upper peat is probably nothing but the surface of the modern marsh,
smothered and much compressed by the eight feet of “loose ground” or
refuse from the neighbouring mines which had accumulated above it.
The cockles probably flourished at the same level (about low-water
mark) as that at which they are now found.
It is not our intention here to deal in any detail with the submerged
land-surfaces noticed on the French coast opposite. The Channel
Islands yield indications of submergence, and if its amount was as
great as that proved on the north shores of the Channel, then the
Channel Islands must have been connected with the mainland up to a
period when the climatic conditions were similar to and the fauna and
flora resembled those of the adjoining parts of France at the present
day.
Further west, recent discoveries on the shores of the Bay of Biscay
are of considerable interest, for submerged forests occur at various
places, though the maximum amount of the submergence has not yet been
satisfactorily made out.
One of the most interesting of the submerged forests seen between
tide-marks on the French coast was that discovered a few years
since by Monsieur Emil Gadeceau in Belle Ile. This island lies off
the mouth of the Loire, and its position some way from the coast and
well out in the Atlantic induced him to make a special study of its
flora. While engaged in this, his attention was drawn to certain hard
peaty deposits seen only at low tide, and he asked me to undertake
the examination of the seeds found in them. This work was gladly
undertaken, as it carried further south the examination which was
then being made into the flora of the submerged forests.
The results were somewhat surprising; out of about 30 species
sufficiently well preserved for identification, six were no longer
living in Belle Ile, though known in Western France. The whole
flora might have come from the north of England, characteristic
French species being entirely missing, though this element is fairly
represented in the living flora of the island. In short, the flora
is a northern one, though in no degree arctic, and in this it agrees
well with the poor assemblage commonly found in the submerged forests
of the south of England.
From still further south, at various points on the shores of the Bay
of Biscay, and from the submerged peaty deposits which underlie the
Landes, seeds have since been collected by my friend, Professor Jules
Welsch, of Poitiers. These also all belong to common living British
plants, except that at Brétignolles, south of latitude 47°, we
meet for the first time one characteristic southern plant—the vine.
Unfortunately the search for traces of man and his works in these
deposits has so far been unsuccessful, and we cannot yet be certain
therefore that they are all of quite the same date, or correspond
exactly with the submerged forests of Britain.
CHAPTER VIII
SUMMARY
To what conclusions do the foregoing somewhat monotonous pages lead?
Do they help us to explain the origin of our fauna and flora? What
light do they throw on the antiquity of man in Britain, or on the
race problems that everywhere confront us? Can the deposits therein
described be in any way connected with written history or with
legend? Do they give us any approach to a measure of geological time?
And, to what extent does the period of the submerged forests tie on
historical times with the Glacial Epoch?
All these questions are connected with the subject-matter of this
little book; but it is not written with the idea of showing how much
we know or pretend to know. Our main object is to draw attention
to a much neglected period in geological history and to suggest
directions in which further research is likely to be profitable. We
have, however, made out several points, and can give an approximate
answer to some of the questions.
It is quite clear that at the opening of the period with which
this volume deals, the greater part of England stood fully 70 feet
above its present level, for the oldest deposit we deal with is
a land-surface covered with oak-forest and lying 60 feet below
tide-level. The oaks cannot have flourished lower, but they may have
grown on a soil well above sea-level. Perhaps taking the whole of the
evidence into account, a subsidence of nearly 90 feet is the most
probable measure of the extent of the subsequent movement.
We do not yet know whether in England this movement was a depression
of the land or a rise of the sea; but the fact that the relative
levels seem to have been quite different in Scotland and in
Scandinavia seems to indicate that it was the land that moved, not
the sea.
We begin, therefore, with a period when the whole of the southern
part of the North Sea was an alluvial flat connecting Britain with
Holland and Denmark, and to some extent with France. The Isle of
Wight was connected with Hampshire, and the Channel Islands with
France. Probably the Isles of Scilly were islands even then, for the
channel between them and Cornwall is both deep and wide, though this
may possibly be due to tidal scour.
The animals and plants yet known from this lowest submerged forest
are disappointingly few; but the prevalence of the oak shows that
the climate was mild, and that we have no clear indication of
conditions approaching to those of the Glacial Epoch. In fact, in all
the submerged forests the fauna and flora seem poor and monotonous,
consisting essentially of living British species, with a few mammals
since locally exterminated by man, and all known to have a wide range
both in climate and latitude.
This in itself, however, is a point gained in the study of the
origin of our flora; for though the deficiency is no doubt largely
due to insufficient collecting, I am convinced that it is a true
characteristic of this period of transition. Much time has been spent
in examining and collecting the fossils of these submerged forests,
and various friends have also worked at them; but everywhere we seem
to get the same result, and many abandon the study because there
is so little to show for it. The deposits certainly contain a much
poorer fauna and flora than either the Pleistocene or the recent
alluvial strata.
If we consider the Britain of the submerged forests as having
lately emerged from a time when the climate was ungenial, we should
naturally expect to find among the first incomers after the change
only such animals and plants as have a wide climatic range or can
migrate freely. It is these species, and these only, which will be
living on the neighbouring lands; it is only an assemblage like
this that can stand the climatic alternations and relapses that are
likely to attend the transition. An assemblage consisting only of
species widely distributed in latitude is probably an assemblage that
has special means of dispersal—even if we do not happen yet to have
discovered these means.
These considerations should lead us to expect to find living, in any
country which has lately undergone a change of climate, a somewhat
peculiar assemblage, consisting mainly of hardy forms of wide range
in latitude, and not characteristically either northern or southern.
Mingled with them, we might expect a few survivors from the previous
warm or cold period. A hardy fauna and flora seem to characterise the
period of the submerged forests; but the absence or great scarcity of
characteristic survivors from a former period suggests that even the
lowest of these deposits is far removed from the Glacial Epoch. The
arctic species had already had time to die out, or had been crowded
out; but the time had not been sufficiently long for the incoming
of the southern forms which now characterise our southern counties.
Then, even less than now, had we reached a perfect adjustment of the
fauna and flora to the climatic conditions; this can only be brought
about by a constant invasion of species from all the surrounding
regions. Some hold their own, most cannot; but as time goes on, the
surviving assemblage consists more and more of species which have
been able to fight against the severe competition and colonize a new
country.
Garden experiments are of little use as tests of the capability of
any plant to survive in this country; the study of cornfield weeds
is no better. In both cases the cultivation of the land produces a
bare place on which a foreign introduction has as good a chance as
a native. But could this foreigner survive if the seed were dropped
on a natural moor or meadow? In this connexion it is noticeable that
great part of the rare British plants occur close to the coast,
opposite the part of the continent in which they are found, though
they are not maritime species. This is probably due to two different
causes, both acting in the same direction. In the first place most
of these local plants are obviously late comers, which have not yet
had time to spread inland or far. And, secondly, on the coast alone
do we find any considerable extent of natural bare land—practically
garden land—which does not at the same time consist of poor soil.
The tumbled undercliffs of our coast are just the places to give a
foreign invader a chance; there only will it find patches of bare
good soil, full of small cracks in which a seed is hidden from birds.
If the view is correct, that a continuous growth of our flora, and
to some extent of our fauna, takes place through transportation to
our coasts, from which such species as can fight their way tend
more slowly to spread inland, it seems to account for the present
curious distribution of species, and this in a way that no continuous
land-connexion will do.
As we have pointed out in a former chapter, the land-connexion across
the North Sea was a wide alluvial plain and swampy delta. What use
could dry-soil plants make of such a bridge? It would be no easier
for them to cross than so much sea; and migrating mammals could
not greatly help in the dispersal, where so many rivers had to be
crossed. The aquatic species would be helped by such a connexion, and
it is curious to note that several of our most interesting aquatic
plants are confined to the eastern counties, which in post-glacial
times had direct connexion with the delta of the Rhine, and probably
with the Elbe.
Aquatic species, however, are not dependent on continuous waterways
for their dispersal; they have great facilities for overleaping
barriers and reaching isolated river-basins and lakes. Every dew-pond
on the downs after a few years’ existence contains aquatic plants and
mollusca, and a still larger number of species, including fish, will
be found in ancient flooded quarries or prehistoric dykes surrounding
some hill-fortification. If an aquatic plant is fairly common on the
continent near by, it is almost certain to occur in some isolated
pond or river in the part of Britain opposite.
Many of our peculiar mollusca and plants are limestone species,
which must have crossed over at a single leap, for no elevation or
depression will connect the various isolated limestone masses of
Britain. A post-glacial elevation would connect the North Downs with
the corresponding chalk-hills of France; but these Downs are isolated
by wide tracts of non-calcareous strata from the areas of Oolite
or Carboniferous limestone to which many of our limestone animals
and plants are now confined. There is also nothing in the present
distribution of our limestone species to suggest that any great
stream of migrants used this bridge of chalk-downs.
It may be asked, Why discuss these questions here, if all these
peculiar species are unknown in the submerged forests? In certain
cases negative evidence is of great value, and the deficient flora
of the submerged forests is a case in point. We find a striking
contrast between this ancient flora and the flora which flourished
when cultivation of the land had begun. The Roman deposits in Britain
yield many species which have not yet been found in the submerged
forests, and even the earlier Celtic deposits have already yielded a
few of them. To a large extent this difference is due to the agency
of man, intentional to a certain extent, but mainly accidental,
through the introduction of weeds and the preparation of the soil
for crops. It must not be forgotten that man not only introduced the
weeds, he prepared the land on which they could establish themselves,
and from thence spread to uncultivated ground where few botanists now
suspect that they are anything but “native.”
In days when the people of Britain were hunters, the only extensive
open country in the south and east seems to have been the chalk-downs
and the sandy heaths. These were not suitable for new additions to
the plant population, for the good land was all oak forest, the
barren heaths were unfavourable for any but heath plants, and the
alluvial flats were largely covered with sallow and alder. The open
downs were clothed with close turf, and until this was broken by
cultivation there would be little chance for migrants. It seems,
therefore, that to obtain a clear idea of the plant population of
this country before man’s influence could be felt, we must study
the flora of the submerged forests and of the associated alluvial
detritus washed from the uplands during the same period. Till this is
done more thoroughly, it is not much use to discuss what species are
“native” and what “introduced”; the submerged forest will yield the
answer to this question.
The next question we have put—What light do these submerged forests
throw on the antiquity of man in Britain, or on the race-problems of
Britain?—is a difficult one to answer in the present state of our
knowledge. Valuable evidence has been lost through the failure to
preserve most of the human remains that have been found; but both
Owen and Huxley recognised the peculiar type of the “river-drift
man.” Unfortunately few implements have been collected, and the
pieces of wood shaped by man, though recorded, have not been
preserved. One implement of polished stone has certainly been found
in the latest submerged land-surface, but it is not clear that
anything except flakes has been obtained in the older deposits.
Still the stratigraphical relations seem to indicate that all these
deposits are of Neolithic age and later than the Palaeolithic
terraces. The relations of Palaeolithic to Neolithic are still very
obscure in this country, and the reason is perhaps to be sought in a
submergence which has tended to carry many of the transition deposits
beneath the sea-level, or has caused them to be silted up under more
modern alluvium. The lowest submerged forest requires careful search
before we can be certain of its true position in the sequence; but
it is seldom exposed, and then only in dock-excavations soon again
hidden.
Before we can attempt to answer the other questions, it is important
to get an estimate of the amount of time occupied in the formation
of these deposits, and of the lapse of time since the last of
them was formed. The newest of them belongs certainly to the age
of polished stone, and the earliest also probably comes within
the Neolithic period. We have already seen that within the period
represented by the submerged forests there has been a rise of the
sea-level, or depression of the land, to the extent of 80 feet,
perhaps a few feet more. If we can obtain some measure of the time
occupied in the formation of such a series of deposits, this should
give us some idea as to the length of the Neolithic period, and also
of the rate at which changes of the sea-level sometimes can take
place.
It is unfortunate that for these calculations so many of the factors
are of uncertain value. We may estimate from the present rate
of erosion of the coast the amount that has been lost since the
sea-level became stationary, or we may take the rate of accumulation
of sand-dunes or shingle-spits; or the rate at which our estuaries,
harbours, and broads are silting up. It all comes, however, to
this—no exact figures can be given; but so many rough calculations
lead to approximately the same date, that the date arrived at may
be trusted to give some idea of the length of the period which has
elapsed since the downward movement ceased.
Working backwards from the present day, step by step, archaeological
evidence gives an undoubted period of 2000 years, to the first
century B.C., during which no measurable change of sea-level has
taken place in the south of England.
To this must be added a few centuries for the growth of the marshes
on which Glastonbury and similar lake-dwellings were built, and
for the growth of various other marshes at present sea-level known
to be earlier than the Roman invasion. Also we must allow for the
accumulation of various shingle-spits and sand-dunes then already
partly formed.
In general, somewhere about one-third or one-half of this
accumulation seems to have taken place before the Roman invasion.
This adds another 1500 years; so that about 3500 years ago, we get
back to the beginning of the period of unchanging sea-level in which
we are still living, and begin to see evidence of earth movements
still in progress.
Whether this 3500 years will take us back to the beginning of the
Bronze Age in Britain is not yet proved; but so far we seem to
discover metals in the whole of the deposits formed whilst the
sea-level remained unchanged, and only stone weapons in even the
newest of the submerged forests. For the present, we may therefore
take it that the two changes nearly coincided. The use of metals
began in Britain about the time that the earth-movements ceased—that
is to say somewhere about 1600 B.C.
Whether this period of 3500 years will really take us back to the
commencement of the Bronze Age is doubtful, for Stonehenge had
already been built, and though only stone hammers seem to have been
there used, yet one slight streak of bronze or copper has been
noticed. Of course, there may have been a similar occasional use of
bronze at the time of the last submerged forest; but we have as yet
no evidence of this, and the possible correspondence in date between
Stonehenge and the last of the submerged forests remains merely a
suggestion.
Perhaps we may still find submerged stone-circles or other
antiquities of the age of Stonehenge beneath the sea-level; but
Stonehenge lies too high above the sea for it in itself to give any
clue as to a change of sea-level. We will only make one suggestion.
It is probable that when Stonehenge was built, a long arm of the
sea extended far up the Avon Valley, so that navigable water was
found not far from Stonehenge. There is in Stonehenge an inner
circle of smaller stones, not composed of the local greywethers but
consisting of large blocks of igneous rock of foreign origin. These
blocks, which are sufficiently large to be awkward for land-carriage,
have been said to be erratics gathered on Salisbury Plain, just as
the greywethers for the main circle were gathered; but there are
no erratics on Salisbury Plain. Large erratic blocks of similar
character occur, however, abundantly on the lowlands of Selsey Bill,
under the lee of the Isle of Wight. Probably a similar erratic-strewn
plain once fringed the coast on the west also, though on the exposed
side the part above the sea-level has now been entirely swept away by
the sea.
I would suggest that the Stonehenge erratics, instead of being
brought from any great distance, may have come from a wide plain at
the mouth of the Avon, then two or three miles further seaward. From
thence they were rafted far up the navigable fjord, not yet silted
up, and were only carried a short distance uphill. Igneous rocks such
as these, found in a country consisting essentially of chalk and
Tertiary strata, would be valuable and probably endowed with magic
properties, hence their employment in this inner circle.
Our next enquiry must be into the length of time represented by
the series of submerged forests and associated deposits described
in the foregoing pages. The newest of them belongs certainly to
the age of polished stone, and the earliest also probably comes
within the Neolithic Period. Within the period represented by the
submerged forests, we have seen that there has been a change of the
sea-level to the extent of 80 feet, or perhaps rather more. If we can
obtain some measure of the time occupied, this should give us some
approximate idea as to the length of the Neolithic period, and of
the rate at which changes of the sea-level can take place.
The first point to be considered is the length of time occupied by
the growth of the series of submerged forests. On first examining,
or reading accounts of, deposits of this sort one obtains a vague
impression of long periods, during which mighty oaks flourished.
Both the movements of submergence and the intervening periods of
vegetable growth seem to require great lapses of time. On closer
study, however, the evidence seems scarcely to support this view,
for estuarine silts are deposits of exceptionally rapid growth, and
one finds that the usual characteristic of a “submerged forest” is
that it shows indications of only a single generation of trees. The
trees also are usually small, except where the submerged forest rests
directly on deposits of much earlier date, or on solid rock.
It should be remembered that the large oak trees which are often
found in the lowest land-surface at any particular place do not
necessarily belong to any one special stage of the submergence.
These same trees may have grown continuously above tide-marks
during several successive stages, until at last the upward creeping
water rose sufficiently to reach this part of the forest. The large
well-grown oaks seen in Mount’s Bay and various other places are, as
far as I have seen, all rooted on ancient gravels, solid rock, or
boulder clay, not on beds of silt.
We cannot speak confidently as to the time needed to form each thin
layer of vegetable soil, marsh peat, or estuarine silt. On comparing
the submerged land-surfaces, however, with similar accumulations
formed within known periods, such as marsh soils grown behind ancient
embankments, or forest-growth over flats silted up at known dates,
we can learn something. No one of the land-surfaces alternating with
the silts would necessarily require more than a century or two for
its formation. Brushwood and swamp growth are the characteristic
features of these deposits, and such growth accumulates and decays
very rapidly. Possibly trees of older growth may still be found, but
I have not succeeded in discovering a tree more than a century old in
any one of the marsh deposits alternating with the estuarine silts.
Oaks of three centuries may be observed rooted in the older deposits;
but this, as above explained, is another matter.
It is useless to pretend to any exact calculations as to the time
needed for the formation of these alternating strata of estuarine
silt and marsh-soil; but looking at the whole of the evidence without
bias either way, it seems that an allowance of 1000, or at most
1500, years would be ample time to allow. A period of 1500 years may
therefore be taken to cover the whole of the changes which took
place during the period of gradual submergence.
If this is approximately correct, the date at which the submergence
began was only 5000 years ago, or about 3000 B.C. The estimate may
have to be modified as we obtain better evidence; but it is as well
to realize clearly that we are not dealing with a long period, of
great geological antiquity; we are dealing with times when the
Egyptian, Babylonian, and Minoan civilizations flourished. Northern
Europe was then probably barbarous, and metals had not come into use;
but the amber trade of the Baltic was probably in full swing. Rumours
of any great disaster, such as the submergence of thousands of square
miles and the displacement of large populations might spread far and
wide along the trade routes. Is it possible that thus originated some
of the stories of the deluge?
We will not now pursue this enquiry; but it is well to bear in mind
the probability that here geology, archaeology, and history meet and
overlap. Any day one of our submerged forests may yield some article
of Egyptian manufacture of known date, such as a scarab, which has
passed from hand to hand along the ancient trade routes, till it
reached a country still living in the Stone Age, where its only use
would be in magic. But it might now serve to give us a definite date
for one of these submerged forests. It might happen to have been
lost with some of the stone implements, or with one of the human
skeletons, apparently belonging to persons drowned, for no trace of
a grave is ever mentioned. A find of this sort is no more improbable
than the discovery of a useless modern revolver in a bag of stone
and bone tools belonging to some Esquimaux far beyond the reach of
ordinary civilized races.
In this connexion it might be worth while systematically to dredge
the Dogger Bank, in order to see whether any implements made by man
can be found there. The alluvial deposits are there so free from
stones that if any at all are found in them they may probably show
human workmanship. The Dogger Bank may have remained an island long
after great part of the bed of the North Sea had been submerged, for
the Bank now forms a submerged plateau. It may even have lasted into
fairly recent times, the final destruction of the island being due
to the planing away of the upper part of the soft alluvial strata
through the attacks of the sea and of boring molluscs. _Pholas_ is
now actively attacking the hard peat-beds at a depth of more than
10 fathoms, and is rapidly destroying this accumulation of moorlog,
wherever the tidal scour is sufficient to lay it bare.
BIBLIOGRAPHY
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GODWIN-AUSTEN, R. A. C. On the Newer Tertiary Deposits of the
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HENWOOD, W. J. On some of the Deposits of Stream Tin-Ore in
Cornwall, with remarks on the Theory of that Formation. _Trans. R.
Geol. Soc. Cornwall_, vol. IV, 1832, p. 57.
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Excursion, _Geol. Assoc._ p. 182.
—— Notes on the Geological Position of the Human Skeleton lately
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IV, 1884, p. 135.
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INDEX
Alder, 1, 8, 16, 32, 64, 112
Alternations of freshwater and marine strata, 4, 7, 16, 29, 35, 44,
51, 57, 94-97
Amberley Wild Brook, 73
Amsterdam, 48
Anchor-ice, 89
Aquatic plants, dispersal of, 110
Archaeological evidence, 61, 115-117, 121
Atlantic waves, effect of, 85, 86
Baltic, 49
Barry Docks, 52-57, 59
Base-level planes, 22, 37, 38, 80, 87, 88
Baynes, D., 13-17
Bay of Biscay, 86, 103-105
Bear, 2
Beaver, 2, 35, 44
Belle Ile, 104
Bideford Bay, 61, 62
Birch, arctic, 46
—— white, 46
Blakeney, 29
Boar, 2, 35
Boats, 99, 102, 103
Bog-bean, 45
Bone needles, 54
Brétignolles, 104
Bridgwater Levels, 60
Bristol Channel, 58-61
Broads, 23-26, 114
Brögger, W. C., 122
Bronze age, 115, 116, 120
Caesar’s invasion, 65, 66, 75
Canoes, 99, 102, 103
Cardiff, 57
Carne, J., 102
Carnon stream-works, 98, 99
Cattewater, 84, 88
Causeway, submerged, 97, 100
Celtic shorthorn, 63, 96
Champion, G. C., 46
Channel Islands, 68, 86, 103, 106
Channels, shifting of, 6, 21, 36, 37, 91, 93, 102
Chara-marl, 8
Charcoal below submerged forest, 97
Chatteris, 30, 35
Cheshire, 50, 51, frontispiece
Chichester Harbour, 76, 77
Clacton, 21, 23, 30, 31
Climatic changes, 10, 46, 66-68, 104, 107, 108
Clyde-beds, 50
Coast-erosion, 114
Colenso, J. W., 91-97, 122
Compression of alluvial strata, 3, 4, 16
Continental platform, origin of the, 69, 70
Coracles, 99
_Corbicula fluminalis_, 11, 23, 31, 35, 37, 74
Cornel, 55
Cornwall, 80-105
_Cotoneaster Pyracantha_, 74
Cromer Forest-bed, 11, 22, 23, 39
Cultivation, effects of, 111, 112
Dart, 98
Dartmoor, 82, 83, 89
Dates, 26, 114-121
Dead Sea depressions, 5
Dee, 51
Deformation of shore-lines, 38, 49
Deluge, 2, 66, 81, 120
Denmark, 48, 106
Depression, extent of, 8, 17, 20, 22, 26, 38, 47-49, 51, 56, 57, 68,
73, 75, 88, 93, 94, 106
Devonport, 6, 83, 90
Dispersal of plants, 108, 109-112
Dogger Bank, 17, 18, 39-49, 69, 121, 124
Domestic animals, 63, 96
Dove Point, 50, 51, frontispiece
Dunge Ness, 71
Eccles Church, 27, 28
Elbe, 40, 110
Elder, 16
Elevation of the land, former, 8, 17, 20, 22, 26, 38, 47-49, 51, 56,
57, 68, 73, 75, 106
Elwes, J. W., 124
Ely, 30, 34
English Channel, 64-79
Estuaries, silting-up of, 25, 26, 114
Fal, 98
Fareham, 76
Fen-deposits, 30-36, 44, 45
Fenland, 4, 30-37
Flint-implements, 21, 35, 54, 62, 74, 113
Flora, changes of the, 9, 10, 110
—— of the submerged forests, 63, 64, 104, 105, 107, 108
Formby, 52
France, 103-106
Gadeceau, E., 104
Gannel, 98
Geographical distribution, 60, 110
Glacial epoch, 10, 23, 36, 67, 68, 83, 87, 89, 90, 94, 107, 108
Glamorgan, 52-57
Glastonbury Levels, 60-62, 115
Godwin-Austen, R. A. C., 122
Gold-dust, 92
Goodchild, H. H., 42, 124
Goodwin Sands, 66, 70
Gothland, 49
Gravesend, 17
Grays, 20, 23, 30, 31, 37
Hamoaze, 84, 85, 88
Hampshire, 74-79, 106
Happisburgh, 39
Happy Union, 92-97
Harbours, origin of, 77, 78
—— silting-up of, 98, 99, 114
Harwich, 22
Hastings, 71
Hawthorn, 55
Hazel, 1, 8, 9, 46, 55, 98, 101, 102
Henwood, W. J., 98, 123
Heysham, 50
Holderness, 38, 44
Holland, 4, 44, 48, 49, 106
Holmes, T. V., 123
Horse Sand Fort, 74
Human remains, 9, 10, 13-15, 17, 18, 52, 62, 96-99, 113, 121
Humber, 20, 35-38, 88
Huxley, Prof. T., 13, 15, 113
Implements, stone, 21, 35, 54, 62, 74, 113
—— wooden, 96, 113
Intermittent depression, 35, 51
Ireland, 49
Irish elk, 21
—— Sea, 80
Isle of Man, 50
—— Wight, 75, 76, 106, 117
Keith, Prof. A., 13, 15, 122
Kent, 65-70
Kessingland, 23
King, Rev. S. W., 27
King’s Lynn, 6
Laira Viaduct, 88
Lancashire, 50, 51
Landes, 104
Land’s End, 100
Landslips, 3
Langer Fort, 22
Langston Harbour, 76, 77
Leasowe, 50, 51, frontispiece
Lewes, 73
Lincolnshire, 35
London, 12, 19
Lorié, Dr. J., 122
Lost Lyonesse, 100
Lowestoft, 23, 26
Lyell, Sir C., 27, 91
Mammoth, 15, 21, 35
Man, remains of, 9, 10, 13-15, 17, 18, 52, 62, 96-99, 113, 121
Man’s influence on our flora, 112
Marazion, 101-103
March, 30, 31, 35, 37
Marine erosion, 84-86
Mersey, 51, 52
Migration of species, 10, 108-112
Miller, S. H., 122
Moorlog, 41, 43, 44, 47
Morton, G. H., 123
Mosses, formation of, 35
Mount’s Bay, 100, 101, 118
Munthe, Dr H., 49, 123
_Najas marina_, 53
Nar Valley Beds, 31, 35
Neolithic, 21, 35, 38, 53, 62, 74, 75, 79, 113-115, 118
Noah’s Woods, 1, 66
Noman Fort, 75
Norfolk broads, 23, 24-26, 49
North Sea, 17, 38-49, 67, 68, 80, 106, 110, 121
Norway, 49
Oak, 1, 8, 16, 19, 32, 34, 54-57, 62, 71, 94-96, 101, 102, 106, 107,
119, 120
—— cut by man, 97
_Osmunda_, 1, 52
Ouse, 6
Owen, Sir R., 13, 15, 18, 113, 123
Oysters on submerged trees, 94, 95
Palaeolithic, 15, 21, 22, 113
Pegwell Bay, 65
Pentuan, 92-97, 99, 100, 122
Penzance, 101
Pepys, S., 19
Pevensey Level, 71
_Pholas_, erosion caused by, 43, 121
Piles below a submerged forest, 97, 100
Pine, 32, 34, 35, 79
Plane of marine denudation, 22
Plym, 82
Plymouth Sound, 83-88, 90, 98
Polished stone implements, 54, 113
Pollen-grains in submerged peat, 45
Poole Harbour, 79
Portsmouth Harbour, 76, 77
Potter, C., 123
Potter Heigham, 26
Prestwich, Sir J., 26, 123
Prevost, Dr E. W., 123
Raised beaches, 38, 50
Rashleigh, P., 123
Reade, T. M., 50, 51, 123
Reindeer, 44, 79
Rhine, 18, 40, 67, 110
Ribble, 51
River-drift man, 15
—— ice, 89
Roadway, submerged, 28, 100
Rogers, I., 61, 62, 124
Roman period, 3, 16, 61, 63, 65, 75, 101, 111, 115
Romney Marsh, 70, 71
Rostock, 49
_Ruppia_, 46
St Austell River, 92-98
St Michael’s Mount, 100, 101
Saltash, 88
Sand-dunes, 3-5, 26, 27, 47, 114
Sangatte, 68
Scandinavia, 49, 106
Scilly, 100, 106
Scotland, 38, 49, 50, 106
_Scrobicularia_-clay, 7, 19, 53, 96
Sea-level, changes of, 3, 4, 8, 17-20, 22, 26, 38, 47, 48, 51, 57,
68, 73, 75, 106, 114
Selsey, 74, 117
Severn Tunnel, 59
Sheep, 63
Shell-marl, 8
Sherringham, 22
Shingle-beach, protection afforded by, 3-5
—— spits, growth of, 26, 114
Shore, T. W., 79, 124
Silting-up, rate of, 25, 26
Skertchly, S. B. J., 31, 122, 124
Solent, 74-76
Somerset, 60, 61
Southampton Water, 74, 76, 79
South Downs, 72
Southwold, 22
Spithead, 74-77
Spurrell, F. C. J., 13, 15-18, 124
Stather, J. W., 124
Stonehenge, 116
Storrie, J., 53, 54
Strahan, Dr A., 52-57, 124
Strait of Dover, 65-70
Stream-tin works, 80, 81, 90-102, 123, 124
Submarine erosion, 69, 84-86, 121
Submerged forest, definition of, 11
Submergence, extent of the, 8, 17, 20, 22, 26, 38, 47-49, 51, 56, 57,
68, 73, 75, 88, 93, 94, 106
Sussex, 65, 68, 71-74
Sweden, 49
Tamar, 82, 84, 85
Tavy, 82, 88
Thames, 11-19, 20, 21, 29, 38, 40, 51, 52, 84, 88, 122, 124
Thaws and floods, 89
Tidal scour, 69, 85, 86
—— wave deadened by obstructions, 4, 5, 28, 29
Tides of the Bristol Channel, 56
Tilbury, 13-19
Time, 114-121
Tin stream-works, 80, 81, 90-102, 123, 124
Trees growing below high-water level, 4, 5, 8
—— rooted into rock, 4
—— succession of, 8, 32
Tyler, A., 124
Vegetable remains, decay of, 4
Vine, 105
Virgin, Wheal, 97
Walrus, 44
Wash, 29, 30
Waves, effect of, 85, 86
Wealden axis, 72
Weeds, introduction of, 112
Welsch, Prof. J., 104
Westward Ho, 61-64
Weybourn, 29
Whitaker, W., 13, 15, 16, 124
Whitehead, H., 42, 124
Wolf, 2, 35
Wood, implements of, 9, 96, 97, 113
Worth, R. H., 124
Yarmouth, Isle of Wight, 75
—— Norfolk, 26, 28
Yew, 19, 32
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