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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.