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.