# Submerged Forests/Chapter 7

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 sandbanks 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. ​here lie 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 appeal's 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 sandbanks.
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 ​readied 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, selfheal, 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 deposits | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | Rather soft peat……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… | 4 |
| Recent estuarine deposits | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | White sand with cockles……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… | 12 |
| Recent or Neolithic | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Layer of trees, principally oak and hazel, all prostrate. 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 forest" | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | Hard solid peat, of closer texture 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.