Submerged Forests/Chapter 5
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 of depression |
Blown sand Recent silts with beds of peat; Scrobicularia, occasional
freshwater shells, red-deer, horse, Bos primigenius, Bos longifrons, and human skull | |
| 2nd period of elevation |
Superior peat- and forest-bed | |
| 2nd period of depression |
Formby and Leasowe marine beds; human skeleton,
bones of horse and red-deer, Scrobicularia, Tellina baltica, Turritella communis, etc. | |
| 1st period of elevation |
Inferior peat- and forest-bed | |
| 1st period of depression |
Washed drift-sand (apparently no contemporaneous Boulder clay
fossils) |
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 aflected this part of South Wales. The following sequence was met with:—
| 1. Blown sand. | Recent subaerial and tidal deposits. | |
| 2. Scrobicularia-clay | ||
| 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
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Rubus fruticosus Callitriche Cornus sanguinea Sambucus nigra Aster Tripolium Solanum Dulcamara Ajuga reptans |
| Sueda maritima Atriplex patula Rumex Urtica dioica Alnus glutinosa Corylus Avellana
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Quercus robur Alisma Plantago Ruppia maritima Eleocharis palustris Scirpus Tabernaemontani Carex 3 sp.
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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.