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:—