Submerged forests

by Clement Reid

These considerations should lead us to expect to find living, in any country which has lately undergone a change of climate, a somewhat peculiar assemblage, consisting mainly of hardy forms of wide range in latitude, and not characteristically either northern or southern. Mingled with them, we might expect a few survivors from the previous warm or cold period. A hardy fauna and flora seem to characterise the period of the submerged forests; but the absence or great scarcity of characteristic survivors from a former period suggests that even the lowest of these deposits is far removed from the Glacial Epoch. The arctic species had already had time to die out, or had been crowded out; but the time had not been sufficiently long for the incoming of the southern forms which now characterise our southern counties. Then, even less than now, had we reached a perfect adjustment of the fauna and flora to the climatic conditions; this can only be brought about by a constant invasion of species from all the surrounding regions. Some hold their own, most cannot; but as time goes on, the surviving assemblage consists more and more of species which have been able to fight against the severe competition and colonize a new country.

Garden experiments are of little use as tests of the capability of any plant to survive in this country; the study of cornfield weeds is no better. In both cases the cultivation of the land produces a bare place on which a foreign introduction has as good a chance as a native. But could this foreigner survive if the seed were dropped on a natural moor or meadow? In this connexion it is noticeable that great part of the rare British plants occur close to the coast, opposite the part of the continent in which they are found, though they are not maritime species. This is probably due to two different causes, both acting in the same direction. In the first place most of these local plants are obviously late comers, which have not yet had time to spread inland or far. And, secondly, on the coast alone do we find any considerable extent of natural bare land—practically garden land—which does not at the same time consist of poor soil. The tumbled undercliffs of our coast are just the places to give a foreign invader a chance; there only will it find patches of bare good soil, full of small cracks in which a seed is hidden from birds.

If the view is correct, that a continuous growth of our flora, and to some extent of our fauna, takes place through transportation to our coasts, from which such species as can fight their way tend more slowly to spread inland, it seems to account for the present curious distribution of species, and this in a way that no continuous land-connexion will do.

As we have pointed out in a former chapter, the land-connexion across the North Sea was a wide alluvial plain and swampy delta. What use could dry-soil plants make of such a bridge? It would be no easier for them to cross than so much sea; and migrating mammals could not greatly help in the dispersal, where so many rivers had to be crossed. The aquatic species would be helped by such a connexion, and it is curious to note that several of our most interesting aquatic plants are confined to the eastern counties, which in post-glacial times had direct connexion with the delta of the Rhine, and probably with the Elbe.

Aquatic species, however, are not dependent on continuous waterways for their dispersal; they have great facilities for overleaping barriers and reaching isolated river-basins and lakes. Every dew-pond on the downs after a few years’ existence contains aquatic plants and mollusca, and a still larger number of species, including fish, will be found in ancient flooded quarries or prehistoric dykes surrounding some hill-fortification. If an aquatic plant is fairly common on the continent near by, it is almost certain to occur in some isolated pond or river in the part of Britain opposite.

Many of our peculiar mollusca and plants are limestone species, which must have crossed over at a single leap, for no elevation or depression will connect the various isolated limestone masses of Britain. A post-glacial elevation would connect the North Downs with the corresponding chalk-hills of France; but these Downs are isolated by wide tracts of non-calcareous strata from the areas of Oolite or Carboniferous limestone to which many of our limestone animals and plants are now confined. There is also nothing in the present distribution of our limestone species to suggest that any great stream of migrants used this bridge of chalk-downs.

It may be asked, Why discuss these questions here, if all these peculiar species are unknown in the submerged forests? In certain cases negative evidence is of great value, and the deficient flora of the submerged forests is a case in point. We find a striking contrast between this ancient flora and the flora which flourished when cultivation of the land had begun. The Roman deposits in Britain yield many species which have not yet been found in the submerged forests, and even the earlier Celtic deposits have already yielded a few of them. To a large extent this difference is due to the agency of man, intentional to a certain extent, but mainly accidental, through the introduction of weeds and the preparation of the soil for crops. It must not be forgotten that man not only introduced the weeds, he prepared the land on which they could establish themselves, and from thence spread to uncultivated ground where few botanists now suspect that they are anything but “native.”

In days when the people of Britain were hunters, the only extensive open country in the south and east seems to have been the chalk-downs and the sandy heaths. These were not suitable for new additions to the plant population, for the good land was all oak forest, the barren heaths were unfavourable for any but heath plants, and the alluvial flats were largely covered with sallow and alder. The open downs were clothed with close turf, and until this was broken by cultivation there would be little chance for migrants. It seems, therefore, that to obtain a clear idea of the plant population of this country before man’s influence could be felt, we must study the flora of the submerged forests and of the associated alluvial detritus washed from the uplands during the same period. Till this is done more thoroughly, it is not much use to discuss what species are “native” and what “introduced”; the submerged forest will yield the answer to this question.

The next question we have put—What light do these submerged forests throw on the antiquity of man in Britain, or on the race-problems of Britain?—is a difficult one to answer in the present state of our knowledge. Valuable evidence has been lost through the failure to preserve most of the human remains that have been found; but both Owen and Huxley recognised the peculiar type of the “river-drift man.” Unfortunately few implements have been collected, and the pieces of wood shaped by man, though recorded, have not been preserved. One implement of polished stone has certainly been found in the latest submerged land-surface, but it is not clear that anything except flakes has been obtained in the older deposits. Still the stratigraphical relations seem to indicate that all these deposits are of Neolithic age and later than the Palaeolithic terraces. The relations of Palaeolithic to Neolithic are still very obscure in this country, and the reason is perhaps to be sought in a submergence which has tended to carry many of the transition deposits beneath the sea-level, or has caused them to be silted up under more modern alluvium. The lowest submerged forest requires careful search before we can be certain of its true position in the sequence; but it is seldom exposed, and then only in dock-excavations soon again hidden.

Before we can attempt to answer the other questions, it is important to get an estimate of the amount of time occupied in the formation of these deposits, and of the lapse of time since the last of them was formed. The newest of them belongs certainly to the age of polished stone, and the earliest also probably comes within the Neolithic period. We have already seen that within the period represented by the submerged forests there has been a rise of the sea-level, or depression of the land, to the extent of 80 feet, perhaps a few feet more. If we can obtain some measure of the time occupied in the formation of such a series of deposits, this should give us some idea as to the length of the Neolithic period, and also of the rate at which changes of the sea-level sometimes can take place.