Page:Early Man in Britain and His Place in the Tertiary Period.djvu/182

154 the arctic division farther down to the south as the cold increased, and, as it waned, to allow of their coming northwards again.

Superficial deposits containing the remains of the late Pleistocene animals swept down by the floods are present in most of the river-valleys in the eastern, central, and southern portions of Great Britain. They consist of sand, gravel, and loam, or brick earth, and are found sometimes below the level of the present streams, as, for example, in the lower Thames in the neighbourhood of London, or, as is usually the case, on the sides at various elevations above the water-level. They are proved by the presence of shells of fresh-water mussels, fresh-water snails, and others, to have been accumulated at the bottom of rivers which have cut their way down to, or below, their present level since they were formed. From this we may conclude that the present system of hill and valley was then sketched out, but that in some places the valleys have been considerably deepened by the erosive action of the stream; while in others their lower parts have been filled up in the late Pleistocene age.

Sometimes the river flowed over what is now the top of a hill or the top of a cliff, as in the case of Bemerton (Fig. 36) and the fluviatile strata on the cliff's near Southampton Water, described by Mr. Codrington, and those of Reculver, described by Professor Prestwich.

It must not, however, be supposed that the rivers in any case occupied the whole of the valley at one time. The swinging of the current from one side to the other,