Page:VCH Surrey 1.djvu/62

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A HISTORY OF SURREY unequal hardness of the strata had its effect in the development of ridge and hollow in agreement with the strike of the rocks ; and thus longi- tudinal depressions were formed in which the surface-waters gathered and flowed until intercepted by an older transverse stream. Hence the trunk-streams crossing the strata gradually developed lateral branches or tributaries running parallel to the strike of the rocks. And this trenching of the land has gone on until in the central tract the soft Weald Clay has be'en reached and partly scooped out, while the broken rim of overlying harder beds forms steep escarpments facing inwards to the hollow. But in the rim also the wasting back has been irregular, so that the present Lower Greensand escarpment west of Dorking attains higher levels than the Chalk escarpment to the north, though in other parts of the county this relation is reversed. PLEISTOCENE AND RECENT DEPOSITS We shall now be better able to appreciate the evidence afforded by the shreds and patches of transported material which, as previously mentioned, have >here and there been left behind during the erosion of the land. We may find such material at all levels, though it is necessarily the more abundant the more nearly we approach the level of the present rivers. Even on the higher slopes and ridges of the Chalk Downs there are patches of gravel and sand which represent the residue of the Chalk and of once-existing Tertiary strata rearranged and modified by the flow of water. In the north and west of the county also, small tracts of high-level gravel and sand occur on the highest ground, often capping the little plateaus and ridges into which the Tertiary strata have been eroded ; and these gravels contain not only the detritus of the Chalk and Tertiary beds, but also many pebbles of quartz- ite and other rocks which must have been transported for long distances. The conditions under which these ' high-level ' or ' plateau gravels ' were deposited have been the subject of much discussion and difference of opinion l ; by some geologists they have been thought to indicate old beaches, and to denote a period of submergence during which the land was planed down to an even surface by the sea ; by others, whose views are now more generally accepted, they are considered to be flood-gravels formed at a time when the rivers were far more powerful than at present, and when the bottoms of the valleys were approximately at the level of these gravels. In some of these high-level deposits, as well as in others at lower levels, the agency of floating ice seems to be indicated by the presence of large blocks of grey- wether sandstone, etc., in positions which it is believed they could not otherwise have attained ; and as we know that after the close of Pliocene times there was a long period dur- ing which the climate in our islands was so inclement and moist that 1 See Prof. J. Prestwich, Quart. Journ. Geol. Soc., xlvi. (1890) p. 159 ; Rev. A. Irving, Proc. Geol. Assoc., vol. viii. (1883) p. 143, and Quart. Journ. Geol. Sac., vol. xlvi. (1890) p. 562 ; H. W. Monckton, ibid. vol. liv. (1898) p. 184 ; and Proc. Geol. Assoc., vol. xvi. (1900) p. 443. Other references will be found in these papers. 24