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. 



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