PLEISTOCENE RIVER-DEPOSITS. 125 



Antiquity of Man, and the example of this eminent geologist 

 was of course soon followed by the greater number of his 

 disciples. Extended observations and the evidence obtained 

 during collateral inquiries have only tended to confirm the 

 general truth of Professor Prestwich's conclusions. My limits 

 will not allow me, however, to give a detailed account of those 

 investigations, which may be said to .have revolutionised Pleis- 

 tocene geology. All I can do is to sketch in outline the main 

 features of the evidence, and to note the chief results arrived at. 

 The occurrence of great sheets of gravel, loam, and sand on 

 the slopes of many valleys in the south of England and the 

 north of France had long been known to geologists, and many 

 were the explanations, advanced from time to time, to account 

 for their presence. Eew could believe that such water-worn 

 materials — often appearing at heights of more than 100 feet 

 above the valley-bottom — could have resulted from the action 

 of the present streams and rivers. It was thought possible 

 that this might well be the origin of the gravel and sand at 

 low levels, but the more elevated deposits were assigned some- 

 times to the action of the sea, during a comparatively recent 

 period of submergence, and at other times they were supposed 

 to be due to the sweep of great cataclysmic rushes of water. 

 Considerable doubt also existed as to the age of the gravels in 

 different valleys, and even of those in one and the same valley. 

 This uncertainty arose chiefly from the nature of the palseonto- 

 logical evidence — the fossils appearing to indicate various ages. 

 Thus, for example, it was thought that the deposits at Brent- 

 ford, in the valley of the Thames, were newer than those of 

 Grays. Professor Prestwich, however, had on physical grounds 

 long been satisfied of the contemporaneity of these deposits, and 

 contended for their posteriority to the "Boulder Clay." In 

 other words, he had come to the conclusion that they were 

 posterior in date to the Glacial Period or Ice Age. This latter 

 point I will not now consider, as it falls to be discussed in suc- 

 ceeding chapters. Eor the present we are concerned simply with 

 the origin of the valley-gravels — high- and low-level deposits 



