LATER GEOLOGICAL CHANGES. 415 



ent gravel - beds. Considerable difference of opinion 

 still exists, however, regarding some of these deduc- 

 tions. Other observers, as remarked above, have been 

 unable to perceive any satisfactory evidence that the 

 rivers were generally more swollen than they are at 

 present, though at exceptional periods of melting 

 snow they may have surpassed in volume any floods 

 chronicled in their valleys during historic time. But 

 Prestwich detected the traces of another transporting 

 agent than that of mere unaided river-water. In the 

 presence of large unworn blocks among the ancient 

 gravels, together with much sharp angular detritus, 

 he recognised the operation of river-ice. Thus all over 

 the south-east of England, where the climate is now 

 so mild, he traced indications that in old times the 

 rivers flowing on the platform of the higher gravels 

 were frozen over ; that ice forming along their margins 

 or over their bottoms lifted and carried along the 

 shingle and boulders lying there ; and that when these 

 Arctic conditions prevailed, man had already appeared, 

 fishing in the rivers, or tracking the mammoth, the 

 bison, and various extinct forms of deer through the 

 surrounding forests and prairies. 



Among Prestwich 's contributions to the history of 

 the latest geological changes that have affected the 

 south of England and the north of France, his num- 

 erous papers on the so-called Raised Beaches of this 

 region [4, 44, 48, 52, 64, 79, 88, 97-99, 128] deserve 

 recognition. The notices of recent uprise of the land 

 in Britain and on the opposite French coast were based 

 on his own personal observations, and they are of 

 value as records of facts formerly visible, but some of 

 which have been obscured or concealed by the progress 

 of building and other changes. Prestwich, however, 



