Tin-: PLEISTOCENE SKI; IKS c,i:, 



into the base of enormous ii e-sheet-, \\hich not only filled 

 the basins of the North Sea and tin- Irish Sea, l>ut were able to 

 take up portion* of tin- sea floor, and carry them up to high 

 levels in Kngland and Iieland. Further it lias l.een maintained 

 that instead of the land being lower and the seas deeper than they 

 are now, the whole of Northern Europe had been raised to a greater 

 elevation than that at which it now stands, and that land-ice was 

 the sole agent concerned in the transport of marine shells as \\ell 

 as of large stones and boulder*. 



These are extreme views, and the truth probably lies in a 

 modification of them. The idea of the land being at a higher 

 level than it is at present, either at the beginning of the Glacial 

 Period or at the time of maximum glaciation, is a mistake. The 

 evidence only proves that the land had previously been at a higher 

 elevation, i.e. in Pliocene and Miocene times, and it favours the 

 conclusion that the land sank as the thickness of ice upon it 

 increased. At the same time it must be admitted that the shells 

 at very high levels may not be exactly in situ, and that perfect 

 -hells have not been found in sufficient number to prove a sub- 

 mergence of more than 500 or 600 feet. Thus at Clava, near 

 Inverness, there is a shell -bearing deposit with Arctic species at 

 about 500 feet, and overlain by boulder-clay. A British Associa- 

 tion Committee, appointed to investigate this and other localitie-, 

 reported that the molluscs at Clava had probably lived on the spot 

 where their shells are now found. 



It is probable, however, that the subsidence was greater in the 

 north than it was farther south, and that while it may have been 

 600 feet in the latitude of Inverness it was not more than 130 or 

 perhaps 160 feet in the south of England. Even such a moderate 

 subsidence, however, would greatly facilitate the movement of ice- 

 sheete over tracts which are now dry land, and would account for 

 the transport of many rock-fragments from lower to higher levels 

 under 500 feet. In spite of this we are still obliged to admit that 

 there was sufficient vis a tergo to force the ice not only over the 

 sea floors but out again and up to considerable heights on the other 

 side. It has also been pointed out that under such conditions shear- 

 planes may have been produced, and portions of the ice thrust 

 forward and upward over parts in front of them. Such upward 

 thrusts have been observed in Spitzbergen at the present day. 1 



Q-laciation of Rock Surfaces. In nearly all districts where 

 boulder-clays occur the rock -surfaces which underlie the lowest 

 clays are glaciated, that is to say they are grooved, scratched, and 

 smoothed as they would be by the passage over them of heavy ice, 

 which, when its sole is full of stones and boulders, becomes a gigantic 



