296 THE ICE AGE W NORTH AMERICA. 



nearer we approach the extreme limits reached by the ice- 

 sheets, the more extensive and the less disturbed do inter- 

 glacial deposits become. In a word, they occur in best preser- 

 vation where the erosive power of the ice was weakest ; they 

 are entirely wanting where we have every reason to believe that 

 the grinding force was strongest. . . . 



It is needless to refer one to the petty glaciers of the Alps 

 and Norway to prove that glacier-ice can not both erode its 

 bed and accumulate debris upon that bed at one and the same 

 time. A mountain-valley glacier is one thing — a glacier ex- 

 tending far into the low grounds beyond the mountains, and, 

 it may be, coalescing with similar extensive ice-flows, is 

 another and very different thing. No considerable deposit 

 could possibly gather below Alpine glaciers like those of Switz- 

 erland and Norway ; but underneath glaciers of the kind that 

 invaded the low grounds of Piedmont and Lombardy we know 

 that thick deposits of tough bowlder-clay, crammed with 

 scratched stones, did accumulate ; and not only so, but that 

 these glaciers flowed over incoherent deposits of sand and clay 

 containing marine shells of late Tertiary age, without entirely 

 obliterating them. The deposits referred to occur now as little 

 patches within the area bounded by the great terminal mo- 

 raines. 



As physicists themselves are not yet quite agreed upon the 

 subject of glacier motion, it is not incumbent upon the geolo- 

 gist to explain the precise mode in which a thick mass of ice 

 can creep over the surface of incoherent beds without entirely 

 demolishing them. It is enough for him to show how the 

 remarkable distribution of the interglacial beds, and the vari- 

 ous phenomena presented by these deposits, indicate that ice 

 has overflowed them. It is useless, therefore, to tell him that 

 the thing is impossible. The statement has been made more 

 than once that an ice-sheet several thousand feet thick is a 

 physical impossibility ; but, unfortunately for this dictum, the 

 geological facts have demonstrated that such massive ice- 

 sheets have really existed, and there appears to be one even 

 now covering up the Antarctic Continent. We used also to be 

 told, not so many years ago, that the abysses of ocean must be 

 void of life for various reasons, among: which one was that the 



