INTERGLACIAL EPOCHS. 295 



ceived notions of what Nature's forces can and cannot do are 

 often enough wide of the mark. 1 



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

 and Norway to prove that glacier-ice cannot 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 extending 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 Switzerland and Norway ; 

 but underneath glaciers of the kind that invaded the low grounds 

 of Piedmont and Lombardy we know that thick deposits of 

 tough boulder-clay, crammed with scratched stones, did accumu- 

 late ; 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 moraines. 2 



As physicists themselves are not yet quite agreed upon the 

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

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

 creep over the surface of incoherent beds without entirely demo- 

 lishing them. It is enough for him to show how the remarkable 

 distribution of the interglacial beds, and the various phenomena 

 presented by these deposits, indicate that ice has overflowed 

 them. It is needless, 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 impossi- 

 bility, 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 



1 It may be doubted whether interglacial deposits, at the time they were over- 

 ridden by ice, were as loose and incoherent as they are at present. My brother 

 has suggested that, when the ice-sheet advanced over a land-surface, the loose 

 superficial deposits might be frozen so hard as to be capable of resisting a very 

 considerable degree of glacial erosion. 



2 See postea, Chapter XIII. 



