V GLACIAL EROSION OF LAKE BASINS 105 



no doubt reduced to nearly level plains by sub-aerial 

 denudation before the ice began its work. The basin of 

 these valleys comprises about two hundred square miles 

 and the watershed to the north is moderately high : but 

 there can be no doubt that a large overflow from the Como 

 glacier poured into it ; and the difficulty seems to me to 

 be purely imaginary if we simply recognise the fact that 

 an essential preliminary to lake-erosion is a pre-existing 

 nearly level valley bottom. 



Another difficulty is said to be the frequent presence of 

 islands in the lakes ; but here again the answer is easy. 

 The islands, always ground down to roches moutonn^e^, were 

 craggy hills in the pre-existing valleys, and such hills 

 existed because they had for ages resisted the sub-aerial 

 denudation which had hollowed out the valleys. The 

 same characters of density or toughness that enabled them 

 to resist ordinary denudation, enabled them also, to some 

 extent, to resist destruction by ice-erosion ; just as the 

 character of the rocks which enabled ordinary denudation 

 to bring them down to a nearly level surface in the valley- 

 bottom, also facilitated the ice-erosion which converted the 

 level valley floor into a rock-basin, and after the ice left it 

 into a lake. 



Every writer brings forward the well-known fact that 

 the ends of glaciers pass over beds of gravel or moraine 

 matter, without destroying or even disturbing it. But 

 there is no reason why they should do more than compress 

 such beds of loose material and roughly level their surfaces. 

 It is the old delusion of a glacier acting like a scoop or a 

 plough that leads to the idea that if it can erode rock 

 slowly it must altogether demolish gravel or boulder-clay. 

 But if we turn to the description I have given of how a 

 glacier erodes a rock-basin and apply this to its passage 

 over a bed of gravel or boulder-clay, we shall see that in 

 the latter case the erosion would be much more difficult, 

 because each ice-embedded stone or rock would press into 

 the yielding material, which would close up instantly behind 

 it under pressure of the ice and thus leave no result. 

 Where the sub-glacial water accumulated, channels would 

 be cut in the gravel or clay, but elsewhere there would 



