GLACIAL EROSION OF LAKE BASINS 



103 



access, it seems probable that at any one 

 time not more than half the entire bottom 

 surface of the glacier would be in actual 

 contact with the rock, thus greatly reducing 

 the total friction : while, as the process of 

 erosion went on, the rock surfaces would 

 become continually smoother and the in- 

 equalities less pronounced, so that even 

 when a rock-basin had been ground out to 

 a considerable depth the onward motion 

 might be almost as great as at the begin- 

 ning of the process. 



If, now, we consider that the erosion I 

 have attempted to describe was going on 

 during a large part of the glacial period, 

 under a weight of ice varying from one to 

 five or six thousand feet in thickness, and 

 in some cases even more ; that the huge 

 grinding tool was at work day and night, 

 winter and summer, century after century, 

 for whatever number of thousands of years 

 we give to the glacial period ; that — as in- 

 numerable other facts prove — the ice moved 

 irresistibly over hill and dale, and up slopes 

 far steeper than any formed by the inclines 

 of the bottoms of our deepest lakes — 

 as shown by the true scale section of Lake 

 Como here given — what is there of 

 impossible, or even of improbable in the 

 belief, that lake-basins were produced by 

 such differential erosion ? To the ordinary 

 observer it seems impossible that a moun- 

 tain valley, half a mile wide and bounded 

 by rocky slopes and precipices two or three 

 thousand feet high, can have been formed 

 without any " convulsion of nature," but 

 merely by the natural agencies he sees still 

 in action — rain and frost, sun and wind — 

 and that the small rock-encumbered stream 

 now flowing along its bottom can have 



Upper End 



CO 



o 

 o 



o 



o 



:5 



5e 





Co 



Lower End 



