THE ICE AGE AND ITS WORK. 47 



was going on during a large part of the Glacial period, under a 

 weight of ice varying from one to three or four thousand feet in 

 thickness ; 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 innumer- 

 able other facts prove, the ice moved irresistibly over hill and dale, 

 and up slopes far steeper than any formed by the upward slopes 

 of the bottom of our deepest lakes, 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 mountain 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 carried away the whole 

 of the cubic miles of solid rock that once filled up the valley. But 

 the geologist knows that these apparently insignificant forces 

 have done the work, through their continuous action always in 

 one direction for thousands or even for millions of years ; and, 

 therefore, having before him so many proofs of the eroding 

 power of ice, in planed and rounded rocks, and in the grooves and 

 furrows which are the latest marks left by the ice tool, and bear- 

 ing in mind the long duration and possibly recurrent phases of 

 the Ice age to be measured certainly by tens, perhaps by hun- 

 dreds of thousands of years he can have little difficulty in ac- 

 cepting the erosion of lake basins as the most satisfactory explana- 

 tion of their origin. 



Objections of Modern Writers considered. Prof. Bon- 

 ney and many other writers ask, why lakes are so few though 

 all the chief valleys of the Alps were filled with ice ; and why, for 

 instance, there is no great lake in the Dora Baltea Valley, whose 

 glacier produced the great moraines of Ivrea opposite its outlet 

 into the plains of Italy, and which form a chain of hills fifteen 

 miles long and fifteen hundred feet high. The answer, in the 

 case of the Dora Baltea, is not difficult, since it almost certainly 

 has had a series of lake basins at Aosta, Verrex, and other places 

 where the broad, level valley is now filled with alluvial gravel. 

 But the more important point is the extreme narrowness of the 

 lower jDart of the valley above Donnas and again near its entrance 

 into the valley of the Po. The effect of this would be that the 

 great glacier, probably two thousand feet thick or more, would 

 move rapidly in its upper layers, carrying out its load of stones 

 and debris to form the terminal moraine, while the lower strata, 

 choked in the defiles, would move very slowly. And once out in 

 the open valley of the Po, then a great inlet of the warm Medi- 



