568 THE ICE AGE IN NORTH AMERICA. 



eroding forces cannot have been at work over this region for 

 much more than 10,000 years, and this is some time previous 

 to the beginning of the work of the present Niagara River. 



Another class of facts which seems to set moderate limits 

 to glacial chronology relates to the amount of superficial ero- 

 sion of glacial deposits of various sorts, and the extent to 

 which the rocks have been disintegrated since that period. 



President T. C. Chamberlin, when State geologist of 

 Wisconsin, remarked that no sensible denudation had taken 

 place there since glacial times.* Even Mr. Croll expresses 

 surprise at the small amount of erosion which has taken 

 place since the kames of Scotland were deposited. Both in 

 Europe and in America these peculiar relics of the Glacial 

 period retain a sharpness of outline which it is difficult to be- 

 lieve could have survived the protracted period of one hun- 

 dred thousand or even of forty thousand years, according to 

 Hitchcock's reckoning. When, also, one considers the chemi- 

 cal agencies at work to decompose the rocks wherever un- 

 protected by a covering of till, the freshness of the glaciated 

 surfaces never ceases to be a cause of astonishment. 



Dr. Geo. F. Becker, of the United States Geological Sur- 

 vey, bears striking testimony to the freshness of the glaciated 

 surfaces of the rocks in the mountains of California on the 

 Pacific Coast. He writes : 



"No one, who has examined the glaciated regions of *the 

 Sierra can doubt that the great mass of the ice disappeared 

 at a very recent period. The immense areas of polished 

 surfaces fully exposed to the severe climate of say from 

 7,000 to 12,000 feet altitude, the insensible erosion of streams 

 running over glaciated rocks, and the freshness of erratic 

 bowlders are sufficient evidence of this. There is also evi- 

 dence that the glaciation began at no very distant geologic 

 date. As Professor Whitney pointed out, glaciation is the 

 last important geological phenomenon and succeeded the 

 great lava flows. 



* "Geology of Wisconsin," vol. ii, p. 632.' 



