THE DATE OF THE GLACIAL PERIOD. 349 



ure of the rate at which the recession of the ice-front went 

 on after it had withdrawn to the international boundary. 

 The distance from there to the mouth of Kelson River is 

 about 600 miles. The recession of the ice-front over that 

 area proceeded, therefore, at the average rate of about 

 half a mile per year. 



There are many evidences that the main period of 

 glaciation west of the Rocky Mountains was considerably 

 later than that in the eastern part of the continent. A 

 portion of the facts pointing to this conclusion have been 

 well stated by Mr. George F. Becker, of the United States 

 Geological Survey. 



" ~No one," he says, " 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 ero- 

 sion of streams running over glaciated rocks, and the fresh- 

 ness of erratic boulders are sufficient evidence of this. 

 There is also evidence 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. There is also much 

 evidence that erosion has been trifling since the commence- 

 ment of glaciation, excepting under peculiar circum- 

 stances. East of the range, for example, at Virginia City, 

 andesites which there is every reason to suppose pregla- 

 cial have scarcely suffered at all from erosion, so that de- 

 pressions down which water runs at every shower are not 

 yet marked with water-courses, while older rocks, even of 

 Tertiary age and close by, are deeply carved. The rainfall 

 at Virginia City is, to be sure, only about ten inches, so 

 that rock would erode only say one third as fast as on the 

 California coast ; but even when full allowance is made 

 for this difference, it is clear that these andesites must be 

 much younger than the commencement of glaciation in 

 24 



