280 



TEE ICE AGE IN NORTH AMERICA. 



by the torrential subglacial streams continually pouring out 

 from the ice-front. It is doubtful if the larger part of the 

 glacial grist is not thus transported far beyond the limit of 

 the glaciated region, 



Notwithstanding the great waste, the extent of the glacial 

 deposits yet remaining over the southern portion of the gla- 

 ciated region is immense. Probably not less than 1,000,000 

 square miles of territory in North America is covered with 

 an average depth of fifty feet of glacial debris, forming the 

 most permanently productive portion of the continent. It 

 is in the extent of these glacial deposits, and in the certainly 

 great amount of transportation by subglacial streams, that we 

 have our most certain and impressive evidence of the enor- 

 mous activity of erosive agencies during the Glacial age. 



Fig. 85 — Towan bowlders south of New Hampton, Iowa. (Photo by Calvin.) 



