62 



A NATURALIST IN THE GREAT LAKES REGION 



tion at Chicago some three hundred miles. Nansen found in cross- 

 ing Greenland that the ice surface rose for the first 75 miles 

 from the west coast nearly 90 feet per mile, then at a decreasing 



Fig. 42. — The moraines of the glacier about south end of Lake ]\Iichigan; 

 old Lake Chicago in heavy horizontal shading, Lake Morris in broken hori- 

 zontal lines. 



rate averaging 26 feet per mile. On this basis of calculation the 

 ice of the Illinoian sheet at Chicago must have been about 

 twelve thousand feet deep. Even if, to be on the safe side, this 

 estimate were cut in half, it would still make the mass more than 

 a mile in thickness, a ponderous thing, capable of over-riding 



