(Blacfal anD ipregiactal iafces, 223 



their long journey from their original homes. 

 Where their home was the geologist can im- 

 mediately tell upon examination. It is only 

 necessary then to examine the bowlders of any 

 particular locality to determine the direction 

 of the ice flow at that point. 



There seem to have existed centers of ice 

 accumulation to the north of all of the great 

 lakes. And when they had grown to a suffi- 

 cient height they joined at their edges, mak- 

 ing one grand glacier, the movements of which 

 were the resultant of the combined pressure 

 exerted by these great centers of power, so that 

 all of North America north of the line of the 

 terminal moraine, with the exception of a 

 small area (heretofore noted) chiefly in Wis- 

 consin, became covered with one vast sheet of 

 ice. 



The glacier north of Lake Superior widened 

 out the old river bed by a process of erosion 

 to its present width. 



There may have existed something of a lake 

 in preglacial times, through which the river 

 ran, but it undoubtedly owes its present width 

 to the grinding action of the irresistible ice- 

 bergs and the piling up of debris on the shores. 

 The river bed was filled up by a glacial drift at 

 the point of its present outlet until the lake 

 was raised in its level much higher than that 

 of Lake Michigan. Another glacier plowed 

 down through Lake Michigan, widening it out 



