THE QUATERNARY PERIOD 



453 



(B, Fig. 471), and even after the ice melted back again the 

 river held its new course. 



Marginal lakes of the retreat stage. Like all ice sheets, 

 those of the Glacial epoch pushed out lobes or tongues along 

 the valleys near their 

 margins. The ice sheet 

 thus came to have scal- 

 loped edges. During the 

 last retreat the great gla- 

 cial lobes which had oc- 

 cupied such depressions 

 ponded the waters be- 

 tween the moraines they 

 had left and the front of 

 the ice, thus producing a 

 series of lakes (Fig. 473). 

 The overflow water from 

 these lakes ran south- 

 ward, largely into tribu- 

 taries of the Mississippi 

 River, Lake Superior 

 draining out past Duluth 

 and Lake Michigan past 

 Chicago. As the ice re- 

 treated slowly northward the lakes grew in size and some joined 

 those next to them to form larger lakes ; while others, having 

 lost the ice wall on one side, disappeared entirely. Our present 

 Great Lakes began as marginal waters of this kind, and it was 

 only after the ice had retreated into Canada that they were all 

 connected and found the St. Lawrence Valley the lowest point 

 of outflow. 



As the ice retreated from Minnesota, the Dakotas, and 

 Manitoba, it left a shallow basin in which another great lake 

 came into existence. Lake Agassiz, as it is called, was once 

 five times as large as Lake Superior, but when the ice sheet 

 which blocked its northern edge finally melted away, the 



FIG. 471. A and B. Diagrammatic maps 

 of South Dakota, showing how the Mis- 

 souri River was displaced by the invasion 

 of an ice sheet. (Modified after Todd.) 



