640 



THE PLEISTOCENE PERIOD 



the sea. Subsequently the outlet was shifted to its present position, 

 probably by a gentle upwarping of the surface at the north. 



Similar complicated histories doubtless attended the retreat 

 of the ice in the Mackenzie and Hudson Bay basins, but little is yet 

 known regarding them. A very important lake was formed in the 

 Red River valley of the North (Lake Agassiz, Fig. 532), discharging, 



Fig. 531. A still later stage of the Great Lakes. The sea is thought to have 

 covered the area shaded by lines at the east. (Taylor.) 



in its earlier history, into the Minnesota River at Lake Traverse. 

 Lake Agassiz had a comparatively simple history. It grew to the 

 northward with the retreat of the ice which held it in at that end, 

 and continued to discharge into the Minnesota River until the 

 retreat of the ice gave it a northerly outlet. It developed beaches 

 while it discharged to the southward, and another set after the out- 

 let was northward. On the final withdrawal of the ice, the lake was 

 drained. 



The evidence which demonstrates the existence of these ex- 

 panded lakes is found chiefly in the deposits which they made, and 

 in the topographic features which they developed about their shores. 

 Many of the former shore-lines have been traced in detail, and most 

 of them depart notably from horizontality. In general, they rise 



