THE GLACIAL LAKES 



tially all outside the basins of the present lakes and all dis- 

 charging southward to the Mississippi and the Gulf of Mexico, 

 grew a series of lakes "which for size and complicated history 

 is not known in any other part of the world." The total area 

 which at some time has been covered by waters of these 

 glacial lakes is considerably greater than the area of the pres- 

 ent Great Lakes (now about 95,000 square miles), although 

 the entire area submerged was not covered at any one time. 

 Only one glacial lake was larger — Lake Agassiz, of which 

 Lake W'innepeg is a remnant. Although it once covered 1 10,- 

 000 square miles, its history is very simple. It also was formed 

 in front of an ice barrier during the retreat of the last glacier 

 — the Keewatin ice sheet which late in the retreat of Wisconsin 

 glaciation had pushed southward from a center west of Hud- 

 son Bay and had reached south as far as Des Moines, Iowa, 

 over-riding the deposits of its earlier neighbor — the Patrician 

 lobe of the Labrador ice. (Figure 2.) The Keewatin 

 glacier persisted for some time after the formation of the in- 

 cipient Great Lakes, thus Lake Agassiz came into existence 

 long after the Great Lakes were well established. Its outlet 

 was to the Mississippi before establishment of the present 

 drainage. The bed of Lake Agassiz now makes the wheat 

 lands of the northwest, in Minnesota, North Dakota, Mani- 

 toba and Saskatchewan. 



The accompanying maps show the manner of development 

 and the changes in the lakes more graphically than words can 

 describe. Figure 1 shows the distribution of the rock forma- 

 tions which determined the location of the lake basins. Figure 

 2 shows the areal extent of and the centers of accumulation 

 of the ice sheets which covered the northern part of North 

 America during Pleistocene time. Figure 3 shows the direc- 

 tions of movement of the ice in the glacial lobes which oc- 

 cupied the several lake basins and the positions of the ice 

 border at successive stages in the retreat. The other maps, in 

 order, show the various strand lines and outlet streams of the 



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