136 



PHYSIOGRAPHY 



the ice-sheets, where the ice itself formed one border of the lake. 

 The melting of the ice brought such lakes to an end. 



One of the largest of the marginal lakes (Lake Agassiz) lay in 

 the valley of the Red River of the North (Fig. 143). When this 



N.DAK. 



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S.DAK. P: 



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Fig. 143. Map of the extinct Lake Agassiz, and other glacial lakes. Lake 

 Winnipeg occupies a part of the basin of Lake Agassiz. (U. S. Geol. Surv.) 



lake was largest, its length was 700 miles, its greatest width about 

 250 miles, and it covered an area of about 110,000 square miles, an 

 area greater than that of all the Great Lakes. The water, however, 

 was shallow. It came into existence when the edge of the retreat- 

 ing ice lay north of the lake, and stopped drainage in that direction. 

 The water rose in the basin until it overflowed to the south, finally 

 reaching the Mississippi River. When the ice at the north melted 

 back far enough, a new and lower outlet was opened to Hudson 

 Bay, and the lake was drained. Lake Winnipeg and several 

 smaller lakes may be looked upon as remnants of this great lake, 

 for they occupy the deepest parts of the old basin. 



