GLACIAL LAKES 381 



lake quickly drains and floods the valley at the end of the 

 glacier. This formerly caused so much damage that a 

 canal has been constructed across the head of the valley, 

 so that now no great quantity of water can accumulate 

 behind the ice dam. When the lake drains, the bottom is 

 left as a comparatively level, dry plain until the crevasse 

 closes and the lake again forms. 



MARJKLEN LAKE. 



Toward the close of the Glacial Period a vast lake of 

 this kind was formed in the northern partv of the United 

 States, the region now drained by the Red River of the 

 North. The slope of the land is here toward the north, 

 and as the ice retreated it formed a barrier to the drain- 

 age and dammed back a great sheet of water in front of' 

 it. When the ice melted, the lake was drained, leaving 

 the flat fertile plain through which the Red River now 

 flows. The ancient glacial lake has received the name of 



