GLACIAL LAKES 



325 



col which was the outlet of the Spean valley at the time. This 

 stage continued until the ice front had retired so far that the waters 

 drained naturally down the river Spean to Loch Lochy and thence 

 to the ocean. 



Only in their far grander scale and in the lesser relief of the land 

 over which they formed, do the complex histories of the great 



; '-'^'^;;;;^ 



FIG. 352. Harvesting time on the fertile floor of the glacial Lake Agassiz (after 



Howell). 



ice-blocked lakes of North America differ from these little valley 

 lakes whose beaches may be visited and the relationships worked 

 out, thanks to Jamieson, in a single day's strolling. 



The glacial Lake Agassiz. The grandest of the temporary lakes 

 referable to blocking by the continental glaciers of the ice age 

 must be looked for in the largest 

 valleys that lay within the terri- 

 tory invaded and which normally 

 drain toward the retiring ice front. 

 In North America these rivers are 

 the Red River of the North in 

 Minnesota, the Dakotas, and Mani- 

 toba ; and the St. Lawrence River 

 system. To the ice dam which lay 

 across the Red River valley we 

 owe the fertility of that vast plain 



of lake deposits where is to-day the FlG - 353 - Ma P of Lake A g assiz 

 most intensive wheat farming of 



the northwest (Fig. 352). Lakes Winnipeg, Winnipegoosis, and 

 Manitoba, and the Lake of the Woods, are all that now remain of 

 this greatest of the glacial lakes, which in honor of the distinguished 

 founder of the glacial theory has been called Lake Agassiz (Fig. 

 353). With their natural outlet blocked by the ice in northern 



