4fi THE GLACIAL LAKE AGASSIZ. 



liHve their course marked in the same way. The growth of wood is here 

 conhned chiefly to the banks of the streams, which have cut hollows 20 to 

 40 feet deep in the broad lacustrine plain. 



Al)out a sixth part of the area of Lake Agassiz, and a larger propor- 

 tion (nearly the whole) of the adjoining country on the south and west, 

 are prairie, this term being commonly used to embrace all tracts destitute 

 of trees and shrubs, but well covered with grass. Groves of a few acres, 

 or sometimes a hundred acres or more, occur here and there upon this 

 prairie region beside lakes, and a narrow line of timber usually borders 

 streams, as just described along the Red River; but many lakes and creeks, 

 and even portions of the course of large streams, have neither bush nor 

 tree in sight, and occasionally none is visible in a view which rang-es from 

 5 to 10 miles in all directions. The contour of the prairie is as varied as 

 that of the wooded region. Within the area of Lake Agassiz the surface 

 is almost absolutely level, but the adjoining prairie country is undulating, 

 rolling, and hilly, having in some tracts a very rough surface of knolls, 

 hills, and ridges of morainic drift that rise steeply 25 to 100 feet or more 

 above the intervening hollows. The material of the greater part of all 

 these areas, whether forest or prairie, is closely alike, being till or unmodi- 

 fied glacial drift, showing no important differences such as might cause the 

 growth of forest in one region and of only gi-ass and herbage in another. 

 Chapter XI will include a discussion of the climatic conditions, as abun- 

 dance or lack of rainfall, and auxiliary causes, as prairie fires, b}" which the 

 limits of these diverse phases of vegetation have been determined. 



EXISTING LAKES WITHIN THE AREA OF LAKE AGASSIZ. 



The glacial Lake Agassiz was gradually reduced in size, first by the 

 lowering of its south.ward outlet, and afterwards by finding successively 

 lower outlets to the northeast, until, with the complete departure of the ice- 

 sheet, it shrank to its present representatives, the great lakes of Manitoba. 

 These are three in number, Lakes Winnipeg, Manitoba, and Winnipegosis. 

 With them are associated several others, comparatively small, as Cedar 

 Lake, through which the Saskatchewan flows near its mouth; Lake 



