182 THE GLACIAL LAKE AGASSIZ. 



varies in its average thickness from 50 to 150 feet, Avliile its central portion 

 rises 400 to 600 feet above its sonth and north ends. Though the greater 

 part of both the modified drift and till have only slight undulations, the 

 former being often nearly flat and the latter moderately uneven, other por- 

 tions are crossed by moraines which have a prominently knolly and hilly 

 contour, rising usually 25 to 75 feet, or occasionally 100 to 200 feet, and 

 in the Leaf Hills 100 to 350 feet, above the adjoining country. In some 

 places the belts of morainic hills, consisting chiefly of till, with abundant 

 bowlders, are bordered on one side by tracts of stratified gravel and sand 

 which slope slowly downward from them and are merged in the extensive 

 plains or moderately undulating areas of this modified drift, showing that 

 a part of the gravel and sand was brought by streams that descended from 

 the ice-sheet dm-ing the time of accumulation of its moraines. Besides 

 these overwash slopes of modified drift, the morainic belts often include - 

 kames, or knolls, hillocks, and short ridges of sand and gravel. 



During the rapid melting of the ice in its times of retreat between 

 successive moraines the glacial streams attained their greatest extent and 

 volume, and brought proportionately extensive deposits of modified di-ift, 

 spreading it mainly in plains or moderately undulating tracts beyond the 

 ice margin, but here and there leaving long esker ridges of gi-avel and 

 sand, which were formed in their channels between walls of ice. The dis- 

 tribution of the modified di-ift thus found upon lai'ge tracts along a broad 

 belt from St. Paul to Winnipeg, while it is very scantily developed on a 

 still wider region of Minnesota, North Dakota, and Manitoba southwest of 

 this belt, and likewise is scanty or wanting on its northeast side in northern 

 Minnesota and about Rainy Lake and the northeast and north portions of 

 the Lake of the Woods, seems to be attributable to converging slopes of 

 the surtace of the ice-sheet and the consequent convergence of its cun-ents, 

 which brought an unusual amount of englacial drift into the ice along this 

 belt, and bv wliich also the streams produced in its melting were caused to 

 flow thither from extensive areas of the ice on the east and west. The 

 glacial strife of these adjoining areas show that on the east the course of 

 the motion and the descent of the surface of the ice-sheet were from north- 

 east to southwest, but that on the west the glacial current moved and the 



