134 THE GLACIAL LAKE AGASSIZ. 



outcrops occur about Rainy Lake and the Lake of th« Woods, westward 

 along the Canadian Pacific Railway nearly to the Whitemouth River, and 

 in the country east of Lake Winnipeg ; and it is probable that the average 

 thickness of the supei'ficial deposits in that extensive district is not more 

 than 30 to 50 feet. West of Lake Agassiz many portions of the plateau 

 bordered by the Pembina Mountain and the Tiger Hills have onlya small 

 depth of drift, ranging from a few feet to 20 or 30 feet, but in some places 

 the drift appears to extend deeper, as shown by stream valleys, and its 

 average thickness may be 40 feet or more. 



Southward from Devils Lake, upon the expanse crossed by the Shey- 

 enne and James rivers, the drift covering its eastern portion, along the 

 Sheyenne, continues thin, varying from 10 or 20 to 50 feet or more; but 

 farther west, along the James, it is again of considerable depth, averaging 

 probably 100 feet, and ranging commonly from 50 to 150 feet on moder- 

 ately undulating tracts. Tlu-oughout this plain-like expanse, as generally 

 upon other parts of the country adjoining Lake Agassiz, the drift-sheet 

 receives an addition of probably 30 to 60 feet along the course of its 

 numerous admirably developed marginal moraines. 



TILL OR BOWLDER-CLAY. 



Till, also called bowlder-clay, constitutes the greater part of the entire 

 sheet of superficial deposits, both within the area of Lake Agassiz and 

 upon the adjoining country. It usually lies on the striated bed-rock, and 

 upon large areas it reaches thence upward to the surface; but elsewhere 

 this unmodified glacial drift is covered by modified drift, the stratified 

 gravel, sand, and clay deposited by streams which flowed down from the 

 ice-sheet during its melting, or by lacustrine and fluvial sediments. Fully 

 half of the area of Lake Agassiz in Minnesota and North Dakota has a 

 surface of till. In the part of this lake area examined by me in Manitoba 

 its proportion is less, because much of this district is covered by the 

 Assiniboine delta and its associated lacustrine beds. Extensive tracts of 

 till, however, occupy the surface on the north and east portions of the 

 Manitoba area, as north of Neepawa, on the east side of the Big Grass 

 Marsh, from the south end of Lake Manitoba eastward by Shoal Lake 



