MODIFIED DRIFT DISTINGUISHED FRO>f ALLUVIUM. 



0"' 



The deltas of Lake Agassiz show that extensive contributions of silt were 

 received in that lake from the englacial drift of the retreating ice both on 

 the east and west ; and these deposits' agree with the terminal moraines of 

 that region in indicating that against this great glacial lake the ice was melted 

 back faster than on the adjoining land areas. On the eastern side of Lake 

 Agassiz only the Buffalo and Sand Hill rivers brought in noteworthy deltas, 

 but several other tributaries from the east are at the present time larger 

 than these. No topographic or other now existing causes for this difference 

 are discoverable ; and we are left to the inference that, during the vicissi- 

 tudes of the glacial recession, exceptionally large rivers poured down from 

 the ice-surface, laden with its drift, to these deltas. Similar conditions seem 

 also to have been largely efficient to produce the three great deltas which I 

 have examined on the western side of Lake Agassiz, namely, the Sheyenne and 

 Pembina deltas in North Dakota and that of the Assiniboine in Manitoba. 

 Each of these demonstrably contains much tribute of modified drift, that is, 

 of drift brought directly from the ice by the rivulets, brooks, and rivers 

 formed in its melting. But this could not have taken place, if a considera- 

 ble embayment in the ice-front had not been made by its more rapid melting 

 where it was washed by the glacial lake. In like manner, the portions of 

 the ice-border washed by the sea in theGulf of St. Lawrence and in Hudson 

 strait and Hudson's bay were undoubtedly melted back exceptionally fast 

 during the departure of the ice-sheet. At the last, indeed, the ice probably 

 became divided into Arctic and Labrador areas on opposite sides of Hud- 

 sou's bay. 



In closing this essay, we may profitably notice the Assiniboine delta of 

 Lake Agassiz and attempt to estimate its proportion of modified drift as dis- 

 tinguished from the alluvium of ordinary river erosion. This remarkable 

 delta of gravel and sand covers an area of about 2,000 square miles and has 

 an estimated average depth of at least fifty feet. Its volume therefore is 

 about twenty cubic miles, which exceeds the combined capacity of the 

 Qu'Appelle valley, which was the outlet of the glacial Lake Saskatchewan, 

 and the Assiniboine valley from the mouth of the Qu'Appelle to this delta. 

 Each of these valleys has an average width of about one mile, and their 

 depth probably averages 250 feet along their extent of about 350 miles, be- 

 ing eroded in drift and the underlying soft Fort Pierre shales. This was 

 doubtless a preglacial and interglacial water-course, which became partly 

 filled with drift in each of the principal epochs of glaciation. Much of the 

 erosion of the upper Qu'Appelle valley during the departure of the last ice- 

 sheet was effected by its glacial river while it emptied into the Lake Souris ; 

 and probably the lower valley and that of the Assiniboine were only filled 

 on the average to the extent of a third or half of their depth by the drift of 



