MODIFIED DRIFT FOKMING DELTAS. 189 



PROPORTION OF MODIFIED DRIFT SUPPLIED TO THE DELTAS OF LAKE AGASSIZ. 



Extensive contributions of fluvial silt were received in Lake Agassiz 

 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 indi- 

 cating 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 vicissitudes 

 of the glacial recession exceptionally large streams poured down from the 

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

 also to have been largely efficient in j^i'oducing the four great deltas which I 

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

 Elk Vallev, and Pembina deltas in North Dakota, and that of the Assini- 

 boine in Manitoba. Each of these demonstrably contains much tribute of 

 modified di'ift; that is, of drift brought directly from the ice by the rivulets, 

 brooks, and rivers formed in its melting. 



It will be especially instructive to notice the Assiniboine delta of Lake 

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

 guished 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 50 feet. Its volume is about 20 

 cubic miles, so that it exceeds the combined capacity of the Qu'Appelle 

 Valley, which was the outlet of the glacial Lake Saskatchewan, and of the 

 Assiniboine Valley from the mouth of the Qu'Appelle to this delta. Each 

 of these valleys has an average width of about 1 mile, and their depth 

 probably averages 250 feet along their extent of about 350 miles, being 

 eroded in drift and tlie underlying soft Fort Pierre shales. This was doubt- 

 less a preglacial watercourse, which, like the Pembina and Minnesota 

 valleys, became only partly filled with drift. ]Much of the erosion of the 

 upper Qu'Appelle Valley during the departure of the ice-sheet was eflPected 

 by its glacial river while it emptied into the Lake Souris, and probably the 

 lower valley and that of the Assiniboine were filled on the average only 



