122 THE GLACIAL LAKE AGASSIZ. 



though "no change in the character of the drift deposits Avas noted, 

 * * * Laurentian pelibles and bowlders were for the first time seen 

 in considerable abundance. * * * East of this point * * * the 

 surface is thickly covered with drift deposits, so much so that exposures 

 of the underlying rocks are, as a rule, only found in tlie larger river 

 valleys."^ No better evidence could be desired by a glacialist, account- 

 ing for the formation of the bowlder-clay by the agency of land ice, to 

 demonstrate the confluence here of two currents of the ice, one flowing 

 eastward from the Cordilleran area and the other flowing westward from 

 the Archean area, whose nearest portion is on Lake Athabasca, about 400 

 miles distant. 



Near the divide between the Liard and Yukon River systems, Dawson 

 found drift on the summit of an isolated mountain 4,300 feet above the sea 

 and about 1,000 feet above this part of the Pacific-Arctic watershed." This, 

 however, is on the west side of the Rocky Mountains proper, which, as 

 defined by Dawson, constitute the northeastern marginal range of the broad 

 mountainous Cordilleran belt. With this definition, the Rocky Mountains 

 are intersected by the Mackenzie River south and west of Great Bear Lake. 

 Farther northward the Laurentide or eastern portion of the ice-sheet 

 pushed northwestward to the extreme limit of the drift. "The till near the 

 lower ramparts of the Mackenzie," according to Mr. R. CI. McConnell, "is 

 in approximately the same latitude as the northern boundary of the Archean 

 area on the east, and the gneissic bowlders which it contains must have 

 traveled either directly west or northwest in order to reach their present 

 situation." He therefore infers that "the ice from the Archean gathering 

 ■ grounds to the east poured westward through the gaps and passes in the 

 eastern flanking ranges of the Rocky Mountains until it reached the barrier 

 formed by the main axial range, when, being unable to pass this, it was 

 deflected northwestward in a stream from 1,500 to 2,000 feet deep down the 

 valley of the Mackenzie and thence out to sea."^ 



All the testimony thus gathered concerning the line of junction and 

 the hniits of the eastern and western di'ift seems to the present writer to 



I Geol. and Nat. Hist. Survey of Canada. Report of Progress, 1879-80, pp. 139, 140 B. 

 ; Unci., Annual Report, Vol. Ill, for 1887-88. p. 119 B. 

 » Bulletin, G. S. X., Vol. I, p. 543. 



