AGE OF THE HIGH-LEVEL DRIFT 707 



spread, as over the broad plateau of Milk Eiver Ridge. The thick de- 

 posits of drift adjacent to the valleys then acted as a protective capping, 

 preventing the lowering of the thickly covered tracts while uncovered or 

 less thickly covered adjacent tracts were further reduced. On this in- 

 terpretation the high-level drift would still be referred to a pre-Wiscon- 

 sin stage of glaciation, but the length of the succeeding interglacial 

 interval would not be so great and less of the sculpturing of the moun- 

 tains would be referred to Pleistocene time. Further investigations will 

 be required to learn the full significance of this high-level drift, includ- 

 ing its relation to the "quartzite gravels," which Dawson and McConnell 

 found underlying the Keewatin drift in southern Alberta and Saskatche- 

 wan and grading westward into drift of the mountain glaciers, to which 

 Doctor Dawson gave the name "Albertan" drift.^^ 



Discussion 



Mr. President: In answer to Doctor Atwood's question whether the 

 pre-Wisconsin drift was deposited by valley glaciers rather than an ice- 

 cap, I would say, in my opinion, it was deposited by valley glaciers, inas- 

 much as it is now found just east of the mountain front capping the 

 ridges closely adjacent to the main valleys heading in the mountains. 

 In these places the drift capping protected the soft Cretaceous rocks 

 from erosion. Beyond, on either hand, where the drift may have been 

 thin or absent the peneplain was reduced. 



Answering the second question, the peneplain is pre-glacial. The 

 drift was laid on the truncated edges of the upturned Cretaceous beds. 

 The age of the drift is independent of the age of the peneplain excepting 

 that it can not be so old as the plain. The important question is as to 

 whether or not the dissection of the peneplain occurred prior or subse- 

 quent to the deposition of the drift. If subsequent to the deposition of 

 the drift,^ then the drift is much older than that of the Wisconsin, and 

 a considerable part of the sculpturing of the mountains must be referred 

 to Pleistocene time, inasmuch as the contours marking elevations of 

 these flat-topped ridges extend far up the valleys, in several instances to 

 the foot of the cliffs below the cirques in the heart of the range. 



The flat-topped mesas vary several hundred feet in maximum eleva- 

 tion, and it is not certain that all mark the same stage of deposition. 

 More work is required before accurate differentiation can be made. 



I have not had the opportunity yet to work out the relations of this old 

 drift to the widespread quartzite gravels which have been regarded as 



13 Geo. M. Dawson and R. G. McConnell: "Glacial deposits of southwestern Alberta. In 

 the vicinity of the Rocky Mountains." Bull. Geol. Soc. America, vol. 7, 1805. pp. 31-GG. 



