60 G. M. DAWSON — GLACIAL DEPOSITS OF SOUTHWESTERN ALBERTA." 



As already mentioned, it is not certainly known how far the lower and 

 upper boulder-claj^s of the plains or either of them extend to the west. 

 Both are found at Lethbridge, 60 miles from the mountains, and, if the 

 line observed in sections on High wood river corresponds with this divis- 

 ion, both are there present to within about 15 miles of the base of the 

 mountains and at an actual elevation of 3,700 feet. One or the other of 

 these boulder-clays, however, extends westward along the Oklman river 

 beyond the longitude of the Porcupine hills, and at least as far west as 

 Calgary, on Bow river, and there is some reason to believe that it is the 

 upper boulder-clay which is thus most widely spread. 



Respecting the conditions indicated b}^ the various deposits, the follow- 

 ing remarks may in the first place be made : 



The Saskatchewan gravels, in their composition and because of the 

 great distance to which they have been carried from the mountains, im- 

 ply the existence at the time of their formation of a considerable east- 

 ward slope of the i)lains, probably greater than that by which the same 

 region of the plains is affected today. The existence of silty deposits 

 and sands in association with them, however, shows that areas of slack 

 water or lacustrhie conditions must in some places have occurred. 



Tlie interglacial deposits give reason to believe that at the time of their 

 deposition, as elsewhere explained,* at least a considerable tract of the 

 western plains had become practically horizontal. 



It remains uncertain to what particular period subsequent to that of 

 the Saskatchewan gravels, and excluding that of the interglacial deposits, 

 the traveled gravels and boulders marking the highest levels of tlie drift 

 deposits on the Porcupines and foothills are referable; but it is certain that 

 this time was one of great relative change of level, taking the form of a 

 depression toward the west or southwest. This is rendered evident in a 

 broad way by the occurrence of Laurentian stones to a height of 5,300 

 feet, or about three times that of the present summit level of the Lauren- 

 tian plateau from which they came. It is reinforced by the association 

 of these with limestones of the still lower Winnipeg basin. 



Pursuing this argument a little further into detail, we may compare 

 some of the levels at which the highest drift is found in several places in 

 the west. In the Porcupine hills this level is undoubtedly that of a water- 

 line, and I believe it to be so also in other places in which it has been 

 noted.f On this assumption a relative depression to the west at this 

 time of 900 feet is indicated between the Cypress hills and the Porcu- 



* Report of Progress, Geol. Survey of Canada, 1882-'84, p. 151 C. 



t Terraces noted by Mr G. E. Culver near Saint Marys lakes, in northern Montana, may repre- 

 sent those here described, although no eastern drift appears to have been found upon them. Mr 

 Culver's description appears to show that the levels are about the same. Trans. Wisconsin Acad. 

 Sci., vol. viii, p. 202. 



