COMPARISON AND EXTENSION OP TWO MEDICINE GLACIER 545 



glacial drift. This, of course, was not present at the time of the first 

 extension of the ice. It might then be a reasonable assumption that the 

 bottom of the valley at the foot of the upper lake lacked at least 800 feet 

 of being as low as at present at the time of the glaciation of the Black- 

 foot peneplain. If this was the case, Saint Mary Valley for some dis- 

 tance farther north and also the other valleys heading in the mountains 

 and those of the various tributaries of Milk Eiver must have been cor- 

 respondingly less deep at the same time and considerable remnants of 

 the Blackfoot peneplain may have still persisted in southern Alberta, 

 although probably much erosion would have been accomplished along 

 most of the valleys and most of the intervening tracts would have been 

 somewhat lowered and rendered uneven in consequence of the denudation. 



Deductions prom Eelations op pre- Wisconsin Drift of the 

 Keewatin Ice-sheet 



If it were known that the recognized earliest advance of the conti- 

 nental ice-sheet into this region 6 was contemporaneous with the first 

 extension of the Cordilleran ice, we would have something of a check on 

 the relative antiquity of the oldest mountain drift in question. Calhoun 

 has shown that there was probably approximate contemporaneity between 

 the last great extension of the mountain glaciers and of the ice from the 

 northeast, and with this conclusion we are in accord from our own obser- 

 vations, as described in a subsequent connection (page 557). We observed, 

 as did Dawson and McConnell farther north, and as did Calhoun near the 

 49th parallel, that drift of the continental ice-sheet overlaps drift of the 

 mountain glaciers. This relation and the fact that the Saskatchewan 

 gravels, which Dawson and McConnell regarded as, in part at least, out- 

 wash from this mountain drift, extend eastward below the lower of the 

 two northeastern drift sheets exposed in the region of Lethbridge, led these 

 latter investigators to regard all the mountain drift east of the mountain 

 front, including the moraines, as the oldest drift of southern Alberta. 



Calhoun, however, pointed out, and with this again our observations 

 agree, that there are in the sections observed no evidences that any con- 



8 It should be stated that the interpretations of the present writers are based on what 

 is now generally considered by geologist* a well founded assumption thai the upper and 

 lower boulder clays, consisting of drift of the Keewatin Glacier, were deposited i>.\ exten- 

 sions of the continental ice-sheet. Doctor Dawson seems not to have been ready to ac 

 cept such an interpretation, for lie stales (Hull. Geol. Soc Am., vol. 7. p. 61) . "I bare 

 elsewhere given reasons for the belief thai both these boulder elays of the western plains 

 are attributable to the agency of floating ice (On the physiographies! geology of die 

 Rocky Mountain region in Canada. Trans. Roy. Soc. Can., vol. viii. see. i. p. i;:; el seq.)i 

 but this need not here be specially insisted on." 



XXXVII— -Bull. Geol. Soc. Am., Vol. 24, 1912 



