152 /. BURR TYRRELL 



disappeared, or merely confined itself to the west side of the 

 Rocky Mountains, I do not know, but at all events it seems to 

 have ceased to be an important element in molding the features 

 of the plains. 



After the withdrawal (or possible disappearance) of the Cor- 

 dilleran glacier, the Keewatin glacier overspread the country,, 

 radiating outwards from a center probably lying somewhere 

 between Doobaunt and Back rivers. The till formed during this 

 period, which is probably synchronous with that of the sub- 

 Aftonian period of Professor Chamberlin, may be easily recog- 

 nized in the scarped banks of many of the streams in Alberta, 

 where it overlies the Saskatchewan gravel of the Albertan period. 

 It is composed largely of material derived from the underlying 

 Cretaceous and Laramie rocks, but at the same time it contains 

 a considerable quantity of other material transported from a dis- 

 tance, some of which, consisting of granite, gneiss, quartzite, and 

 similar rocks, has been derived from the Archaean nucleus to the 

 northeast, while some has been derived from the Cambrian sand- 

 stones, and Cambro-Silurian and Silurian limestones that extend 

 around the edge of the Archaean. 



After the deposition of the sub-Aftonian till the Aftonian 

 period of deglaciation set in, during which the Keewatin glacier 

 became greatly diminished, and interglacial deposits were laid 

 down, both in extraglacial lakes, and in lakes and swamps at 

 some distance from the face of the glacier. How far the foot 

 of the glacier withdrew in this interglacial time I do not know, 

 but I am inclined to think that most of Manitoba still remained 

 covered with ice, for in the western part of that province I have 

 not been able to find evidence of more than one main Keewatin 

 interglacial period, which is probably later than the Aftonian 

 period. 



After this period of diminution the Keewatin glacier again 

 began to increase, and it spread southward and westward until 

 it had reached about the same limits that it had reached during 

 the sub-Aftonian period, and had spread another sheet of till 

 over the earlier till and subsequent interglacial deposits. The 



