THE GEXESfS OF LAKE AG A SSI Z 815 



hatl ncit retired further east during the time when Lake Agassiz 

 stood at a higher level is shown by the absence of stratified 

 lacustral deposits above the 150-170-foot line, in man}^ places 

 only a \ery few miles from Lake Winnipeg, an absence peculiarly 

 noticeable as these deposits occur in such abundance below that 

 line. 



It is thus seen that the Keewatin glacier, which centered west 

 of the northern part of Hudson Bay, had extended southward to its 

 furthest limit, and had then retired many hundreds of miles, prob- 

 ably more than half way to its gathering ground, before the 

 Laurcntide glacier had reached its greatest extension. 



Dr. Dawson' has also shown that the Cordilleran g-lacier 

 reached its greatest extent and retired before bowlder-clay that 

 generally underlies the western plains was deposited. This 

 bowlder-clay I take to be the true till or ground moraine of the 

 Keewatin glacier, when this glacier had reached its greatest 

 extent in a southwesterl}- direction. 



The ex'idence at present at hand would therefore seem to 

 strengthen the view that in the northern part of this continent 

 during the glacial period there were three great centers of snow 

 and ice accumulation, one, the Cordilleran in the mountains of 

 British Columbia, a second, the Keewatin, on the comparatively 

 low land northwest of Hudson Bay, a third, the Laurentide, in 

 the Labrador Peninsula. 



Beginning at the west, and going eastward, these three 

 great glaciers would seem to have reached their widest extent 

 and retired in succession. Still further east, across Davis Straits, 

 a fourth great glacier, probably similar in character to those 

 that have disappeared from the American continent, covers 

 Greenland at the present time. 



J. Burr Tyrrell. 



'Glacial Deposits of Southwestern Alberta, in the vicinit}- of the Rocky Moun- 

 tains, by George M. Dawson, Bull. Geol. Sur. Am., Vol. VII, pp. 31-66. Nov. 1895. 



