87 



The question of the extension of the continental ice 

 sheet is still an open one and the glacial till west of the 

 Coteau is believed by many to have been carried by 

 floating ice. The closing stage of glaciation was no doubt 

 one in which the ice front held back large lake-like basins, 

 of which the best known is glacial Lake Agassiz which 

 occupied the basin at the eastern edge of the Cretaceous 

 plateau. This lake at first drained southward to the 

 Mississippi valley, and at that stage formed many beaches 

 along its western and southern margin. These beaches 

 show a gradual rise to the north due to an upwarp 

 of the crust, which caused the waters to continue 

 their discharge southward until on the retreat of the 

 ice to the north an outlet was found in that direction. 

 Many of the former drainage channels were ice blocked, 

 and the lake received a large inflow from the southern 

 part of the plateau to the west of it. As a result of the 

 valley cutting which ensued at this time, a great burden 

 of fine-grained material was deposited in this basin to 

 form the lacustrine deposits of the Manitoba plains. 



SUMMARY DESCRIPTION OF FORMATIONS. 



ORDOVICIAN. 



In Manitoba the Ordovician includes the following 

 formations : — 



Stony Mountain formation, consisting 

 of yellowish and reddish lime- 

 stones overlying dark shales. 



Exposed at Stony Mountain no feet (33-5 m.) 



Upper mottled limestone 150 " (45-7 m.) 



Cat Head limestone 70 " (21 m.) 



Lower mottled limestone 70 " (21 m.) 



The divisions of the Trenton here indicated are made 

 on physical grounds. The exposures are best seen on 

 Lake Winnipeg. No deposits of this age occur in the outer 

 ranges of the Rocky mountains. 



