1916] The Ottawa Naturalist. 13 



feet were removed. The cycle of denudation was not completed, 

 as is shown by fragments of the first surface which still remain. 



The coming of the ice sheet of the glacial period is thought 

 to have altered the general topography but little, with the 

 exception perhaps of a smoothing of the uneven surface or a 

 filling up of sharply cut valleys. The period during which the 

 ice was wasting or melting is marked by many drainage chan- 

 nels that are now abandoned. The occupation by the glacier 

 of the valleys of the principal streams which have a north- 

 eastward trend, caused no doubt a damming up of the water 

 which, together with that from the melting ice, overflowed along 

 the ice front and sought channels that were almost at right 

 angles to the original channel. Many of these are still used 

 as part of the present river courses, but in the southern portion 

 of the Canadian plains there are many of these glacially-induced 

 channels that are now abandoned, and have apparently no 

 other reason for their existence. The Saskatchewan drainage 

 was diverted to the Missouri for a short period while its former 

 valley through the Coteau was blocked by ice. The diversion 

 filled lakes Chaplin and Johnston and proceeded south, scouring 

 out the valley now occupied by Lake of the Rivers, Willow- 

 bunch and Big Muddy lakes. A little later -the outlet was 

 shifted to east of the Coteau, and the Regina plain was a lake 

 basin drained by the Souris river probably to the Red River 

 valley. This lake was lowered by the retreat of the ice to a 

 position farther north, and a new channel was again adopted. 

 This was deeply cut by the flowing stream, and is now used by 

 the Qu'Appelle and Assiniboine rivers, which have but a small 

 flow at present. 



The melting of the ice in the lowlands of the Red River 

 valley created a lake along its front that was not as readily 

 drained as was the case in the retreat of the ice cap across the 

 prairies. In the Red River valley there seemed no outlet, and 

 the basin filled until it spilled over its lowest point, far south 

 in Minnesota at Lake Traverse. The removal or melting of 

 a vast mass of ice in the north seems to have resulted in a slight 

 elevation of the crust that had been depressed by the weight of 

 the ice. This recovery, which means an actual tipping of the 

 lake basin, lowered the lake by spilling its water to the south, 

 and as the lake at its several stages formed beaches, the levels 

 of these give us the amount of tilt that occurred between their 

 dates of formation. This outlet was abandoned when the lake 

 secured a lower northern outlet. The greatest depth of water 

 over the site of the city of Winnipeg was about 560 feet. 



The benefit of this old lake to the agricultural value of 

 the Red River valley can hardlv be measured. Over the sur- 



