124 THE ICE AGE IN CANADA. 



feature, and which fringes the margin of the third plateau, 

 about 400 miles west of Winnipeg, is now known to be 

 continuous with similar ridges extending southward into 

 the United States and eastward towards the Atlantic, 

 and which have been described as the terminal moraine 

 of a great continental glacier. In the western plains, 

 however, where it has its greatest development, it cannot 

 be explained in this way, but must mark the margin of 

 an ancient glacial sea, or at least of that deeper portion of 

 such sea in which heavy ice could float, while in its upper 

 portion it shows evidence of having been, in the later 

 periods of its formation, an actual water-margin. 



The railway, taking advantage of the oblique valley of 

 Thunder creek, crosses the coteau at one of its least- 

 marked portions, but where it still presents very definite 

 and striking characters. On entering it, the railway 

 passes for nearly thirty miles through a rolling or broken 

 country, consisting of successive ridges and mounds inter- 

 spersed with swales and alkaline ponds without outlet. 

 To this class belongs a somewhat extensive series of lakes 

 known as the " Old Wives' Lakes." The highest point of 

 the coteau on this section is near Secretan Station. 



As seen in the road-cutting, the basis of the ridges 

 appears to consist of thick beds of imperfectly stratified 

 clay, derived from the disintegration of the local Creta- 

 ceous beds, but with many Laurentian boulders. In one 

 place the -clay was observed to be crumpled as if by 

 lateral pressure. Above the clay are stratified gravels, 

 also with large boulders, most abundant at top. The 

 ridges are highest and most distinct at the eastern or 

 lower side, and gradually diminish towards the upper or 

 western margin, where they terminate on the broadly 

 rolling surface of the upper prairie. 



