PHYSICAL AND CLIMATAL CONDITIONS. 125 



Tlie history of the coteau would seem to have been as 

 follows : 



1. The excavation in pre-glacial times of an edge or 

 escarpment in the gently sloping surface of the Cretaceous 

 and Laramie beds, and the cutting by subaerial causes of 

 coul(^es and valleys of streams in this escarpment. 



2. Submergence in the glacial period, in such a manner 

 as to permit heavy ice loaded with Laurentian debris to 

 ground on the edge of the escarpment and deposit its 

 burden there, while at the period of greatest submergence 

 deep water must have extended much further westward. 

 These conditions must have continued for a long time and 

 with somewhat variable depth of water. 



3. Ee-elevation, during which gravel ridges were 

 formed, until at length the coteau became the coast-line 

 of a shallow sea, which lingered at a later date along the 

 line already referred to in advance of the coteau. 



4. On the re-elevation of the country, the transverse 

 ravines and valleys were so effectually dammed up by 

 the glacial ridge, that the surface waters of the region, 

 now comparatively arid, have to remain as alkaline lakes 

 and ponds behind the coteau. 



The upper prairie plateau, extending from the coteau 

 to the Rocky mountains, has, on its general surface, com- 

 paratively few boulders ; yet these are locally numerous, 

 especially on the eastern and northern sides of some 

 gentle elevations of the prairie. They consist, as before, 

 of Laurentian gneiss, Huronian schists, and yellow Silurian 

 limestone, all derived from the eastern side of the plains, 

 some of the boulders of Laurentian gneiss being of great 

 dimensions. Some of these have been used in modern times 

 by the buffalo as rubbing-stones, and are surrounded by basin- 

 shaped depressions formed by the feet of these animals. 



