616 G. M. DAWSON ON THE SUPERFICIAL GEOLOGY OF 



foot of the Tertiary plateau has blocked them up and prevented 

 the waters finding their way northward as before ; and since glacial 

 times the rainfall of the district has never been sufficiently great 

 in proportion to the evaporation to enable the streams to cut through 

 the barrier thus formed. The existence of these old valleys, and 

 the arrangement of the drift-deposits with regard to them, throw 

 important light on the former history of the plains. 



Northward, the Coteau ceases to be identified with the Tertiary 

 plateau, and rests on a slope of Cretaceous rocks. It can be followed 

 by Palliser's and Hector's descriptions of the country to the elbow 

 of the South Saskatchewan, and thence in a line nearly due north 

 through the Eagle and Thickwood Hills ; beyond the North Sas- 

 katchewan, however, it appears to become more broken and less 

 definite. In Dr. Hector's description of certain great valleys with- 

 out outlet in this northern region, I believe I can recognize there 

 too the existence of old blocked-up river-courses similar to those 

 just described. 



South of the forty-ninth parallel the continuation of the belt of 

 drift material can also be traced. It runs south-eastward, charac- 

 terizing the high ground between the tributaries of the Missouri 

 and the Red River, which has already been noticed in connexion 

 with the watershed of the continent ; but wanting the backing of 

 the Lignite-Tertiary plateau, it appears to become more diffuse, and 

 spread more widely over the country. That the drift-deposits do 

 not form the high ground of the watershed, but are merely piled 

 upon it, is evident, as Cretaceous rocks are frequently seen in its 

 neighbourhood at no great depth. From what I can learn of the 

 region it would appear that the so-called Coteau des Prairies and 

 Coteau de Missouri, between which a distinction is made on the maps, 

 are parts of the same great feature. Their elevation is similar, and 

 nearly the same as that of the Coteau on the line ; and they are 

 equally characterized by the immense profusion of erratics with 

 which they are strewn, and by basin-like swamps and lakes. The 

 Coteau des Prairies, however, stretches furthest, and dies away only 

 in the south-western corner of Minnesota. 



In the Coteau, then, we have a natural feature of the first magni- 

 tude — a mass of glacial debris and travelled blocks with an aver- 

 age breadth of perhaps 30 to 40 miles, and extending diagonally 

 across the central region of the continent for a distance of about 

 800 miles. 



Third or Highest Prairie- Plateau. 



Passing the Coteau and ascending the plateau of the Tertiary (PL 

 XXXII., d), we notice at once a change in the character of the drift 

 deposits. They are much thinner, and, area for area, perhaps do not 

 equal one twentieth of those on the second prairie-steppe. They are 

 also now largely composed of quartzite drift from the Rocky Mountains, 

 of the nature of shingle, and seldom showing much trace of glaciation. 

 With this western drift, however, a smaller proportion of that from 



