1916] The Ottawa Naturalist. 11 



THE FORMATION OF THE GREAT PLAINS OF NORTH- 

 WESTERN CANADA* 



D. B. Dowling. 



The plains of Canada form but a portion of the larger 

 plains of the continent, which occupy a large part of the interior 

 and are divided into an eastern and a western portion by a 

 great central valley. The eastern plains which extend to the 

 St. Lawrence lowlands are forested and, therefore, seldom refer- 

 red to as plains. Westward, the rainfall being lighter, there is 

 a thinning of the forests and there are more open areas. These 

 are generally referred to as the plains. In Canada the open 

 prairie of the plains is being invaded by the forests from the 

 north, so that a division can be ma ' oi treeless plains, plains 

 with scattered trees, and forested plains. 



The first requisite in a definition for these plains would 

 haps be a nearly level surface, supplemented by a soil covering, 

 and a climate admitting of the pro some vegetation, 



for the absence of moisture soon produces desert conditions. 

 The formation of a level ee, to take a homely exam; 



suggests either -planing or plastering. The planing process oi 

 nature is a slow decay of the old surface and its removal by 

 erosion. The surface thus planed is inclined to be rockv. a- 

 as it is losing its rock waste, the soil is to be found sparingly 

 in the hollows or valleys. In plastering the nature process 

 consists of the spreading out, generally by large bodies of wa 

 of the rock waste poured in by the streams. This produces a 

 more perfectly even surface outline than is ever produced by 

 the planing process, but our surface features are the product 

 of both. If the surface were a part of a perfectly rigid sphere, it 

 would be difficult to explain the presence of large areas con- 

 taining the rock waste, or of those plains built up by the spread- 

 ing action of the sea, but as there is a vast amount of evidence 

 showing that the continent has not been stable but sank in 

 certain areas, rose in others, and repeated the sinking and 

 rising several times, we are forced to believe that the crust is 

 flexible, and that its equilibrium is influenced by tangential 

 strains or the shifting of load. To this we owe the submergence 

 of those parts which received a coating of rock waste deposited 

 by the sea. Much of this rock waste underlies the great agri- 

 cultural areas or plains, so that we may say that the flexibility 

 of the crust made possible the peopling of the earth by pro- 

 viding soil covered areas for the plant growth necessary 



Resume of lecture before the O.F.X.C., March 21st, 1916. 



C4> 



MBRAR 



