Section IV, 1920 [61] Trans. R.S.C. 



The Origin of the Rocky Mountain Trench, B.C. 



By S. J. ScHOFiELD, M.A., B.Sc, Ph.D. 

 Presented by W. McInnes, LL.D., F.R.S.C. 



(Read May Meeting, 1920) 



Introduction 



In 1889, Dawson presented before this Society a paper entitled, 

 "On the Later Physiographical Geology of the Rocky Mountain 

 Region in Canada with Special Reference to Changes in Elevation 

 and the History of the Glacial Period," in which he outlined in a 

 general way the history of the Cordilleran drainage, especially that of 

 the Glacial period. Since that time, many new facts have been 

 recorded by numerous observers, which open new avenues of thought 

 on the problem concerning the drainage systems of British Columbia, 

 and permit us to trace this drainage with a greater degree of certainty 

 than was possible in Dawson's time. It must not be understood, 

 however, that the problem is in any way completely solved, and it is 

 hoped that this paper will be considered simply as a preliminary contri- 

 bution to one of the greatest and most interesting problems in Cordil- 

 leran geology. 



Summary 



1 . The Rocky Mountain trench is the most remarkable structural 

 feature of the Canadian Cordillera. It extends from the 49th parallel 

 of latitude at least to the boundary between British Columbia and 

 Yukon. 



2. At the close of the Jurassic, the Cordilleran topography was 

 made up of two main highlands (the area now occupied by the Coast 

 Range and the area now represented by the Selkirk Mountains and 

 their continuation northward) separated by an interior basin now 

 designated the Interior Plateaus. 



3. The drainage naturally was from the highlands to the basins, 

 the products of erosion being deposited as Cretaceous sediments along 

 the flanks of the highlands in the areas now represented by Vancouver 

 Range, Interior Plateaus, the Rocky Mountains, and the Great Plains. 



4. The highlands were reduced to a condition of old age or 

 peneplanation, by erosion throughout the whole of Cretaceous time. 

 Towards the close of the Cretaceous, the rivers flowing eastwards 

 from the easterly highland eroded headwards, until it is possible they 

 reached the Interior Plateaus. 



