92 THE ROYAL SOCIETY OF CANADA 



the Rocky Mountains which now border the eastern edge of the 

 ancient highland were slowly raised. The western portion of the 

 Rocky Mountains is characterized by open folds while the eastern 

 portion has a structure in which overthrust faulting is the predominant 

 feature. At the same time the two ancient highlands as well as the 

 Interior Plateaus suffered an uplift in which folding and faulting 

 played a minor role. This uplift rejuvenated the Cretaceous streams 

 and they began to entrench themselves in the uplifted land mass. 

 With the increased velocity and with the material derived from the 

 accelerated erosion the main streams were able to keep their course 

 across the uprising Rocky Mountains thus forming the great "through" 

 valleys which traverse the Rocky Mountains from west to east at 

 right angles to the structure. Such valleys are the Peace, and the 

 valleys occupied by the Grand Trunk Pacific, the main line of the 

 Canadian Pacific Railway and the Crowsnest branch line of the same 

 railway. Many streams which were unable to keep their course across 

 the uprising Rocky Mountains were diverted and became part of the 

 drainage of the larger through streams. Subsequent streams which 

 conformed in direction with the strike of the underlying structures 

 gradually pushed headwards along the weak strata, or structures 

 giving rise to the peculiar rectangular drainage systems so character- 

 istic of the Rocky Mountain drainage system. It is very probable 

 that subsequent streams separated by watersheds or divides occupied 

 portions at least of the depression which later developed into the 

 Rocky Mountain trench. Thus we have the first faint outline of 

 what was to prove one of the largest and most imposing valleys on this 

 continent. 



An examination of a present day map shows that the Peace River 

 has a course which was characteristic of all the above-mentioned 

 rivers just after the building of the Rocky Mountains was completed 

 while a great rearrangement has taken place in courses of the "through" 

 going rivers such as those which occupied the valleys in which are 

 located the Yellowhead, Kickinghorse, and Crowsnest Passes in 

 southern British Columbia. The drainage of the last mentioned 

 through valleys is westerly in the western part of the Rocky Moun- 

 tains and easterly in the eastern portion of the mountains. Hence the 

 course of the Peace River may be taken as normal. The recognition 

 of this fact gives us a point of departure from which may be attempted 

 a discussion of the origin of the peculiar characters of the abnormal 

 courses pursued by the other rivers. 



