THE PURCELL RANGE AND THE ROCKY MOUNTAINS 137 



thrust, are examples of indipping structure on the other side of the 

 Rocky Mountain wedge. 



The cause of this valley in the northern section is partly the 

 zone of weakness produced by the intersection of such a large 

 number of fault planes, causing erosion to be more rapid here than 

 elsewhere. Also the t3^e of folding on the sides of the trench 

 produced a depression between (Fig. 2). 



South of Canal Flats the zone which separates the structure of 

 the Rocky Mountains from the Purcell Range leaves the trench, 

 and swings to the east. This zone does not die out, but appears in 

 the Hughes Range of the Rockies some 12 miles from the trench. 

 It was traced as far south as Elko. In at least two places, this 

 zone has overthrust faults from the west. Fifteen miles up into the 



Fig. 6. — Section at Fort Steele (through C — C, Fig. i). Scale: i inch = 14.63 

 miles. 



Rockies from Fort Steele, there is an overthrust with a throw of 

 several thousand feet (Fig. 6). No valley exists along this fault, 

 which shows that the faulting does not always control topography. 



2. The central part. — From Canal Flats to Bull River the trench 

 does not follow structural Hnes, neither in regard to the strike of the 

 bordering formations, which is roughly parallel on both sides, nor, 

 so far as could be found, in regard to the faults. The conclusion 

 to be drawn from this point is that this part of the trench was 

 developed by a stream flowing on an old erosion surface which 

 was flat enough to allow the river to develop its course inde- 

 pendently of structure. 



This conclusion is strengthened by several factors. To begin 

 with, the Purcell Range is thought by Schofield^ to have been 

 deformed in the Jurassic and peneplained in the Cretaceous, and 

 at this time to have developed many valleys out of accord with the 

 structure developed in the Jurassic mountain-building. After the 



^ S. J. Schofield, Geol. Surv. Can. Mem. y6, pp. 101-2. 



