THE PURCELL RANGE AND THE ROCKY MOUNTAINS 139 



dipping fault plane. As the upthrow is on the east, the relation 

 is that of a thrust fault. 



Also along the eastern wall south of Bull River there is a place 

 where the Mississippian appears to dip under the Galton series on 

 the east. The common occurrence of the east-dipping thrust faults 

 in the northern zone on the east side of the trench suggests the 

 probabiHty of this fault being one of the thrust type. It was 

 thought by Dawson' that the fault continued north of Bull River 

 because of a depression along the east side of the valley. The 



Fig. 7. — Sketch contour map showing trace of fault east of Gateway 



writer considers that this depression is one of many instances of 

 drainage along the flanks of the retreating valley glaciers. The 

 valley in this southern section was probably originally defined by 

 this fault plane, and subsequent erosion has broadened the valley 

 chiefly to the west. 



In conclusion, the Rocky Mountain trench does not appear to 

 be the unit in its development and structure that it has been 

 thought to be by Daly and Schofield. It appears instead to have 

 been produced partly by normal erosion, partly by erosion along 

 lines of structural weakness, and partly by the escarpment of a 

 fault. Even in this last instance the faulting is probably of the 

 thrust rather than of the normal type. 



' G. M. Dawson, Geol. Surv. Can., Ann. Kept. (1885), p. 190 B. 



