THE PURCELL RANGE AND THE ROCKY MOUNTAINS 135 



At Beavermouth there is a wide zone separating the two series in 

 question, and the fault was inferred entirely on lithological grounds. 

 It seems unlikely that such a fault exists. As will be seen from the 

 section, thrust-faulting on the east is important in this region, 

 which is rather significant in view of the dying out of the west- 

 dipping thrust faults. 



In the southern portion of this northern zone there are cases 

 on the east side of the trench where there are intersecting east- and 

 west-dipping thrust faults. In all such cases the west-dipping 

 thrusts cut those from the east. There is an interesting example 



Rocky Mountain 

 I'renoh. 



L wen Or do vie ian. 



Fig. 5. — Section at Sinclair 



of this at Sinclair Springs (Fig. 5). To the east the west-dipping 

 thrust faults always become less conspicuous, and finally die out. 

 At Brisco an east-dipping thrust fault was found on the west side 

 of the trench. Thus there is considerable intersection of the east- 

 and west-dipping thrust faults. 



If the general rule for thrust faults is to have dips of much less 

 than 45°, then these faults are exceptional since their dips average 

 about 50° and in some cases are as high as 70°. This is in contrast 

 to the low dips of the thrust faults on the east side of the Rockies 

 in the northern United States and Canada. Experiments, on 

 folding and faulting, which I have made, show that faults which 

 develop at the intersection of two folds may have dips which 

 exceed 45° due to the upward relief being much easier than the 

 lateral. The faults developing between two separate mountain 

 ranges might be subjected to the same influences. 



