136 FRANCIS PARKER SHEPARD 



The age of the faulting is not known definitely, but there are 

 several factors bearing on the age, which should be mentioned. 

 In the first place the faulting must have been subsequent to most 

 of the folding in the Rocky Mountains, because in almost no 

 instances are the fault planes involved in the severe folding found 

 on the east side of the trench. The folding of the Rockies is 

 thought to have been mostly during the Laramide revolution. In 

 several cases thrust faults of very great throw were found to exist 

 in such a location that the upthrow side of the thrust is in the 

 valley, while the downthrow side forms the mountain wall (Fig. 3). 

 This feature was found in so many cases that it is obvious that 

 faulting does not directly control the topography. Therefore this 

 faulting must be old. It seems probable that it took place in early 

 Eocene times, at the close of the Laramide deformation. If the 

 overthrusts on the east side of the Rockies are contemporaneous 

 with those on the west, the great amount of subsequent erosion 

 lends some weight to the problem of the age of the Lewis over- 

 thrust, which has been considered either early Eocene or 

 Miocene.^ 



The slaty cleavage found in the vicinity of the trench in this 

 section is very constant in character in that it persistently dips to 

 the east on the east side of the trench, while on the west side it dips 

 to the west. Up Canyon Creek, where the section was almost 

 uninterrupted, the cleavage, as shown by the dotted lines (Fig. 4), 

 dips to the west in the Purcell Mountains, becomes gradually 

 vertical toward the middle of the trench, and farther east dips at 

 high angles to the east. Across the trench the cleavage dips to 

 the east at lower angles. 



Thus the folding, the thrust-fault system, and the slaty cleavage 

 all point toward the indipping structures and are evidence in favor 

 of Chamberlin's hypothesis of the wedge-shaped deformation 

 masses formed in mountain-building. Here in the trench two 

 intersecting wedges occur, one the Rocky Mountain and the other 

 the Purcell-Selkirk system. The great overthrusts on the east side 

 of the Rockies, such as the Lewis thrust and the Cascade Mountain 



'B. Willis, Bull. Geol. Soc. of America, Vol. XIII (1902), p. 2>2>z; R- A. Daly, 

 Geol. Surv. Can. Mem. 38, pp. 90-95. 



