180 THE ROYAL SOCIETY OF CANADA 



the general scheme of normal faults so evident in the Selkirk system 

 and a longer period for denudation and trenching. The period of 

 compression at the close of the Eocene might well be credited with 

 minor folding in the fault blocks and also some faulting as noted 

 below. To this period is assigned the elevation of the plains and the 

 formation of a great anticlinorium on the site of the Rocky Moun- 

 tains. The western edge of this arch rests against the older moun- 

 tains and in places the outer normal fault block within the Rocky 

 Mountains appears to have been further tilted westward as a part 

 of the arch, thus intensifying the grade of the drainage from the east 

 into the trench. Where this has occurred, as along the Kicking 

 Horse valley, the hinge of the tilting may well have been near the 

 Rocky Mountain trench and have developed there the structure 

 which now appears as a graben but which in reality may be a slight 

 elevation of the eastern block from the effect of the compression. 

 Along the outer normal fault forming the eastern edge of the fault 

 block of the western part of the Rocky Mountains, the arching has 

 in places modified the fault line and the compression appears to 

 have produced some overthrusting to the east which is, probably, 

 most evident in the Athabaska section where the compression has 

 thrown the fault block into a syncline, but is also in evidence near 

 Mount Assiniboine, though in less pronounced form. 



The age of the major normal fault forming the boundary between 

 the two divisions of the mountains is somewhat uncertain. This dis- 

 placement if, as already suggested, it resulted in the production of 

 a coastal or land wall, should be marked by coarse material deposited 

 along its front. The deposits which might have shown the exact 

 foot of the supposed escarpment pobably would be removed in the 

 process of mountain building, but an examination of the nearest 

 Cretaceous deposits of the suspected period of great displacement is 

 of interest. 



The Cretaceous deposits so much in evidence on the plains 

 indicate periods either of shallowing of the sea or of revived denu- 

 dation of land areas, followed by subsidence. The maximum sub- 

 sidence and the greatest areal extent were reached in the Colorado 

 period. The best available sections of the deposits near the western 

 margin are those remnants found within the mountains and of these 

 the most important are in the Elk River valley. 



Very coarse conglomerates there appear in places to cover the 

 marine shales of the Jurassic and are found in various parts of the 

 land deposits of the Lower Cretaceous (Kootenay formation). They 

 occur in greater amount above this formation in the Elk conglomerates 



