[MACKENZIE] HISTORICAL AND STRUCTURAL GEOLOGY 119 



The foothills south of Blairmore, Alberta, are "characterized by a 

 parallel series of ridges, often maintaining uniform heights for several 

 miles; attaining an altitude of 6,000 feet or more in the western part 

 of the area, where they are often of knife-like sharpness, and gradually 

 decreasing in elevation and steepness of slope as the plains are ap- 

 proached (24,d). The fact that the ridges are higher and steeper 

 in that part of the area adjacent to the mountain front is significant, 

 and may indicate that the present foothill region was formerly covered 

 by the overthrust block, which protected the western ridges from 

 erosion for a longer time than it did the eastern ones. 



In his discussion of the antecedents of the Lewis thrust by folding 

 and erosion, Willis postulates two episodes of compression, between 

 which the peneplanation took place (45,b). I do not know of any 

 facts in the structure or physiography of the region that necessitate 

 two episodes of compression for their explanation, but on the other 

 hand, all the known facts can be explained on the basis of a single 

 episode. This compression may have begun slowly, continued to a max- 

 imum; and ended gradually, or it may have been pulsatory. Reason- 

 ing from what we know of the larger geologic processes, the latter is 

 the more probable supposition, but that there was any sufficient 

 interval between pulsations for peneplanation to take place is not 

 considered probable. Evidence for two episodes of compression 

 during the Laramide revolution is known from Wyoming (51), but the 

 two are very close together, and may be considered as effects of the 

 usual rhythm of geologic processes. Willis, on the other hand, 

 supposes a first episode in the early Tertiary causing moderate folding 

 (45, c) and that this folding was followed by a quiescent period long 

 enough for peneplanation. Moderate folding in the Cretaceous is 

 truaonly for a limited area adjacent to the boundary. The structure 

 of the Cretaceous in general is that of strong folds and steep reverse 

 faults of large displacement, so that according to his hypothesis, the 

 region must first have been mountain-built to a large degree during 

 the first period of compression in order to give the structures now found 

 in the Cretaceous of southwest Alberta, and then a great deal of 

 erosion must have taken place to peneplain these mountains. The 

 time that can be allowed for this erosion, a limited interval preceding 

 or early in the Eocene, as demonstrated above, is hardly sufficient for 

 peneplanation of a mountain-built region. 



The evidence with regard to an early Tertiary peneplain in the 

 Rocky mountains, as recapitulated above, shows that the strati- 

 graphic facts do not allow of sufficient time for it to have formed; 

 the physiography does not require a peneplain for its explanation, 



