CANADIAN AND MONTANA ROCKIES 



309 



ceive of thrusts such as the Lewis to originate in any other way than by 

 compression of a considerable thickness of the crust. The concept of 

 compression is deeply ingrained in the literature of the Rocky Mountains, 

 and the representations in this and following chapters reflect these views. 

 They are challenged only if recent workers have taken a different view or 

 if the writer feels strongly in favor of gravity induced movements. 



West of the Lewis thrust and between the two tear faults that bound 

 the Reltian segment is a broad syncline in the Reltian strata. See section 

 I-I', Fig. 20.4. West of the syncline, or on its west flank, a number of fairly 

 high-angle faults that dip eastward repeat the formations ( Clapp, 1932 ) . 

 The beds dip eastward at angles ranging from 20 to 50 degrees, and the 

 faults dip more steeply than the beds, generally at angles of 60 to 80 de- 

 grees. These western faults, together with the eastern, form a set of huge 

 downward-pointing wedges. Clapp estimates the amount of throw of the 

 western high-angle thrusts to be from about 10,000 to 30,000 feet. 



Each of the western faults follows closely the west base of one of the 

 ranges which appear to have been uplifted along the faults. The faults 

 have been named for their respective ranges (Clapp, 1932). 



In addition to the eastern and western thrusts, there are steeply dip- 

 ping transverse faults of both reverse and normal categories. They have 

 displacements up to 10,000 feet. The transverse faults are most abundant 

 in the southern part of the area north and northeast of the batholiths. One 

 fault of a singular category has been mapped that parallels the beds but 

 dips at a very low angle and is normal (Clapp, 1932). 



Clapp relates the various groups of faults in the following way: 



It appears as if the forces causing faulting acted from the southwest, first 

 uplifting and folding the rocks, then breaking them along the longitudinal 

 (western) thrust faults. Later, the deformation continued to such an extent that 

 relief from the stresses came by overthrusting (to the east). The two sets of 

 transverse faults seem to be a still later effect of the continued pressure from the 

 southwest, and consequent elongation to the northwest and southeast. ... As 

 the compressive forces acting from the southeast lessened, normal strike faults 

 with low dip relieved the vertical pressures resulting from the great height of 

 the uplifted rocks. 



Normal faults, probably of late Cenozoic age, are also present and will 

 be discussed under a later heading. Most of this faulting appears to have 

 taken place along the much earlier longitudinal thrust faults. 



In Canada, at least from Jasper at latitude 53° N southward to the bor- 

 der, the eastward and westward thrusts are found much in the same rela- 

 tions as in northwestern Montana. See section B-B', Fi<4. 20.3. The strata 

 instead of being mostly Beltian are mostly Cambrian, which in a broad 

 way are synclinal, although a distinct and great anticline occurs west of 

 Banff along the British Columbia-Alberta border. 



A difference from the Montana division, however, is the nature of the 

 western boundary. In Montana, the Beltian rocks continue westward un- 

 der the entire terrane until the great intrusions make their appearance. 

 In Canada, the belt of longitudinal thrust faults in Paleozoic formations 

 is bounded on the west by the Rocky Mountain trench. 



The belt of great imbricate thrust faults may be traced northward to 

 Mountain Park (section A-A', Fig. 20.5) and from there to ranges west of 

 Fort Nelson. The Alaska Highway west of Fort Nelson, between miles 

 380 and 497, crosses two ranges, the Sentinel on the west and the Stone on 

 the east. Here various rocks from Precambrian to Jurassic are exposed. 

 The section, according to Laudon and Chronic (1949), is: 



Triassic strata 



Black shale and black limestone 500 feet 



Unconformity 

 Mississippian strata 



Kindle formation: gray, silty limestone and chert 300—400 



Unconformity 

 Devonian strata 



Ft. Creek shale: black, pyritic shale 800 



Ramparts limestone: massive, tan, gray, and black 4imestone 1500 



Muncho limestone: gray and black limestone 600 



Unconformity 

 McConnell limestone: gray and black limestone 680 



Unconformity 

 Silurian strata, entirely Niagaran in age 



Roninng limestone: gray and black, cherty, dolomitic limestone 1200 



Cambrian (?) strata 



MacDougal sandstone: tan sandstone "thin" 



Precambrian rocks 



Quartzite, slate, marble, and schist, intruded by basic igneous rocks 



