108 THE ROYAL SOCIETY OF CANADA 



The Western Structural Area 



The clearly defined portion of the western area, as distinguished 

 from its less definite northward extension, lies between Elk and 

 Kootenay rivers on the west and Flathead river on the east. It is 

 characterized in its southern part by homoclinal fault-blocks separated 

 by large normal faults (12, j), (26,c). Farther north folding is the 

 more characteristic structure, but there is also normal faulting (23). 

 At the boundary the surface rocks of the western area are Precam- 

 brian, but northward younger ones appear, and Cretaceous measures 

 are exposed in the Crowsnest pass. 



The Central Structural Area 



At the boundary the central structural area is a great syncline 

 of Precambrian strata, and this general structure probably holds as 

 far north as the North Kootenay pass. The boundary between the 

 central and the western structural areas in this distance is a great 

 normal fault or zone of faults along the east side of Flathead valley. 

 These faults have a downthrow to the west, so that, as Daly has 

 pointed out, the Clarke range — the central area of this paper — "is 

 in what may be called horst relationship to the block underneath 

 the Flathead valley (12, k)". From the North Kootenay pass to the 

 Crowsnest pass the central structural area is very much narrower 

 than it is farther south, because of the wide western swing of the Lewis 

 thrust at this point. The eastern part of this huge syncline of massive 

 Precambrian sediments, and perhaps the whole of it, is thrust over 

 Cretaceous rocks to the east on a thrust surface of very great extent, 

 the outcrop of which forms the eastern boundary of the central struc- 

 tural area. This enormous displacement is known as the Lewis thrust 

 and its sinuous outcrop has been traced for many miles in Montana 

 and Alberta. 



The Eastern Structural Area 



The third and eastern structural area consists principally of the 

 foothills of western Alberta adjacent to the Crowsnest pass. In it 

 also is included a part of the Great Plains immediately adjacent to 

 the mountains for 20 miles north of the International Boundary. 

 In this area foothills are lacking, but geologically the region belongs 

 to the "disturbed belt". 



In the eastern structural area the rocks are Cretaceous, except 

 for a few areas of Devono-Carboniferous limestone in the Crowsnest 

 pass, and some Eocene beds near the eastern edge. Structurally it is 

 characterized by numerous nearly parallel reverse strike faults, of 



