[MACKENZIE] HISTORICAL AND STRUCTURAL GEOLOGY 129 



The effects of the final major event in the structural history is 

 shown in Section G, which illustrates diagrammatically the present 

 conditions near the 49th parallel. Consequent on the relaxation 

 following the compression of the Laramide revolution the western 

 arched zones collapsed along a series of normal faults, some of which 

 have been mapped by Daly. The easternmost fault, that along the 

 east wall of the Flathead valley, was one of the greatest of these 

 breaks, and dropped the block under the Flathead valley in such a 

 way as to cause ponding of the consequent stream that occupied the 

 depression, and in these shallow freshwater basins, the Tertiary Kish- 

 enena formation was deposited. That the episode of relaxation ac- 

 companied by normal faulting continued into the Tertiary is shown 

 by the tilting of these Tertiary sediments in a constant eastward 

 direction (45, d), (26, c). The integrity of the major syncline of the 

 Clarke range may be explained as being caused by the thickening and 

 strengthening of this central structural area by folding and over- 

 thrusting. 



The structure at the North Kootenay pass section, illustrated 

 in Figure 3, is different in certain respects. For example, Mesozoic 

 rocks here occupy the surface of the western structural area; the central 

 structural area is narrow, and there are many more reverse faults in 

 the eastern structural area. The development of the present structure 

 is shown by the successive sections. The significant process illus- 

 trated is the development of "soles" from some of the reverse faults 

 in a manner analogous to that indicated in Cadell's experiments 

 (20,b). The development of these soles furnishes the key to the 

 explanation of the present steep attitude of the reverse faults of the 

 region. The slices of the earth's crust above these "soles" are sup- 

 posed to have rotated so that the faults and the strata were given 

 westward dips steeper than those caused by the compression alone. 

 This is the explanation of the rotation observed in the southern sec- 

 tions of Rose's map of the Blairmore area, which has been previously 

 referred to in this paper (39). The steepening of the "soles" as they 

 reach the surface is in accordance with the experimental results of 

 Quirke (30) and of Chamberlin and Miller (10). The last section 

 shows the effects caused by normal faulting consequent on the relaxa- 

 tion of the compressive stresses of the Laramide revolution. 



I realize that an explanation of the complex structural relations 

 of the Rocky mountains can only be of the most tentative sort until 

 our knowledge of the field relations of the region, and our knowledge 

 of theoretical and experimental geological mechanics is more nearly 

 complete. The explanation attempted in this paper will be of value, 



9— D 



