98 THE ROYAL SOCIETY OF CANADA 



The general problem for which a solution is advanced in this 

 paper is the origin of the present structures of the Rocky mountains 

 of Canada south of the Crowsnest pass. A particular part of this 

 problem is the origin of the parallel steep reverse strike faults of the 

 foothill region in Alberta. It was in seeking an explanation for the 

 attitude of these faults that my attention was directed to the more 

 general problem as stated above. . 



Summary 



The southernmost Rocky mountains of Canada are composed 

 of a succession of virtually parallel sedimentary rock formations 

 belonging to the Precambrian, the Palaeozoic, and the Mesozoic eras. 

 The rocks of each of these eras form units that are separated from each 

 other by breaks which are essentially disconformities, though in some 

 places they may be unconformities. From the earliest Precambrian 

 known until at least the beginning of the Tertiary the accumulation 

 of sediments proceeded, without interruption by pronounced deforma- 

 tion, varied from time to time by broad and relatively slight oscilla- 

 tions which caused local erosion and resulted in the disconformities 

 mentioned above. After the deposition of the Fort Union formation, 

 the age of which is either late Cretaceous or early Tertiary, this great 

 mass of strata, 40,000 feet thick, was severely deformed by a thrust 

 acting from the west. This thrust was one of the consequences of 

 the Laramide revolution. These compressive stresses folded, faulted, 

 and overthrust the strata, and the effect of the compression and the 

 relaxation following it has been to leave the rocks in the positions 

 they now occupy. 



The significant structural elements of the region may be described 

 in terms of three areas elongated parallel to the mountains. These 

 are: (1) a western area, with structures characteristic of tension 

 following compression ; (2) a central area with structures characteristic 

 of severe compression, the eastern boundary of which is the great 

 Lewis overthrust; and (3) an eastern area, the cause of the structures 

 of which is not at once apparent. The very steep i;everse strike faults 

 of this area are explained as being due to a rotation of slices of the 

 earth's crust above flat overthrusts or "soles" in a manner analogous 

 to the formation of the more complex but generally similar structures 

 of the Scottish highlands. 



Willis explained the Lewis overthrust as being an erosion thrust 

 consequent on early Tertiary peneplanation following an earlier epi- 

 sode of compression and followed by a later episode, when renewed 

 compression drove the western limb of an anticline over folded terri- 



