[MACKENZIE] HISTORICAL AND STRUCTURAL GEOLOGY 115 



vation to the steeply dipping disturbed rocks in the western half is very abrupt. 

 Where exposures are good, especially along the major stream valleys, this change 

 can be seen to occur within a few feet, there being no intermediate zone of gentle 

 folding. 



The disturbed belt of rocks adjacent to the mountains occupying the western 

 third of the region here described is a small part of a structural area from 15 to 

 20 miles wide, lying at the base of the Rocky mountains, which extends at least 

 80 miles southeastward to and beyond Sun river and a much greater distance north- 

 westward across Alberts. Throughout this area the rocks have been intensely 

 folded and faulted by thrust stresses that acted from the southwest. In many 

 places the individual formations are so much crushed and broken that it is impossible 

 to identify them with certainty. The one constant feature in this whole disturbed 

 area is the uniform northwesterly strike of the rocks. Because of this parallelism 

 of strike the more resistant sandstones of the several formations appear as numerous 

 parallel strike ridges, the same formation being repeated within short distances. 



The disturbed belt mentioned by Stebinger as extending into 

 Alberta has been studied in some detail by several workers in that 

 province. This area forms the eastern structural area of this paper. 

 It is to be recalled that the western portion of this belt is certainly 

 the overthrust mass of the Precambrian strata, and that it was 

 formerly covered by this block to a greater extent than is now the 

 ease. Its structures, therefore, are to be considered as underlying 

 the thrust surface, and any hypothesis explaining the mechanics of 

 the overthrusting must give them weighty consideration. 



Date of the Lewis Thrust. — From considerations which were fully 

 discussed above it was concluded that the Laramide revolution took 

 place not earlier than the uppermost Cretaceous nor later than the 

 latest Eocene. The interpretation of the structural history of the 

 region as elaborated in this paper indicates that the Lewis thrust 

 was one of the latest effects of the compressive stresses of the Laramide 

 revolution. Its age can not be fixed precisely from the evidence in 

 hand, but a probability of Eocene date is indicated. That it took 

 place before the deposition of the conglomerate of the Cypress hills 

 seems established beyond doubt. 



In the Rocky mountains of Idaho, Montana, and Wyoming 

 several similar flat overthrusts have been studied, one at least of 

 which, the Heart Mountain thrust, is comparable in its dimensions 

 with the Lewis thrust. The age of these great breaks has been de- 

 termined from stratigraphie evidence which is in general more con- 

 clusive than that on which the age of the Lewis thrust is based. 



In southeastern Idaho, Richards and Mansfield date the Bannock 

 overthrust before the deposition of the lower Eocene Wasatch beds 

 (31). In southwestern Wyoming Veatch dates the Absaroka thrust 

 in the lower Eocene (42). In northern Wyoming Hewett places the 



