6io ROLLIN T. CHAM BERLIN 



From the foregoing inspection of the problem, it would seem 

 clear that this remarkable strip of oblique faults has not been pro- 

 duced by simple torsion alone. The observed facts are at variance 

 in several essential particulars with what the theory of pure torsion 

 should require. The genesis of the faults must have involved 

 in addition other important factors. These are to be sought in 

 an analysis of the larger structural features of the general region, 

 and a consideration of the strains involved in their genesis. 



A study of the map of Montana shows that, while the Rocky 

 Mountain chain loses much of its regularity and individual linear 

 character amidst the scattered mountain groups of west central 

 Montana, there is nevertheless a prominent Rocky Mountain 

 trend line which swings sharply eastward in southern Montana, 

 and thence turns south again in the Big Horn range. This par- 

 ticular line of the Rockies thus follows a sigmoidal curve which is 

 perhaps more conspicuous on a small-scale map than a map of Ja 

 larger scale, for the reason that the details of the minor ranges, if 

 they are too prominent, tend to obscure the larger relations. 

 It was suggested by T. C. Chamberlin that this pronounced bend 

 in the range may have been an important factor in the present 

 problem. The great Lewis overthrust of northern Montana 

 shows that the Glacier National Park region has been transported 

 bodily eastward for at least 15 miles, and possibly much farther.^ 

 Southward from Glacier National Park the easterly overriding 

 upon the thrust plane gradually diminished, but in any case, 

 even where the faulting has given way to folding as the dominant 

 process, the formation of the Rocky Mountain structure involved 

 an eastward movement of the crumpled and faulted materials. 

 To accomplish the crustal shortening involved in the folding, the 

 main mass of the Big Horn Mountains should have moved eastward 

 to some extent, on the assumption that the deformation was due 

 to thrusting from the Pacific. 



The northwestern extremity of the Big Horn anticline, as 

 already stated, extends into the southeastern corner of the area 

 under consideration. Thence westward to the Crazy Mountains 



'M. R. Campbell, "The Glacier National Park," U.S. Geol. Survey, Bull. 600 

 (1914), p. 12. 



