270 G. R. MANSFIELD STRUCTURE OF ROCKY MOUNTAINS 



thrust is of later age than Horizon A of the Bridger formation and is 

 therefore not earlier than middle Eocene nor later than Oligocene. 



One fact that appears to be established by reviewing the age relation- 

 ships of the various thrust-faults is that these faults were in all proba- 

 bility not synchronous. This fact indicates that the compressive stresses 

 that produced the overthrusts were maintained over a considerable period 

 and that they 'found relief at more or less distinct, successive intervals — 

 a conclusion to be naturally expected, in view of the greatness of the 

 region affected and of the tremendous stresses involved. 



-&' 



Possible Eelationships of Overthrusts 



In a valuable paper on structural conditions in central and western 

 Montana, Thom^** mentions briefly most of the overthrusts above re- 

 viewed. He thinks that the continuity of the mountain ranges, the 

 relative topographic position of the terranes involved, and the observed 

 magnitude of the horizontal displacements indicate that these faults have 

 a common origin and are parts of a continuous major fracture. He de- 

 picts central Montana as a region where the simple overthrust seen in the 

 Canadian Front Eange gives place to more complex structural conditions 

 probably because the edge of the overthrust sheet there impinged on older 

 east-west uplifts and was disrupted by them. According to Thom's view, 

 waves of deformation and intiTision, which began on the Pacific coast in 

 Upper Jurassic time, affected areas progressively farther east, but were 

 not vigorously active in the Front Eange until the Upper Cretaceous. 



Kober-^ regards the eastern ranges of the Eocky Mountains northward 

 from the Yellowstone National Park as border chains of a great geosyn- 

 cline, which have been thrust forward on a foreland of older and more 

 rigid rocks. His idea of the structure of border chains, derived from a 

 study of the Alps,^^ is that of a highly folded and faulted mass, parts of 

 which have been torn from their foundations, piled up on each other to 

 a greater or less extent, and moved bodily forward toward or on the fore- 

 land. He accounts for the difference in the aspect of the eastern chain 

 in Montana and Colorado by assuming that the mountains in Colorado 

 east of the AYasatch Eange, which he calls the Colorado plateaus and the 

 pre-Cordillera, represent earlier folded and eroded mountains, which had 

 become incorporated in the foreland and were little, if any, affected by 



19 W. T. Thorn, Jr. : The relation of deep-seated faults to the surface structural fea- 

 tures of central Montana. Manuscript submitted for publication to Am. Assoc. Petrol. 

 Geols., November, 1922. 



20 Leopold Kober : Der Bau der Erde. Berlin, 1921, pp. 160-161. 



21 Op. cit., pp. 86-89. 



