POSSIBLE EELATIONSHIPS OF OVERTHRUSTS 271 



the supposed!}^ later orogenic forces which produced the mountains 

 farther north. 



In the paper on "Types of Eocky Mountain structure" previously cited 

 the writer has shown that certain Alpine characteristics are present in 

 the mountains of southeastern Idaho. The presence of great overthrusts 

 in the Eocky Mountain region points to additional similarities of struc- 

 ture, but thus far nothing comparable in complexity with the more 

 highly developed Alpine structures — the so-called decke — has there been 

 recognized. 



Thom, in his consideration of conditions in central Montana, seems to 

 assume that the movement of the disturbed parts of the outer earth's 

 shell has all been eastward, but that some parts were more retarded than 

 others. The evidence presented on figure 1 shows that there were at least 

 two well defined directions of movement — an easterly movement, which 

 obtained as far south as the southwest corner of Wyoming and parts of 

 Utah immediately west, and a northerly movement, represented by the 

 zones of thrusting and faulting along the north border of the Uinta 

 Mountains. The change of direction of movement is very abrupt, form- 

 ing nearly a right angle. The faults of the Cottonwood district, which 

 are very complex, appear to occupy a strategic position at the apex of the 

 angle. It is possible that the direction of faulting along the Uintas may 

 be due merely to the resolution of the generally eastward-acting forces 

 which there impinged on an earlier opposing structure. On the other 

 hand, it may indicate that the orogenic forces which built the southern 

 Eockies were differently oriented from those which built the northern 

 division and may have originated under different conditions. 



DEriciEN"CY OF Knov^ledge 



In figure 1 it is apparent that great gaps interrupt the continuity of 

 examined areas. There are thousands of square miles in the northern 

 Eocky Mountain region which have either not been studied at all geolog- 

 ically or in which geologic observations have been made only in a cursory 

 way. This deficiency of knowledge makes it obvious that generalizations 

 regarding overthrusts and other crustal disturbances in the northern 

 Eocky Mountains should be tentative. Generalizations such as those cited 

 above are valuable, however, as working hypotheses. 



