504 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



CARIBOU RANGE 



SNAKE RIVER RANGE 



'"o^e'af '^cilea Vr'bo'tory S~ "" "PrlfJFe o7"»c'ised tributary 

 Intrenched Snake River 

 Grand Valley before faulting in upper miocene time 



p«»J!^^ 



First stage or faulting, volcanism and sedimentation 



Second stage of faulting and culmination of deposition 



JnoAe ftiver 



Unio 



„ Pea sorface 



Fig. 31.12. Idealized diagrams showing the late Cenozoic evolution of the Grand Valley 

 trench in Wyoming and Idaho. 



mountain fronts appear surprisingly like the classical Wasatch scarp in 

 Utah. Basin beds are widespread in the large intermontane valleys, and 

 in part were deposited before the faulting and have been displaced by 

 it, but in part are a direct consequence of it. In the erosion that followed 

 the faulting, the basin beds have been stripped away in places from 

 bedrock against which they had been faulted or in other places deposited, 

 and fault-line scarps have formed, as in Fig. 31.12. The basin beds in 

 which fossils have been found are upper Eocene, middle Oligocene, lower 

 Miocene, and uppermost Miocene or lower Pliocene, and have a large 

 tuffaceous and volcanic ash content, and even sills or lava flows in places. 



The Tertiary history is reviewed under the heading "Southwestern Mon- 

 tana," in Chapter 22. See also Fig. 31.14. 



Northwestern Montana, British Columbia, and the Yukon 



The Rocky Mountain Trench of British Columbia is described in Chap- 

 ters 21 and 33. It continues the zones of great trenches to the Yukon and 

 probably to Alaska. 



Fig. 31.13. Relation of late Tertiary faulting to the laramide elements in northwestern Wye 

 ming and eastern Idaho. After Bayless, 1947. 



