CANADIAN AND MONTANA ROCKIES 



311 



Le w/ 5^ thrust Folded and faulted foothi// be/t of Cretaceous and Tert/ory 



~$^^Z~ r\ j. /o ~-t ~j formations 



rau^ — ■ Pe t a/ is not mapped 



* F 1 



Oweetgrass ' 



arch — >- 



KJs Kc 



Kmb 



Kv 



at, 



_H<L 



H' 



J CIs 



Kk Kc Kv 



SCALE IN MILES 

 5 



10 



Fig. 20.7. Cross sections of Foothill structure in northwestern Montana. Upper section, F— F', 

 after Stebinger, 1916. Lower two sections, H— H' and J— J', after Stebinger, 1918. KTsm, 

 St. Mary River formation; Kh, Horsethief sandstone; Kb, Bearpaw shale; Ktm, Two Medicine 



The structure of the two ranges is one of folding and thrust faulting 

 typical of the Canadian Rockies farther south. The cross section of Fig. 

 20.10 illustrates the structure along the highway from miles 375 to 443. 



See Figs. 37.1 and 39.14 and related text for brief discussion of the 

 Mackenzie, Franklin, and Richardson Mountains in the far north. 



FOOTHILL BELT 



Sections R-R', C-C, and D-D', Figs. 20.3 and 20.5, are typical of the 

 folded and faulted foothill belt in Canada. Sections F-F', H-H', and G-C, 

 Figs. 20.4 and 20.7 are examples of the structure of the foothills in north- 

 western Montana. The belt ranges in width from 5 to 25 miles and extends 

 from north central Montana ( southwest of Cutbank, Fig. 20.2 ) northwest- 



formation,- Kmb, Bearpaw and Two Medicine undifferentiated; Kv, Virgelle sandstone; Kc and 

 Keb, Colorado shale; KK, Kootenai formation; KJs shales and sandstones undifferentiated, 

 belonging to Colorado, Kootenai, and Ellis (Upper Jurassic). 



ward to at least the 54th parallel, a distance of 500 miles or more. The foot- 

 hills preserve remnants of early erosion surfaces, and are topographically 

 low and related more to the Great Plains than to the mountains, but inter- 

 nally their structure is complex and reveals a great deal of compressional 

 deformation. They are composed for the most part of the Cretaceous 

 shales which have been easily eroded. The prevalence of the weak shales 

 probably explains the reduction of the belt to one dominated bv low, 

 graded slopes. Only in the cores of a few anticlines are Paleozoic beds ex- 

 posed (section H-H', Fig. 20.7). The most common conception is that 

 small reverse faults of a few hundred feet displacement are numerous, 

 and that these terminate downward in major low-angle thrusts (Hume, 

 1926, 1931; Goodman, 1932). The anticlines and svnclines that exist 



