CANADIAN AND MONTANA ROCKIES 



.307 



branches as can be seen on the map, Fig. 20.2, and it is not obvious every- 

 where which should carry the name. It is a low-angle thrust. See sections 

 G— C, I-F and K-K'. In general, metamorphosed rocks of Beltian age 

 have been thrust up and over shales and sandstones of Mesozoic age. The 

 Lewis thrust in places is complex, consisting of several closely spaced 

 parallel faults with considerable drag folding. In other places the fault is 

 a single fracture, and the rocks on either side have not been greatly dis- 



turbed. For example, on the north side of Cut Bank Creek Valley, the 

 Altyn limestone of Beltian age appears to rest almost conformably upon 

 relatively uncrushed carbonaceous shale of the Colorado formation of 

 middle Cretaceous age. 



The fault in Glacier National Park and southern Alberta is very well 

 known from the writings of Willis ( 1902), Campbell ( 1914 ) , Daly ( 1912) , 

 Clapp (1932), and Billings (1938). Here the fault has a lower' dip than 



10,000' -, 

 JeoJeve.1 



SCALE tN MILES 



10 



20 



i— 



30 



—i 



p io 



Fig. 20.4. Cross sections in northwestern Montana showing Lewis thrust and related structures. 

 Al to A8, the Beltian formations; Al, Prichard argillite; A2, Altyn siliceous limestone; A3, 

 Appekunny quartzite and argillite; A4, Grinnell argillite; A5, Newland limestone and argillite; 



A6, Spokane argillite and quartzite; A7, Helena argillaceous limestone; A8, Missoula group, 

 chiefly argillites, quartzites, and sandstones; PI, Lower Paleozoic formations; P2, Upper Paleozoic 

 formations; Kl, Lower, Middle and Upper Cretaceous. After Clapp, 1932. 



