CENTRAL MONTANA ROCKIES 



355 



BIG 5N0WY MOUNTAINS 



5ca/e in miles 



36 mile 5 



BIG 5N0WY MT5. 



SW 



SHAWMUT 

 ANTICLINE 



CRAZY 

 5YNCL 



NE 



-30,000 

 - 15,000 



feet 



Fig. 23.4. Upper diagram is section across the Big Snowy Mountains, after Reeves, 1931. Lower diagram 

 illustrates the relation of flexures in the surficial sedimentary rocks to deep-seated faults in the Musselshell 

 Valley region of Montana, after Thorn, 1923. 



the Reartooth uplift. See Figs. 23.1 and 23.2. The Cat Creek anticline 

 (monoclinal flexure) extends eastward from the Judith Mountains, and 

 the strata of the steep flank are broken by a series of small faults. It ap- 

 pears that the north block, the Rlood Creek syncline, moved slightly 

 westward in the downward movement. 



The Lake basin fault zone extends east and west of Rillings, is the larg- 

 est of the three, and has a length of over 100 miles. On the west end it 

 cuts the south flank of the Rig Coulee-Hailstone dome, and on the east 

 end it cuts the northward dipping strata from the Rig Horn uplift. For the 

 most part, the southeast side of each fault is downthrown, but there are 

 many exceptions (Hancock, 1918). Along some of the faults, the direction 



of throw changes from one end to the other. In any event, the throw is 

 small. The fault planes are generally inclined 30 to 80 degrees. It would 

 seem that the zone of faults came into existence after the dome and flexure 

 that it cuts and that a deep-seated fault with horizontal movement was the 

 cause. The surficial strata over the horizontallv displaced blocks broke in 

 numerous small tensional faults oriented obliquely to the master fault 

 beneath ( Chamberlin, 1919 ) . As with the Cat Creek zone, the north block 

 moved westward. 



The Nye-Rowler zone consists of a series of anticlines, domes and half 

 domes in perfect alignment for 56 miles, extending from the Reartooth 

 Mountain front to the Pryor Mountains. Dips on the south limb are 



