STRUCTURE 



343 



Date of the Lewis thrust. — On the hypothesis of a single episode of com- 

 pression, from which resulted all the phenomena of folding and thrusting 

 in Cretaceous and Algonkian rocks in the district, the Lewis thrust and 

 the associated structures must be assigned to a date closely following 

 the Laramie deposition. The growth of the Front ranges and the de- 

 velopment of the Blackfoot plain must be placed later, and the expres- 

 sion of the Lewis thrust must be considered subordinate at the surface 

 to these later effects of orogeny and erosion. 



On the other hand, on the hypothesis of two episodes of compression, 

 separated by the Blackfoot cycle, the Lewis thrust must result from the 

 second episode, and falls probably in mid-Tertiary. Its orogenic effects 

 are then dominant in the Front ranges, and the physiographic history is 

 to be read in terms of structure as well as of erosion. 



It is concluded that the data of the Lewis thrust may be placed in 

 either late Cretaceous or mid-Tertiary time, and the principal criteria 

 for determining which date is correct are to be found in the relations of 

 structure to physiography. 



NORTH FORK NORMAL FAULT 



Topographic relations. — The valley of North fork of Flathead river is 

 apparently a structural valley of much greater length than the water- 

 shed which coincides with its southern portion. The form of the valley 

 is long as compared with its width. The North fork has no extended 

 tributaries, but streams from the western side have more extensive basins 

 than those from the eastern, which are notably short. There is marked 

 contrast in the aspect of the mountains, west and east. Those on the west, 

 the Galton range, are broad and massive (distance of figure 2, plate 50) ; 

 they attain heights of 7,000 to 7,500 feet above sea, and they are trav- 

 ersed by open, V-shaped valleys. Those on the east, the Livingston 

 range, are abruptly precipitous, vary in altitude from 8,000 to 10,000 

 feet, and present acute peaks rising from U-shaped valleys. Over the 

 summits of the Galton range the lowland topography of an earlier cycle 

 is easily traced ; in the Livingston range that cycle has not been recog- 

 nized. A long slope extends from the heights of Galton to the valley; 

 abrupt rise from hills of drift characterizes the western spurs of Living- 

 ston. 



Geologic relations. — The geologic relations in a cross-section of North 

 Fork valley are simple. Proceeding from southwest to northeast in the 

 vicinity of Hay and Bowman creeks, one encounters : In Galton range, 

 calcareous argillite, probably of the Siyeh formation, lying with a dip of 



