[schofield] rocky MOUNTAIN TRENCH 75 



a zone of combined faulting and down flexure. In all probability the 

 faulting has had the dominant control in locating the trench." 



Daly^ gives the following details: "At the western edge of the 

 drift covered Tobacco Plains (Rocky Mountain trench) a block of 

 fossiliferous Devonian limestone has been faulted down into contact 

 with the Gateway formation. On the west and south the limestone is 

 covered by drift and alluvium. The main fault which limits the block 

 on the east can be sharply located, the strikes of the limestone and the 

 Gateway formation being nearly at right angles to each other. This 

 fault is marked on the map sheet where it will be seen to run roughly 

 parallel to the other faults that are responsible for the local graben 

 character of the Rocky Mountain trench. The limestone itself is 

 affected by numerous minor slips so that it is impossible to obtain its 

 thickness; in general the block is monoclinal with an average north- 

 easterly dip of about 45 degrees." 



No fault exists in the trench itself at its western side as described 

 by Daly, but the Devonian limestone overlaps the Purcell series. 

 This contact can be seen on the low range of hills which lies immedi- 

 ately west of the trench between the Rocky Mountain trench and 

 the valley of Gold Creek. The valley of Gold Creek which parallels 

 the trench is located along a zone of faulting. The throw which con- 

 sists only of a few hundred feet has the down-throw on the east side. 

 Hence the block between the fault on Gold Creek and that at the 

 base of the eastern flank of the Rocky Mountain trench acted as a 

 unit in the earth movements which initiated the trench. The block 

 consists of a long, narrow mass which has been tilted on a longitudinal 

 axis which was nearer the western border of the block so that the 

 inclination or dip of the block is to the east. Hence the normal fault 

 of large throw is on the eastern side of the block and the small reverse 

 fault is on the western side of the block in the valley of Gold Creek. 



2. At Bull River. — The stratigraphie series of the Rocky Moun- 

 tain trench at Bull River is the same as that at the 49th parallel. 

 Dawson^ makes the first reference to faulting in the trench (Fig. 1) 

 in the following words : "On the lower part of Bull River near the 

 banks of the Kootenay to the south and elsewhere, rocks of the lime- 

 stone series occur. The area affording these exposures is outlined in a 

 general way on the map and is supposed to be bounded on the east 

 by an extensive fault which must run near the base of the moun- 

 tains." 



1 Daly, R. A., Geol. Surv., Can., Mem. 38, 1912, p. 112. 



2 Dawson, G. M., Geol. Surv., Can., Ann. Rept., 1885, p. 190B. 



