78 THE ROYAL SOCIETY OF CANADA 



"It thus appears likely that the trench is floored by Upper 

 Cambrian and Ordovician sediments, which have been faulted into 

 contact with the Beltian Cougar formation (See structural section)." 



The structure along the Rocky Mountain trench in the vicinity 

 of Golden (Figure 1) is described by Daly^ in the following words: 



"The origin of the Rocky Mountain trench is certainly connected 

 with the great strike-fault separating the Beltian sediments from the 

 Upper Cambrian-Ordovician beds of the Columbia valley (Map No. 

 1450 and Structural section). The sharp contact of these two sets of 

 rocks can only be explained by a fault, with a vertical displacement of 

 about 20,000 feet. Probably the contact has not been established by 

 overthrust. No trace of a thrust plane could be found at or near the 

 contact. In the Beaverfoot range, across the trench, the Palaeozoic 

 (Ordovician or Silurian) strata are strongly overturned to the south- 

 west, that is, in the direction opposite to that expected if the Beltian 

 rocks of the Purcells had been driven upward and eastward over the 

 Cambrian-Ordovician beds of the trench.^ 



"It seems more likely that the master fault of the trench is normal, 

 with downthrow on the northeast. 



"A similar relation between the rocks of the Purcell range and 

 those of the trench has been established at the 49th parallel, where 

 Devonian limestones on the east are dropped down into contact with 

 pre-Upper Cambrian volcanics and metargillites on the west.' Scho- 

 field has recently determined that the same zone of faulting extends 

 at least 35 miles to the north-northwest, where the Lower Carbonifer- 

 ous limestone on the east contacts with the Cambrian ( ?) Gateway 

 formation on the west.* 



"Thus, for at least 250 miles of its length, the Rocky Mountain 

 trench is probably located on a continuous zone of faulting, which is 

 characterized by very great upthrow on the southwest. 



"However, it may well be that the trench displacement is not 

 merely that due to normal faulting. This greatest known trench is 

 nearly 1,000 miles in length, stretching from Montana to Yukon 

 Territory, and it lies in rocks of many different ages, though keeping 

 its Cordilleran trend throughout. So far as known it is nowhere a 

 typical graben, with equivalent formations left at the same level on 

 the sides (two) of the fault-zone. Finally, the trench is very narrow 



1 Daly, R. A., Geol. Surv., Can., Mem. 68, 1915, p. 113. 



^ See J. A. Allan, Guide-book No. 8, Congrès Géologique International, Ottawa, 

 1913, showing structure section across the Rocky Mountains. 



3 Geol. Surv., Can., Mem. No. 38, 1912, p. 118. 



* Schofield, S. J., Guide-book No. 9, Congrès Géologique International, Ottawa, 

 1913, p. 53. 



