[schofif.ld] origin OF THE PURCELL TRENCH 25 



Quaternary Till, unconsolidated gravels and 



sands. 



Unconformity 



Jurassic West Kootenay Batholith Granite. 



(Nelson) 



Intrusive Contact 



Carboniferous .Slocan Series (fossiliferous). 



Carboniferous or 



Pre-Carboniferous Ainsworth Series Mainly quartzites, argillaceous 



quartzites, limestones and their 

 metamorphosed equivalents. 



Structure 



Viewing the geology of the whole trench as shown on the accom- 

 panying geologic map (Plate III), it will be seen that the sedimentary 

 series forms a huge, almost symmetrical bow, the apex of the bow 

 being to the east. The general dip is almost everywhere about 45 

 degrees towards the inside of the bow so that the rocks in the northern 

 part of the trench strike northwest with a dip to the southward, 

 bending gradually until the middle of the trench is reached where the 

 strike is north and south with a dip to the west, and finally in the 

 southern part of the trench where the strike is southwest with a dip 

 to the northwest. The granite masses, the eastern fringe of the west 

 Kootenay (Nelson) batholith, have had no effect on the general 

 strike of the sedimentary series. Hence, it is seen that the Purcell 

 trench is carved into the terrane with no reference to its structure. 



Origin 



The origin of the Purcell trench has been described by Daly^ in 

 the following words: "The Rocky Mountain trench and the Purcell 

 trench are likewise located on zones of profound faulting; in each case 

 the constructional profiles may have been grabens as typical as that 

 of the middle Rhine or that of the Dead Sea." The geology of the 

 Purcell trench in the neighbourhood of the International Boundary 

 line was carefully examined by the writer in 1913, and it was found 

 that the faults marked by Daly on the geological map as occurring 

 on each side of the Purcell trench were not present and that the 

 valley in this locality was not a graben^. The area was again examined 

 in 1915 and these results were confirmed. In 1916, Drysdale^ made 



' Daly, R. A., Geol. Surv., Can., Mem. 38, pt. 2, 1912, p. 600. 

 Schofield, S. J., Geol. Surv., Can., Sum. Rept., 1914, p. 41, Geol. Surv., Can., 

 Mem. 76, 1915, p. 168. 



3 Drysdale, C. VV., Geol. Surv., Can., Sum. Rept. 1916, p. 61. 



Sec. IV. Sig. 3 



