Section IV, 1919 [23] Trans. R.S.C. 



The Origin of the Purcell Trench, British Columbia 

 (Kootenay Lake Valley)^ 



By Stuart J. Schofield 



Presented by William McInnes, P\R.S.C. 



(Read May Meeting, 1919.) 



Summary 



1. Orogenic movements affected the Selkirk range in southern 

 British Columbia for the first time in the late Jurassic period. 



2. The first granodiorite intrusion of the West Kootenay (Nelson) 

 batholith took place in the late Jurassic period. 



3. This batholith was unroofed in the Upper Blairmore times. 



4. The upland surface of the present Selkirk range was formed 

 by erosion during the period from late Jurassic to early Tertiary. 

 This period of erosion was brought to a close by the Laramide orogenic 

 movements which built the Rocky Mountains and raised the upland 

 surface of the Selkirk range to its present height. 



5. The Purcell trench and similar valleys of the Selkirk range 

 were cut into this upland surface during the Tertiary and Quaternary 

 periods and hence are antecedent river valleys. 



Geography 



Everyone who crosses the Canadian Cordillera by way of the 

 Crow's Nest pass or the southern route is struck with the great longi- 

 tudinal valleys or trenches, not only on account of their beauty but 

 also on account of their even continuity to the north and south. 

 From east to west the trenches may be listed as follows, the Rocky 

 Mountain trench (Kootenay River Valley) separating the Rocky 

 Mountains on the east from the Selkirk mountains on the west, the 

 Purcell trench (Kootenay Lake valley), the Selkirk valley (Columbia 

 River valley), the Okanagan Lake valley and the Fraser River valley 

 (Plate I). 



The Purcell trench is situated in the Selkirk mountains^ and crosses 

 the International boundary line, longitude 116 degrees 30 minutes, 

 (Plate II). It extends from this point in a northwesterly direction 



1 Published by permission of the Director of the Geological Survey of Canada. 

 ^ Nomenclature of the Mountains of Western Canada. Geographic Board of 

 Canada, April 2 1918. 



