CANADIAN AND MONTANA ROCKIES 



317 



mountains west of the trench and an Eocene age for the Rockies east of 

 i it. The Rocky Mountain trench is probably still younger and of mid- or 

 late Cenozoic age. 



THE ROCKY MOUNTAIN TRENCH 



In British Columbia. As previously indicated, a deep, wide valley 

 separates the opposing Canadian Rockies on the east from the Selkirk 

 system of ranges on the west in southern British Columbia. The Dogtooth, 

 Purcell, and McGillivray ranges ( see sections B-B' and K-K', Fig. 20.3 ) 

 are parts of the Selkirk system that flanks the valley on the west, and the 



; Van Horn, Brisco, and Galton ranges are examples of the Rocky Mountain 

 system on the east. The great valley is so regular and continuous that it 



: was called the Rocky Mountain trench by Daly. 



It does not have a continuous downhill gradient, but within the trench 

 are low divides that separate courses of several great rivers. The Ketchika 

 River drains the trench northward from latitude 58° into the Liard River. 



i South of latitude 58°, the Finlay River drains the trench into the Peace 

 River which flows eastward through great canyons in the Rockies. The 



! Parsnip River is a tributary of the Peace that extends southward nearly 



, to the 54th parallel. The Frazer River occupies the trench from 54 to 53 

 N. Lat, and then the Columbia and its tributaries flow in the trench 



: nearby to the international border. 



Except for about 60 miles between the big bend of the Frazer River 

 and latitude 55° the trench is sharply or fairly sharply defined from 

 Kalispell, Montana, to beyond latitude 58°, nearly to the Yukon border, a 

 distance of over 900 miles. 



The Rocky Mountain trench lies at the boundary approximately be- 

 tween the Nevadan and Laramide orogenic belts. According to Bostock 

 etal. (1957): 



Throughout most of its length it forms the approximate boundary between 

 intensely deformed, altered, and intruded rocks characteristic of the western 

 Cordillera, and the moderately deformed and comparatively unmetamorphosed 

 strata that typify the eastern Cordillera. However, the trench does not every- 

 where coincide with this geological boundary; in several places it obliquely 

 transects structures on both sides and, south of about latitude 50 degrees, the 

 geological boundary lies east of the trench. North of this latitude the trench 

 is known in several places to be the locus of extensive faulting and may have 



^BRITISH COLUMBI 

 IDAHO 



CRETACEOUS (') 



I 1 I BATHOUTHS '• \ N 



DEVONIAN AND LATER '• 'I 



PRE ■ DEVONIAN 



WINDER MERE 



4 TOBY HORSC THltF C8£l K 



PURCELL (BELT) 

 UPPER 



KlTCMtNSIt. S/V£W WALLACC. 

 DUTCH C««. MISSOULA. £TC. 



10WE 



m 

 m 



LOWER 



CfteSTON. BAVALUCTC 



ALDMDGt, PRITCHARO 



FAULT cW-w. 



Fig. 20.14. Geology on either side of the southern part of the Rocky Mountain trench. 

 Reproduced from Leech, 1959. Stippled zone is trench. 



