206 



KUomttres at hand. Its habit is somewhat abnormal 

 on account of unusually intense cleaving under 

 tangential stress. Where seen outside this 

 Prairie Hills syncline, the Ross quartzite is 

 more massive. All or nearly all of the formation 

 here exposed is of Beltian age; the younger, 

 Lower Cambrian beds may not appear at 

 this low level in the fold. 



The southwestern limb of the syncline becomes 

 identifiable at a point nearly 2 kilometres beyond 

 the bridge over the river, where the Nakimu 

 limestone with steep northeasterly dip crosses 

 the railway. The train then runs over the 

 Cougar formation with similar dips until, 

 at a point about 2 kilometres beyond Sixmile 

 Creek station, the dip becomes vertical or 

 disordered. At that locality is the axis of the 

 Beaver River anticline, trending N. 30 W. 



Purcell Trench — As the train slowly climbs 

 the steep grade to Bear Creek station, an excel- 

 lent view of the Purcell Trench in its northern, 

 relatively narrow development, is obtained. 

 With remarkable straightness this primary 

 feature of the Cordillera continues 40 kilometres 

 S.S.E., to the head of the Beaver river and 

 then down the Duncan to Duncan lake and 

 the long Kootenay lake. The trench ends 

 at Bonner's Ferry, Idaho, where it is entered 

 by the transverse valley of the Upper Kootenay 

 river. The total length of the trench is about 

 350 kilometres. West of it is the Selkirk 

 Mountain system; east of it the group of ranges 

 to which the inclusive name, Purcell system, 

 has recently been applied. 



Here in its northern part the trench is an 

 erosion trough opened on the axis of a broad 

 anticline which has been demonstrated for a 

 distance of 30 kilometres and probably extends 

 still farther south. At the International Boun- 

 dary, the trench is considerably broader and is an 

 erosion trough located on a longitudinal fault 

 of the first order. Elsewhere, the origin of 

 this depression has not been determined. 



