600 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



faulting.f His view, however, gives no explanation of the strictly local char- 

 acter of the Miocene beds. The observed deformation of the clays can be 

 explained by the late Miocene lateral pressure which affected the Cordillera 

 in tbis latitude. * 



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 those of grabens as typical as that of the middle Rhine or that of 

 the Dead Sea. The dating of these faults cannot be made with assurance but 

 the resemblance of the present topographic relations of fault-planes and 

 retreating escarpments to the same relations at the Flathead suggests that in all 

 three instances, the faulting was post-Laramie and pre-Miocene. In the two 

 trenches there is no evidence of their having been floored with Tertiary sedi- 

 ments. The absence of such sediments is most simply, though of course not 

 surely, explained by the hypothesis that the trenches have undergone erosion 

 ever since their formation. Each of them has been much widened and deepened 

 by erosion; considering the hardness of the rocks it is not incredible that all 

 of late Miocene and post-Miocene time has been occupied with that task of 

 excavation. 



We have seen that the Christina lake valley is probably located on a meri- 

 dional fault of considerable, perhaps very great throw. The Pasayten valley 

 may be due to excavation along the outcrop of the relatively weak Pasayten 

 "Volcanic formation, and, if so, may possibly be classed among the subsequent 

 valleys developed by adjustments to soft belts of rock. The Skagit river valley 

 is located on a zone of strong faulting, which at the Forty-ninth Parallel sepa- 

 rates the Hozomeen range from the Skagit range. 



On the other hand, several of the master valleys in the section have no 

 direct explanation in the visible structures of the terranes over which the 

 respective rivers flow. The Columbia river in the Selkirk Valley has been 

 ' superposed/ through a complex of volcanic and stratified rocks, upon the 

 Trail batholith or upon the chaotic Pend D'Oreille schists lying unconform- 

 ably beneath the lavas and younger sediments of the Rossland group. This 

 part of the Columbia valley can, with much confidence, be referred to a post- 

 Laramie and pre-Glacial date; we have no facts compelling a closer dating 

 within that long period. Nor is it yet possible to choose between the hypothesis 

 that the river is here consequent on the relief initiated during the Laramide 

 orogenic revolution, and the hypothesis that it has been located by some subse- 

 quent adjustment to a soft belt in the roof of the Trail batholith. A like problem 

 and a like uncertainty prevail in the case of the great Okanagan valley, a part of 

 which separates the Okanagan range from the Belt of Interior Plateaus. The 

 lower Eraser river valley, as well as the lower Chilliwack valley may be located 

 on the axes of east- west folds which here seem to be unusually common in the 

 Cordillera; the poor rock-expesures do not give data sufficient to establish this 

 view finally. The upper Chilliwack river has been superposed on the Chilli- 

 wack batholith through a cover of folded Paleozoics. 



tB. Willis, ibid., p. 344. 



