2 GEORGE V. SESSIONAL PAPER No. 25a A. 1912 



CHAPTER XXII. 



PHYSIOGRAPHIC NOTES ON THE FORTY-NINTH PARALLEL 



SECTION. 



A general account of the physiography of the Cordillera in the vicinity 

 of the International Boundary, even if it embraced merely the facts now in 

 hand, would alone occupy a stout volume. The present report would grow out 

 of its intended proportion if such additional matter should enter it. In the 

 present chapter it has seemed better to restrict the physiographic treatment 

 of the region to a brief discussion of some of the observations made in the field, 

 together with an equally brief note on the general theory of the topographic 

 development in the Cordillera. Restricted in scope as this chapter is intended 

 to be, it does not cover, except in the most incidental way, the climatological 

 side of the physiography. The following notes relate essentially to the geo- 

 morphology, that is, to the genetic discussion of the land-forms encountered 

 in the traverse across the mountain chain. 



Origin of the Master Valleys. 



In the chapters dealing with the bed-rock geology we have seen that the 

 subdivision of the Cordillera on a purely topographic basis is to some extent 

 supported by the structural geology determined along the Forty-ninth Parallel. 

 This support consists in the fact that the topographic subdivision is partly a 

 genetic subdivision and perhaps so far a final one. 



West of the eastern faces of the Clarke and Lewis ranges, where the ragged 

 escarpments on the blocks overlying the great thrust-planes are slowly retreat- 

 ing, the first principal valley is that occupied by Waterton lake and Mineral 

 creek in Montana. As stated by Willis, this valley seems to be constructional 

 in the sense that it occupies the floor of the wide syncline forming the twin 

 Clarke and Lewis ranges.* The two ranges are separated by this valley. 



The Clarke and MacDonald ranges are separated by the wide Flathead 

 valley which is a ' graben,' or trough bounded on both sides by normal faults. 

 Since the Kishenehn formation is, so far as known, rigorously confined to the 

 trough, the principal faulting is to be referred to early Miocene or pre-Miocene 

 time. Ever since, the graben seems to have existed as an actual topographic 

 depression, though some deformation of the fresh-water beds flooring the 

 trough has taken place in late Miocene or still later time. This latter fact has 

 led Willis to suggest a Miocene or possibly Pliocene date for the principal 



* B. Willis, Bull. Geol. Soc, America, Vol. 13, 1902, page 347 and Plate 53. 



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