THE STRUCTURAL RELATION OF THE PURCELL RANGE 

 AND THE ROCKY MOUNTAINS OF CANADA 



FRANCIS PARKER SHEPARD 

 University of Chicago 



It has been found repeatedly, in recent years, that on the 

 flanks of many elongated mountain ranges there are thrust faults 

 dipping in under the range. In the absence of these faults, over- 

 turned folds, with their axial planes dipping into the range, and 

 indipping slaty cleavage are common. Partly from observation of 

 this sort. Dr. R. T. Chamberlin developed his hypothesis of the 

 wedge-shaped zones of deformation in mountain building.^ With 

 this idea in mind, I attempted to determine whether the Rocky 

 Mountain trench, a long, narrow depression between two mountain 

 ranges, would show a connection with this type of structure. 



This great valley has a length of about 800 miles. It starts 

 somewhat south of the boundary line between the United States 

 and Canada and runs in almost a straight hne to the northwest 

 between the Rocky Mountains on the east and a succession of 

 ranges on the west. From what is known about the trench it 

 has been estimated to have an average width from wall to wall 

 of 5 miles. The portion of the trench which I studied was from 

 Gateway, Montana, at the boundary line, to Golden, British Colum- 

 bia, 180 miles to the north. Study of the trench has been hitherto 

 quite Hmited. Dr. Daly's section at the 49th parallel crosses the 

 trench. Dr. Schofield's " Cranbrook Area "^ borders the west side of 

 it for about 60 miles, and the section along the main hne of the 

 Canadian Pacific across the Cordillera, made by Allen and Daly,'' 

 crosses it about 200 miles north of the boundary. Others have 

 made rapid reconnaissances of the trench. 



^ R. T. Chamberlin, Jour. Geol., Vol. XVIII (1910), p. 228. 

 " R. A. Daly, Geol. Siirv. Can. Mem. 38 (1912). 

 3 S. J. Schofield, Geol. Surv. Can. Mem. 65 (19 15). 

 ^Guidebook No. 8, Congres Geologique International, 19 13. 



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