IdowlingI eastern BELT OF THE CANADIAN CORDILLERAS 179 



the slight denudation in evidence in the outer ranges. Therefore, 

 for this southern area of the "Rockies" the assumption is made 

 that previous to the formation of the outer Rocky Mountains the 

 land mass as it rose widened mainly by the successive appearance of 

 great blocks separated by normal faults, and that at about the begin- 

 ning of Cretaceous time one long block appeared which widened the 

 land mass along the western side of the sinking basin and by its 

 gradual rise and straight bounding fracture, preserved the general 

 straight and steep western shore of the basin. 



This block was possibly the last to appear and is now incised, 

 trenched, and slightly folded and on account of its position east of 

 the Rocky Mountain Trench is included in the Rocky Mountains.^ 

 It extends northward from somewhere near Fernie, i.e., between 

 Fernie and Elko, and attains its greatest width west of Castle Moun- 

 tain. It is not definitely shown in the sections of the mountains at 

 Peace river and on the Liard, but at Jasper Pass it maintains its 

 width and is compressed into a broad syncline from the central part 

 of which is carved the mass of Mount Robson. It is not clear that 

 its western edge is separated from the old land mass by a fault, but 

 along its eastern edge the great downthrow of a normal fault is in 

 evidence. It thus appears from these several sections that a dis- 

 tinction can be made in the Rocky Mountains between an eastern 

 terrane consisting of measures in part as young as Cretaceous and 

 showing overthrusting and folding, and a western terrane eroded 

 from a large elevated section of the stratified crust whose deforma- 

 tion outside the intense sculpturing consists of gentle folding parallel 

 to the general mountain structure.^ This gentle folding resulted 

 during a period of compression followed by the intrusion of an alka- 

 line complex of laccolitic nature and a period of normal faulting. 

 The age of this deformation has been stated to be as that of the 

 Laramide revolution, i.e., late Eocene. 



It has been argued that previous to this deformation, the drainage 

 was eastward across the block and that the formation of a drainage 

 system normal to this direction was due to normal faulting in a period 

 of tension in Tertiary time^. It is not very evident that there has been 

 a general period of tension since the overthrusting at the close of the 

 Eocene and there seems no reason to assume that all the great valleys 

 tributary to the Rocky Mountain trench were excavated since that 

 time. It seems just as reasonable to suppose an earlier history for 



^See Fig. 1. 



»J. A. Allan, Field Map area, Memoir .55, p. 207. 



'Schofield, Rocky Mountain trench. Trans. R.S.C., Vol. XIV, Sec. 4, p. 81. 



