JDOWLiNG] EASTERN BELT OF THE CANADIAN CORDILLERAS 183 



Columbia. The deformation of this time includes the elevation and 

 upthrust of the Rocky Mountains. A northern limit for the region 

 affected seems indicated by the available information because the 

 folding and displacement in the Rockies show a decided decrease north- 

 ward, and at the Liard river consist only of an anticlinal fold with 

 Mesozoic rocks on either limb and an arch in the foothills showing 

 Triassic rocks on its crown. Beyond this to the north, uplift and 

 deformation appear to have been of an earlier date. At least a partial 

 uplift occurred before the Cretaceous sea was expelled from the 

 southern basin. 



The Northern Mountains 



North of the sixtieth degree of latitude the mountains do not 

 lie along the western limb of the great geosyncline as in Alberta, 

 but proceeding northward cross it diagonally from west to east and 

 reach to near the eastern edge. Compared with the southern region 

 the thickness of the sediments is reduced, partly by a lessening in 

 the amount of sedimentation during the period between Devonian 

 and early Cretaceous and partly by early denudation. Deposition of 

 Cretaceous sediments was also limited to an early period. A great 

 part of the mountain structure is due to compressive strain and must 

 have formed during intervals characterized by the development of 

 such types of strain. 



The pre-Cietaceous land to the west was of low relief judging 

 from the occurrence of marine Jurasso-Cretaceous measures over such 

 wide areas in British Columbia and Yukon, The earliest intrusions 

 which might have accompanied disturbances affecting the topo- 

 graphy of the time, were the granites of the Coast range, pebbles 

 of which are found in the Lower Cretaceous measures. Intrusions 

 that cut and deform these measures were more widespread and are 

 believed to have occurred previous to the deposition of the Kenai^ 

 series, the supposed equivalent of the early Eocene. 



In the Mackenzie valley the evidence of the age of the period 

 of compression and mountain building is more definite and indicates 

 it as being the close of the Cretaceous. It seems reasonable to sup- 

 pose that the mountain mass extending westward from this valley 

 was formed in the period represented elsewhere by the latest Cre- 

 taceous deposits. The principal evidence of this late Cretaceous 

 date for the deformation is the folded Cretaceous beds, probably not 

 including beds younger than Colorado, and the undisturbed con- 

 dition of the Tertiary sediments abutting against the fault plane 

 ^D. D. Cairnes, Memoir 50, GeoL Surv., Can., p. 115. 



