Section IV, 1922 [175] Trans. R.S.C 



The Eastern Belt of the Canadian Cordilleras,'^ An Inquiry into the 



Age of the Deformation 



By D. B. DowLiNG, D.Sc, F.R.S.C. 



(Read May Meeting, 1922) 



Introduction 



The threefold division of the Canadian Cordilleras into Eastern, 

 Central, and Western Belts, is based mainly on topographical charac- 

 teristics, though age and structure have left an impress that is in 

 general harmony with the division as made. The Eastern belt was 

 the last to be formed and in the main is the most rugged, hence the 

 collective name "the Rockies". Its structural features are unlike 

 those of the older ranges to the west, being predominantly the pro- 

 duct of great crustal compression which produced not only close 

 folds but also great overthrust faults giving rise to a type of structure 

 that has acquired the designation Rocky Mountain structure. The 

 eastern part of the Eastern Belt has this Rocky Mountain structure 

 which thus associates three unit members into the Rocky Mountain 

 system, viz., the Rocky Mountains, Mackenzie Mountains, and Franklin 

 Mountains, each with a similar structure but probably not a common 

 history. These mountains although rudely aligned are not directly 

 connected except by the older ranges against which they rest. 



The Rocky Mountains consist of a well aligned and almost 

 parallel series of ranges which have their maximum width at the 

 western border of central Alberta, but become much reduced and 

 practically die out before reaching as far north as the northern 

 boundary of that Province. The Mackenzie Mountains, which are 

 much wider and have a curving outline, consist of somewhat irregularly 

 folded masses flanked on the east by approximately parallel ranges 

 of folds and overthrusts which extend far to the east of the general 

 alignment of the Rocky Mountains. The Franklin Mountains are 

 a minor range of folds in front of the Mackenzie Mountains. Struc- 

 turally they are genetically related to the Mackenzie Mountains 

 and may be merely an early expression of the forces which later built 

 the mountain masses to the west. 



The formation of the above series of mountains is generally 

 supposed to have taken place during the latest period of Canadian 

 mountain building, when the interior of the continent was raised 

 ^Published by permission of the Director, Geological Survey, Ottawa. 



