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timber. The general arrangement is a belt of varying 

 width and elevation consisting of parallel ridges cut here 

 and there by streams rising in the mountains behind. 



ROCKY MOUNTAINS. 



On the basis both of form and of structure the Rocky 

 Mountain system is divisible into two parts: — a western, 

 and an eastern part. The axial ranges constituting the 

 western part have been carved from a slightly folded 

 but greatly elevated block, the denudation of which was 

 probably inaugurated before the eastern ranges were 

 elevated. The eastern part is made up of monoclinal 

 blocks, the beds of which they are composed being gener- 

 ally younger in age than those of the western part of the 

 mountains. 



Outer Ranges. — As topographic features these ranges 

 are in a general way merely blocks more elevated than 

 those of the foothills, from which nearly all the younger 

 soft series of rocks have been removed, exposing the more 

 consolidated Palaeozoic sediments beneath. The fault 

 blocks are, as a rule, tilted westward, and along their 

 eastern scarped faces remnants are often found of the 

 anticlines which were broken near the crest, showing that 

 these blocks were the western limbs of folds overturned 

 and broken. The plane of the overthrust faults is fre- 

 quently inclined at a comparatively low angle indicating 

 that the thrust was from the west. The outer fault is 

 often of this character, and the overthrust, although 

 great in Montana, becomes modified in the Canadian 

 ranges and decreases northward. In southern Alberta the 

 Palaeozoic rocks of the watershed range on the British Col- 

 umbia boundary line overlap the Cretaceous beds of 

 the western part of the fault block, forming the Livingstone 

 range ; and Crowsnest mountain, which is an erosion remnant 

 of Palaeozoic superposed upon Cretaceous rocks, stands as 

 an example of this overthrust. The westward slopes of 

 of these fault blocks depend to a great extent on the dip 

 of the beds, so that a similarity in outline of their slopes 

 is repeated along the range: The eastern slopes are often 

 more abrupt, and their form depends largely on resistance 

 of the strata to erosion or a disposition of fractures. Local 

 glaciers have, moreover, etched this face into cirques and 

 thereby contributed to the irregularity of the crest line. 



