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STRUCTURAL GEOLOGY OF NORTH AMERICA 



the Coast Mountains: on the north it joins Yukon Plateau between Atlin and 

 Teslin Lakes, and elsewhere is bounded by the northerly extensions of Babine 

 and Bulkley Mountains or by Omineca and Cassiar Mountains. Its gently undu- 

 lating surface averages 4,000 feet or more above sea-level, and is dissected into 

 a number of smaller plateaux by the larger stream and river valleys. 



The Interior Plateau stretches from Bulkley, Babine, and Omineca Moun- 

 tains approximately 500 miles southeasterly to the International Boundary. At 

 its north end it extends from the Coast Mountains 200 miles east to the Rocky 

 Mountains. Toward the south it becomes progressively restricted by the Cascade 

 Mountains, on the west, and by Cariboo and Monashee Mountains, on the east, 

 and at the Boundary near Okanagan and Ketde Bivers is less than 50 miles 

 wide. This great plateau region, with a general elevation of 3,000 to 4,000 feet 

 is composed of a succession of plateau surfaces interrupted by the deeply cut 

 valleys of a drainage system whose main channels lie 1,000 feet or more below 

 the remnants of the upland surface. 



Cariboo, Monashee, and Purcell Mountains form a mountain group within a 

 triangular area between the Interior Plateau on the west and the Bocky Moun- 

 tain Trench on the east; the apex is in the big bend of Fraser Biver, and the 

 base at the International Boundary. The various members of the group are sepa- 

 rated by deep valleys or trenches trending northward and northwestward. Sel- 

 kirk Mountains are exceedingly rugged, with summits rising to elevations of 

 11,000 feet and more above sea-level. 



Eastern System 



The eastern system, or the Canadian and Montana Rockies, is the sub- 

 ject of the present chapter because it is basically Laramide in origin. It 



. . . includes Bichardson, Mackenzie, Franklin, and Bocky Mountains, and 

 intervening plateau and plain areas. 



In British Columbia the eastern and central systems are separated by the 

 Bocky Mountain Trench, a great trough that extends northwesterly from the 

 International Boundary nearly to the southern boundary of Yukon, and includes 

 aligned parts of Kootenay, Columbia, Fraser, Parsnip, and Finlay Bivers. The 

 boundary between these systems is less well defined beyond the northern end 

 of the trench; it enters Yukon near longitude 126 degrees, extends northerly 

 into Northwest Territories, and swings to the northwest between Selwyn Moun- 

 tains on the southwest and Mackenzie Mountains on the northeast to re-enter 

 Yukon near latitude 65 degrees, and thence proceeds northwesterly on a sinuous 

 course to pass west of Bichardson Mountains and enter Alaska near latitude 69 

 degrees. 



Bichardson Mountains form a straight wall 175 miles long extending northerly 

 from Peel Biver near longitude 136 degrees nearly to the Arctic coastal plain 

 west of Mackenzie Biver delta. In the north they are more than 40 miles wide, 

 and contain rugged, northerly trending asymmetrical ridges with peaks rising to 



heights of 5,000 feet or more. Throughout most of their length, however, they 

 comprise a much narrower belt of steep-sided ridges, the flat tops of which lie 

 mainly below 4,000 feet. No cirques or other evidence of alpine glaciation has 

 been found in aerial photographs of even the highest peaks. 



Mackenzie Mountains occupy a broad crescentic area, convex towards the 

 northeast, stretching 425 miles southeasterly from south of Peel Biver near 

 longitude 134 degrees nearly to Liard Biver at latitude 61 degrees. Their maxi- 

 mum width exceeds 100 miles. They are distinguished from Selwyn Mountains, 

 which adjoin them on the southwest, not by any abrupt topographic boundary, 

 but by absence of intrusions, conspicuous stratification, and more youthful to- 

 pography. On the north and northeast they rise abrupdy from the Mackenzie 

 Biver lowland. In the main they comprise a compact mass of conspicuously 

 layered, northwesterly trending ridges topped by peaks that commonly rise to 

 elevations of more than 7,000 feet, and in some places are reported to exceed 

 elevations of 9,000 to 10,000 feet. Small alpine glaciers are widespread. The 

 Canyon Banges, which form their northeastern front and occupy a belt up to 40 

 miles wide, include more subdued mountains and high plateau areas traversed 

 by deeply incised river valleys. 



Peel Plateau is a great triangular terrace occupying the angle between the 

 east front of Bichardson Mountains and the north front of Mackenzie Moun- 

 tains. Its northeastern edge is in part a scarp rising 200 to 1,000 feet above the 

 Plains region. The major rivers traversing the plateau, such as the Peel and 

 Arctic Bed, are deeply entrenched in the otherwise rather flat, glaciated upland 

 surface. 



Throughout most of their length Franklin Mountains lie a short distance east 

 of and parallel with Mackenzie River. They extend from Fort Good Hope more 

 than 400 miles southeasterly to the mouth of South Nahanni Biver and average 

 less than 30 miles wide. They include, from north to south, Norman, Franklin, 

 Camsell, and Nahanni Banges, each comprising a number of parallel north to 

 northwesterly trending ridges. In places they reach heights of 5,000 feet. 



The Bocky Mountains form the eastern front of the Cordilleran region in 

 British Columbia. Here they rise sharply from the comparatively flat Plains 

 region, through a Foothills belt, to peaks reaching elevations of 10,000 to nearly 

 13,000 feet. These mountains, with their eastern foothills, have a maximum 

 width of about 100 miles, and extend from the International Boundary at longi- 

 tude 114 degrees 850 miles northwesterly to Liard Biver. At their northwest 

 end, they are separated from Selwyn and Mackenzie Mountains by a distance 

 of more than 1O0 miles. They have been carved from a thick series of sedimen- 

 tary strata of rather simple structure, and the resulting layering, visible from 

 great distances, at once distinguishes them from most other mountains in British 

 Columbia. They consist of a series of overlapping ranges that trend northwest 

 and, on the whole, have precipitous eastern faces and much less steep western 

 slopes. Individual ranges are broken or terminated by deep cross-valleys, and 

 the whole mountain mass is crossed by several deep depressions having com- 

 paratively low heights at the divides (Lord et al., 1947). 



