NATURAL AREAS AND REGIONS 



151 



In the south it includes the Purcell and 

 the Selkirk Trenches, each of which is 

 threaded by large rivers and lakes 

 North of these are the Cariboo Range, 

 the Peace and the Stikine Mountains. 

 Geologically the Columbia system repre- 

 sents an earlier upheaval and is formed 

 of an entirely different series of rocks 

 than the Rockies. The average height 

 is almost that of the Rockies. 



Within the Rocky Mountain Trench is 

 found the headwaters of most of our im- 

 portant river systems. The Kootenay, 

 Columbia, Fraser; the Parsnip, and 

 Finlay, which unite to form the Peace; 

 and the Kachika and Frances, which 

 form (with the Dease) the Liard, either 

 rise in or flow for some distance through 

 this trench. These streams are mostly 

 glacier-fed and therefore subject to 

 rapid variations in run-off, while their 

 tributaries flowing from the stepper 

 side-valleys are turbulent mountain 

 torrents. These systems, therefore, 

 are not important for log transporta- 

 tion except when diverted and used in 

 conjunction with log flumes. In the 

 lower courses, however, numerous lake 

 expansions become of prime importance 

 for transportation. 



The central regions of the plateau and 

 low mountains, 200 mi. wide, merge into 

 the Columbia and Rocky Mountain sys- 

 tems on the east and Coast Mountains 

 on the west. Originally it was a table- 

 land elevated about 3500 ft. above sea- 

 level, but it has been so dissected and 

 eroded that from the valley-bottoms it 

 now appears mountainous. Much of the 

 soil is composed of disintegrated lava, 

 which once overflowed large areas, and 

 glacial drift. This plateau is largely 

 drained by the central portion of the 

 Fraser River system. The Skeena and 

 Stikine Rivers drain the northern sec- 

 tion, while the south-eastern section is 

 drained by the Okanagan and Kettle 

 Rivers. The characteristic topography 

 is of two types: First, the deep-cut U- 

 shaped valleys with elevation ranging 

 from, 1000 to 2000 ft. above sea-level and 

 occupying one-third of the area; second, 



the rolling upland plateau, 3500 to 5000 

 ft. in elevation, dissected with V-shaped 

 valleys where the rivers debouch from 

 the uplands into the main valleys. The 

 plateau is generally found covered with 

 glacial drift, while the valleys show dis- 

 tinct evidence of glaciation. The fauna 

 and flora of the greater part of this 

 region resembles closely that of the 

 interior of Canada to the eastward and 

 is discussed in connection with that 

 area. (See pp. 288, 293.) 



The coast range forms a barrier between 

 the Interior Plateau and the ocean, 6000 

 to 7000 ft. high and averaging about 100 

 mi. in width. The mountains are com- 

 posed of massive crystalline rocks of 

 an ancient period. Lying to the west of 

 the Mainland is a partially submerged 

 mountain range the Island Range or 

 insular system now represented by an 

 archipelago of islands, of which Van- 

 couver and Queen Charlotte are the 

 largest. Between this system and the 

 Mainland is the deep submerged Coastal 

 Trench. 



The Coastal Trench, like the Rocky 

 Mountain Trench, extends the entire 

 length of the Province. It was formerly 

 a land trench and still appears as such 

 south from Puget Sound. Pre-glacial 

 submergence, however,, sunk the main 

 trench and the lower reaches of its tribu- 

 tary valleys far below sea-level, thus 

 forming that remarkable system of 

 straits and fiords which penetrate into 

 the very heart of the Coast Range and 

 which characterizes the British Colum- 

 bia Coast.. Twelve per cent of produc- 

 tive timber area of the Province, con- 

 taining 65 per cent of the standing 

 timber, is tributary to this trench, which 

 becomes of the greatest economic im- 

 portance because these submerged val- 

 leys form navigable sheltered waterways 

 for the transportation of this timber to 

 mills. Eighty per cent of the timber 

 cut of the Province is derived from tim- 

 ber tributary to the Coast Trench. 



During the glacial period the whole 

 Province, except the highest peaks, was 

 covered by a continuous ice-sheet. The 



