Bulletin of the Geological Society of America . pie Trench The present North Thompson River flows through a steep-walledtrench, deeply incised in the upland plateau. The trench varies in widthfrom rim to rim from 2 to 10 miles, with an average width of about 5miles. The bottom of the valley is comparatively even, consisting ofbroad river flats and low benches. Its width varies from one-half mileto 5 miles, with an average of approximately 2 miles. The average slopeof the rocky walls of the valley is about 45 j)er cent, representing a riseof 45 feet per 100 horizontal. The elevation of the upla
Bulletin of the Geological Society of America . pie Trench The present North Thompson River flows through a steep-walledtrench, deeply incised in the upland plateau. The trench varies in widthfrom rim to rim from 2 to 10 miles, with an average width of about 5miles. The bottom of the valley is comparatively even, consisting ofbroad river flats and low benches. Its width varies from one-half mileto 5 miles, with an average of approximately 2 miles. The average slopeof the rocky walls of the valley is about 45 j)er cent, representing a riseof 45 feet per 100 horizontal. The elevation of the upland plateau varies in this district from 3,500to 4,500 feet above sealevel. In general, it is 500 to 600 feet higher onthe east than on the west side of the trench. The North ThompsonRiver has an elevation of 1,320 feet at its junction with the Clearwater TOPOGRAPHY OF THE TRENCH 563 and of 1,120 feet at Kamloops. It flows, therefore, along a gradient of3 feet per mile and is entrenched in the plateau to a depth varying from2,300 to 3,200 FiauRE 1.—Key Mat) of Area studied The upland plateau is characterized by gently undulating ground oflow relief, usually drift-covered and thickly timbered. A few bold peaks 564 ^y. l. ugloav—peneplaix in British Columbia and rocky ridges stand as monadnocks in high rehef above its generallevel. In places where the valley consists of a single depression it is broadand up to 3 miles in width at its base. In other places, as near LouisCreek and Mount Olie, the valley consists of two or more longitudinaltrenches, separated from one another by high ridges, whose tops repre-sent isolated remnants of the plateau. In these cases the river is foundto flow through one of these subsidiary trenches and is consequentlyquite constricted (see figure 2). General Geology of the Trei^ch ^the rock formations The rock formations of the vicinity may be divided into three maingroups: (a) the pre-Tertiary complex, {h) the Tertiary sedimentaryand volcanic
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectgeology, bookyear1890
