582 AV. L. UGLOW PEXEPLAIN IN BRITISH COLUMBIA 



of the beds might reveal some precise information concerning the char- 

 acter and date of the Laramide revolution and Tertiary faulting in the 

 southern interior, special importance was attached to their study, and 

 fairly complete collections of fossil plant remains were obtained. 



The upland surface into which the North Thompson River has cut a 

 valley about 3,000 feet deep is, at least locally, a peneplain. The age of 

 the coal-bearing beds has been definitely determined as Middle or Upper 

 Eocene. The topographical position of the beds at the bottom of the 

 trench establishes the fact that the peneplain was uplifted about 3,000 

 feet at or before the beginning of Eocene time. This uplift in the 

 interior plateau country is believed to have synchronized with the 

 mountain-building of the Laramide revolution. 



In order to make the evidence on which these conclusions are based 

 available and intelligible to the reader, the following outline of the 

 topography and general geology of the trench is presented : 



Location" 



The North Thompson Eiver runs through south-central British Co- 

 lumbia, rising on the eastern side of the Eocky Mountains in the vicinity 

 of Tete Jaune, and flowing in a general southerly direction for a distance 

 of nearly 200 miles to join the South Thompson at Kamloops. From 

 this point the combined rivers flow westerly and then southerly as the 

 Thompson, joining the Eraser at Lytton. 



In this paper only that portion of the North Thompson River which 

 lies within the region of the interior plateaus from its junction with the 

 Clearwater River to its mouth at Kamloops is discussed (figure 1). 



Topography of tpie Trench 



The present North Thompson River flows through a steep-walled 

 trench, deeply incised in the upland plateau. The trench varies in width 

 from rim to rim from 2 to 10 miles, with an average width of about 5 

 miles. The bottom of the valley is comparatively even, consisting of 

 broad river flats and low benches. Its width varies from one-half mile 

 to 5 miles, with an average of approximately 2 miles. The average slope 

 of the rocky walls of the valley is about 45 j)er cent, representing a rise 

 of 45 feet per 100 horizontal. 



The elevation of the upland plateau varies in this district from 3,500 

 to 4,500 feet above sealevel. In general, it is 500 to 600 feet higher on 

 the east than on the west side of the trench. The North Thompson 

 River has an elevation of 1,320 feet at its junction with the Clearwater 



