GEOLOGY OF THE TRENCH 



567 



into a series of flats and terraces clinging to the sides of the main valley 

 and its tributaries to a height of 1,000 feet above the bottom, and (3) 

 Recent lava flows of small ascertained extent, covering some of the Recent 

 valley fill in U-shaped glacial tributary trenches. 



Cretaceous Peneplaix 



AYhen one views the topography in the vicinity of the trench from an 

 altitude of 4,000 or 4,500 feet, he is struck by the fact that he is stand- 

 ing on a gently undulating elevated plain, into which the North Thomp- 



FiGURK 4. — 2\ort]i Thompson River at Ch:i Chua 



'L'he view was taken looking east across the river. The foothills on the east side of the 

 river are underlain by early Eocene sediments. 



son and its tributary streams have incised deep youthful channels. From 

 such an elevated point in the vicinity of Chu Chua, for example, a view 

 down the valley to the south reveals four important physiographic fea- 

 tures : (1) a dissected upland plain, (2) several steep rocky monad- 

 nocks, (3) the deep youthful valleys, and (4) a very broad, shallow, 

 elongated depression of the upland surface, paralleling the course of the 

 North Thompson Valley, intermediate in position between the top of the 

 present river valley and the general level of the undulating plain. This 

 depression varies from 15 to 25 miles in width, and its axis lies along 

 the course of the present North Thompson River. It represents the 



