586 



W. L. UGLOW PENEPLAIN IN BRITISH COLUMBIA 



ridges, constituting foothills to the main valley slopes, on either side of 

 the river. In all of the exposures the formation strikes parallel to the 

 trend of that part of the trench in which it lies, but in each case the beds 

 dip 20 to 25 degrees toward the eastern side of the trench. No complete 

 stratigraphic section is exposed from which the thickness of the forma- 

 tion could be determined. The limited evidence which is available, 

 however, points to a thickness of at least 2,000 feet. 



Andesitic and basaltic lavas of Miocene age occur in the lower portions 

 of the valley, chiefly below the 2, 5 00 -foot contour. On Skull Hill, in 

 the vicinity of Louis Creek, the lavas are found between elevations of 



..-^ 



Figure 3. — North TJwmpsoii Trench from Skull Hill 



The view was taken looking south down the trench and shows the side slopes of the 

 present valley and portion of upland plateau. 



2,000 and 3,000 feet, producing a flat, mesa-like top. The erosion rem- 

 nants of these rocks may be seen as rock-defended terraces along both 

 sides of the valley, in many places overlying the sediments of the Cbu 

 Chua formation. The separate flows are nearly horizontal, but locally 

 show dips up to 20 degrees. 



POST-TERTIARY ROCKS 



These are of three types: (1) the glacial drift and erratics wliich 

 mantle the gently undulating uplands beyond the rim of the trench; (2) 

 the giacio-fluvial and Eecent gravels, sands, and silts are now carved 



