PHYSIOGRAPHY OF THE BEAVERDELL MAP-AREA. 
13 
ally, the topographic surface of the present upland shows a 
partial relation to the structure of the rocks below, both in the 
drainage pattern, and in the differences in the topography de- 
veloped upon the oldest rocks. 
These characteristics indicate that the upland surface has 
reached the stage of late maturity, or the beginning of old age in 
the geographic cycle. We shall discuss its physiographic develop- 
ment in more detail later on. 
THE VALLEYS. 
From wide and shallow valleys upon the uplands the streams 
flow through narrow canyons to the floors of the large flat- 
bottomed steep-sided valleys. There is, therefore, an upland type, 
a V-shaped canyon type, and a U-shaped, deep trench type of 
valley. The last two types have walls upon which the per- 
centages of slopes are five to ten times that found upon the sides 
of the upland valleys (Figure 2, profiles 1 to 6, and the topo- 
graphic map in pocket). They form breaks on the upland sur- 
face and their floors are often hundreds or even several thousand 
feet below it (Plate I). 
Drainage Pattern . 
The valleys have in certain parts of the Plateaus a marked 
rectilinearity in plan ; that is, they are made up of straight stretches 
and the streams turn sharply from one to the other of these 
stretches. A number of such intersecting straight stretches will 
trend in one direction for a long distance, and then the valley will 
turn at nearly right angles into the next series of “straights.” 
Thus the North Thompson, which occupies a southerly trending 
valley for over 100 miles, joins the South Thompson where its 
valley is trending to the west. Forty miles below their junction 
the South Thompson makes a big turn and trends southerly with 
minor deviations until it joins the Fraser. In detail the longer 
valley stretches of these two rivers are made up of a series of 
straight reaches from a few miles to 20 miles long. 1 
1 Kamloop8 map-sheet, British Columbia. Geol. Surv., Canada. 
