AVERAGE REGIONAL SLOPE 



29 



nental ice sheet which removed the soil covering from the upland, 

 carved a few shallow rock basins, and left a thin irregular blanket 

 of drift upon its retreat, but which does not appear to have modified 

 the upland slopes in any essential manner. This old upland sur- 

 face has all the essential characters which are commonly used as 

 criteria to distinguish peneplains, but the average slopes on it, 

 measured from the higher areas or ridges toward main drainage 

 lines, vary from 150 to over 300 feet to the mile, and these slopes 



Fig. 3. — View of a portion of the Interior Plateaus near the Tulameen Valley 



are found on the surface as a whole, not merely on isolated portions 

 of it. 



If the slopes on this land form had not been measured, the 

 dominant discordance of topography and structure and the general 

 evenness of sky line would have caused it to be classed as a peneplain. 

 The degree of slope present, however, led to a search for indications 

 that would point to the fact that in this instance the stage of old 

 age had just begun. It was found that the drainage system upon 

 the upland was apparently related to a system of shear zones in the 

 underlying rocks, and that in some of the areas underlain by certain 

 Tertiary sediments and lavas, topographic form was governed by 



