PHYSIOGRAPHY OF THE BEAVERDELL MAP-AREA. 
43 
upland is more closely related to mountains than to peneplains. 
We, therefore, place it as a land form at the stage of late maturity. 
It may be of interest to note here that since in the Beaverdell, 
Tulameen, and Kamloops map-areas there seems to be no evi- 
dence of the existence of more than one land form above the 
deep valleys, one may in those areas apply the term upland to 
that form with more propriety than uplands. On the same 
grounds the term Interior Plateau, first applied to this region 
by Dawson, is, as far as our evidence goes, more appropriate 
than Interior plateaus. 
THE PERIOD FOLLOWING THE GREAT UPLIFT. 
Consequences of the Uplift. — At the beginning of the third 
and last period of physiographic development a change occurred 
in the intensity and manner of attack of the erosive agencies 
upon the land. Instead of the mere wearing down of a mature 
landscape the main streams began cutting canyons in their 
beds. These canyons moved gradually up stream, and sideways 
along tributary streams, and started to destroy the old surface 
(Plate II). The reason for this change in the intensity and 
manner of erosion is ascribed to uplift of the land and the con- 
sequent change in base level of the streams, which caused them 
to cut down and form canyons. A measure of the extent of the 
uplift was obtained in the following manner : the slope of the old 
surface toward the deep valleys of to-day indicates that the deep 
valleys are situated upon the sites of large valleys in the upland 
surface. By projecting these old slopes to meet over the pre- 
sent deep valleys it appears that in certain places their present 
bottoms lie 1,000 feet below the bottoms of the old valleys. This 
was the case in the Beaverdell area, and indicates that that part 
of the plateaus was raised up at least 1,000 feet. While the 
period we are treating of is chiefly concerned with the develop- 
ment of the deep valleys, it must not be forgotten that away 
from the deep canyons and valleys the upland was also being 
worn down. Here erosion acted upon it at about the same 
rate as just before the uplift. But the changes brought about 
on a mature surface like the upland are insignificant in com- 
