PHYSIOGRAPHY OF THE BEAVERDELL MAP-AREA. 
37 
southwestern and south-central part of the plateaus, we shall 
extend our conclusions to cover all of the southern portion of the 
Interior plateaus . 1 
The development of the physiography may be conveniently 
divided into three periods which are separated by two rather 
obvious change* in topographic form. The first is the period 
preceding the laying down of the Tertiary lavas; the second 
period begins at the time the lavas were erupted and ends at the 
beginning of the great uplift which caused the cutting of the 
present deep valleys; the last period is concerned with the 
physiographic development following the uplift. The first 
and second periods are separated by the eruption of the lavas. 
A break in the geographic cycle is suggested there, because of 
the difference in the topography upon, and off, lava areas. 
The second and third are separated by the uplift which formed 
the valleys. We shall for convenience refer to this as the 
“great uplift.” The forms developed in the second and third 
periods are generally easily separable by difference of slopes. 
THE PERIOD PRECEDING THE ERUPTION OF TERTIARY LAVAS. 
The evidence relating to the land forms existing in this 
period is rather fragmentary. We have some evidence, how- 
ever, which indicates the type of land surface that existed in 
the Beaverdell area at the beginning of Oligocene .time, and also 
just before the eruption of the lavas. 
Deposits of conglomerates, agglomerates, sandstones, and 
fine-grained tuffs occur in the southern part of the Beaverdell 
quadrangle. They are of Oligocene age, and have the character- 
istic structure and make up of a continental deposit of the Pied- 
mont type. The association of coarse conglomerates and fine- 
grained, thin bedded tuffs suggests the presence of steep moun- 
tain valleys, lakes, and nearby volcanic cones. We may, there- 
fore, infer that the topography at that time was mountainous. 
X A comparison of the Beaverdell, Tulameen, Kamloops and Shuts wap 
map-areas has been made in the regional description of the Interior plateaus, 
pp. 6 to 15. According to all available information the portions of the plateau 
lying between these widely scattered areas do not differ essentially from them. 
