PHYSIOGRAPHIC EXTENSION OF UNITED STATES 177 
-A large area of the Interior Plateaus is covered by basaltic 
lava flows of the same age as those of the Columbia Plateau, and 
where these are favorably disposed for observation it may be seen 
that they obscure a former rugged relief.‘ The present topography 
is conditioned, however, by factors other than lava flows and 
anterior relief. It represents in part dissected lava tables, in part 
dissected local peneplains of pre-Miocene age, and in part dissected 
mountain torsos reduced during early Tertiary and Mesozoic times.? 
In general, the topography may be described as a series of gently 
undulating and plateau-like uplands, from 4,000 to 6,000 feet in 
altitude, within which the streams have cut wide and deep valleys. 
A significant feature of the Interior Plateaus, in fact, of the 
entire belt of Intermontane Plateaus, except the Great Basin, is 
the antecedent drainage. The basalt cover of the plateaus, 
originally horizontal, was later deformed, but the deformation was 
not rapid enough to affect seriously the streams that had been laid 
out on nearly flat lava sheets in response to initial slopes, and which 
have maintained their courses to the present day. Where there 
have been great uplifts we now find deep canyons. The Columbia 
River where it crosses the province is typical of this condition, as 
are also the Fraser, Skeena, Nano, Stikine, and Taku rivers.’ 
_ The Interior Plateaus lie below two adjacent and limiting 
provinces and are similar in this respect to the Columbia Plateau 
and the Great Basin. The eastern boundary is not marked by a 
prominent topographic break at any place, but a difference of sur- 
face features may be easily distinguished within a few miles. On 
the west, where the Interior Plateaus are bordered by the Coast 
Range, the boundary of the province is difficult to determine. 
The mountains and plateaus merge insensibly in many places‘ and 
present a case analogous to that of the Cascades and the Columbia 
Plateau in Washington. 
The Interior Plateaus of British Columbia are continued north- 
ward into Yukon Territory and Alaska under the name of Yukon 
tL. Reineke, Geol. Survey Canada, Mus. Bull. rr, Fig. 1 and p. 38. 
2R. A. Daly, Geol. Survey Canada, Guide Book No. 8, Part II, p. 164. 
3 A. C. Spencer, Bull. Geol. Soc. Am., XIV, 125-28. 
4R. G. McConnell, Geol. Survey Canada, Guide Book No. 10, p. 11. 
