178 W. N. THAYER 
Plateau. There is no natural dividing line between these two units, 
and any line that is drawn to separate them must be arbitrary. 
No dividing line is necessary except for convenience in discussion, 
and for this purpose the political boundary between British Colum- 
bia and Yukon Territory is as satisfactory as any other, particularly 
as it practically coincides with the watershed that separates the 
headwaters of the Yukon River from the rivers flowing southward. 
Topographically this division is a dissected plateau. The 
summits are accordant on the whole, though here and. there isolated 
residuary masses rise above the general level.t Structurally the 
area has the form of a “‘broad shallow trough pitching to the north, 
whose axis coinciding with the valley of the Yukon trends northwest 
to the Arctic circle and then bends to the southwest. In other 
words, the trough makes nearly a right-angled bend and pitches 
toward Bering Sea.’” | 
The Yukon Plateau has an altitude of 4,000 to 5,000 feet where 
it is bordered by the Coast Range, falling near the center to about 
3,000 feet, and rising again as it approaches the Rockies. Along 
a part of the southwest margin the plateau abuts almost directly 
against the slopes of high mountain ranges, and so abrupt is this 
change from the smooth, flat summits of the upland to the rugged 
mountains that it is very suggestive of a fault-scarp. 
It is apparent that the Yukon Plateau may be correlated with 
the other members of the Intermontane Plateaus, not only because 
of general similarity of surface features, but also because it lies 
between two mountain provinces. The probable fault-scarp along 
the western border suggests an analogy to that between the Great 
Basin and the Sierra Nevada. In fact, Brooks states specifically 
that the ‘““Yukon Plateau is coextensive (continuous) with the 
plateau of British Columbia and can be regarded as belonging to 
the same physiographic province as the Great Basin.’’ 
Along the remainder of the western border, however, the rela- 
tion between plateau and mountains is more like that existing 
between the Coast Range and the Interior Plateaus or between the 
Cascades and the Columbia Plateau. This is particularly true 
«F. E. Wright, Geol. Survey Canada, Guide Book No. 10, pp. 53, 54- 
2 A. H. Brooks, op. cit., p. 278. 3 A. H. Brooks, op. cit., p. 41. 
