PHYSIOGRAPHIC EXTENSION OF UNITED STATES 230 
Great Bear Lake the boundary is essentially the contact of Meso- 
zoic and younger sediments on the west with old pre-Cambrian 
rocks on the east. In the Anatuvuk Plateau section, the contact 
of Mesozoic and Tertiary rocks on the south with later Tertiary 
.sediments of the Arctic Coastal Plain on the north may be regarded 
as the boundary line. ‘This line is not easy to recognize everywhere 
in the field. In many places the merging of plain and plateau is 
imperceptible; in other places the coastal plain is wanting and 
the waves of the Arctic Ocean dash against the base of the plateau 
scarp." 
Many of the features of the Great Plains in the northern part 
of the United States owe their origin and character either to base- 
leveling or glacial accumulation, or to both. In brief, the topog- 
raphy may be described as a broad expanse of moderately rolling 
plateau broken here and there by valleys and irregularly dissected 
tracts, and rising westward to the foot of the Rocky Mountains at 
a rate of four or five feet to the mile. Standing above the general 
surface are a few residuals that resisted the Tertiary base-leveling. 
Turtle Mountain near the boundary of North Dakota and Manitoba 
is typical of these. Glacial moraines with their accompanying 
bowlder ridges, lakes, and ponds are characteristic. 
. Approximately the same conditions are continued northward 
into Canada to about the fifty-fourth parallel. The topography 
of this section is also irregular and rolling. The plateau surface 
rises from 2,000 or 2,500 feet at the eastern escarpment to about 
4,000 feet, where it meets the mountains on the west. Residual 
masses, Such as the Cypress Hills and Wood Mountain, represent 
the resistant and unreduced portions of a once higher plain.* The 
numerous ponds and lakes scattered over the surface are glacial 
features. 
The Anatuvuk Plateau of Alaska is also a dissected peneplain 
(Eocene or Miocene) which has been elevated to an altitude of 
about 2,500 feet at the northern base of the Endicott Mountains, 
from which elevation it descends gradually to about 800 feet, 
where it merges with, or overlooks, the Arctic Coastal Plain. 
1 A. H. Brooks, Joc. cit. 
2C. A. Young, op. cit., p. 107. 3 A. H. Brooks, op. cit., pp. 279,.280. 
