PHYSIOGRAPHIC EXTENSION OF UNITED STATES 183 
of the boundary, however, they are more rugged, and in places 
peaks attain heights of 7,000 feet.” 
The fourth orographic unit lies between the Selkirk Valley on the 
east and the edge of the Intermontane Plateaus on the west, 
excepting a distance of about 75 miles south of the International 
Boundary, where it abuts against the Cascade Mountains. It 
embraces Daly’s Columbia System, which includes the Colville 
Mountains of Washington. ‘This unit is characterized by compara- 
tively low mountains that commonly show a certain uniformity of 
summit levels. ‘Topographically they are not sharply distinguished 
from the mountains of the Interior Plateaus; however, there are no 
remnant plateaus, and they are to be regarded as a group distinct 
from any association with the mountains of that division.’ 
These four units form a group which writers on the physiography 
of the United States have frequently referred to as the “‘ Northern 
Rockies.” This term fails to convey a proper idea of their relative 
location on the continent, however, and in the absence of a better 
name, as well as for convenience in discussion, they will be referred 
to hereafter in this paper as the ‘Boundary Group.” 
Just north of the sixtieth parallel rise the Mackenzie Mountains, 
the greatest mountain group of Canada. This group consists of 
two ranges—an older range against the eastern edge of which a 
newer or front range has been built up. It has a crescentic axis, 
paralleling the general trend of the Cordilleran provinces, and 
extends from the valley of the Liard River to the valley of the 
Porcupine River. The group has a maximum width of about 300 
miles. ‘There is no well-defined crest line, and it appears to be 
a complex of mountain masses which are the result of deformation 
and erosion following an uplift. 
The topography of the older western range is governed to some 
extent by structure, many of the wider valleys being cut in soft 
strata, and the higher ridges and peaks formed by uptilted hard 
beds. The highest peaks and most-rugged crests are built of granite 
stocks. The surface features in general are those which result 
from long-continued differential erosion acting on an uplifted and 
IBC. Calkins op cits, pi 12: 
20. E. LeRoy, Geol. Survey Canada, Mem. 21, p. 23. 
