PHYSIOGRAPHIC EXTENSION OF UNITED STATES 185 
valleys are sharply cut. The longitudinal valleys are broad, of 
gentle slopes—some of them of the basin type—and divide the group 
into several units. The northern or Front Range has an altitude 
of more than 6,000 feet, and in places peaks reach 8,000 feet. The 
altitude of the southern range reaches scarcely 5,000 feet. The 
southern range shows a remarkably even sky-line, which strongly 
suggests that it has been carved from a former plateau." 
Important topographic features that may be used in classifying 
all the groups herein discussed into one large major division of the 
North American continent are: (1) definite delimitation of each 
group on two sides respectively by the Great Plains and Intermon- 
tane Plateaus; (2) a high, bold ‘‘Front Range” separating the 
mountains from the Great Plains; (3) a range of lesser altitude and 
of a dissected plateau type standing behind each of the Front 
Ranges; and (4) a persistent en échelon arrangement of the Front 
Ranges from the Sangre de Cristo Mountains of Colorado to the 
Endicott Mountains of Alaska. 
Within the United States the physiographic history of the 
Front Ranges begins with the post-Laramie deformation. This 
event raised the Jurassic-Cretaceous sediments into positions where - 
they were subject to active erosion. Mature dissection of these 
sediments, and of the igneous bodies which had been intruded into 
them, of varying degrees of hardness and standing in diverse struc- 
tural attitudes, has wrought the present topographic forms. The 
dissected plateau uplands lying west of the Front Ranges are prob- 
ably remnants of a Pliocene or late Tertiary peneplain. 
Schofield? implies a similar history for the Boundary Group, 
and the work of Brooks and Schrader in Alaska indicates a similar 
history for the Endicott Mountains. The Mackenzie group has 
been studied only in reconnaissance, but from what is known of its 
structure and stratigraphy and its relation to adjoining provinces 
it seems that we may be warranted in drawing the tentative deduc- 
tion that it also participated in the events outlined for the United 
States. 
t A. H. Brooks, ibid. 
2S. J. Schofield, Geol. Survey Canada, Guide Book No. o, p. 21. 
[To be continued| 
