128 
UTAH PENEPLAINS 
unconsumed hills of some former cycle of erosion. Of these Long 
Peak, James Peak, Arapahoe, and other peaks are 2,000 to 2,500 
feet higher than the lofty flats. 
Singularly and unfortunately the Front Range of the Rocky 
Mountains in Colorado is especially notable because of being the 
theme of extended geographic description in which geologic terms 
and geologic features have no essential role. Were it not for the 
fact that in this method of description there lurks great inherent 
dangers of erroneous visualization of the real sequence of geologic 
events the plan is commendable. By completely ignoring the 
geologic aspects of a strictly geologic problem grave misinterpreta¬ 
tion seems inevitable. The genetic relationships of diastrophic 
changes are thus missed. Peneplanation levels are mistaken and 
confused. Danger of wrong deduction is vastly magnified for 
reason of the fact that curiously enough in the Colorado Front 
Range critical criteria for the determination of the questions 
presented are wholly wanting. For decisive testimony we have 
to look beyond the boundaries of the Centennial state. 
The geological age of this flat top is the subject of much specu¬ 
lation. Chronological fixation of the flat crest of the Rocky 
Mountains is really not nearly so simple an affair as has been 
commonly supposed. This summital plain is not to be developed 
out of the finale of the geographic cycle immediately precding the 
present one; nor out of some phase of that cycle. There appear 
to be five distinct and widely separated peneplains with which it 
must be compared before the most probable correlation is deter¬ 
mined. 
The notion that the Summit Plain of the Rockies once extended 
far out at about its present level over the Great Plains but that 
now all vistiges of it there are now lost appears to have no very 
substantial foundation. By faulty analogy with conditions obtain¬ 
ing in other regions the Summit Plain is also regarded as belong¬ 
ing to the cycle immediately preceding the present one. Before 
the last regional uplifting, according to Professor Davis, for in¬ 
stance, ‘‘The surface of the peneplain on the crystalline rocks of 
the mountain area must have been continued by a smoother pene¬ 
plain on the weak rocks of the [Great] Plains area; it is only since 
the uplift of the mountains and Plains together that the Plains 
have been worn down lower than the mountain highland. The 
