UTAH PENEPLAINS 
127 
The present general plains-surface of the Cordilleran tract is 
generally misunderstood. The Great Plains lying east of the 
Rocky Mountains is an area the origin of which is, and long has 
been, variously interpreted. Since the early days of Hayden 
when the flat configuration was thought to be due to the existence 
of vast Tertic lakes from which the waters had been recently 
withdrawn, diametrically opposite explanations find strong sup¬ 
port. Davis’ conception of an aggraded fluvial plain emphasizes 
the constructional phase. Later, the well known leveling tenden¬ 
cies of eolic activities under the stimulus of aridity are found to 
have had potent eflfect in shaping the dominant topography. 
Eolian processes of leveling are both constructive and destructive 
in nature. 
Immediately in front of the Rocky ranges the plain is one of 
rapid denudation. Deflation here is typical. It is conservatively 
estimated that not less than a mile of rock material has been 
removed since the mountains were last elevated. In other por¬ 
tions of the Great Plains there is perhaps equal thickness of re¬ 
cently deposited waste. Consequently the Great Plains surface 
of today finds no representative in any plains phenomena within 
the limits of the Cordillera itself. There the stream beds are the 
only extensions of the Great Plains surface. 
The relief of the Rocky Mountain crest presents some unsus¬ 
pected features. Notwithstanding the fact that the Rocky Moun¬ 
tains in Colorado are broken up into numerous subordinate ranges 
and individually are so notably rugged there appears to be still 
retained over the summit remnants of an old regional plain of 
erosion. So marked a feature of the relief is this highland plain 
that it is especially and widely recognized by dwellers in the 
region. Flat-Top Mountain, near Long Peak, is only one of the 
many local titles which give expression to this peculiar character 
of the topography. 
For a considerable distance in Colorado the continental divide 
is occupied by this summit flat. The latter is often bordered by 
precipitous cliflfs — the high walls of innumerable cirques. It is 
in strong contrast to the steep-sided peaks which are the chief 
characteristic of the region. 
Above the highland plain-level rise numerous monadnocks — the 
