UTAH PENEPLAINS 
129 
heights of the mountains over the Plains is therefore not so much 
the result of the local uplift of the mountains as of the wide-spread 
erosion of the Plains after a broad uplift of the whole region/’ 
The possible extension of a Summit Plain eastward over the 
Great Plains area is not entirely hypothetical and unsupported 
by fact. Although there now remains no vestige of it in Colorado 
such a highland plain finds actual representation farther south 
in the terreplein of the Mesa de Maya, 3500 feet above the present 
Plains surface. Upon careful analysis, however, the Maya plain 
does not appear to be really an extension of the Summital Plain 
of the Rockies. When it was - formed thick sediments probably 
still mantled the present mountain area. So that its actual posi¬ 
tion over the Cordilleran uplift was in fact a level at least several 
thousands of feet above the highest crest of the mountains. 
There are other cogent reasons for not believing the Summit 
Plain and the Mesa de Maya identical. 
Identification of the Summit Plain with the Great Plains sur¬ 
face leads to other difficulties. The presence of an eastward, 
steeply sloping plain half way up the Front Range leads to the 
surmise that the latter is a warped continuation of the Summit 
Plain, and, inferentially, identical with the present Great Plains 
surface. This is the view which seems to be taken by Prof. R. T. 
Chamberlin. The time of the planation is thus given a late 
Tertic date. Through differential uplift the Front Range is as¬ 
sumed to attain its present superior elevation above the Great 
Plains to the east. A necessary consequence of this explanation is 
the restriction of erosion during all later time to the work of re¬ 
juvenated streams. Evidences set forth in support of this view 
are notably meagre and not at all satisfactory. 
In the mountains the nearest approach to the Great Plains base- 
level is clearly the beds of the existing rivers. For a long time 
the stream-work on the Great Plains appears to have been main¬ 
ly constructive. Hence in the two districts it is strictly antitheti¬ 
cal in results. Because of the semi-arid climatic conditions 
prevailing over the Great Plains another very potent factor of 
erosion comes into play. In general land depletion deflation is 
many times, probably hundreds or even thousands of times, more 
effective than streani-corrasion. It is likely, therefore, that the 
great hypsometric differences between the crest of the Cordillera 
