UTAH PENEPLAINS 
139 
tion is not an accomplishment of a single simple cycle of erosion as 
he fondly fancies. All does not take place so recently as he 
supposes. Neither is the general depletion of the region a direct 
result of the same pluvial graving conditions with which most of 
us are most familiar, as he urges. He errs in merging into one the 
effects of what are really four complete and grand geographic 
cycles supperposed one upon the other. 
Erosion affecting the general lowering of the land about proves 
to have an altogether different origin from that involved in the 
development of the canyon. Instead the latter following in 
natural sequence the former, both not only could go on 
simultaneously, but probably did operate side by side as they do 
today. General denudation is accomplished with the wind as the 
chief and tremendous erosive agency; while the local canyon¬ 
cutting is the pigmy result of stream corrasion. The relative 
efficiencies of the two erosive powers under the same climatic con¬ 
ditions but where aridity is the controling factor are thus strongly 
contrasted. 
The peneplain upon which was initiated the latest geographic 
cycle of erosion in the region is inferred from the surrounding 
country to have stood about a mile above the present surface of 
the Navajo Dome. Therefore, while the Colorado River has 
been sinking its narrow channel vertically a distance of two miles, 
as a maximum figure, the entire region about has been undergoing 
deflation until at least one-half of this distance has been removed 
in the same time. Stream-corrasion has been merely local and 
linear; wind abrasion has been broadly general and regional. 
The cubic mileage of rock removal has been measurable by a few 
tens in the instance of the river; by many thousands in the case 
of the wind. Regional leveling and lowering of the Grand Canyon 
country should be re-examined in the light of aridity — an angle 
which Dutton and his confreres never touched. 
Over the Navajo Dome all evidences of the three early great 
cycles of erosion are vanquished because of the fact that, unlike 
in the cases of the Mesa de Maya and the High Plateaux, removal 
of the initial plains-surface is complete. There are no uncon¬ 
sumed sections of strata in which unconformities are preserved by 
such accidental circumstances attending old lava-flows. The 
Navajo Dome is in the same condition as the other two regions 
