138 
UTAH PENEPLAINS 
untrenched by important drainage ways. If any stream channels 
were ever let down from the shales above their impress has long 
since completely vanished. The Colorado River’s canyon is the 
sole noteworthy exception. Being a large master-stream the 
Rio Colorado probably had cut well down into the limestone be¬ 
fore the shales had altogether disappeared from the general sur¬ 
face. 
Now in the High Plateaux region the shales mentioned are not 
yet completely denuded and some great sections are preserved 
by old lavas. Eastward, in the great San Juan syncline between 
the Navajo Dome and the southern Rocky Mountains, or Sangre 
de Cristo Sierra, the enormous thickness of shales is well pre¬ 
served and gives adequate conception of its vast bulk, wide extent 
and large stratigraphic importance. 
One rather remarkable feature of the stratigraphy which usually 
escapes notice, but one which is probably the real basis of the 
curious desert relief, is the segregation of all the hard rocks in 
the bottom of the geological column, and all the weak rocks in 
the top of the succession. There is little actual alternation of 
resistant and weak strata. Paleozoic and pre-Cambrian forma¬ 
tions are all highly indurated and hence have strong resistant 
powers to erosive agencies. Mesozoic and Cenozoic strata are 
mainly incoherent and are easily denuded by rains, and especially 
by the winds in that excessively dry climate. 
In the Arizona stretch of the Colorado River there are readily 
distinguishable two notable stages of canyon-cutting — one finding 
expression in the broad, upper esplanade, and the other in the 
narrow inner gorge. The formation of the esplanade is some¬ 
times referred to as an epoch of local depression, or temporary 
base-leveling. Removal of the thick shale section, which is calculat¬ 
ed to have been over 10,000 feet above the present brink of the 
Canyon, is assumed to have mainly taken place prior to the canyon¬ 
cutting and during a single cycle of erosion. 
In the “Tertiary History of the Grand Canyon” Dutton makes 
much of the immensity of the regional depletion just before the 
great chasm was initiated, and he devotes much effort to prove 
that not less than two miles of strata were removed at that time. 
The “Great Denudation” is the title by which he designated the 
phenomenon. However, it now transpires that the Great Denuda- 
