132 
UTAH PENEPLAINS 
The Corazon peneplain is especially noteworthy because of the 
fact that it serves as an exact determinant of the amount of de¬ 
formation which the Rocky Mountains have undergone since 
Paleozoic times. It is the best geologic directrix which we know 
of for guaging isostatic compensation. Prior to its first upraise 
this erosion plain was depressed below sea-level and continued 
to sink until nearly two miles of sediments were laid down upon it. 
Laramie erosion removed fully one-half of the entire Mesozoic 
section. Then enormous thicknesses of Eocene deposits were 
formed, which Miocene planation reduced to a thickness of about 
2000 feet. 
Present erosion has to do with two things in particular. Over 
the Great Plains and among the High Plateaux it is engaged in 
removing four or five thousand feet of rock. Over the Cordil- 
leran uplift, besides disposing of last lingering traces of Mesozoic 
strata, it is, through means of stream-work, cutting deeply into the 
uplift. Its rate of progress is slow. Recent uprising enables 
profound canyons to be excavated on every hand. In the moun¬ 
tains this is indeed the principal result of present cycle erosion. 
The Summital plain of the Front Range is not, therefore, an 
extension of the Maya peneplain of Miocene times; nor of a 
hypothetical plain of latter date of approximately this position, as 
urged by Davis. Neither does it appear that it is the extension 
of the present surface of the Great Plains, as inferred by Chamber¬ 
lin, because this leaves no interval for the present stream canyon 
cutting in the mountains. It seems to be really the remnant of 
the Jurassic Corazon peneplain recently exhumed, and to be 
repeatedly betrayed around the isolated patches of heavy sand¬ 
stone which over the highlands are the all but vanquished Dakotan 
formation. 
Turning now to the old planation levels in the High Plateaux, 
the stratigraphic horizons of major unconformities are of special 
interest. In the High Plateaux country Powell, and other workers 
of the last quarter of a century, descern above the Paleozoic rocks 
only two unconformity planes which they regard as at all con¬ 
sequential. One of these is at the base, and the other is at the 
summit, of the Mesozoic succession. Later observers recognize 
at least two other major planation levels, besides a host of minor 
ones. 
