360 
SUMMIT PLAIN OF ROCKIES 
days it is certain, according to the latest evidences, that it has un¬ 
dergone several notable deformations. 
Each orographic uprising appears to have been immediately fol¬ 
lowed by profound regional planation. Five such major plana- 
tion-levels are now on display. Therefore the outlook is not near¬ 
ly so simple as it once seemed to be. Only by careful consider¬ 
ation of the geologic sequence of events does it appear possible to 
connect widely separated remnants of each erosion-level; or to 
correlate the several plains-surfaces. 
The most pretentious of all the peneplanations affecting the 
Rocky Mountain region is the one which took place during Co- 
manchan times. Its areal influence reaches from the Mexican 
boundary to Hudson Bay and from the Sierra Nevada to the 
Mississippi River. It is almost continental in extent. It is per¬ 
haps the broadest peneplain of which we have knowledge. The 
position of this old erosion plain is well marked stratigraphically 
because of the fact that the massive Dakotan sandstone rests di¬ 
rectly upon it. By means of the latter all of the later movements 
connected with the Cordilleran uplift are easily evaluated. 
Other peneplanations of the region appear to have taken place, 
after each orographic disturbance, in Laramian times, in the 
Miocene, and in Pliocene times. The last uprising seems to be in 
full swing at the present time and to be acting about as fast as 
mountain genesis ever goes on. Production of a new base-level is 
now already initiated. With it enter new and complex compli¬ 
cations in the geographic cycle never before even fancied. 
It is really beyond the boundaries of the Cordilleran tract that 
the best evidences of the existence of the several peneplains are 
presented. The present vertical distance between the First, or 
Comanchan, and the Second, or Raton, levels is about 4000 feet. 
Between the Second and the Third, or Maya, plain the interval is 
approximately 2000 feet. The latter’s position is 3500 feet above 
the existing general plains-surface. What volume of sediments 
is represented by the successive parts removed through erosion is 
not very difficult to estimate. In each instance it is a plate not 
less than one to two miles in thickness. 
The level of the Comanchan plain, deformed as it now is, is 
easily determined in the field by the position of the bottom of the 
great Dakotan sandstone which reclines upon it. Without going 
