SUMMIT PLAIN OF ROCKIES 
359 
SUMMIT PLAIN OF THE COLORADO ROCKIES 
By Charles Keyes 
Of four great and relatively lately formed peneplains, which 
especially characterize the southern Rocky Mountain uplift, the 
so-called Summit Plain receives from geographers most and often 
sole consideration. In a region of sharp peaks and cirque-walled 
heights the presence of such extensive highland plain appears, if 
not a distinct anomaly, at least quite out of place. For adequate 
explanation we are almost compelled at the outset to seek some¬ 
thing else than mere simple epeirogenic elevation of a recent sea- 
level savannah. 
That the Rocky Mountain region is a tract which has been sub¬ 
ject to oft-repeated diastrophic movement is a circumstance wide¬ 
ly recognized. From earliest geologic times this broad segment 
of earth-crust appears to have undergone almost continuously both 
orogenic and epeirogenic disturbance. No less than a dozen times 
have the ancestral Rockies reared themselves, as majestically, per¬ 
haps, as they stand today, only to be speedily planed off to the 
level of the sea. So frequently does it seem that the region has 
been a land area that first workers in this field long regarded it as 
a great continental island lasting quite through all the ages. 
In recent years a wholly different view gains credence. It is 
now generally conceded that sediments of nearly all periodic times 
have entirely covered the area. Infolded Paleozoics in narrow 
belts along the Front Range seem to afford ample clue to the 
former extensive presence of some of the ancient strata. 
It is to the later periods of orogenic uprising that attention par¬ 
ticularly turns. Since the time when the great limestone plate 
which saddles the southern Rocky Mountains was laid down in 
the shallow waters of the epi-continental seas of Mid-Carbonic 
