UTAH PENEPLAINS 
125 
Iowa as majestically as do today the Rockies above the Colorado 
plain, is razed to ocean-level. 
In the south, on the Gulf savannah, the Early Cretacic sediments 
transgress the old Jurassic plain so far as Oklahoma, but north 
of that line Jurassic and Comanchean planation effects are super¬ 
posed, and seemingly constitute a single cycle. Over the Rocky 
Mountain region the two plains are apparently rather widely 
separated, the evidences of which are not entirely obliterated even 
to the present day. 
Curiously enough, from an isostatic angle, the ancestral Rockies 
should, with the prodigious removal of their tops, become more 
buoyant and grow higher. Instead, they immediately proceed to 
sink beneath the epicontinental waters until their evenly planed 
surface reaches a depth of a full mile below sea-level. 
This planation surface is everywhere marked by the thick, 
massive Dakotan sandstone which reposes upon it. By this sand¬ 
stone it is easily and widely recognizable. In the later and re¬ 
peated upliftings of the Rockies this planation level is carried 
nearly two miles above tide-level. Its warped surface now ap¬ 
pears to coincide closely with the highland plain of the Rocky 
Mountains. The flat highlands of the Front Range seem to be in 
reality the Tucumcari plane from which the Mid and Late Cretacic 
overburden has been only recently stripped. This interpretation 
appears to be strongly supported by the presence of remnantal 
patches of the Dakotan sandstone on the back of the mountains 
at about the same level as the highland level in the neighbor¬ 
hood. The relations are well displayed around Pike’s Peak, to the 
southwest of Long Peak, to the west of the Arapahoe Peaks, near 
Corona, around Breckenridge, and at numerous other places. 
Raton leveling marks extensive peneplanation. It is now gen¬ 
erally recognized that the time of Laramie deposition in the Rocky 
Mountain region is characterized by marked orogenic movements. 
It is not so widely appreciated that this epoch is also a time of 
wide regional planation. Evidences bearing upon this point are 
as yet meager or entirely wanting in Colorado. Beyond the state 
boundaries both to the north and to the south they are well dis¬ 
played. Over the present Rocky Mountain area no less than sev¬ 
eral thousands of feet of strata appear to have been removed 
