UTAH PENEPLAINS 
131 
Recent exhuming of Corazon peneplain indicated along the 
Rocky Mountain front where there is upturned a thick sandstone 
— the Dakotan sandstone, the base of which marks the position 
of the old Jurassic erosion surface. The same sandstone likewise 
rises on edge along the west limb of the mountain arch. Over 
the back of the Cordilleran uplift the sandstone is also found in 
more or less extended patches. 
That the Dakotan formation' formerly extended unbrokenly 
over the entire uplifted area is indicated by many facts. Long 
before W. T. Lee marshalled the details of evidence the Dakotan 
sandstone and overlying Cretacic formations were regarded as 
once completely covering the uplift. At the southern end of the 
Rockies, where its folds pitch beneath the Mexican Tableland, 
the phenomenon is especially well displayed. Ceja Glorietta, one 
of the most magnificant escarpments in the world, is the broken 
margin of the Dakotan sandstone. Its brow rises a thousand 
feet above the grade of the Atchison, Topeka and Santa Fe rail¬ 
road which, for many miles, follows at its foot. Even from the 
train the sloping and warped peneplain now on hard Carbonic 
limestone is presented in fine perspective rising quite to the highest 
summit of the Rockies to the north. From this surface is peeled, 
as it were, down to the foot of the escarpment the Dakotan sand¬ 
stone and overlying beds. 
Over the highland of central Colorado remnants of the Dakotan 
sandstone still persist in many places. Curiously enough the 
patches of the Summital Plain which are best preserved are closely 
accompanied by remnantal areas of this sandstone. From be¬ 
neath the sandstone the Jurassic plane emerges, and the surface 
is often continuous with the Summital Plain. 
The necessary inference is that the Jurassic peneplain once 
constituted the original surface of the entire Rocky Mountain 
area. 
In place, then, of a single plain with which to correlate the 
Summital Plain of the Front Range there are in the Rocky 
Mountain region four great peneplains. At maximum departures 
from one another the Laramian Plain is approximately 4000 feet 
above the Jurassic peneplain. The Maya, or Miocene, Plain is 
2000 feet above the Laramian. Present General Plains Surface 
of the region is 3500 feet below the Miocene level. 
