124 
UTAH PENEPLAINS 
The ancient Corazon erosion plain is most noteworthy. Mid 
Jurassic time in the southern Rocky Mountain region is a period 
characterized by extensive denudation rather than by sedimenta¬ 
tion. Shore and river deposits of the Morrisonian Series are the 
first strata to be laid down on an old Jurassic erosion surface. 
The unconformity line is well displayed in the mural faces of the 
Cerro Tucumcari, in northeastern New Mexico, where Jules 
Marcou, as early as 1853, first correctly identified the existence 
of Jurassic beds in America. 
To the north of the Cerro Tucumcari the same unconformity 
plane is conspicuously exposed for a distance of 100 miles along 
the broad Canadian River valley, the north wall of which con¬ 
stitutes the Corazon cliff, perhaps the finest escarpment in the 
world. As one stands on its brow he looks down 2,000 feet to 
the rocky bottom below. Away to the south 75 miles, is the 
opposite cliff of this valley, the edge of the Llano Estacado, the 
vast walled or staked plain of Texas. 
Along the Rocky Mountain front the Corazon planation hori¬ 
zon is easily recognizable far to the northward. Over the present 
Cordilleran uplift it now seems more than probable that the plane- 
level marking the tops of the monadnocks which surmount the 
present summital plain of the mountains, in central Colorado, 
belongs to the Corazon peneplain. 
Far to the south along the Rio Grande, at the mouth of the 
Rio Pecos in Texas, this planation is still in notable evidence. 
At Hayford, west of Del Rio, the folded and crumpled Paleozoics 
are shaved off as by some giant planer, and the Early Cretacic 
sediments rest in horizontal lines upon the highly tilted strata 
beneath. In the mountain wall the structural relations are dis¬ 
played with startling diagrammatic clearness. 
The Tucumcari planation surface displays some special features. 
In the North American interior general planation of Mesozoic 
times is the most expansive known in all geologic history. Its 
effects extend from the Mexican Tableland to Hudson Bay, and 
from the Sierra Nevada to the Mississippi River. It is the most 
continental in scope of any peneplain of which we know. Not 
only are the ancestral Rocky Mountains worn down to the sea, 
but a mighty mountain which rose above the surface of prairie 
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