The Geologic Story of the Great Plains / A nontechnical description of the origin and evolution of the landscape of the Great Plains

by Donald E. Trimble

THE COLORADO PIEDMONT

The Colorado Piedmont lies at the eastern foot of the Rockies, ([fig. 1]) largely between the South Platte River and the Arkansas River. The South Platte on the north and the Arkansas River on the south, after leaving the mountains, have excavated deeply into the Tertiary (65- to 2-million-year-old) sedimentary rock layers of the Great Plains in Colorado and removed great volumes of sediment. At Denver, the South Platte River has cut downward 1,500 to 2,000 feet to its present level. Three well-formed terrace levels flank the river’s floodplain, and remnants of a number of well-formed higher land surfaces are preserved between the river and the mountains. Along the western margin of the Colorado Piedmont, the layers of older sedimentary rock have been sharply upturned by the rise of the mountains. The eroded edges of these upturned layers have been eroded differentially, so that the hard sandstone and limestone layers form conspicuous and continuous hogback ridges ([fig. 28]). North of the South Platte River, near the Wyoming border, a scarp that has been cut on the rocks of the High Plains marks the northern boundary of the Colorado Piedmont. Pawnee Buttes ([fig. 29]) are two of many butte outliers of the High Plains rocks near that scarp, separated from the High Plains by erosion as is Scotts Bluff, farther north in Nebraska. To the east, about 10 miles northwest of Limon, Colo., Cedar Point forms a west-jutting prow of the High Plains.

The Arkansas River similarly has excavated much of the Tertiary piedmont deposits and cut deeply into the older Cretaceous marine rocks between Canon City and the Kansas border. The upturned layers along the mountain front, marked by hogback ridges and intervening valleys, continue nearly uninterrupted around the south end of the Front Range into the embayment in the mountains at Canon City. Skyline Drive, a scenic drive at Canon City, follows the crest of the Dakota hogback for a short distance and provides a fine panorama of the Canon City embayment.

Figure 28.—Hogback ridges along the Front Range west of Denver, Colo. South Platte River emerges from the mountains and cuts through hogbacks in middle distance. Photograph courtesy of Eugene Shearer, Intrasearch, Inc.

Extending eastward from the mountain front at Palmer Lake, a high divide separates the drainage of the South Platte River from that of the Arkansas River. The crest of the divide north of Colorado Springs is generally between 7,400 and 7,600 feet in altitude, but Interstate Highway 25 crosses it at about 7,350 feet, nearly 1,500 feet higher than Colorado Springs and more than 2,000 feet higher than Denver. From the crest of the divide to north of Castle Rock, resistant Oligocene Castle Rock Conglomerate (which is equivalent to part of the White River Group of the High Plains) is preserved in many places and forms a protective caprock on mesas and buttes. This picturesque part of the Colorado Piedmont looks quite different from the excavated valleys of the South Platte and Arkansas Rivers.

Much of the terrain in the two river valleys has been smoothed by a nearly continuous mantle of windblown sand and silt. Northwesterly winds, which frequently blow with near-hurricane velocities, have whipped fine material from the floodplains of the streams and spread it eastward and southeastward over much of the Colorado Piedmont. Well-formed dunes are not common, but alined gentle ridges of sand and silt and abundant shallow blowout depressions inform us of the windblown origin of this cover.

Figure 29.—Pawnee Buttes in northeastern Colorado. Buttes isolated by erosion from High Plains in the background. Ogallala Formation caps top of Buttes. White River Group forms lower part. The top of the highest butte is about 240 feet above the saddle between the two buttes. Photograph by R. D. Miller, U. S. Geological Survey.

In the Colorado Piedmont, then, the erosional effects of streams are the most conspicuous features of the landscape, but these are enhanced by the steep tilting of the layered rocks along the western margin as a result of earth movement and modified by the nearly ubiquitous products of wind action, which have softened the landscape with a widespread cover of windblown sand and silt.