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.
[Illustration: _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.
[Illustration: _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.