At the end of Ogallala deposition, some 5 million years ago, the Great
Plains, with the exception of the uplifted and the volcanic areas, was a
vast, gently sloping plain that extended from the mountain front
eastward to beyond the present Missouri River in some places. Regional
uplift of the western part of the continent forced the streams to cut
downward; land near the mountains was stripped away by the Missouri, the
Platte, the Arkansas, and the Pecos Rivers, and the eastern border of
the plains was gnawed away by lesser streams. A large central area of
the plain is preserved, however, essentially untouched and unaffected by
the streams, as a little-modified remnant of the depositional surface of
5 million years ago. This Ogallala-capped preserved remnant of that
upraised surface is the High Plains. In only one place does that old
surface still extend to the mountains—at the so-called “Gangplank” west
of Cheyenne, Wyo. (fig. 15). In places, as at Scotts Bluff National
Monument, Nebr. (fig. 16), small fragments of this surface have been
isolated from the High Plains by erosion and now stand above the
surrounding area as buttes.
[Illustration: _Figure 14.—Spanish Peaks, southwest of Walsenburg, Colo.
Igneous rocks and many radiating dikes exposed by erosion. Photograph by
R. B. Taylor, U.S. Geological Survey._]
[Illustration: _Figure 15.—Looking east toward Cheyenne at “the
Gangplank.” Interstate Highway 80 and the Union Pacific Railroad follow
the Gangplank from the High Plains in the distance onto the Precambrian
rocks (older than 570 m.y.) of the Laramie Mountains in the foreground.
Photograph by R. D. Miller, U.S. Geological Survey._]
[Illustration: _Figure 16.—Aerial view of Scotts Bluff National
Monument, Nebr. Buttes on the south side of the valley of the North
Platte River isolated by erosion from High Plains in the background.
Highest butte stands about 800 feet above valley floor._]
The High Plains extends southward from the Pine Ridge escarpment, near
the South Dakota-Nebraska border (fig. 3), to the Edwards Plateau in
Texas. The Platte, the Arkansas, and the Canadian Rivers have cut
through the High Plains. That part of the High Plains south of the
Canadian River is called the Southern High Plains, or the Llano Estacado
(staked plain). The origin of this name is uncertain, but it has been
suggested that the term Llano Estacado was applied by early travelers
because this part of the High Plains is so nearly flat and devoid of
landmarks that it was necessary for those pioneers to set lines of
stakes to permit them to retrace their routes.
The Llano Estacado is bounded on the west by the Mescalero escarpment
(fig. 4) and on the east by the Caprock escarpment. The southern margin
with the Edwards Plateau is less well defined, but King Mountain, north
of McCamey, Tex., is a scarp-bounded southern promontory of the High
Plains. The remarkably flat surface of the Llano Estacado is abundantly
pitted by sinks and depressions in the surface of the Ogallala
Formation; these were formed by solution of the limestone by rainwater
and blowing away or deflation by wind of the remaining insoluble
particles. Many of these solution-deflation depressions are aligned like
strings of beads, suggesting that their location is controlled by some
kind of underlying structure, such as intersections of joints in the
Ogallala Formation.
The solution-deflation depressions are less abundant north of the
Canadian River, but occur on the High Plains surface northward to the
Arkansas River and along the eastern part of the High Plains north of
the Arkansas to the South Fork of the Republican River.
Covering much of the northern High Plains, however, are sand dunes and
windblown silt deposits (loess) that mantle the Ogallala Formation and
conceal any solution-deflation depressions that might have formed. The
Nebraska Sand Hills (fig. 17), the largest area of sand dunes in the
western hemisphere, is a huge area of stabilized sand dunes that extends
from the White River in South Dakota southward beyond the Platte River
almost to the Republican River in western Nebraska but only to the Loup
River in the northeast part of the High Plains (fig. 18). Loess covers
the western High Plains southward from the sand dunes almost to the
Arkansas River, and to the South Fork of the Republican in the eastern
part. This extensive cover of loess has created a fertile land that
makes it an important part of America’s wheatlands (fig. 19).
[Illustration: _Figure 17.—Aerial view, looking northwest, of the
Nebraska Sand Hills west of Ashby, Nebr._]
Other, smaller areas of sand dunes lie south of the Arkansas River
valley. The only large areas of sand dunes on the Llano Estacado, or
Southern High Plains, are along the southwestern margin near Monahans,
southwest of Odessa, Tex.
Oil and gas are present in the Paleozoic rocks that underlie the High
Plains at depth. Gas fields are ubiquitous in much of the eastern part
of the High Plains between the Arkansas and Canadian Rivers. Just south
of the Canadian River, at the northeast corner of the Southern High
Plains, a huge oil and gas field has been developed near Pampa, Tex. Oil
and gas fields also are abundant in the southwestern part of the
Southern High Plains, south of Littlefield, Tex.
[Illustration: _Figure 18.—The Sand Hills region of Nebraska. Arrows
show inferred direction of dune-forming winds. Map from Wright (1970),
used by permission._]
WYOMING
Badlands National Monument
Missouri River Valley
JAMES RIVER LOBE
MINNESOTA
IOWA
SOUTH DAKOTA
NEBRASKA
Rosebud
Valentine
DES MOINES LOBE
NEBRASKA
Ashby
SANDHILLS
Platte River Valley
IOWA
MISSOURI
NEBRASKA
KANSAS
COLORADO
Muscotah
TOPEKA
EXPLANATION
Transverse dunes
Longitudinal dunes
Wind-blown sand
Loess thickness (in feet)
[Illustration: _Figure 19.—Little-modified loess plain in southeastern
Nebraska. Photograph by Judy Miller._]
The surface of the High Plains, then, has been little modified by
streams since the end of Ogallala deposition. It has been raised by
regional uplift and pitted by solution and deflation, and large parts of
it have been covered by wind-blown sand and silt. It has been drilled
for oil and gas and extensively farmed, but it is still a geological
rarity—a preserved land surface that is 5 million years old.