Within the Great Plains are many large areas that differ greatly from
adjoining areas (fig. 3). The Black Hills stands out distinctively from
the surrounding lower land, and its dark, forested prominence can be
seen for scores of miles from any direction. At the southern end of the
Great Plains is another, less imposing, forested prominence—the Central
Texas Uplift. Most impressive, perhaps, is the huge, nearly flat plateau
known as the High Plains, which extends southward from the northern
border of Nebraska through the Panhandle of Texas, and which forms the
central part of the Great Plains. The east and west rims of the southern
High Plains are at high, cliffed, erosional escarpments—the Caprock
escarpment on the east and the Mescalero escarpment on the west. The
north edge of the High Plains is defined by another escarpment, the Pine
Ridge escarpment, which separates the High Plains from a region that has
been greatly dissected by the Missouri River and its tributaries. There,
several levels of rolling upland are surmounted by small mountainous
masses and flat-topped buttes and are entrenched by streams. This region
is the Missouri Plateau. The continental glacier lapped onto the
northeastern part of the Missouri Plateau and altered its surface.
The South Platte and Arkansas Rivers and their tributaries have
similarly dissected an area along the mountain front that is called the
Colorado Piedmont, and the Pecos River has excavated a broad valley
trending southward from the Sangre de Cristo Mountains in New Mexico
into Texas. The Mescalero escarpment separates the Pecos Valley from the
southern High Plains (fig. 4). South and east of the Pecos Valley,
extending to the Rio Grande and the Coastal Plain, is a broad plateau of
bare, stripped, flat-lying limestone layers bearing little but cactus
that is called the Edwards Plateau. Green, crop-filled valleys with
gently sloping valley walls and rounded stream divides trend eastward
from the High Plains of western Kansas and characterize a Plains Border
section. And finally, between the Colorado Piedmont on the north and the
Pecos Valley on the south, volcanic vents, cinder cones, and lava fields
form another distinctive terrain in the part of the Great Plains called
the Raton section.
[Illustration: _Figure 3.—The Great Plains province and its sections._]
[Illustration: _Figure 4.—Mescalero escarpment and the southern High
Plains (Llano Estacado) south of Tucumcari, N. Mex., Photograph by C. D.
Miller, U. S. Geological Survey._]
Can such diverse parts of our land have a sufficiently common origin to
justify their being considered part of one unified whole—the Great
Plains? Probably so, but to understand why, we must examine some of the
earlier geologic history of the Great Plains as well as subsequent
events revealed in the present landforms. We will find that all parts of
this region we call the Great Plains have a similar early history, and
that the differences we see are the results of local dominance of
certain processes in the ultimate shaping of the landscape, mostly
during the last few million years. The distinctive character of the
landscape in each section is determined in part by both the early events
and the later shaping processes.