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 Pecos River, and to a lesser extent the Devils and Nueces Rivers, particularly in their lower courses, have entrenched themselves deeply in the plateau in remarkable meandering courses of a type that is usually found only in broad, low-lying floodplains. These stream courses reflect the stream environment prior to regional uplift.

Sinkholes pit the relatively undissected limestone plateau surface in the northeast part of the Edwards Plateau, and some underground solution cavities in the limestone are well-known caves, such as the Caverns of Sonora, southwest of Sonora, Tex.

Oil and gas fields are widely developed in the northern part of the Edwards Plateau, but only cattle ranches are found in the bare southern part.

Ancient oceans deposited the limestones that now cap the Edwards Plateau; streams planed off the surface of the flat-lying limestone layers and entrenched themselves in steep-walled valleys; and ground water dissolved the limestone and created the solution cavities that are the caves and sinks of the Edwards Plateau. Water has created this landscape.

PLAINS BORDER SECTION

The Missouri Plateau, the Colorado Piedmont, the Pecos Valley, and the Edwards Plateau all were outlined by streams that flowed from the mountains. On the eastern border of the Great Plains, however, headward cutting by streams that have their source areas in the High Plains has dissected a large area, mainly in Kansas. This Plains Border Section comprises a number of east-trending river valleys—of the Republican, Solomon, Saline, Smoky Hill, Arkansas, Medicine Lodge, Cimarron, and North Canadian Rivers—and interstream divides, most of which are intricately dissected.

North of the Arkansas River, the east-flowing Republican, Solomon, Saline, and Smoky Hill Rivers have incised themselves a few hundred feet below the Tertiary High Plains surface and have developed systems of closely spaced tributary draws. The interstream divides are narrow, and the tributary heads nearly meet at the divides. This intricately dissected part of the Plains Border section is called the Smoky Hills. Some isolated buttes of Cretaceous rocks left in the upper valley of the Smoky Hill River are called the Monument Rocks. A large area of rounded boulders exposed by erosion south of the Solomon River, southwest of Minneapolis, Kans., is called “Rock City.” These boulders originated as resistant nodules (concretions) within the Cretaceous rocks that contained them.

South of the Arkansas River is a broad, nearly flat upland sometimes referred to as the Great Bend Plains. The Medicine Lodge River has cut headward into the southeastern part of the Great Bend Plains and created a thoroughly dissected topography in Triassic red rocks that is locally called the Red Hills. In a few places, badlands have formed in the Red Hills.

Some large sinks or collapse depressions have formed because of solution of salt and gypsum at depth by ground water. Big and Little Basins, in Clark County in south-central Kansas, were formed in this way.

Sand dunes have accumulated in places, especially near stream valleys. Dunes are common, for example, along the north side of the North Canadian River.