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.
EARLY HISTORY
The Interior Plains, of which the Great Plains is the western, mostly unglaciated part ([fig. 2]), is the least complicated part of our continent geologically except for the Coastal Plain. For most of the half billion years from 570 million ([fig. 5]) until about 70 million years ago, shallow seas lay across the interior of our continent ([fig. 6]). A thick sequence of layered sediments, mostly between 5,000 and 10,000 feet thick, but more in places, was deposited onto the subsiding floor of the interior ocean ([table 1]). These sediments, now consolidated into rock, rest on a floor of very old rocks that are much like the ancient rocks of the Superior Upland.
About 70 million years ago the seas were displaced from the continental interior by slow uplift of the continent, and the landscape that appeared was simply the extensive, nearly flat floor of the former sea.
WARPING AND STREAM DEPOSITION
Most of these rocks of marine origin lie at considerable depth beneath the land surface, concealed by an overlying thick, layered sequence of rocks laid down by streams, wind, and glaciers. Nevertheless, their geologic character, position, and form are exceptionally well known from information gained from thousands of wells that have been drilled for oil. The initial, nearly horizontal position of the layers of rock beneath the Interior Plains has been little disturbed except where mountains like the Black Hills were uplifted about 70 million years ago. At those places, which are all in the northern and southern parts of the Great Plains, the sedimentary layers have been warped up and locally broken by the rise of hot molten rock from depth. Elsewhere in the Interior Plains, however, earth forces of about the same period caused only a reemphasis of gentle undulations in the Earth’s crust.
These undulations affected both the older basement rocks and the overlying sedimentary rocks, and they take the form of gentle basins and arches that in some places span several States. (See sketch map, [figure 7].) A series of narrow basins lies along the mountain front on the west side of the Great Plains. A broad, discontinuous arch extends southwest from the Superior Upland to the Rocky Mountain front to form a buried divide that separates the large Williston basin on the north from the Anadarko basin to the south.
While the flat-lying layers of the Interior Plains were being only gently warped, vastly different earth movements were taking place farther west, in the area of the present Rocky Mountains. Along a relatively narrow north-trending belt, extending from Mexico to Alaska, the land was being uplifted at a great rate. The layers of sedimentary rock deposited in the inland sea were stripped from the crest of the rising mountainous belt by erosion and transported to its flanks as the gravel, sand, and mud of streams and rivers. This transported sediment was deposited on the plains to form the rocks of the Cretaceous Hell Creek, Lance, Laramie, Vermejo, and Raton Formations. Vegetation thrived on this alluvial plain, and thick accumulations of woody debris were buried to ultimately become coal. This lush vegetation provided ample food for the hordes of three-horned dinosaurs (Triceratops) that roamed these plains. Their fossilized remains are found from Canada to New Mexico.