Today’s traveler, who has less time for contemplation, races past a changing kaleidoscope of landscape. The increased awareness created by this rapidity of change perhaps is even more likely to stimulate questions about the origin of this landscape.
Figure 1.—Index map of the Great Plains showing route of Lewis and Clark and the Santa Fe and Oregon Trails.
[[This map in a higher resolution]]
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For instance, the westbound traveler on Interstate Highway 70 traverses nearly a thousand miles of low, rounded hills after leaving the Appalachians; the rolling landscape is broken only by a few flat areas where glacial ice or small lakes once stood. Suddenly, near Salina, Kans., the observant traveler senses a difference in the landscape. Instead of rounded hills, widely or closely spaced, he sees on the skyline flat surfaces, or remnants of flat surfaces. As he climbs gently westward these broken horizontal lines stand etched against the sky. About 35 miles west of Salina he finds himself on a broad, flat plateau, where seemingly he can see forever. True, in places he descends into stream valleys, but only briefly, for he soon climbs back onto the flat surface.
This plateau surface continues for 300 miles to the west—to within 100 miles of the abrupt front of the Rocky Mountains. East-flowing streams, such as the Smoky Hill, the Saline, the Solomon, and the Republican Rivers and their tributary branches, have cut their valleys into this surface, but these valleys become increasingly shallow and disappear entirely near the western rim of the plateau in eastern Colorado.
The distant peaks of the Rockies are seen for the first time as the traveler approaches the escarpment that forms the western edge of this great plateau. After crossing the escarpment near Limon, Colo., he begins the long gentle descent to Denver, on the South Platte River near the foot of the mountains that loom so awesomely ahead. He has crossed the Great Plains. The distances have been great, but the contrasts have been marked.
Had our traveler selected a different route, either to the north or south, he would have found even greater contrasts, for the Great Plains has many parts, each with its own distinctive aspect. Why should such diverse landscapes be considered parts of the Great Plains? What are their unifying features? And what created this landscape? Has it always been this way? If not, when was it formed? How was it formed?
We will look here at some of the answers to those questions. The history of events that produced the landscape of the Great Plains is interpreted both from the materials that compose the landforms and from the landforms themselves. As we will see, all landforms are the result of geologic processes in action. These processes determine not only the size and shape of the landforms, but also the materials of which they are made. These geologic processes, which form and shape our Earth’s surface, are simply the inevitable actions of the restless interior of the Earth and of the air, water, and carbon dioxide of the atmosphere, aided by gravity and solar heating (or lack of it). They all have helped sculpture the fascinating diversity of the part of our land we call the Great Plains.