[1. Generalized chart of rocks of the Great Plains] 15
The GEOLOGIC STORY of
The GREAT PLAINS
By Donald E. Trimble
INTRODUCTION
The Great Plains! The words alone create a sense of space and a feeling of destiny—a challenge. But what exactly is this special part of Western America that contains so much of our history? How did it come to be? Why is it different?
Geographically, the Great Plains is an immense sweep of country; it reaches from Mexico far north into Canada and spreads out east of the Rocky Mountains like a huge welcome mat. So often maligned as a drab, featureless area, the Great Plains is in fact a land of marked contrasts and limitless variety: canyons carved into solid rock of an arid land by the waters of the Pecos and the Rio Grande; the seemingly endless grainfields of Kansas; the desolation of the Badlands; the beauty of the Black Hills.
Before it was broken by the plow, most of the Great Plains from the Texas panhandle northward was treeless grassland. Trees grew only along the floodplains of streams and on the few mountain masses of the northern Great Plains. These lush prairies once were the grazing ground for immense herds of bison, and the land provided a bountiful life for those Indians who followed the herds. South of the grasslands, in Texas, shrubs mixed with the grasses: creosote bush along the valley of the Pecos River; mesquite, oak, and juniper to the east.
The general lack of trees suggests that this is a land of little moisture, as indeed it is. Nearly all of the Great Plains receives less than 24 inches of rainfall a year, and most of it receives less than 16 inches. This dryness and the strength of sunshine in this area, which lies mostly between 2,000 and 6,000 feet above sea level, create the semiarid environment that typifies the Great Plains. But it was not always so. When the last continental glacier stood near its maximum extent, some 12,000-14,000 years ago, spruce forest reached southward as far as Kansas, and the Great Plains farther south was covered by deciduous forest. The trees retreated northward as the ice front receded, and the Great Plains has been a treeless grassland for the last 8,000-10,000 years.
For more than half a century after Lewis and Clark crossed the country in 1805-6, the Great Plains was the testing ground of frontier America—here America grew to maturity ([fig. 1]). In 1805-7, explorer Zebulon Pike crossed the south-central Great Plains, following the Arkansas River from near Great Bend, Kans., to the Rocky Mountains. In later years, Santa Fe traders, lured by the wealth of New Mexican trade, followed Pike’s path as far as Bents Fort, Colo., where they turned southwestward away from the river route. Those pioneers who later crossed the plains on the Oregon Trail reached the Platte River near the place that would become Kearney, Nebr., by a nearly direct route from Independence, Mo., and followed the Platte across the central part of the Great Plains.
Although these routes may have seemed long and tedious to those dusty travelers, they provided relatively easy access to the Rocky Mountains and had a continuous supply of fresh water, an absolute necessity in these plains. The minds of those frontiersmen surely were occupied with the dangers and demands of the moment—and with dreams—but the time afforded by the slow pace of travel also gave them ample opportunity for thought about the origins of their surroundings.