Saturday, June 25, 2011

Great Plains geography 3

The High Plains: You know 'em when you see 'em.

Comparing the seven maps that follow shows the challenge of defining the Great Plains. The first map is the most vague; the last map, the most specific. Nevertheless, each map locates the region's boundaries differently, especially its soft eastern margin that merges into the lower Central Plains and tall grass prairie.

The Interior Plains, which make up half of the Continental U.S., are divided into two parts: the Central Plains/Lowlands to the east and the Great Plains to the west. The boundary between the two parts is not distinct. But this summation holds true: The Central Lowlands' rich soils have made the region famous as America's breadbasket; the Great Plains' sea of grass became legendary for trail drives, ranches, and the cowboy cattle empire. Also the Great Plains are considerably higher in elevation than the Central Lowlands. Viewed from the Mississippi River Valley at, say, St. Louis, the plains are a plateau of sedimentary rock that rises gently west to the foothills of the Rocky Mountains. At Denver, Colorado, the land is one mile high.   

This depiction of the Great Plains shows a huge rural expanse covering almost one-forth of the Continental U.S. Lying astride the 100th meridian, the Great Plains extend over all or parts of ten states west of the Mississippi River and are part of the vast Interior Plains physiographic province.

Indigenous grassland biomes on the Great Plains include the steppe-like short grass prairie (light green) and mixed grasses between the 100th and 98th parallels. The tall grass prairie to the east (dark green) is often used to distinguish the eastern boundary of the Great Plains from the Central Lowlands. However, there are hardly any remnants of tall grass prairie left.
Another map of the North American prairie. The short grass prairie is in yellow.

Geomorphically, the Great Plains are much more complex than most Americans realize.
Visit the U.S.G.S. site for a remarkable topographic map of and map key to the physiographic regions of the United States. Geologically, the High Plains, as the term suggests, are composed of numerous layers of relatively horizontal sedimentary rock that slopes gently east from the Rockies toward the Missouri-Mississippi rivers. But erosion has produced complex terrain (geomorphology), especially near larger rivers. Among the landforms on the High Plains are plateaus, mesas, buttes, hogbacks, bluffs, badlands, loess formations, moraines left by continental glaciers, and driftless plains that largely escaped glaciation. Depending on where one puts the boundaries of the region, elevations range from 7,000 feet in Colorado (the Black Forest) to under 1,000 feet in North Dakota (along the Red River) and Texas (Edwards Plateau).

This map of the grasses, crops, and other vegetation of the Great Plains extends farther east than the geologic and most geographic depictions of the Great Plains; the coniferous patch in South Dakota represents the Black Hills. Note that the region's boundaries, viewed from the perspective of agriculture and land use, extend into Mexico and Canada.

A consortium of scholars at the University of Michigan, Colorado State University, and University of Saskatchewan has defined the eastern boundary of the Great Plains with a concession: "The only agreement on this boundary is that no authoritative line exists. Numerous people have attempted to define this border in both physiographical and cultural terms, using such demarcators as the 100th, 98th, 95th, and 88th meridians; the Missouri, Arkansas, and Mississippi rivers; and various levels of annual rainfall. Our boundaries depend on a combination of climatic, topographical, political, and cartographic criteria, and are ultimately drawn along county borders. According to these definitions, the Great Plains region contains about 475 counties in twelve states."

As land use maps suggest, the Great Plains have the lowest population density east of the Rockies; in many places fewer than 2 inhabitants per square mile live there. One keenly experiences the loneliness of the land in eastern Montana, eastern Wyoming, and the western Dakotas in the U.S., and great stretches of Alberta and Saskatchewan in Canada. Their rural feel contrasts with significant cities around the perimeter of the Great Plains that serve as the region's entrepots: Fort Worth, Oklahoma City, Tulsa, Kansas City, Omaha, Sioux Falls, Fargo, Winnipeg, Saskatoon, Edmonton, Calgary, Cheyenne, Fort Collins, Greeley, Boulder, Denver, Colorado Springs, Pueblo, San Antonio, Austin, and Waco. There are also significant cities within the Great Plains, but on average they are smaller than the perimeter cities. The interior cities include Moose Jaw, Regina, Billings, Minot, Bismarck, Rapid City, Pierre, Lincoln, Wichita, Abilene (Kansas), Topeka, Amarillo, Lubbock, Midland, Odessa, Abilene (Texas), and Roswell.

*     *     *

This photo-essay is part of a series on Great Plains geography.

No comments:

Post a Comment