Occupying 502,000 square miles of west-central North America---and partly owing their genesis to a Cretaceous inland sea once covering this area---the Plains tilt gently downward from the Rocky Mountain Front east to the Central Lowlands of the Midwest. The Rockies provide an abrupt, magnificent western border to the physiographic province. The Plains extend north to the great delta of the Mackenzie River in Canada and south to the Rio Grande. On their eastern front, they merge less dramatically with the Central Lowlands, which are lower and moister.
The U.S. Geological Survey splits the Great Plains into the following sections (roughly listed north-to-south):
•Missouri Plateau (glaciated and unglaciated)
•Central Texas Section
While many presume the Great Plains to be relentlessly level, the province actually contains much topographic variety. There are island mountain ranges, especially on the Missouri Plateau---the Bears Paw Mountains, the Sweetgrass Hills, the Black Hills, for example. There are badlands formations scattered throughout, exceptionally displayed in Badlands and Theodore Roosevelt national parks; stretches of buttes and mesas, and the evocative Sand Hills of Nebraska, a rolling belt of mixed-grass prairie and wetlands atop the Oglalla Aquifer; and scenic drainages, like the Missouri Breaks of north-central Montana and Palo Duro Canyon in Texas. The uniformity of the High Plains reaches its zenith on the Llano Estacado of New Mexico and Texas, an immensely flat tableland between the Canadian River and the Edwards Plateau.
The Buffalo Ranges
Millions of American bison roamed the Great Plains before their near-extermination by market hunters in the late 1800s. Historical bison bones are sometimes exhumed from shifting riverbanks, phantom relics---like the scattered traces of old buffalo roads and wallows---of these enormous herds. Through dedicated conservation efforts, bison have made an impressive comeback both on public and private lands.
Mingling with the bison masses were pronghorn, elk and mule deer, all of which are still found (in diminished numbers) in parts of the Plains. White-tailed deer more typically favor the river bottoms. Until the 19th century, grizzly bears ranged widely onto the grasslands, especially along drainages; remarkably, this still occurs along Montana's Rocky Mountain Front. Gray wolves have been reappearing on the Plains in recent years, dispersing from the Rockies and the upper Great Lakes. Big raptors like ferruginous hawks sometimes nest directly on the grasslands in areas where tree or rock roosts are scarce.
Kingdom of Grass
Cast in the rainshadow of the Rockies, the Great Plains harbor the greatest remaining expanses of prairie, a highly-endangered habitat, in the country. The tall-grass prairies of the Central Lowlands transition to the short-grass steppe of the western Plains, which is still quite extensive, via the mixed-grass prairie, containing species from both ecosystems. Badlands National Park and the Nebraska Sand Hills contain large, intact portions of mixed-grass prairie. But the Plains aren't just about grass---there are conifers and aspens in the island ranges and higher buttes, cottonwood gallery forests along the rivers, cacti and yucca, sagebrush and mesquite, and other plant communities.