Plains are level or gently rolling expanses of terrain.
A plain refers to any flat or subtly-rolling tract of terrain, though in North America the same word is sometimes used to specifically describe grasslands or steppe. On account of their level, uncomplicated topography, plains often support homogenous ecological landscapes such as vast prairie, dense woods or broad wetlands.
Floodplains shoulder mature, meandering rivers.
Plains in the U.S. demonstrate the diverse geologic processes that can produce such landforms. The Atlantic-Gulf Coastal Plain of the East and Southeast grades into the continental shelf that extends well offshore; as sea levels have fluctuated in recent millennia, parts of the present-day coastal plain have occasionally been underwater. Sediments from a Mesozoic Era inland sea as well as more recent depositions from streams draining the Rocky Mountains blanket the Great Plains of the continental interior. The alluvial materials deposited by large rivers overflowing their banks build the floodplains that define their valleys. Indeed, a relentlessly level plain roamed over by a meandering river is sometimes the mark of a geologically old and stable landscape that has been thoroughly eroded and weathered to flatness.
Frontiers of Plains
The margins of plains landscapes may be subtle, as when they merge gently with foothills. But they can be quite dramatic, depending on the underlying geology. For example, the transition from the Piedmont Plateau (an Appalachian province) to the Atlantic-Gulf Coastal Plain is a sharp frontier called the Fall Line. Here, the more resistant rocks of the Piedmont drop to the geologically younger, unconsolidated layers of the plain. Large rivers heading in the Appalachians form cataracts, rapids or waterfalls as they traverse the Fall Line. The belt is marked by some of the East’s major cities, established along the Fall Line because of the hydropower resources.
Depending on climate, plains develop different ecological communities. The Great Plains are semiarid steppe of shortgrass and mixed-grass prairie partly because of the rain shadow cast by the Rocky Mountains to the immediate west (they are also far from marine moisture sources). The poorly-drained nature of river floodplains means they are often draped in swamps or marshes. Sandy soils of the Atlantic-Gulf Coastal Plain support extensive pine flatwoods, while the semiarid plains of the Intermountain West grow sparse cover of bunchgrasses and sagebrush. Barren alkaline flats, pebbly pavements and stunted shrub communities define desert basins of the Southwest.
Cheetahs in the Serengeti scan from a kopje.
Within the large-scale landform of the plain itself, micro-topography adds considerable variation. Isolate outcrops may not cover much acreage on a given tract of plain, but they can wield substantial ecological influence because of the microclimate and habitat diversity they foster. Granite and gneiss formations called kopjes stud the Serengeti Plains of East Africa, used by big cats as vantages and resting places. Buttes, mesas and outcrops on the North American grasslands serve as nesting sites for golden eagles, ferruginous hawks, great horned owls and other raptorial birds.