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plain,large area of level or nearly level land. Elevated plains are called plateausplateau,
elevated, level or nearly level portion of the earth's surface, larger in summit area than a mountain and bounded on at least one side by steep slopes, occurring on land or in oceans.
..... Click the link for more information. , or tablelands, and very low, wet plains are called swampsswamp,
shallow body of water in a low-lying, poorly drained depression, usually containing abundant plant growth dominated by trees, such as cypress, and high shrubs. Swamps develop in moist climates, generally in such places as low-lying coastal plains, floodplains of rivers,
..... Click the link for more information. . Plains have different names in different climates and countries. They include the tundrastundra
, treeless plains of N North America and N Eurasia, lying principally along the Arctic Circle, on the coasts and islands of the Arctic Ocean, and to the north of the coniferous forest belt.
..... Click the link for more information. , steppessteppe
, temperate grassland of Eurasia, consisting of level, generally treeless plains. It extends over the lower regions of the Danube and in a broad belt over S and SE European and Central Asian Russia, stretching E to the Altai and S to the Transbaykal and Manchurian plains.
..... Click the link for more information. , prairiesprairies,
generally level, originally grass-covered and treeless plains of North America, stretching from W Ohio through Indiana, Illinois, and Iowa to the Great Plains region.
..... Click the link for more information. , pampaspampas
, wide, flat, grassy plains of temperate S South America, c.300,000 sq mi (777,000 sq km), particularly in Argentina and extending into Uruguay. Although the region gradually rises to the west, it appears mostly level. Precipitation decreases from east to west.
..... Click the link for more information. , savannassavanna
, tropical or subtropical grassland lying on the margin of the trade wind belts. The climate of a savanna is characterized by a rainy period during the summer when the area is covered by grasses, and by a dry winter when the grasses wither.
..... Click the link for more information. , llanosllanos
, Spanish-American term for prairies, specifically those of the Orinoco River basin of N South America, in Venezuela and E Colombia. The llanos of the Orinoco are a vast, hot region of rolling savanna broken by low-lying mesas, scrub forest, and scattered palms.
..... Click the link for more information. , floodplains of rivers, coastal plains, loess plains, arid plains (see desertdesert,
arid region, usually partly covered by sand, having scanty vegetation or sometimes almost none, and capable of supporting only a limited and specially adapted animal population.
..... Click the link for more information. ), and lacustrine plains. The erosive action of water, glaciation, the draining of a lake, deposition of sediment, and the uplift of a continental shelf are some of the causes of the formation of plains. The extensive area comprising the western part of the Mississippi watershed, very gradually rising to the foothills of the Rockies, and having, largely, a steppe climate, is called the Great Plains region of the United States. The coastal plains region of the United States along the Gulf Coast and the Atlantic seaboard is widest in the south and southeast.
one of the most important relief features on dry land and on the floor of seas and oceans, characterized by small differences in elevation and slight dips.
On land plains are classified as below sea level (for example, the Caspian Lowland); lowland plains, with elevations from sea level to 200 m (Western Siberian Plain); elevated plains, lying between 200 m and 500 m (Ustiurt); and highland plains, occurring above 500 m (interior of the Iranian Plateau). The surface of a plain may be horizontal (the western part of the Betpak-Dala Desert), sloping (submontane trains), or down-warped (the central part of the Kashgar Plain). Depending on their mesorelief, plains are classified as flat, stepped, terraced, rolling, ridgy, hilly, or hummocky.
Plains differ in origin, geological structure, and development. In terms of the predominant exogenic processes at work, plains are divided into denudation plains, formed by the breakup and erosion of irregularities in the relief (such as mountains), and accumulation plains, created by the accumulation of layers of loose sediment.
Denudation plains that cut unconformably across the surface of a crystalline basement (surface of shields) or a folded foundation are called socle plains. Denudation plains whose surfaces are close to the structural surfaces of a slightly disrupted mantle are called stratified plains. By genesis of planation, denudation plains are subdivided into erosion, abrasion, exaration (glacial erosion), and deflation (wind action) plains. Denudation plains are also classified as peneplains or pediplains according to the mechanism of planation. Tiered plains are formed where the process of denudation planation is discontinuous because of the irregularity of tectonic uplift.
Accumulation plains are usually subdivided according to the predominant agent of endogenic accumulation (volcanic plains) or exogenic accumulation (marine, alluvial, lacustrine, glacial plains). Accumulation plains of combined origin (lacustrine-alluvial, delta-marine, and alluvial-proluvial) are also common. A more detailed breakdown of accumulation plains is also possible, for example, glacial plains may be subdivided into moraine, fluvioglacial, and lacustrine-glacial plains. Differences also occur in underwater accumulation plains. For example, there are abyssal plains, confined primarily to ocean platforms (thalassocratons), and the plains of the shelf and basins of marginal seas.
Structurally, plains are divided into those of platform regions and those of orogenic regions. Platforms, with their relatively tranquil tectonic conditions, are more conducive to the formation of a plains relief. On platforms, the relationship between relief forms and tectonic elements, between the river drainage pattern and the divides separating the river basins, may be direct or complex. Tectonic movements have a major impact on the relief of platform plains; especially noticeable in present-day plains relief are the tectonic movements of Recent (Neo-gene-Anthropogene) time. As a result of these movements, platform plains (also called plains country) include stretches with rugged relief in addition to the predominant level areas.
Pediment accumulation plains and denudation plains are formed within orogenic regions, in intermontane and submontane troughs. (Pediment accumulation plains are usually alluvial-marine, lacustrine-alluvial, or proluvial plains.) The pediment plains form sloping surfaces at the boundary of orogenic and platform regions or constitute the floors of intermontane depressions and large basins. In mountain country there are stretches of denudation plains that have been involved in intensive uplifting but have not yet been dissected by erosion (highland plains, tablelands, and mountain plateaus). Such stretches are orogenic and preorogenic planation surfaces.
Plains occupy the largest part of the earth’s surface. The basins of the greatest rivers and the largest lakes are located on plains, and in terms of terrain plains are the most suitable areas for human habitation. The largest plains on dry land are the Great and Central plains in North America, the Amazon and Guyana lowlands in South America, the East European Plain in Europe, the Western Siberian, North China, and Indo-Gangetic plains in Asia, the Sahara and Sudan plains in Africa, and the Central Lowlands in Australia.
REFERENCESShchukin, I. S. Obshchaia geomorfologiia, vol.2. Moscow, 1964.
Rel’ef Zemli (Morfostruklura i morfoskul’ptura). Moscow, 1967.
Meshcheriakov, Iu. A. Strukturnaia geomorfologiia ravninnykh stran. Moscow, 1965.
A. A. ASEEV