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In geography, a plain (pleyn) are fertile regions. They are vast and flat and are very conductive to habitation. For these reasons, the plains of the world have the highest density of population. Some of them have been cradles of famous civilizations. Large rivers and their tributaries that provide enough water for human and animal consumption usually drain plains.
Plains occur as lowlands and at the bottoms of valleys but also on plateaus at high elevations. In a valley, a plain is enclosed on two sides but in other cases a plain may be delineated by a complete or partial ring of hills, by mountains or cliffs. Where a geological region contains more than one plain, they may be connected by a pass (sometime termed a gap). Plains may have been formed from flowing lava, deposited by water, ice or wind, or formed by erosion by these agents from hills and mountains.
Plains in many areas are important for agriculture because where the soils were deposited as sediments they may be deep and fertile, and the flatness facilitates mechanization of crop production; or because they support grasslands which provide good grazing for livestock.
Types of terrestrial plains
- Coastal plain, an area of low-lying land adjacent to a sea; the term is used especially where they contrast with hills, mountains or plateau further inland.
- Alluvial plains, formed by rivers, and may be one of these overlapping types:
- Alluvial plain, formed over a long period of time by a river depositing sediment on its flood plain or bed which becomes alluvial soil. The difference between a flood plain and an alluvial plain is that the flood plain represents the area experiencing flooding fairly regularly in the present or recently, whereas an alluvial plain includes areas where the flood plain is now and used to be, or areas which only experience flooding a few times a century.
- Flood plain, adjacent to a stream, river, lake or wetland that experiences occasional or periodic flooding.
- Scroll plain, a plain through which a river meanders with a very low gradient.
- Lacustrine plain, a plain that originally formed in a lacustrine environment, that is, as the bed of a lake.
- Lava plain, formed by sheets of flowing lava.
- Glacial plains, formed by the movement of glaciers under the force of gravity:
- Grassland plain, formed by moss and algae, which look like grasslands.
- Mud plain, usually found close to swamps and quicksand. Mud plains are usually found near the jungle or lakes.
- Sandur (plural sandar), a glacial out-wash plain formed of sediments deposited by melt-water at the terminus of a glacier. Sandar consist mainly of stratified (layered and sorted) gravel and sand
- Till plain, a plain of glacial till that forms when a sheet of ice becomes detached from the main body of a glacier and melts in place depositing the sediments it carries. Till plains are composed of unsorted material (till) of all sizes.
- Folklore: It was believed that in England in the 14th century, that plains of any kind is where the dead wandered searching for their revenge. This was thought to be true because so many people went missing when wandering the plains at night and were never heard of again.
The term may also be used for flat areas of the ocean floor or for flat areas on moons and planets.
- Abyssal plain, a flat or very gently sloping area of the deep ocean basin
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