In geography, a plain is land with relatively low relief, that is flat or gently rolling. Prairies and steppes are types of plains, and the archetype for a plain is often thought of as a grassland, but plains in their natural state may also be covered in shrublands, woodland and forest, or vegetation may be absent in the case of sandy or stony plains in hot deserts. Types of flatlands for which the term is not generally used include those covered entirely and permanently by swamps, marshes, playas, or ice sheets.
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.
Coastal plain, an area of low-lying land adjacent to a sea; the term is used especially where they contrast with hills, mountains or plateaux further inland.
Aluvial plains are formed by rivers, and may be one of these overlapping types:
Flood plain, adjacent to a stream, river, lake or wetland that experiences occasional or periodic flooding.
Alluvial plain, formed over a long period of time by a river depositing sediment on its floodplain or bed which becomes alluvial soil. The difference between a floodplain and an alluvial plain is that the floodplain represents the area experiencing flooding fairly regularly in the present or recently, whereas an alluvial plain includes areas where the floodplain is now and used to be, or areas which only experience flooding a few times a century.
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 are formed by the movement of glaciers under the force of gravity:
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.
Sandur (plural sandar), a glacial outwash plain formed of sediments deposited by meltwater at the terminus of a glacier. Sandar consist mainly of stratified (layered and sorted) gravel and sand.
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