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In geography, a plain is a flat area. Plains occur as lowlands and at the bottoms of valleys but also on plateaus or uplands 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, 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 plains== *Structural plains: relatively undisturbed horizontal surfaces of the earth. They are structurally depressed areas of the world that make up some of the most extensive natural lowlands on the Earth's surface. *Erosional plains that have been leveled by various agents of denudation such as running water, rivers, wind and glacier which wear out the rugged surface and smoothens them. Plain resulting from the action of these agents of denudation are called ''peneplains'' (almost plain) while plains formed from wind action are called ''pediplains''. *Depositional Plains formed by the deposition of materials brought by various agents of transportation such as rivers, wind, waves, and glaciers. Their fertility and economic relevance depend greatly on the types of sediments that are laid down. Depositional Plains are grouped into the following: *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: * *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. *Abyssal plain, a flat or very gently sloping area of the deep ocean basin *Planitia, the Latin word for plain, is used in the naming of plains on extraterrestrial objects (planets and moons), such as Hellas Planitia on Mars or Sedna Planitia on Venus. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Plain」の詳細全文を読む スポンサード リンク
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