plain, large area of level or nearly level land. Elevated plains are called plateaus, or tablelands, and very low, wet plains are called swamps. Plains have different names in different climates and countries. They include the tundras, steppes, prairies, pampas, savannas, llanos, floodplains of rivers, coastal plains, loess plains, arid plains (see desert), 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.

Plain, the, in French history, term designating the independent members of the National Convention during the French Revolution. The name was applied to them because, in contrast to the radical Mountain, they occupied the lower benches of the chamber. The Plain was a leaderless mass and a pliable instrument, but it was numerically in the majority and consequently determined many votes. It played an important role in bringing about the overthrow (9 Thermidor; July 27, 1794) of Maximilien Robespierre, but after this effort it again lost its cohesion.

Any relatively level area of the Earth's surface that exhibits gentle slopes and small local relief (differences in elevation). Occupying slightly more than one-third of the terrestrial surface, plains are found on all continents except Antarctica. Some are tree-covered, and others are grassy. Still others support scrub brush and bunch grass, and a few are nearly waterless deserts. With certain exceptions, plains have become the sites of major centres of population, industry, commerce, and transportation.

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Flat seafloor area at a depth of 10,000–20,000 ft (3,000–6,000 m), generally adjacent to a continent. The larger plains are hundreds of miles wide and thousands of miles long. The plains are largest and most common in the Atlantic Ocean, less common in the Indian Ocean, and even rarer in the Pacific Ocean, where they occur mainly as small, flat floors of marginal seas or as long, narrow bottoms of trenches. They are thought to be the upper surfaces of land-derived sediment that accumulates in abyssal depressions.

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In geography, a plain is an area of land with relatively low relief — meaning that it is flat. 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 cse 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. They 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 mechanisation 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 coast; the term is used especially where they contrast with hills, mountains or plateaux further inland.
• Fluvial plains are formed by rivers and streams, 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.
• 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 carried.

Other types of plain

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 floor.

Notes and references

See also

• Field
• Flooded grasslands and savannas
• Flood-meadow
• Grassland
• Liverpool Plains
• Machair
• Meadow
• Pasture
• Plateau
• Prairie
• Rangeland
• Savanna
• Steppe
• Water-meadow
• Wet meadow
• Veld