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.

abyssal plain: see ocean.

West Siberian Plain, an extensive region of continuous flatland, N Asia, comprising the western third of Siberia, Russia. It is bounded on the east by the Yenisei River, on the north by the Arctic Ocean, on the west by the Urals, and on the south by Kazakh hill country and the Altai Mts. Tundra and taiga cover the northern section, while the southern section is steppe. It was the source of most of the USSR's oil and gas production during the 1970s and 80s.

Staked Plain: see Llano Estacado.

Salisbury Plain, undulating, mostly barren chalk plateau, c.300 sq mi (780 sq km), Wiltshire, S England. It is noted chiefly as the site of ancient monuments, of which Stonehenge is the most famous. The region is also an army training ground.

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.

Plain of Jars, region, N Laos, at the northern end of the Annamese Cordillera. Over 300 jars, c.1,500 to 2,000 years old, are scattered across its landscape. Averaging 272 lbs (600 kg), the jars range from 3 to 10 ft (1-3 m) in height. Their origin and function remain mysteries. From the mid-1960s through the early 70s, the Plain of Jars was the scene of heavy fighting between the Pathet Lao and U.S.-backed troops.

North China Plain: see Huang He.

Carrizo Plain National Monument: see National Parks and Monuments (table).

Adana, Plain of, fertile region along the Mediterranean coast, S central Turkey. It has a subtropical climate and receives rainfall mainly during the autumn and winter months. The plain, traversed and irrigated by the Seyhan River, is a major agricultural region, producing a large variety of crops including most of the cotton grown in Turkey. The city of Adana is the commercial and marketing center of the region.

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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