PLAIN (OR plain, from Lat. planum. level ground, plain, nett. sg. of planus, level, even; connected with Lith. pldnas, fiat, OPruss. threshing-floor. OLat. patina, Gk. raNdyn, Warne', flat of the band, 7:-Nci.E, plus. plain, rXavay. planar, to wander, OHO. flail. 1:er. flock lint. 011G. r/uor, Ger. Flue, AS. yo,, Eng. floor, HIr.
floor). An expanse of level or gently sloping land. In contrast with a the surface of a plain is disposed at a low elevation, generally less than Woo feet. altIwugh no sharp line Of division can be drawn between the two. The Great Plains of the ited :states have an tion on the western border of 2000 feet or more. In respect to their origin, plains may be classed as marine. lacustrine, 1111Viatile, and plains of denudation. .1.1 ri or plains are formed by the of sediment beneath the sea. The dAris brought down by rivers to their mouths is carried seaward by tides and currents and dis tributed Ozer a wide area. Thus in time a broad belt of sedimentary strata accumulates along the margins of continental lands. and these sediments may subsequently be raised into land. The Coastal Plain (q.v.I which extends along the Atlantic and Gulf borders of the United States and the North German Plain have been formed in this manner. The marine origin may be demonstrated by the character of the fossils in cluded in the strata. La cast rine plains occupy the basins of former lakes whose disappearance may have been brought. about in various ways. The celebrated Vale of Kashmir in Northwestern India and the great Hungarian Plain are covered with fine sediments that were deposited in bodies of fresh water before the outlets had been cut down. In Nevada and Utah there are extensive
lacustrine plains which have been laid bare owing to a change of climate from moist to arid condi tions, and similar plains are known to occur in , Central Asia. A peculiar type is that illustrated by the valley of the Red River of the North: during the Glacial period a great lake was formed here only to disappear with the retreat of the obstructing ice-sheet. (See LIKE, LAKE ACAS :41Z.) Fla ciatilc plains are built up by the depo sition of silt along the borders and at the mouths of rivers. The Alississippi, the Nile. the Ganges, and most of the large rivers of the world have constructed extensive finviatile plains. (See }loot-PLAIN: DELTA.) Plains of denuda t owe their origin to erosion. When land has acquired considerable elevation above the sea the surface is dissected by the action of running streams which are constantly engaged in widen ing and deepening their channels. This process when carried to completion results in the reduc tion of the divides between the streams. produc ing a broad lowland whose surface lies but little above sea level. If the former elevations are still indicated by uneonsumed hills the lowland is called a peneplain. Th, plain thus produced may again be elevated and once more subjected to the process of dissection and base-leveling. The great plain of Central Russia and the lowlands border ing the Appalachians have probably been formed in this way. See GEOGRAPHY; PHYSIOGRAPHY.