one of the most important relief features on dry land and on the floor of seas and oceans, characterized by small differences in elevation and slight dips.
On land plains are classified as below sea level (for example, the Caspian Lowland); lowland plains, with elevations from sea level to 200 m (Western Siberian Plain); elevated plains, lying between 200 m and 500 m (Ustiurt); and highland plains, occurring above 500 m (interior of the Iranian Plateau). The surface of a plain may be horizontal (the western part of the Betpak-Dala Desert), sloping (submontane trains), or down-warped (the central part of the Kashgar Plain). Depending on their mesorelief, plains are classified as flat, stepped, terraced, rolling, ridgy, hilly, or hummocky.
Plains differ in origin, geological structure, and development. In terms of the predominant exogenic processes at work, plains are divided into denudation plains, formed by the breakup and erosion of irregularities in the relief (such as mountains), and accumulation plains, created by the accumulation of layers of loose sediment.
Denudation plains that cut unconformably across the surface of a crystalline basement (surface of shields) or a folded foundation are called socle plains. Denudation plains whose surfaces are close to the structural surfaces of a slightly disrupted mantle are called stratified plains. By genesis of planation, denudation plains are subdivided into erosion, abrasion, exaration (glacial erosion), and deflation (wind action) plains. Denudation plains are also classified as peneplains or pediplains according to the mechanism of planation. Tiered plains are formed where the process of denudation planation is discontinuous because of the irregularity of tectonic uplift.
Accumulation plains are usually subdivided according to the predominant agent of endogenic accumulation (volcanic plains) or exogenic accumulation (marine, alluvial, lacustrine, glacial plains). Accumulation plains of combined origin (lacustrine-alluvial, delta-marine, and alluvial-proluvial) are also common. A more detailed breakdown of accumulation plains is also possible, for example, glacial plains may be subdivided into moraine, fluvioglacial, and lacustrine-glacial plains. Differences also occur in underwater accumulation plains. For example, there are abyssal plains, confined primarily to ocean platforms (thalassocratons), and the plains of the shelf and basins of marginal seas.
Structurally, plains are divided into those of platform regions and those of orogenic regions. Platforms, with their relatively tranquil tectonic conditions, are more conducive to the formation of a plains relief. On platforms, the relationship between relief forms and tectonic elements, between the river drainage pattern and the divides separating the river basins, may be direct or complex. Tectonic movements have a major impact on the relief of platform plains; especially noticeable in present-day plains relief are the tectonic movements of Recent (Neo-gene-Anthropogene) time. As a result of these movements, platform plains (also called plains country) include stretches with rugged relief in addition to the predominant level areas.
Pediment accumulation plains and denudation plains are formed within orogenic regions, in intermontane and submontane troughs. (Pediment accumulation plains are usually alluvial-marine, lacustrine-alluvial, or proluvial plains.) The pediment plains form sloping surfaces at the boundary of orogenic and platform regions or constitute the floors of intermontane depressions and large basins. In mountain country there are stretches of denudation plains that have been involved in intensive uplifting but have not yet been dissected by erosion (highland plains, tablelands, and mountain plateaus). Such stretches are orogenic and preorogenic planation surfaces.
Plains occupy the largest part of the earth’s surface. The basins of the greatest rivers and the largest lakes are located on plains, and in terms of terrain plains are the most suitable areas for human habitation. The largest plains on dry land are the Great and Central plains in North America, the Amazon and Guyana lowlands in South America, the East European Plain in Europe, the Western Siberian, North China, and Indo-Gangetic plains in Asia, the Sahara and Sudan plains in Africa, and the Central Lowlands in Australia.
A. A. ASEEV