PLAINS

PLAINS A plain is a relatively flat and low-lying land surface with least difference between its highest and lowest points. The plains are usually lowlands, seldom rising more than a few hundred feet above sea level. Some of them may be smooth, while others are slightly rolling. Plains can be placed according to their position and surface relief but are better classified on the basis of their mode of formation.

They are sub-divided into structural, erosional and depo­sitional plains.

1. Structural Plains These are the structurally de­pressed areas of the world, that make up some of the most extensive natural lowlands on the earth's surface. They are formed by horizontally-bedded rocks, relatively undis­turbed by the crustal movement of the earth. Examples include the Russian Platform, the Great Plains of USA, and the central lowlands of Australia.

2. Erosional Plains Erosional plains are formed when an elevated tract of land, such as a mountain, a hill or a plateau, is worn down to a plain by the process of erosion. Rain, rivers, ice and wind help to smooth out the irregu­larities of the earth's surface, and in terms of millions of years, even high mountains can be reduced to low undu­lating plains. Examples include northern Canada, northern Europe and western Africa (all ice-eroded plains) and parts of Sahara in Africa (wind-eroded plain).

3. Depositional Plains These are plains formed by the deposition of materials brought by various agents of transportation, such as rivers, glaciers, wind and sea waves. A list of different types of depositional plains and their depositional agents is given below.

The Indo-Ganga Plains in India, the Hwang Ho Plains of North China, the Po River Plains or Lombardy in North Italy and that of Nile River are examples of some of the great alluvial plains. Plains of north-western Eurasia and that of Ladakh to the east of Shyok River and north of Chang Chenmo River are examples of plains made by glacial depositon. Plains in West Rajasthan, Turkmenistan and north-western China represent loesses.