Plains are often classified into three main groups according to the way in which they have been formed: 1. Erosional Plain 2. Depositional Plain 3. Uplifted and Down-Tilted or Basin Plain.
1. Erosional Plain:
The various natural agencies such as rain, rivers, sea wave, ice in the form of glacier, frost and melting snow and the wind gradually wear away the surface of the land. Some plains may have been formed by this process from the area that were once much higher. Such plains are not perfectly level and hence they are sometimes called ‘Peneplains’.
The lowlands of Canada, West Siberia plain, Finland plain, Hudson Bay and Lowlands of U.S.A. are the good examples of such plains. Peneplains are also seen in parts of peninsular India. Plains are also formed by the action of sea waves in the coastal regions. A good example of such plains is the East Coastal plain of south India.
2. Depositional Plain:
Some plains may be formed by the gradual deposition of silt in sea or lake floor brought down by rivers, glaciers or winds. Plains formed by this way are called depositional plains. Thus the plains formed by silt deposit brought down by the rivers, are called alluvial plains.
The plains formed by the action of glaciers are called glacial plains and plains formed due to action of winds are known as loess.
The Indo-Ganga-Brahmaputra plains in India, the po valley of Italy and the Nile delta are the examples of alluvial plains. The North-western part of Europe and Asia and the Canadian plains are good examples of glacial plains.
The Gobi desert of China is an example of loess plain. Lake beds may be filled up by silt laid down by rivers. They are called lacustrine plains. The plains formed by the deposits of lava are called lava plains. The Manipur plain and Kashmir Valley of India are lacustrine plains. The Snake River Valley of U.S.A. is a lava plain.
3. Uplifted and Down-Tilted or Basin Plain:
Sometimes due to movement of the earth a depression or a shallow part of the sea floor may be raised to form a plain. Such a plain is called an uplifted plain. The Steppe of USSR is an example of this kind.
For the same reason some area of the surface of the earth may be depressed to form a certain kind of plain. This is called basin plain. Good examples of such plains are Turan lowland near the Caspian Sea and Tarim Plain of Central Asia.