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Plains

By : brainykey   September 25, 2013
PLAINS
Plains are flat and broad land areas on the earth’s surface, e.g. Prairies, Steppes, Great Plains of India, etc. The elevations of these landforms are relatively low, when measured with reference to the mean sea level.

Types of plains

  1. Erosional Plains : Erosional plains are developments on the Earth’s surface caused by natural weathering of glacier activity wind movement or water (sea, river & stream) torrent and are subdivided on the basis of the type of erosional agent. Here we will discuss the glacial plain, winderoded plain & alluvial plain. Erosional plains are likely to develop on landscapes that are relatively flat with low height elevations and shallow depressions. Some plains may develop below sea level but not higher than the surrounding region; this is a result of endogenetic factors and diastrophic movements.

Types of Erosional Plains

a)    Alluvial Plain : The alluvial plain is an erosional plain that occurs from weathering caused by water currents in the sea, river or stream. Flu, vial (water) movement comes from higher land regions and wear away landmasses to produce low relief plains. This is what is known as the alluvial plain. These landforms are made up of the deposition of sediment over a long period of time from the fluvial movement to form alluvial soil. An alluvial plain is characterized by its relatively flat and gently sloping landform and is normally formed at the base of a range of hills.

b)    Wind-Eroded Plain : Wind-eroded plains develop in dry land expanses. These formations are created by continuous abrasive wind activity wearing down land formations by a grinding action and a sandblasting of wind-borne particles to shape and disfigure landscape surfaces. Wind-eroded plains are found in arid regions with large supplies of unconsolidated sediment. Vegetation is sparse, as a result of such severe wind erosion. These plains are usually infertile since the most fertile parts of the soil are removed by abrasive wind activity. Soil productivity and maintenance is poor, which impacts seedling survival and growth and ultimately the survival of any type of plant life.

c)    Glacial Plain : These are structures formed by the movement of ice from highland areas to low land levels. Slow and continuous glacial movement takes place over time to cause sizable erosive work to produce a glacial plain. There are two types of glacial structures – true glacial plains, formed of pure glacial material and outwash plains, formed from deposition of materials such as sand, marl, gravel, silt & clay after the ablation (melting) of glaciers and ice sheets. Outwash glacial plains often form an alluvial plain when the ice has melted. As ablation occurs, the water rises and moves away from the glacial plain and takes with it fine, eroded sediment. As the speed of the water decreases, so does its capacity to carry objects in suspension. The water then gradually deposits the sediment across the landscape and creates an alluvial plain.

  1. Depositional Plains – Deposition is carried on by rivers, glaciers, wind, sea waves, etc. Plains result as a product of deposition. Five types of plains are described here:

        a)   Plains of Fluvial Deposition- The Rivers start deposition as a result of the decrease in the speed of rivers and the volume of water. An accumulation of sediment also contributes to deposition. There are three areas of deposition-the floor, the mouth and the valley of the river where the slope suddenly decreases. The following plains are described as belonging to this type:

  • A flood plain is the floor of a river valley beyond the riverbed. A flood plain is formed of mud, sand, and silt that are left behind when the river overflows its banks. These materials are carried off by the river as it erodes the land upstream. A river in flood conditions can carry a large amount of eroded material, which the overflow waters deposit onto the flood plain.
  •  Piedmont alluvial plains are formed at the foothill zones of the mountains.
  • Delta plains are formed by rivers through gradual deposition of sediments while entering the seas and oceans.

 

      b)    Plains of Glacial Deposition – Many plains are formed due to glacial deposition. These plains have great importance. These plains are found in North America and Europe in areas, which were affected by glacial action. Till covers mainly the plains. The size of the till particles varies from fine particles to boulders. Though a lot of till is local yet erratics are also common. The base rock of till have been eroded by ice. The surface is lightly undulating and has low and broad ridges and depressions. Drumlins, Eskers, Moraines, etc., are a common feature of these plains.

    c)    The Desert Plains: The plains where sand accumulates in large quantity are desert plains and are developed by wind action. The wind produces sand by sand-abrasion of sandstones. The sand does not blow much far away from its place but accumulates in various form. The main characteristics of these plains depend upon the accumulation of sand, the strength of the winds, the persistence of the direction of winds and vegetational cover. A few examples of such plains are the Sahara of Africa, the Koum of Russian Turkistan the north-central Nebraska, etc.

     d)    Lava Pains: These are formed due to deposition of thin sheets of lavas coming out through fissure flows. These are found in Iceland, USA, Deccan plateau of India etc.

     e)    Lacustrine Plains: These are formed when the lakes are filled with sediments either due to filling of lakes by the sediments brought by the rivers or by the upliftment of the beds of lakes due to diastrophic movements caused by endogenetic forces.

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