The ice sheet had been covering North America and northwest Eurasia for millions of years. When this ice sheet melted; the area eroded by ice was exposed. Though the running water started modifying the plains yet a large number of such plains are more or less in the original conditions.
The rivers have adjusted themselves before the extension of ice-sheet. Glaciation disturbs the whole scene and the landscape presents the stage of youth again. Lakes, swamps, waterfalls and rapids are formed here and there. For example, there are more than 35,000 lakes in Finland. In fact 11% in Finland is lake infested. Muskeg swamp covers a large part of Canada. Waterfalls occur in areas of hard rocks. The waterfalls and the rapids have contributed a lot in the development of hydro-electricity and irrigation in Canada and U.S.A.
Glaciated Plains - The divides have a great economic value. Agriculture, colonies, transport and urban development are encouraged by the peculiarities of landforms. Big river valleys encourage agriculture but the floors of smaller river valleys are not suitable for it. The Colorado, Kansas, Nebraska, etc., east of the Rockies, belong to this category of plains.
(b) The Dissected Plains of Maturity. These are the plains where the development of tributary valleys has dissected the high lands. Areas of gentle slope are very much limited in these plains. Plain areas are available more in the valleys. Gradually the plain area of the divides goes on decreasing. Finally, the divides are reduced to small ridges. Fields, buildings, roads, villages, etc., develop in the broad areas of valleys. The plains of this stage are found in North Missouri, Southern Iowa, Eastern Nebraska, etc.
Depositional Plains - Deposition is carried on by rivers, glaciers, winds, sea waves, etc. Plains result as a product of deposition. Five types of plains are described here:
(i) 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:
Alluvial plains - These are different types of plains. The shape of plains changes according - to the method and place of deposition. The great plains of the world are mainly alluvial. It is necessary to differentiate between three types of plains- Flood plains, Deltaic plains and Piedmont alluvial plains.
(ii) 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.
(iii) The Desert Plains of Wind Erosion. 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.
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