Useful notes on the Classification of Plains based on their origin

(i) Diastrophic plains, (ii) Flood plains and delta plains, (iii) Peneplains and Pediplanes, (iv) Lacustrine plains, (v) Glaciated plains such as drift or till-plains, outwash plains etc, (vi) Loess plains, and (vii) Minor plains.

(i) Diastrophic plains:

Diastrophic plains refer to coastal plains that were formed due to the relative changes of the sea level as well as the land surface.

Sediment borne to the sea and spread over its floor tends to make it level by filling the depressions and smoothing over the elevations. Thus there are plains of deposit formed on the ocean bottom.

Where a part of the ocean bottom is brought above sea-level due to diastrophism it is normally added to the land as a coastal plain. That is the reason why so many coasts in regions of recent uplift are flanked by coastal plains.

The Coastal Plains Province, a physiographic province of the United States is a fine example of coastal plain caused by diastrophism.

It has all been uplifted since Miocene and much of it since Pliocene times, as is clearly demonstrated by marine beds of these ages which underlie most of the plain.

(ii) Flood plains and delta plains:

Even though a river is confined between its banks, but at times of flood the river water overflows them and spreads beyond them. During floods, the river is apt to be over laden with sediment.

Extra load of sediment that a river is unable to transport is deposited. As a result of this process, flood plains are built adjacent to the channel. It is a zone of low relief and gentle gradient.

Flood plain is composed of alluvium which buries the rock floor of the valley to different depths.

Such plains are characterised by the presence of distributary’s streams, ox-bow lakes, or swamps etc. The soils are fine and fertile. However, such a type of plain is subject to occasional floods and flood hazards.

Delta plains are formed at a river mouth formed by the deposition of successive layers of sediment. (For details of delta formation, see the chapter on landforms made by rivers.)

(iii) Peneplains and Pediplanes:

See the chapter on cycle of erosion where they are discussed in detail.

(Iv) Lacustrine plains:

Lacustrine plains are formed by the infilling of lakes. It is a low- lying tract of land formed from the sediments deposited in a lake. Such plains are usually marshy and can only be used if they are drained artificially.

In our country such plains are common on the Himalayas and other mountains. The Valley of Kashmir and the Imphal basin are excellent examples of old lakes which have been filled up and afterwards uplifted to become lacustrine plains. Such plains have been the bed of old lakes and have a centripetal drainage.

(v) Glaciated Plains:

All the glaciated plains are formed due to glacial deposition. Drift or till plains are excellent examples of such plains. Outwash plains of sands and gravels found in Michigan and heartlands of Western Europe are the product of fluvio-glacial deposition.

As a matter of fact, the great glaciated plains of central North America and Western Europe were plains prior to their glaciations.

The relief features of glaciated plains are dominated by drumlins, various types of moraines, lake basins, marshes and lacustrine plains. Actually the glaciers superimposed a new topography on the pre-existing ones.

Glaciers have made important changes in the pre-glacial drainage. While the glaciated regions present a youthful topography, the non-glaciated adjacent areas show a well-drained, mature topography.

(v) Loess plains:

It is believed that the loess plains of central Europe, Russia and North China were formed by windblown dust blowing from the anticyclones of central Asian regions. The largest known deposits are found is China.

Loess plains are the product of the transportational and depositional work of wind. They are found, besides Russia and China, in other parts of the world such as Rhine and Rhone valleys in Europe, in Austria, Argentina, Asia Minor and many other parts of the world.

(vi) Minor Plains:

In addition to major types of plains, there are other plains which are included in the category of minor plains. This category includes the wind-swept plains of bolsons, playas, bajadas, pediments, lava and ash plains, lagoon and marsh plains, and uplifted wave-cut plains.

All these minor plains differ from one another in respect of their mode of origin and other present characteristics.