Plains: Origin, Meaning and Classification |Plains | Geography

In this article we will discuss about:- 1. Origin and Development of Plains 2. Meaning of Plains 3. Classification.

Origin and Development of Plains:

Plains found on the earth’s surface have been formed in different ways as follows:

(1) Most of the plains have been originated due to upliftment or emergence of submerged landmasses under epicontinental seas due to diastrophic move­ments. The Great Plains Province of the USA is a burning example of this process of the origin of plains. Northern portion of the Atlantic coastal plains is also supposed to have been formed due to emergence of submerged coastal land having marine deposits.

(2) Plains are formed due to filling of depres­sions with sediments, which are created in front of the mountains during the process of orogenesis. In other words, the land areas in front of a newly originated mountain are depressed due to diastrophic movements and an extensive trench-like depression or foredeep is formed.

This foredeep is filled with sediments in due course of time and ultimately a broad and extensive depositional plain is formed. The northern plains or the Ganga-Yamuna plains of India are typical examples of such plains.

(3) Sometimes, the middle part of the geosynclines remain unfolded after the formation of bordering ranges on both the margins of the geosynclines. Such area unaffected by folding activities, very often known as median mass, becomes plains.

It may be pointed out that not all the median masses become plains; rather most of them become plateau (like Ti­betan plateau, Iranian plateau, Anatolian plateau, Colo­rado plateau etc.). A median mass becomes plain only when it is characterized by flat surface and compara­tively lower height. Hungarian plain between Dinaric Alps and Carpathian Alps is an example of plain formed by a median mass.

(4) Sometimes, coastal lands are submerged under sea water because of transgressional phase of sea. Such coastal lands submerged under shallow wa­ter receive sediments regularly and after long time of continuous sedimentation narrow coastal plains are formed. The eastern coastal plains of India, mainly North Circar, are supposed to have been formed due to sedimentation over submerged coastal lands.

(5) Submerged coastal lands emerge as marine coastal plains due to withdrawal of sea water during regressional phase of sea. The Kutch plain of India is the example of such plain. It may be pointed out that Rann of Kutch is gradually prograding (extending) towards sea and hence the marshy plain is also gradu­ally extending in area.

(6) Extensive plateaux after prolonged denuda­tion become plains.

(7) Deposition of enormous volume of lavas over extensive area gives birth to lava or volcanic plains.

Meaning of Plains:

Plains are the simplest relief features of all the reliefs of the second order. These are easily identifi­able and are easily classified into distinct categories. Plains are flat areas with low height. The height of plains cannot be precisely defined in reference to the sea-level because some plains are sometimes even below sea-level e.g., coastal plains of Netherlands (where sea walls and sea embankments of concrete have been constructed to protect the coastal plains against possible submergence during high tidal waves), some plains are only a few metres above the sea-level e.g., southern deltaic plains of Bangladesh (the most devastating hazardous cyclonic storms of the century hit the coast in May, 1991 and submerged most part of the deltaic coastal plains killing more than 5,00,000 people) while some plains are hundreds of metres above mean sea-level e.g., eastern Mississippi plain is 450m high which is definitely higher than the pied­mont plateau situated to the east of the Applachians.

It may be mentioned that plains may be above or below sea-level but they cannot be higher than the surround­ing regions. The plains are generally characterized by almost flat surface but it also becomes undulating because of erosion. Plains are also dominated by level to gentle slopes. On an average, plains are composed of same type of rocks but these are generally overlain by sediments.

There is a wide range of variations in the plains in relation to their height, situation, size and extent, topographic forms, lithological characteristics, mode of origin etc.

It may be pointed out that plains are formed mainly in two ways e.g.:

(i) Through endogenetic forces, and

(ii) Through exogenetic processes.

The plains grouped under the category of the reliefs of the second order include only those plains which are formed due to endogenetic forces coming from within the earth whereas the plains formed through erosional and depositional works of exogenetic processes (denudational processes) are grouped under the cat­egory of the reliefs of the third order.

Classification of Plains:

Diastrophic Plains:

It may be pointed out that a particular plain is seldom formed by a single process. Erosion or deposi­tion, in one way or the other, always plays an important role in the development of plains. Thus, the nomencla­ture of a plain is based on the dominant process playing the most significant role in the origin and evolution of that plain.

Under the diastrophic events plains are formed due to upliftment or subsidence of land areas or emergence of land areas from beneath the oceanic water or submergence of coastal lands under oceanic water caused by epeirogenetic movements.

The Great Plains (USA) are the examples of plains formed due to upliftment or emergence of sub­merged landmasses under epicontinental sea. The great plains are bordered by the Rockies in the west, by Central Lowland Province or Mississippi-Missouri plains in the east and by Rio Grande River in the south.

The plains further extend northward into Canada. It is believed that the Great Plains remained under water in Cretaceous period for fairly long period and hence horizontal thick beds of marine sediments were depos­ited. The submerged land mass began to rise due to diastrophic movement and the landmass appeared above sea-level by the end of Cretaceous period and thus the plains were formed which soon developed various lakes, ponds, swamps and marshes and vegetation’s.

These plains were further uplifted along with the origin of the Rockies during Tertiary period. To some extent the Cretaceous sedimentaries were folded and faulted due to orogenetic force coming from the west. Ero­sional agencies deposited alluvial materials over low-lying Cretaceous formations.

The northern part of Great Plains falling in the states of North Dakota and Montana (USA) was greatly modified by southward advancing ice sheets during Pleistocene glaciation. It is obvious that the Great Plains were definitely origi­nated due to diastrophic movements but they have also been developed and modified by fluvial erosion and deposition and glaciation from time to time.

Plains are also formed due to upliftment or emergence of submerged coastal lands caused by diastrophic movements. The Atlantic Coastal Plains of the USA are considered to have been formed under this process. It is believed that the present Atlantic Coastal Plains were submerged under oceanic water upto Miocene period, with the result there was continued deposition of marine sediments on the submerged landmass.

The emergence and upliftment of submerged landmass began by the end of Miocene period and was completed by Pliocene. The Atlantic Coastal Plains extend from New York to the Gulf of Mexico. These plains together with the continental shelves extend for an average width of 480km. It may be pointed out that there is unique relationship between the widths of the coastal plains and the continental shelves.

The narrow coastal plains are associated with wider continental shelves whereas wider coastal plains are associated with narrow continental shelves. The coastal plains gently slope towards Atlantic Ocean with average gradient of 10 feet per mile. The distinct marine ter­races along the coast denote the phases of upliftment or emergence of submerged landmasses above sea water.

Coastal plains are also formed due to deposition of sediments over submerged continental shelves. Such plains are not placed under the category of diastrophic plains. Sometimes, land area along the sea coast is subjected to mild subsidence and thus the submerged land area gets sediment deposits to such extent that it rises above sea water.

The Coromandal and Northern Circar Coastal Plains were formed due to mild subsid­ence and consequent sedimentation. It is believed that the north-western part of Peninsular India was raised upward with the result the south-eastern part was submerged due to mild subsidence. The eastern coastal plain was thus formed due to subsidence, sedimenta­tion and then upliftment due to diastrophic move­ments.

Erosional Plains:

(1) Erosional Plains Formed By River Erosion:

Peneplains are the most characteristic plains formed by river erosion at the end of ‘normal cycle of erosion’. Peneplains are characterized by convexo-concave residua] hills, known as monadnocks, which project above the general surface. The peneplains are formed by lateral erosion by the rivers and weathering processes. The weathered materials are deposited over the peneplains. Rivers are well adjusted to the struc­ture.

(2) Glaciated Plains:

Glaciers transform highland areas through their slow but continued erosive works into lowland with flat surface but with sufficient low reliefs of rounded shape. Glaciated plains are charac­terized by rounded peaks, broad and flat valleys and small depressions which become lakes when filled with water.

Swamps and marshes are also developed over glaciated plains. Several examples of glaciated plains are found in the northern part of North America and north-western Eurasia because of advancement and retreat of ice sheets during Pleistocene glaciation.

(3) Wind Eroded Plains:

Rocks become loose due to disintegration caused by mechanical (physical) weathering in the hot desert areas. Strong winds pick up these weathered and commented fine particles and deposit them elsewhere. The repetition of this process over longer period of time results in the transformation of stony areas into plains. Wind eroded plains are called reg, serir and hamada in Sahara.

Hamada is, in fact, nude stony flat surface. Hamada is formed due to joint actions of wind erosion and sheet erosion during floods caused by occasional strong rainstorms. The plains developed at the hillslopes due to twin processes of scarp retreat and pedimentation (pediment forma­tion) are called pediplains.

(4) Karst Plains:

The plateaux composed of massive limestones are subjected to chemical weather­ing and erosion by groundwater and are ultimately transformed into subdued topographic surface of very low relief. Such plains are called karst plains which are formed at the end of karst cycle of erosion. The surface of karst plain is corrugated and undulating.

Depositional Plains:

Depositional plains are formed due to gradual deposition of sediments by different geological agents e.g., rivers, wind, glaciers etc. Depositional plains range in size and extent which depend upon area and nature of sedimentation. The Ganga-Yamuna plains, Mississippi-Missouri plains, Yangtze plain, Yellow plain (Hwang Ho plain) etc. are very extensive depositional plains which have been formed due to alluviation by big rivers and their tributaries. Depositional plains include alluvial plains (by rivers), aeolian plains or loess plains (by wind), outwash plains (by gleciers) wave-built platforms (by sea waves), lacustrine plains, lava plains etc.

(1) River Deposited Plains:

The alluvial plains deposited by rivers are most extensive depositional plains. Nearly all the alluvial plains of the major rivers of the world are the members of this group. Alluvial plains are formed due to filling of extensive and deep trenches or depressions caused by tectonic movements with sediments of various sorts brought by the rivers.

The North Indian Plains or the Ganga-Yamuna Plains were formed due to deposition of eroded mate­rials into extensive foredeep which was formed due to Himalayan orogenetic movements. River-deposited plains include piedmont alluvial plains, flood plains and delta plains.

(A) Piedmont Alluvial Plains:

Piedmont alluvial plains are formed at the foothill zones of the mountains. The gradient of the rivers is remarkably reduced at the foothill zones where the river leaves the mountain and enters the plains topography of level to gentle slope (less than 5 degree of slope), with the result sediments of fairly large size, e.g., cobbles, pebbles, boulders together with silt and gravels are deposited at the foot of the mountains because the rivers are unable to carry them further downstream due to decrease in transportation capacity because of marked decrease in the channel gradient.

Thus, the materials are deposited at the foot­hill zones in the form of alluvial cones and alluvial fans which grow in size with time. If several rivers descend through a mountain range numerous alluvial fans are formed parallel to the mountain range. These alluvial fans gradually and gradually grow in size and thus coalesce to form one extensive fan which becomes piedmont alluvial fan plain.

The appex of the fans is characterized by larger particles, the size of which decreases further away from the appicers of the allu­vial fans. The mountain ward portion of the piedmont alluvial plains is called Bhabar in India. The rivers coming from the Himalayas generally disappear in the bhabar zone because water disappears in the large- sized materials like boulders, cobbles and pebbles. These rivers repappear in the Tarai region to the south of bhabar region. Bhabar plain is also called dry delta plain.

(B) Flood Plains:

Flood plains are formed due to deposition of fine sediments consisting of sands, silts, muds, clays etc. in the flood zones of the rivers. Flood plains are also known as alluvial plains because they are formed of the argillaceous sediments or alluvial soils. Agricul­turally, flood plains are very significant because they have become the granaries of the world e.g. flood plains of Mississippi, Yangtze, Ganga, Yellow, Salveen, Meekong etc.

The flood plains are very fertile because their fertility is renewed every year due to yearly alluivation during flood period.

In India flood plains are divided into:

(i) khadar plains and

(ii) bhangar plains.

Khadar plains are those which are submerged under flood waters almost every year. These are al­ways situated on either side of the river.

Bhangar plains are not affected by flood waters.

(C) Delta Plains:

Rivers while debouching in the seas and oceans form deltas through gradual depo­sition of sediments brought by them. The Ganga delta is the largest delta of the world as it covers an area of 1,29,000 square kilometres (50,000 square miles). The Ganga delta is shared by both India and Bangladesh but the latter accounts for the largest area. The delta plains are drained by numerous tributaries of the master river. These tributaries are called distributaries. In West Bengal, higher parts of the delta are called ‘char’ and lowlying areas are called ‘beels’.

(2) Lacustrine Plains:

Lacustrine plains are formed when the lakes are filled with sediments.

Plains are formed in two ways e.g.:

(i) due to filling of lakes by the sediments brought by the rivers and

(ii) due to upliftment of the beds of lakes due to diastrophic movements caused by endogenetic forces.

(3) Lava Plains:

Lava plains are formed due to deposition of thin sheets of lavas coming out through fissure flows. Several examples of lava plains are found in France, New Zealand, Iceland, US A, Argentina etc. Lava plains are economically very important because black soils
are formed due to weathering of lavas. These black soils (regur soils) support good cotton crops.

(4) Wind Deposited Plains:

Wind deposited plains fall in two categories e.g.:

(i) sandy plains or desert plains and

(ii) loess plains.

Sahara Desert of Africa, Thar Desert of India etc. are characterized by extensive sandy plains or desert plains. The surfaces of desert plains are irregular and undulating because of the presence of numerous sand dunes and ripple marks.

Loess plains are formed away from the deserts. Loess plains of Shansi province of China are most extensive loess plains of the world.

Loess plains are divided into:

(i) desert loess plains, when the sands are derived from the deserts and

(ii) glacial loess plains, when the sediments are of glacial origin. Glacial loess plains are found in North America and North West Europe.

(5) Glacial Plains:

Glacial plains are formed due to deposition of glacial and other debris by glacial ice sheets mainly during ice ages. Extensive glacial plains were formed in North America and North-West Europe during Pleistoncene glaciation of the northern hemisphere.

Glacial plains are generally uneven, undulating and marshy and are of two types e.g.:

(i) True glacial plains formed of pure glacial materials, and

(ii) Outwash plains, which are formed due to deposition of materials after the ablation of glaciers and ice sheets.

Glacial plains are divided into 3 types on the basis of composition and structure:

(i) Till plains- Finer to coarser materials brought by glaciers are called ’tills’. The plains formed of tills are called till plains which are characterized by undulating wave-like surface having small ridges and peaks. Eskers and drumlins are significant landforms of till plains,

(ii) Morainic plains are deposition of finer glacial materials,

(iii) Outwash plains are composed of the mixture of sands, gravels, silts and clays.

Finch and Trewartha have classified plains into four types e.g.:

(i) Flat plains, relative reliefs not more than 15m,

(ii) Undulating plains, relative reliefs ranging between 15m and 50m,

(iii) Rolling plains, relative reliefs between 50m 75m and

(iv) Dissected plains, relative reliefs between 75m and 100m.