GEOMORPHOLOGICAL FEATURES

FORCES AFFECTING LANDFORMS
 

• ENDOGENETIC (ENDOGENIC) FORCES: The ‘internal’ forces of contraction, expansion, uplift, depression, distortion and outpouring which have contributed to the present landforms.
• EXOGENETIC (EXOGENIC) FORCES: The ‘external’ forces of denudation, (weathering, mass-wasting, erosion, transport, deposition), which combine with internal (ENDOGENETIC) forces to produce landforms.

Endogenetic (Endogenic) Forces

These forces are caused by contraction of the Earth on cooling, or change in the rotation of the Earth, or due to action of the radioactive elements.
On the basis of INTENSITY, these forces can be divided into two sub-groups.

• SUDDEN ENDOGENETIC FORCES: These main forces are volcanic or of earthquake. Landscape undergoes disintegration suddenly. For example, production of deep fissures in plain areas, the sudden changes in the routes of river, formation of small hills, etc.
• DIASTROPHIC FORCES : The forces which have disturbed or deformed the Earth’s crust, including folding, faulting, uplift and depression, though not vulcanism. These forces act very slowly. Their effect becomes discernible after thousands of years. For example, the coast of the Baltic Sea rises 1.3 metres in a century.
   

The Diastrophic Forces can be divided into two further sub-groups – EPEIROGENETIC and OROGENETIC.

• EPEIROGENETIC (EPEIROGENIC) FORCES: This word is derived from Greek ‘epeiros’ which means a continent. Large-scale continent building forces, hence applied to all en masses vertical or radial crustal movements, both of uplift and depression. Strata are not thereby folded or crumpled, thought they may be slightly tilted or warped.
• OROGENETIC (OROGENIC) FORCES: These forces act on the Earth’s mantle in a horizontal direction. Hence, these forces are also called TANGENTIAL FORCES.

When TENSION is produced by these forces, the Earth’s surface displays fissures and when COMPRESSION is produced, folds result in the sedimentary rocks.

Exogenous Forces

These are external forces that produce physical features-i.e. those forces which operate externally on earth’s surface and change it by- (i) Denudation or by (ii) Deposition. The forces of denudation are weathering and erosion. The forces of deposition are water, ice, wind and living organisms.

Weathering

Weathering is the combined action of all processes whereby a rock is decomposed and disintegrated in situ, because of exposure at or near earth’s surface. This process changes hard, massive rock material into finely fragmented, soft residual material, the parent matter of soil. For this reason, weathering is often described as the preparation of rock materials for transport by the agents of land erosion-flowing water, glacial ice, and wind. Due to the gravitational force, both bedrock and the products of weathering tend to slide, roll, flow or creep down all slopes in a variety or type of earth and rock movements grouped under the term mass washing.
Weathering may result from (i) physical (mechanical) processes e.g. heating, cooling, freezing and mechanical action of plants and animals (ii) chemical processes, e.g. carbonation, hydration, hydrolysis, solution, Organic weathering, which consists of both physical and chemical weathering, is caused by plants, and animals, particularly when they bury or release acidic substances into the rocks. Lichens, for example, cause breakdown as they extract nutrients directly from the rocks.

Mechanical Weathering

Erosion
Erosion refers to the disintegration of rocks which lie exposed to what are called the agents of erosion, i.e. gravity, running water, wind, and moving ice, and on the coasts, waves, tides and currents.

Exfoliation
Exfoliation is aptly called onion weathering. It is a process of physical weathering in which the outer layers of a rock split off into thin sheets or scales. It has been suggested that it is caused by the alternate expansion and contraction resulting from a wide diurnal range of temperature in arid regions, which would lead to stresses in the rock.

Frost action
Frost action shatters rocks through repeated freezing and thawing of water which seeps into rocks and expands on freezing. This is common in winter in the temperate regions and in some high mountains all the year.

Repeated wetting and drying
In tropical areas or in coastal areas where rocks are repeatedly exposed to water weathering occurs. When the rocks are wetted, outer layer absorbs moisture and expand. When the moisture dries, shrinkage occurs. This repeated expansion and shrinkage produces stresses leading to peeling/splitting of outer layers.

Chemical weathering

Exposure of rocks to air and water often sets up chemical reactions thereby leading to either weakening or entire dissolution of exposed surfaces. Rocks with a pitted surface and pore spaces are more affected by chemical weathering as water and air gets trapped into them thereby speeding up chemical reactions. Also chemical weathering is most potent in hot and wet climate and least in dry climate.
Chemical weathering can be classified under following heads-

Solution- Rain water contains some carbon dioxide that makes it slightly acidic. In limestone regions, this weak acid dissolves the calcium carbonate of which limestone is chiefly formed. Though solution is most evident in limestone, other rocks are also subject to solution but at a slower rate.

Oxidation- Certain rocks contain iron or oxides of iron. When exposed to the oxygen in the air iron oxide is formed which easily crumbles or peels thereby loosening the overall structure of rock.

Decomposition by organic acids- Organic acids are released by bacteria thriving on plant and animal matter present in the soil which lead to chemical weathering of rocks.
The weathered material is transported to other places by agents like wind, running water, glaciers, waves etc. and deposited to give rise to depositional geographical features like alluvial plains, beaches etc.

MOUNTAINS

A mountain is that land block whose height is at least 900 meters above sea level and its slope makes an angle of 250 to 350 with the horizontal plane. The lower mountains are called hills. In fact mountain and hill are relative terms.
Some Definitions
1. Ridge: It is a high and narrow continuation of hills. It has many passes.
2. Mountain Range: It is a big and complex ridge. It is a series of ridges, which are mutually related, form a compact unit and stand over an extensive area.
3. Mountain System: It is a group of mountain ranges, which are similar in their form, structure and extension. Besides, they should owe their origin to the same cause.
4. Mountain Chain: It is an elongated unit of these mountain ranges or chains whose size and age need not necessarily be similar or the same.
5. Cordillera: It is a Spanish word and is the highest unit of mountain.

Classification of Mountains
Mountains are classified on many bases. Some classifications are given here.

1. Classification based on Height: Mountains are usually divided into 4 classes on the basis of height.
i. Low Mountains: The heights of low mountains range from 1,000 to 1,500 meters.
ii. Mountains of Ordinary height: The height of such mountains varies from 1,500 to 2,000 meters.
iii. High Mountains: The height of these mountains is above 2,000 meters.

2. Classification based upon Structure: On the basis of structure, mountain can be divided into the following classes:

Fold Mountains
These mountains are formed by compression generated by plate tectonics. When the compression acts upon the accumulated sediment, the latter begins to rise. In the process of uplifting, the layers of sediment are folded. Mountains formed in this way are called Fold Mountains. There are many folds in these mountains, which break down under great pressure. Such folds are called Nappe.

Block Mountains
These are also known as Fault-block landforms (mountains, hills, ridges, etc.) and are formed when large areas of bedrock are widely broken up by faults creating large vertical displacements of continental crust.
Vertical motion of the resulting blocks, sometimes accompanied by tilting, can then lead to high escarpments. These mountains are formed by the Earth's crust being stretched and extended by tensional forces. Fault block mountains commonly accompany rifting, another indicator of tensional tectonic forces.
The uplifted blocks are called block mountains or horsts. The intervening dropped blocks are termed graben. These can be small or form extensive rift valley systems. This form of landscape can be seen in East Africa, the Vosges, the Basin and Range province of Western North America, in south-central New England, and the Rhine valley. These areas often occur when the regional stress is extensional and the crust is thinned.

Mountains of Accumulation
These are the mountains which come into the existence due to the accumulation of the material. They can be of various types according to their geological features and accumulated material, such as volcanic mountains, mountains of glaciers etc.

ISLAND TYPES

Islands are broadly divided into four types :-
(i) Continental Islands,
(ii) Oceanic Islands,
(Iii) Tectonic Island, And
(Iv) Coral Island.
CONTINENTAL ISLANDS are those islands that rise from the ‘continental shelf’, for example, the British Isles and Newfoundland. These islands have the same geological structure, as the continents to which they are related.
OCEANIC ISLAND are those that rise from the BOSOM of the oceans. Their geological structure will have no geological relation to that of the nearest shores. They are very often the tops of submarine mountains or submarine volcanoes.

ASSENSION and TRISTAN DE CUNHA, for example, rise from the Central Atlantic ridge (mountain), while ST. HELENA and TENERIFFE are the islands formed by submarine volcanoes.
TECTONIC ISLAND are created by the movements in the Earth’s crust. The outer-most layer of the earth made of rigid plates are in very slow, but constant, motion. When one plate is pushed under another plate, the top plate may scrape off pieces of the bottom plate. Over millions of years, this material piles up to form an island. BARBADOS in the West Indies and KODIAK ISLANDS near Alaska were formed in the same manner.
CORAL ISLANDS are the work of minute sea organisms called CORAL POLYPS. They congregate in large colonies. When the organisms (constituting ecological community of coral reefs) die, their skeletons, which are made of a substance resembling limestone, form big clusters, some of which rise above the water.

WORLD’S LARGEST ISLANDS

DESERTS

When people think about deserts, they usually imagine vast sand dunes under a burning sun, with nothing to break the monotony of the landscape except for an occasional palm-filled oasis. Although places like this certainly exist, the word `desert´ encompasses a wide range of landscapes, many of which are rich in wild plants, animals, and people, adapted to difficult conditions. Characteristics of deserts include very low rainfall and humidity, high evaporation rates, and little cloud cover. Many deserts are hot but some can be bitterly cold; there are ice deserts in the polar regions.
Desert soils are generally poor because they contain little vegetable matter; if water is made available, most deserts are capable of sustaining agriculture.
Natural deserts occur when geography and climate combine to prevent significant amounts of rain falling on an area.

Tropical deserts are caused by the descent of air that is heated over warm land and has previously lost all its moisture.
Continental deserts, such as the Gobi in Mongolia, are too far from the sea and large inland waters to receive any significant moisture.
Rain-shadow deserts lie in the lee of mountains where the rain falls on the windward slope, as is the case in the Mojave Desert beyond the Sierra Nevada in California.
Coastal deserts, such as the Namib in southern Africa, are formed when cold ocean currents cause local dry air masses to descend thereby leading to high pressure.
Animals have also developed special adaptive strategies. In hot deserts, most lie dormant during the day; if the nights are cold, small mammals confine their activities to the hours of sunrise and sunset, and spend the rest of the time sheltered in their burrows. Birds migrating across desert regions follow routes that have remained the same for hundreds or thousands of years, stopping at occasional oasis of water and vegetation along the way.

Desert Cultures
Human cultures survive under harsh conditions. Human societies also exist in deserts, although, like any other animal, they have had to adapt. Most live on the edges of desert regions, and the only settled communities are around oasis. Truly desert cultures are nomadic, moving constantly to find food for their livestock or fresh supplies of water, and living in temporary accommodation. The Bedouin of Arabia and N Africa (the word Bedouin means `desert dweller ´ in Arabic) lived for centuries in tents made of animal skins, and traded in camels and horses, although they are now increasingly being forced to settle in one place. In the Gobi Desert in Mongolia, nomadic tribes lived in temporary structures known as yurts. The people of the Kalahari Desert in Botswana (formerly known as Bushmen) have no permanent dwellings at all and traditionally live a hunter-gatherer existence; their knowledge of desert survival is so acute that they spend only a small amount of time every day on gathering food and drink.

Desertification
Deserts are expanding. Unfortunately deserts can also be formed as a result of human mistakes. Degradation of a dry-land area - through deforestation, overgrazing, poorly designed irrigation systems, and soil erosion - can lead to a complex cycle of changes that result in desertification. Loss of trees and ground cover means that water is no longer trapped by vegetation, increasing the evaporation rates and often also the frequency and severity of drought. Mismanagement of irrigation can lead to a raised salt content in the soil, preventing crop growth and adding to risks of desert formation. Overgrazing, particularly by animals like goats that can strip vegetation bare, is often a contributory factor. Throughout the world, desertification is accelerating, and according to the United Nations, a hundred countries are affected.
Main Regions Of Natural Desert
The Earth has five main regions of natural desert.
By far the largest is the Afro-Asian desert, a vast belt that stretches from the Atlantic Ocean to China, and includes the African Sahara, the Arabian, Iranian, and deserts of the Middle East and central Asia, the Thar Desert in Pakistan and India, and the Takla Makan and Gobi deserts of China and Mongolia.
Also in Africa, the Namib and Kalahari deserts cover a large area in the southwest.
The North American desert covers much of southern USA and northern Mexico.
Further south, the Atacama Desert is a thin, arid strip on the coastal side of the Andes, and the Patagonian desert covers much of Argentina east of the Andes.
The Australian desert covers much of the interior of the world's largest island.
The main categories of DESERT are:
(i) THE HOT (TROPICAL) DESERT-These are the areas of high atmospheric pressure around the subtropical high belt, with rainfall less than 25 cm, high summer temperatures; e.g., the Sahara, Arabian, Thar desert.
(ii) THE COASTAL DESERTS-These are on the western margins of continents in latitudes 15-30, with cold offshore currents, and low summer temperature; e.g., Atacama, Namib, Kalahari, Mojave deserts.
(iii) THE MID-LATITUDE DESERTS-These are the deserts of continental interiors far from rain bearing winds and moderating impact of oceans, with high summer and low winter temperature; e.g., Gobi, Taklamakan, Turkestan, Australian deserts.
(iv) THE ICE AND SNOW DESERTS-These are the cold deserts of polar lands : e.g., the Greenland, the Antarctica.

Classification of deserts

RIVER ACTION

Upper stages
Rivers are one of the greatest sculpturing agents at work in humid regions. In its youthful stage the river flows turbulently in a narrow, steep-sided valley whose floors are broken by pot holes and waterfalls. A youthful valley is ‘V’ Shaped, with steep gradient. The water of a fast-flowing river swirls if its bed is uneven. The pebbles carried by a swirling river cut circular depressions in the riverbed. These gradually deepen and are called potholes. Much larger but similar depressions form at the base of a waterfall under the consistent impact of falling water; these are called plunge pools.
Interlocking spurs are another feature of a youthful valley formed due to erosion of softer portions of bedrock in the valleys.
Some valleys have very steep sides and are both narrow and deep; these are called gorges. A gorge is often formed when a waterfall retreats upstream. One on the most famous gorges formed in this way lies below the Victoria Falls. A gorge will also form when a river maintains it course across a terrain which is being uplifted. The Indus, the Brahamaputra and the headwaters of the Ganga have cut deep gorges in the Himalayas. A
huge gorge is called a canyon; they usually occur in dry regions where large rivers are actively eroding vertically and where weathering of the valley sides is at a minimum.
Valleys of the mature stage have the shape of a ‘V’ in cross-section. The gradient is gentler, river beds are more pronounced, spurs are removed by lateral erosion and there remains a line of bluffs on each side of the valley floor.

Middle and lower stages
After the stage of maturity is reached the river begins to overflow its banks and it deposits fine silts and mud on the valley floor. This is the final stage in the growth of a flood plain. Meanders are pronounced and cutoffs develop and produce ox-bow lakes. The river builds up its banks with alluvium (the banks are called levees). The river thus flows between pronounced banks and above the level of the flood plain. In process of time, river erosion, transportation, and deposition turn the original surface into an almost level plain, which is called a peneplain.
River and their tributaries drain an area, which is called a ‘river basin’; its boundary formed by the crest line of the surrounding highland is the watershed of the basin.
A river at any stage of its development from youth to old age may be rejuvenated, and a young valley may occur in an old landscape. Where the river crosses from the original flood plain to the new flood plain, there may be a waterfall or rapids: this point is called knickpoint.

Deltas
A delta is formed at the mouth of a river where it deposits more material than can be carried away, as the speed of the river is reduced by the time it enters a sea or lake. Also, fine clay particles carried in suspension in the river coagulate in the presence of salt water and get deposited.
In the first stage, deposition of sediment chokes the flow and divides the river into several distributaries. Spits and bars arise and lagoons are formed. (Lagoons are shallow stretches of water separated from the sea by a barrier such as a spit). In the next stage the lagoons begin to get filled in with sediments, and they become swampy. The delta begins to assume a more solid appearance. In the third stage the old part of the delta becomes colonized by plants. As a delta grows larger, the old parts merge imperceptibly with the flood plain, and they no longer have the appearance of a delta.

Types of Delta

Arcuate: composed of coarse sediments such as gravel and sand and is triangular in shape. It always has a number of distributaries. River having this type of delta are “Nile”. “Ganga”, “Indus”, “Irrawady”, “Mekong”, “Hwnag Ho”, “Niger”.
Bird’s Foot/Digitate: It is composed, of very fine sediment called silt. The river channel divides into few distributaries only and maintains clearly defined channels across the delta. The “Mississippi Delta” is one of the best example.
Estuarine: Develops in the mouth of a submerged river i.e. river falling into deep sea. Rivers like “Amazon”, “Ebe”, “Ob”, “Vestula” from this type of Delta.
Cuspate: Only few rivers like Ebro of Spain form such type of delta. These have tooth-like projections and are formed when a river drops sediments on a straight shoreline with strong waves.
Delta can and do form on the shores of high tidal seas e.g. river Colorado (Gulf of California) and River Fraser (British Columbia)
Any rivers, irrespective of its development can build a delta. The “Kander” whose valley is in stage of youth has built delta lake in lake Thun (Switzerland)

DRAINAGE SYSTEM AND PATTERNS

Drainage refers to a body of flowing water, ranging in scale from a rill to river. On the basis of the origin and evolution of drainage system, Geological Structure, there are two broad categories of drainage systems.

Sequent Drainage System
Sequent Drainage System are system of stream which follow the regional slope and are well adjusted to geological structure. These are four types
 1. Consequent Stream: Consequent streams are streams whose course is a direct consequence of the original slope of the surface upon which it developed, i.e., streams that follow slope of the land over which they originally formed. Most of the streams draining the coastal plains of India are of this type. The ideal landscape for the origin and development of consequent drainage systems are domes and volcanic cones.

2. Subsequent Stream: When the master consequent stream is joined by its tributary at right angels, it is called subsequent stream. For example, the river “Asan”, a tributary of Yamuna and river “Son”, a tributary of the Ganga are the subsequent streams..

3. Obsequent Stream: The stream which flows following the direction of slopes opposite to master consequent stream. For example, the Mahabharata Range of the Lesser Himalayas has originated several streams from its northern slope which flow as obsequent stream opposite to the direction of flow of master consequent streams like Ganga and Yamuna

4. Resequent Stream: Such streams whose course follows the original relief, but at a lower level than the original slope (e.g., flows down a course determined by the underlying strata in the same direction).

Insequent Drainage System
The streams which do not follow the regional slopes and drain across the geological structure randomly constitute insequent drainage system. These are of two types

1. Antecedent Drainage: The streams which originated before the upliftment of the surface on which they flow. For example, Indus; Sutlej and Brahmputra are antecedent rivers as they originated before the upliftment of Himalayas ranges and hence create deep gorges along the mountain ranges.

2. Superimposed Drainage: It is formed when the nature and characteristics of the valley and flow direction of a consequent stream develop on the upper geological formation and structure are superimposed on the lower geological formation of entirely different characteristic. For example, river Subarnarekha is superimposed on Dalma Hills to the west of Chandil in the Chota Nagpur plateau region of Bihar.

DRAINAGE PATTERNS

(On the basis of Nature of original surface and slope)
Dendritic Pattern: A drainage pattern consisting of a single main stream with tributaries resembling the branches of a tree. Such pattern is most fully developed where the underlying rocks are of uniform type and the structures are relatively simple.

Trellis Drainage: It is a rectangular pattern of river channels. It may develop where a slope is crossed at right angles by the strike of alternating hard and soft rock strata. Long streams develop along the soft rock strata, parallel to the strike and the short streams follow the slopes.

Radial Drainage: It is a drainage pattern in which streams radiate from a central peak or upland mass in all directions. It is also called centrifugal pattern. Dome structure commonly develops radial drainage as in the English Lake District. The entire drainage network of Sri Lanka, Hazaribagh Plateau, Parasnath Hills, Panchet Hills and Dalma Lava Plateaus are of such type.

Rectangular Drainage: A pattern of drainage consisting of two main directions of flow at right angles to one another. The pattern is most commonly caused by streams following fault lines that have a rectangular pattern. Here, both the mainstreams and its tributaries display right angled bends.

Annular Drainage: Here, streams follow roughly a circular pattern. Such patterns are usually produced on domed structures where the rivers follow the outcrops of weaker beds of rocks, alternating bands of hard and soft beds. It is also known as circular pattern, e.g., Sonapet Dome of Bihar.

Parallel Drainage: A pattern in which the mainstream and the tributary streams follow virtually parallel courses. This develops where there is a strong structural control in one direction, for example, where strata are gently dipping.

Barbed Drainage: In this pattern, the tributaries flow in opposite direction to their master streams. The tributaries join their master streams in a hook-shaped bend. Such pattern in generally developed due to river capture.

Centripetal Drainage: When the streams coverage at a point, which is generally a depression or a basin, they form centripetal or inland drainage pattern, for example, the Kathmandu Valley of Nepal.

Pinnate Drainage: It is developed in a narrow valley flanked by steep ranges. The tributaries originating from the steep side of parallel ridges join the longitudinal master consequence, occupying the valley at acute angles, e.g., the drainage network of angles, e.g., the drainage network of the upper Sone and Narmada river.

LIMESTONE AND CHALK FEATURES

Action of the carbon dioxide in rain water on limestone converts the insoluble calcium carbonate into soluble calcium bicarbonate. Limestone is a well-jointed rock and its joints and bedding planes are opened up by rain and water, and in time the surface consists of broken and rugged rocks. One of the most noticeable features of the limestone landscape is the almost complete absence of surface drainage. Rivers rising in non-limestone areas, on entering the limestone region, disappear into vertical holes on the surface and continue to flow as underground rivers. The vertical holes called swallow holes or sink holes are formed by rivers and they are usually widened vertical joints. Swallow holes may join together to give a very large opening, called doline. Dolines may join together to give even larger opening called uvula. Rivers that flow inside limestone develop underground caves and caverns as they flow along joints and bedding planes. In the caves, deposits of minerals, especially calcium carbonate, are formed by evaporation of calcium carbonate charged water. The deposits hanging from the ceiling are known as stalactite, and those that grow from the floor of the caves are known as stalagmites. Sometimes the two join to form pillars or columns. Apart from these, there are flowstones in underground caves.

Limestone regions are generally barren with a very thin soil cover. The porous nature of rocks and absence of surface drainage means that vegetative cover is minimal. Only short grasses and turfs are able to grow and hence human settlements are also very few. But limestone regions yield important building material for the cement industry. Also some lead is found in limestone rocks.

Chalk features

Chalk is extremely pure form of limestone and being made of calcium carbonate is susceptible to solution by rainwater. But since chalk is of extremely Friable nature, it crumbles easily and is not able to form swallow-holes, underground caves and other features associated with limestone.

GLACIER ACTION

The level above which there is perpetual snow cover is called the snowline. The snowline varies with altitude and latitude. In the polar region it is at sea-level; in East Africa it is at 5000 m; in the northern hemisphere it is lower on the shady north-facing side of a mountain than the south-facing side. When the accumulation of snow in a region increases year by year it gradually turns into ice by its own weight. Masses of ice that cover large areas of a continent are called ice sheets, and those, which occupy mountain valleys, are called valley glaciers.
A glacier is defined as a mass of ice that moves under the influence of gravity along a confined course away from its source area, however, the movement is not of the glacier as a whole. Throughout the glacier bits of ice are melting, trickling downvalley and then turning back into ice the whole time. This means that within the glacier there is a gradual down valley movement.
Glacial erosion consists of two processes:

(i) plucking or the tearing away of blocks of rock which have become frozen into the base and sides of a glacier, and (i) abrasion or the wearing away of rocks beneath a glacier by the scouring action of the rocks embedded in the glacier.
The erosional features produced by glaciers include the cirque or corrie. A glacier while moving downhill, abrades the floor thereby steepening it; and there is the process of Plucking on the back-wall. The resultant action leads to creation of horse shoe shaped basin called cirque. A cirque or corrie originates as a small hollow where snow accumulates. The snow becomes compacted to glacial ice, forming a cirque glacier, and eventually flows downslope under the influence of gravity. Many cirques form small circular lakes called tarns. Sometimes corries develop on adjacent slopes and only a knife-edge ridge, called an arete, separates them. Where three or more cirques cut back together, a pyramidal peak or angular horn is formed.

If a glacier extending down has to negotiate a bend or a precipitous slope, there develop cracks in it. These cracks are very deep and are called crevasses. As the amount of ice in valley increases, the power to erode by a valley glacier also increases. This results in the glacier deepening, straightening and widening a river valley. The over deepening of the valley gives it a characteristic U shape marked by wide flat floor and steep sides.
Hanging valleys are another common feature in areas that have been glaciated. These are tributary valleys that lie above the main valley and are separated from it by steep slopes down which streams may flow as a waterfall or a series of rapids. The hanging valleys are formed when the main valley is eroded much more rapidly than the valleys made by tributary glaciers. As a result these smaller valleys hang above the main valley.(Hanging valleys may also form during the retreat of a coastline under rapid erosion.).
A valley glacier carries a large amount of rock waste called moraines. The moraine forming along the sides of a glacier is called lateral moraine; that along the front of a glacier is called terminal moraine; that at the bottom of a glacier is the ground moraine. When two glaciers join together their inner moraines coalesce to give a medial moraine. Terminal moraine material is carried down-valley by the melting waters issuing from the glacier’s snout (front) and is deposited as a layer called an outwash plain. One of the most conspicuous features of lowlands, which have been glaciated by ice sheets, is the widespread morainic deposits. Because of the numerous boulders in the clay these are called boulder clay deposits. The deposits are sometimes several hundred meters thick and their surface is marked by long rounded hills, called drumlins. Large blocks of rock of a material quite different to that of the rocks of the region often occur in areas, which lay under ice sheets. These blocks are known as erratics. Rivers and streams occur inside most glaciers and these are heavily loaded with rock debris. As an ice front retreats the rivers build up ridge-like deposits called eskers. They develop on top of the boulder clay deposits.

WIND ACTION

Wind is a very effective agent of erosion, transportation and deposition and more so in arid areas where there is little vegetation or moisture to bind the loose surface materials. Erosion by wind is carried out by deflation, abrasion and attrition.
Deflation involves lifting and blowing away of loose materials by wind action thereby lowering the ground levels and producing depressions called deflation hollows. Abrasion takes place when the loose sand particles are carried by wind and hurled against the rocks thereby leading to abrasion or scratching of the exposed surfaces. Attrition takes place when wind born particles of sand wear each other away to form small rounded sand grains.

Landforms of Wind Erosion
Due to abrasion or sand blasting by wind borne sand particles produces grooves and hollows in the rock surfaces thereby leading to formation of Rock Pedestals. Since the erosion is maximum near the base where most of the wind born sand particles strike, there results an undercutting in the rocks thereby giving it the form of a mushroom shaped rock.
Some regions have tabular masses of soft rocks lying beneath a layer of more resistant rocks. Wind action erodes the softer layers at a faster rate thereby giving rise to a ridge and furrow landscape called Zeugen. At times instead of lying over one another, the hard and soft rocks exist as alternate vertical bands. Wind abrasion excavates the bands of softer rocks into long narrow corridors separated by ridges of hard rocks called Yardangs.
Mesas or buttes are other features of wind erosion. These are flat table like land masses with a very resistant horizontal top layer that resists erosion and protects the softer layers underneath it. Generally a butte is smaller than a mesa but geographically both are similar.
Inselbergs are Island Mountains or isolated residual hills. They are the relics of an old plateau which has been entirely eroded away by the action of wind. A part of the original plateau being made up of hard granite or gneiss resists erosion and stands out as an isolated hill with rounded tops and steep sides and is called an Inselberg.
Ventifacts or dreikanters are pebbles polished and smoothened by abrasive action of wind.
At times the winds lower the ground surface by blowing away loose unconsolidated materials, thereby giving rise to depressions called the deflation hollows. At times the deflating action of wind lowers the deflation hollows to the extent that the water table is reached thereby leading to formation of oasis or swamps in arid regions.

Landforms of Wind deposition
The sand particles being carried away by wind may get deposited around an obstacle thereby leading to formation of hills of sands called sand dunes. Dunes rooted with vegetation are fixed dunes; while the ones which are on constant move are active or live dunes (The wind action keeps shifting the sand thereby causing the movement of the dunes).
Sand dunes are of two types namely Barchans and Seifs. Barchans are crescent or moon shaped dunes which occur transversely to the wind direction such that their horns thin out and become lower in the direction of the wind due to reduced frictional retardation of wind around the edges. The windward side is convex and gently sloping while the leeward side being sheltered is concave and steep. The sand is driven up on the windward side and after reaching the crest of the dune it falls down towards the concave side so that the dune advances or moves. Migrating sand dunes often threaten human habitations and trees and oasis.
Seifs are longitudinal dunes which are like narrow ridges of sand lying parallel to the direction of winds.
Loess plains are another depositional feature of the wind action. The fine dust of deserts is often blown away beyond the desert limits and is deposited on neighboring lands as loess. Loess is generally fine loam, rich in lime and highly porous. Loess plains are found in the basins of the Hwang-ho.

COASTAL LANDFORMS

Action of waves, tides and currents
Waves are the most powerful agents of marine erosion as well as transportation. Tides and currents are weaker agents of erosion and mainly play the role of transporting the eroded material and depositing it as silt, sand and gravel along the coasts.

These agents of marine erosion transform the coastal landscape by the processes of –
• Corrosion- waves armed with rock debris and other fine materials strike the coast line and wear them by corrosion. The waves and tides remove the eroded material.
• Attrition- the movement of eroded material by waves leads to abrasion against one another leading to attrition.
• Hydraulic action- wave waters while striking the coastline enters into the joints and crevices of coastal rocks and compresses the sir trapped within. When the water recedes the air expands explosively. This repeated contraction and expansion widens the joints eventually leading to breaking of rocks.
• Solution- the salt laden sea water also erodes the coastal landscape by chemical solution.

Coastal features of erosion
The repeated action of waves against a coastal cliff gives rise to wave cut platforms. These are narrow flat areas often found at the base of a sea cliff or along the shoreline created by the erosive action of waves. Wave-cut platforms form when destructive waves hit against the cliff face, causing undercutting between the high and low water marks mainly by the actions of corrosion and hydraulic action thereby forming a notch on the cliff. This notch under repeated wave action develops to form a flat platform.

Similarly, on coasts containing rocks of varying resistance to erosion, the continuous wave action gives birth to capes and bays. Under erosive action of waves the softer rocks are worn back giving rise to bays and the resistant ones project out of the coastline as capes.

Continued wave action on cliffs excavates holes in the softer portion of cliffs. Such holes gradually enlarge to form caves. When two caves approach one another back to back they give rise to an arch. Eventually under continued wave action, the arch collapses to form a pillar of rock called stack. Further erosion of stacks give rise to short pillars called stumps.

Repeated erosive action of waves on the roof of a cave may give rise to blowholes or gloups. The continued action of waves on the cave roof will lead to enlargement of the gloup to such an extent that the roof will collapse giving rise to a geo.

Coastal landforms of deposition
Sand and gravel eroded from the coastlines is deposited along the shore by waves and tides as beaches. The Longshore Drift moving obliquely to the shore deposits the material along it. The coarser and heavier material like shingles, pebbles, boulders are deposited at the top of the beach while the finer materials like sand are carried by the backwash and deposited closer to the sea.
At times the material carried by waves is deposited at the mouth of a river or a bay thereby giving rise to spits and bars. The water body enclosed by a bar is called a lagoon.

The strong onshore winds often carry fine sand from the coastal beaches inland thereby giving rise to marine dunes another landform associated with coastal landscapes.