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LANDFORMS AND THEIR EVOLUTION
Small to medium tracts or parcels of the earth’s surface is known landforms.
Several related landforms combine together to make landscapes.
Each landform has its own characteristic features such as shape, size, and materials and is a result of the action of certain geomorphic processes and agents of change.
Running water, groundwater, glaciers, wave, and winds are five agents of change which bring about changes on the surface of the earth and give birth to various types of landforms.
These agents of change work through the processes of erosion, transportation, and deposition.
They cause erosion of rock materials by virtue of their force and transport the eroded material to some other place. These two processes are followed by deposition.
RUNNING WATER OR RIVER
When rainfall occurs, water flows down the slope of the land and removes rock material in the direction of flow.
The gullies further deepen, widen and lengthen and unite to form a network of valleys.
At the early stage down cutting dominates during which irregularities are removed.
In the middle stage, streams cut their beds slower and lateral erosion becomes dominant.
At a later stage, the valley sides are reduced to lower and lower slopes.
It is clear from the above discussion that the work of a river can be divided into three stages viz. youth, mature and old.
YOUTH
In the youthful stage, streams are few with poor integration which flows in shallow V-shaped valleys over the original slopes.
Down cutting and headward erosion are more prominent. Stream divides or watersheds are broad. Gorges, Canyon, V-shaped valley, waterfall, rapids, etc are the main features in this stage.
There are no floodplains.
Meanders, if present develops over the upland represented by the stream divides. River capture or stream piracy is a major event of this stage.
MATURE
Streams are plenty with good integration.
Down cutting gradually give way to lateral cutting.
Stream divides start becoming narrow and V-shaped valleys become broad.
Trunk streams are broad enough to have wider flood plains within which streams may flow in meanders confined within the valley.
Deposition starts along with erosion. River meandering, ox-bow lakes, and flood plains are the main features of this stage. Waterfalls and rapids disappear.
OLD
Almost all the features of the youthful, stage disappear and the river reaches its old stage.
The river does not do any erosional work and deposition is the main work done by the river.
The valley becomes still broader and the stream divides become still narrower.
The river flows in peneplain.
Delta is the most prominent feature of this stage.
Sometimes flood plains and natural levees are also formed.
EROSIONAL LANDFORMS
Erosional work by the river is a dynamic process that is involved in the removal of the rock material from the area over which it flows.
The erosion work of the river can either be physical or chemical.
The river does the erosional work by the energy of its water which is known as its physical or mechanical erosion.
The water also dissolves many rock minerals in its valley which is called chemical erosion.
The water adopts different processes of erosion under different sets of conditions.
1. HYDRAULIC ACTION
The Pressure and drag of flowing water exerted upon grains projecting from the bed and banks result in hydraulic action.
Weak bedrock and various forms of regolith are easily carved out by hydraulic action.
2. ABRASION
The mechanical wearing of the rocks is called abrasion.
It occurs when rock particles carried in the current strike against the exposed bedrocks of the channels.
Small particles are further reduced by crushing and grinding when caught between larger cobbles and boulders.
3. SOLUTION
Chemical reactions between ions carried and solution in river water and exposed minerals surfaces result in the form of erosional which is called a solution.
TYPES OF EROSION
Following three types of erosion by the river are normally recognized:
(i) HEADWARD EROSION
This is the process of cutting back upstream of a valley above its original source by rain-wash, bullying, and spring-sapping.
Thus, the source of the stream gradually recedes and ultimately may notch the ridge which forms the original watershed. This may ultimately lead to river capture.
(ii) VERTICAL EROSION
When the river cuts the rock material in a vertical direction e., when the river is engaged in down cutting, it is called vertical erosion. This leads to the deepening of the river valley.
(III) LATERAL EROSION
Lateral erosion takes place when the river cuts the sides of its valley.
Lateral erosion results in the widening of the river.
DEPOSITIONAL WORK OF RIVER
The moment the transporting power of the river decreases due to a decrease in the velocity or volume of its water, the river starts depositing its load.
Most of its load is deposited along the banks in the lower valley or at its mouth.
Nearly one-fourth reaches the sea.
The first to be deposited are big boulders which are followed by pebbles sand and clay.
1. GORGE
In highlands when the river passes through a bed of hard rocks, the main action of the river is down cutting.
This results in a narrow and steep-sided valley which is known as The Sutlej, the Indus, the Brahmaputra, the Gandak, and the Kosi rivers have cut deep gorges in the Himalayas.
2. CANYON
This is just a magnified form of a gorge.
In areas of arid climate with little rainfall, the valley sides fail to be widened at all and the river cuts deeper only in its floor.
A canyon is a variant of a gorge, still, there is some distinction between the two.
A gorge has very steep straight sides while a canyon is characterized by steep step-like side slopes.
A gorge is almost equal in width at its top than at its bottom.
Canyons commonly form in horizontal-based sedimentary rocks and gorges form in hard rocks.
3. V-SHAPED VALLEY
Most of the world’s rivers have formed V-shaped valleys.
Such valleys are normally formed in areas of sufficient rainfall where the rocks are not very hard.
Down cutting and side or lateral cutting are done simultaneously by the river and V-shaped valley are formed.
4. WATERFALLS
When the river waterfalls down almost vertically from a sufficient height along the course of the river, it forms a waterfall, Hard and soft rocks are found at several places in the course of the river.
The soft rocks are eroded easily and quickly and the river bed is lower at the place of soft rock.
Thus, the waterfalls from a
5. RAPIDS
Sometimes there is a band of hard-rock along the path of a river which makes it jump over or fall downwards.
This leads to the formation of rapids at places where the hard valley bottom offers greater resistance to erosion than the strips above and below it.
A series of rapids is called a cascade.
6. POTHOLE AND PLUNGE POOL
Whenever a soft rock comes in the way of the river, it is eroded and scattered all around.
Eddies are formed and water whirls around and produces depressions by plucking the sediment up.
This makes the depression deep and cylindrical.
Sometimes these depressions look like discs and are known as potholes. Stone pieces also enter the potholes along with water and act as grinders.
The potholes have a diameter varying from a few centimeters, to many meters.
The depth of a pothole is greater than its diameter.
Potholes of over 7 meters depth have been observed. When the potholes grow in size they are called plunge pools.
7. INCISED OR ENTRENCHED MEANDERS
These are very deep and wide meanders cut in hard rocks by the action of a river.
Such meanders develop over the original land surface in the initial stages of the development of streams and get entrenched into rocks due to erosion or uplift of land.
They widen and deepen in the course of time.
8. RIVER TERRACES
River terraces are terraces found on both sides of the river valley and represent old valley floor or phases of plain levels.
They may be bedrock surfaces without any alluvial cover or alluvial terraces consisting of stream deposits.
The river terraces may occur at the same elevation on either side of the rivers in which case they are called paired terraces.
The terraces may result due to
(i) receding water after a peak flow;
(ii) change in hydrological regime due to climatic changes;
(iii) tectonic uplift of land;
(iv) sea-level changes in case of rivers closer to the sea.
LANDFORMS FORMED BY RIVER:
The river forms a large variety of landforms through its erosion, transportation, and deposition throughout its source course from its source to the course of the river can be divided into the following three parts:
- Upper Course
- Middle Course
- Lower Course
1. Landforms by the Upper Course River.
The upper of the mountain course begins at the source of the river in hills or the mountain areas.
The river tumbles down the steep slopes and as a result of its high velocity, its erosion and transporting power are at the maximum.
(i) ALLUVIAL CONES AND ALLUVIAL FANS
The velocity of river water is suddenly reduced due to a reduction in slope when it enters the plains after leaving the mountains after leaving the mountain.
Hence, the rock material brought down by the river from highlands is deposited at the foothills. This is generally coarser material composed of sand, gravel, and boulders.
In due course of time, the deposited material projects like a cone from the foothill and are known as an alluvial cone.
The river crosses alluvial cones’ by dividing itself into many channels and forms alluvial fans.
The alluvial fan is semi-circular in form and points upstream.
It is broader than the alluvial cone but its height is less than the alluvial cone.
Almost all the rivers coming out of the Himalayas, Rockies, and Andes mountains form alluvial fans.
(ii) RIVER MEANDERING AND OXBOW LAKE
A river normally follows a curved path rather than going straight.
To and fro movement of the river channel across its plain results in developing S-shaped meanders, common to all rivers of large size.
The speed of the river water slows down and the river does both the works of erosion as well as deposition.
LANDFORMS IN THE LOWER COURSE OF THE RIVER
The load-bearing capacity of the river is drastically reduced in its lower course due to the sluggish flow of water as a result of the gentle slope of the land.
Therefore, the work of the river in this course is entirely that of deposition and it does not do any erosion at all.
(i) BRAIDED STREAM
The lower river plain is characterized by an excess deposit of the load on the floor of the channel because of the reduction in the carrying capacity of the slow-moving stream.
The stream which thus gets divided into a network of channels, forming bars of sand and islands,
(ii) NATURAL LEVEES AND FLOOD PLAINS
The current of the river is slowest at the sides and bottom because of retardation due to friction, in the lower course of the river, the slightest slackening of speed at once results in the deposition so that silt accumulates at the bottom and the sides of the river.
After a long time, the bed of the river is raised and it flows between raised banks along a bed which is at a higher level than the plains on either side, such raised banks or embankments are known as natural levees.
Most of the sediment of the river is deposited there after the flood is over. In this way, a vast plain is formed which is known as a flood plain.
The flood plains receive fresh soil every year and are very fertile. Rivers Huang Ho, Mississippi, Ganga, Indus, Nile, etc. have formed extensive flood plains.
(iii) DELTA
When the river enters the sea or a lake, it deposits the entire load at its mouth giving rise to the formation of a delta. It is a triangular feature with its apex pointing upstream and is marked as a fan-shaped area of fine alluvium.
The mud deposited along the banks because of the slow carrying capacity of water becomes an obstacle and divides the river into numerous branches called distributaries.
The development of the network of such channels and sub-channels goes on extending the delta area. The river has no alternative but to discharge its water sluggishly only through these distributaries.
The size, shape, and rate of growth of the delta are variable as it depends upon the interplay of a number of factors.
The Ganga-Brahmaputra Delta, 1,25,000 sq. km. in area, is one of the biggest deltas in the world. Other well-known deltas are those of rivers Nile, Mississippi, Po, Volga, and the Yangtze.
GROUNDWATER
Part of the surface water percolates in the ground and is termed groundwater.
All types of rocks do not hold the same quantity of underground water. The water-holding capacity of a rock depends upon the pore spaces which are known as its porosity.
Permeability is another property of a rock which means its capacity to allow water to pass through it.
The pore spaces or openings if connected with each
EROSIONAL WORK OF UNDERGROUND WATER
Underground water flows slowly and its physical erosional power is negligibly small. However, underground water is an important agent of erosion.
Erosion by underground water includes four different activities: corrosion, attrition, solution, and hydraulic action.
1. LAPIES
When underground water carrying carbon dioxide enters cracks and joints in a limestone area, it dissolves the surrounding rock and widens the cracks and joints.
Thus, long furrows with almost vertical walls are formed which are known as lapies in French.
2. SINKHOLES
A sinkhole is a funnel-shaped depression that has an average depth of three to nine meters.
Sinkholes are developed by enlargement of the cracks found in such rocks, as a result of continuous solvent action of the rainwater.
The surface rock gradually subsidises creating deep and wide sinks on the hillsides and valley floor indicating the intense activity of underground water.
There are many areas of limestone, dolomite, and gypsum where sinkholes are found in large numbers. The best example is that of the limestone plateau of Kentucky in the United States of America, where over 60,000 sinkholes are found. In India, these are observed along the southern edge of Meghalaya’s limestone strata.
3. SWALLOW HOLES
Swallow holes are cylindrical in shape lying underneath the sinkhole at some depth.
The surface streams en sink suddenly disappears underground through them.
It is so because these are linked with underground caves in rocks through vertical shafts.
4. DOLINE
When a sinkhole is enlarged due to the solution of the rocks by underground water, it becomes a doline. Dolines are funnel-shaped at the surface and cylindrical below it.
5. UVALAS
The walls of adjacent dolines collapse due to solution by underground water and they coalesce into a bigger hole known as Uvalas.
Uvalas are bigger in size than dolines.
They have vertical walls, a closed basin, and an oval shape.
6. PONOR
Sometimes uvalas are filled with water obtained from heavy rainfall.
They suffer large-scale erosion and many uvalas combine to form a sort of cave which is known as ponor. Sometimes a river falls into it and forms a bigger ponor.
7. CAVES AND CAVERNS
In certain areas, there are hard insoluble rocks at the surface and soluble limestone below it.
The underground water dissolves the limestone from below while the upper rock remains intact like a roof. Thus, a cave is formed.
A large limestone cave formed by solution due to
NATURAL BRIDGE
A part of the roof of a cavern collapses, but part of its remains intact and looks like a bridge. Since this bridge is made by nature, it is called natural bridge.
DEPOSITIONAL WORK OF UNDERGROUND WATER
The underground water dissolves a large number of minerals but the same is deposited elsewhere later on. Following features are formed due to depositional work of underground water:
1. STALACTITE
The water, containing limes one in solution, seeps through the roof of the caverns in the form of a continuous chain of drops.
A portion of the drops hangs on the roof and on the evaporation of water, a small deposit of limestone is left behind contributing to the formation of a stalactite.
A stalactite grows downwards from the roof. Its thickness is maximum near the roof and it thins out downwards.
2. STALAGMITE
The remaining portion of the drop falls to the floor of the cavern.
This also evaporates, leaving behind a small deposit of limestone aiding the formation of stalagmite, thicker and flatter, rising upwards from the floor.
3. CAVERN PILLARS OR COLUMNS
Sometimes a stalactite from above and stalagmite form below develop towards each other and combine together to form this is known as cavern pillar or column.
4. GLACIAL PROCESSES
Like a river, a glacier does the work of erosion, transportation, and deposition on the surface of the earth and forms a variety of landforms through these actions.
LANDFORMS CREATED BY THE EROSIONAL WORK OF A GLACIER
1. CREVASSES
It has been observed that there is the unequal movement of ice in a glacier.
The top of ice moves much faster than its bottom and ice in the center also moves faster than ice along the sides of the glacier.
The cracks marked on the glacier because of the splitting up of ice as a result of its unequal movement are known as crevasses.
2. BERGSCHRUND
When the upper part of a valley glacier comes out of the ice field along a steep slope, it develops a big crack known as bergschrund.
3. CIRQUE
It is a steep-walled rock basin on the side of a mountain resembling the shape of a vast amphitheater or an armchair.
It is open at one end, has a flat bottom and very steep slopes on three sides.
Often water accumulates in a cirque forming a lake. Such a lake is called cirque or tarn lake.
4. HORN
When several cirques erode cutting a mountain back towards a common height from different sides, a pyramidal peak is formed.
It is known as a horn because it resembles that shape. The Metter-horn peak of the Swiss Alps is its best example.
5. COLOR PASS
When two cirques from opposite sides of a hill develop and meet each other, col or a pass is many passes in the Alps were formed by glacial action. Some of the world’s passes are used for transportation.
6. COMB RIDGE OR ARETE
When the cirques from two sides of a mountain ridge are enlarged and extend towards each other, the ridge top is eroded and takes the shape of a comb.
Hence, it is known as comb ridge or arête.
7. GLACIAL TROUGH AND TROUGH LAKE
Plucking of bedrocks by the overriding glacier leads to the formation of a glacial trough, a channel of the valley glacier. The glacial trough, if filled up by water gives rise to Trough Lake.
8. FIORD
The glacial troughs formed near the sea get filled in by the seawater giving rise to floods.
9. U shaped Valley
Mountain glaciers cannot dig a new valley but deepen, straighten as well as widen the pre-existing valley by eliminating irregularities during its passage.
Such a trough has steep sides and a wider floor. This enables us to see over a longer distance in a glaciated valley.
The original V-shaped valley becoming narrower towards its head is turned into a
10. HANGING VALLEY
A steep slope develops at the confluence of the tributary glacier with the main glaciers.
After the snow melts, the water of the tributary glacier makes a waterfall when it enters the main valley and it seems that the tributary valley is hanging when seen from the floor of the main This is known as hanging valley.
DEPOSITIONAL WORK OF A GLACIER MORAINE
1. MORAINES
When the glacier melts, it starts depositing its sediments known as moraines.
Moraines consist of heterogeneous rock material of unsorted nature.
It is a mixture of fine sediments called glacier flour, angular stones, and boulders of different sizes and shapes ranging from
Moraines are of the following types.
(i)LATERAL MORAINES
Material deposited on either side of the glacier known as lateral moraine.
(ii) MEDIAL MORAINES
When two glaciers join, their lateral moraines also join near their confluence and are called medial moraines. Many alpine pastures in the Himalayas like Margs of Kashmir occupy the sites of morainic deposits of old valleys.
(iii) TERMINAL MORAINES
Material dropped at the end of a valley glacier is in the form of a ridge called a terminal moraine. Each time a glacier retreats, a fresh terminal moraine is left at a short distance behind the first one.
2. DRUMLINS
Drumlins are a peculiar type of low round hillocks that resemble the shape of an inverted boat or a half egg split lengthwise.
Drumlins are smooth and elongated hillocks of boulder clay, with their long axis parallel to the direction of the moving ice which deposited it.
Drumlins may measure up to 1 km in length and 30 m or so in height.
FLUVIO-GLACIAL DEPOSITS
Glacial deposition produces a variety of forms as a result of not only glacial but fluvioglacial action subsequent to the melting of ice in the lower parts of the glaciated region.
1. KETTLE HOLES
Usually, pebbles and other fragments are found lying over the glacier. When the glacier melts, pebbles and fragments subside and form a small depression.
A large number of kettle holes are found in the prairies of North America.
2. ESKER
When glaciers melt in summer, the water flows on the surface of the ice or seeps down along the margins, or even moves through holes in the ice.
These waters accumulate beneath the glacier and flow like streams in a channel beneath the ice.
Very coarse materials like boulders and blocks along with some minor fractions of rock debris carried into this stream settle in the valley of ice beneath the glacier and after the ice, melts can be found as a sinuous ridge called esker.
3. KAME
It is a sort of mound formed along the ice front.
A kame is formed by the sub-glacial streams and consists of assorted material-gravel and sands.
Many ridges are formed at the glacier margin and are known as Kame terraces.
4. OUTWASH PLAIN
It is so named because the material has been washed out of the morainic deposit. Since it spreads over the valley bottom from side to side, it is also given the name of ‘valley trains’.
WAVES AND CURRENTS
Sea waves and currents are most active along the sea coast and bring about drastic changes When waves break, the water is thrown with great force onto the shore.
Also, there is great force onto the shore. Also, there is great churning of sediments on the sea bottom. The constant impact of breaking waves brings about considerable changes on the coasts.
Storm waves and tsunami waves can cause far-reaching changes in a short period of time than normal breaking waves.
EROSIONAL LANDFORMS
1. SEA CLIFF
A scarp face of the coast facing the sea is known as a sea cliff.
In the beginning, the sea waves cut a groove in the rock at sea level which is known as a notch. This notch keeps on widening with the passage of time.
It thus undermines the overhanging rock until it falls into the sea and forms a cliff.
2. WAVE CUT PLATFORM
As the cliff retreats further by the erosional work of the waves, a wave-cut platform is produced. This is also known as Terrace.
3. SEA CAVES
Sea waves can erode soft rocks faster than the rocks.
At several places, rocks are traversed by joints, faults or bands of weak rocks.
Sea waves easily erode these weak points leading to the formation of sea caves.
4. MARINE ARCHES OR NATURAL BRIDGES
A marine arch or a natural bridge is formed when the sea waves working from opposite directions are able to cut through the caves.
5. STACK
When the roof of a marine arch collapses, a portion of the arch keeps standing in the sea like a pillar. This is known as
6. BLOWHOLES OR SPOUTING HORNS
If a hole developed in the roof of a sea cave, it is known as blowhole or spouting horn.
When the seawater enters the sea caves, the air of the cave is pressed up by seawater and the air passes through the hole with a noise.
Spouting horn is the name given because of the voice the blowholes make.
DEPOSITIONAL LANDFORMS
1. BEACH
A beach is formed due to the deposition of sand, gravel, and pebbles on the shore between low tide level and the coastline.
A beach grows in size when waves are less active but it may be completely destroyed by strong waves in a storm.
Most of the beaches are made up of sand-sized materials.
Beaches called shingle beaches to contain excessively small pebbles and even cobbles.
2. SAND DUNES
Just behind the beach, the sands lifted and winnowed from over the beach surfaces will be deposited sand dunes.
Sand dunes forming long ridges parallel to the coastline are very common along low sedimentary coasts.
3. SPIT
Spit is a ridge or embankment of sediments deposited by the waves and attached to the land at one end and projecting in the open sea at the other.
It is formed when the waves deposit the rock waste tangentially to the headland.
4. BAR
A bar is a sort of embankment of sand and gravel built by the depositional work of sea waves.
It normally extends between two adjoining headlands of hard rocks or runs roughly parallel to the shoreline.
A bar formed in the off-shore zone lying approximately parallel to the coast is called an off-shore bar.
An off-shore bar that is exposed due to further addition of sand is known as a barrier bar.
5. LAGOON OR HAFF
When bars further extend, the seawater is partially enclosed between the coast and the bar which is known as a lagoon lake or haff.
A lagoon generally maintains a connection with the open sea through a narrow gap between the bar and the headlands.
The Chilka and the Pulicat lakes along the East Coast and Vembanad on the Kerala Coast in the west are examples of lagoons in India.
WINDS
Work of wind is more prominent in desert areas where soil particles are loose due to a lack of moisture and vegetation.
Plenty of loose material is provided to be picked up by the blowing wind. Topography created by the erosional and depositional wind is called Aeolian topography.
EROSIONAL LANDFORMS
1. PEDIMENTS AND PEDIPLAINS
Pediments are gently inclined rocky floors close to the mountains at their foot with or without thin cover or debris.
These are formed as a result of erosion of the mountain front through a combination of lateral erosion by streams and sheet flooring.
Once the pediments are formed, the steep wash slope and free face retreat backward through parallel retreat of slopes and the pediments extend backward at the expense of the mountain front.
Gradually the mountain gets reduced leaving an inselberg which is a remnant of the mountain.
In this way, high relief in the desert areas is reduced to low featureless plains called pediplains.
2. PLAYAS
Playa is an inland drainage basin with a shallow saline lake of fluctuating volume, encircled usually by mud sheets.
Water remains for a short period only due to the high rate of evaporation in the dry climate of the desert. Quite often the playas contain deposits of salt and such playas are called alkali flats.
3. DEFLATION BASIN
Many depressions are formed by the deflation action of wind. These depressions are called deflation basins.
4. BLOW OUT
In several desert areas which lack vegetal cover, strong winds from eddies and blow away the loose and soft rocks.
Consequently, a saucer-shaped depression is formed which is known as a blowout.
5. MUSHROOM
The maximum erosion of an upstanding mass of rock occurs slightly above its ground level, where friction close to the ground is absent and sand content in the air is yet high.
The intensity of cutting into the rock decreases, both upwards as well as downwards from this level of maximum erosion by wind.
Due to greater erosion of the lower portion, the rock
DEPOSITIONAL LANDFORMS
When the velocity of the wind decreases, it starts dropping its load.
Some of the important depositional features due to wind action are the sand dunes and loess deposits.
1. SAND DUNES
A sand dune is the mo important feature formed by the depositional work of wind.
It is a mound, a hill, or a ridge of sand with a crest or a definite summit.
When the carrying capacity of sand-laden wind is reduced due to the reduction of its speed, it deposits its material whenever some obstacle comes in its way.
A dune has a long and gentle windward slope and a much steeper leeward slope.
2. BARCHAN OR BARKHAN
Barchan is the typical sand dune with a crescent-shaped front, having two horns or wings towards the leeward slope.
It has a convex windward side.
As the extremities of this dune move more rapidly than the middle portion, it gives it such a characteristic shape.
A constant wind direction and a limited supply of sand are two essential conditions for the formation of Barkhan.
PARABOLIC DUNES
Parabolic dunes are reverse barchans that are formed when sandy surfaces are partially covered with vegetation.
SEIF
Seif is similar to barchans with the only difference that it has only one wing or point. It is formed when there is a shift in wind conditions.
The long wing of seifs grows very long and high.
LONGITUDINAL DUNES
Longitudinal dunes are formed when the supply of sand is poor and wind direction is constant.
They have considerable length but have low height.
TRANSVERSE DUNES
Transverse dunes are in directions perpendicular to the wind directions.
These dunes are formed when the wind direction is constant and the source of sand is an elongated feature at right angles to the wind.
