Chapter-7- LANDFORMS AND THEIR EVOLUTION
Landform:- In simple words, small to medium tracts or parcels of the earth’s surface are called
landforms.
Each landform has its own physical shape, size, materials and is a result of the action of certain
geomorphic processes and agent
Several related landforms together make up landscapes, (large tracts of earth’s surface)
geomorphic agents are capable of erosion and deposition, two sets —
1-erosional or destructional and 2- depositional or constructional — of landforms are
produced by them.
varieties of landforms develop by the action of each of the geomorphic agents depending upon
especially the type and structure i.e. folds, faults, joints, fractures, hardness and softness,
permeability and impermeability, etc.
There are some other independent controls like (i) stability of sea level; (ii) tectonic
stability of landmasses; (iii) climate, which influence the evolution of landforms. Any
disturbance in any of these three controlling factors can upset the systematic and
sequential stages in the development and evolution of landforms
RUNNING WATER
In humid regions, which receive heavy rainfall running water is considered the most
important of the geomorphic agents in bringing about the degradation of the land surface. There
are two components of running water.
1. overland flow on general land surface as a sheet.
2. linear flow as streams and rivers in valleys
Most of the erosional landforms made by running water are associated with
vigorous and youthful rivers flowing over steep gradients. With time, stream channels
over steep gradients turn gentler due to continued erosion, and as a consequence, lose
their velocity, facilitating active deposition.
The gentler the river channels in gradient or slope, the greater is the deposition. When
the stream beds turn gentler due to continued erosion, downward cutting becomes
less dominant and lateral erosion of banks increases and as a consequence the hills
and valleys are reduced to plains.
Overland flow :-causes sheet erosion.
Depending upon irregularities of the land surface, the overland flow may concentrate into
narrow to wide paths.
Because of the sheer friction of the column of flowing water, minor or major quantities of
materials from the surface of the land are removed in the direction of flow and gradually small
and narrow rills will form.
These rills will gradually develop into long and wide gullies; the gullies will further deepen,
widen, lengthen and unite to give rise to a network of valleys
The divides between drainage basins are likewise lowered until they are almost completely
flattened leaving finally, a lowland of faint relief with some low resistant remnants called
monadnocks standing out here and there.
This type of plain forming as a result of stream erosion is called a peneplain (an almost plain.
Youth
Streams are few during this stage with poor integration and flow over original slopes
showing shallow V-shaped valleys with no floodplains or with very narrow floodplains
along trunk streams.
Streams divides are broad and flat with marshes, swamp and lakes.
Meanders if present develop over these broad upland surfaces. These meanders may
eventually entrench themselves into the uplands.
Waterfalls and rapids may exist where local hard rock bodies are exposed
Mature
During this stage streams are plenty with good integration.
The valleys are still V-shaped but deep;
trunk streams are broad enough to have wider floodplains within which streams may flow in
meanders confined within the valley.
The flat and broad inter stream areas and swamps and marshes of youth disappear and the
stream divides turn sharp. Waterfalls and rapids disappear
Old
Smaller tributaries during old age are few with gentle gradients.
Streams meander freely over vast floodplains showing natural levees, oxbow lakes, etc.
Divides are broad and flat with lakes, swamps and marshes. Most of the landscape is at or slightly
above sea level
EROSIONAL LANDFORMS:-
1. Valleys
Valleys start as small and narrow rills; the rills will gradually develop into long and wide
gullies;
the gullies will further deepen, widen and lengthen to give rise to valleys. Depending upon
dimensions and shape, many types of valleys like V-shaped valley, gorge, canyon, etc. can be
recognised.
A gorge is a deep valley with very steep to straight sides
and a canyon is characterised by steep step-like side slopes and may be as deep as a gorge.
A gorge is almost equal in width at its top as well as its bottom. In contrast, a canyonis wider at
its top than at its bottom. In fact, a canyon is a variant of gorge.
Valley types depend upon the type and structure of rocks in which they form. For example,
canyons commonly form in horizontal bedded sedimentary rocks and gorges form in hard
rocks
2. Potholes and Plunge Pools
Over the rocky beds of hill-streams more or less circular depressions called potholes form
because of stream erosion aided by the abrasion of rock fragments.
Once a small and shallow depression forms, pebbles and boulders get collected in those
depressions and get rotated by flowing water and consequently the depressions grow in
dimensions.
A series of such depressions eventually join and the stream valley gets deepened. At the foot
of waterfalls also, large potholes, quite deep and wide, form because of the sheer impact of
water and rotation of boulders. Such large and deep holes at the base of waterfalls are called plunge
pools.
These pools also help in the deepening of valleys. Waterfalls are also transitory like any other
landform and will recede gradually and bring the floor of the valley above waterfalls to the
level below.
3. Incised or Entrenched Meanders
In streams that flow rapidly over steep gradients, normally erosion is concentrated on the
bottom of the stream channel.
Also, in the case of steep gradient streams, lateral erosion on the sides of the valleys is not
much when compared to the streams flowing on low and gentle slopes.
Because of active lateral erosion, streams flowing over gentle slopes, develop sinuous or
meandering courses.
It is common to find meandering courses over floodplains and delta plains where stream
gradients are very gentle.
But very deep and wide meanders can also be found cut in hard rocks. Such meanders are
called incised or entrenched meanders.
Meander loops develop over original gentle surfaces in the initial stages of development of
streams and the same loops get entrenched into the rocks normally due to erosion or slow,
continued uplift of the land over which they start.
They widen and deepen over time and can be found as deep gorges and canyons in hard rock
areas. They give an indication on the status of original land surfaces over which streams
have developed.
4. River Terraces
River terraces are surfaces marking old valley floor or floodplain levels.
They may be bedrock surfaces without any alluvial cover or alluvial terraces consisting
of stream deposits.
River terraces are basically products of erosion as they result due to vertical erosion by
the stream into its own depositional floodplain.
The river terraces may occur at the same elevation on either side of the rivers in which
case they are called paired terraces (Figure 7.3).
When a terrace is present only on one side of the stream and with none on the other
side or one at quite a different elevation on the other side, the terraces are called unpaired
terraces.
Unpaired terraces are typical in areas of slow uplift of land or where the water
column changes are not uniform along both the banks.
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.
DEPOSITIONAL LANDFORMS
1- Alluvial Fans:
Alluvial fans (Figure 7.4) are formed when streams flowing from higher levels break
into foot slope plains of low gradient.
Normally very coarse load is carried by streams flowing over mountain slopes. This load
becomes too heavy for the streams to be carried over gentler gradients and gets dumped
and spread as a broad low to high cone shaped deposit called alluvial fan.
Usually, the streams which flow over fans are not confined to their original channels for
long and shift their position across the fan forming many channels called distributaries.
Alluvial fans in humid areas show normally low cones with gentle slope from head to toe and
they appear as high cones with steep slope in arid and semi-arid climates
2 -Deltas
Deltas are like alluvial fans but develop at a different location.
The load carried by the rivers is dumped and spread into the sea. If this load is not carried away
far into the sea or distributed along the coast, it spreads and accumulates as a low cone.
Unlike in alluvial fans, the deposits making up deltas are very well sorted with clear
stratification.
The coarsest materials settle out first and the finer fractions like silts and clays are carried
out into the sea. As the delta grows, the river distributaries continue to increase in length
and delta continues to build up into the sea.
3-Floodplains, Natural Levees and Point Bars
Deposition develops a floodplain just as erosion makes valleys.
Floodplain is a major landform of river deposition. Large sized materials are deposited
first when stream channel breaks into a gentle slope. Thus, normally, fine sized materials like
sand, silt and clay are carried by relatively slow moving waters in gentler channels usually
found in the plains and deposited over the bed and when the waters spill over the banks
during flooding above the bed.
A river bed made of river deposits is the active floodplain. The floodplain above the bank is
inactive floodplain.
Inactive floodplain above the banks basically contain two types of deposits — flood
deposits and channel deposits.
The flood deposits of spilled waters carry relatively finer materials like silt and clay.
The flood plains in a delta are called delta plains.
Natural levees - are found along the banks of large rivers. They are low, linear and parallel
ridges of coarse deposits along the banks of rivers, quite often cut into individual mounds.
During flooding as the water spills over the bank, the velocity of the water comes down
and large sized and high specific gravity materials get dumped in the immediate vicinity of
the bank as ridges. They are high nearer the banks and slope gently away from the river.
The levee deposits are coarser than the deposits spread by flood waters away from the
river. When rivers shift laterally, a series of natural levees can form.
Point bars are also known as meander bars.-They are found on the convex side of meanders
of large rivers and are sediments deposited in a linear fashion by flowing waters along the
bank.
almost uniform in profile and in width and contain mixed sizes of sediments.
If there more than one ridge, narrow and elongated depressions are found in between the point
bars.
As the rivers build the point bars on the convex side, the bank on the concave side will erode
actively.
4-Meanders
In large flood and delta plains, rivers rarely flow in straight courses. Loop-like channel
patterns called meanders develop over flood and delta plains
Meander is not a landform but is only a type of channel pattern. This is because
of
(i) propensity of water flowing over very gentle gradients to work laterally on the banks;
(ii) unconsolidated nature of alluvial deposits making up the banks with many irregularities
which can be used by water exerting pressure laterally;
(iii) coriolis force acting on the fluid water deflecting it like it deflects the wind.
When the gradient of the channel becomes extremely low, water flows leisurely and
starts working laterally. Slight irregularities along the banks slowly get transformed
into a small curvature in the banks;
the curvature deepens due to deposition on the inside of the curve and erosion
along the bank on the outside.
Normally, in meanders of large rivers, there is active deposition along the
convex bank and undercutting along the concave bank.
The concave bank is known as cut-off bank which shows up as a steep scarp and
the convex bank presents a long, gentle profile and is known as slip-off bank
As meanders grow into deep loops, the same may get cut-off due to erosion at the
inflection points and are left as ox-bow lakes
5. Braided Channels
When rivers carry coarse material, there can be selective deposition of coarser materials causing
formation of a central bar which diverts the flow towards the banks; and this flow increases
lateral erosion on the banks.
As the valley widens, the water column is reduced and more and more materials get deposited
as islands and lateral bars developing a number of separate channels of water flow.
Deposition and lateral erosion of banks are essential for the formation of braided pattern.
Or, alternatively, when discharge is less and load is more in the valley, channel bars and
islands of sand, gravel and pebbles develop on the floor of the channel and the water flow is
divided into multiple threads. These thread-like streams of water rejoin and subdivide repeatedly
to give a typical braided pattern
6. GROUNDWATER
The surface water percolates well when the rocks are permeable, thinly bedded and highly
jointed and cracked.
After vertically going down to some depth, the water under the ground flows horizontally
through the bedding planes, joints or through the materials themselves.
It is this downward and horizontal movement of water which causes the rocks to erode.
Physical or mechanical removal of materials by moving groundwater is insignificant in
developing landforms. That is why, the results of the work of groundwater cannot be seen in all
types of rocks.
rocks like limestones or dolomites rich in calcium carbonate, the surface water as well as
groundwater through the chemical process of solution and precipitation deposition develop
varieties of landforms.
Any limestone or dolomitic region showing typical landforms produced by the action of
groundwater through the processes of solution and deposition is called Karst topography
after the typical topography developed in limestone rocks of Karst region in the Balkans
adjacent to Adriatic sea.
The karst topography is also characterised by erosional and depositional landforms.
EROSIONAL LANDFORMS
Pools, Sinkholes, Lapies and Limestone Pavements
Small to medium sized round to sub-rounded shallow depressions called swallow holes form on
the surface of limestones through solution.
Sinkholes are very common in limestone/karst areas. A sinkhole is an opening more or less
circular at the top and funnel-shapped towards the bottom with sizes varying in area from a
few sq. m to a hectare and with depth from a less than half a metre to thirty metres or more.
if the bottom of a sinkhole forms the roof of a void or cave underground, it might collapse
leaving a large hole opening into a cave or a void below (collapse sinks). The term doline is
sometimes used to refer the collapse sinks
.When sink holes and dolines join together because of slumping of materials along their
margins or due to roof collapse of caves, long, narrow to wide trenches called valley sinks or
Uvalas form.
Caves
In areas where there are alternating beds of rocks (shales, sandstones, quartzites) with
limestones or dolomites in between or in areas where limestones are dense, massive and
occurring as thick beds, cave formation is prominent.
Water percolates down either through the materials or through cracks and joints and
moves horizontally along bedding planes.
It is along these bedding planes that the limestone dissolves and long and narrow to wide
gaps called caves result.
There can be a maze of caves at different elevations depending upon the limestone
beds and intervening rocks.
Caves normally have an opening through which cave streams are discharged. Caves
having openings at both the ends are called tunnels
Depositional Landforms
Many depositional forms develop within the limestone caves. The chief chemical in
limestone is calcium carbonate which is easily soluble in carbonated water (carbon
dioxide absorbed rainwater). This calcium carbonate is deposited when the water
carrying it in solution evaporates or loses its carbon dioxide as it trickles over
rough rock surfaces.
Stalactites, Stalagmites and Pillars
Stalactites hang as icicles of different diameters. Normally they are broad at their bases
and taper towards the free ends showing up in a variety of forms.
Stalagmites rise up from the floor of the caves. In fact, stalagmites form due to dripping water
from the surface or through the thin pipe, of the stalactite, immediately below it
Stalagmites may take the shape of a column, a disc, with either a smooth, rounded bulging
end or a miniature crater like depression.
The stalagmite and stalactites eventually fuse to give rise to columns and pillars of different
diameters
GLACIERS
Masses of ice moving as sheets over the land (continental glacier or piedmont glacier if a vast
sheet of ice is spread over the plains at the foot of mountains) or as linear flows down the
slopes of mountains in broad trough-like valleys (mountain and valley glaciers) are called
glaciers
The movement of glaciers is slow. Glaciers move basically because of the force of gravity
Erosion by glaciers is tremendous because of friction caused by sheer weight of the ice.
The material plucked from the land by glaciers (usually large-sized angular blocks and
fragments) get dragged along the floors or sides of the valleys and cause great damage
through abrasion and plucking.
Glaciers can cause significant damage to even un-weathered rocks and can reduce high
mountains into low hills and plains.
We have many glaciers in our country moving down the slopes and valleys in
Himalayas. Higher reaches of Uttaranchal, Himachal Pradesh and Jammu and
Kashmir, are places to see some of them. Do you know where one can see river
Bhagirathi is basically fed by meltwaters from under the snout (Gaumukh) of the
Gangotri glacier. In fact, Alkapuri glacier feeds waters to Alakananda river. Rivers
Alkananda and Bhagirathi join to make river Ganga near Deoprayag.
EROSIONAL LANDFORMS
1-Cirque
Cirques are the most common of landforms in glaciated mountains. The cirques quite often are
found at the heads of glacial valleys.
The accumulated ice cuts these cirques while moving down the mountain tops. They
are deep, long and wide troughs or basins with very steep concave to vertically
dropping high walls at its head as well as sides.
A lake of water can be seen quite often within the cirques after the glacier disappears.
Such lakes are called cirque or tarn lakes. There can be two or more cirques one leading
into another down below in a stepped sequence.
Horns and Serrated Ridges
Horns form through head ward erosion of the cirque walls.
If three or more radiating glaciers cut headward until their cirques meet, high, sharp
pointed and steep sided peaks called horns form.
The divides between cirque side walls or head walls get narrow because of
progressive erosion and turn into serrated or saw-toothed ridges sometimes referred
to as arêtes with very sharp crest and a zig-zag outline.
The highest peak in the Alps, Matterhorn and the highest peak in the
Himalayas, Everest are in fact horns formed through headward erosion of
radiating cirques.
Glacial Valleys/Troughs
Glaciated valleys are trough-like and U-shaped with broad floors and relatively smooth, and
steep sides.
The valleys may contain littered debris or debris shaped as moraines with swampy
appearance.
There can be hanging valleys at an elevation on one or both sides of the main glacial valley. The
faces of divides or spurs of such hanging valleys opening into main glacial valleys are quite often
truncated to give them an appearance like triangular facets.
Very deep glacial troughs filled with sea water and making up shorelines (in high
latitudes) are called fjords/fiords.
What are the basic differences between glacial valleys and river valleys?
Depositional Landforms
The unassorted coarse and fine debris dropped by the melting glaciers is called glacial till.
Most of the rock fragments in till are angular to sub- angular in form. Streams form by melting
ice at the bottom, sides or lower ends of glaciers
Some amount of rock debris small enough to be carried by such melt-water streams is
washed down and deposited. Such glacio- fluvial deposits are called outwash deposits.
Unlike till deposits, the outwash deposits are roughly stratified and assorted. The rock
fragments in outwash deposits are somewhat rounded at their edges.
Moraines
are long ridges of deposits of glacial till.
Terminal moraines are long ridges of debris deposited at the end (toe) of the glaciers.
Lateral moraines form along the sides parallel to the glacial valleys. The lateral moraines may
join a terminal moraine forming a horse-shoe shaped ridge. There can be many lateral moraines
on either side in a glacial valley.
These moraines partly or fully owe their origin to glacio- fluvial waters pushing up materials to the
sides of glaciers.
Many valley glaciers retreating rapidly leave an irregular sheet of till over their valley floors.
Such deposits varying greatly in thickness and in surface topography are called ground moraines.
The moraine in the centre of the glacial valley flanked by lateral moraines is called medial
moraine. They are imperfectly formed as compared to lateral moraines. Sometimes medial
moraines are indistinguishable from ground moraines.
Eskers
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. Such streams flow over the ground (not in a valley cut in the ground) with ice forming
its banks.
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.
Outwash Plains
The plains at the foot of the glacial mountains or beyond the limits of continental ice sheets
are covered with glacio-fluvial deposits in the form of broad flat alluvial fans which may join to
form outwash plains of gravel, silt, sand and clay.
Drumlins
Drumlins are smooth oval shaped ridge-like features composed mainly of glacial till with
some masses of gravel and sand.
The long axes of drumlins are parallel to the direction of ice movement.
They may measure up to 1 km in length and 30 m or so in height.
One end of the drumlins facing the glacier called the stoss end is blunter and steeper than the
other end called tail.
The drumlins form due to dumping of rock debris beneath heavily loaded ice through fissures
in the glacier. The stoss end gets blunted due to pushing by moving ice.
Drumlins give an indication of direction of glacier movement.
WAVES AND CURRENTS
When waves break, the water is thrown with great force onto the shore, and
simultaneously, there is a great churning of sediments on the sea bottom.
Constant impact of breaking waves drastically affects the coasts. Storm waves and
tsunami waves can cause far-reaching changes in a short period of time than
normal breaking waves. As wave environment changes, the intensity of the force of
breaking waves changes.
Other than the action of waves, the coastal landforms depend upon (i) the configuration
of land and sea floor; (ii) whether the coast is advancing (emerging) seaward or
retreating (submerging) landward.
Assuming sea level to be constant, two types of coasts are considered to explain the
concept of evolution of coastal landforms: (i) high, rocky coasts (submerged coasts);
(ii) low, smooth and gently sloping sedimentary coasts (emerged coasts).
HIGH ROCKY COASTS
Along the high rocky coasts, the rivers appear to have been drowned with highly irregular
coastline.
The coastline appears highly indented with extension of water into the land where glacial
valleys (fjords) are present.
The hill sides drop off sharply into the water. Shores do not show any depositional landforms
initially.
Erosion features dominate Along high rocky coasts, waves break with great force against the
land shaping the hill sides into cliffs.
With constant pounding by waves, the cliffs recede leaving a wave-cut platform in front of
the sea cliff. Waves gradually minimise the irregularities along the shore.
bars (long ridges of sand and/or shingle parallel to the coast) in the nearshore zone. Bars are
submerged features and when bars show up above water, they are called barrier bars.
Barrier bar which get keyed up to the headland of a bay is called a spit.
When barrier bars and spits form at the mouth of a bay and block it, a lagoon forms.
The lagoons would gradually get filled up by sediments from the land giving rise to a coastal
plain
LOW SEDIMENTARY COASTS
Along low sedimentary coasts the rivers appear to extend their length by building coastal plains
and deltas.
The coastline appears smooth with occasional incursions of water in the form of lagoons and
tidal creeks. The land slopes gently into the water.
Marshes and swamps may abound along the coasts. Depositional features dominate.
When waves break over a gently sloping sedimentary coast, the bottom sediments get churned
and move readily building bars, barrier bars, spits and lagoons.
Lagoons would eventually turn into a swamp which would subsequently turn into a coastal
plain. The maintenance of these depositional features depends upon the steady supply of
materials
Storm and tsunami waves cause drastic changes irrespective of supply of sediments. Large
rivers which bring lots of sediments build deltas along low sedimentary coasts.
The west coast of our country is a high rocky retreating coast. Erosional forms dominate
in the west coast. The east coast of India is a low sedimentary coast. Depositional forms
dominate in the east coast.
What are the various differences between a high rocky coast and a low sedimentary coast
in terms of processes and landforms?
EROSIONAL LANDFORMS
Cliffs, Terraces, Caves and Stacks
Wave-cut cliffs and terraces are two forms usually found where erosion is the dominant
shore process.
Almost all sea cliffs are steep and may range from a few m to 30 m or even more. At the foot
of such cliffs there may be a flat or gently sloping platform covered by rock debris derived from
the sea cliff behind.
Such platforms occurring at elevations above the average height of waves is called a wave-cut
terrace.
The lashing of waves against the base of the cliff and the rock debris that gets smashed
against the cliff along with lashing waves create hollows and these hollows get widened and
deepened to form sea caves.
The roofs of caves collapse and the sea cliffs recede further inland. Retreat of the cliff may
leave some remnants of rock standing isolated as small islands just off the shore. Such
resistant masses of rock, originally parts of a cliff or hill are called sea stacks.
DEPOSITIONAL LANDFORMS
Beaches and Dunes
Beaches are characteristic of shorelines that are dominated by deposition, but may occur as
patches along even the rugged shores.
Most of the sediment making up the beaches comes from land carried by the streams and
rivers or from wave erosion. Beaches are temporary features.
Most of the beaches are made up of sand sized materials. Beaches called shingle beaches
contain excessively small pebbles and even cobbles.
Just behind the beach, the sands lifted and winnowed from over the beach surfaces will be
deposited as sand dunes. Sand dunes forming long ridges parallel to the coastline are very
common along low sedimentary coasts.
Bars, Barriers and Spits
A ridge of sand and shingle formed in the sea in the off-shore zone (from the position of low
tide waterline to seaward) lying approximately parallel to the coast is called an off-shore bar.
An off-shore bar which is exposed due to further addition of sand is termed a barrier bar.
The off-shore bars and barriers commonly form across the mouth of a river or at the entrance
of a bay. Sometimes such barrier bars get keyed up to one end of the bay when they are called
spits.
Spits may also develop attached to headlands/hills. The barriers, bars and spits at
the mouth of the bay gradually extend leaving only a small opening of the bay
into the sea and the bay will eventually develop into a lagoon.
The lagoons get filled up gradually by sediment coming from the land or from the
beach itself (aided by wind) and a broad and wide coastal plain may develop replacing
a lagoon.
the coastal off-shore bars offer the first buffer or defence against storm or
tsunami by absorbing most of their destructive force. Then come the barriers,
beaches, beach dunes and mangroves, if any, to absorb the destructive force
of storm and tsunami waves. So, if we do anything which disturbs the
‘sediment budget’ and the mangroves along the coast, these coastal forms will
get eroded away leaving human habitations to bear first strike of storm and
tsunami waves.
WINDS
Wind is one of the two dominant agents in hot deserts. The desert floors get heated
up too much and too quickly because of being dry and barren.
The heated floors heat up the air directly above them and result in upward
movements in the hot lighter air with turbulence, and any obstructions in its
path sets up eddies, whirlwinds, updrafts and downdrafts.
Winds also move along the desert floors with great speed and the obstructions in
their path create turbulence. Of course, there are storm winds which are very
destructive.
Winds cause deflation, abrasion and impact.
Deflation includes lifting and removal of dust and smaller particles from the surface of
rocks. In the transportation process sand and silt act as effective tools to abrade the
land surface.
The impact is simply sheer force of momentum which occurs when sand is blown into
or against a rock surface. It is similar to sand- blasting operation.
The desert rocks devoid of vegetation, exposed to mechanical and chemical weathering
processes due to drastic diurnal temperature changes, decay faster and the torrential rains help
in removing the weathered materials easily.
That means, the weathered debris in deserts is moved by not only wind but also by rain/sheet
wash.
The wind moves fine materials and general mass erosion is accomplished mainly through
sheet floods or sheet wash. Stream channels in desert areas are broad, smooth and indefinite
and flow for a brief time after rains.
EROSIONAL LANDFORMS
Pediments and Pediplains
Gently inclined rocky floors close to the mountains at their foot with or without a thin cover
of debris, are called pediments.
Such rocky floors form through the erosion of mountain front through a combination of lateral
erosion by streams and sheet flooding.
Once, pediments are formed with a steep wash slope followed by cliff or free face above it, the
steep wash slope and free face retreat backwards.
This method of erosion is termed as parallel retreat of slopes through back wasting. So,
through parallel retreat of slopes, the pediments extend backwards at the expense of mountain
front, and gradually, the mountain gets reduced leaving an inselberg which is a remnant of the
mountain. That’s how the high relief in desert areas is reduced to low featureless plains called
pediplains.
Playas
Plains are by far the most prominent landforms in the deserts. In basins with mountains and
hills around and along, the drainage is towards the centre of the basin and due to gradual
deposition of sediment from basin margins, a nearly level plain forms at the centre of the
basin.
In times of sufficient water, this plain is covered up by a shallow water body. Such types of
shallow lakes are called as playas where water is retained only for short duration due to
evaporation and quite often the playas contain good deposition of salts. The playa plain
covered up by salts is called alkali flats.
Deflation Hollows and Caves
Weathered mantle from over the rocks or bare soil, gets blown out by persistent movement of
wind currents in one direction. This process may create shallow depressions called
deflation hollows.
Deflation also creates numerous small pits or cavities over rock surfaces. The rock
faces suffer impact and abrasion of wind-borne sand and first shallow depressions called
blow outs are created, and some of the blow outs become deeper and wider fit to be called
caves.
Mushroom, Table and Pedestal Rocks
Many rock-outcrops in the deserts easily susceptible to wind deflation and abrasion are
worn out quickly leaving some remnants of resistant rocks polished beautifully in the
shape of mushroom with a slender stalk and a broad and rounded pear shaped cap above.
Sometimes, the top surface is broad like a table top and quite often, the remnants stand out
like pedestals.
Depositional Landforms
Wind is a good sorting agent. Depending upon the velocity of wind, different sizes of grains are
moved along the floors by rolling or saltation and carried in suspension and in this process
of transportation itself, the materials get sorted.
When the wind slows or begins to die down, depending upon sizes of grains and their
critical velocities, the grains will begin to settle. So, in depositional landforms made by wind,
good sorting of grains can be found.
wind is there everywhere and wherever there is good source of sand and with constant wind
directions, depositional features in arid regions can develop anywhere.
Sand Dunes
Dry hot deserts are good places for sand dune formation. Obstacles to initiate dune
formation are equally important. There can be a great variety of dune forms
Crescent shaped dunes called barchans with the points or wings directed away
from wind direction i.e., downwind, form where the wind direction is constant and
moderate and where the original surface over which sand is moving is almost
uniform.
Parabolic dunes form when sandy surfaces are partially covered with vegetation. That
means parabolic dunes are reversed barchans with wind direction being the same.
Seif is similar to barchan with a small difference. Seif has only one wing or point.
This happens when there is shift in wind conditions. The lone wings of seifs can grow
very long and high.
Longitudinal dunes form when supply of sand is poor and wind direction is
constant. They appear as long ridges of considerable length but low in height.
Transverse dunes are aligned perpendicular to wind direction. These dunes form
when the wind direction is constant and the source of sand is an elongated feature at
right angles to the wind direction. They may be very long and low in height.
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