LANDFORMS AND THEIR

EVOLUTION

After weathering processes have hadtheir actions on the earth materialsmaking up the surface of the earth, the

geomorphic agents like running water, groundwater, wind, glaciers, waves perform erosion.It is already known to you that erosion causeschanges on the surface of the earth. Depositionfollows erosion and because of deposition too,changes occur on the surface of the earth.

As this chapter deals with landforms andtheir evolution ‘first’ start with the question,what is a landform? In simple words, small tomedium tracts or parcels of the earth’s surfaceare called landforms.

If landform is a small to medium sizedpart of the surface of the earth, what is alandscape?

Several related landforms together makeup landscapes, (large tracts of earth’s surface).Each landform has its own physical shape,size, materials and is a result of the action ofcertain geomorphic processes and agent(s).Actions of most of the geomorphic processesand agents are slow, and hence the resultstake a long time to take shape. Every landformhas a beginning. Landforms once formed maychange in their shape, size and nature slowlyor fast due to continued action of geomorphicprocesses and agents.

Due to changes in climatic conditions andvertical or horizontal movements of land-masses, either the intensity of processes or theprocesses themselves might change leading tonew modifications in the landforms. Evolutionhere implies stages of transformation of either

a part of the earth’s surface from one landforminto another or transformation of individuallandforms after they are once formed. Thatmeans, each and every landform has a historyof development and changes through time. Alandmass passes through stages of developmentsomewhat comparable to the stages of life —youth, mature and old age.

What are the two important aspects of

the evolution of landforms?

RUNNING WATER

In humid regions, which receive heavy rainfall

running water is considered the most importantof the geomorphic agents in bringing aboutthe degradation of the land surface. There are

two components of running water. One is

overland flow on general land surface as asheet. Another is 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 steepgradients. With time, stream channels over

steep gradients turn gentler due to continued

erosion, and as a consequence, lose their

velocity, facilitating active deposition. Theremay be depositional forms associated with

streams flowing over steep slopes. But these

phenomena will be on a small scale compared

to those associated with rivers flowing overmedium to gentle slopes. 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

C H A P T E R

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cutting becomes less dominant and lateralerosion of banks increases and as aconsequence the hills and valleys are reduced

to plains.

Is complete reduction of relief of a high

land mass possible?

Overland flow causes sheet erosion.Depending upon irregularities of the landsurface, the overland flow may concentrate intonarrow to wide paths. Because of the sheerfriction of the column of flowing water, minoror major quantities of materials from thesurface of the land are removed in the directionof flow and gradually small and narrow rillswill form. These rills will gradually develop intolong and wide gullies; the gullies will furtherdeepen, widen, lengthen and unite to giverise to a network of valleys. In the early stages,down-cutting dominates during whichirregularities such as waterfalls and cascadeswill be removed. In the middle stages, streamscut their beds slower, and lateral erosion ofvalley sides becomes severe. Gradually, thevalley sides are reduced to lower and lowerslopes. The divides between drainage basinsare likewise lowered until they are almostcompletely flattened leaving finally, a lowlandof faint relief with some low resistant remnantscalled monadnocks standing out here andthere. This type of plain forming as a result ofstream erosion is called a peneplain (an almostplain). The characteristics of each of the stagesof landscapes developing in running waterregimes may be summarised as follows:

Youth

Streams are few during this stage with poor

integration and flow over original slopesshowing shallow V-shaped valleys with nofloodplains or with very narrow floodplains

along trunk streams. Streams divides arebroad and flat with marshes, swamp andlakes. Meanders if present develop over these

broad upland surfaces. These meanders mayeventually entrench themselves into theuplands. Waterfalls and rapids may exist where

local hard rock bodies are exposed.

Mature

During this stage streams are plenty with goodintegration. The valleys are still V-shaped butdeep; trunk streams are broad enough to havewider floodplains within which streams mayflow in meanders confined within the valley.The flat and broad inter stream areas andswamps and marshes of youth disappear andthe stream divides turn sharp. Waterfalls andrapids disappear.

Old

Smaller tributaries during old age are fewwith gentle gradients. Streams meander freelyover vast floodplains showing natural levees,oxbow lakes, etc. Divides are broad and flatwith lakes, swamps and marshes. Most ofthe landscape is at or slightly above sea level.

EROSIONAL LANDFORMS

Valleys

Valleys start as small and narrow rills; therills will gradually develop into long and widegullies; the gullies will further deepen, widenand lengthen to give rise to valleys. Dependingupon dimensions and shape, many types ofvalleys like V-shaped valley, gorge, canyon,etc. can be recognised. A gorge is a deep valleywith very steep to straight sides (Figure 7.1)and a canyon is characterised by steep step-like side slopes (Figure 7.2) and may be asdeep as a gorge. A gorge is almost equal inwidth at its top as well as its bottom. Incontrast, a canyon is wider at its top than atits bottom. In fact, a canyon is a variant ofgorge. Valley types depend upon the type andstructure of rocks in which they form. Forexample, canyons commonly form inhorizontal bedded sedimentary rocks andgorges form in hard rocks.

Potholes and Plunge Pools

Over the rocky beds of hill-streams more orless circular depressions called potholes formbecause of stream erosion aided by theabrasion of rock fragments. Once a small and

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shallow depression forms, pebbles andboulders get collected in those depressions andget rotated by flowing water and consequentlythe depressions grow in dimensions. A seriesof such depressions eventually join and thestream valley gets deepened. At the foot of

Figure 7.1 : The Valley of Kaveri river near Hogenekal,Dharmapuri district, Tamil Nadu in the form of gorge

Figure 7.2 : An entrenched meander loop of river Coloradoin USA showing step-like side slopes of its valley

typical of a canyon

waterfalls also, large potholes, quite deep andwide, form because of the sheer impact ofwater and rotation of boulders. Such largeand deep holes at the base of waterfalls arecalled plunge pools.

Incised or Entrenched Meanders

In streams that flow rapidly over steepgradients, normally erosion is concentratedon the bottom of the stream channel. Also, inthe case of steep gradient streams, lateralerosion on the sides of the valleys is not muchwhen compared to the streams flowing onlow and gentle slopes. Because of active lateralerosion, streams flowing over gentle slopes,develop sinuous or meandering courses. It iscommon to find meandering courses overfloodplains and delta plains where streamgradients are very gentle. But very deep andwide meanders can also be found cut in hardrocks. Such meanders are called incised or

entrenched meanders (Figure 7.2).

River Terraces

River terraces are surfaces marking old valleyfloor or floodplain levels. They may be bedrocksurfaces without any alluvial cover or alluvialterraces consisting of stream deposits. Riverterraces are basically products of erosion asthey result due to vertical erosion by thestream into its own depositional floodplain.There can be a number of such terraces atdifferent heights indicating former river bedlevels. The river terraces may occur at thesame elevation on either side of the rivers inwhich case they are called paired terraces..

DEPOSITIONAL LANDFORMS

Alluvial Fans

Alluvial fans (Figure 7.4) are formed whenstreams flowing from higher levels break intofoot slope plains of low gradient. Normallyvery coarse load is carried by streams flowingover mountain slopes. This load becomes tooheavy for the streams to be carried overgentler gradients and gets dumped andspread as a broad low to high cone shaped

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Deltas

Deltas are like alluvial fans but develop at adifferent location. The load carried by therivers is dumped and spread into the sea. Ifthis load is not carried away far into the seaor distributed along the coast, it spreads and

accumulates as a low cone. Unlike in alluvialfans, the deposits making up deltas are verywell sorted with clear stratification. Thecoarsest materials settle out first and the finerfractions like silts and clays are carried outinto the sea. As the delta grows, the riverdistributaries continue to increase in length(Figure 7.5) and delta continues to build upinto the sea.

Floodplains, Natural Levees and Point Bars

Deposition develops a floodplain just aserosion makes valleys. Floodplain is a majorlandform of river deposition. Large sizedmaterials are deposited first when streamchannel breaks into a gentle slope. Thus,normally, fine sized materials like sand, siltand clay are carried by relatively slowmoving waters in gentler channels usuallyfound in the plains and deposited over thebed and when the waters spill over thebanks during flooding above the bed.A river bed made of river deposits is the activefloodplain. The floodplain above the bank isinactive floodplain. Inactive floodplain abovethe banks basically contain two types ofdeposits — flood deposits and channeldeposits. In plains, channels shift laterallyand change their courses occasionally leavingcut-off courses which get filled up gradually.Such areas over flood plains built up byabandoned or cut-off channels contain coarsedeposits. The flood deposits of spilled waterscarry relatively finer materials like silt andclay. The flood plains in a delta are calleddelta plains.

Figure 7.4 : An alluvial fan deposited by a hill streamon the way to Amarnath, Jammu and Kashmir

deposit called alluvial fan. Usually, the streamswhich flow over fans are not confined to theiroriginal channels for long and shift theirposition across the fan forming manychannels called distributaries. Alluvial fansin humid areas show normally low cones withgentle slope from head to toe and they appearas high cones with steep slope in arid andsemi-arid climates.

Figure 7.5 : A satellite view of part of Krishna riverdelta, Andhra Pradesh Figure 7.6 : Natural levee and point bars

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LANDFORMS AND THEIR EVOLUTION 59

Natural levees and point bars (Figure 7.6)are some of the important landforms foundassociated with floodplains. Natural levees arefound along the banks of large rivers. Theyare low, linear and parallel ridges of coarsedeposits along the banks of rivers, quite oftencut into individual mounds. Point bars arealso known as meander bars. They are foundon the concave side of meanders of large riversand are sediments deposited in a linearfashion by flowing waters along the bank.They are almost uniform in profile and in widthand contain mixed sizes of sediments.

In what way do natural levees differ from

point bars?

Meanders

In large flood and delta plains, rivers rarelyflow in straight courses. Loop-like channelpatterns called meanders develop over floodand delta plains (Figure 7.7).

the banks slowly get transformed into a smallcurvature in the banks; the curvaturedeepens due to deposition on the inside ofthe curve and erosion along the bank on theoutside. If there is no deposition and noerosion or undercutting, the tendency tomeander is reduced. Normally, in meandersof large rivers, there is active deposition alongthe concave bank and undercutting along theconvex bank. The concave bank is known ascut-off bank which shows up as a steep scarpand the convex bank presents a long, gentleprofile (Figure 7.8). As meanders grow intodeep loops, the same may get cut-off due toerosion at the inflection points and are leftas ox-bow lakes.

GROUNDWATER

Here the interest is not on groundwater as aresource. Our focus is on the work ofgroundwater in the erosion of landmasses andevolution of landforms. The surface water

Figure 7.7 : A satellite scene showing meanderingBurhi Gandak river near Muzaffarpur, Bihar, showing

a number of oxbow lakes and cut-offs

Meander is not a landform but is only atype of channel pattern. This is because of(i) propensity of water flowing over very gentlegradients to work laterally on the banks;(ii) unconsolidated nature of alluvial depositsmaking up the banks with many irregularitieswhich can be used by water exerting pressurelaterally; (iii) coriolis force acting on the fluidwater deflecting it like it deflects the wind.When the gradient of the channel becomesextremely low, water flows leisurely and startsworking laterally. Slight irregularities along

Figure 7.8 : Meander growth and cut-off loops andslip-off and undercut banks

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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 horizontallythrough the bedding planes, joints or throughthe materials themselves. It is this downwardand horizontal movement of water whichcauses the rocks to erode. Physical ormechanical removal of materials by movinggroundwater is insignificant in developinglandforms. That is why, the results of the workof groundwater cannot be seen in all types ofrocks. But in rocks like limestones ordolomites rich in calcium carbonate, thesurface water as well as groundwater throughthe chemical process of solution andprecipitation deposition develop varieties oflandforms. These two processes of solutionand precipitation are active in limestones ordolomites occurring either exclusively orinterbedded with other rocks. Any limestoneor dolomitic region showing typical landforms

produced by the action of groundwaterthrough the processes of solution anddeposition is called Karst topography after thetypical topography developed in limestonerocks of Karst region in the Balkans adjacentto Adriatic sea.

The karst topography is also characterisedby erosional and depositional landforms.

EROSIONAL LANDFORMS

Pools, Sinkholes, Lapies andLimestone Pavements

Small to medium sized round to sub-roundedshallow depressions called swallow holes formon the surface of limestones through solution.Sinkholes are very common in limestone/karst areas. A sinkhole is an opening more orless circular at the top and funnel-shappedtowards the bottom with sizes varying in areafrom a few sq. m to a hectare and with depth

Figure 7.10 : Various karst features

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from a less than half a metre to thirty metresor more. Some of these form solely throughsolution action (solution sinks) and othersmight start as solution forms first and if thebottom of a sinkhole forms the roof of a voidor cave underground, it might collapse leavinga large hole opening into a cave or a voidbelow (collapse sinks). Quite often, sinkholesare covered up with soil mantle and appearas shallow water pools. Anybody steppingover such pools would go down like ithappens in quicksands in deserts. The termdoline is sometimes used to refer the collapsesinks. Solution sinks are more common thancollapse sinks. Quite often the surface run-off simply goes down swallow and sink holesand flow as underground streams and re-emerge at a distance downstream through acave opening. When sink holes and dolinesjoin together because of slumping of materialsalong their margins or due to roof collapse ofcaves, long, narrow to wide trenches calledvalley sinks or Uvalas form. Gradually, mostof the surface of the limestone is eaten awayby these pits and trenches, leaving it extremelyirregular with a maze of points, grooves andridges or lapies. Especially, these ridges orlapies form due to differential solution activityalong parallel to sub-parallel joints. The lapiefield may eventually turn into somewhatsmooth limestone pavements.

Caves

In areas where there are alternating beds ofrocks (shales, sandstones, quartzites) withlimestones or dolomites in between or in areaswhere limestones are dense, massive andoccurring as thick beds, cave formation isprominent. Water percolates down eitherthrough the materials or through cracks andjoints and moves horizontally along beddingplanes. It is along these bedding planes thatthe limestone dissolves and long and narrowto wide gaps called caves result. There canbe a maze of caves at different elevationsdepending upon the limestone beds andintervening rocks. Caves normally have anopening through which cave streams aredischarged. Caves having openings at boththe ends are called tunnels.

Depositional Landforms

Many depositional forms develop within thelimestone caves. The chief chemical inlimestone is calcium carbonate which is easilysoluble in carbonated water (carbon dioxideabsorbed rainwater). This calcium carbonateis deposited when the water carrying it insolution evaporates or loses its carbon dioxideas it trickles over rough rock surfaces.

Stalactites, Stalagmites and Pillars

Stalactites hang as icicles of dif ferentdiameters. Normally they are broad at theirbases and taper towards the free endsshowing up in a variety of forms. Stalagmitesrise up from the floor of the caves. In fact,stalagmites form due to dripping water fromthe surface or through the thin pipe, of thestalactite, immediately below it (Figure 7.11).

Stalagmites may take the shape of acolumn, a disc, with either a smooth, roundedbulging end or a miniature crater like

Figure 7.11 : Stalactites and stalagmites in limestone caves

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depression. The stalagmite and stalactiteseventually 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 avast sheet of ice is spread over the plains atthe foot of mountains) or as linear flows downthe slopes of mountains in broad trough-likevalleys (mountain and valley glaciers) arecalled glaciers (Figure 7.12). The movementof glaciers is slow unlike water flow. Themovement could be a few centimetres to afew metres a day or even less or more. Glaciersmove basically because of the force of gravity.

fragments) get dragged along the floors or

sides of the valleys and cause great damagethrough abrasion and plucking. Glaciers cancause significant damage to even un-weathered rocks and can reduce highmountains into low hills and plains.

As glaciers continue to move, debris getsremoved, divides get lowered and eventuallythe slope is reduced to such an extent thatglaciers will stop moving leaving only a massof low hills and vast outwash plains alongwith other depositional features. Figures 7.13and 7.14 show various glacial erosional anddepositional forms described in the text.

EROSIONAL LANDFORMS

Cirque

Cirques are the most common of landformsin glaciated mountains. The cirques quiteoften are found at the heads of glacial valleys.The accumulated ice cuts these cirques whilemoving down the mountain tops. They aredeep, long and wide troughs or basins withvery steep concave to vertically dropping highwalls at its head as well as sides. A lake ofwater can be seen quite often within thecirques after the glacier disappears. Suchlakes are called cirque or tarn lakes. Therecan be two or more cirques one leading intoanother down below in a stepped sequence.

Horns and Serrated Ridges

Horns form through head ward erosion ofthe cirque walls. If three or more radiatingglaciers cut headward until their cirques meet,high, sharp pointed and steep sided peakscalled horns form. The divides between cirqueside walls or head walls get narrow becauseof progressive erosion and turn into serratedor saw-toothed ridges sometimes referred toas arêtes with very sharp crest and a zig-zagoutline.

The highest peak in the Alps, Matterhornand the highest peak in the Himalayas,Everest are in fact horns formed through

headward erosion of radiating cirques.

Figure 7.12 : A glacier in its valley

We have many glaciers in our countrymoving down the slopes and valleys inHimalayas. Higher reaches of Uttaranchal,Himachal Pradesh and Jammu andKashmir, are places to see some of them.Do you know where one can see riverBhagirathi is basically fed by meltwatersfrom under the snout (Gaumukh) of theGangotri glacier. In fact, Alkapuri glacierfeeds waters to Alakananda river. RiversAlkananda and Bhagirathi join to makeriver Ganga near Deoprayag.

Erosion by glaciers is tremendous becauseof friction caused by sheer weight of the ice.

The material plucked from the land by glaciers

(usually large-sized angular blocks and

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Glacial Valleys/Troughs

Glaciated valleys are trough-like and U -

shaped with broad floors and relatively

smooth, and steep sides. The valleys maycontain littered debris or debris shaped asmoraines with swampy appearance. There

may be lakes gouged out of rocky floor orformed by debris within the valleys. Therecan be hanging valleys at an elevation on one

or both sides of the main glacial valley. Thefaces of divides or spurs of such hangingvalleys opening into main glacial valleys are

quite often truncated to give them anappearance like triangular facets. Very deepglacial troughs filled with sea water and

making up shorelines (in high latitudes) arecalled fjords/fiords.

What are the basic differences betweenglacial valleys and river valleys?

Depositional Landforms

The unassorted coarse and fine debrisdropped by the melting glaciers is called glacial

till. Most of the rock fragments in till areangular to sub-angular in form. Streams formby melting ice at the bottom, sides or lowerends of glaciers. Some amount of rock debrissmall enough to be carried by such melt-water streams is washed down and deposited.Such glacio-fluvial deposits are calledoutwash deposits. Unlike till deposits, theoutwash deposits are roughly stratified andassorted. The rock fragments in outwashdeposits are somewhat rounded at their edges.Figure 7.14 shows a few depositionallandforms commonly found in glaciated areas.

Moraines

They are long ridges of deposits of glacial till.Terminal moraines are long ridges of debris

Figure 7.13 : Some glacial erosional and depositional forms (adapted and modified from Spencer, 1962)

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deposited at the end (toe) of the glaciers.Lateral moraines form along the sides parallelto the glacial valleys. The lateral moraines mayjoin a terminal moraine forming a horse-shoeshaped ridge (Fig. 7.13). There can be manylateral moraines on either side in a glacial valley.These moraines partly or fully owe their originto glacio-fluvial waters pushing up materialsto the sides of glaciers. Many valley glaciersretreating rapidly leave an irregular sheet oftill over their valley floors. Such deposits varyinggreatly in thickness and in surface topographyare called ground moraines. The moraine in thecentre of the glacial valley flanked by lateralmoraines is called medial moraine. They areimperfectly formed as compared to lateralmoraines. Sometimes medial moraines areindistinguishable from ground moraines.

Eskers

When glaciers melt in summer, the waterflows on the surface of the ice or seeps downalong the margins or even moves throughholes in the ice. These waters accumulatebeneath the glacier and flow like streams ina channel beneath the ice. Such streams flow

over the ground (not in a valley cut in theground) with ice forming its banks. Verycoarse materials like boulders and blocksalong with some minor fractions of rock debriscarried into this stream settle in the valley ofice beneath the glacier and after the ice meltscan be found as a sinuous ridge called esker.

Outwash Plains

The plains at the foot of the glacial mountainsor beyond the limits of continental ice sheetsare covered with glacio-fluvial deposits in theform of broad flat alluvial fans which mayjoin to form outwash plains of gravel, silt,sand and clay.

Distinguish between river alluvial plainsand glacial outwash plains.

Drumlins

Drumlins are smooth oval shaped ridge-like

features composed mainly of glacial till withsome masses of gravel and sand. The longaxes of drumlins are parallel to the direction

of ice movement. They may measure up to 1

Figure 7.14 : A panoramic diagram of glacial landscape with various depositional landforms(adapted and modified from Spencer, 1962)

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km in length and 30 m or so in height. Oneend of the drumlins facing the glacier calledthe stoss end is blunter and steeper than the

other end called tail. The drumlins form dueto dumping of rock debris beneath heavilyloaded ice through fissures in the glacier. The

stoss end gets blunted due to pushing bymoving ice. Drumlins give an indication ofdirection of glacier movement.

What is the difference between till and

alluvium?

WAVES AND CURRENTS

Coastal processes are the most dynamic and

hence most destructive. So, don’t you thinkit is important to know about the coastalprocesses and forms?

Some of the changes along the coasts takeplace very fast. At one place, there can beerosion in one season and deposition in

another. Most of the changes along the coastsare accomplished by waves. When wavesbreak, the water is thrown with great force

onto the shore, and simultaneously, there isa great churning of sediments on the seabottom. Constant impact of breaking waves

drastically affects the coasts. Storm wavesand tsunami waves can cause far-reachingchanges in a short period of time than normal

breaking waves. As wave environmentchanges, the intensity of the force of breakingwaves changes.

Do you know about the generating forces

behind waves and currents? If not, refer

to the chapter on movements in ocean

waters.

Other than the action of waves, the coastallandforms depend upon (i) the configuration

of land and sea floor; (ii) whether the coast isadvancing (emerging) seaward or retreating(submerging) landward. Assuming sea level

to be constant, two types of coastsare considered to explain the concept ofevolution 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 appearto 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 sidesdrop off sharply into the water. Shores do not

show any depositional landforms initially.

Erosion features dominate.

Along high rocky coasts, waves break withgreat 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. Wavesgradually minimise the irregularities along

the shore.

The materials which fall off, and removedfrom the sea cliffs, gradually break intosmaller fragments and roll to roundness, will

get deposited in the of fshore. A fter aconsiderable period of cliff development andretreat when coastline turns somewhat

smooth, with the addition of some morematerial to this deposit in the offshore, awave-built terrace would develop in front of

wave-cut terrace. As the erosion along thecoast takes place a good supply materialbecomes available to longshore currents and

waves to deposit them as beaches along theshore and as bars (long ridges of sand and/or shingle parallel to the coast) in the nearshore

zone. Bars are submerged features and whenbars show up above water, they are calledbarrier bars. Barrier bar which get keyed up

to the headland of a bay is called a spit. Whenbarrier bars and spits form at the mouth of abay and block it, a lagoon forms. The lagoons

would gradually get filled up by sedimentsfrom the land giving rise to a coastal plain.

LOW SEDIMENTARY COASTS

Along low sedimentary coasts the riversappear to extend their length by buildingcoastal plains and deltas. The coastlineappears smooth with occasional incursions

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of water in the form of lagoons and tidal

creeks. The land slopes gently into the water.Marshes and swamps may abound along thecoasts. Depositional features dominate.

When waves break over a gently slopingsedimentary coast, the bottom sediments getchurned and move readily building bars,

barrier bars, spits and lagoons. Lagoonswould eventually turn into a swamp whichwould subsequently turn into a coastal plain.The maintenance of these depositional featuresdepends upon the steady supply of materials.Storm and tsunami waves cause drasticchanges irrespective of supply of sediments.Large rivers which bring lots of sedimentsbuild 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 dif ferences

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 formsusually found where erosion is the dominantshore process. Almost all sea cliffs are steepand may range from a few m to 30 m or evenmore. At the foot of such cliffs there may bea flat or gently sloping platform covered byrock debris derived from the sea cliff behind.Such platforms occurring at elevations abovethe average height of waves is called a wave-

cut terrace. The lashing of waves against thebase of the cliff and the rock debris that getssmashed against the cliff along with lashingwaves create hollows and these hollows getwidened and deepened to form sea caves. Theroofs of caves collapse and the sea cliffs recedefurther inland. Retreat of the cliff may leave

some remnants of rock standing isolated assmall islands just off the shore. Such resistantmasses of rock, originally parts of a cliff orhill are called sea stacks. Like all otherfeatures, sea stacks are also temporary andeventually coastal hills and cliffs will disappearbecause of wave erosion giving rise to narrowcoastal plains, and with onrush of depositsfrom over the land behind may get coveredup by alluvium or may get covered up byshingle or sand to form a wide beach.

DEPOSITIONAL LANDFORMS

Beaches and Dunes

Beaches are characteristic of shorelines thatare dominated by deposition, but may occuras patches along even the rugged shores. Mostof the sediment making up the beaches comesfrom land carried by the streams and riversor from wave erosion. Beaches are temporaryfeatures. The sandy beach which appears sopermanent may be reduced to a very narrowstrip of coarse pebbles in some other season.Most of the beaches are made up of sandsized materials. Beaches called shinglebeaches contain excessively small pebbles andeven cobbles.

Just behind the beach, the sands lifted andwinnowed from over the beach surfaceswill be deposited as sand dunes. Sand dunesforming long ridges parallel to the coastline arevery common along low sedimentary coasts.

Bars, Barriers and Spits

A ridge of sand and shingle formed in the seain the off-shore zone (from the position oflow tide waterline to seaward) lyingapproximately parallel to the coast is calledan off-shore bar. An off-shore bar which isexposed due to further addition of sand istermed a barrier bar. The off-shore bars andbarriers commonly form across the mouth ofa river or at the entrance of a bay. Sometimessuch barrier bars get keyed up to one end ofthe bay when they are called spits (Figure7.15). Spits may also develop attached toheadlands/hills. The barriers, bars and spitsat the mouth of the bay gradually extend

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leaving only a small opening of the bay intothe sea and the bay will eventually developinto a lagoon. The lagoons get filled upgradually by sediment coming from the landor from the beach itself (aided by wind) and abroad and wide coastal plain may developreplacing a lagoon.

Do you know, 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 hotdeserts. The desert floors get heated up toomuch and too quickly because of being dryand barren. The heated floors heat up the airdirectly above them and result in upwardmovements in the hot lighter air withturbulence, and any obstructions in its pathsets up eddies, whirlwinds, updrafts anddowndrafts. Winds also move along the desertfloors with great speed and the obstructions intheir 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 dustand smaller particles from the surface of rocks.In the transportation process sand and silt actas effective tools to abrade the land surface.The impact is simply sheer force of momentumwhich occurs when sand is blown into oragainst a rock surface. It is similar to sand-blasting operation. The wind action creates anumber of interesting erosional anddepositional features in the deserts.

In fact, many features of deserts owe theirformation to mass wasting and running wateras sheet floods. Though rain is scarce indeserts, it comes down torrentially in a shortperiod of time. The desert rocks devoid ofvegetation, exposed to mechanical andchemical weathering processes due to drasticdiurnal temperature changes, decay fasterand the torrential rains help in removing theweathered materials easily. That means, theweathered debris in deserts is moved by notonly wind but also by rain/sheet wash. Thewind moves fine materials and general masserosion is accomplished mainly through sheetfloods or sheet wash. Stream channels indesert areas are broad, smooth and indefiniteand flow for a brief time after rains.

EROSIONAL LANDFORMS

Pediments and Pediplains

Landscape evolution in deserts is primarilyconcerned with the formation and extensionof pediments. Gently inclined rocky floorsclose to the mountains at their foot with orwithout a thin cover of debris, are calledpediments. Such rocky floors form throughthe erosion of mountain front through acombination of lateral erosion by streams andsheet flooding.

Erosion starts along the steep margins ofthe landmass or the steep sides of thetectonically controlled steep incision featuresover the landmass. Once, pediments areformed with a steep wash slope followed bycliff or free face above it, the steep wash slopeand free face retreat backwards. This method

Figure 7.15 : A satellite picture of a part of Godavaririver delta showing a spit

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of erosion is termed as parallel retreat of slopesthrough backwasting. So, through parallelretreat of slopes, the pediments extendbackwards at the expense of mountain front,and gradually, the mountain gets reducedleaving an inselberg which is a remnant ofthe mountain. That’s how the high relief indesert areas is reduced to low featurelessplains called pediplains.

Playas

Plains are by far the most prominentlandforms in the deserts. In basins withmountains and hills around and along, thedrainage is towards the centre of the basinand due to gradual deposition of sedimentfrom basin margins, a nearly level plain formsat the centre of the basin. In times of sufficientwater, this plain is covered up by a shallowwater body. Such types of shallow lakes arecalled as playas where water is retained onlyfor short duration due to evaporation andquite often the playas contain good depositionof salts. The playa plain covered up by saltsis called alkali flats.

Deflation Hollows and Caves

Weathered mantle from over the rocks or baresoil, gets blown out by persistent movementof wind currents in one direction. This processmay create shallow depressions calleddeflation hollows. Deflation also createsnumerous small pits or cavities over rocksurfaces. The rock faces suffer impact andabrasion of wind-borne sand and firstshallow depressions called blow outs arecreated, and some of the blow outs becomedeeper and wider fit to be called caves.

Mushroom, Table and Pedestal Rocks

Many rock-outcrops in the deserts easilysusceptible to wind deflation and abrasionare worn out quickly leaving some remnantsof resistant rocks polished beautifully in theshape of mushroom with a slender stalk anda broad and rounded pear shaped cap above.Sometimes, the top surface is broad like atable top and quite often, the remnants standout like pedestals.

List the erosional features carved out bywind action and action of sheet floods.

Depositional Landforms

Wind is a good sorting agent. Depending uponthe velocity of wind, different sizes of grainsare moved along the floors by rolling or

saltation and carried in suspension and inthis process of transportation itself, thematerials get sorted. When the wind slows

or begins to die down, depending upon sizesof grains and their critical velocities, the grainswill begin to settle. So, in depositional

landforms made by wind, good sorting ofgrains can be found. Since wind is thereeverywhere and wherever there is good source

of sand and with constant wind directions,depositional features in arid regions candevelop anywhere.

Sand Dunes

Dry hot deserts are good places for sand duneformation. Obstacles to initiate dune formationare equally important. There can be a greatvariety of dune forms (Figure 7.16).

Figure 7.16 : Various types of sand dunesArrows indicate wind direction

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Crescent shaped dunes called barchans

with the points or wings directed away from

wind direction i.e., downwind, form where thewind direction is constant and moderate andwhere the original surface over which sand is

moving is almost uniform. Parabolic dunes

form when sandy surfaces are partially coveredwith vegetation. That means parabolic dunes

are reversed barchans with wind directionbeing the same. Seif is similar to barchan witha small difference. Seif has only one wing or

point. This happens when there is shift inwind conditions. The lone wings of seifs cangrow very long and high. Longitudinal dunes

form when supply of sand is poor and winddirection is constant. They appear as longridges of considerable length but low in

height. Transverse dunes are alignedperpendicular to wind direction. These dunesform when the wind direction is constant and

the source of sand is an elongated feature atright angles to the wind direction. They maybe very long and low in height. When sand is

plenty, quite often, the regular shaped dunescoalesce and lose their individualcharacteristics. Most of the dunes in the

deserts shift and a few of them will getstabilised especially near human habitations.

EXERCISES

1. Multiple choice questions.

(i) In which of the following stages of landform development, downward cuttingis dominated?

(a) Youth stage (c) Early mature stage

(b) Late mature stage (d) Old stage

(ii) A deep valley characterised by steep step-like side slopes is known as

(a) U-shaped valley (c) Blind valley

(b) Gorge (d) Canyon

(iii) In which one of the following regions the chemical weathering process ismore dominant than the mechanical process?

(a) Humid region (c) Arid region

(b) Limestone region (d) Glacier region

(iv) Which one of the following sentences best defines the term ‘Lapies’ ?

(a) A small to medium sized shallow depression

(b) A landform whose opening is more or less circular at the top andfunnel shaped towards bottom

(c) A landform formed due to dripping water from surface

(d) An irregular surface with sharp pinnacles, grooves and ridges

(v) A deep, long and wide trough or basin with very steep concave high wallsat its head as well as in sides is known as:

(a) Cirque (c) Lateral Moraine

(b) Glacial valley (d) Esker

2. Answer the following questions in about 30 words.

(i) What do incised meanders in rocks and meanders in plains of alluviumindicate?

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(ii) Explain the evolution of valley sinks or uvalas.

(iii) Underground flow of water is more common than surface run-off inlimestone areas. Why?

(iv) Glacial valleys show up many linear depositional forms. Give theirlocations and names.

(v) How does wind perform its task in desert areas? Is it the only agentresponsible for the erosional features in the deserts?

3. Answer the following questions in about 150 words.

(i) Running water is by far the most dominating geomorphic agent in shapingthe earth’s surface in humid as well as in arid climates. Explain.

(ii) Limestones behave differently in humid and arid climates. Why? What isthe dominant and almost exclusive geomorphic process in limestone areasand what are its results?

(iii) How do glaciers accomplish the work of reducing high mountains into lowhills and plains?

Project Work

Identify the landforms, materials and processes around your area.

2020-21