IESO Preparation Material (Geomorphic Processes and Evolution of Landforms)

IESO Preparation Material

Geomorphic Processes and Evolution of Landforms

GEOMORPHIC PROCESSES

C H A P T E R

After learning about how the earth wasborn, how it evolved its crust and otherinner layers, how its crustal plates

moved and are moving, and other informationon earthquakes, the forms of volcanism andabout the rocks and minerals the crust iscomposed of, it is time to know in detail aboutthe surface of the earth on which we live. Letus start with this question.

Why is the surface of the earth uneven?

First of all, the earth’s crust is dynamic. Youare well aware that it has moved and movesvertically and horizontally. Of course, it moveda bit faster in the past than the rate at which itis moving now. The differences in the internalforces operating from within the earth whichbuilt up the crust have been responsible forthe variations in the outer surface of the crust.The earth’s surface is being continuouslysubjected to external forces induced basicallyby energy (sunlight). Of course, the internalforces are still active though with differentintensities. That means, the earth’s surface isbeing continuously subjected to by externalforces originating within the earth’s atmosphereand by internal forces from within the earth.The external forces are known as exogenicforces and the internal forces are known asendogenic forces. The actions of exogenicforces result in wearing down (degradation) ofrelief/elevations and filling up (aggradation) ofbasins/depressions, on the earth’s surface. Thephenomenon of wearing down of reliefvariations of the surface of the earth througherosion is known as gradation. The endogenic

forces continuously elevate or build up partsof the earth’s surface and hence the exogenicprocesses fail to even out the relief variationsof the surface of the earth. So, variations remainas long as the opposing actions of exogenic andendogenic forces continue. In general terms,the endogenic forces are mainly land buildingforces and the exogenic processes are mainlyland wearing forces. The surface of the earth issensitive. Humans depend on it for theirsustenance and have been using it extensivelyand intensively. So, it is essential to understandits nature in order to use it effectively withoutdisturbing its balance and diminishing itspotential for the future. Almost all organismscontribute to sustain the earth’s environment.However, humans have caused over use ofresources. Use we must, but must also leave itpotential enough to sustain life through thefuture. Most of the surface of the earth had andhas been shaped over very long periods of time(hundreds and thousands of years) andbecause of its use and misuse by humans itspotential is being diminished at a fast rate. Ifthe processes which shaped and are shapingthe surface of the earth into varieties of forms(shapes) and the nature of materials of whichit is composed of, are understood, precautionscan be taken to minimise the detrimental effectsof human use and to preserve it for posterity.

GEOMORPHIC PROCESSES

You would like to know the meaning ofgeomorphic processes. The endogenic andexogenic forces causing physical stresses andchemical actions on earth materials and

FUNDAMENTALS OF PHYSICAL GEOGRAPHY46

bringing about changes in the configurationof the surface of the earth are known asgeomorphic processes. Diastrophism andvolcanism are endogenic geomorphicprocesses. These have already been discussedin brief in the preceding unit. Weathering, masswasting, erosion and deposition are exogenicgeomorphic processes. These exogenicprocesses are dealt with in detail in this chapter.

Any exogenic element of nature (like water,ice, wind, etc.,) capable of acquiring andtransporting earth materials can be called ageomorphic agent. When these elements ofnature become mobile due to gradients, theyremove the materials and transport them overslopes and deposit them at lower level.Geomorphic processes and geomorphic agentsespecially exogenic, unless stated separately,are one and the same.

A process is a force applied on earthmaterials affecting the same. An agent is amobile medium (like running water, moving icemasses, wind, waves and currents etc.) whichremoves, transports and deposits earthmaterials. Running water, groundwater,glaciers, wind, waves and currents, etc., canbe called geomorphic agents.

Do you think it is essential to distinguishgeomorphic agents and geomorphicprocesses?

Gravity besides being a directional forceactivating all downslope movements of matteralso causes stresses on the earth’s materials.Indirect gravitational stresses activate wave andtide induced currents and winds. Withoutgravity and gradients there would be nomobility and hence no erosion, transportationand deposition are possible. So, gravitationalstresses are as important as the othergeomorphic processes. Gravity is the force thatis keeping us in contact with the surface and itis the force that switches on the movement ofall surface material on earth. All the movementseither within the earth or on the surface of theearth occur due to gradients — from higherlevels to lower levels, from high pressure to lowpressure areas etc.

ENDOGENIC PROCESSES

The energy emanating from within the earth isthe main force behind endogenic geomorphicprocesses. This energy is mostly generated byradioactivity, rotational and tidal friction andprimordial heat from the origin of the earth.This energy due to geothermal gradients andheat flow from within induces diastrophismand volcanism in the lithosphere. Due tovariations in geothermal gradients and heat flowfrom within, crustal thickness and strength,the action of endogenic forces are not uniformand hence the tectonically controlled originalcrustal surface is uneven.

Diastrophism

All processes that move, elevate or build upportions of the earth’s crust come underdiastrophism. They include: (i) orogenicprocesses involving mountain buildingthrough severe folding and affecting long andnarrow belts of the earth’s crust; (ii) epeirogenicprocesses involving uplift or warping of largeparts of the earth’s crust; (iii) earthquakesinvolving local relatively minor movements;(iv) plate tectonics involving horizontalmovements of crustal plates.

In the process of orogeny, the crust isseverely deformed into folds. Due to epeirogeny,there may be simple deformation. Orogeny isa mountain building process whereasepeirogeny is continental building process.Through the processes of orogeny, epeirogeny,earthquakes and plate tectonics, there can befaulting and fracturing of the crust. All theseprocesses cause pressure, volume andtemperature (PVT) changes which in turninduce metamorphism of rocks.

Epeirogeny and orogeny, cite thedifferences.

Volcanism

Volcanism includes the movement of moltenrock (magma) onto or toward the earth’ssurface and also formation of many intrusiveand extrusive volcanic forms. Many aspects ofvolcanism have already been dealt in detail

GEOMORPHIC PROCESSES 47

processes and their respective driving forces.It should become clear from this chart that foreach process there exists a distinct driving forceor energy.

As there are different climatic regions onthe earth’s surface owing to thermal gradientscreated by latitudinal, seasonal and land andwater spread variations, the exogenicgeomorphic processes vary from region toregion. The density, type and distribution ofvegetation which largely depend upon

under volcanoes in the Unit II and underigneous rocks in the preceding chapter in thisunit.

What do the words volcanism andvolcanoes indicate?

EXOGENIC PROCESSES

The exogenic processes derive their energyfrom atmosphere determined by the ultimateenergy from the sun and also the gradientscreated by tectonic factors.

Why do you think that the slopes orgradients are created by tectonic factors?

Gravitational force acts upon all earthmaterials having a sloping surface and tend toproduce movement of matter in down slopedirection. Force applied per unit area is calledstress. Stress is produced in a solid by pushingor pulling. This induces deformation. Forcesacting along the faces of earth materials areshear stresses (separating forces). It is thisstress that breaks rocks and other earthmaterials. The shear stresses result in angulardisplacement or slippage. Besides thegravitational stress earth materials becomesubjected to molecular stresses that may becaused by a number of factors amongst whichtemperature changes, crystallisation andmelting are the most common. Chemicalprocesses normally lead to loosening of bondsbetween grains, dissolving of soluble mineralsor cementing materials. Thus, the basic reasonthat leads to weathering, mass movements,erosion and deposition is development ofstresses in the body of the earth materials.

As there are different climatic regions onthe earth’s surface the exogenic geomorphicprocesses vary from region to region.Temperature and precipitation are the twoimportant climatic elements that controlvarious processes.

All the exogenic geomorphic processes arecovered under a general term, denudation. Theword ‘denude’ means to strip off or to uncover.Weathering, mass wasting/movements, erosionand transportation are included in denudation.The flow chart (Figure 6.1) gives the denudation

precipitation and temperature exert influenceindirectly on exogenic geomorphic processes.Within different climatic regions there may belocal variations of the effects of different climaticelements due to altitudinal differences, aspectvariations and the variation in the amount ofinsolation received by north and south facingslopes as compared to east and west facingslopes. Further, due to differences in windvelocities and directions, amount and kind ofprecipitation, its intensity, the relation betweenprecipitation and evaporation, daily range oftemperature, freezing and thawing frequency,depth of frost penetration, the geomorphicprocesses vary within any climatic region.

What is the sole driving force behind allthe exogenic processes?

Climatic factors being equal, the intensityof action of exogenic geomorphic processesdepends upon type and structure of rocks. Theterm structure includes such aspects of rocksas folds, faults, orientation and inclination ofbeds, presence or absence of joints, beddingplanes, hardness or softness of constituentminerals, chemical susceptibility of mineralconstituents; the permeability or impermeability

Figure 6.1 : Denudational processes and theirdriving forces


etc. Different types of rocks with differences intheir structure offer varying resistances tovarious geomorphic processes. A particularrock may be resistant to one process and non-resistant to another. And, under varyingclimatic conditions, particular rocks mayexhibit different degrees of resistance togeomorphic processes and hence they operateat differential rates and give rise to differencesin topography. The effects of most of theexogenic geomorphic processes are small andslow and may be imperceptible in a short timespan, but will in the long run affect the rocksseverely due to continued fatigue.

Finally, it boils down to one fact that thedifferences on the surface of the earth thoughoriginally related to the crustal evolutioncontinue to exist in some form or the other dueto differences in the type and structure of earthmaterials, differences in geomorphic processesand in their rates of operation.

Some of the exogenic geomorphic processeshave been dealt in detail here.

WEATHERING

Weathering is action of elements of weather andclimate over earth materials. There are anumber of processes within weathering whichact either individually or together to affect theearth materials in order to reduce them tofragmental state.

Weathering is defined as mechanicaldisintegration and chemical decom-position of rocks through the actions ofvarious elements of weather and climate.

As very little or no motion of materialstakes place in weathering, it is an in-situ oron-site process.

Is this little motion which can occursometimes due to weathering synonymouswith transportation? If not, why?

Weathering processes are conditioned bymany complex geological, climatic, topographicand vegetative factors. Climate is of particularimportance. Not only weathering processesdiffer from climate to climate, but also the depthof the weathering mantle (Figure 6.2).

Figure 6.2 : Climatic regimes and depth of weatheringmantles (adapted and modified from Strakhov, 1967)

Activity

Mark the latitude values of differentclimatic regimes in Figure 6.2 andcompare the details.

There are three major groups of weatheringprocesses : (i) chemical; (ii) physical ormechanical; (iii) biological weathering processes.Very rarely does any one of these processes everoperate completely by itself, but quite often adominance of one process can be seen.

Chemical Weathering Processes

A group of weathering processes viz; solution,carbonation, hydration, oxidation andreduction act on the rocks to decompose,dissolve or reduce them to a fine clastic statethrough chemical reactions by oxygen, surfaceand/or soil water and other acids. Water andair (oxygen and carbon dioxide) along withheat must be present to speed up all chemicalreactions. Over and above the carbon dioxidepresent in the air, decomposition of plants andanimals increases the quantity of carbondioxide underground. These chemicalreactions on various minerals are very muchsimilar to the chemical reactions in a laboratory.

Solution

When something is dissolved in water or acids,the water or acid with dissolved contents is


called solution. This process involves removalof solids in solution and depends uponsolubility of a mineral in water or weak acids.On coming in contact with water many solidsdisintegrate and mix up as suspension inwater. Soluble rock forming minerals likenitrates, sulphates, and potassium etc. areaffected by this process. So, these minerals areeasily leached out without leaving any residuein rainy climates and accumulate in dryregions. Minerals like calcium carbonate andcalcium magnesium bicarbonate present inlimestones are soluble in water containingcarbonic acid (formed with the addition ofcarbon dioxide in water), and are carried awayin water as solution. Carbon dioxide producedby decaying organic matter along with soilwater greatly aids in this reaction. Commonsalt (sodium chloride) is also a rock formingmineral and is susceptible to this process ofsolution.

Carbonation

Carbonation is the reaction of carbonate andbicarbonate with minerals and is a commonprocess helping the breaking down offeldspars and carbonate minerals. Carbondioxide from the atmosphere and soil air isabsorbed by water, to form carbonic acid thatacts as a weak acid. Calcium carbonates andmagnesium carbonates are dissolved incarbonic acid and are removed in a solutionwithout leaving any residue resulting in caveformation.

Why are clay minerals easily erodible?

Hydration

Hydration is the chemical addition of water.Minerals take up water and expand; thisexpansion causes an increase in the volume ofthe material itself or rock. Calcium sulphatetakes in water and turns to gypsum, which ismore unstable than calcium sulphate. Thisprocess is reversible and long, continuedrepetition of this process causes fatigue in therocks and may lead to their disintegration.

Many clay minerals swell and contract duringwetting and drying and a repetition of thisprocess results in cracking of overlyingmaterials. Salts in pore spaces undergo rapidand repeated hydration and help in rockfracturing. The volume changes in mineralsdue to hydration will also help in physicalweathering through exfoliation and granulardisintegration.

Oxidation and Reduction

In weathering, oxidation means a combinationof a mineral with oxygen to form oxides orhydroxides. Oxidation occurs where there isready access to the atmosphere andoxygenated waters. The minerals mostcommonly involved in this process are iron,manganese, sulphur etc. In the process ofoxidation rock breakdown occurs due to thedisturbance caused by addition of oxygen. Redcolour of iron upon oxidation turns to brownor yellow. When oxidised minerals are placedin an environment where oxygen is absent,reduction takes place. Such conditions existusually below the water table, in areas ofstagnant water and waterlogged ground. Redcolour of iron upon reduction turns to greenishor bluish grey.

These weathering processes are inter-related. Hydration, carbonation and oxidationgo hand in hand and hasten the weatheringprocess.

Can we give iron rusting as an exampleof oxidation? How essential is water inchemical weathering processes? Canchemical weathering processes dominatein water scarce hot deserts?

Physical Weathering Processes

Physical or mechanical weathering processesdepend on some applied forces. The appliedforces could be: (i) gravitational forces such asoverburden pressure, load and shearing stress;(ii) expansion forces due to temperaturechanges, crystal growth or animal activity;(iii) water pressures controlled by wetting and


drying cycles. Many of these forces are appliedboth at the surface and within different earthmaterials leading to rock fracture. Most of thephysical weathering processes are caused bythermal expansion and pressure release. Theseprocesses are small and slow but can causegreat damage to the rocks because ofcontinued fatigue the rocks suffer due torepetition of contraction and expansion.

Unloading and Expansion

Removal of overlying rock load because ofcontinued erosion causes vertical pressurerelease with the result that the upper layers ofthe rock expand producing disintegration ofrock masses. Fractures will develop roughlyparallel to the ground surface. In areas ofcurved ground surface, arched fractures tendto produce massive sheets or exfoliation slabsof rock. Exfoliation sheets resulting fromexpansion due to unloading and pressurerelease may measure hundreds or eventhousands of metres in horizontal extent. Large,smooth rounded domes called exfoliationdomes (Figure 6.3) result due to this process.

temperatures, this internal movement amongthe mineral grains of the superficial layers ofrocks takes place regularly. This process ismost effective in dry climates and highelevations where diurnal temperature changesare drastic. As has been mentioned earlierthough these movements are very small theymake the rocks weak due to continued fatigue.The surface layers of the rocks tend to expandmore than the rock at depth and this leads tothe formation of stress within the rock resultingin heaving and fracturing parallel to thesurface. Due to differential heating andresulting expansion and contraction of surfacelayers and their subsequent exfoliation fromthe surface results in smooth rounded surfacesin rocks. In rocks like granites, smoothsurfaced and rounded small to big boulderscalled tors form due to such exfoliation.

What is the difference between exfoliationdomes and exfoliated tors?

Freezing, Thawing and Frost Wedging

Frost weathering occurs due to growth of icewithin pores and cracks of rocks duringrepeated cycles of freezing and melting. Thisprocess is most effective at high elevations inmid-latitudes where freezing and melting isoften repeated. Glacial areas are subject to frostwedging daily. In this process, the rate offreezing is important. Rapid freezing of watercauses its sudden expansion and high pressure.The resulting expansion affects joints, cracksand small inter granular fractures to becomewider and wider till the rock breaks apart.

Salt Weathering

Salts in rocks expand due to thermal action,hydration and crystallisation. Many salts likecalcium, sodium, magnesium, potassium andbarium have a tendency to expand. Expansionof these salts depends on temperature andtheir thermal properties. High temperatureranges between 30 and 50oC of surfacetemperatures in deserts favour such saltexpansion. Salt crystals in near-surface pores

Figure 6.3 : A large exfoliation dome in granite rocknear bhongir (Bhuvanagiri) town in Andhra Pradesh

Temperature Changes and Expansion

Various minerals in rocks possess their ownlimits of expansion and contraction. With risein temperature, every mineral expands andpushes against its neighbour and astemperature falls, a corresponding contractiontakes place. Because of diurnal changes in the


cause splitting of individual grains withinrocks, which eventually fall off. This process offalling off of individual grains may result ingranular disintegration or granular foliation.

Salt crystallisation is most effective of allsalt-weathering processes. In areas withalternating wetting and drying conditions saltcrystal growth is favoured and the neighbouringgrains are pushed aside. Sodium chloride andgypsum crystals in desert areas heave upoverlying layers of materials and with the resultpolygonal cracks develop all over the heavedsurface. With salt crystal growth, chalk breaksdown most readily, followed by limestone,sandstone, shale, gneiss and granite etc.

BIOLOGICAL ACTIVITY AND WEATHERING

Biological weathering is contribution to orremoval of minerals and ions from theweathering environment and physical changesdue to growth or movement of organisms.Burrowing and wedging by organisms likeearthworms, termites, rodents etc., help inexposing the new surfaces to chemical attackand assists in the penetration of moisture andair. Human beings by disturbing vegetation,ploughing and cultivating soils, also help inmixing and creating new contacts between air,water and minerals in the earth materials.Decaying plant and animal matter help in theproduction of humic, carbonic and other acidswhich enhance decay and solubility of someelements. Algae utilise mineral nutrients forgrowth and help in concentration of iron andmanganese oxides. Plant roots exert atremendous pressure on the earth materialsmechanically breaking them apart.

SOME SPECIAL EFFECTS OF WEATHERING

This has already been explained underphysical weathering processes of unloading,thermal contraction and expansion and saltweathering. Exfoliation is a result but not aprocess. Flaking off of more or less curvedsheets of shells from over rocks or bedrockresults in smooth and rounded surfaces(Figure 6.4). Exfoliation can occur due toexpansion and contraction induced by

temperature changes. Exfoliation domes andtors result due to unloading and thermalexpansion respectively.

SIGNIFICANCE OF WEATHERING

Weathering processes are responsible forbreaking down the rocks into smallerfragments and preparing the way for formationof not only regolith and soils, but also erosionand mass movements. Biomes and bio-diversity is basically a result of forests(vegetation) and forests depend upon the depthof weathering mantles. Erosion cannot besignificant if the rocks are not weathered. Thatmeans, weathering aids mass wasting, erosionand reduction of relief and changes inlandforms are a consequence of erosion.Weathering of rocks and deposits helps in theenrichment and concentrations of certainvaluable ores of iron, manganese, aluminium,copper etc., which are of great importance forthe national economy. Weathering is animportant process in the formation of soils.

When rocks undergo weathering, somematerials are removed through chemicalor physical leaching by groundwater andthereby the concentration of remaining(valuable) materials increases. Withoutsuch a weathering taking place, theconcentration of the same valuablematerial may not be sufficient andeconomically viable to exploit, process andrefine. This is what is called enrichment.

Fig.6.4 : Exfoliation (Flacking) and granulardisintegration


the three forms of movements. Figure 6.5 showsthe relationships among different types of massmovements, their relative rates of movementand moisture limits.

Figure 6.5 : Relationships among different types ofmass movements, their relative rates of movement

and moisture limits (after Whitehead, 2001)

MASS MOVEMENTS

These movements transfer the mass of rockdebris down the slopes under the directinfluence of gravity. That means, air, water orice do not carry debris with them from place toplace but on the other hand the debris maycarry with it air, water or ice. The movementsof mass may range from slow to rapid,affecting shallow to deep columns of materialsand include creep, flow, slide and fall. Gravityexerts its force on all matter, both bedrock andthe products of weathering. So, weathering isnot a pre-requisite for mass movement thoughit aids mass movements. Mass movements arevery active over weathered slopes rather thanover unweathered materials.

Mass movements are aided by gravity andno geomorphic agent like running water,glaciers, wind, waves and currents participatein the process of mass movements. That meansmass movements do not come under erosionthough there is a shift (aided by gravity) ofmaterials from one place to another. Materialsover the slopes have their own resistance todisturbing forces and will yield only when forceis greater than the shearing resistance of thematerials. Weak unconsolidated materials,thinly bedded rocks, faults, steeply dippingbeds, vertical cliffs or steep slopes, abundantprecipitation and torrential rains and scarcityof vegetation etc., favour mass movements.

Several activating causes precede massmovements. They are : (i) removal of supportfrom below to materials above through naturalor artificial means; (ii) increase in gradient andheight of slopes; (iii) overloading throughaddition of materials naturally or by artificialfilling; (iv) overloading due to heavy rainfall,saturation and lubrication of slope materials;(v) removal of material or load from over theoriginal slope surfaces; (vi) occurrence ofearthquakes, explosions or machinery;(vii) excessive natural seepage; (viii) heavydrawdown of water from lakes, reservoirs andrivers leading to slow outflow of water fromunder the slopes or river banks; (ix) indis-criminate removal of natural vegetation.

Heave (heaving up of soils due to frostgrowth and other causes), flow and slide are

Mass movements can be grouped underthree major classes: (i) slow movements;(ii) rapid movements; (iii) landslides.

Slow Movements

Creep is one type under this category whichcan occur on moderately steep, soil coveredslopes. Movement of materials is extremelyslow and imperceptible except throughextended observation. Materials involved canbe soil or rock debris. Have you ever seen fenceposts, telephone poles lean downslope fromtheir vertical position and in their linearalignment? If you have, that is due to the creepeffect. Depending upon the type of materialinvolved, several types of creep viz., soil creep,talus creep, rock creep, rock-glacier creep etc.,can be identified. Also included in this groupis solifluction which involves slow downslopeflowing soil mass or fine grained rock debrissaturated or lubricated with water. This processis quite common in moist temperate areaswhere surface melting of deeply frozen groundand long continued rain respectively, occurfrequently. When the upper portions getsaturated and when the lower parts areimpervious to water percolation, flowing occursin the upper parts.


Rapid Movements

These movements are mostly prevalent inhumid climatic regions and occur over gentleto steep slopes. Movement of water-saturatedclayey or silty earth materials down low-angleterraces or hillsides is known as earthflow.Quite often, the materials slump making step-like terraces and leaving arcuate scarps at theirheads and an accumulation bulge at the toe.When slopes are steeper, even the bedrockespecially of soft sedimentary rocks like shaleor deeply weathered igneous rock may slidedownslope.

Another type in this category is mudflow.In the absence of vegetation cover and withheavy rainfall, thick layers of weatheredmaterials get saturated with water and eitherslowly or rapidly flow down along definitechannels. It looks like a stream of mud withina valley. When the mudflows emerge out ofchannels onto the piedmont or plains, they canbe very destructive engulfing roads, bridgesand houses. Mudflows occur frequently on theslopes of erupting or recently erupted volcanoes.Volcanic ash, dust and other fragments turninto mud due to heavy rains and flow down astongues or streams of mud causing greatdestruction to human habitations.

A third type is the debris avalanche, whichis more characteristic of humid regions withor without vegetation cover and occurs innarrow tracks on steep slopes. This debrisavalanche can be much faster than themudflow. Debris avalanche is similar to snowavalanche.

In Andes mountains of South Americaand the Rockies mountains of NorthAmerica, there are a few volcanoes whicherupted during the last decade and verydevastating mudflows occurred downtheir slopes during eruption as well asafter eruption.

Landslides

These are known as relatively rapid andperceptible movements. The materials involvedare relatively dry. The size and shape of thedetached mass depends on the nature of

discontinuities in the rock, the degree ofweathering and the steepness of the slope.Depending upon the type of movement ofmaterials several types are identified in thiscategory.

Slump is slipping of one or several units ofrock debris with a backward rotation withrespect to the slope over which the movementtakes place (Figure 6.6). Rapid rolling or sliding

of earth debris without backward rotation ofmass is known as debris slide. Debris fall isnearly a free fall of earth debris from a verticalor overhanging face. Sliding of individual rockmasses down bedding, joint or fault surfacesis rockslide. Over steep slopes, rock sliding isvery fast and destructive. Figure 6.7 showslandslide scars over steep slopes. Slides occuras planar failures along discontinuities likebedding planes that dip steeply. Rock fall isfree falling of rock blocks over any steep slopekeeping itself away from the slope. Rock fallsoccur from the superficial layers of the rock

Figure 6.6 : Slumping of debris with backward rotation

Figure 6.7 : Landslide scars in Shiwalik Himalayan rangesnear river Sarada at India-Nepal border, Uttar Pradesh


face, an occurrence that distinguishes it fromrockslide which affects materials up to asubstantial depth.

Between mass wasting and massmovements, which term do you feel ismost appropriate? Why? Can solifluctionbe included under rapid flow movements?Why it can be and can’t be?

In our country, debris avalanche andlandslides occur very frequently in theHimalayas. There are many reasons forthis. One, the Himalayas are tectonicallyactive. They are mostly made up ofsedimentary rocks and unconsolidatedand semi-consolidated deposits. Theslopes are very steep. Compared to theHimalayas, the Nilgiris borderingTamilnadu, Karnataka, Kerala and theWestern Ghats along the west coast arerelatively tectonically stable and aremostly made up of very hard rocks; but,still, debris avalanches and landslidesoccur though not as frequently as in theHimalayas, in these hills. Why? Manyslopes are steeper with almost verticalcliffs and escarpments in the WesternGhats and Nilgiris. Mechanical weatheringdue to temperature changes and rangesis pronounced. They receive heavyamounts of rainfall over short periods.So, there is almost direct rock fall quitefrequently in these places along withlandslides and debris avalanches.

EROSION AND DEPOSITION

Erosion involves acquisition and transportationof rock debris. When massive rocks break intosmaller fragments through weathering andany other process, erosional geomorphicagents like running water, groundwater,glaciers, wind and waves remove andtransport it to other places depending uponthe dynamics of each of these agents. Abrasionby rock debris carried by these geomorphicagents also aids greatly in erosion. By erosion,relief degrades, i.e., the landscape is worndown. That means, though weathering aids

erosion it is not a pre-condition for erosion totake place. Weathering, mass-wasting anderosion are degradational processes. It iserosion that is largely responsible forcontinuous changes that the earth’s surface isundergoing. As indicated in Figure 6.1,denudational processes like erosion andtransportation are controlled by kinetic energy.The erosion and transportation of earthmaterials is brought about by wind, runningwater, glaciers, waves and ground water. Ofthese the first three agents are controlled byclimatic conditions.

Can you compare the three climaticallycontrolled agents?

They represent three states of matter —gaseous (wind), liquid (running water) andsolid (glacier) respectively. The erosion can bedefined as “application of the kinetic energyassociated with the agent to the surface of theland along which it moves”. Kinetic energy iscomputed as KE = 1/

2 mv2 where ‘m’ is the mass

and ‘v’ is the velocity. Hence the energyavailable to perform work will depend on themass of the material and the velocity withwhich it is moving. Obviously then you will findthat though the glaciers move at very lowvelocities due to tremendous mass are moreeffective as the agents of erosion and wind,being in gaseous state, are less effective.

The work of the other two agents of erosion-waves and ground water is not controlled byclimate. In case of waves it is the location alongthe interface of litho and hydro sphere —coastal region — that will determine the workof waves, whereas the work of ground water isdetermined more by the lithological characterof the region. If the rocks are permeable andsoluble and water is available only then karsttopography develops. In the next chapter weshall be dealing with the landforms producedby each of the agents of erosion.

Deposition is a consequence of erosion. Theerosional agents loose their velocity and henceenergy on gentler slopes and the materialscarried by them start to settle themselves. Inother words, deposition is not actually the workof any agent. The coarser materials get


deposited first and finer ones later. Bydeposition depressions get filled up. The sameerosional agents viz., running water, glaciers,wind, waves and groundwater act asaggradational or depositional agents also.

What happens to the surface of the earthdue to erosion and deposition is elaborated inthe next chapter on landforms and theirevolution.

There is a shift of materials in massmovements as well as in erosion from oneplace to the other. So, why can’t both betreated as one and the same? Can therebe appreciable erosion without rocksundergoing weathering?

SOIL FORMATION

Soil and Soil Contents

You see plants growing in soils. You play inthe ground and come into contact with soil.You touch and feel soil and soil your clotheswhile playing. Can you describe it?

A pedologist who studies soils defines soilas a collection of natural bodies on the earth’ssurface containing living matter andsupporting or capable of supporting plants.

Soil is a dynamic medium in which manychemical, physical and biological activities goon constantly. Soil is a result of decay, it is alsothe medium for growth. It is a changing anddeveloping body. It has many characteristicsthat fluctuate with the seasons. It may bealternatively cold and warm or dry and moist.Biological activity is slowed or stopped if thesoil becomes too cold or too dry. Organic matterincreases when leaves fall or grasses die. Thesoil chemistry, the amount of organic matter,the soil flora and fauna, the temperature andthe moisture, all change with the seasons aswell as with more extended periods of time.That means, soil becomes adjusted toconditions of climate, landform and vegetationand will change internally when thesecontrolling conditions change.

Process of Soil Formation

Soil formation or pedogenesis depends first onweathering. It is this weathering mantle (depth

of the weathered material) which is the basicinput for soil to form. First, the weatheredmaterial or transported deposits are colonisedby bacteria and other inferior plant bodies likemosses and lichens. Also, several minororganisms may take shelter within the mantleand deposits. The dead remains of organismsand plants help in humus accumulation. Minorgrasses and ferns may grow; later, bushes andtrees will start growing through seeds broughtin by birds and wind. Plant roots penetratedown, burrowing animals bring up particles,mass of material becomes porous and sponge-like with a capacity to retain water and to permitthe passage of air and finally a mature soil, acomplex mixture of mineral and organicproducts forms.

Is weathering solely responsible for soilformation? If not, why?

Pedology is soil science. A pedologist is asoil-scientist.

Soil-forming Factors

Five basic factors control the formation of soils:(i) parent material; (ii) topography; (iii) climate;(iv) biological activity; (v) time. In fact soilforming factors act in union and affect theaction of one another.

Parent Material

Parent material is a passive control factor insoil formation. Parent materials can be any in-situ or on-site weathered rock debris (residualsoils) or transported deposits (transportedsoils). Soil formation depends upon the texture(sizes of debris) and structure (disposition ofindividual grains/particles of debris) as wellas the mineral and chemical composition of therock debris/deposits.

Nature and rate of weathering and depth ofweathering mantle are important considerationunder parent materials. There may bedifferences in soil over similar bedrock anddissimilar bedrocks may have similar soilsabove them. But when soils are very youngand have not matured these show strong links


with the type of parent rock. Also, in case ofsome limestone areas, where the weatheringprocesses are specific and peculiar, soils willshow clear relation with the parent rock.

Topography

Topography like parent materials is anotherpassive control factor. The influence oftopography is felt through the amount ofexposure of a surface covered by parentmaterials to sunlight and the amount ofsurface and sub-surface drainage over andthrough the parent materials. Soils will be thinon steep slopes and thick over flat uplandareas. Over gentle slopes where erosion is slowand percolation of water is good, soil formationis very favourable. Soils over flat areas maydevelop a thick layer of clay with goodaccumulation of organic matter giving the soildark colour. In middle latitudes, the southfacing slopes exposed to sunlight have differentconditions of vegetation and soils and the northfacing slopes with cool, moist conditions havesome other soils and vegetation.

Climate

Climate is an important active factor in soilformation. The climatic elements involved in soildevelopment are : (i) moisture in terms of itsintensity, frequency and duration ofprecipitation - evaporation and humidity;(ii) temperature in terms of seasonal anddiurnal variations.

Precipitation gives soil its moisture contentwhich makes the chemical and biologicalactivities possible. Excess of water helps in thedownward transportation of soil componentsthrough the soil (eluviation) and deposits thesame down below (illuviation). In climates likewet equatorial rainy areas with high rainfall,not only calcium, sodium, magnesium,potassium etc. but also a major part of silica isremoved from the soil. Removal of silica fromthe soil is known as desilication. In dry climates,because of high temperature, evaporationexceeds precipitation and hence ground wateris brought up to the surface by capillary actionand in the process the water evaporates leavingbehind salts in the soil. Such salts form into acrust in the soil known as hardpans. In tropical

climates and in areas with intermediateprecipitation conditions, calcium carbonatenodules (kanker) are formed.

Temperature acts in two ways — increasingor reducing chemical and biological activity.Chemical activity is increased in highertemperatures, reduced in cooler temperatures(with an exception of carbonation) and stopsin freezing conditions. That is why, tropical soilswith higher temperatures show deeper profilesand in the frozen tundra regions soils containlargely mechanically broken materials.

Biological Activity

The vegetative cover and organisms that occupythe parent materials from the beginning and alsoat later stages help in adding organic matter,moisture retention, nitrogen etc. Dead plantsprovide humus, the finely divided organic matterof the soil. Some organic acids which formduring humification aid in decomposing theminerals of the soil parent materials.

Intensity of bacterial activity shows updifferences between soils of cold and warmclimates. Humus accumulates in cold climatesas bacterial growth is slow. With undecomposedorganic matter because of low bacterial activity,layers of peat develop in sub-arctic and tundraclimates. In humid tropical and equatorialclimates, bacterial growth and action is intenseand dead vegetation is rapidly oxidised leavingvery low humus content in the soil. Further,bacteria and other soil organisms take gaseousnitrogen from the air and convert it into achemical form that can be used by plants. Thisprocess is known as nitrogen fixation.Rhizobium, a type of bacteria, lives in the rootnodules of leguminous plants and fixes nitrogenbeneficial to the host plant. The influence of largeanimals like ants, termites, earthworms, rodentsetc., is mechanical, but, it is neverthelessimportant in soil formation as they rework thesoil up and down. In case of earthworms, asthey feed on soil, the texture and chemistry ofthe soil that comes out of their body changes.

Time

Time is the third important controlling factorin soil formation. The length of time the soilforming processes operate, determines


maturation of soils and profile development. Asoil becomes mature when all soil-formingprocesses act for a sufficiently long timedeveloping a profile. Soils developing fromrecently deposited alluvium or glacial till areconsidered young and they exhibit no horizonsor only poorly developed horizons. No specificlength of time in absolute terms can be fixedfor soils to develop and mature.

Is it necessary to separate the process of

soil formation and the soil forming control

factors?

Why are time, topography and parent

material considered as passive control

factors in soil formation?

EXERCISES

1. Multiple choice questions.

(i) Which one of the following processes is a gradational process?

(a) Deposition (c) Volcanism

(b) Diastrophism (d) Erosion

(ii) Which one of the following materials is affected by hydration process?

(a) Granite (c) Quartz

(b) Clay (d) Salts

(iii) Debris avalanche can be included in the category of:

(a) Landslides (c) Rapid flow mass movements

(b) Slow flow mass movements (d) Subsidence

2. Answer the following questions in about 30 words.

(i) It is weathering that is responsible for bio-diversity on the earth. How?

(ii) What are mass movements that are real rapid and perceptible? List.

(iii) What are the various mobile and mighty exogenic geomorphic agents andwhat is the prime job they perform?

(iv) Is weathering essential as a pre-requisite in the formation of soils? Why?


(i) “Our earth is a playfield for two opposing groups of geomorphic processes.”Discuss.

(ii) Exogenic geomorphic processes derive their ultimate energy from the sun’sheat. Explain.

(iii) Are physical and chemical weathering processes independent of eachother? If not, why? Explain with examples.

(iv) How do you distinguish between the process of soil formation and soil-forming factors? What is the role of climate and biological activity as twoimportant control factors in the formation of soils?

Project Work

Depending upon the topography and materials around you, observe and recordclimate, possible weathering process and soil contents and characteristics.

LANDFORMS AND THEIR

EVOLUTION

C H A P T E R

A fter 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 sized partof the surface of the earth, what is a landscape?

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 of certaingeomorphic processes and agent(s). Actionsof most of the geomorphic processes andagents are slow, and hence the results take along time to take shape. Every landform has abeginning. 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 eithera part of the earth’s surface from one landforminto another or transformation of individuallandforms after they are once formed. That

means, each and every landform has a historyof development and changes through time. Alandmass passes through stages ofdevelopment somewhat comparable to thestages of life — youth, mature and old age.

What are the two important aspects ofthe evolution of landforms?

The evolutionary history of the continuallychanging surface of the earth is essential to beunderstood in order to use it effectively withoutdisturbing its balance and diminishing itspotential for the future. Geomorphology dealswith the reconstruction of the history of thesurface of the earth through a study of itsforms, the materials of which it is made up ofand the processes that shape it.

Changes on the surface of the earth owemostly to erosion by various geomorphicagents. Of course, the process of deposition too,by covering the land surfaces and filling thebasins, valleys or depressions, brings changesin the surface of the land. Deposition followserosion and the depositional surfaces too areultimately subjected to erosion. Running water,ground-water, glaciers, wind and waves arepowerful erosional and depositional agentsshaping and changing the surface of the earthaided by weathering and mass wastingprocesses. These geomorphic agents actingover long periods of time produce systematicchanges leading to sequential development oflandforms. Each geomorphic agent producesits own assemblage of landforms. Not only this,each geomorphic process and agent leave theirdistinct imprints on the landforms they

LANDFORMS AND THEIR EVOLUTION 59

produce. You know that most of thegeomorphic processes are imperceptiblefunctions and can only be seen and measuredthrough their results. What are the results?These results are nothing but landforms andtheir characteristics. Hence, a study oflandforms, will reveal to us the process andagent which has made or has been makingthose landforms.

Most of the geomorphic processes areimperceptible. Cite a few processes whichcan be seen and a few which can’t beseen.

As the geomorphic agents are capable oferosion and deposition, two sets — erosionalor destructional and depositional orconstructional — of landforms are producedby them. Many varieties of landforms developby the action of each of the geomorphic agentsdepending upon especially the type andstructure i.e. folds, faults, joints, fractures,hardness and softness, permeability andimpermeability, etc. come under structure ofrocks. There are some other independentcontrols like (i) stability of sea level; (ii) tectonicstability of landmasses; (iii) climate, whichinfluence the evolution of landforms. Anydisturbance in any of these three controllingfactors can upset the systematic andsequential stages in the development andevolution of landforms.

In the following pages, under each of thegeomorphic regimes i.e. running water;groundwater, glaciers, waves, and winds, firsta brief discussion is presented as to howlandmasses are reduced in their relief througherosion and then, development of some of theerosional and depositional landforms is dealtwith.

RUNNING WATER

In humid regions, which receive heavy rainfallrunning water is considered the mostimportant of the geomorphic agents inbringing about the degradation of the landsurface. There are two components of runningwater. One is overland flow on general landsurface as a sheet. Another is linear flow as

streams and rivers in valleys. Most of theerosional landforms made by running waterare associated with vigorous and youthfulrivers flowing along gradients. With time,stream channels over steep gradients turngentler due to continued erosion, and as aconsequence, lose their velocity, facilitatingactive deposition. There may be depositionalforms associated with streams flowing oversteep slopes. But these phenomena will be ona small scale compared to those associatedwith rivers flowing over medium to gentleslopes. The gentler the river channels ingradient or slope, the greater is the deposition.When the stream beds turn gentler due tocontinued erosion, downward cutting becomesless dominant and lateral erosion of banksincreases and as a consequence the hills andvalleys are reduced to plains.

Is complete reduction of relief of a highland 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 give riseto 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 poorintegration and flow over original slopesshowing shallow V-shaped valleys with nofloodplains or with very narrow floodplainsalong trunk streams. Streams divides are broadand flat with marshes, swamp and lakes.Meanders if present develop over these broadupland surfaces. These meanders mayeventually entrench themselves into theuplands. Waterfalls and rapids may exist wherelocal 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 few withgentle gradients. Streams meander freely overvast floodplains showing natural levees, oxbowlakes, etc. Divides are broad and flat with lakes,swamps and marshes. Most of the landscapeis at or slightly above sea level.

EROSIONAL LANDFORMS

Valleys

Valleys start as small and narrow rills; the rillswill 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) anda canyon is characterised by steep step-likeside slopes (Figure 7.2) and may be as deep asa gorge. A gorge is almost equal in width at itstop as well as its bottom. In contrast, a canyon

is wider at its top than at its bottom. In fact, acanyon is a variant of gorge. Valley types dependupon the type and structure of rocks in whichthey form. For example, canyons commonlyform in horizontal bedded sedimentary rocksand gorges form in hard rocks.

Figure 7.1 : The Valley of Kaveri river near Hogenekal,Dharmapuri district, Tamilnadu 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


Potholes and Plunge Pools

Over the rocky beds of hill-streams more or lesscircular depressions called potholes formbecause of stream erosion aided by the abrasionof rock fragments. Once a small and shallowdepression forms, pebbles and boulders getcollected in those depressions and get rotatedby flowing water and consequently thedepressions grow in dimensions. A series of suchdepressions eventually join and the streamvalley gets deepened. At the foot of waterfallsalso, large potholes, quite deep and wide, formbecause of the sheer impact of water androtation of boulders. Such large and deep holesat the base of waterfalls are called plunge pools.These pools also help in the deepening of valleys.Waterfalls are also transitory like any otherlandform and will recede gradually and bringthe floor of the valley above waterfalls to thelevel below.

INCISED OR ENTRENCHED MEANDERS

In streams that flow rapidly over steepgradients, normally erosion is concentrated onthe bottom of the stream channel. Also, in thecase of steep gradient streams, lateral erosionon the sides of the valleys is not much whencompared to the streams flowing on low andgentle slopes. Because of active lateral erosion,streams flowing over gentle slopes, developsinuous or meandering courses. It is commonto find meandering courses over floodplainsand delta plains where stream gradients arevery gentle. But very deep and wide meanderscan also be found cut in hard rocks. Suchmeanders are called incised or entrenchedmeanders (Figure 7.2). Meander loops developover original gentle surfaces in the initial stagesof development of streams and the same loopsget entrenched into the rocks normally due toerosion or slow, continued uplift of the landover which they start. They widen and deepenover time and can be found as deep gorges andcanyons in hard rock areas. They give anindication on the status of original landsurfaces over which streams have developed.

What are the differences between incisedmeanders and meanders over flood anddelta plains?

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 the streaminto its own depositional floodplain. There canbe a number of such terraces at differentheights indicating former river bed levels. Theriver terraces may occur at the same elevationon either side of the rivers in which case theyare called paired terraces (Figure 7.3).

Figure 7.3 : Paired and unpaired river terraces

When a terrace is present only on one sideof the stream and with none on the other sideor one at quite a different elevation on the otherside, the terraces are called non-pairedterraces. Unpaired terraces are typical in areasof slow uplift of land or where the water columnchanges are not uniform along both the banks.The terraces may result due to (i) receding waterafter a peak flow; (ii) change in hydrologicalregime due to climatic changes; (iii) tectonicuplift of land; (iv) sea level changes in case ofrivers closer to the sea.

DEPOSITIONAL LANDFORMS

Alluvial Fans

Alluvial fans (Figure 7.4) are formed whenstreams flowing from higher levels break intofoot slope plains of low gradient. Normally verycoarse load is carried by streams flowing overmountain slopes. This load becomes too heavyfor the streams to be carried over gentler


gradients and gets dumped and spread as abroad low to high cone shaped deposit calledalluvial fan. Usually, the streams which flowover fans are not confined to their originalchannels for long and shift their position acrossthe fan forming many channels calleddistributaries. Alluvial fans in humid areasshow normally low cones with gentle slope from

as a low cone. Unlike in alluvial fans, thedeposits making up deltas are very well sortedwith clear stratification. The coarsest materialssettle out first and the finer fractions like siltsand clays are carried out into the sea. As thedelta grows, the river distributaries continueto increase in length (Figure 7.5) and deltacontinues to build up into 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, silt andclay are carried by relatively slow movingwaters in gentler channels usually found in theplains and deposited over the bed and whenthe waters spill over the banks during floodingabove the bed. A river bed made of riverdeposits is the active floodplain. The floodplainabove the bank is inactive floodplain. Inactivefloodplain above the banks basically containtwo types of deposits — flood deposits andchannel deposits. In plains, channels shiftlaterally and change their courses occasionallyleaving cut-off courses which get filled upgradually. Such areas over flood plains builtup by abandoned or cut-off channels containcoarse deposits. The flood deposits of spilledwaters carry relatively finer materials like siltand clay. The flood plains in a delta are calleddelta plains.

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. They arelow, linear and parallel ridges of coarse depositsalong the banks of rivers, quite often cut intoindividual mounds. During flooding as thewater spills over the bank, the velocity of thewater comes down and large sized and highspecific gravity materials get dumped in theimmediate vicinity of the bank as ridges. Theyare high nearer the banks and slope gentlyaway from the river. The levee deposits arecoarser than the deposits spread by floodwaters away from the river. When rivers shiftlaterally, a series of natural levees can form.

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

head to toe and they appear as high cones withsteep slope in arid and semi-arid climates.

Deltas

Deltas are like alluvial fans but develop at adifferent location. The load carried by the riversis dumped and spread into the sea. If this loadis not carried away far into the sea or distributedalong the coast, it spreads and accumulates

Figure 7.5 : A satellite view of part of Krishna riverdelta, Andhra Pradesh


Point bars are also known as meander bars.They are found on the convex side of meandersof large rivers and are sediments deposited ina linear fashion by flowing waters along thebank. They are almost uniform in profile and inwidth and contain mixed sizes of sediments. Ifthere more than one ridge, narrow and elongateddepressions are found in between the point bars.Rivers build a series of them depending uponthe water flow and supply of sediment. As therivers build the point bars on the convex side,the bank on the concave side will erode actively.

In what way do natural levees differ frompoint bars?

Meanders

In large flood and delta plains, rivers rarely flowin straight courses. Loop-like channel patternscalled meanders develop over flood and deltaplains (Figure 7.7).

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. Whenthe gradient of the channel becomes extremelylow, water flows leisurely and starts workinglaterally. Slight irregularities along the banksslowly get transformed into a small curvaturein the banks; the curvature deepens due todeposition on the inside of the curve anderosion along the bank on the outside. If thereis no deposition and no erosion or undercutting,the tendency to meander is reduced. Normally,in meanders of large rivers, there is activedeposition along the convex bank andundercutting along the concave bank.

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

Figure 7.6 : Natural levee and point bars


The concave bank is known as cut-off bankwhich shows up as a steep scarp and theconvex bank presents a long, gentle profile andis known as slip-off bank (Figure 7.8). Asmeanders grow into deep loops, the same mayget cut-off due to erosion at the inflection pointsand are left as ox-bow lakes.

Braided Channels

When rivers carry coarse material, there can beselective deposition of coarser materials causingformation of a central bar which diverts the flowtowards the banks; and this flow increaseslateral erosion on the banks. As the valleywidens, the water column is reduced and moreand more materials get deposited as islandsand lateral bars developing a number ofseparate channels of water flow. Depositionand lateral erosion of banks are essential forthe formation of braided pattern. Or,alternatively, when discharge is less and load

is more in the valley, channel bars and islandsof sand, gravel and pebbles develop on the floorof the channel and the water flow is dividedinto multiple threads. These thread-like streamsof water rejoin and subdivide repeatedly to givea typical braided pattern (Figure 7.9).

Figure 7.9 : Satellite scenes showing braided channelsegments of Gandak (left) and Son (right) rivers

Arrows show the direction of flow

Figure 7.10 : Various karst features


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 waterpercolates well when the rocks are permeable,thinly bedded and highly jointed and cracked.After vertically going down to some depth, thewater 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 or dolomitesrich in calcium carbonate, the surface wateras well as groundwater through the chemicalprocess of solution and precipitationdeposition develop varieties of landforms. Thesetwo processes of solution and precipitation areactive in limestones or dolomites occurringeither exclusively or interbedded with otherrocks. Any limestone or dolomitic regionshowing typical landforms produced by theaction of groundwater through the processesof solution and deposition is called Karsttopography after the typical topographydeveloped in limestone rocks of Karst regionin the Balkans adjacent to 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/karstareas. A sinkhole is an opening more or lesscircular at the top and funnel-shapped towardsthe bottom with sizes varying in area from afew sq. m to a hectare and with depth from aless than half a metre to thirty metres or more.Some of these form solely through solutionaction (solution sinks) and others might start

as solution forms first and if the bottom of asinkhole forms the roof of a void or caveunderground, it might collapse leaving a largehole opening into a cave or a void below(collapse sinks). Quite often, sinkholes arecovered up with soil mantle and appear asshallow water pools. Anybody stepping oversuch pools would go down like it happens inquicksands in deserts. The term doline issometimes used to refer the collapse sinks.Solution sinks are more common than collapsesinks. Quite often the surface run-off simplygoes down swallow and sink holes and flow asunderground streams and re-emerge at adistance downstream through a cave opening.When sink holes and dolines join togetherbecause of slumping of materials along theirmargins or due to roof collapse of caves, long,narrow to wide trenches called valley sinks orUvalas form. Gradually, most of the surface ofthe limestone is eaten away by these pits andtrenches, leaving it extremely irregular with amaze of points, grooves and ridges or lapies.Especially, these ridges or lapies form due todifferential solution activity along parallel tosub-parallel joints. The lapie field mayeventually turn into somewhat smoothlimestone 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 can bea 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 both theends are called tunnels.

Depositional Landforms

Many depositional forms develop within thelimestone caves. The chief chemical in limestone


is calcium carbonate which is easily soluble incarbonated water (carbon dioxide absorbedrainwater). This calcium carbonate is depositedwhen the water carrying it in solutionevaporates or loses its carbon dioxide as ittrickles over rough rock surfaces.

Stalactites, Stalagmites and Pillars

Stalactites hang as icicles of differentdiameters. Normally they are broad at theirbases and taper towards the free ends showingup in a variety of forms. Stalagmites rise upfrom the floor of the caves. In fact, stalagmitesform due to dripping water from the surface orthrough the thin pipe, of the stalactite,immediately below it (Figure 7.11).

GLACIERS

Masses of ice moving as sheets over the land(continental glacier or pidmont glacier if a vastsheet of ice is spread over the plains at the footof mountains) or as linear flows down theslopes of mountains in broad trough-likevalleys (mountain and valley glaciers) are calledglaciers (Figure 7.12). The movement of glaciers

Figure 7.12 : A glacier in its valley

is slow unlike water flow. The movement couldbe a few centimetres to a few metres a day oreven less or more. Glaciers move basicallybecause of the force of gravity.

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 andfragments) get dragged along the floors or sidesof the valleys and cause great damage throughabrasion and plucking. Glaciers can causesignificant damage to even un-weathered rocksand can reduce high mountains into low hillsand plains.

Figure 7.11 : Stalactites and stalagmites in a limestone cave

Stalagmites may take the shape of acolumn, a disc, with either a smooth, roundedbulging end or a miniature crater likedepression. The stalagmite and stalactiteseventually fuse to give rise to columns andpillars of different diameters.


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 along withother depositional features. Figures 7.13 and7.14 show various glacial erosional anddepositional forms described in the text.

EROSIONAL LANDFORMS

Cirque

Cirques are the most common of landforms inglaciated mountains. The cirques quite oftenare found at the heads of glacial valleys. Theaccumulated 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 of watercan be seen quite often within the cirques after

the glacier disappears. Such lakes are calledcirque or tarn lakes. There can be two or morecirques one leading into another down belowin a stepped sequence.

Horns and Serrated Ridges

Horns form through head ward erosion of thecirque walls. If three or more radiating glacierscut headward until their cirques meet, high,sharp pointed and steep sided peaks calledhorns form. The divides between cirque sidewalls or head walls get narrow because ofprogressive erosion and turn into serrated orsaw-toothed ridges sometimes referred to asarêtes 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 throughheadward erosion of radiating cirques.

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


Glacial Valleys/Troughs

Glaciated valleys are trough-like and U-shapedwith broad floors and relatively smooth, andsteep sides. The valleys may contain littereddebris or debris shaped as moraines withswampy appearance. There may be lakesgouged out of rocky floor or formed by debriswithin the valleys. There can be hanging valleysat an elevation on one or both sides of the mainglacial valley. The faces of divides or spurs ofsuch hanging valleys opening into main glacialvalleys are quite often truncated to give theman appearance like triangular facets. Very deepglacial troughs filled with sea water andmaking up shorelines (in high latitudes) arecalled fjords/fiords.

What are the basic differences betweenglacial valleys and river valleys?


The unassorted coarse and fine debris droppedby the melting glaciers is called glacial till. Mostof the rock fragments in till are angular to sub-angular in form. Streams form by melting iceat the bottom, sides or lower ends of glaciers.

Some amount of rock debris small enough tobe carried by such melt-water streams iswashed down and deposited. Such glacio-fluvial deposits are called outwash deposits.Unlike till deposits, the outwash deposits areroughly stratified and assorted. The rockfragments in outwash deposits are somewhatrounded at their edges. Figure 7.14 shows afew depositional landforms commonly foundin glaciated areas.

Moraines

They are long ridges of deposits of glacial till.Terminal moraines are long ridges of debrisdeposited at the end (toe) of the glaciers. Lateralmoraines form along the sides parallel to theglacial valleys. The lateral moraines may join aterminal moraine forming a horse-shoe shapedridge. There can be many lateral moraines oneither side in a glacial valley. These morainespartly or fully owe their origin to glacio-fluvialwaters pushing up materials to the sides ofglaciers. Many valley glaciers retreating rapidlyleave an irregular sheet of till over their valleyfloors. Such deposits varying greatly in thicknessand in surface topography are called groundmoraines. The moraine in the centre of the

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


glacial valley flanked by lateral moraines iscalled medial moraine. They are imperfectlyformed as compared to lateral moraines.Sometimes medial moraines are indistinguishablefrom ground moraines.

Eskers

When glaciers melt in summer, the water flowson the surface of the ice or seeps down alongthe margins or even moves through holes inthe ice. These waters accumulate beneath theglacier and flow like streams in a channelbeneath the ice. Such streams flow over theground (not in a valley cut in the ground) withice forming its banks. Very coarse materials likeboulders and blocks along with some minorfractions of rock debris carried into this streamsettle in the valley of ice beneath the glacierand after the ice melts can be found as asinuous 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 may jointo form outwash plains of gravel, silt, sand andclay.

Distinguish between river alluvial plainsand glacial outwash plains.

Drumlins

Drumlins are smooth oval shaped ridge-likefeatures composed mainly of glacial till withsome masses of gravel and sand. The long axesof drumlins are parallel to the direction of icemovement. They may measure up to 1 km inlength and 30 m or so in height. One end ofthe drumlins facing the glacier called the stossend is blunter and steeper than the other endcalled tail. The drumlins form due to dumpingof rock debris beneath heavily loaded icethrough fissures in the glacier. The stoss endgets blunted due to pushing by moving ice.Drumlins give an indication of direction ofglacier movement.

What is the difference between till andalluvium?

WAVES AND CURRENTS

Coastal processes are the most dynamic andhence most destructive. So, don’t you think itis 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 inanother. Most of the changes along the coastsare accomplished by waves. When waves break,the water is thrown with great force onto theshore, and simultaneously, there is a greatchurning of sediments on the sea bottom.Constant impact of breaking waves drasticallyaffects the coasts. Storm waves and tsunamiwaves can cause far-reaching changes in ashort period of time than normal breakingwaves. As wave environment changes, theintensity of the force of breaking waves changes.

Do you know about the generating forcesbehind waves and currents? If not, referto the chapter on movements in oceanwaters.

Other than the action of waves, the coastallandforms depend upon (i) the configurationof land and sea floor; (ii) whether the coast isadvancing (emerging) seaward or retreating(submerging) landward. Assuming sea level tobe constant, two types of coasts are consideredto explain the concept of evolution of coastallandforms: (i) high, rocky coasts (submergedcoasts); (ii) low, smooth and gently slopingsedimentary coasts (emerged coasts).

HIGH ROCKY COASTS

Along the high rocky coasts, the rivers appearto have been drowned with highly irregularcoastline. The coastline appears highlyindented with extension of water into the landwhere glacial valleys (fjords) are present. Thehill sides drop off sharply into the water. Shoresdo not show any depositional landformsinitially. Erosion features dominate.


Along high rocky coasts, waves break withgreat force against the land shaping the hillsides into cliffs. With constant pounding bywaves, the cliffs recede leaving a wave-cutplatform in front of the sea cliff. Wavesgradually minimise the irregularities along theshore.

The materials which fall off, and removedfrom the sea cliffs, gradually break into smallerfragments and roll to roundness, will getdeposited in the offshore. After a considerableperiod of cliff development and retreat whencoastline turns somewhat smooth, with theaddition of some more material to this depositin the offshore, a wave-built terrace woulddevelop in front of wave-cut terrace. As theerosion along the coast takes place a goodsupply material becomes available to longshorecurrents and waves to deposit them as beachesalong the shore and as bars (long ridges of sandand/or shingle parallel to the coast) in thenearshore zone. Bars are submerged featuresand when bars show up above water, they arecalled barrier bars. Barrier bar which get keyedup to the headland of a bay is called a spit.When barrier bars and spits form at the mouthof a bay and block it, a lagoon forms. Thelagoons would gradually get filled up bysediments from the land giving rise to a coastalplain.

LOW SEDIMENTARY COASTS

Along low sedimentary coasts the rivers appearto extend their length by building coastalplains and deltas. The coastline appearssmooth with occasional incursions of water inthe form of lagoons and tidal creeks. The landslopes gently into the water. Marshes andswamps may abound along the coasts.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 sediments builddeltas along low sedimentary coasts.

The west coast of our country is a highrocky retreating coast. Erosional formsdominate in the west coast. The eastcoast of India is a low sedimentary coast.Depositional forms dominate in the eastcoast.

What are the various differences betweena high rocky coast and a low sedimentarycoast in terms of processes andlandforms?

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 be aflat or gently sloping platform covered by rockdebris derived from the sea cliff behind. Suchplatforms occurring at elevations above theaverage height of waves is called a wave-cutterrace. The lashing of waves against the baseof 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 leavesome remnants of rock standing isolated assmall islands just off the shore. Such resistantmasses of rock, originally parts of a cliff or hillare called sea stacks. Like all other features,sea stacks are also temporary and eventuallycoastal hills and cliffs will disappear becauseof wave erosion giving rise to narrow coastalplains, and with onrush of deposits from overthe land behind may get covered up byalluvium or may get covered up by shingle orsand to form a wide beach.


DEPOSITIONAL LANDFORMS

Beaches and Dunes

Beaches are characteristic of shorelines that aredominated by deposition, but may occur aspatches along even the rugged shores. Most ofthe sediment making up the beaches comesfrom land carried by the streams and rivers orfrom 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 sand sizedmaterials. Beaches called shingle beachescontain excessively small pebbles and evencobbles.

Just behind the beach, the sands lifted andwinnowed from over the beach surfaces will bedeposited as sand dunes. Sand dunes forminglong ridges parallel to the coastline are verycommon 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 of lowtide waterline to seaward) lying approximatelyparallel to the coast is called an off-shore bar.An off-shore bar which is exposed due tofurther addition of sand is termed a barrierbar. The off-shore bars and barriers commonlyform across the mouth of a river or at theentrance of a bay. Sometimes such barrier barsget keyed up to one end of the bay when theyare called spits (Figure 7.15). Spits may also

develop attached to headlands/hills. Thebarriers, bars and spits at the mouth of thebay gradually extend leaving only a smallopening of the bay into the sea and the baywill eventually develop into a lagoon. Thelagoons get filled up gradually by sedimentcoming from the land or from the beach itself(aided by wind) and a broad and wide coastalplain may develop replacing a lagoon.

Do you know, the coastal off-shore barsoffer the first buffer or defence againststorm or tsunami by absorbing most oftheir destructive force. Then come thebarriers, beaches, beach dunes andmangroves, if any, to absorb thedestructive force of storm and tsunamiwaves. So, if we do anything whichdisturbs the ‘sediment budget’ and themangroves along the coast, these coastalforms will get eroded away leaving humanhabitations to bear first strike of stormand 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 obstructionsin their path create turbulence. Of course, thereare 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 theirFigure 7.15 : A satellite picture of a part of Godavari

river delta showing a spit


formation to mass wasting and running wateras sheet floods. Though rain is scarce in deserts,it comes down torrentially in a short period oftime. The desert rocks devoid of vegetation,exposed to mechanical and chemicalweathering processes due to drastic diurnaltemperature changes, decay faster and thetorrential rains help in removing the weatheredmaterials easily. That means, the weathereddebris in deserts is moved by not only windbut also by rain/sheet wash. The wind movesfine materials and general mass erosion isaccomplished mainly through sheet floods orsheet wash. Stream channels in desert areasare broad, smooth and indefinite and flow fora brief time after rains.

EROSIONAL LANDFORMS

Pediments and Pediplains

Landscape evolution in deserts is primarilyconcerned with the formation and extension ofpediments. Gently inclined rocky floors closeto the mountains at their foot with or withouta thin cover of debris, are called pediments.Such rocky floors form through the erosion ofmountain front through a combination oflateral erosion by streams and sheet flooding.

Erosion starts along the steep margins ofthe landmass or the steep sides of thetectonically controlled steep incision featuresover the landmass. Once, pediments are formedwith a steep wash slope followed by cliff or freeface above it, the steep wash slope and free faceretreat backwards. This method of erosion istermed as parallel retreat of slopes throughbackwasting. So, through parallel retreat ofslopes, the pediments extend backwards at theexpense of mountain front, and gradually, themountain gets reduced leaving an inselbergwhich is a remnant of the mountain. That’s howthe high relief in desert areas is reduced to lowfeatureless plains called pediplains.

Playas

Plains are by far the most prominent landformsin the deserts. In basins with mountains andhills around and along, the drainage is towardsthe centre of the basin and due to gradual

deposition of sediment from basin margins, anearly level plain forms at the centre of thebasin. In times of sufficient water, this plain iscovered up by a shallow water body. Suchtypes of shallow lakes are called as playaswhere water is retained only for short durationdue to evaporation and quite often the playascontain good deposition of salts. The playaplain covered up by salts is 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 first shallowdepressions called blow outs are created, andsome of the blow outs become deeper andwider fit to be called caves.

Mushroom, Table and Pedestal Rocks

Many rock-outcrops in the deserts easilysusceptible to wind deflation and abrasion areworn out quickly leaving some remnants ofresistant rocks polished beautifully in theshape of mushroom with a slender stalk and abroad and rounded pear shaped cap above.Sometimes, the top surface is broad like a tabletop and quite often, the remnants stand outlike pedestals.

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


Wind is a good sorting agent. Depending uponthe velocity of wind, different sizes of grains aremoved along the floors by rolling or saltationand carried in suspension and in this processof transportation itself, the materials get sorted.When the wind slows or begins to die down,depending upon sizes of grains and theircritical velocities, the grains will begin to settle.So, in depositional landforms made by wind,good sorting of grains can be found. Since


wind is there everywhere and wherever thereis good source of sand and with constant winddirections, depositional features in arid regionscan develop anywhere.

Sand Dunes

Dry hot deserts are good places for sand duneformation. Obstacles to initiate dune formation

are equally important. There can be a greatvariety of dune forms (Figure 7.16).

Barchans

Crescent shaped dunes called barchans withthe points or wings directed away from winddirection i.e., downwind, form where the winddirection is constant and moderate and wherethe original surface over which sand is movingis almost uniform. Parabolic dunes form whensandy surfaces are partially covered withvegetation. That means parabolic dunes arereversed barchans with wind direction beingthe same. Seif is similar to barchan with a smalldifference. Seif has only one wing or point. Thishappens when there is shift in wind conditions.The lone wings of seifs can grow very long andhigh. Longitudinal dunes form when supplyof sand is poor and wind direction is constant.They appear as long ridges of considerablelength but low in height. Transverse dunesare aligned perpendicular to wind direction.These dunes form when the wind direction isconstant and the source of sand is anelongated feature at right angles to the winddirection. They may be very long and low inheight. When sand is plenty, quite often, theregular shaped dunes coalesce and lose theirindividual characteristics. Most of the dunesin the deserts shift and a few of them will getstabilised especially near human habitations.

Figure 7.16 : Various types of sand dunesArrows indicate wind direction

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 forms 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


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

(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?


(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.

Documents

IESO Preparation Material (Geomorphic Processes and Evolution of Landforms)