Landforms made by Running Water

Denudation is general lowering of earth’s surface by agents of erosion such as Wind, Water, Ice, Waves etc. Unlike glaciers & snow, which are confined to cold & temperate latitudes;  waves, which acts only on coastlines;  winds, are only efficient in deserts; the effect of running water is felt all over the world, thus, making it the most important agent of denudation.

• The source of river is generally found in an upland region with a slope down for the run offs
• Hence, the uplands form the catchment areas of the rivers & the crest of mountains become the divide or watershed from which the streams flows down the slope
• The initial stream that exists as a consequence of the slope is called the consequent stream
• As the consequent stream wears down the surface, it is joined by several tributaries from either side

 

Processes of river action

• When a river flows it carries with it eroded materials which can be divided into 3 distinct types

 

Materials in Solution	Minerals which are dissolved in water
Materials in Suspension	Suspension of Sand, Silt & Mud in water
The Traction Load	Coarser materials such as pebbles, stones, rocks & Boulders

 

• The ability of river to move the various grades of materials depends greatly on the volume of the water, velocity of the flow & size, shape and weight of the load
• It is said that by doubling the velocity of a river, its transporting power is increased by more than 10 times.

 

River Erosion & Transportation Processes

Corrasion / Abrasion

• Mechanical grinding of river’s traction load against the banks & bed of the river.
• The rock fragments hurdle against the sides as well as bottom of the river leading to lateral & vertical corrasion
• Lateral corrasion is sideways erosion which widens the V shaped valley.
• Vertical corrasion is the downward action which deepens the river channel.

 

Corrosion / Solution

• Chemical action of water on soluble or partly soluble rocks with which river come into contact
• For ex. in case of Calcium carbonate

 

Hydraulic Action

• Mechanical loosening & sweeping away of materials by river water
• Mainly by surging into the crevices & cracks of rocks & disintegrating them

 

Attrition

• Wear & tear of transported material among them when they roll and collide into one another

 

Upper or Mountain Course (Youth stage)

• Begins at the source of the river near the watershed, generally at the crest of mountain range
• Flow is very swift as it descends the steep slopes & predominant action of the river is vertical erosion
• Valley developed is thus deep, narrow & distinctively V shaped which sometimes results in formation of gorges & canyons

Some of the features associated with the upper course of the river

River Capture

• Also known as river piracy or river beheading
• Its development depends upon different rate of back cutting (headward erosion) into a divide mainly due to difference in precipitation received by streams.
• If one side of the divide cut more rapidly than the other then its greater erosive power will succeed in enlarging its basin at the expense of weaker stream.
• For example in given figure, Stream A may eventually break through the divide & capture & pirate stream B.
• The bend at which the piracy occurs is termed as Elbow of the capture & the beheaded stream is called as misfit.
• The valley below the elbow is wind gap which may be useful for road & rail route.

Rapids, Cataracts & Waterfalls

• Liable to occur in any part of river course but most numerous in mountains course where changes in gradient are more abrupt & frequent.
• Due to unequal resistance of hard & soft rocks transverse by a river, the outcrop of hard rock may cause a river to jump & fall, known as
• Similar falls of greater dimensions are referred as
• When river plunges down in a sudden fall via. some height, they are called
• Their greater force usually wears out a plunge pool beneath.

Middle or Valley Course (Maturity stage)

• In the middle course, lateral corrasion tends to replace vertical corrasion; active corrasion of the bank thus widens V shaped valleys.
• Volume of water increases with the confluence of many tributaries which increases river’s load.
• The work of the river is predominantly transportation with some deposition in clearer manner, although velocity does not decrease.

 

Some of the features associated with the Middle course of the river

Interlocking spurs

• Downstream, interlocking spurs that project from both side of the valley are cut back into a line of bluffs.
• Rainwash, soil creeps, landslides & gullying gradually widens the valley, cutting back the sides
• As the stream flows on, the meanders migrate progressively outward with the interlocking spurs alternating with the undercut slopes.
• Meanders in the middle course are only the beginning of the downward swing as bends are restricted by the interlocking spurs.
• In the lower course, the loops are enlarged across the level plain & meanders are fully developed.

River cliffs & Slip off slopes

• When the flow of water PQ enters the bend of the river, it dashes straight into Q, eroding the outer bank into a steep river cliff at Q.
• The water piles up on the outside of the bend due to centrifugal force.
• A bottom current RS is setup in a cork screw motion & is hurled back into midstream & inner bank. Shingle is thus deposited here at S, where the slip off is gentle.
• The outer bank is therefore the bank of continuous erosion & the inner bank is the bank of continuous deposition.

Meanders

• As water flowing under gravity seldom flows straight for long distance, a winding course soon develops
• The irregularities of the ground forces the river to swing in loops forming Meanders

Lower or Plain Course (Old Stage)

• The river moving downstream across a broad, level plain is heavy with debris brought down from the upper course.
• Vertical corrasion has almost ceased though lateral corrasion still goes on to erode its banks further.
• Volume of water is greatly swelled with work of the river is mainly depositional, building up its bed & forming flood plains.

 

Some of the features associated with the plain course of the river

Flood Plain

• During sporadic floods, large quantity of sediments are spread over the low lying adjacent areas by the rivers, thus gradually building up a fertile flood plain.
• When the river flows normally its bed is raised through accumulation of deposits.
• Material is also deposited on the sides forming raised banks called Levees.
• In an attempt to minimize the risk of the floods, artificial embankments are erected on the natural levees.
• Nowadays, huge dredgers are also brought up in use to deepen the channels to avoid excessive sedimentation.

Ox-Bow Lakes (Dead Lake)

• In the lower course of the river, a meander becomes very much pronounced.
• The outside bank is so rapidly eroded that the river becomes almost a complete circle.
• There comes a time, when the river cuts through the narrow neck of the loop, abandoning an Ox-bow lake & then flows straight.
• The ox-bow lake will later degenerate into a swamp through subsequent floods that may silt up the lake, thus becoming marshy & eventually dries up.

Delta

• When the river reaches the sea, the fine materials it has not dropped yet are deposited at its mouth, forming a fan shaped alluvial area called a delta.
• The alluvial tract is infact the seaward extension of the flood plains.
• Due to obstruction caused by the deposited alluvium, the river may discharge its water through several channels called distributaries.

Favorable conditions for delta are

• Active vertical & lateral erosion in upper course of the river to provide extensive sediments
• Coast should be sheltered preferably tideless & no strong current at right angle to the mouth of the river washing away the sediments
• Sea adjoining the delta should be shallow or else the load will disappear in deep waters
• No large lakes in the river course to filter off sediments

River Rejuvenation

• When in the course, if the river parts are uplifted or depressed, they rejuvenate the river & make it young again
• Rejuvenation mainly occurs when there is either a fall in sea level relative to the level of the land or a rise of the land relative to the sea known as negative movements leading to fall in river’s base level
• This steepens the slope so that river’s eroding power or down cutting is renewed
• River with its renewed vigour cuts into the former plain, leaving behind traces on both sides of the river
• Point where the old & rejuvenated profile meet is called Knick point, which can be seen as waterfalls and rapids
• If rejuvenation occurs in upper course, the river valleys are deepened & steep sided gorges are formed
• In middle & lower course vertical corrasion replaces lateral corrasion & thus the existing meanders are vertically eroded by the rejuvenated stream

A distinct new trench is cut in old valley & the river develops a deep valley with incised meanders; which are of 2 types’ entrenched meanders & ingrown meanders

Entrenched meanders	§  Are symmetrical & form when the river downcuts particularly quickly §  Due to the speed which the river downcuts, there is little opportunity for lateral erosion to occur giving them symmetrical shape
Ingrown meanders	§  Are asymmetrical & form when river downcuts at a less rapid pace §  Give river opportunity to erode laterally + vertically

 

• A rejuvenating river can erode vertically into the former flood plain to produce features called river terraces.
• If vertical erosion is rapid then pairedterraces are formed either side of the channel.
• If vertical erosion is slower though, unpairedterraces form as the river is given opportunity to meander.
• River terraces are particularly useful for settlements as they provide flat areas above the present floodplain.
• Oxford, Cambridge and London all are developed on the river terraces of the Isis, Cam and Thames respectively.
• A positive movement occurs when there is either a rise in sea level relative to the level of the land or a fall of the land relative to the sea level.
• This will submerge the land along the coast, drown the valleys & weaken the erosive power of the river.
• The lower course of the river may be partly in the sea & features of deposition are shifted in the middle course; with the upper course affected only a little.
• Rise in the sea level mainly occurs due to release of the water locked up in the ice masses during the Quaternary ice ages.

 

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Landforms of Glaciaton

The Ice-Age and types of ice masses

• Today, only 2 major ice caps are present in this world – Antarctica & Greenland, along with many highlands above the snowline surviving in the world.
• The peaks of the loftiest mountains projecting above the ice surface are known as Nunataks
• Ice from ice cap creeps out in all directions to escape as glaciers
• When the ice sheets reaches down to the sea they float as ice shelves in polar waters.
• When ice sheets breaks into individual blocks, these are called icebergs.
• While afloat in the sea, only 1/9^thof the iceberg’s mass is visible above the surface.

• They diminish in size when reaching warm waters & eventually melted, dropping the rock debris that was frozen inside them on the sea bed.
• Permanent snowfield is sustained by heavy snowfall in winters & ineffective snow melting & evaporation in summers as part of snow that melt during the day is refrozen during the night.
• This refreezing process repeats until it forms a hard, granular substance known as neve or firn.
• Owing to the gravitational forces, neve of the upland snowfield is drawn towards the valley below, which marks the beginning of the flow of glacier (river of ice).

Glaciers

• Glaciers normally assume the shape of a tongue, broadest at the source & becoming narrower downhill.
• Though glacier is not liquid, but it moves gradually under the continual pressure from the snow accumulated above.
• Rate of movement is greatest in the middle where there is little obstruction
• The sides & bottom are held back by the frictions due to valley sides & valley floors.
• If a row of stakes is planted across a glacier in a straight line, they will eventually take a curved shape down the valley, showing that glacier moves faster at the centre than at sides.

Types of Glaciers

 

Piedmont Glacier	At the foot of the mountain ranges, several glaciers may converge to form an extensive ice mass
Cirque Glacier	§  Formed in a cirque, a bowl-shaped depression on the side of or near mountains §  Snow and ice accumulation in corries often occurs as the result of avalanching from higher surrounding slopes
Valley Glacier	Streams of flowing ice that are confined within steep walled valleys, often following the course of an ancient river valley

 

Landforms of Highland Glaciation

• Glaciation generally gives rise to erosional features in the highlands & depositional features on lowlands
• It erodes its valley by two processes viz. plucking & abrasion.
• Plucking → Glacier freezes the joints & beds of underlying rocks, tears out individual blocks & drags them away.
• Abrasion → Glacier scratches, scrapes, polishes & scours the valley floor with the debris frozen into it.

 

Characteristic features of Glaciated Highland

Corrie, Cirque or cwm

• The downslope movement of a glacier from its snow covered valley head & the intensive shattering of the upland slopes, tend to produce a depression where neve or firn accumulate
• Plucking & abrasion further deepen the depression into a steep horse shoe shaped basin called Cirque (in French), cwm (in wales) & Corrie (in Scotland)
• There is a rocky ridge at the exit of the corrie & when the ice eventually melts, water collect behind this barrier known as Corrie Lake or tarn

Aretes and Pyramidal Peaks

• When two corries cut back on opposite sides of the mountain, knife edged ridges are formed called aretes
• When three or more cirques cut back together, recession will form an angular horn or pyramidal peak

Bergschrund

• At the head of a glacier, where it begins to leave the snowfield of a corrie, a deep vertical crack opens up called a Bergschrund or Rimaye
• This happens in summer when although the ice continues to move out of the corrie, there is no new snow to replace it
• In some cases not one but several such cracks occur which present a major obstacle to climbers
• Further down, where the glacier negotiates a bend or a precipitous slope, more crevasses or cracks are formed

U shaped glacial Troughs & Ribbon lakes

• Glaciers on their downward journey are fed by several corries scratches & grind the bedrock with straightening out any portruding spurs.
• The interlocking spurs are thus blunted to form truncated spurs with floor of the valley deepened.
• Hence, the valley which has been glaciated takes the characterstic U shape, with a wide flat floor & very steep sides.
• After the disappearance of the ice, the deep sections, of these long, narrow glacial troughs may be filled with waterforming Ribbon lakes also known as Trough lakes or Finger Lakes.

Hanging Valleys

• The main valley is eroded much more than the tributary valley as it contains much larger glacier.
• After the ice has been melted, a tributary valley hang above the main valley & plunges down as waterfall. Such Tributary valleys are termed as Hanging valleys.
• Hanging valleys may form a natural head of water for generating hydroelectric power.

Rock Basins and Rock Steps

• A glacier erodes & excavates the bed rock in an irregular manner.
• The unequal excavation gives rise to many rock basins later filled by lakes in valley trough.
• Where a tributary valley joins a main valley, the additional weight of ice in the main valley cuts deeper into the valley floor & deepest at the point of convergence forming rock steps.
• A series of such rock steps may also be formed due to different degrees of resistance to glacial erosion of the bedrocks.

Moraines

• Moraines are made up of the pieces of rock that are shattered by frost action, imbedded in the glaciers & brought down the valley.
• Those that fall on the sides of the glacier form lateral moraines.
• When two glaciers converge, their inside lateral moraines unite to form a medial moraine.
• The rock fragments which are dragged along, beneath the frozen ice, are dropped when the glacier melts & spread across the floor of the valley as ground moraine.
• The glacier eventually melts on reaching the foot of the valley & the pile of transported materials left behind at the snout is terminal moraine or end moraine.
• The deposition of end moraines may be in several succeeding waves, as the ice may melt back by stages so that a series of recessional moraines are formed.

Fjord

• If the glacier flows right down to the sea, it drops its load of moraine in the sea.
• If section break off as icebergs, moraine material will only be dropped when they melt
• Where the lower end of the trough is drowned by the sea, it forms a deep, steep side inlet called a Fjord, a typical of Norway & Chilean coast.

Landforms of Lowland Glaciation

Most of the glaciated lowlands have depositional features, but where rock masses project above the level surface, they result in striking features of erosion.

Roche Moutonnee

• Basically a resistant residual rock hummock or mound, striated by the ice movement
• Its upstream or stoss side is smoothened by abrasion & its downward or leeward side is roughened by plucking & is much steeper.
• It is believed that plucking may have occurred on leeward side due to a reduction in pressure of the glacier moving over the stoss slope
• Therefore providing the opportunity for water to refreeze on the lee side and pluck the rock away.

Crag & Tail

• A crag and tail is a larger rock mass than a Roche moutonnee
• Like a Roche moutonnee, it is formed from a section of rock that was more resistant than its surroundings.
• Crag is a mass of hard rock with a steep slope on the upward side, which protects the softer leeward slope from being completely worn down by the oncoming ice.
• It therefore has a gentle tail strewn with the eroded rock debris.

Boulder clay or Glacial till

• This is an unsorted glacial deposit comprising a range of eroded materials such as boulders, sticky clays & fine rock flour.
• It is spread out in sheets, not mounds, & forms gently undulating till or drift plains with monotonous landform.
• The degree of fertility of such glacial plains depends very much on the composition of the depositional materials.

Erratics

• Boulders of varying size that are transported by ice & left stranded in the regions of deposition when the ice melted.
• Called erratics because they are composed of the materials entirely different from those of the regions in which they have been transported.
• Useful in tracing the source & direction of ice movements but their presence in large numbers causes hindrance in farming.
• Also known as perched blocks as sometimes they are found perched in precarious positions as the ice dropped them.

 

Drumlins

• Elongated whale back type hummocks composed wholly of boulder clay with elongation in the direction of ice flow i.e. on the downward side.
• They are low hills upto 1.5 km long and 60 mm tall & appear steeper on the onset side & taper off at leeward side.
• They are arranged diagonally & commonly referred as basket of eggs topography.

Eskers

• Eskers are the sinuous ridges composed of glacial material mainly sands & gravel deposited by meltwater currents in glacial tunnels
• Glacial tunnels marks the former sites of sub glacial melt water streams
• Their orientation is generally parallel to the direction of glacial flow, and they sometimes exceed 100 kilometres in length.

Outwash Plains

• Made up of fluvio glacial deposits washed out from the terminal moraines by the streams of stagnant ice mass.
• The melt waters sort & redeposit the material mainly consisted of layers of sand and other fine sediments.
• Such plains with their sandy soils are often used for specialized kinds of agriculture, such as the potato.

 

Kettle lake

• A depressions formed when the deposition takes place in the form of alternating ridges
• Shallow, sediment filled body of water formed by retreating glaciers

Kames

• Small rounded hillocks of sand & gravel which cober part of the plain
• Kames are often associated with kettles, and this is referred to as kame & kettle topography

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ARID OR DESERT LANFORMS

Deserts

• About 1/5^thof the world’s land is made up of deserts.
• Deserts which are absolutely barren, where nothing grows are known as true deserts.
• Insufficient & irregular rainfall, high temperature & rapid rate of evaporation are the main causes of desert’s aridity.
• Almost all the deserts are confined within 15* – 30* parallels to N – S of equator known as trade wind desert or tropical deserts.
• They lie in the trade wind belt on the western parts of the continents.
• Off shore trade winds are often bathed in cold currents which produces a desiccating (dehydrating) effect, hence moisture is not easily condensed into precipitation.

Types of deserts

Hamada / Rocky Desert

• Consist of large stretches of bare rocks, swept clear of sand & dust by wind.
• Exposed rocks are thoroughly smoothened, polished & highly sterile.

 

Reg / Stony Desert

• Composed of extensive sheets of angular pebbles & gravels which the wind is not able to blow off.
• Stony deserts are more accessible than sandy deserts & large herds of camels kept there.

 

Erg / Sandy Desert

• Also known as sea of sand
• Winds deposit vast stretches of undulating sand dunes in the direction of winds.

 

Badlands

• Consists of gully & ravines formed on hill slopes & rock surfaces by the extent of water action
• Not fit for agriculture & survival
• Finally leads to the abandonment of entire region by its inhabitants

 

Mountain Deserts

• Deserts which are found on the highlands such as on plateaus & mountain ranges, where erosion has dissected the desert highland into rough chaotic peaks & uneven ranges.
• Their steep slopes consist of Wadis (dry valleys) with sharp & irregular edges carved due to action of frost.

 

Mechanism of Desert/Arid Erosion

Weathering

• Most potent factor in reducing rocks to sand in arid regions.
• Even though the amount of rain that falls in a desert is small, but manage to penetrate into rocks & sets up chemical reactions in various minerals it contains.
• Intense heating during the day & rapid cooling during the night by radiations, set up stresses in already weakened rocks, hence they eventually crack.
• When water gets into cracks of a rock, it freezes at night as the temperature drops below the freezing point & expands by 10 % of its volume.
• Successive freezing will prise of fragments of rocks which get accumulated as screes.
• As heat penetrates rock, its outer surface gets heated & expands, leaving its inner surface comparatively cool.
• Hence, outer surface prise itself from the inner surface & peels off in successive thin layers, known as exfoliation.

 

Action of Wind

• Efficient in arid regions as little vegetation or moisture to bind the loose surface materials
• Carried out in following ways –

Deflation 

• Involves lifting & blowing away of loose materials from the ground
• Blowing capacity depending largely on the size of the material lifted from the surface
• Finer dust & sands may be removed miles away from their place of origin & may get deposited even outside the desert margins.
• Deflation results in the lowering of the land surface to form large depressions called Deflation hollows

 

Abrasion 

• Sand blasting of rock surfaces by wind when they hurl sand particles against them
• This results in rock surfaces being scratched, polished & worn away
• Abrasion is most effective near the base of the rocks, where the amount of material the wind is able to carry is greatest.
• This explains why telegraphic poles in the deserts are protected by covering of metal for a foot or two above the ground.

 

Attrition 

• When wind borne particles roll against one another in collision, they wear each other away
• Hence their sizes are greatly reduced & grains are rounded into millet seed sand

Landforms of Wind Erosion in Desert

Rock pedestals / Mushroom rocks

• Formed by the sand blasting effect of winds against any projecting rock masses
• It wears down the softer layer leading to formation of irregular edges on alternate bands of softer & harder rocks.
• Grooves & hollows cut in the rock surfaces, carve them into grotesque looking pillar known as rock pedestals.
• Such rock pillars will be further eroded near their bases where friction is greatest.
• This process of undercutting produces rocks of mushroom shape called mushroom rocks.

Zeugen

• Tabular masses which have a layer of soft rocks lying beneath a surface layer of more resistant rocks
• Difference in erosional effect of the wind on soft & resistant rock surfaces, carve them into weird looking ridge & furrow landscape
• Mechanical weathering initiates their formation by opening up joints of the surface rocks
• Wind abrasion further eats into underlying softer layer so that deep furrows are developed
• The hard rocks then stand above the the furrows as ridges or Zeugen
• Zeugen may stand 10 to 100 feet above the sunken furrows
• Continuous abrasion by winds gradually lowers the Zeugen & widens the furrow

 

Yardangs

• Yardangs looks quite similar to Zeugen but instead of lying in horizontal starta upon one another, the hard & soft rocks of Yardangs are vertical bands
• Rocks are aligned in the direction of prevailing winds.
• Winds abrasion excavates the bands of softer rocks into long, narrow corridors, separating the steep sided overhanding ridges of hard rocks called Yardangs.

Mesas & Buttes

• Mesa is a flat, table like land mass with a very resistant horizontal top layer & very steep sides, may be formed in canyon region.
• The hard stratum on the surface resist denudation by both wind & water thus protects the underlying layer of rocks from being eroded.
• Continuous denudation through ages may reduce Mesas in area so that they become isolated flat topped hills called Buttes.
• Many of which are separated by deep gorges & canyons.

Isenberg (Island Mountain)

• They are basically isolated residual hills rising abruptly from the ground level
• Characterized by very steep slopes & rather rounded tops
• They are often composed of granite or gneiss
• Are probably relics of an original plateau, which has been almost entirely eroded away

Ventifacts & Dreikanter

• Ventifacts are generally pebbles faceted & edged by sand blasting
• Rock fragments weathered from mountains
• Are shaped & polished thoroughly by wind abrasion
• Smoothened on windward side
• If wind direction changes another facet is developed.
• Among the ventifacts, those with the three wind faceted surfaces are known as Dreikanter.

Deflation Hollows

• Wind lowers the ground by blowing away the unconsolidated material & hence forms small depressions.
• Similarly, minor faulting can also initiate depressions which along with the eddying action of oncoming winds will wear off the weaker rocks until water table is reached.

• Water then seeps out forming oasis or swamps in deflation hollows or depressions.
• Large areas in western USA, was stripped of their natural vegetation & was completely deflated by strong winds, that moved materials as dust storms & creating what is now known as Great dust bowl.

Landforms of Wind Deposition in Deserts

• Materials eroded & transported by winds must come to rest somewhere.
• The finest dust travels enormous distances in the air sometimes as long as 2300 miles before they settle down.
• The dust from Sahara desert is sometimes blown across the Mediterranean to fall as blood rains in Italy or on the glaciers of Switzerland.
• Dust that settles in Hwang Ho basin (also known as Hwangtu – the yellow earth) from the Gobi desert has been accumulated over past centuries to a depth of several hundred feet
• As wind borne materials are shifted according to their coarseness, it can be expected that the coarser sands will be too heavy to be blown out of desert limits.
• They remain as dunes or other depositional landforms within desert themselves.

Dunes

• Hills of sand formed by the accumulation of sand & shaped by the movement of winds, a striking characteristic of erg or sandy desert
• Can be classified as active or live dunes, constantly on move or inactive fixed dunes, rooted with vegetation
• Two most common types of dunes are Barchan & Seifs

Barchan dune

• Crescent or moon shaped live dunes which advance steadily in the particular direction of prevailing winds.
• Initiated probably by a chance accumulation of sand across an obstacle, such as patch of grass or a heap of rocks
• They occur transversely to the wind, so that their horns thin out & become lower in the direction of the wind
• Mainly due to reduced frictional retardation of the winds around the edges.

• The windward side is convex & gently sloping whiles the leeward side, being sheltered, is concave & steep.
• The crest of sand dunes moves forward as more sand is accumulated by the prevailing wind.
• The sand is driven up the windward side & on reaching the crest slips down the leeward side so that the dune advances

• The migration of Barchans may be a threat to desert life as they may encroach on an oasis burying palm trees & houses.
• Long rooted sand holding trees & grasses are therefore planted to halt the advancement of the dunes to prevent areas of fertile land from being devastated.

 

Seif or longitudinal dunes

• Long narrow ridges of sand, often over a hundred miles long, lying parallel to the direction of the prevailing winds, with their crustline rises & falls in alternate peaks & saddles in regular successions.
• Dominant wind blows straight along the corridor between the lines of the dunes so that they are swept clear of sand & remain smooth.
• The eddies that are setup in corridors, blow towards the side of the corridor & drops the sand to form the dune.
• In this manner, prevailing winds increases the length of the dunes into tapering linear ridges while occasional cross winds tends to increase their heights & width.

 

Loess

• The fine dust blown beyond the desert limits is deposited on neighbouring lands as loess.
• It is a yellow, friable (softly crumbled) material rich in lime, very coherent, extremely porous & is usually very fertile.

• Water sinks in readily so the surface is always dry, with streams may cut into thick mantle of soft loess to develop badland topography.

Landform of water actions in desert

• Although rainfall is scanty in desert areas but thunderstorm & cloudburst do occur which leads to torrential downpour of rain, producing devastating effects
• A single rainstorm may bring several inches of rain within a few hours, drowning people who camp there & flooding mud baked houses into oasis;
• Also leads to formation of gullies & ravines (badland topography)
• As deserts have little vegetation to protect the surface soil, large quantity of rock waste are transported in sudden raging torrents known as flash floods.
• There is so much material in flash flood that the flow becomes liquid mud.

 

When the masses of debris are deposited at the foot of the hill or at the mouth of the valley, an alluvial cone or fan or dry delta is formed, over which temporary stream discharges through several channels, depositing more material. Alluvial deposits are subjected to rapid evaporation by the hot sun & downward percolation of water into porous ground, & soon dry up leaving mounds of debris.

Temporary lakes

• Also known as Playas, Salina or Salars
• Formed in arid or semi-arid areas by intermittent streams flowing into depressions
• Contain high percentage of salts due to high evaporation & lower precipitation

 

Bajada & Pediment

The floor of desert depression is made up of two features viz. Bajada & Pediment.

• Bajda–Depositional feature made up of alluvial material lay down by intermittent streams.
• Pediment –An erosional plain formed at the base of the surrounding mountain scarps -steep slope

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Limestone and Chalk Landform

• Limestone & chalk are sedimentary rocks of organic origin derived from the accumulation of corals & shells in the sea.
• In its pure state, limestone is made up of calcite or calcium carbonate; along with magnesium present in form of dolomite.
• Chalk is pure form of limestone, white & soft.
• Limestone is soluble in rain water, which, with carbon dioxide from the air, forms a weak acid.
• A region with a large stretch of limestone therefore possesses a very distinct type of topography, termed as Karst region.

Features of Karst Topography

• Generally, Karst regions have a bleak landscape, occasionally broken by precipitous slopes.
• General absence of surface drainage as most of the surface water percolate underground, hence surface valleys are generally dry.
• Streams generally cut their way along the joints & fissures of the rock wearing out a system of underground channels.
• When the water penetrates to the base of the limestone & meets the non-porous rocks, it re-emerges onto the surfaces as a spring or resurgence.
• Limestones are well jointed & it is through these joints & cracks that rain water finds its way into the underlying rock.
• Progressive widening by the solution enlarges these cracks into trenches & a most intriguing feature called limestone pavementis developed.
• The enlarged joints are called Grikes & the isolated, rectangular blocks are termed as clints
• On the surface of limestone are numerous swallow holes, which are small depressions carved out by solution where rainwater sinks into limestone at the point of weakness, also known as sink holes. Once water has sunk into limestone, it etches out caverns & passages along joints.
• When a number of swallow holes coalesce, a larger hollow is formed & is called a Doline.
• Several dolines may merge as a result of subsidence (gradual caving) to form an even larger depression called an Uvala
• In Yugoslavia, some very large depressions called Polje,may be as large as 100 square miles, but produced partly due to faulting.
• Subterranean streams which descent through swallow holes to the underground passes leads to the formation of caves & caverns which may contain ponds or lakes.
• The most spectacular underground features that adorn the limestone caves are
• Stalactites
• Stalagmites
• Calcite pillars
• Water carries calcium in solution & when this lime charged water evaporates, it leaves behind solidified crystalline calcium carbonate.
• Stalactitesare sharp, slender, downward growing pinnacles that hang from the cave roofs.
• When moisture drips from the roof, it trickles down the stalactites & drops to the floor, where calcium is deposited to formStalagmites, Which are shorter, fatter & more round.
• Over a longer period, the stalactite hanging from the roof is eventually joined to stalagmite growing from the floor to form a pillar.

 

Human activities of Karst region

• Karst regions are often barren & at best carry a thin layer of soil.
• The porosity of the rocks & the absence of surface drainage make vegetative growth difficult, hence limestone can usually support only poor grass.
• Limestone vegetation in tropical regions is luxuriant because of heavy rainfall all the year around.
• The only mineral found in association of limestones is lead.
• Good quality limestone is used as building materials & quarried for cement industry.

 

Chalk

• Landforms of chalk are rather different from other limestones.
• There is little or no surface drainage & valleys which once contained rivers are now dry often called as Coombes.
• Chalk is covered with short turf & is used for pasture & sometimes for arable farming.

Because of the friable nature of the chalk rocks, swallow holes & underground cave networks do not generally develop.

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Lakes

General

Lakes

• Lakes occupy the hollows of the land surface in which water accumulates & vary tremendously in size, shape, depth & mode of formation.
• The tiny ones are no bigger than ponds or pools, but the large ones are so extensive that they merit the name of the seas e.g. Caspian Sea.
• It must be noted that the lakes are only temporary feature of the earth crust & eventually be eliminated by draining & silting up.
• Most of the lakes in the world are fresh water lakes fed by the rivers
• But in regions where evaporation is greater than precipitation & only few streams filling up the lake, form saline water lakes such as Dead sea & Great Salt Lake of Utah.

 

The Formation and Origin of lakes

Lakes formed by earth movement

Tectonic lakes

• Due to warping, bending & fracturing of earth crust, tectonic depressions occur which give rise to lakes of immense sizes & depths
• Examples → Lake Titicaca (at Andes) – Highest lake of the world, Caspian Sea (Largest lake of the world & 5 times larger than its nearest rival i.e. lake superior)

 

Rift Valley Lakes

• Due to faulting, a rift valley is formed by sinking of land between two parallel faults which is deep, narrow & elongated in character.
• Water collect in these troughs & their floors are often below sea level
• Lake Tanganyika (World’s deepest lake) & Dead Sea (World’s lowest lake)

 

Lakes formed by Glaciation

Cirque lakes / Tarns

• A glacier on its way down the valley leaves behind circular hollows in the heads of the valleys up in the mountains known as corries or cirque
• Their over deepened floors may be filled with water to form cirque lakes
• Those that occupy long & deep glacial troughs down the valley are termed as Ribbon lakes

 

Kettle Lakes

• They are basically depressions in the outwash plain left by the melting of masses of stagnant ice
• They are irregular because of the uneven moraine surface & are never of any great size or depth

 

Rock Hollow Lakes

• Formed by ice scouring when valley glaciers or ice sheets scoop out hollows on the rock surface (lakes enclosed within a rock hollow)
• Such lakes are abundant in Finland (Land of lakes)

 

Lakes formed due to Moraine damming of Valleys

• Valley glaciers often deposit moraine debris across a valley so that lakes are formed when water accumulates behind the barrier.
• Both lateral & terminal moraines are capable of damming valleys.

 

Lakes formed due to deposition of glacial drifts

• In glaciated lowlands with a predominant drumlin landscape, where drainage is poor
• There are intervening depressions which are often waterlogged, forming small lakes

 

Lakes formed by volcanic activity

Crater & Caldera Lakes

• During a volcanic explosion, top of the cone may be blown off, leaving behind a natural hollow called a crater, which may be enlarged by subsidence into a caldera
• In dormant & extinct volcanoes, rain falls straight into the crater or caldera which has no superficial outlet & forms a crater or caldera lake

Lava Blocked Lakes

• In volcanic regions a stream of lava may flow across a valley which may solidify
• Solidifying of lava may dam the river, leading to the formation of lava blocked lakes.

 

Lakes formed due to subsidence of volcanic land surface

• The crust of hollow lava flow may collapse
• Subsidence leaves behind a wide & hollow depression in which a lake may form.

 

Lakes formed by Erosion

Karst lakes

• The solvent action of rain water on limestone carves out solution hollow.
• When these become clogged with debris, lakes may form in them.
• The collapse of limestone roofs of underground caverns may result in the exposure of long, narrow lakes that were once underground.
• The large depressions called Polje, which normally do not have any outlet, may contain lakes.

 

Wind deflated lakes

• The deflating action of winds in deserts creates deep hollows which may reach water table via which water seeps out forming small shallow lakes.
• Excessive evaporation causes these to become salt lakes or Playas.

 

Lakes formed by Deposition

Lakes formed due to river deposits

• A river may shorten its course during a flood by cutting its meandering loops leaving behind a horseshoe shaped channel called ox – bow lake.

 

Lakes formed due to marine deposits

• The action of wind & waves may isolate lagoons along the coasts, enclosed by narrow spit of land known as lagoon lakes.
• Lagoonis a shallow body of water separated from a larger body of water by barrier islands or reefs.
• In East Germany & Poland lagoons are called Haffs.

 

Lakes formed due to landslides, screes & avalanches

• Landslides or screes may block valleys so that rivers are dammed, leading to formation of temporary lakes.
• Lakes formed by these processes are also known as barrier lakes.
• Such lakes are short lived because the loose fragments that pile up across the valleys will soon rupture under pressure & will give way to water.
• When they suddenly give way, the dammed water rushes down causing floods

 

Lakes formed by Human & Biological activities

Man-made lakes

• Besides the natural lakes, man has created artificial lakes by erecting a concrete dam across a river valley
• This is done so that the river water can be kept in check to form reservoirs.

 

Lakes made by animals

• Animals like Beavers are particularly interesting.
• They live in communities & construct dams across the rivers with timber, mud & soil.
• Such Beaver dams are quite permanent & modify the natural environment in such a way that the overall ecosystem builds upon the change, making beavers a keystone species.

 

Other type of man-made lakes

• Ornamental lakes → Especially made to attract tourists
• Lakes made by men mining activities
• Inland fishing lakes to develop inland fish culture