Chapter 23 — बहिर्जात प्रक्रियाएँ तथा उत्पन्न स्थलरूप

The landforms we see around us are the result of two opposite groups of forces. Endogenetic forces originate inside the earth. They are driven by heat within the crust and mantle and they act through earth movements, folding, faulting, earthquakes and volcanic activity. They build the land up: they raise mountains such as the Himalayas, form plateaus and rift valleys, and create the great unevenness of the earth's surface. Because they construct relief, they are called constructive forces.

Exogenetic forces originate outside the crust, at or near the surface. Their energy comes from the sun, which drives the water cycle and the winds, and from gravity, which pulls water, ice and loose rock downhill. They act through the agents of running water (rivers), moving ice (glaciers), wind, sea waves and groundwater. These agents wear away the high ground and deposit the material in the low ground, tending always to reduce the unevenness created by endogenetic forces. Because they destroy relief in one place and build it in another, they are called destructive or, more correctly, levelling forces.

The combined result of exogenetic work is gradation, the process by which the earth's surface is brought towards a uniform level or grade. Gradation has two parts. Degradation is the lowering of high land by weathering, mass wasting and erosion; aggradation is the raising of low land by the deposition of the eroded material. A river that cuts a gorge in the mountains is degrading; the same river that builds a delta at its mouth is aggrading. The end result, if endogenetic forces stopped entirely, would be a nearly flat plain close to sea level, which the geographer W. M. Davis called a peneplain.

Gradation happens very slowly by human standards, but over millions of years it has removed whole mountain ranges. The Aravalli range of Rajasthan, once as high as the Himalayas, has been worn down to low hills, while the sediments removed from it lie in the plains and the sea. The chapter that follows examines how each agent carries out its share of this work.

The distinction between the two groups is important in the examination: a fold mountain, a rift valley or a volcano is endogenetic; a waterfall, a moraine, a sand dune or a delta is exogenetic.

Weathering is the breaking down and decay of rocks in place, without any movement of the broken material, by the action of the atmosphere, water and living things. It has three types. In physical or mechanical weathering rocks break into smaller pieces without change in their chemical composition. Daily heating and cooling in deserts makes the outer layers expand and contract until they peel off like the skin of an onion, a process called exfoliation. Water freezing in cracks expands by about nine per cent and shatters rock (frost action) in the high Himalayas. In chemical weathering minerals are altered by water, oxygen and carbon dioxide: oxidation rusts iron-bearing rocks, carbonation dissolves limestone in weak carbonic acid, hydration makes minerals swell and crumble, and solution removes salts. In biological weathering plant roots wedge rocks apart, burrowing animals loosen soil and lichens produce acids.

Mass wasting or mass movement is the downhill movement of weathered rock and soil under gravity, without any transporting agent. It includes slow soil creep, faster earth flows and mudflows after heavy rain, rock falls and the sudden slides of whole hillsides called landslides, which are common in the monsoon on the steep slopes of Darjeeling and Sikkim.

Erosion is the wearing away of rock and the removal of the loosened material by a moving agent: a river, a glacier, wind, waves or groundwater. Erosion differs from weathering in that it always involves transport. Erosion, transportation and deposition are the three stages of an agent's work.

Denudation is the wider term that includes all of these: weathering, mass wasting, erosion and transportation together strip the surface bare. The word comes from the Latin for 'to lay bare'. The rate of denudation depends on climate (heavy rainfall and extreme temperatures speed it up), on rock type (soft shale erodes faster than hard granite), on slope (steeper is faster), on vegetation cover (which protects the surface) and on human activity such as deforestation, quarrying and road cutting, which have greatly increased soil loss in India's hill regions.

These processes prepare the material that rivers, glaciers and winds then shape into landforms, so they are the foundation of everything that follows in this chapter.

A river is a natural stream of water flowing in a channel from higher to lower ground, fed by rain, springs, melting snow or lakes. The area drained by a river and its tributaries is its drainage basin or catchment, and the high ground separating two basins is a watershed or water divide. A river has three tasks: erosion, transportation and deposition, and the balance among them changes from source to mouth.

River erosion takes place in four ways. Hydraulic action is the force of the moving water itself, which loosens rock and dashes into cracks. Abrasion or corrasion is the wearing of the bed and banks by the sand, pebbles and boulders the river drags along, which act like sandpaper. Attrition is the grinding of the rock fragments against one another so that they become smaller and rounder. Solution or corrosion is the chemical dissolving of soluble rocks such as limestone by the water. Erosion works in three directions: vertical erosion (downcutting) deepens the valley, lateral erosion widens it, and headward erosion lengthens the river at its source.

Transportation carries the load in four ways. Heavy boulders are rolled along the bed (traction); pebbles bounce along (saltation); fine sand and silt float within the water (suspension); and dissolved minerals are carried invisibly (solution). The maximum size of particle a river can move is its competence, and the total quantity it can carry is its capacity; both rise sharply with velocity, which is why floods move huge loads.

Deposition occurs wherever velocity falls: where the gradient becomes gentle, where the channel widens, on the inside of a bend, where the river enters a lake or the sea, or when the volume drops in the dry season. The coarsest material is dropped first and the finest last, so boulders and gravel lie near the mountains and fine clay reaches the delta. The material a river deposits is called alluvium.

The course of a river is conventionally divided into three parts. In the upper or mountain course the gradient is steep, velocity is high, vertical erosion dominates and the valley is narrow and deep. In the middle or plain course the gradient eases, lateral erosion widens the valley and transportation dominates. In the lower or deltaic course the gradient is almost nil, velocity is low and deposition dominates. The landforms of each course are described in the following topics.

In the mountains the river flows swiftly down a steep slope carrying boulders and pebbles, and its energy is spent mainly in vertical erosion. The result is a set of dramatic erosional landforms.

A V-shaped valley forms because the river cuts downward faster than the valley sides are worn back. The bed is narrow and the sides slope steeply to form a V in cross-section. Weathering and mass wasting on the sides keep the V open; the more resistant the rock and the faster the downcutting, the steeper the V. Where the sides are almost vertical and the valley is very narrow and deep, the feature is a gorge. Gorges form where hard rock resists lateral erosion, where a river keeps pace with the rising of a mountain (as the Indus, Sutlej and Brahmaputra have done through the Himalayas), or in dry climates where the sides are not worn back by rain. A canyon is a very large gorge with stepped sides, formed where a river cuts through horizontal layers of rock in a dry region; the Grand Canyon of the Colorado river in the USA, about 446 km long and up to 1.8 km deep, is the most famous.

A river's course down a mountain is rarely smooth. Where it crosses a band of hard rock lying over soft rock, the soft rock is worn away faster and the river falls vertically over the hard ledge, forming a waterfall. The falling water excavates a plunge pool at the base, undercuts the ledge, and the fall slowly retreats upstream, leaving a gorge below it. Waterfalls also form where a tributary valley hangs above a glaciated main valley, at a fault line, or where a river plunges off a plateau edge. Examples are Jog (Gersoppa) on the Sharavathi in Karnataka, about 253 m high, Angel Falls in Venezuela, the highest in the world at 979 m, and Niagara on the USA–Canada border. Where the drop is small and broken into steps, the feature is a cascade; where the bed is uneven and the water tumbles turbulently over rocks without a clear drop, it forms rapids.

A pothole is a rounded, cylindrical hole drilled into the rocky bed of a river. Pebbles caught in a small hollow are whirled round by eddies and grind the hole deeper and wider, a process called pothole drilling. Potholes are common on the bed of the Subarnarekha and in the rocky beds of streams of the Chota Nagpur plateau. When many potholes join, they can deepen the whole channel.

An interlocking spur is a further feature: the young river winds around projections of hard rock from either side, which overlap each other when seen from downstream, so that the valley appears to be blocked.

When the river leaves the mountains and enters the plain its gradient falls sharply. Vertical erosion almost stops, lateral erosion widens the valley, and the river carries a large load of finer material. Both erosion and deposition operate, and the landforms of this course are the result of their interplay.

Where a river leaves the hills it drops its coarse load in a fan-shaped heap called an alluvial fan. Several fans joining together along a mountain foot form a piedmont plain; the Bhabar belt at the foot of the Himalayas in north Bengal and Uttar Pradesh is such a zone of coarse gravels.

The most characteristic feature of the middle course is the meander, a large sweeping bend. The name comes from the winding river Menderes in Turkey. On a gentle slope the slightest obstruction turns the current to one side; on the outside of the resulting bend the water flows faster and erodes the bank, forming a steep river cliff or cut bank, while on the inside the water is slower and deposits sand to form a gentle slip-off slope or point bar. The bend grows larger and moves slowly downstream. Over time the neck of land between two meander loops narrows until, during a flood, the river cuts straight across it. The old loop is abandoned, silt seals its ends, and it becomes a crescent-shaped lake called an ox-bow lake, also known as a cut-off lake, mortlake or, in Bengal, a beel. Such lakes gradually fill with vegetation and become marshes. Ox-bow lakes are numerous along the Ganga and its distributaries in Murshidabad and Nadia and along the Kosi and Gandak in Bihar.

Every year in the monsoon the river overflows its banks. As the water spreads out, it slows and drops its load, coarsest nearest the channel and finest farthest away. The flat area covered by this flood alluvium on both sides of the river is the floodplain, the most fertile farmland in the world. The ridge of coarser sediment built up along the immediate banks is the natural levee. Levees may raise the river bed above the plain, so that a breach during a flood sends water rushing over the countryside; this is why the Hwang Ho of China is called China's Sorrow and why the Damodar earned the name Sorrow of Bengal before its dams were built.

Where the load exceeds what the river can carry, it drops sand bars in its channel and splits into many shallow shifting channels, a pattern called braiding, common in the Tista and the Brahmaputra.

As the river approaches the sea the land is nearly flat, the current is slow, and the river has little energy left for anything except deposition. Its last and greatest work is the delta, a triangular or fan-shaped deposit of alluvium at its mouth. The name was given by the Greek historian Herodotus, who saw that the mouth of the Nile resembled the Greek capital letter delta, Δ.

A delta forms when the river brings a large load of fine sediment to a sea that is calm and shallow, without strong tides or currents that would sweep the sediment away. As the fresh water meets salt water the fine clay particles clump together (flocculate) and sink, and the sediment builds up at the mouth faster than the sea can remove it. The river, blocked by its own deposits, splits into several channels called distributaries, which spread the sediment further. Conditions favouring a delta are: a long river with a large catchment and heavy sediment load, a gentle gradient in the lower course, a shallow sheltered coast, weak tides and currents, and a slowly subsiding or stable shore.

Deltas are classified by shape. An arcuate delta is fan-shaped with a curved outer edge, made of coarser sediment; the Nile, the Ganga-Brahmaputra and the Po deltas are arcuate. A bird's-foot delta has long finger-like distributaries reaching into the sea because the fine sediment and weak waves let each channel build its own levees; the Mississippi delta is the classic example. A cuspate delta is tooth-shaped, formed where waves from both sides push the deposit into a point, as at the Tiber mouth in Italy. An estuarine delta fills a drowned river mouth, as at the Seine.

The Ganga-Brahmaputra delta, shared by West Bengal and Bangladesh, is the largest delta in the world, covering about 105,000 sq km. Its seaward part, the Sundarbans, is a maze of tidal creeks and mangrove islands. The delta is still growing, especially in its eastern, active part, while the western part around the Hooghly is the moribund delta where distributaries have silted up. Deltas are densely populated because their soil is renewed every year by flood silt.

Where the coast is sinking, or the tides and currents are strong, the river cannot build a delta; instead the sea enters the river mouth and forms a funnel-shaped estuary. The Narmada and Tapti on India's west coast, and the Thames and St Lawrence, have estuaries. Estuaries make good harbours because they are deep and free of silt.

A glacier is a large mass of ice that moves slowly downhill under its own weight. It forms wherever snowfall exceeds melting over many years, above the snow line, which lies at about 4,500–6,000 m in the Himalayas, 2,700 m in the Alps and at sea level in polar regions. Fresh snow is compressed by the weight of later snow into granular firn or névé, and then into dense, blue glacier ice.

Glaciers are of three main kinds. Continental glaciers or ice sheets cover vast areas of land, hiding the relief beneath a dome of ice thousands of metres thick; the Antarctic ice sheet (about 14 million sq km) and the Greenland ice sheet are the only ones today, though ice sheets covered much of Europe and North America during the Pleistocene Ice Age. Valley or mountain glaciers (also called alpine glaciers) flow down pre-existing river valleys in high mountains like rivers of ice; India's Siachen glacier in the Karakoram, about 76 km long, is the largest outside the polar regions, and the Gangotri glacier, about 30 km long, gives birth to the Bhagirathi. Piedmont glaciers form when several valley glaciers spread out and merge at the foot of a mountain range, like the Malaspina glacier of Alaska.

A glacier moves at a few centimetres to a few metres a day, faster in the centre and at the surface than at the sides and bottom, where friction is greatest. Movement occurs by basal sliding over a film of meltwater and by internal deformation, in which ice crystals slip past one another under pressure. The brittle upper surface cracks into deep fissures called crevasses where the glacier bends over a step. The lower end, where melting equals supply, is the snout.

Glaciers erode in two ways. Plucking occurs when meltwater freezes into cracks in the bedrock and the moving ice tears out blocks. Abrasion occurs when these blocks and smaller fragments, frozen into the base of the ice, scratch, groove and polish the rock beneath, producing striations and smooth roches moutonnées (rock knobs smooth on the upstream side and plucked on the downstream side). Because ice is solid and heavy it erodes the whole floor and sides of its valley at once, unlike a river which erodes only a narrow channel, and it can even deepen its bed below sea level to form fjords.

The erosional work of mountain glaciers produces some of the most striking scenery on the earth, and the examination often asks for these features with diagrams.

A cirque (also called a corrie in Scotland or cwm in Wales) is a deep, semicircular, armchair-shaped hollow with a steep back wall at the head of a glacial valley. It begins as a hollow where snow collects; frost shattering steepens the back wall, plucking and abrasion by the rotating ice deepen the floor, and a raised lip is left at the mouth. When the ice melts the hollow may hold a small round lake called a tarn. Many cirques are found around the peaks of the Himalayas and the Alps.

When two cirques form back to back or side by side and cut into the same ridge, the ridge between them is narrowed into a sharp, knife-edged crest called an arête. When three or more cirques cut into a mountain from different sides, they leave a sharp pyramid-shaped peak called a horn or pyramidal peak. The Matterhorn in the Swiss Alps is the type example; many Himalayan peaks, including parts of the Kangchenjunga massif, show the same form.

A glacier flowing down an old river valley straightens and widens it because ice cannot turn sharp bends and cuts off the interlocking spurs, leaving truncated spurs. It deepens the floor and steepens the sides until the valley is broad and flat-bottomed with steep walls: a U-shaped valley or glacial trough. After the ice melts, a stream far too small for the valley flows along the floor and is called a misfit stream. Lakes strung along the trough, filling hollows scooped by the ice, are called ribbon or finger lakes. The upper Bhagirathi valley above Gangotri and the Lachen valley in North Sikkim show the U-shaped form.

A tributary glacier, being smaller, cannot erode as deeply as the main glacier, so its valley floor is left high above the floor of the main valley. When the ice melts the tributary valley opens onto the side of the main trough as a hanging valley, and its stream drops into the main valley as a waterfall. The Bridalveil Fall in Yosemite, California, falls from a hanging valley.

Where a glacial trough reaches the coast and its floor lies below sea level, the sea floods it after the ice melts to form a long, narrow, deep inlet called a fjord. Norway's Sogne fjord (about 200 km long and over 1,300 m deep), and the fjords of Chile and New Zealand are examples.

Everything a glacier erodes it carries frozen within, on top of or beneath the ice, and it lays this material down wherever the ice melts. The mixture of clay, sand, gravel and large unsorted boulders left by ice is called glacial till or boulder clay, and all landforms built of it are called moraines. Till is unsorted, unlike river alluvium, because ice does not separate particles by size.

Moraines are named by position. Lateral moraines are ridges of rock debris along the two sides of a valley glacier, made of material that falls from the valley walls by frost action. Where two glaciers join, their inner lateral moraines unite to form a medial moraine, a dark stripe running down the middle of the combined glacier. The terminal or end moraine is the crescent-shaped ridge of debris dumped at the snout, marking the farthest point the glacier reached; a series of smaller ridges left as the ice retreats in stages are recessional moraines. Material carried and dropped beneath the ice forms the ground moraine, a rolling sheet of till spread over the valley floor or over vast lowlands where ice sheets once lay, as in the plains of northern Germany, Poland and the American Midwest. Huge boulders carried far from their source rock and left stranded on different rock are erratics.

Some depositional features are shaped by the moving ice or its meltwater. A drumlin is a smooth, elongated, oval hill of till, shaped like an inverted spoon or half an egg, with its steep blunt end facing the direction from which the ice came and its gentle tapering end pointing the way the ice went. Drumlins usually occur in groups, forming a 'basket of eggs' topography, as in Northern Ireland and New York State.

Meltwater flowing in tunnels beneath or within the ice deposits sorted sand and gravel in the tunnel; when the ice melts, this is left as a long, winding, steep-sided ridge called an esker, which may run for many kilometres like a raised railway embankment. Kames are irregular mounds of sand and gravel left where meltwater dumped debris in hollows in the ice or at its edge. Kettle holes are depressions, often filled by lakes, left where buried blocks of ice melted.

Beyond the terminal moraine the meltwater streams spread out and deposit sorted sand and gravel in a broad gently sloping sheet called the outwash plain or sandur, common in Iceland and in front of Himalayan glaciers. Together these features tell a geologist how far the ice advanced and how it retreated.

Wind is the chief agent of erosion in hot deserts such as the Sahara, the Arabian desert and the Thar, and in semi-arid and coastal areas, because these places have little rainfall, little vegetation to bind the soil, wide daily temperature ranges that shatter rock, and loose sand ready to be moved. Wind cannot erode solid rock by itself; it works with the sand it carries.

Wind erosion has three processes. Deflation is the lifting and removal of loose dry particles from the surface; it lowers the ground and leaves behind hollows and a surface of pebbles too heavy to move, called a desert pavement or reg. Abrasion is the sandblasting of rock surfaces by sand grains driven by the wind; because sand is heavy and travels mostly within a metre or two of the ground, abrasion is strongest near the base of a rock. Attrition is the wearing of the sand grains against one another as they fly, which makes desert sand round, frosted and fine.

Wind transports material in the same three ways as a river. Fine dust is lifted high and carried hundreds of kilometres in suspension; Sahara dust falls on Europe and Thar dust reaches Delhi. Sand grains bounce along in a series of hops (saltation), each landing grain knocking others into the air. Larger grains and small pebbles are pushed and rolled along the surface (surface creep). The carrying power of wind rises very steeply with speed, so a dust storm (andhi in Rajasthan) can move enormous quantities of material in a few hours.

Wind deposits its load wherever its speed falls, behind an obstacle such as a bush, a rock or a fence, or when the wind dies down. Coarse sand is dropped first and forms dunes; fine dust travels far and settles as loess.

The desert landscape is therefore made up of three surfaces: the hamada, a rocky desert of bare rock swept clean; the reg, a stony desert of pebbles left after deflation; and the erg, a sandy desert of dunes where the removed sand collects. The Thar desert of Rajasthan has all three. Although wind is the dominant agent, occasional violent rainstorms in deserts also do much work, and the landforms of the next topics show the combined effect.

Wind, armed with sand, carves the rocks of the desert into a number of distinctive forms.

A mushroom rock or pedestal rock (called gara in the Sahara) is an isolated rock whose base has been worn thin by sand abrasion while its top remains broad. Because saltating sand travels close to the ground, the lower part of a rock outcrop is sandblasted far more than the upper part; if the upper layer is also harder, the effect is greater. The result looks like a mushroom on a stalk. Such rocks are found in the Thar, the Sahara and the deserts of Egypt and Saudi Arabia.

Zeugen (singular zeuge) form where a hard rock layer lies horizontally over a soft one and the surface is broken by joints. Wind abrasion widens the joints and cuts away the soft rock beneath, leaving flat-topped, tabular blocks of hard rock standing on narrower stems of soft rock, separated by furrows. They may be from a few metres to 30 m high. In the Sahara they resemble rows of tables.

Yardangs form where hard and soft rock layers stand vertically and run parallel to the prevailing wind. The wind scoops out the soft bands into long troughs and leaves the hard bands standing as sharp parallel ridges, like the keels of upturned boats, often 5–15 m high. Yardangs are common in the Atacama, the Iranian deserts and Central Asia (the word is of Turkic origin).

An inselberg (German for island mountain) is an isolated, steep-sided, rounded hill of hard resistant rock, usually granite or gneiss, rising abruptly from a flat desert or savanna plain. It is what remains after the softer surrounding rock has been removed by weathering, wind and sheet floods; the rounded shape comes from exfoliation. Inselbergs are found in the Kalahari, in the Sahara, in Australia (Uluru or Ayers Rock is the most famous) and in India around the Aravallis, in the Deccan and in the Bankura–Purulia region of West Bengal.

A deflation hollow or basin is a depression scooped out by deflation, where the wind has removed loose material down to the water table or to hard rock. If the hollow reaches the water table an oasis forms. The Qattara Depression in Egypt, over 130 m below sea level, and the Big Hollow of Wyoming are deflation basins. Salt lakes in Rajasthan such as Sambhar occupy shallow basins partly formed in this way.

Wind-faceted pebbles called ventifacts or dreikanter, with polished flat faces, litter desert pavements and record the direction of the prevailing wind.

When wind slows down it drops its sand, building heaps called dunes. Any obstacle, a bush, a rock or a small hollow, can start a dune. Every dune has a gentle windward slope up which the sand creeps and saltates, and a steep leeward slip face where the sand falls and settles at its angle of rest, about 30–34°. Because sand is continually moved from the windward side over the crest to the leeward side, the whole dune migrates slowly downwind, sometimes covering roads, fields and villages.

A barchan is a crescent-shaped dune formed where the wind blows steadily from one direction and the sand supply is limited. Its convex side faces the wind, and its two horns point downwind because the thin ends of the dune move faster than the thick centre. Barchans are typically 5–30 m high and may move 10–20 m a year. They are found in the Thar, the Sahara and Turkestan, sometimes in long chains.

A seif (Arabic for sword) or longitudinal dune is a long, narrow, straight-crested ridge of sand running parallel to the prevailing wind, formed where two winds blow from slightly different directions or where a strong wind is combined with a cross wind. Seifs may be over 100 km long and 100–200 m high, separated by bare corridors of rock or reg along which the wind is funnelled. The great sand seas of the Sahara, Arabia and Australia are largely seifs.

Transverse dunes are long ridges lying at right angles to the wind, formed where sand is plentiful and the wind is steady; they resemble waves on the sea. Parabolic dunes are U-shaped dunes whose horns point upwind because vegetation anchors the ends while the centre blows forward; they are common on coasts and in the semi-arid margins of the Thar. Where dunes become fixed by vegetation they are called stabilised dunes, and much of the eastern Thar consists of them. Coastal dunes also line the shores of Digha and Puri.

The finest desert dust is carried far beyond the desert in suspension and settles as a thick, yellowish, unstratified but very fertile deposit called loess. Loess is porous, stands in vertical cliffs when cut, and can be tens of metres thick. The Loess Plateau of northern China, built from dust blown out of the Gobi desert, is the largest loess area on earth and gives the Hwang Ho its yellow colour and its name. Loess also covers parts of Central Europe, the Mississippi valley and the Pampas of Argentina, and its fertility makes these regions great wheat lands.

Deserts are dry but not rainless. Rain, when it comes, falls in sudden violent storms on bare ground, and the water rushes off in sheet floods and through short-lived channels, doing in a few hours the erosion and deposition that a humid-region river spreads over the year. Many desert landforms are therefore the joint product of wind and occasional running water.

A wadi (called a nullah or nala in India) is a dry, steep-sided desert stream channel that carries water only after rain. Flash floods in wadis cut sharp gullies and carry a heavy load of debris to the foot of the hills. Wadis are typical of the Sahara, Arabia and the hill margins of the Thar.

A pediment is a gently sloping, bare rock surface at the foot of a desert mountain or inselberg. It is cut into the solid rock by sheet floods, by lateral swinging of wadi streams and by weathering that causes the mountain front to retreat; wind then sweeps it clean. The pediment slopes at only 1° to 7°, and the break of slope between the steep mountain and the gentle pediment is sharp, giving desert mountains their characteristic look of rising straight out of a plain.

Below the pediment, the wadi streams emerging from the mountain drop their load in fan-shaped deposits called alluvial fans. When many neighbouring fans grow and merge along the mountain foot they form a continuous apron of sand and gravel called a bajada (Spanish for slope), which may be several kilometres wide and slopes gently down into the basin. Bajadas fringe the ranges of the Mojave desert and the Basin and Range country of the USA, and the hill margins of Baluchistan and the Thar.

At the lowest part of an enclosed desert basin, water from the surrounding slopes collects after rain to form a shallow temporary lake. Under the intense sun this lake evaporates within days or weeks, leaving a flat, hard floor of clay and salt called a playa (also salina, or salar in South America, and rann in Gujarat). The playa surface, cracked and glittering with salt, is one of the flattest natural surfaces on earth. The Sambhar Lake in Rajasthan, the Rann of Kutch in Gujarat, the Bonneville Salt Flats of Utah and the Salar de Uyuni of Bolivia are playa-type features, and their salt is often harvested.

Where wind removes the dried playa clay and salt it may form deflation hollows and clay dunes, and where groundwater comes near the surface in such hollows, oases with date palms and settlement appear. Thus in the desert, wind and water are partners rather than rivals.

Having studied rivers, glaciers and wind separately, it helps to place them side by side. All three erode, transport and deposit, but they differ in the way they do it, and these differences show in the landforms and in the deposits they leave.

A river works in a narrow channel with liquid water, so it cuts deep and narrow valleys (V-shaped), sorts its load by size as its velocity changes, and produces stratified, rounded, well-sorted alluvium. Its work is continuous and it is found in every climate with rainfall. A glacier works with solid ice over its entire bed, so it erodes the whole valley floor and sides at once, producing broad U-shaped troughs, and it cannot sort its load, so till is unstratified and contains angular boulders of every size mixed with clay. It operates only in cold regions above the snow line. Wind works only with dry loose material and has no fixed channel, so it erodes by sandblasting near the ground, producing pedestal and streamlined forms, and it sorts its load extremely well: sand into dunes, dust into loess. Desert sand grains are round and frosted, river sand is sub-rounded, glacial fragments are angular and striated.

These landforms shape human life. River floodplains and deltas support the densest farming populations in the world: the Ganga plain and the Bengal delta feed hundreds of millions of people, and their levees, ox-bow lakes and shifting channels decide where villages can stand and where floods will strike. Waterfalls and gorges are sites for hydroelectric dams, such as those on the Sharavathi, and gorges make natural sites for reservoirs. Glacial valleys hold ribbon lakes that supply water and attract tourists; moraines and outwash plains provide gravel; and the melting of Himalayan glaciers such as Gangotri and Zemu feeds the perennial rivers of North India, so their retreat under global warming is a serious concern. Glacial lakes dammed by moraines can burst, causing the glacial lake outburst floods that have devastated valleys in Sikkim and Uttarakhand. In deserts, shifting barchans threaten roads, railways and the Indira Gandhi Canal in Rajasthan and must be fixed by planting; loess soils, though fragile, are among the most productive wheat lands; playas yield salt; and oases in deflation hollows are the only places where settlement is possible.

Understanding exogenetic processes therefore is not only about naming landforms. It explains why people live where they do, why some places flood or slide, and how careless deforestation, mining or construction can speed up erosion and turn slow natural processes into disasters.