Chapter 4 — 1.4 Works of Wind

Wind is air in motion, and moving air, like moving water, can pick up loose particles and carry them along. Wind blows everywhere on earth, but it is an effective agent of erosion only where certain conditions are met, and those conditions are found chiefly in the hot deserts of the tropics and in some cold deserts and coastal belts. The geographical work of wind is therefore called aeolian work, from Aeolus, the Greek god of the winds.

The conditions that make wind the dominant agent are these. First, scanty rainfall, usually below 250 mm a year, so that there are no permanent rivers to do the work and the ground surface is dry; dry particles are loose and light, whereas wet particles stick together. Second, absence of vegetation, because plant roots bind the soil and plant cover shelters the surface from the wind; in a desert the bare ground is exposed to the full force of the air. Third, a supply of loose material, which the desert climate itself produces: the great daily range of temperature, from over 45 °C at noon to near 10 °C at night, breaks the rocks by expansion and contraction into sand and dust, and the occasional torrential rain washes further debris into the basins. Fourth, strong and steady winds, which are common in deserts because there are no obstacles to slow them and because intense heating of the ground creates strong convection and pressure differences; the trade winds blow steadily across the tropical deserts. Where these conditions coincide, as in the Sahara, Arabia, the Thar, the Kalahari, the deserts of central Asia, Atacama and central Australia, wind does more work than any other agent.

Deserts are not, however, entirely without water. Rain falls rarely but, when it does, it often comes as a violent cloudburst that produces sudden flash floods. These floods erode and deposit far more in a few hours than the wind does in years, and the landforms of a desert are always the joint product of wind and occasional water. The chapter therefore treats the work of wind first and then the combined work of wind and water.

Wind differs from the other agents in three ways that shape its work. It can move only small particles, sand, silt and dust, and cannot carry pebbles, so its erosion is chiefly by sand-blasting near the ground. It is not confined to a channel or valley but sweeps across the whole surface, so it erodes wide areas rather than lines. And it is not bound by gravity to flow downhill, so it can carry material uphill and over great distances, and its deposits do not collect in valleys but wherever the wind slackens.

Wind erodes in three ways, which correspond closely to the hydraulic action, abrasion and attrition of a river.

Deflation is the lifting and blowing away of loose, dry particles of sand, silt and dust from the surface by the wind. The word means blowing away. It removes the finer material and leaves the coarser behind, so it lowers the surface unevenly and produces shallow hollows, and it exposes the bedrock. Deflation is the wind's way of starting its work, just as hydraulic action is the river's; it is most effective where the surface is loose and completely dry, and it is the process responsible for dust storms. In the Thar, the loo of summer raises dust storms called andhi that darken the sky for hours. Over the centuries deflation can scoop out large basins, such as the Qattara Depression of Egypt, which lies 134 m below sea level.

Abrasion or corrasion is the wearing away of rock surfaces by the sand grains carried by the wind, which strike the rock like the grit of a sandblasting machine. Because sand grains are heavy, the wind carries most of them within a metre or so of the ground, so abrasion is strongest near the base of rocks and dies away upward. This concentration of erosion at the base gives many wind-carved features their undercut shape. Abrasion polishes and pits rock surfaces, grooves and fluttes them along the wind direction, and cuts away soft layers faster than hard ones, so that hard bands stand out in ridges. Telegraph poles in deserts are sometimes cut through at the base by abrasion, and window glass in desert towns becomes frosted.

Attrition is the wearing down of the sand grains themselves as they strike against one another and against the rock while in transit. The grains become smaller, rounder and smoother; desert sand grains, having been bounced about for thousands of years, are far rounder and better polished than river sand, and are described as millet-seed sand. Attrition also reduces sand to dust, which the wind can then carry in suspension over great distances.

The rate of wind erosion depends on the velocity of the wind, the amount and coarseness of the sand it carries, the hardness and structure of the rock, and the length of time the wind blows from one direction. A wind of about 20 km per hour is enough to move sand; a gale can carry vast quantities. In the trade-wind deserts the wind blows from nearly the same direction for most of the year, so the erosional and depositional forms are aligned with it and record the prevailing wind direction.

Deflation, abrasion and attrition rarely act alone. Deflation removes the fine cover and exposes the rock; abrasion then attacks the exposed rock; attrition prepares finer material for deflation to lift; and the cycle goes on.

The simplest erosional features of wind are those made by deflation alone, together with the pebbles it leaves behind.

A deflation hollow or blow-out is a shallow depression in the desert surface formed where the wind has blown away the loose sand and silt. It begins at any patch of bare, loose ground, often where the vegetation has been destroyed, and grows as the wind scoops out more and more material, until it reaches either the bedrock or the water table, where the damp sand can no longer be lifted. If the water table is reached, water seeps into the hollow and forms an oasis, around which date palms and settlements grow; many of the oases of the Sahara and the Thar lie in deflation hollows. Deflation hollows range from a few metres across, as in the blow-outs of coastal dunes, to enormous basins like the Qattara Depression of Egypt, about 300 km long and 134 m below sea level, and the Turfan depression of China, 154 m below sea level. Deflation is a strong process on cultivated land too: the Dust Bowl of the United States in the 1930s was created when ploughing destroyed the grass cover of the Great Plains and drought let the wind strip the topsoil.

Where deflation has removed all the fine material from a surface and left behind only the pebbles and stones that the wind cannot lift, the ground is covered with a tightly packed layer of stones called desert pavement, known as reg in the Sahara, serir in Libya and gibber in Australia. The pavement, once formed, protects the finer material beneath it from further deflation, and it makes a hard surface over which vehicles can travel. The stones on it are often coated with a dark, shiny desert varnish of iron and manganese oxides.

The pebbles lying on the desert surface are attacked by abrasion. Sand blown against a pebble wears a smooth, polished, flat face on the side towards the wind; if the pebble is turned over by animals or by undercutting, or if the wind changes direction, a second and third face are cut, and the pebble becomes a many-faced stone with sharp edges between the faces. Such a wind-faceted pebble is a ventifact, and the three-faced form is called a dreikanter, a German word meaning three-edged. Ventifacts are excellent evidence of past wind action and are found not only in present deserts but in regions that were deserts in the past.

Rock surfaces exposed to sand blasting are also cut into small grooves and hollows, and where softer patches are eroded out of a rock face it becomes pitted and honeycombed, a pattern sometimes called stone lattice. All these are small features, but they are the surest signs that wind, not water, has been at work.

The most familiar and most frequently drawn landform of wind erosion is the mushroom rock, also called a pedestal rock or, from the German, pilzfelsen. It is an isolated rock mass with a broad, rounded top perched on a narrow, worn stem, so that the whole looks like a mushroom or a table on a thin leg.

The mushroom rock is the direct result of the way wind abrasion works. Sand grains are heavy, and the wind carries the bulk of them by saltation within about a metre of the ground; above that height the air carries only fine dust, which has little cutting power. Therefore an isolated rock standing in the path of the wind is sand-blasted most severely near its base and hardly at all near its top. Year after year the base is worn away faster than the upper part, until the rock is undercut on all sides, or on the windward side if the wind is constant, and stands on a narrow neck. The effect is strongest where the rock is made of horizontal layers with a harder, more resistant layer at the top and softer layers below, because the soft lower layers yield quickly to abrasion while the hard cap protects the top. Some part is also played by weathering: the base of a desert rock is kept slightly damp by dew and moisture in the ground, and chemical weathering and salt crystallisation weaken it, so that abrasion removes it more easily.

Mushroom rocks are usually a few metres high, though some reach 10 m or more. Eventually the stem becomes too thin to support the cap, the cap topples, and the process starts again on the fallen block. They are found in all the great deserts; famous examples are in the White Desert of Farafra in Egypt, at Timna in the Negev of Israel, in the Saudi Arabian desert and, in India, in the Jaisalmer and Barmer districts of the Thar, where they are locally called chhatri (umbrella) rocks. In cold deserts and along coasts, similar pedestal rocks are formed by wind-blown sand and salt spray.

The diagram of a mushroom rock is a standard examination item. It should show a horizontal rock layered with a hard cap, the direction of the prevailing wind with an arrow, the zone of maximum sand-blasting marked within about one metre of the ground, the undercut neck and the overhanging cap. A brief label should note that the base is eroded more because most sand travels near the ground.

The mushroom rock illustrates a principle that runs through all of wind erosion: the intensity of abrasion decreases with height above the surface, so that wind-carved features are typically undercut, narrow at the base and broad above, the exact opposite of a water-carved cliff, which is worn most at the top where rain and weathering attack it.

Where the desert bedrock is made of alternating hard and soft layers, wind abrasion attacks the soft layers preferentially and leaves the hard ones standing out as ridges. Two landforms result, depending on whether the layers stand vertically or lie horizontally.

Yardangs form where the hard and soft rock layers are arranged vertically, in bands that run parallel to the direction of the prevailing wind. The wind, blowing along the bands, scours out the soft layers into long, narrow furrows or corridors and leaves the hard layers as sharp-crested, elongated ridges between them. The ridges are streamlined by the wind, with a steep, blunt end facing the wind and a long tapering tail downwind, like the inverted hull of a boat. Yardangs are usually a few metres high and tens of metres long, but the giant yardangs of the Lut desert of Iran and of the Tibesti region of the Sahara are up to 100 m high and several kilometres long. The word is of Turkic origin and was first applied to the ridges of the Taklamakan desert in central Asia. Yardangs run parallel to the wind and so are a reliable indicator of its direction; they have also been photographed on the surface of Mars.

Zeugens form where the layers lie horizontally, with a hard layer at the top and a soft layer beneath, and where the surface is crossed by joints or cracks. Weathering first opens the joints; then the wind, blowing through the cracks, abrades the soft lower layer and widens the cracks into furrows, leaving the hard cap rock standing on blocks of the soft rock beneath. The result is a tabular or flat-topped mass of hard rock resting on a pedestal of soft rock, separated from its neighbours by furrows, rather like a row of anvils or tables; the word zeugen is German for witnesses, because the blocks stand as witnesses to the former height of the surface. Zeugens may be from a few metres to 30 m high. As undercutting continues, the cap collapses and the zeugen is destroyed, so that a zeugen may be regarded as a large, tabular mushroom rock arranged in rows along joints. Zeugens run across the wind direction, along the joints, whereas yardangs run parallel to it.

The difference between the two is a frequent examination question and can be put simply:

Both landforms show how rock structure controls the shape that erosion produces, a lesson already met in the waterfall of the river section.

An inselberg is an isolated, steep-sided, often dome-shaped hill of hard rock that rises abruptly from a flat or gently sloping desert plain. The word is German and means island mountain, because these hills stand out of the plain like islands out of the sea. Inselbergs are usually made of very resistant rocks such as granite or gneiss, and they range from small knobs to great masses hundreds of metres high; Uluru or Ayers Rock in central Australia, 348 m high and about 9 km round, is the best-known example in the world.

The origin of inselbergs has been debated, but the standard explanation for the examination is as follows. The desert surface is worn down by the combined action of wind and occasional running water. The softer rocks are eroded rapidly, but a mass of very hard, unjointed rock resists both abrasion by the wind and the sheet floods of the rare rains. As the surrounding plain is lowered, the resistant mass is left standing above it. Its sides are kept steep by the undercutting action of sand-blasting and of sheet-wash at the base, and its top is rounded by exfoliation, the onion-skin weathering caused by the great daily changes of desert temperature. Some geographers add that the sharp junction between the hill and the plain is due to the slope retreating parallel to itself, that is, the whole side of the hill wearing back at a constant angle, until only the core remains. The gently sloping rock surface that surrounds the inselberg at its foot, thinly covered with debris, is the pediment, described in a later topic.

Inselbergs are therefore residual landforms: they are the remnants of a former higher surface, and they belong to the old-age stage of the desert landscape, just as monadnocks belong to the old-age stage of a humid landscape. In fact the two terms overlap, and an inselberg may be regarded as the desert equivalent of a monadnock. Where a group of inselbergs stands close together, the surface is sometimes described as inselberg landscape, typical of the Kalahari, the Namib, the Sahara and the interior of Australia. Small, rounded, boulder-strewn inselbergs of granite are called bornhardts or tors in some texts.

In India, inselbergs are found in the Thar, in the granite country of the Aravalli fringe in Rajasthan, and, formed under a former drier climate, on the Deccan and the Chotanagpur plateau; the rounded granite hills of Purulia and Bankura in West Bengal, and the boulder hills around Hyderabad, are often cited as inselbergs shaped by exfoliation and sheet wash. This shows that inselbergs are not confined to present-day deserts but survive from earlier arid periods.

A diagram of an inselberg should show the flat desert plain, the steep-sided hill rising sharply from it, its dome-shaped top, and the pediment with a thin debris cover sloping gently away from its base.

Wind transports the material it has loosened in three ways, depending on the size of the particles and the speed of the wind. These correspond to the suspension, saltation and traction of a river, but there is one great difference: the wind cannot carry anything larger than coarse sand, so pebbles and boulders are beyond its power.

Suspension is the carrying of the finest particles, silt and dust smaller than about 0.1 mm, high in the air, held up by the turbulence of the wind. Dust can be lifted thousands of metres and carried for hundreds or thousands of kilometres before it settles; the dust of the Sahara reaches Europe and the Caribbean, the dust of the Thar reaches Delhi and the Gangetic plain, and the dust of the Gobi is carried to Beijing and even across the Pacific. Dust storms in the Thar during April and May, called andhi, are suspension on a huge scale. The material carried in suspension is the source of loess deposits.

Saltation is the movement of sand grains, from about 0.1 to 1 mm in size, in a series of low hops or bounces. The wind lifts a grain a few centimetres or tens of centimetres into the air; it is carried forward a short distance and falls back to the ground, where its impact knocks other grains into the air, which in turn hop forward and dislodge more. In this way a moving curtain of sand travels close to the ground. Saltation is by far the most important mode of transport in a desert, accounting for the bulk of the moving sand, and because it is confined to within about a metre of the ground, it explains why abrasion is concentrated near the base of rocks and why the mushroom rock and the zeugen are undercut. The word comes from the Latin saltare, to jump.

Surface creep or traction is the rolling and sliding along the ground of the coarser sand grains, larger than about 1 mm, that are too heavy to be lifted. They are pushed by the wind and by the impact of saltating grains striking them from behind, and they move slowly, a few centimetres at a time. Roughly a quarter of the sand moved by a strong wind travels by creep.

The quantity of sand a wind can carry increases very rapidly with its speed, roughly as the cube of the velocity; a wind of 30 km per hour moves about eight times as much sand as one of 15 km per hour. Sand begins to move at about 15 to 20 km per hour, and above 40 km per hour the desert surface is in motion. Because the wind's power increases so steeply, most of the transport is done by a few strong windy days in the year rather than by the steady light breezes. The sand is deposited when the wind slackens, when it meets an obstacle such as a bush or a rock, or when it blows over a surface of damp ground or vegetation that traps the grains. These are the conditions of dune formation, described next.

A sand dune is a hill or ridge of sand piled up by the wind. Dunes are the chief depositional landform of deserts and also occur along sandy coasts, where they are built by on-shore winds. In the great sand deserts, called ergs in the Sahara, dunes cover thousands of square kilometres in fields called sand seas; in the Thar, dunes cover roughly half the surface of Jaisalmer, Barmer and Bikaner districts.

A dune begins wherever the wind, carrying sand by saltation, meets an obstacle that slows it, a bush, a rock, a dead animal, a slight rise of the ground. The wind speed drops on the lee side of the obstacle and the sand is deposited there. The heap itself is then an obstacle, so more sand collects and the dune grows. Once formed, a dune has a characteristic shape: a gentle windward slope of about 10 to 15 degrees up which the sand is blown, and a steep leeward slope or slip face of about 30 to 34 degrees, the angle of rest of dry sand, down which the sand slides after passing over the crest. Because sand is continually blown up the windward side and slides down the lee side, the whole dune slowly migrates downwind; small dunes may move 20 to 30 m a year, and their migration can bury roads, railways, fields and villages, which is why the shifting sands of the Thar are checked by planting grasses and shelter belts.

The barchan or barkhan is the classic desert dune, crescent-shaped in plan, with its convex side facing the wind and its two horns pointing downwind. It forms where the wind blows steadily from one direction, the supply of sand is limited and the ground is hard and flat, so that the dune is a separate heap on a bare floor. The sand at the two edges of the heap, being thinner, is moved faster by the wind than the thick sand in the middle, so the edges run ahead of the centre and become the horns; the centre lags behind, and the whole takes the form of a crescent. The windward face is gentle and convex; the leeward face inside the crescent is the steep slip face. Barchans range from a few metres to about 30 m in height and up to 400 m across between the horns, and they migrate steadily downwind, faster when small. They occur singly or in groups, sometimes in long chains, in the Sahara, the deserts of Turkestan (where the word originated), Arabia, Peru and the Thar, where they are well developed around Jaisalmer and Bikaner. Rarely, when the wind reverses, the horns may face upwind, and the dune becomes a parabolic dune, described in the next topic.

A barchan diagram should show, in plan, the crescent with the wind arrow striking the convex side and the horns pointing away; and in section, the gentle windward slope, the crest and the steep slip face on the lee.

Dunes take several forms besides the barchan, depending on the wind regime, the supply of sand and the vegetation.

A seif dune or longitudinal dune is a long, narrow, sharp-crested ridge of sand running parallel to the direction of the prevailing wind, often for many kilometres and sometimes for over 100 km, with a height of tens of metres and occasionally over 200 m in the Sahara and the Rub al Khali. The word seif is Arabic for sword, from the ridge's knife-edged crest. Seifs form where the wind blows from two directions that alternate, a dominant wind and a cross wind from one side; one explanation is that the cross wind extends one horn of a barchan into a long ridge, which then grows along the resultant direction, while the other horn is starved. Between the parallel seif ridges lie long corridors of bare rock or gravel called gassi, along which caravans travel. Seif dunes are dominant in the Sahara, in Arabia, in the Simpson desert of Australia, where the parallel red ridges run for hundreds of kilometres, and in parts of the Thar in Rajasthan.

A transverse dune is a long ridge of sand lying at right angles to the wind, with a gentle windward and a steep lee slope, formed where the wind blows steadily from one direction and the supply of sand is very large, so that the barchans coalesce sideways into a wave-like ridge. Transverse dunes are typical of sand seas and of coastal belts; the coastal dunes at Digha and along the Odisha and Tamil Nadu coasts are of this kind.

A parabolic dune is U-shaped in plan, like a barchan turned round: its horns point upwind and its nose points downwind. It forms where partial vegetation anchors the arms of the dune while the bare centre blows forward. Parabolic dunes are common in semi-arid areas and on coasts, and are the commonest dune type in the Thar, where scrub vegetation is present.

Star dunes are large pyramid-shaped mounds with several arms radiating from a central peak, formed where the wind blows from many directions; they do not migrate but grow upward, and reach heights of 300 m in the Sahara and the Badain Jaran desert of China. A whaleback is a broad, rounded, elongated ridge of coarse sand on which smaller dunes ride.

Dunes are also classified as active or live dunes, which are bare and moving, and fixed or stabilised dunes, held in place by vegetation. Fixing the sand by planting grasses, shrubs and trees is the chief method of controlling desertification, used along the Indira Gandhi canal and in the Rajasthan desert afforestation programmes. Dune sands are porous and store rain water, and ancient dunes buried under later deposits form good aquifers.

Not all the material moved by the wind stays in the desert. The finest particles, silt and dust, are carried in suspension far beyond the desert margins and deposited wherever the wind slackens or rain washes them down. Where such wind-blown dust has accumulated over thousands of years into a thick blanket, the deposit is called loess, a German word (from lösen, loose) first applied to the deposits of the Rhine valley.

Loess is a fine, yellowish or buff-coloured, unstratified deposit of silt-sized particles, mainly quartz with some feldspar, mica and calcium carbonate. It is porous and permeable, so it absorbs water readily, and yet, because its angular grains interlock and are cemented by lime, it stands in vertical cliffs when cut by streams or roads. It contains no layers, since it was dropped from the air grain by grain, and often preserves the vertical tubes left by grass roots. Loess is extremely fertile, because it is fine, rich in minerals and easily worked, and loess regions are among the world's great grain lands.

The largest loess region in the world is the Loess Plateau of northern China, in the middle basin of the Hwang Ho (Yellow River), covering some 6 lakh square kilometres in the provinces of Shanxi, Shaanxi and Gansu, with deposits up to 300 m thick. The dust was carried by winds from the Gobi and the deserts of central Asia over hundreds of thousands of years. The Hwang Ho cuts into this loess and carries away enormous quantities of yellow silt, which give the river and the Yellow Sea their names and their colour; the silt is deposited downstream to build the North China plain and to raise the river bed above the surrounding land, causing catastrophic floods. On the plateau, people have for centuries cut cave dwellings into the vertical loess cliffs, which are cool in summer and warm in winter. The loess is also easily gullied by rain, and the plateau is one of the most severely eroded regions on earth.

Other loess deposits occur in the Mississippi valley of the United States and the Pampas of Argentina, derived from glacial outwash blown by winds at the end of the Ice Age, and in Ukraine, central Europe and the Rhine valley, where they support rich black soils. The loess of Europe and North America is thus of glacial origin, whereas that of China is of desert origin. In India, thin deposits of wind-blown dust from the Thar are spread over the Punjab, Haryana and the western Ganga plain and add to the fertility of the alluvium, but no thick loess plateau exists.

Loess is important for the examination as the only depositional landform of wind that lies outside the desert, as the material that made the Hwang Ho the Yellow River, and as an illustration of the great distances over which wind can carry its finest load.

Rain in deserts is rare, but it is not absent, and when it comes it usually falls in short, violent downpours on ground that has no vegetation to slow the run-off and is too hard-baked to absorb it. The result is a sudden flash flood that sweeps across the surface, carrying sand, gravel and boulders, and dies away within hours as the water sinks into the ground or evaporates. Such floods do more erosional and depositional work in a few hours than the wind does in years, and the larger features of the desert landscape are the joint work of wind and water. This topic and the next describe them in the order in which they occur from the mountains to the basin.

A wadi is a dry desert valley or stream channel that carries water only during and immediately after a rainstorm and lies dry for the rest of the year. It is cut by the flash floods, which erode powerfully because of their speed and their load of debris; the channel has steep sides, a flat gravelly floor and no permanent stream. Wadis are the Arabic name used in the Sahara and Arabia; in the American deserts the same feature is called an arroyo, and in the Thar the dry beds of seasonal streams such as the tributaries of the Luni are the equivalent. Wadis are dangerous places to camp, because a storm far upstream can send a wall of water down a dry channel under a clear sky. Between floods, the wind reworks the sand and silt left on the wadi floor. Wadis often end in the desert, their water lost by percolation and evaporation, without reaching the sea.

Where a wadi leaves the mountains and enters the basin, the flood spreads out, loses velocity and drops its coarse load in a cone-shaped heap, exactly as a mountain river does in a humid region. This is the desert alluvial fan. Because the floods are violent and carry boulders, desert fans are coarse and steep near the apex and grade to sand and silt at the foot. Fans are the most favourable sites for desert settlement, because flood water sinks into their gravels and can be reached by wells, and their gentle slopes can be irrigated; many of the oases at the foot of the Atlas, the Zagros and the Kunlun stand on fans.

Where many wadis issue from a mountain front side by side, their fans grow until they merge laterally into a continuous, gently sloping apron of alluvium along the whole foot of the mountains. This coalesced fan belt is called a bajada (Spanish, pronounced bahada), a term from the deserts of the south-western United States and Mexico. The bajada slopes gently from the mountain front towards the centre of the basin, and its surface, between floods, is reworked and partly veneered by the wind. It is the desert equivalent of the piedmont plain, and it forms the upper part of the depositional slope leading down to the playa.

Below the mountain front and the bajada the desert landscape continues down to the centre of the basin, and three more features complete it.

A pediment is a gently sloping surface of bare rock, or rock thinly covered with debris, that extends outward from the foot of a desert mountain or inselberg towards the basin. Its slope is only a few degrees, and it meets the steep mountain front at a sharp angle called the knick. The pediment is an erosional surface: it is cut across the bedrock by the sheet floods that pour off the mountain after rain, spreading out as a sheet of water that planes the rock, helped by wind abrasion and by the weathering of the rock beneath the thin debris cover. As the mountain front is worn back parallel to itself, the pediment grows wider at its expense, and when two pediments from opposite sides meet, the mountain between them is reduced to an inselberg and finally disappears. A surface made of merging pediments is a pediplain, the desert equivalent of the peneplain of a humid region. The pediment is often confused with the bajada, which lies just below it; the difference is that the pediment is cut in rock (erosional) while the bajada is built of alluvium (depositional), and the two together form a continuous slope from the mountain to the basin floor.

A playa is a flat, dry lake bed in the lowest part of a desert basin. After a rainstorm the flood water from the wadis and the bajada collects there as a shallow, temporary lake called a playa lake, which may be a few centimetres deep and many kilometres across. Within days or weeks the water evaporates, leaving a hard, flat, cracked floor of clay and silt and a crust of salts, chiefly common salt, gypsum and sodium carbonate, that were dissolved in the water. When salt is the main deposit the playa is called a salina or salt pan; the Sambhar Lake of Rajasthan, the largest inland salt lake of India, and the salt lakes of the Thar near Didwana and Pachpadra are playas from which salt has been harvested for centuries. The Rann of Kutch is a vast salt-encrusted flat of similar character, though of marine origin. In the United States the playas are called dry lakes or, when salty, alkali flats; the Bonneville Salt Flats of Utah are used for land speed records because of their perfectly level surface.

The whole assemblage, a basin of inland drainage surrounded by mountains, with pediments and bajadas sloping from the mountains to a playa at the centre, is called a bolson, a Spanish word for purse, since the basin is closed like a purse and its streams never reach the sea. The bolson is the typical landscape of the deserts of Nevada, Arizona and Mexico, of Iran, and of the interior basins of central Asia, and it shows every landform of this section in order: mountain and inselberg, pediment, bajada with fans, dunes reworked from the alluvium, wadis crossing the slope, and the playa at the bottom.

Thus the desert landscape, though it appears lifeless and unchanging, is a system in which wind and occasional water work together, the water cutting and carrying in a few violent hours, the wind sorting, sculpting and spreading the material through the long dry years between.

The Indian examples of wind landforms come from the Thar Desert or Great Indian Desert, which covers western Rajasthan, parts of Gujarat, Punjab and Haryana, and extends into the Sindh and Punjab provinces of Pakistan, an area of about 2 lakh square kilometres. It lies between the Aravalli range on the east and the Indus valley on the west, and its Indian districts include Jaisalmer, Barmer, Bikaner, Jodhpur, Churu and Nagaur. It is the most densely populated desert in the world.

The Thar receives 100 to 500 mm of rain a year, less than 150 mm in Jaisalmer, almost all of it from the south-west monsoon in July to September, and the rain is highly unreliable. Summer temperatures reach 45 to 50 °C and winter nights fall to near freezing, so the daily and annual range is extreme, and physical weathering by expansion and contraction is intense. From April to June the hot dry wind called the loo blows from the west and south-west, raising the andhi dust storms that carry Thar dust to Delhi and beyond. Vegetation is thin scrub of khejri, ber and thorny bushes, and much of the surface is bare.

The Thar shows both erosional and depositional forms of wind. Around Jaisalmer and Barmer the flat-lying sandstones and limestones have been carved into mushroom rocks, called chhatri, and small zeugen-like tables, and the rocky hills of the Jaisalmer plateau rise as inselbergs from the plain; deflation hollows around the base of the hills hold small oases and the traditional tanks of the desert towns. About half the surface, especially in the west, is covered with dunes: barchans and transverse dunes on the bare floors near Jaisalmer and Bikaner, and parabolic dunes with vegetated arms over most of the semi-arid east, some of them fixed and cultivated. The Luni is the only river of the desert; it rises in the Aravalli near Ajmer, flows south-west for about 500 km and dies in the Rann of Kutch without reaching the sea, its water salty in its lower course, and its tributaries are seasonal wadi-like streams. The salt lakes of Sambhar, Didwana, Pachpadra and Lunkaransar are playas that supply salt.

The Thar is also the scene of India's largest effort to control the work of wind. The Indira Gandhi Canal, bringing water from the Sutlej and Beas, has turned parts of Ganganagar, Hanumangarh and Bikaner into farmland, and its banks are protected against drifting sand by shelter belts of trees. Dune stabilisation by planting grasses such as sewan and shrubs, the Desert Development Programme and the network of village tanks are all attempts to fix the sand and stop the desert from advancing eastward, a process called desertification that is quickened by overgrazing, deforestation and the loo.

A West Bengal student should also know the coastal dunes of Digha, Mandarmani and Bakkhali, built by on-shore winds from the beach sand, as the nearest local example of the work of wind.

The three agents studied in this unit, river, glacier and wind, do the same three kinds of work but in different ways, and the examination often asks for their comparison.

In the Madhyamik examination, this section is tested as follows. One-mark items ask for terms: the process of removal of loose material by wind (deflation), the crescent-shaped dune (barchan), the sword-shaped dune (seif), the German name of mushroom rock (pilzfelsen), the wind-deposited yellow silt of China (loess), the dry lake in a desert basin (playa), the island-like hill of the desert (inselberg), the dry valley (wadi), the only river of the Thar (Luni), the hot wind of Rajasthan (loo). Two-mark questions ask for definitions of ventifact, yardang, zeugen, bajada, pediment, loess and playa. Three-mark questions ask for a formation with a diagram, most often the mushroom rock, the barchan and the inselberg, or for a difference, most often yardang versus zeugen, barchan versus seif, and pediment versus bajada. Five-mark questions ask for the erosional landforms of wind with diagrams, the depositional landforms of wind, the landforms of the combined action of wind and water, or the comparison of wind with river or glacier.

Answering guidance: always state that wind abrasion is concentrated near the ground because sand travels by saltation within about a metre of the surface, since this single fact explains the mushroom rock, the zeugen and the undercut base of the inselberg. Always mark the wind direction with an arrow in every diagram; a barchan or yardang drawn without a wind arrow is incomplete. Distinguish erosional from depositional forms clearly and give one example each from the Thar and one from the Sahara or China. Use the exact figures of the chapter: the 250 mm rainfall limit, the 30 to 34 degree slip face, the 300 m thickness of Chinese loess, the 134 m depth of the Qattara Depression. Remember that the combined work of wind and water, wadi, fan, bajada, pediment, playa and bolson, is part of this section and not of the river section, and that loess is the one wind deposit found outside deserts.