Arid and semi-arid landforms are the erosional and depositional forms shaped by wind and by rare, violent runoff in regions receiving under about 250 mm (arid) and 250–500 mm (semi-arid) of rain a year. Dune types, loess, playas and the arid cycle recur in Paper I, and the Thar and the Rann of Kachchh give the Indian examples examiners expect.

Each entry opens with a definition you can lift into an answer, then formation, examples and a sketch line. Terms are grouped by process: wind erosion, wind deposition, and water in the desert. UPSC asked the contrast between the normal and arid cycles of William Morris Davis in 2013, and the origin of the Thar in Paper II in 1997.

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Term	Meaning in one line	Example
Deflation	Wind lifting and removing loose, dry, fine particles	Qattara Depression, Egypt
Aeolian abrasion	Sand-laden wind scouring and polishing rock surfaces	Ventifact fields, McMurdo Dry Valleys, Antarctica
Blowout	Shallow hollow scooped by deflation in loose sediment	Blowouts at the heads of Thar parabolic dunes
Desert pavement	Surface mosaic of closely packed stones over finer sediment	Cima volcanic field, Mojave Desert, USA
Reg, hamada & erg	Stony plain, bare rock plateau and sand sea	Tanezrouft (reg); Grand Erg Oriental (erg), Sahara
Mushroom rock	Rock pillar undercut near its base into a pedestal	White Desert, Farafra, Egypt
Demoiselle	Soft-rock pillar protected by a resistant capstone	Fairy chimneys of Cappadocia, Turkey
Zeugen	Tabular ridge of hard caprock on undercut soft strata	Horizontally bedded plateaus of the Sahara
Yardang	Streamlined wind-cut ridge aligned parallel to the wind	Kaluts of the Lut Desert, Iran
Ventifact	Stone faceted and polished by wind-driven sand	McMurdo Dry Valleys, Antarctica
Sand dune	Mound of wind-blown sand with a windward slope and slip face	Thar Desert, Rajasthan
Barchan	Crescentic dune with horns pointing downwind	Pampa de la Joya, southern Peru
Transverse dune	Sand ridge at right angles to a unidirectional wind	Crescentic mega-dunes of the Taklimakan, China
Seif	Linear dune parallel to the resultant of two winds	Simpson Desert, Australia
Star dune	Pyramidal dune with three or more radiating arms	Badain Jaran Desert, China
Parabolic dune	U-shaped dune whose vegetated arms point upwind	Western Rajasthan, about half its area
Draa	Giant compound dune bearing smaller dunes on its flanks	Namib Sand Sea, Namibia
Nebkha & lunette	Shrub-anchored mound; crescentic dune on a lake’s lee shore	Walls of China lunette, Lake Mungo, Australia
Loess	Wind-laid, unstratified, buff silt blanket	Loess Plateau, China; Kashmir Valley
Wadi	Steep-walled, flat-floored channel dry for most of the year	Wadi Hadhramaut, Yemen
Bolson	Closed intermontane desert basin draining inward	Basin and Range Province, USA
Bajada	Apron of coalesced alluvial fans at a mountain foot	Panamint Range front, Death Valley
Playa, salina & sabkha	Dry lake floor; salt-crusted playa; coastal salt flat	Sambhar Lake; Salar de Uyuni; Abu Dhabi coast
Arid cycle of erosion	Davis’s desert variant of the cycle, ending in a desert plain with residual hills	Basin and Range Province, USA
Dhand, dhrian & rann	Thar playa; shifting dune; Kachchh salt flat	Didwana; western Thar; Great Rann of Kachchh

Wind Erosion: Processes and Landforms

Deflation

Deflation is the removal of loose, dry, unconsolidated particles of silt, clay and fine sand by wind, lowering the ground surface and sorting the sediment left behind. It is the aeolian equivalent of entrainment: it needs dry, bare, fine material and wind speeds above the threshold for lifting grains.

Mechanism: Fine grains are lifted into suspension (dust) or bounced along by saltation; coarser gravel is left behind as a lag.
Key features: Deflation stops where it reaches the water table or a coarse armour, because moist or stony surfaces resist lifting.
Examples: The Qattara Depression, Egypt, reaches about 133 m below sea level, excavated largely by salt weathering and deflation working together; in India, pre-monsoon dust storms over Rajasthan export deflated silt eastwards, and those of 2–3 May 2018 killed more than a hundred people across Rajasthan and Uttar Pradesh.
Significance / Hazard link: Deflation strips topsoil from ploughed semi-arid land, the core process of desertification on the Thar margins.

Aeolian Abrasion (Sandblasting)

Aeolian abrasion is the mechanical wearing of rock surfaces by sand and grit carried in the wind, which pits, flutes, grooves and polishes whatever stands in the path of saltating grains. It is wind corrasion, and it works only where wind carries tools of erosion.

Mechanism: Most saltating grains travel within about a metre of the ground, so abrasion is concentrated close to the surface and fades rapidly with height.
Key features: Pure wind without sand is almost harmless to solid rock; attrition then rounds and frosts the grains themselves.
Examples: Faceted boulders in the McMurdo Dry Valleys, Antarctica; fluted rock faces in the Lut Desert, Iran.
Don’t confuse with: Deflation removes loose grains; abrasion wears solid rock.

Blowout (Deflation Hollow)

A blowout is a saucer-, trough- or bowl-shaped depression hollowed out by deflation in loose sand or silt, usually where a break in the vegetation or a disturbance exposes bare sediment to the wind. Blowouts range from a few metres across to several kilometres.

Formation: Wind funnels into a gap, removes sand, enlarges the hollow and deposits the sand downwind as a rim or dune.
Key features: The floor is often limited by the water table or a coarse lag.
Examples: The pang kiang hollows of the Gobi, Mongolia; in the Thar, blowouts form at the upwind heads of parabolic dunes.
Sketch: Plan view of a hollow with an arrow for wind and a crescent of sand on its downwind side.

Desert Pavement

A desert pavement is a tightly interlocking surface layer of pebbles and cobbles, one or two stones thick, resting on a stone-poor layer of fine silt and clay. It armours the ground against further wind erosion and is one of the most extensive surface types in stony deserts.

Formation (classical view): Deflation winnows out fines, leaving a lag of coarse fragments.
Formation (current view): Research by Leslie D. McFadden and Stephen G. Wells in 1987 showed that wind-blown dust settles between and beneath the stones, so the stones are lifted on a growing layer of dust; cosmogenic exposure dating in 1995 found pavement stones on Cima lava flows, Mojave Desert, had stayed at the surface throughout, and were in effect “born at the surface”.
Key features: Stones often carry desert varnish.
Examples: Cima volcanic field, California; the serir of Libya.

Reg, Hamada and Erg

Reg, hamada and erg are the three surface types of a desert, named from Arabic: a reg is a gravel or stony plain, a hamada is a bare rocky plateau swept clean of loose debris, and an erg is a sand sea covered by dunes. Sand seas cover only about a fifth of the world’s deserts; rock and stone dominate the rest.

Reg (serir in Libya and Egypt): Formed by deflation of fines from alluvial or weathered gravels, often pavement-armoured; Tanezrouft, Algeria–Mali.
Hamada: Structural rock surfaces, often capped by resistant strata or duricrust; Hamada du Draa, Morocco–Algeria.
Erg: Sand accumulates where winds converge and slacken; the Rub’ al Khali of Arabia is the largest continuous sand sea, and the Grand Erg Oriental is one of the largest in the Sahara.
Indian context: The Thar is chiefly an erg with dunes, interrupted by rocky hills and gravel plains in its east and south.

Mushroom Rock (Pedestal Rock)

A mushroom rock, also called a pedestal rock, gara in the Sahara and pilzfelsen in German, is an isolated rock mass whose base has been cut back more rapidly than its top, leaving a broad cap on a narrow stem. It forms where erosion is concentrated close to the ground.

Formation: Sand-laden wind abrades the lowest metre or two most intensely, while the cap stands above the saltation layer.
Current view: Moisture and salt weathering are also concentrated at ground level, so the undercutting is usually a combined weathering and abrasion effect rather than sandblasting alone.
Examples: The chalk pedestals of the White Desert, Farafra, Egypt.
Sketch: A pillar with a wide cap and pinched base, with an arrow showing the zone of maximum abrasion near the ground.

Demoiselle

A demoiselle is a pillar of soft, erodible material such as tuff, clay or conglomerate that survives beneath a protective capstone of resistant rock, while the unprotected ground around it is lowered. Demoiselles are the result of differential erosion, not of one agent alone.

Formation: Rainwash and sheetwash strip the soft material everywhere except under the capstone, which shelters the column beneath it; wind contributes only in the driest settings.
Key features: The pillar survives only while the cap stays in place, and collapses once it falls.
Examples: The fairy chimneys of Cappadocia, Turkey; the demoiselles coiffées of the French Alps.
Don’t confuse with: A mushroom rock is undercut at its base; a demoiselle is protected from above.

Zeugen

Zeugen (singular zeuge) are tabular, flat-topped ridges of horizontally bedded rock in which a resistant caprock rests on softer strata, separated from one another by wind-scoured furrows. They form where hard and soft layers lie horizontally and are broken by vertical joints.

Formation: Weathering, including salt and thermal stress, opens the joints; sand-laden wind abrades the soft layers beneath the cap and deflation removes the debris, deepening the furrows until isolated tabular blocks remain.
Key features: Flat tops mark the caprock; the cap overhangs the undercut soft base; ridges run along the dominant joint set.
Examples: Horizontally bedded sandstone–shale plateaus of the Sahara and Arabian deserts.
Don’t confuse with: Yardangs are cut in soft sediments and aligned with the wind; zeugen depend on horizontal hard-over-soft layering and joint pattern.

Yardang

A yardang is a streamlined, elongated ridge carved by wind abrasion and deflation from soft or weakly consolidated material, aligned parallel to a strong, unidirectional wind, with a steep blunt face upwind and a lower, tapering tail downwind. The Swedish explorer Sven Anders Hedin brought the Turkic term, meaning “steep bank”, into English in 1903.

Formation: Wind cuts corridors along lines of weakness in lake beds or soft rock, leaving the intervening ridges.
Key features: Usually at least three times as long as they are wide; from centimetre-scale forms to mega-yardangs several kilometres long and more than 100 m high.
Examples: The kaluts of the Lut Desert, Iran; the Qaidam Basin, China; yardang fields on Mars.
Sketch: Plan view of parallel boat-shaped ridges with a wind arrow along their long axes.

Ventifact (Dreikanter)

A ventifact is a stone or boulder lying on the desert surface that has been abraded by wind-driven sand into one or more flat, polished, sharp-edged faces, called facets. A ventifact with one facet is an einkanter; one with three facets meeting at sharp edges is a dreikanter.

Formation: Each facet grows at right angles to a prevailing wind; a change in wind direction, or a stone rolling over, produces a new facet.
Significance: Facet orientation records palaeowind direction, so fossil ventifacts are used to reconstruct ancient wind regimes.
Examples: McMurdo Dry Valleys, Antarctica; Mojave Desert, California.
Sketch: A pebble with three flat faces meeting along sharp ridges, arrows showing the winds that cut each face.

Wind Deposition: Dunes and Loess

Sand Dune (Formation)

A sand dune is a mound or ridge of wind-blown sand, typically with a gentle windward slope of about 5–15° and a steep lee slip face standing at the angle of repose of dry sand, about 32–34°. Dunes form wherever sand supply, wind strong enough to move sand, and a site that slows the wind coincide.

Formation: Saltating grains pile up behind an obstacle or on a patch of rough ground; the growing mound itself deflects the airflow, grains climb the windward slope, and avalanche down the slip face.
Migration: Repeated climbing and avalanching moves the whole dune downwind; small crescentic dunes can advance more than 100 m a year.
Types (control): Dune shape depends chiefly on wind regime, sand supply and vegetation. Ralph Alger Bagnold (1941) recognised barchans and seifs as the basic forms; Edwin Dinwiddie McKee (1979) classified dunes by the number and orientation of slip faces.
Wind regime rule: unidirectional wind gives barchans and transverse dunes, bidirectional wind gives seifs, multidirectional wind gives star dunes, and vegetation with a steady wind gives parabolic dunes.
Sketch: Cross-section with gentle windward slope, crest, slip face and internal cross-bedding dipping downwind.

Barchan

A barchan is an isolated crescent-shaped dune whose horns point downwind, with a gentle convex windward slope and a steep, concave slip face between the horns. It forms under a steady, unidirectional wind where sand is limited and the desert floor is hard.

Formation: Sand moves faster round the lower flanks than over the higher centre, so the flanks run ahead as horns.
Key features: Barchans are the most mobile dunes; their speed varies inversely with height, and they form chains when sand supply rises.
Examples: Pampa de la Joya, southern Peru; Kharga Depression, Egypt; barchans and barchanoids occur among the younger, mobile dunes of western Rajasthan.
Sketch: Plan view of a crescent, horns downwind, arrow for wind, slip face shaded.
Don’t confuse with: In a parabolic dune the arms point upwind, the reverse of a barchan.

Transverse Dune

A transverse dune is a long, wavy sand ridge lying at right angles to a unidirectional wind, with a single slip face on its lee side. It forms where sand is abundant enough to cover the floor, so individual barchans merge laterally into continuous ridges.

Formation: With more sand, barchans link horn to horn into barchanoid ridges and then into straighter transverse ridges.
Key features: Ridges are separated by troughs; crestlines sinuous; they are common on coasts and on sandy river beds.
Examples: The crescentic mega-dunes of the Taklimakan Desert, China; transverse dunes are recorded in the Thar.
Sketch: Parallel ridges across a wind arrow, slip faces on the lee side.

Seif (Longitudinal Dune)

A seif, from the Arabic for sword, is a long, narrow, sharp-crested linear dune aligned roughly parallel to the resultant direction of two winds that blow from different directions, with slip faces that alternate from one side to the other. Linear dunes may run for more than 160 km.

Formation: Winds from two directions each build a slip face on one flank, and the dune extends along its axis.
Key features: Parallel ridges separated by sand-free corridors, called gassi in the Sahara, used as caravan routes.
Examples: Simpson Desert, Australia; Namib Sand Sea; linear dunes of the western Thar, many of them now vegetated forms built in the late Pleistocene.
Sketch: Parallel linear ridges between two converging wind arrows.

Star Dune

A star dune is a pyramidal sand mountain with a high central peak and three or more sinuous arms radiating outwards, each carrying its own slip face. It forms where winds blow from several directions over the year, so the dune grows upwards instead of migrating.

Formation: Winds from changing directions reverse slip faces again and again, piling sand at the centre.
Key features: Star dunes are the tallest dunes on Earth, reaching about 500 m in the Badain Jaran Desert, China. A 2024 study dated the base of the Lala Lallia star dune in Morocco’s Erg Chebbi, about 100 m high, to about 13,000 years, although the dune itself built up within the last millennium.
Examples: Grand Erg Oriental, Algeria; linked star dunes in the Thar.
Sketch: Plan view of a peak with three to five arms, arrows from several directions.

Parabolic Dune

A parabolic dune is a U- or V-shaped dune whose curved nose points downwind and whose long trailing arms, anchored by vegetation, point upwind. It forms where a steady wind blows over sand that is partly held by grass and shrubs, usually starting from a blowout.

Formation: Wind breaks through the vegetation, deflates a blowout and pushes the bare centre forward, while plants pin down the flanks.
Key features: Arms point upwind, the opposite of a barchan; common on coasts and on semi-arid desert margins.
Examples: Amal Kar (1996) found parabolic dunes to be the most widespread dune type in the Thar, covering about half of western Rajasthan; coastal parabolic dunes are common around the world’s sandy coasts.
Sketch: Plan view of a hairpin with arms upwind and a blowout between them.

Draa

A draa is a mega-dune: a very large compound or complex sand ridge, often hundreds of metres high and spaced more than half a kilometre apart, whose flanks carry smaller dunes superimposed on them. Draas are the largest bedforms in major sand seas.

Formation: Draas record long-lived wind regimes; the smaller dunes on them respond to present winds, the draa itself to winds averaged over millennia.
Key features: Linear draas with broad, rounded crests are called whalebacks; complex draas mix types, such as star dunes on linear ridges.
Examples: Namib Sand Sea, Namibia; Rub’ al Khali, Arabia.
Don’t confuse with: A draa is a whole landform on which ordinary dunes ride, just as ripples ride on a dune; the three form a hierarchy of bedforms.

Nebkha and Lunette

A nebkha (coppice dune) is a small mound of sand trapped around and behind a shrub or tussock, while a lunette is a crescentic dune built on the downwind shore of a lake or playa from sand, silt and clay pellets blown off its dry floor. Both are obstacle-controlled dunes.

Nebkha formation: The plant slows the wind; sand accumulates and the plant grows up through it; nebkhas are typically a few metres high.
Lunette formation: When the lake dries seasonally, pellets of salty clay and sand are deflated and heaped on the lee shore, concave towards the basin.
Examples: Nebkhas are part of the younger dune system of the Thar; the Walls of China lunette at Lake Mungo, New South Wales, Australia.
Significance: Lunettes preserve lake-level and palaeoclimate records.

Loess

Loess is a thick, homogeneous, unstratified, buff to yellowish deposit of wind-blown silt, dominantly quartz grains about 0.01–0.05 mm across with some clay and carbonate, that blankets pre-existing relief far from its source. It is the most widespread wind-laid sediment on Earth.

Formation: Silt is deflated from deserts or from glacial outwash and river floodplains, carried in suspension, and trapped by vegetation downwind.
Types: Desert loess comes from arid basins, as in China; glacial loess comes from the outwash of Pleistocene ice sheets, as in Europe and the Mississippi valley, where the silt was winnowed from outwash plains.
Key features: Stands in vertical cliffs because of calcareous cementation; collapses when saturated; gullies readily, giving badland topography.
Examples: The Loess Plateau of China covers about 400,000 km², with loess typically 50–80 m thick and far thicker near Lanzhou, and its erosion gives the Huang He one of the highest sediment loads of any river; European loess is called limon in France and Belgium; in India, loess mantles the karewa terraces of the Kashmir Valley.
Significance: Loess weathers into fertile soils; loess–palaeosol sequences are among the longest continental climate records.

Water in the Desert and the Arid Cycle

Wadi

A wadi is a steep-sided, flat-floored desert valley or channel, normally dry, that carries water only briefly after intense rainstorms, when flash floods sweep coarse debris down it. The term is Arabic; arroyo is the equivalent in the American south-west.

Formation: Rare, violent floods with high sediment loads cut vertically into unvegetated ground and dump gravel when they spread out.
Key features: Braided gravel floors, abrupt banks, and channels that end in a basin rather than the sea.
Examples: Wadi Hadhramaut, Yemen; in India, the Luni and its tributaries behave like wadis, flowing briefly after monsoon storms before drying out towards the Rann of Kachchh.
Hazard link: Flash floods in wadis are among the deadliest desert hazards.

Bolson

A bolson is an enclosed intermontane basin in an arid or semi-arid region, bordered by mountain ranges and drained internally towards a central playa, so that its floor acts as a local base level. The term is Spanish, meaning “purse”, and comes from the deserts of Mexico and the south-western United States.

Formation: Typically a fault-bounded basin between block mountains, filled by debris from the surrounding ranges.
Key features: From the mountain front inwards: pediment, bajada, and playa, each a zone in the Davisian arid cycle.
Examples: The Basin and Range Province, Nevada and Utah; Death Valley, California.
Sketch: Cross-section: mountain front, pediment, bajada, playa, bajada, mountain front.

Bajada

A bajada is a broad, gently sloping depositional apron formed by the lateral coalescence of alluvial fans along an arid mountain front, lying between the rock-cut pediment above and the playa below. It is wholly depositional, unlike the erosional pediment.

Formation: Ephemeral streams leaving the mountain front lose velocity and water by infiltration, dumping debris as individual alluvial fans that grow sideways until they merge.
Key features: Slopes are a few degrees near the fan heads and flatten to almost level at the toe; sediments fine downslope, from boulders to sand and clay.
Examples: The bajadas below the Panamint Range, Death Valley, California.
Sketch: Several fans from canyon mouths merging into one continuous apron.

Playa, Salina and Sabkha

A playa is the flat, vegetation-free floor of an inland desert basin, briefly covered by a shallow lake after rain and then dried by evaporation; a salina is a playa crusted with salts; a sabkha is a coastal salt flat just above normal high tide, built of evaporites and wind-blown sediment. All three are among the flattest natural surfaces on Earth.

Formation: Runoff carries clay and dissolved salts into the basin centre; evaporation leaves mud cracks, gypsum, halite and sodium carbonate.
Local names: Shott in the Sahara, khabari and mamlaha in Arabia, salar in the Andes.
Examples: Lop Nur, Tarim Basin; Salar de Uyuni, Bolivia, the largest salt flat; coastal sabkhas of Abu Dhabi; in India, Sambhar Lake, Rajasthan, is a saline playa lake and the country’s largest inland salt lake.
Don’t confuse with: A playa is inland and fed by runoff; a sabkha is coastal and fed partly by seawater.

Arid Cycle of Erosion

The arid cycle of erosion is the desert variant of the geographical cycle proposed by William Morris Davis in 1905, in which a block-faulted, mountain-girt arid region is worn down through youth, maturity and old age by sheetfloods, ephemeral streams and wind, ending in a low desert plain studded with residual hills. It is a climatic modification of his humid normal cycle, not a separate theory.

Stages

Initial stage: Uplifted block mountains enclose separate basins (bolsons); each drains inwards to its own playa, which serves as a local base level.
Youth: Mountains are dissected by rills and gullies; fans and bajadas grow; basins fill and relief falls rapidly.
Maturity: Divides are breached and higher basins are captured by lower ones, so drainage becomes integrated; pediments widen as mountain fronts retreat.
Old age: Mountains are reduced to isolated inselbergs rising over a desert plain; wind becomes the dominant agent and deflation lowers the surface, a lowering unrelated to sea level and limited only by the water table.

Normal cycle and arid cycle compared

Basis	Normal (humid) cycle	Arid cycle
Base level	Sea level, a single ultimate base	Local basin floors (playas); later the water table limits deflation
Drainage	External to the sea; integration increases towards maturity	Internal (centripetal) at first, integrated in maturity, disrupted again in old age
Dominant agent	Perennial running water	Sheetfloods and ephemeral streams; wind dominant in old age
Weathering	Chemical weathering under vegetation	Mechanical weathering on bare rock
Slope development	Slope decline by downwearing	Mountain-front retreat leaving pediments
End-form	Peneplain with monadnocks	Desert plain or pediplain with inselbergs

Evaluation: Davis’s model fits basin-and-range deserts but not open, low-lying shield deserts such as the Sahara or the Thar; Lester Charles King went further, arguing that scarp retreat and pedimentation, not downwearing to a peneplain, are the normal mode of landscape development.
Current view: Many desert forms are relict from wetter or windier Quaternary phases; the linear dunes of the Thar, for instance, were built mainly in the late Pleistocene, so present landscapes mix inherited and active forms.
Sketch: Four cross-sections showing basin, youth, maturity and old age, with bajada, pediment, playa and inselbergs labelled.

UPSC 2013: “Write short note: Differences between Normal cycle and Arid cycle of Davis.” — Read the model answer

Thar and Kachchh Vocabulary

Dhand, Dhrian and Rann

Dhand, dhrian and rann are regional terms for Indian desert landforms: a dhand is a shallow, often saline playa lake or depression in the Thar that fills briefly in the monsoon, a dhrian is a shifting, mobile sand dune of the western Thar, and a rann is a vast saline mud flat, above all the Rann of Kachchh in Gujarat.

Dhand: Playa-type depressions between dunes, some fed by saline groundwater and worked for salt; Didwana, Pachpadra and Lunkaransar are salt lakes of this kind.
Dhrian: The active, unvegetated dunes that advance over fields and roads, in contrast with the older, stabilised parabolic and linear dunes.
Rann: The Great Rann of Kachchh, about 7,500 km² at an average height of about 15 m above sea level, was a shallow arm of the Arabian Sea until uplift cut it off; today it is a salt flat in the dry season and a shallow wetland in the monsoon, drained by the Luni, Rupen and West Banas, with raised islands called bets.
Tectonic link: The 1819 Kachchh earthquake raised the Allah Bund scarp across the Rann.
Significance: The Little Rann of Kachchh, the salt-pan belt of Gujarat, is the last home of the Indian wild ass.

PYQs Built on These Terms

Write short note: Differences between Normal cycle and Arid cycle of Davis. (2013)
Name the climatically controlled agents of erosion. Explain how they differ in terms of properties of matter. Compare the landforms produced by each one of them. (2011)
Discuss the origin and main geomorphic features of Thar Desert of India. (Paper II, 1997)

Frequently Asked Questions

What is the difference between a barchan and a parabolic dune?

Their horns point in opposite directions. A barchan’s horns point downwind because its flanks, carrying less sand, move faster than its centre; it needs a bare floor and limited sand. A parabolic dune’s arms point upwind because vegetation pins them while the bare nose advances. Barchans mark true deserts; parabolic dunes mark semi-arid or coastal sand held by plants, as across much of western Rajasthan.

How are star dunes formed?

Star dunes form where winds blow from three or more directions during the year. Each wind builds a slip face, and the next wind reverses it, so sand piles up at the centre instead of travelling downwind. The dune grows vertically, sending out radiating arms, which is why star dunes, reaching about 500 m in China’s Badain Jaran Desert, are the tallest on Earth.

What is the difference between reg, hamada and erg?

They are the three desert surfaces. A reg is a stony or gravel plain, often armoured by desert pavement; a hamada is bare rock plateau swept clean of debris; an erg is a sand sea of dunes. Most desert area is rock or stone, not sand: ergs cover only about a fifth of the world’s deserts, although the Thar is largely sandy.

What is loess and where is it found in India?

Loess is wind-blown silt, unstratified and buff-coloured, deposited downwind of deserts or glacial outwash plains. The largest deposits are on the Loess Plateau of China. In India, loess mantles the karewa terraces of the Kashmir Valley, where loess–palaeosol layers record Quaternary climate change. Loess forms fertile soil but gullies rapidly once vegetation is removed.

What are dhands in the Thar Desert?

Dhands are shallow playa lakes and saline depressions between the dunes of the Thar. They hold water briefly after monsoon rain and then dry, leaving salt crusts; several, such as Didwana and Pachpadra, are worked for salt. They are the Indian equivalent of the playas and salinas of other deserts, and belong to the fluvial part of arid geomorphology.

Why is wind considered a weak agent of erosion?

Air is about 800 times less dense than water, so wind can lift only sand and finer grains, and only within about a metre of the ground. It cannot cut valleys or plane mountains. Wind is dominant only in forming dunes, loess, pavements and small abraded forms; most large desert landforms, including pediments and inselbergs, are the work of water acting on weathered rock.