Desert Landforms: Erosional and Depositional

• Desert landforms are the erosional and depositional features shaped in arid and semi-arid lands by wind (aeolian processes), mechanical weathering and occasional running water.
  • Sparse vegetation leaves dry, loose material exposed, so wind works here as nowhere else, but sheetfloods and ephemeral streams still do much of the heavy work.
• Wind is a weaker agent than rivers or waves because air is far less dense than water; it lifts only fine grains and abrades only close to the ground.
• In India the Thar is the hot-desert laboratory and Ladakh the cold-desert one.

Desert Environments and Processes

Meaning and Extent of Deserts

• A desert is a barren or thinly vegetated land where precipitation is low and highly variable and evaporation far exceeds it.
  • True deserts get less than 250 mm of rain a year (often about 100 mm, with rainless years); semi-arid lands get about 250–500 mm.
  • Other traits: large daily and annual temperature ranges, strong winds, dust storms and rare torrential rainstorms.
• Extremely arid and arid lands cover about one-fifth of the land surface; with semi-arid lands, about one-third.
  • Adding dry sub-humid land, drylands cover 40.6% of land (excluding Antarctica); a UN Convention to Combat Desertification assessment (COP16, Riyadh, December 2024) found that 77.6% of land became drier over 1990–2020 than in the previous 30 years.
• Cold deserts (polar lands, high plateaus such as Ladakh) are dry too, but aeolian work is limited where the ground is frozen.
• Five great desert provinces: Sahara–Central Asia (Sahara, Arabian, Thar, Kara Kum, Taklamakan, Gobi); Southern Africa (Namib, Kalahari, Karoo); South America (Atacama, Patagonia); North America (Mojave, Sonoran); Australia.
• The aridity index (AI) is mean annual precipitation divided by potential evapotranspiration (P/PET); the UNEP World Atlas of Desertification (1992) uses it to grade drylands.
  • Hyper-arid below 0.05; arid 0.05–0.20; semi-arid 0.20–0.50; dry sub-humid 0.50–0.65.
  • India’s hot arid zone lies mostly in western Rajasthan and Kutch.

Causes of Aridity and Types of Desert

• Most hot deserts lie between 15° and 30° N and S in the trade-wind belt, on the western margins of continents.
  • Subsiding, warming air of the subtropical highs suppresses cloud; offshore trade winds and cold currents (Humboldt, Benguela, Canary) cool the lower air, so fog forms but rain does not.

• Desert surfaces are classified by ground cover; sand covers only about a fifth of the world’s deserts.

• Ergs are fed by ephemeral river beds, older fluvial and coastal sands, older dunes and weathered sandstone.
  • The Rub’ al Khali (about 650,000 km²) is the largest continuous sand sea; stony regs are easier to cross, so caravan routes favour them.

Weathering in Deserts

• Mechanical weathering prepares material for wind and water.
  • Thermal stress: daytime heating and rapid night cooling make the outer shell of a rock expand and contract more than its interior, so it peels (exfoliation) or crumbles grain by grain.
  • Frost wedging: in high or continental deserts, water in cracks freezes at night and expands by about 9%, prising off fragments that gather as scree.
  • Salt weathering: salts drawn up by evaporation crystallise in pores and joints and wedge rock apart; it is among the most effective desert processes.
• Chemical weathering is slow but real: dew and rare rain alter minerals and weaken rock.
  • Heating experiments by David Tressel Griggs (1936) showed that dry heating alone rarely shatters rock; moisture and salt are needed, revising the older insolation-only view.
• Tafoni are cavernous, honeycomb hollows in sandstone and granite faces, enlarged mainly by salt weathering while wind clears the loosened grains.
• Desert (rock) varnish is a thin, dark coating of clay cemented by manganese and iron oxides on long-exposed stones and cliffs; it is darker where manganese is richer.
  • It builds up over thousands of years, probably with help from microbes; petroglyphs are made by pecking through it to the pale rock beneath.
• Duricrusts are hard crusts formed where dissolved minerals are precipitated in or on the soil, by downward leaching or by capillary rise and evaporation.
  • They are named by the dominant cement: calcrete (calcium carbonate), gypcrete (gypsum), salcrete (salt), silcrete (silica), ferricrete (iron) and alcrete (aluminium).
  • Calcrete, gypcrete and salcrete mark arid lands; ferricrete and alcrete belong mainly to seasonally humid tropics.
  • Calcrete (kankar) is widespread in the Thar, cementing older dunes and alluvium.

Erosional Work of Wind

• Controls: wind velocity and turbulence, the amount and size of tools (sand, grit), rock composition and structure, vegetation cover and moisture.
• Abrasion is strongest about 20–25 cm above the ground and fades out by roughly 1.5–2 m: at the ground wind is slow, higher up it carries little sand.
  • Hence wooden telegraph poles in deserts were sheathed with metal near the base. Wind also attacks from all sides, since its direction varies.
• Deflation is the lifting and blowing away of loose, dry particles.
  • It lowers the land into blowouts, down to the moist ground near the water table.
  • By removing fines and leaving gravel it creates desert pavement, an armour of packed stones.
• Abrasion (sandblasting): wind hurls sand against rock, pitting, fluting, grooving and polishing it; upstanding rocks are undercut near the base.
• Attrition: wind-borne grains collide and wear one another into small, well-rounded, frosted “millet-seed” sand.

Transport by Wind

• Saltating grains rarely rise above a metre over sand, which is why abrasion hugs the ground; shifting winds make transport multidirectional.

Landforms of Wind Erosion

• Wind carves distinctive but mostly small forms; large features such as inselbergs and pediments owe more to weathering and running water.

Mushroom Rocks (Rock Pedestals)

• Mushroom rocks have a broad top on a narrow neck; they are also called pedestal rocks, gara (Sahara) and pilzfelsen (German).
  • Sandblasting grooves the softer bands, and because abrasion peaks just above the ground and comes from shifting directions, the base is undercut all round.
• Demoiselles are pillars of soft rock protected by a resistant capstone; they survive only while the cap stays in place.

Mesas and Buttes

• A mesa (Spanish, “table”) is a flat-topped upland with a resistant horizontal caprock and steep sides; retreat of the sides reduces it to a smaller isolated hill, a butte, and finally a pinnacle.
  • Classic examples stand in Monument Valley (Arizona–Utah); the flat-topped hills of the Deccan Traps are similar caprock forms.
  • These are structural forms shaped mainly by weathering and running water, with wind adding finishing touches.

Zeugen

• Zeugen are tabular ridges of hard rock on pedestals of softer rock (shale, mudstone) where layers lie horizontally.
  • Mechanical weathering, often night-time freeze–thaw, opens joints in the hard cap; abrasion then cuts deep furrows into the soft layer and deflation clears the debris.
  • The caps stand as ridges roughly 3–30 m high until continued abrasion lowers them and widens the furrows.

Yardangs

• Yardangs are steep-sided, streamlined ridges separated by parallel corridors, formed where hard and soft bands stand vertically and are aligned with a steady wind.
  • The term was introduced by Sven Hedin (1903) from a Turkic word for “steep bank”; in Iran they are kaluts.
  • Abrasion scoops out the soft bands and deflation clears the corridors; ridges are blunt upwind and taper downwind like an upturned hull.
  • Their hallmark is parallelism. They range from centimetre-scale forms to mega-yardangs kilometres long, as in the Lut Desert and around Tibesti, and occur on Mars.

Inselbergs

• Inselbergs (“island mountains”) are isolated residual hills rising abruptly from a plain, steep-sided and round-topped, usually of granite or gneiss.
  • The term was coined by Wilhelm Bornhardt (1900) in East Africa; bare dome-shaped ones are called bornhardts after him.
  • Siegfried Passarge (1904) and William Morris Davis read them as residuals of the late arid cycle.
  • Uluru (Australia) is the best-known example; the granite domes of the Ranchi plateau are Indian ones.
• Their origin is polygenetic, not aeolian: they also occur in savannas and humid tropics, explained by scarp retreat and pedimentation (Lester Charles King) or by deep weathering and stripping around resistant cores.

Ventifacts, Stone Lattice and Wind Windows

• Ventifacts are pebbles and boulders faceted and polished by sandblasting; a new facet forms whenever the wind shifts or the stone turns.
  • Stones with one, two or three facets are einkanter, zweikanter and dreikanter; packed together they form a desert pavement.
  • They also form in periglacial and coastal settings, and on Mars.
• Stone lattice is a honeycombed rock face left where abrasion removes weaker parts; deepened pits pierce the rock as wind windows, which widen into wind bridges (arches).

Deflation Hollows (Blowouts)

• Deflation hollows are basins lowered by removal of loose material; deflation stops near the water table, where water may form an oasis, marsh or salt flat.
  • Minor faulting or weathering can start a hollow; eddying winds enlarge it.
• Sizes range from the “buffalo wallows” of the American Great Plains and pang kiang hollows of the Gobi to the Qattara Depression of Egypt (floor about 133 m below sea level), deepened by salt weathering plus wind removal.
• Human-induced deflation: ploughing of the southern Great Plains of the USA stripped the grass, and 1930s droughts turned the topsoil into dust storms, the Dust Bowl.

Landforms of Wind Deposition

Dust and Sand Deposition

• Wind drops its load when its speed falls or it meets an obstacle, sorting material by size.
  • Dust travels vast distances: Saharan dust falls as “blood rain” in southern Europe, and NASA satellite estimates (2015) put about 27.7 million tonnes a year over the Amazon basin, supplying phosphorus to its soils.
  • Coarse sand rarely leaves the desert and stays as dunes and sheets.
• Sand shadows form beside fixed obstacles and sand drifts in gaps between them.
• Ripples are small wave-like ridges built by saltation impact, usually transverse to the wind.

Sand Dunes

• Sand dunes are hills or ridges of wind-blown sand, found in deserts and also on coasts, river banks and lake shores.
  • They are live (moving) or fixed (anchored by vegetation); star dunes in China’s Badain Jaran reach about 500 m.
• Requirements: abundant sand, winds strong enough to move it, traps or obstacles, and room to accumulate.
  • A heap trapped behind a shrub becomes an obstacle itself and grows, though many regular dunes form on open sand sheets without one; dunes occur in dune fields or chains.
• Profile: gentle windward slope (5°–15°) and steep slip face (about 30°–34°), the angle of repose of dry sand.
• Migration: grains saltate up the windward slope and avalanche down the slip face, so the dune creeps downwind at a few to tens of metres a year; small crescentic dunes in China’s Ningxia moved over 100 m a year in the 1950s.
  • Moving dunes bury fields, roads and oases; shelterbelts and sand-binding grasses fix them.

Classifications of Dunes

Types of Dunes

• Barchans are crescents with a convex, gentle windward face and a concave slip face between two horns that thin out downwind, where sand moves fastest past the edges.
  • They form where sand is limited over a hard floor under a constant wind; with more sand they merge into barchanoid ridges and transverse dunes.
  • Sven Hedin noted that a barchan advances steadily when supply is constant, slows as it grows with rising supply, and speeds up as supply dwindles.
• Parabolic dunes form where vegetation anchors the arms while the bare nose advances, so their arms point upwind, the reverse of a barchan; they usually start from a blowout.

• Seifs are long, sharp-crested ridges parallel to the prevailing wind, often hundreds of kilometres long, their crests rising and falling like a saw.
  • The main wind sweeps the sand-free corridors (gassi) used as caravan routes; cross-winds build the ridges higher and wider.
• Star dunes grow upward rather than forward: dating of the 100 m Lala Lallia dune in Morocco’s Erg Chebbi (2024) found a base about 13,000 years old, with its top 65 m built in the last millennium.
• Whalebacks are broad ridges of coarse sand left by migrating seifs; draas are giant compound dunes hundreds of metres high (Sahara, Namib); nebkhas form behind shrubs and lunettes on the downwind rims of playas.

• Topographic (obstacle) dunes form where hills and cliffs disturb the wind.
  • Echo dunes stand a short distance upwind of a steep cliff, kept apart from it by a reverse-flow eddy; climbing dunes ramp sand up a windward slope; falling dunes spill it down the lee side; lee (shadow) dunes trail downwind of a hill.

Dunes in India

• In the Thar, sand sheets and parabolic dunes dominate, with linear dunes, barchans (around Jaisalmer and Ramgarh) and transverse and star dunes in the drier west.
  • Many older dunes are partly stabilised; pre-monsoon winds reactivate bare sand every year and push it east through gaps in the Aravalli.
  • The Aravalli Green Wall (launched 2023), a planned 1,400 km long, 5 km wide belt across Gujarat, Rajasthan, Haryana and Delhi, aims to check this spread; the Desertification and Land Degradation Atlas (2021) put 29.77% of India under degradation, with wind erosion the leading process in western Rajasthan.
• Coastal dunes line Kutch, Odisha, Kerala and Tamil Nadu; riverine dunes flank the Ganga, Kosi, Godavari and Krishna; cold-desert dunes occur at Hunder in Ladakh’s Nubra valley.

Loess

• Loess is a thick, unstratified, buff-yellow deposit of wind-blown silt (mostly quartz, with feldspar, mica and carbonate), loose but coherent and highly porous.
  • The name comes from German Löss (“loose”); Karl Cäsar von Leonhard described the Rhine valley silts in 1823–24.
  • Angular, interlocking grains let it stand in vertical walls, yet it gullies fast when wet, giving badlands; its lime-rich loam is very fertile.
• Loess and similar deposits cover about 10% of the land, far from their desert, glacial-outwash or floodplain sources.

• The Loess Plateau covers about 640,000 km² around the middle Yellow River, with deposits well over 100 m thick.
  • Gullying fed the river about 1.6 billion tonnes of silt a year, raising its bed and levees and earning it the name “China’s sorrow”; terracing, check dams and the Grain for Green programme have cut this to roughly 0.3 billion tonnes.
• In India, loess caps the Karewa deposits of the Kashmir valley.

Fluvial Landforms of Deserts

• Water is often the chief sculptor of deserts: rain is rare but falls in violent bursts on bare ground.
  • Flash floods fill dry channels within hours and can become mudflows; sheetfloods spread thinly over gentle slopes; most streams end in inland drainage basins.
• Badlands: weak, bare rocks are cut into dense gullies and ravines separated by sharp ridges, as in the Chambal ravines of Madhya Pradesh, Rajasthan and Uttar Pradesh.
• Wadis are steep-sided dry valleys with ephemeral flow; the Luni, the Thar’s main river, flows only after monsoon rain and dies out in the Rann of Kutch.
• Canyons are deep, narrow valleys cut where exotic rivers cross deserts, like the Grand Canyon of the Colorado.
• Alluvial fans are cone-shaped spreads of gravel and sand where a wadi leaves the mountains and drops its load (a “dry delta”).

Bolsons and Playas

• Bolsons are intermontane basins; from the mountain front down they show pediment, bajada and playa.
• Playas are temporary lakes on the bolson floor, lost to evaporation within days to months; also called salinas, salars, khabari (Arabia) and shatts (Sahara), as at Lop Nur in the Tarim basin.
  • Evaporation leaves salt and alkali crusts (borax, sodium carbonate), giving salt or alkali flats.
  • Indian examples: the Sambhar, Didwana and Pachpadra salt lakes of Rajasthan and the salt flats of the Rann of Kutch.

Bajadas

• A bajada is a gently sloping depositional apron formed by the coalescence of alluvial fans, lying between pediment and playa.
  • It slopes about 8°–10° near the mountains, flattening to nearly 0° at the playa, and may hold small playas.

Pediments

• A pediment is a gently sloping (about 0.5°–7°), slightly concave rock-cut surface at the foot of a mountain front, bare or with a thin veneer of debris in transit.
  • A sharp piedmont angle separates it from the mountain front; unlike a fan, it is erosional.
  • Grove Karl Gilbert (1877) first described such surfaces in Utah’s Henry Mountains; Kirk Bryan (1922) called them slopes of transportation.
  • They also occur in savanna and temperate lands.
• As the mountain front retreats parallel to itself, the pediment extends headward; residual hills become inselbergs and coalesced pediments form a pediplain.

Theories of Pediment Formation

• Johnson’s three zones: an inner zone of degradation (vertical cutting at the mountain front), an intermediate zone of lateral corrasion where rock fans widen into pediments, and an outer zone of aggradation (bajada).
• Current view: pediments are polygenetic; numerical models reproduce them where granite weathers to grus and episodic runoff strips the mantle.

Arid and Savanna Cycles of Erosion

Arid Cycle of Erosion (Davis)

• Davis (1905) adapted his cycle of erosion to mountain-girt deserts of the Basin and Range type, not to open lowland deserts.
• Assumptions: uplift by faulting or folding creates enclosed basins; drainage is inland; each basin floor is a local base level; water, not wind, dominates early.

• The end product is a graded erosion surface called panfan by Lawson (1915), pediplain by King and pediplane by Arthur D. Howard.
  • Because wind exports dust, deflation can lower the plain below any fluvial base level.

• Evaluation: it rightly stresses water over wind and local base levels, but fits only fault-block deserts.
  • Most deserts swung between wet and dry phases in the Quaternary; the Thar became markedly arid only about 4,000–5,000 years ago, so few deserts have completed a cycle.
  • Modern work treats inselbergs and pediments as polygenetic, not as stages of one cycle.

Savanna Cycle of Erosion

• African savannas show wide plains studded with inselbergs, bornhardts and castle koppies.
• Charles Andrew Cotton (1942) proposed a separate savanna cycle driven by lateral corrasion, giving plains with inselbergs but no true pediments.
• King explained them instead by pediplanation (scarp retreat and pedimentation).
• The deep-weathering school stresses rock rotted to tens of metres down to a basal weathering surface; stripping the mantle exposes domes as bornhardts and corestones as tors.
  • J. C. Pugh (1966): repeated weathering and stripping after base-level falls give dome-on-dome inselbergs.
  • Michael F. Thomas (1966): Nigerian “pediments” are wash slopes left by removal of weathered material, forming an etchplain (a term of E. J. Wayland, 1934).
• Current view: savanna plains are largely etchplains, with scarp retreat locally important.