Chapter 4 – Deserts, Dunes, Salt Flats

Chapter 4: Deserts, Dunes, Salt Flats

Created by Sarah Choi (prompt writer using ChatGPT)

Deserts, Dunes, Salt Flats — Formation Logic for Environment Concept Artists

Why arid‑land logic matters

Desert scenes can look like random sand and rocks unless the forces that build and erase them are clear. When you anchor forms to wind regimes, sediment supply, rock resistance, and water’s rare but catastrophic appearances, silhouettes, patterns, and materials line up. This article distills desert geomorphology into a practical grammar for concept and production artists so your wastelands feel inevitable, not arbitrary.

The four levers of arid landscapes

Aridity source: Subtropical high‑pressure belts, continental interiors, cold‑current coasts, rain‑shadows behind mountains, and polar deserts each produce different textures and colors.
Wind regime: Prevailing wind direction(s) and seasonal variability determine dune type and yardang orientation.
Sediment budget: Availability of sand‑sized grains (vs. gravel or silt) controls whether you get ergs (seas of dunes), stony regs/pavements, or bare rock hamadas.
Rare water: Flash floods, ephemeral rivers (wadis/arroyos), and endorheic basins (playas) impose channel patterns, fans, and salt flats that reorganize sand and dust.

Substrates and surfaces: the three desert floors

Erg (sand sea): Vast dune fields requiring abundant, well‑sorted sand and wind energy. Interdune areas may expose hardpans or shallow water after storms.
Reg (desert pavement): Armored surfaces of closely packed gravel/pebbles over fine sediments; forms via deflation and wetting–drying. Pavements are darkened by desert varnish.
Hamada (bare bedrock): Resistant rock plateaus and scarps with thin debris—think cliffs, mesas, and stepped cuestas.

Dune physics in one page

Wind moves sand by saltation (grains hop), creep (roll), and suspension (dust). A dune forms where the wind’s transport capacity drops. The upwind stoss slope is gentle and rippled; the leeward slipface is steep (≈30–34°, angle of repose) and avalanches as the brink migrates.

Dune types and when to use them

Barchan (crescent, horns downwind): One dominant wind, limited sand, hard ground. Spacing is regular; dunes migrate quickly and can encircle obstacles. Horns trail downwind; interdunes are firm.
Transverse ridges: One dominant wind, abundant sand. Long, continuous crests perpendicular to wind; broad slipfaces all facing leeward.
Linear/longitudinal (seif) dunes: Two similar wind directions seasonally; crests align with resultant long‑axis downwind. Slipfaces develop alternately on flanks. Interdunes can be sand‑starved.
Star dunes: Multidirectional winds; tall, pyramid forms with radiating arms and multiple slipfaces. Interdunes often lower and sand‑rich.
Parabolic/U‑shaped dunes (horns upwind): Vegetated or moist surfaces in coastal or semi‑arid zones; wind blows out a blowout and plants anchor the horns.
Dome dunes: Low mounds with no slipface—very limited wind or young forms.

Concept cues: Keep slipfaces coherent in orientation; align ripple crests perpendicular to recent wind on stoss slopes and parallel to brink lines near slipfaces. Show avalanche tongues on lee slopes after gust events. Let dunes climb obstacles and cast wind shadows with small lee dunes.

Production tips: Drive ripple normal maps by a world‑space wind vector; vary wavelength with grain size (coarser = longer ripples). Use height‑blend at dune toes to reveal firm interdune crusts and small polygons after rain. Maintain slipface slopes within 30–34° and keep crest continuity across tiles.

Yardangs, rock pedestals, and tafoni: wind’s bedrock sculpture

Yardangs: Streamlined ridges carved from weakly cemented silt/sandstone by abrasive winds; long axes parallel to wind. Expect furrows and sharp lee cliffs. Place in bands with regular spacing.
Ventifact fields: Faceted, polished stones with abraded faces oriented to wind; collect on pavements and near dune margins.
Pedestal (mushroom) rocks: Differential abrasion and weathering—soft base undercuts beneath harder caprock.
Tafoni & honeycomb: Salt‑weathering pits on coastal/arid sandstones; develop on shaded faces and beneath overhangs.

Water’s rare violence: wadis, fans, and pediments

Wadis/Arroyos: Dry channels that flow during storms; steep banks, armored beds, and bar‑and‑swale topography. Tributaries join at acute angles pointing downstream. Place fresh driftwood/silt crusts after floods.
Alluvial fans: At mountain fronts, stream energy drops and deposits a cone of gravel/sand. Channels braid and shift across fan surfaces (avulsion). Neighboring fans coalesce into bajadas. Fan apices are coarsest; distal toes grade to sand/silt.
Pediments: Gently sloping bedrock surfaces at range feet, mantled by thin veneers; transition from steep scarps to open basins.

Concept cues: Route caravan paths along fan crests and pediments, not across active wash bottoms. Show fresh debris lobes and levees after storms; add varnish‑free “scar” where new flows cleaned surfaces.

Production tips: Use flow maps to localize rills and bars; blend clast size downslope on fans. Place tamarisk/mesquite vegetation in shallow groundwater zones along abandoned channels.

Playas and salt flats: endgames of closed basins

Endorheic basins collect water and dissolved ions; evaporation leaves salts.

Playa facies: Muddy central flat with polygonal desiccation cracks; outer belts of silt/sand; ephemeral shorelines marked by strandlines.
Evaporite minerals: Halite (cubic crusts and hopper crystals), gypsum (soft, fibrous selenite), sodium carbonate/bicarbonate (soda lakes), and mixed salts. Crust textures vary from smooth “sheet” to loose “popcorn” to hard polygonal ridges.
Salt pans & sabkhas (coastal flats): Capillary rise brings brines to the surface and precipitates salt; expect efflorescent crusts, microbial mats, and tepee structures where crust buckles.

Concept cues: Give salt flats a subtle crown and drainage to the basin low; place mirage shimmer midday, and wheel/rut scars that heal after rains. At margins, show shore dunes (lunettes) downwind of playas where dry silt/sand piles.

Production tips: Build a layered shader: damp mud sheen → dry cracked polygons → salt bloom/efflorescence. Align polygon ridges into hexagonal/irregular meshes ~1–5 m across. Add travertine/tufa terraces where carbonate springs enter basins.

Rock controls in deserts

Sandstones: Cross‑bedded cliffs, alcoves, arches; color from iron oxides (orange/red). Expect varnish panels on stable, sun‑struck faces.
Limestones/dolomites: Karst towers and sinkholes even in aridity; bright talus and solution pits.
Basalts: Dark flows with columnar joints; clean, blocky talus; glassy black sand sources.
Granites/gneiss: Bornhardts and inselbergs—rounded domes and tors from sheet joints; grus (granite sand) mantles pediments.

Climate flavors of aridity

Hot subtropical deserts: Strong insolation, clear skies, high diurnal range. Bright, crisp forms; varnish common.
Coastal fog deserts: Low clouds/fog feed strip vegetation; dunes may be moist and crusted; salt weathering intense.
Cold/winter deserts: Sparse vegetation, frost‑shattering, patterned ground, and seasonal snowdrifts in shade.
Monsoon margins: Long dry season punctuated by violent convective storms—fresh fan lobes, ephemeral lakes after downpours.

Color, light, and FX from process

Color: Iron‑rich sands (red/orange), basaltic sands (black/olive), gypsum dunes (white), evaporites (blinding white with subtle blues). Varnish deepens toward chocolate; dust mutes distant values.
Light: Low sun carves ripples and yardang ribs; high sun flattens dunes but intensifies salt glare. Heat shimmer and dust veils compress depth.
Atmospherics: Dust devils leave helical streaks; haboobs (wall clouds) advance from storm outflows; salt spray glows along sabkhas.

Map‑scale coherence

Place ergs downwind of large sand sources (deflation hollows, glacial outwash, river deltas now dry). Ergs avoid high relief; they pond in basins and against windward slopes.
Align yardang belts and linear dunes with the same resultant wind. Put star dunes at wind‑confluence zones (passes, capes).
Situate playas at closed‑basin lows with feeder wadis and fan aprons. Flank them with lunettes on the lee side.

Traversal & settlement logic

Water dictates life: springs at faulted contacts, fan toes, and bedrock nickpoints; cisterns in slot canyons; wells in interdune swales where the water table rises.
Roads follow pediments, fan crests, and dune corridors parallel to linear dunes; avoid slipfaces. Waystations cluster at perennial seeps, shade, and defensible rock knolls.

Common pitfalls and fixes

Dunes pointing every which way: Choose a wind rose; keep slipface aspects consistent. Introduce star dunes only if winds are truly multidirectional.
Featureless sand oceans: Add interdune flats, bar‑and‑swale micro‑relief, moisture patches, and vegetation clumps in swales.
Salt flats too uniform: Layer crack sizes, moisture sheen gradients, and salt blooms; add strandlines and shore dunes.
Random rock piles: Tie block fields to specific scarps, joints, or fan lobes; grade clast size downslope.
Wadis climbing divides: Recut a continuous downhill path to the basin; add knickpoints where needed.

From thumbnail to level build: a workflow

Pick aridity type and wind regime (dominant vectors, seasonal shifts).
Choose sediment story: sand‑rich vs. sand‑poor; identify sources and sinks.
Block macroforms: basins, fans/bajadas, pediments, erg extents, inselbergs, and yardang belts aligned to wind.
Place water logic: wadis with alluvial steps, ephemeral lakes, springs at faulted contacts.
Assign material families: sandstone/basalt/granite/limestone; set varnish potential and fracture spacing.
Bake masks: slope, curvature, flow, aspect, wind exposure, wetness. Drive ripples and dust decals from wind mask; set slipface shader to higher roughness/looser scatter.
Scatter intelligently: big blocks near cliffs and slide paths; ventifacts on pavements; driftwood/silt in wash bends; salt blooms in basin lows and around damp seeps.

Field exercises for concept practice

Paint a barchan train marching around a bedrock knob. Show dune horns wrapping the obstacle and slipface avalanches catching the light.
Block out a mountain front with fans merging into a bajada, then a playa with polygon cracks and a downwind lunette dune.
Design a yardang corridor: parallel, elongated ridges with lee cliffs; pepper the upwind stoss slopes with ventifact clusters.

Final checklist

Do dunes, yardangs, and ripples all respect the same wind logic?
Do fans and wadis connect plausibly to basin lows and playas?
Are colors/materials tied to lithology and salt/varnish processes?
Is there a credible water strategy for settlements and routes?

When wind, rock, and rare water negotiate over time, deserts write rhythms into every crest, notch, and crack. Let those rhythms lead your design, and your arid worlds will feel both hostile and hauntingly real.