Arid or Desert Landforms

Desert landscapes are shaped by two agents working together, not by wind alone. Rainfall in a desert is scarce, but when it does come it arrives torrentially, in short, violent bursts, and desert rock, stripped of the vegetation cover that would otherwise protect it and exposed to drastic day-night temperature swings, weathers quickly into loose debris. That debris is moved both by this sudden, sheet-like rainwater runoff and by wind, and stream channels in a desert are consequently broad, poorly defined and flow only briefly after rain, rather than carrying water year-round the way a humid-region river does. Wind itself works through three linked actions: deflation, the lifting and removal of loose dust, sand and fine particles from the surface; abrasion, in which that lifted sand and silt acts as an active grinding tool against exposed rock, essentially a natural sand-blasting; and impact, the sheer momentum a grain of sand carries when the wind drives it directly into a rock face.

Erosional landforms

Pediments and pediplains describe how a desert's high relief is worn down over time. A pediment is a gently sloping rocky floor at the foot of a mountain front, bare or covered only by a thin veneer of debris, formed through a combination of lateral erosion by streams and sheet flooding rather than by wind. The process is one of parallel retreat of slopes: erosion first attacks the steep margins of the highland, cutting a wash slope backed by a steeper cliff or free face; rather than the whole mountain simply wearing down in place and flattening from the top, this cliff and wash slope retreat steadily backward together, backwasting at the expense of the mountain front while the pediment extends outward to take its place. Repeated over a long enough time, this steady backward retreat consumes the mountain almost entirely, leaving only isolated remnants standing above the surrounding flat surface, called inselbergs, and reduces the whole landscape to a low, near-featureless plain called a pediplain.

Playas are the most prominent landform of desert basins proper, wherever mountains and hills ring a closed basin with drainage running inward toward its centre. Sediment washed down from the basin margins accumulates toward the middle, building a nearly level plain there; when enough water is present, this plain is covered by a shallow lake, but because desert evaporation is intense, the water is retained only briefly, and what is left behind is called a playa. Because the water that filled a playa evaporates rather than draining away, it leaves behind the dissolved salts it carried, and a playa plain crusted over with those salts is specifically called an alkali flat.

Wind's own erosional signature is the deflation hollow: as persistent wind currents blow steadily in one direction, they progressively strip weathered debris from bare rock or soil, hollowing out shallow depressions across the surface. The same wind-driven sand also abrades rock faces directly, first pitting them with small cavities called blow-outs, which can deepen and widen over time into true caves. Rock outcrops subjected to prolonged deflation and abrasion are frequently worn into distinctive shapes: mushroom rocks have a slender stalk supporting a broader, rounded cap, since the wind's sand load is concentrated closer to the ground and so erodes the base faster than the top; table rocks keep a broad, flat top surface reminiscent of a tabletop; and pedestal rocks are a more extreme version of the same undercutting, with a markedly narrower base than the resistant remnant standing above it.

Depositional landforms

Wind is an unusually effective sorting agent: as it slows, coarser and finer grains settle out at different critical velocities, so wind-built deposits show a distinct grading by grain size that is largely absent from till or debris left directly by other agents. This sorting is at its most visible in sand dunes, which form wherever there is an abundant sand supply, a steady prevailing wind, and some obstacle to interrupt that wind's flow and force it to drop its load; because these conditions can occur anywhere with the right combination of aridity and wind, dune fields are not restricted to any one region of a desert. Dune shapes vary with wind pattern and sand supply. A barchan is a crescent-shaped dune with its horns pointing downwind, formed under a single, steady wind direction over a hard, relatively sand-poor surface. A seif is a long, narrow, sword-like ridge running roughly parallel to the dominant wind direction, typically forming where wind direction varies somewhat rather than staying perfectly constant. Transverse dunes form as long ridges lying at right angles to the wind, in areas with abundant sand and comparatively little vegetation to interrupt the sand sheet. Parabolic dunes are, in a sense, the mirror image of a barchan: crescent-shaped but with the horns pointing into the wind rather than away from it, typically anchored and dragged into that shape by vegetation or moisture holding the horns in place while the dune's centre continues to advance. Longitudinal dunes are long ridges aligned parallel to the wind direction, generally forming where sand supply is more limited than in a transverse-dune field.

Quick revision points

• Deserts are shaped by both sudden, torrential rainwater runoff (sheet floods) and wind; desert streams are broad, indistinct and short-lived rather than perennial.
• Wind erosion works through deflation (lifting loose material), abrasion (sand-blasting rock), and impact (direct grain-on-rock momentum).
• Pediment: a gently sloping rocky foot-slope formed by lateral stream erosion and sheet flooding, extending outward as the mountain front retreats backward (parallel retreat of slopes), not by the mountain simply flattening in place.
• Inselberg: an isolated remnant of the original highland left standing after pediment formation has consumed the rest of it. Pediplain: the resulting low, featureless plain.
• Playa: a nearly level basin-floor plain covered briefly by a shallow lake after rain, which evaporates and leaves salts behind; a salt-crusted playa is an alkali flat.
• Deflation hollow: a shallow depression stripped out by persistent one-direction wind. Blow-outs deepen into caves; wind-eroded rock outcrops form mushroom, table and pedestal rocks depending on how the base is undercut relative to the cap.
• Wind sorts sediment by grain size as it slows, unlike till or debris dumped directly by other agents.
• Dune shapes: barchan (crescent, horns downwind, steady single wind direction), seif (long ridge parallel to wind, variable wind direction), transverse (ridges at right angles to wind, abundant sand), parabolic (crescent, horns pointing into the wind, anchored by vegetation), longitudinal (ridges parallel to wind, limited sand supply).