The shapes of the desert

A lone crescent-shaped dune with its horns pointing downwind. Barchans march across flat ground as the wind blows sand up the gentle back and spills it down the steep face.

A pyramid of sand with arms reaching out in several directions, built where the wind blows from many angles. Star dunes grow upward rather than travel — the tallest on Earth top 450 m.

Long, straight ridges of sand running for tens of kilometres in parallel lines. The Simpson Desert in Australia has the longest parallel dunes in the world.

A huge sea of wind-blown sand and dunes covering a broad basin. Ergs are what most people picture as "desert", yet they cover only about a fifth of the world's dry lands.

A bare, stony plateau swept clean of sand by the wind, leaving exposed bedrock and boulders. Much more of the Sahara is rocky hamada and gravel than golden dunes.

A flat plain armoured with a tight mosaic of pebbles and gravel. Wind blows the fine sand away, leaving the heavier stones locked together like a natural cobbled floor.

Streamlined ridges of soft rock sculpted by sand-laden wind, all pointing the same way like a fleet of stone ships. The Lut and Taklamakan deserts have spectacular yardang fields.

A valley or channel that stays dry for most of the year, then floods in a flash after a rare storm. Wadis carry the water — and life — that shapes desert landscapes.

The flat, cracked floor of a lake that has evaporated away, often crusted with glittering salt. After rare rains a thin sheet of water can turn a playa into a giant mirror.

A pocket of water and greenery where underground springs reach the surface. For thousands of years oases have been lifelines for desert travellers, farmers and caravan routes.