Barchan dunes creep downwind as crescent hills of sand
Where wind usually blows one way over sparse sand, loose grains pile into crescent barchans — horns pointing downwind, steep slip faces on the lee side.
Sand climbs the gentle windward slope, then avalanches over the crest, so the whole dune migrates grain by grain across flat desert floors.
A crescent on the map is a moving machine. Barchans show arid landscapes are not frozen scenery; they are sand conveyor belts written by wind.
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