Sand dunes form through aeolian (wind-driven) transport: wind exceeding roughly 4-5 m/s "threshold friction velocity" lofts loose 0.1-1mm sand grains into short hops a few cm to tens of cm high, a process called saltation. Bouncing grains dislodge neighbors, building ridges whose windward slope stays gentle (~10-15°) while sand cascading over the crest piles onto a steep leeward slip face near the sand's angle of repose (~30-34°). Crescent-shaped barchan dunes can migrate several meters per year under strong, steady winds, and in erg fields like Morocco's Erg Chebbi or the Sahara's Grand Erg, dune ridges can exceed 300m in scale.
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