Wind can organize loose sand into migrating mounds. A sand dune is an accumulation of sand shaped by eolian transport and deposition. Dunes form where there is a supply of sand, sufficient wind energy, and an obstacle or irregularity that initiates deposition.

Common forms include barchan, transverse, longitudinal, and star dunes, each reflecting a particular wind regime and sand supply. Sand moves up the gentle windward slope and avalanches down the steeper lee face, causing the dune to migrate. Internal cross-bedding records this migration.

Coastal dunes protect inland areas from storms; desert dunes cover vast regions of the subtropics. Human activities can destabilize dunes or, conversely, be threatened by their migration. The geometry and internal structure of ancient dune deposits are important clues in the interpretation of sandstone reservoirs.

• Wind-blown mound of sand
• Migrates by erosion and deposition
• Form depends on wind and sand supply
• Occurs in deserts and along coasts