Wind is the agent of erosion that creates sand dunes. The wind blows grains of sand into a sheltered or otherwise secure location, allowing gradual accumulation.
Each dune features a windward face and a slip face. The windward face is the area where the sand is blowing and pushing materials upward. The slip face is the side that does not experience wind. Dunes that are formed underwater experience a similar phenomenon but through currents of water instead.
Sand dunes are classified by shape. Crescentric dunes are crescent-shaped, while linear dunes are mostly straight. Star dunes occur when wind from many different directions blows sand. Dome dunes are circular, lacking a slip face. Parabolic dunes are formed by wind that blows outward from their center.