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A land-form produced by the action of wind on unconsolidated sediment, normally sand. Aeolian dune forms range from small ripples less than 1cm in height to the draa forms of the Sahara, which rise to more than 300 m. Such dunes may be divided into three basic categories: barchans; longitudinal or ‘seif’ dunes, which parallel the wind direction; and transverse dunes, which are aligned normally to the dominant wind. Transverse dunes are initial forms on sandy coastlines in temperate regions. They migrate inland and may be eroded locally by the wind to form a damp hollow or *‘dune slack’. The enclosing crescentic dune is a ‘parabolic’ dune whose form reverses that of the barchan. See also aklé dune; dune bedform; star dune.

Subjects: Earth Sciences and Geography — Ecology and Conservation.

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