The most common dune type is called a barchan which is a crescent-shaped sand dune with its points in the downwind direction. The upwind side has a shallow slope and the downwind side is called the slipface , which rests at a 35 angle . When sand is so abundant, it may form a sand sea in which transverse dunes occur. These are dunes that have a wavelike structure with deep troughs in-between. Star dunes are common in Africa in the Sahara Desert and consist of a sand hill with four or more radiating arms.
Parabolic dunes occur when the bow crest of the dune points in the downwind direction.
One common type of parabolic dune is the coastal blowout which forms adjacent to beaches. Sometimes the wind direction causes sand to be heaped into long ridges called longitudinal dunes. Landward of sea beaches, one fines a narrow belt of sand dunes in irregularly shaped hills covered with vegetation. These are called foredunes , and provide a protective barrier for areas inland from storm and wave activity. Occasionally, one finds very deep deposits of wind-blown silt in various parts of the world which can be up to 100 feet thick.