Dune Landscape. In physical geography, a dune is a hill of loose sand built by aeolian processes or the flow of water. Dunes occur in different shapes and sizes, formed by interaction with the flow of air or water. Most kinds of dunes are longer on the stoss side, where the sand is pushed up the dune, and have a shorter slip face in the lee side. The valley or trough between dunes is called a slack. An erg or sand sea is a large broad, flat area covered with wind-swept sand and/or dunes with little or no vegetation. Smaller areas are called dune fields. Dunes occur in some deserts, inland and along some coasts. Some coastal areas have one or more sets of dunes running parallel to the shoreline directly inland from the beach. In most cases, the dunes are important in protecting the land against potential ravages by storm waves from the sea. Although the most widely distributed dunes are those associated with coastal regions, the largest complexes of dunes are found inland in dry regions and associated with ancient lake or sea beds. Dunes can form under the action of water flow, and on sand or gravel beds of rivers, estuaries and the sea-bed. The modern word dune came into English from French around 1790, which in turn came from Middle Dutch dune. Dunes are made of sand-sized particles, and may consist of quartz, calcium carbonate, snow, gypsum, or other materials. The upwind/upstream/upcurrent side of the dune is called the stoss side; the downflow side is called the lee side. Sand is pushed or bounces up the stoss side, and slides down the lee side. A side of a dune that the sand has slid down is called a slip face. The Bagnold formula gives the speed at which particles can be transported. Five basic dune types are recognized: crescentic, linear, star, dome, and parabolic. Dune areas may occur in three forms: simple, compound, and complex. Main article: barchan Barchan dunes are crescent-shaped mounds which are generally wider than they are long. The lee-side slipfaces are on the concave sides of the dunes. These dunes form under winds that blow consistently from one direction. They form separate crescents when the sand supply is comparatively small. When the sand supply is greater, they may merge into barchanoid ridges, and then transverse dunes. Some types of crescentic dunes move more quickly over desert surfaces than any other type of dune. A group of dunes moved more than 100 metres per year between 1954 and 1959 in China's Ningxia Province, and similar speeds have been recorded in the Western Desert of Egypt. The largest crescentic dunes on Earth, with mean crest-to-crest widths of more than three kilometres, are in China's Taklamakan Desert. See lunettes and parabolic dunes, below, for dunes similar to crescent-shaped ones. Abundant barchan dunes may merge into barchanoid ridges, which then grade into linear transverse dunes, so called because they lie transverse, or across, the wind direction, with the wind blowing perpendicular to the ridge crest. Seif dunes are linear dunes with two slip faces. The two slip faces make them sharp-crested. They are called seif dunes after the Arabic word for sword. They may be more than 160 kilometres long, and thus easily visible in satellite images. Seif dunes are associated with bidirectional winds. The long axes and ridges of these dunes extend along the resultant direction of sand movement. Some linear dunes merge to form Y-shaped compound dunes. Formation is debated. Bagnold, in The Physics of Blown Sand and Desert Dunes, suggested that some seif dunes form when a barchan dune moves into a bidirectional wind regime, and one arm or wing of the crescent elongates. Others suggest that seif dunes are formed by vortices in a unidirectional wind. In the sheltered troughs between highly developed seif dunes, barchans may be formed, because the wind is constrained to be unidirectional by the dunes. Seif dunes are common in the Sahara. They range up to 300 m in height and 300 km in length. In the southern third of the Arabian Peninsula, a vast erg, called the Rub' al Khali or Empty Quarter, contains seif dunes that stretch for almost 200 km and reach heights of over 300 m.