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Longitudinal (linear, sand-ridge, or seif) dunes form long ridges of sand which are oriented parallel to the prevailing wind. The smaller dunes are 3 m high and 60 m long; the larger dunes are 100 m high and may extend for 100 km. The crests of these larger dunes, which are called seif dunes and are found in the Libyan and Arabian deserts, are usually knife-like ridges with many peaks and sags.
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