Deserts covering vast expanses covered by thick sands, including sand dunes of several types and by an absence of other geographic features, are known as sand seas, or locally as ergs in the North African sahara. Interdune areas may be covered by relatively flat tabular sand sheets, or even evaporite basins (sab-khas). Sand seas are abundant in parts of the Sahara of North Africa, the Namib of southern Africa, the Rub' al-Khali (Empty Quarter) of Arabia, the Great Sandy Desert of Australia, the Gobi Desert of Asia, and the Nebraska Sand Hills of Nebraska.
Sand seas form where the velocity of the transporting wind decreases, dropping its load. The decreased velocity may be caused by a number of factors, including in topographic lows, or adjacent to topographic barriers, such as mountains that cut across the direction of sand transport. A striking example of this process is found in the Wahiba Sand Sea of Oman. Here the Eastern Hajar Mountains terminate the northward-flowing Wahiba sands, and an intermittent river system at the base of the mountains removes sand that gets close to the mountain front, carrying it to the coast of the Arabian Sea. Longshore transport then carries this sand southward where winds pick it up from beaches and cause it to reenter the Wahiba sand sheet in the south, forming a sort of sand gyre. Sand seas may also form where a large body of water intercepts drifting sand, or where the sand is carried into shifting climate zones where the wind strength decreases.
Surface features in sand seas include bed forms of a variety of scales ranging from several different types of ripples that may be up to an inch (several cm) high, to dunes that are typically up to 300 feet (100 m) tall, to huge bedforms called draa that are giant dunes up to 1,650 feet (500 m) tall, with wavelengths of up to several kilometers. These bedforms are typically superimposed on each other, with dunes migrating over and on top of draa and several different sets of ripples migrating over the dunes. The wind directions inferred from the different sets may also be different, with ripples reflecting the most recent winds, dunes the dominant winds over different seasons, and draa reflecting the very long-term direction of wind in the basin. See also deserts.
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