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Closed desert depressions, called pans, have an extensive distribution in the world’s dry lands, particularly on certain types of susceptible surface, which include shales, fine-grained sandstones and unconsolidated materials. They also occur in certain characteristic geomorphological situations, including interdunal areas, old river channels, and deflated lake beds. Pans often, but not invariably, have lunette dunes on their lee sides. Aridity is an important predisposing factor for pan development, and permits deflation, salt weathering, and disruption of drainage systems, all of which contribute to pan formation. Large numbers of hypotheses for pan development have been proposed, including animal activities and solution, but all these can be incorporated in a general model of pan development.