A mutual relationship exists between the morphology of longitudinal (seif) dunes and their dynamics and internal structure. The basic mechanism of longitudinal dune movement results from the deflection of oblique wind flow into a longitudinal flow on the lee flank. The magnitude of the longitudinal flow depends on the angle of the wind incidence with the crest line. Deposition occurs when this angle of incidence increases through meandering of the longitudinal dune crest line. After heavy rain exposed the internal structure of the dune, two kinds of depositional unit were found: the main unit forms in the meandering area on both sides of the dune; the other, secondary unit is a narrow belt on both sides and parallel to the crest line. The two units have different strikes. Because the laminae deposited on the dune are of the three primary stratification types--"climbing translatent strata," "sand flow cross-strata," and "grainfall laminae"--the range of dips is wide (10 degrees to 30 degrees ). These dips accord with topographic profiles across the dune. A model of the internal structure of a longitudinal dune has been developed from the field data. The model shows the erosional and depositional areas along this linear dune.

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