A fan-shaped mass of ALLUVIUM (SAND, GRAVEL, COBBLES and sometimes BOULDERS) formed where a rapidly flowing stream leaves a steep and narrow valley and enters a lowland or broad valley. At such points a reduction in gradient, and thus stream velocity, occurs, causing overloading and DEPOSITION. Alluvial fans form in many different locations – for example, where a small tributary valley joins a major glacial trough, or where a desert stream passes through the MOUNTAIN FRONT to the PIEDMONT Zone.
Alluvial fan in southern Iran.
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