alluvial fan

Alluvial fans are cone-shaped landforms that form at the base of slopes, near the mouth of a mountain canyon. Material is deposited by a river as it emerges from the gorge. There is a reduction in the velocity of the river, and it drops some of its load. The heavy, coarse material is deposited first, and the finest material is deposited later, further from the mountain face, helping to produce the conical slope.

Roughly triangular sedimentary formation found at the base of slopes. An alluvial fan results when a sediment-laden stream or river rapidly deposits its load of gravel and silt as its speed is reduced on entering a plain.

The surface of such a fan slopes outward in a wide arc from an apex at the mouth of the steep valley. A small stream carrying a load of coarse particles builds a shorter, steeper fan than a large stream carrying a load of fine particles. Over time, the fan tends to become destroyed piecemeal by the continuing headward and downward erosion levelling the slope.