What is the primary composition of alluvial fans?

Alluvial fans are primarily composed of coarse sand and gravel due to the processes that form them. They develop in arid or semi-arid environments where streams emerge from mountainous regions onto flatter land. As the water velocity decreases upon entering the gentler slope, it loses its ability to carry sediment, resulting in the deposition of larger particles like coarse sand and gravel at the fan's apex. This leads to a characteristic conical or fan-shaped landform radiating outward from the stream's mouth.

In contrast, fine clay and silt are typically associated with slower-moving water bodies, like lakes or floodplains, where lighter particles settle out. Solid rock and boulders are often found in mountainous areas and are not the primary materials deposited in fan structures. Organic materials and soil mainly accumulate in areas with higher vegetation cover, and their presence is minimal in alluvial fan deposits which are actively shaped by flowing water and sediment transport. Thus, the distinctive composition of alluvial fans is dominated by coarse sand and gravel, reflecting the dynamic environment in which they form.