What does an alluvial fan primarily consist of?

An alluvial fan primarily consists of alluvium at footslopes. Alluvial fans are landforms created by the deposition of sediment carried by water from a stream or river as it flows from a steeper gradient onto a flatter area, such as a valley floor. The process involves the transportation of materials during periods of high flow, where the reduction in velocity causes particles to settle out, forming a fan-shaped deposit at the base of a mountainous area.

Alluvium refers to the loose, unconsolidated soil or sediment that has been deposited by running water. The fan shape results from the spread of sediment over a broad area, and this sediment typically consists of a mix of sand, silt, clay, and gravel deposited in layers over time. Therefore, describing alluvial fans in terms of alluvium at footslopes accurately captures their fundamental characteristics and formation process.

The other choices involve different geological processes and depositional environments, which do not accurately define the composition of an alluvial fan. For example, windblown silt refers to loess deposits, debris spread by meltwater pertains to glacial outwash rather than alluvial processes, and lacustrine deposits are sedimentary materials formed in lake environments.