Alluvial fans are orogenic deposits whose geometry is influenced by the rate and duration of uplift of the adjacent mountains and by climatic changes.

Three longitudinal shapes are common. A fan may be a wedge that is thickest (or thinnest) near the mountains, or it may be lenticular.

An alluvial fan may consist of water-laid sediments, debris-flow deposits, or both. Water-laid sediments occur as channel, sheetflood, or sieve deposits. Main stream channels commonly are backfilled with coarse-grained sediments. Sheets of finer grained sediments are deposited downslope from the channel. The fine-grained sediments may be cross-bedded, massive, or thin bedded; the coarse-grained sediments may be imbricated, massive, or thick bedded. Sieve deposits consist of intertonguing lobes of very permeable gravel.

Debris flows are poorly sorted and may have graded bedding or preferred particle orientation. Boulders weighing many tons may be present in an unsorted matrix. Mudflows are fine-grained debris flows. Platy fragments are oriented parallel with the bedding in low-viscosity flows. In high-viscosity flows, fragments are oriented vertically, and normal to the direction of flow.

Individual beds may be traced for long distances along radial sections, and channel deposits are scarce. Cross-fan sections reveal beds of limited extent that are interrupted by cut-and-fill structures, which are most common near the fan apex.

Logarithmic plots of the coarsest 1-percentile and median-particle size make patterns which are distinctive of fan environments. Sinuous patterns indicate tractive-current environments. Rectilinear patterns indicate mudflow environments.

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