Silt refers to soil or rock particles of a certain very small size range (see grain size). On the Wentworth scale, silt particles fall between 0.004 and 0.063 mm (4 - 62.5 μm), larger than clay but smaller than a sand. In actuality, silt is chemically distinct from clay, and their size ranges overlap. According to the USDA Unified Soil Classification System (USCS) and the AASHTO Soil Classification system, the sand-silt distinction is made at the 0.075 mm particle size (i.e. material passing the #200 sieve), and silts and clays are distinguished by their plasticity.
Silt is produced by the mechanical weathering of rock, as opposed to the chemical weathering that results in clays. This mechanical weathering can be due to grinding by glaciers, eolian abrasion (sandblasting by the wind) as well as water erosion of rocks on the beds of rivers and streams. Silt is sometimes known as 'rock flour' or 'stone dust', especially when produced by glacial action.
Silt can occur as a deposit or as material transported by a stream or by a current in the ocean. Silt is easily transported in water and is fine enough to be carried long distances by air as 'dust'. Thick deposits of silty material resulting from aeolian deposition are often called loess (a German term) or limon (French). Silt and clay contribute to turbidity in water.
Sedimentary rock composed mainly of silt is known as siltstone.