Silt is granular material of a size between sand and clay.Silt particles range between 0.0039 and 0.0625 mm, larger than clay but smaller than sand particles. ISO 14688 grades silts between 0.002 mm and 0.063 mm.The origin trace back to whose is quartz and feldspar. Silt may be found as a soil or as sediment mixed in suspension with water (also known as a suspended load) in water body such as a river. It may also exist as soil deposited at the bottom of a water body. Silt usually has a floury feel when dry, and a slippery feel when wet. Silt can be seen with a hand lens.
Silt is created by a variety of physical processes capable of splitting the generally sand-sized quartz crystals of primary rocks These involve chemical weathering of rocks and regolith, and a number of physical weathering processes such as frost shattering and haloclasty. The main process is abrasion through transport, including fluvial comminution, aeolian attrition and glacial grinding. It is in semi-arid environments that substantial quantities of silt are produced. Silt is sometimes known as “rock flour” or “stone dust”, especially when produced by glacial action .Sedimentary rock composed mainly of silt is known as siltstone.
Silts and clays are distinguished mechanically by their plasticity.
Silt is easily transported in water or other liquid and is fine enough to be carried long distances by air in the form of dust. Thick deposits of silty material resulting from deposition by aeolian processes are often called loess. Silt and clay contribute to turbidity in water. Silt is transported by streams or by water currents in the ocean. When silt appears as a pollutant in water the phenomenon is known as siltation.
There is also a historical role of Silt in development and origin of agriculture.Silt, deposited by annual floods along the Nile River, created the rich, fertile soil that sustained the Ancient Egyptian civilization and the same role in Mesapotamia where the silt deposited by Tigris and Euphrates done the same job. Silt deposited by the Mississippi River throughout the 20th century has decreased due to a system of levees, contributing to the disappearance of protective wetlands and barrier islands in the delta region surrounding New Orleans.
A main source of silt in urban streams is disturbance of soil by construction activity.A main source in rural rivers is erosion from plowing of farm fields, clear cutting or slash and burn treatment of forests.
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