Silt

[5] The upper size limit of 1/16 mm or 63 microns corresponds to the smallest particles that can be discerned with the unaided eye.

[9] The proportions of coarse and fine silt in a sediment sample are determined more precisely in the laboratory using the pipette method, which is based on settling rate via Stokes' law and gives the particle size distribution accordingly.

Civil engineers in the United States define silt as material made of particles that pass a number 200 sieve (0.074 mm or less) but show little plasticity when wet and little cohesion when air-dried.

[15] The term silt is also used informally for material containing much sand and clay as well as silt-sized particles, or for mud suspended in water.

[21] A simple explanation for silt formation is that it is a straightforward continuation to a smaller scale of the disintegration of rock into gravel and sand.

[26] Theoretically, particles formed by random fracturing of an isotropic material, such as quartz, naturally tend to be blade-shaped.

Both materials form under conditions promoting ideal crystal growth, and may lack the Moss defects of quartz grains in granites.

[5] However, the main process is likely abrasion through transport, including fluvial comminution, aeolian attrition and glacial grinding.

[35] However, laboratory experiments show eolian and fluvial processes can be quite efficient at producing silt,[24] as can weathering in tropical climates.

Part of the problem may be the conflation of high rates of production with environments conducive to deposition and preservation, which favors glacial climates more than deserts.

[16] Silt, deposited by annual floods along the Nile River, created the rich, fertile soil that sustained the Ancient Egyptian civilization.

The closure of the Aswan High Dam has cut off this source of silt, and the fertility of the Nile delta is deteriorating.

[39] Quick clay (a combination of very fine silt and clay-sized particles from glacial grinding) is a particular challenge for civil engineering.

[42][43] Silt is easily transported in water[44] and is fine enough to be carried long distances by air in the form of dust.

[47] 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.

[48] In southeast Bangladesh, in the Noakhali district, cross dams were built in the 1960s whereby silt gradually started forming new land called "chars".

With Dutch funding, the Bangladeshi government began to help develop older chars in the late 1970s, and the effort has since become a multi-agency operation building roads, culverts, embankments, cyclone shelters, toilets and ponds, as well as distributing land to settlers.

[50] A main source in rural rivers is erosion from plowing of farm fields,[51] clearcutting or slash and burn treatment of forests.

[52] The fertile black silt of the Nile river's banks is a symbol of rebirth, associated with the Egyptian god Anubis.