Page 90 - Louisiana Loop (manuscript Edition)

Silt is granular material of a size between sand and clay, whose mineral origin is quartz and feldspar. Silt may occur as a soil (often
mixed with sand or clay) or as sediment mixed in suspension with water (also known as a suspended load) and soil in a body of
water such as a river. It may also exist as soil deposited at the bottom of a water body, like mudflows from landslides. Silt has a
moderate specific area with a typically non-sticky, plastic feel. Silt usually has a floury feel when dry, and a slippery feel when wet.
Silt can be visually observed 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 by
exploiting deficiencies in their lattice.[2] These involve chemical weathering of rock[3] and regolith, and a number of physical
weathering processes such as frost shattering[4] and haloclasty.[5] The main process is abrasionthrough transport,
including fluvial comminution, aeolian attrition and glacial grinding.[6] It is in semi-arid environments[7] that substantial quantities
of silt are produced. Silt is sometimes known as "rock flour" or "stone dust", especially when produced by glacial action.
Mineralogically, silt is composed mainly of quartz and feldspar. Sedimentary rockcomposed mainly of silt is known
as siltstone. Liquefaction created by a strong earthquake is silt suspended in water that is hydrodynamically forced up from below
ground level.
In the Udden–Wentworth scale (due to Krumbein), 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. In actuality, silt is chemically distinct from clay, and
unlike clay, grains of silt are approximately the same size in all dimensions; furthermore, their size ranges overlap. Clays are formed
from thin plate-shaped particles held together by electrostatic forces, so present a cohesion. According to the U.S. Department of
AgricultureSoil Texture Classification system, the sand-silt distinction is made at the 0.05 mm particle size.[8] The USDA system has
been adopted by the Food and Agriculture Organization(FAO). In the 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). 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.

Silt, deposited by annual floods along the Nile River, created the rich, fertile soil that sustained the Ancient Egyptian civilization. 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.[9]

In south east Bangladesh, in the Noakhali district, cross dams were built in the 1960s whereby silt gradually started forming new
land called "chars". The district of Noakhali has gained more than 73 square kilometres (28 sq mi) of land in the past 50 years.
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. By fall 2010, the program will have allotted some 100 square kilometres (20,000 acres) to 21,000 families.[10]