Silt
Silt is
soil or
rock derived
granular material of a specific
grain size).
On the
Wentworth scale, silt particles fall between
1⁄
256 and
1⁄
16 mm (3.9 to 62.5 μm), larger than
clay but smaller than a
sand. In actuality, silt is chemically distinct from clay, and unlike clay, grains of silt are roughly the same size in all dimensions, and their size ranges overlap. According to the
USDA Soil Texture Classification system, the sand-silt distinction is made at the 0.05 mm particle size.
The USDA system is also used 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 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. Mineralogically, silt is composed mainly of quartz and feldspar.
Sedimentary rock composed mainly of silt is known as
siltstone.
Silt, deposited by annual floods along the
Nile River, created the rich and fertile soil that sustained the ancient
Egyptian civilization. This silt was depended on for this purpose. A decrease in silt deposited by the
Mississippi River throughout the
20th century has contributed to the disappearance of protective
wetlands and
barrier islands in the
delta region surrounding
New Orleans.[
1]
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.
One of the main causes of river siltation in the year 2006 is as a result of
slash and burn treatment of
tropical forests. When the total ground surface is stripped of vegetation and then seared of all living organisms, the upper soils are vulnerable to both wind and water erosion. In a number of regions of the earth, entire sectors of a country have been rendered unproductive; for example, on the
Madagascar high central
plateau, comprising approximately ten percent of that country's land area, virtually the entire landscape is sterile of
vegetation, with gully erosive furrows typically in excess of 50 meters deep and one kilometer wide.
Shifting cultivation is a farming system which sometimes incorporates the
slash and burn method in some regions of the world. The resulting sediment load in rivers flowing to the west is ongoing, with most rivers a dark red brown colour. The resulting fish kills in most of these rivers have resulted in the process of extinction of a variety of
Madagascar's fish
species.
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Sediment
