Silt is granular material of a size between sand and clay, whose mineral origin is quartz[1] 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, exhibiting a sparkly appearance. It also can be felt by the tongue as granular when placed on the front teeth (even when mixed with clay particles).


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 abrasion through 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 rock composed 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.

Grain size criteria

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 (sub-divided up into three grades fine, medium and coarse 0.002 mm to 0.006 mm to 0.020 mm to 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. Pure silts are not cohesive. According to the U.S. Department of Agriculture Soil 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.

Environmental impacts

Burgwall Jatzke3
A silted lake located in Eichhorst, Germany

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 southeast 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]

A main source of silt in urban rivers is disturbance of soil by construction activity.[11] A main source in rural rivers is erosion from plowing of farm fields, clearcutting or slash and burn treatment of forests.


The fertile black silt of the Nile river's banks is a symbol of rebirth, associated with the Egyptian god Anubis.[12]

See also


  1. ^ Assallay, A. (November 1998). "Silt: 2–62 μm, 9–4φ". Earth-Science Reviews. 45 (1–2): 61–88. Bibcode:1998ESRv...45...61A. doi:10.1016/S0012-8252(98)00035-X.
  2. ^ Moss, A. J.; Green, Patricia (1975). "Sand and silt grains: Predetermination of their formation and properties by microfractures in quartz". Journal of the Geological Society of Australia. 22 (4): 485–495. Bibcode:1975AuJES..22..485M. doi:10.1080/00167617508728913.
  3. ^ Nahon, D.; Trompette, R. (February 1982). "Origin of siltstones: glacial grinding versus weathering". Sedimentology. 29 (1): 25–35. Bibcode:1982Sedim..29...25N. doi:10.1111/j.1365-3091.1982.tb01706.x.
  4. ^ Lautridou, J. P.; Ozouf, J. C. (19 August 2016). "Experimental frost shattering". Progress in Physical Geography. 6 (2): 215–232. doi:10.1177/030913338200600202.
  5. ^ Goudie, A. S.; Watson, A. (January 1984). "Rock block monitoring of rapid salt weathering in southern Tunisia". Earth Surface Processes and Landforms. 9 (1): 95–98. Bibcode:1984ESPL....9...95G. doi:10.1002/esp.3290090112.
  6. ^ Wright, J.; Smith, B.; Whalley, B. (May 1998). "Mechanisms of loess-sized quartz silt production and their relative effectiveness: laboratory simulations". Geomorphology. 23 (1): 15–34. Bibcode:1998Geomo..23...15W. doi:10.1016/S0169-555X(97)00084-6.
  7. ^ Haberlah, D. (June 2007). "A call for Australian loess". Area. 39 (2): 224–229. doi:10.1111/j.1475-4762.2007.00730.x.
  8. ^ "Particle Size (618.43)". National Soil Survey Handbook Part 618 (42-55) Soil Properties and Qualities. United States Department of Agriculture - Natural Resource Conservation Service. Archived from the original on 2006-05-27. Retrieved 2006-05-31.
  9. ^ "Mississippi River". USGS Biological Resources. Archived from the original on 2005-10-28. Retrieved 2006-03-08.
  10. ^ "Bangladesh fights for survival against climate change". Archived from the original on February 12, 2010. Retrieved October 22, 2009.
  11. ^ Leedy, Daniel L.; Franklin, Thomas M.; Maestro, Robert M. (1981). Planning for Urban Fishing and Waterfront Recreation. U.S. Department of the Interior, Fish and Wildlife Service, Eastern Energy and Land Use Team. Archived from the original on 2017-12-24.
  12. ^ Hart 1986, p. 22; Freeman 1997, p. 91.

Alluvium (from the Latin alluvius, from alluere, "to wash against") is loose, unconsolidated (not cemented together into a solid rock) soil or sediment that has been eroded, reshaped by water in some form, and redeposited in a non-marine setting. Alluvium is typically made up of a variety of materials, including fine particles of silt and clay and larger particles of sand and gravel. When this loose alluvial material is deposited or cemented into a lithological unit, or lithified, it is called an alluvial deposit.


Brickearth is a term originally used to describe superficial windblown deposits found in southern England. The term has been employed in English-speaking regions to describe similar deposits.

Brickearths are periglacial loess, a wind-blown dust deposited under extremely cold, dry, peri- or postglacial conditions. The name arises from its early use in making house bricks, its composition being suitable for brick-making without additional material being added and unlike clay its bricks can be hardened (fused) at lower temperatures, including in wood-fired kilns.

The brickearth is normally represented on 1:50,000 solid and drift edition geological maps. In the Thames valley, in broad patches brickearth overlies fluvial terrace gravel; it has been reclassified on later maps as the "Langley Silt Complex".


Clay is a finely-grained natural rock or soil material that combines one or more clay minerals with possible traces of quartz (SiO2), metal oxides (Al2O3 , MgO etc.) and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure. Clays are plastic due to particle size and geometry as well as water content, and become hard, brittle and non–plastic upon drying or firing. Depending on the soil's content in which it is found, clay can appear in various colours from white to dull grey or brown to deep orange-red.

Although many naturally occurring deposits include both silts and clay, clays are distinguished from other fine-grained soils by differences in size and mineralogy. Silts, which are fine-grained soils that do not include clay minerals, tend to have larger particle sizes than clays. There is, however, some overlap in particle size and other physical properties. The distinction between silt and clay varies by discipline. Geologists and soil scientists usually consider the separation to occur at a particle size of 2 µm (clays being finer than silts), sedimentologists often use 4–5 μm, and colloid chemists use 1 μm. Geotechnical engineers distinguish between silts and clays based on the plasticity properties of the soil, as measured by the soils' Atterberg limits. ISO 14688 grades clay particles as being smaller than 2 μm and silt particles as being larger.

Mixtures of sand, silt and less than 40% clay are called loam. Loam makes good soil and is used as a building material.


An earthflow (earth flow) is a downslope viscous flow of fine-grained materials that have been saturated with water and moves under the pull of gravity. It is an intermediate type of mass wasting that is between downhill creep and mudflow. The types of materials that are susceptible to earthflows are clay, fine sand and silt, and fine-grained pyroclastic material.When the ground materials become saturated with enough water, they will start flowing (soil liquefaction). Its speed can range from being barely noticeable to rapid movement. The velocity of the flow is dictated by water content: the higher the water content is, the higher the velocity will be. Because of the dependency on water content for the velocity of the flow, it can take minutes or years for the materials to move down the slope.

Grain size

Grain size (or particle size) is the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. This is different from the crystallite size, which refers to the size of a single crystal inside a particle or grain. A single grain can be composed of several crystals. Granular material can range from very small colloidal particles, through clay, silt, sand, gravel, and cobbles, to boulders.


Loam is soil composed mostly of sand (particle size > 63 µm), silt (particle size > 2 µm), and a smaller amount of clay (particle size < 2 µm). By weight, its mineral composition is about 40–40–20% concentration of sand-silt-clay, respectively. These proportions can vary to a degree, however, and result in different types of loam soils: sandy loam, silty loam, clay loam, sandy clay loam, silty clay loam, and loam. In the USDA textural classification triangle, the only soil that is not predominantly sand, silt, or clay is called "loam". Loam soils generally contain more nutrients, moisture, and humus than sandy soils, have better drainage and infiltration of water and air than silt and clay-rich soils, and are easier to till than clay soils. The different types of loam soils each have slightly different characteristics, with some draining liquids more efficiently than others. The soil's texture, especially its ability to retain nutrients and water are crucial. Loam soil is suitable for growing most plant varieties.

Bricks made of loam, mud, sand, and water, with an added binding material such as rice husks or straw, have been used in construction since ancient times.


Loess (US: , UK: ; from German Löss [lœs]) is a clastic, predominantly silt-sized sediment that is formed by the accumulation of wind-blown dust. 10% of the Earth's land area is covered by loess or similar deposits.Loess is an aeolian sediment formed by the accumulation of wind-blown silt, typically in the 20–50 micrometer size range, twenty percent or less clay and the balance equal parts sand and silt that are loosely cemented by calcium carbonate. It is usually homogeneous and highly porous and is traversed by vertical capillaries that permit the sediment to fracture and form vertical bluffs.

The word loess, with connotations of origin by wind-deposited accumulation, came into English from German Löss, which can be traced back to Swiss German and is cognate with the English word loose and the German word los. It was first applied to Rhine River valley loess about 1821.


Mudrocks are a class of fine grained siliciclastic sedimentary rocks. The varying types of mudrocks include: siltstone, claystone, mudstone, slate, and shale. Most of the particles of which the stone is composed are less than 0.0625 mm (1/16th mm or 0.0025 inches) and are too small to study readily in the field. At first sight the rock types look quite similar; however, there are important differences in composition and nomenclature. There has been a great deal of disagreement involving the classification of mudrocks. There are a few important hurdles to classification, including:

Mudrocks are the least understood, and one of the most understudied sedimentary rocks to date

It is difficult to study mudrock constituents, due to their diminutive size and susceptibility to weathering on outcrops

And most importantly, there is more than one classification scheme accepted by scientistsMudrocks make up fifty percent of the sedimentary rocks in the geologic record, and are easily the most widespread deposits on Earth. Fine sediment is the most abundant product of erosion, and these sediments contribute to the overall omnipresence of mudrocks. With increased pressure over time the platey clay minerals may become aligned, with the appearance of parallel layering (fissility). This finely bedded material that splits readily into thin layers is called shale, as distinct from mudstone. The lack of fissility or layering in mudstone may be due either to the original texture or to the disruption of layering by burrowing organisms in the sediment prior to lithification.

From the beginning of civilization, when pottery and mudbricks were made by hand, to now, mudrocks have been important. The first book on mudrocks, Geologie des Argils by Millot, was not published until 1964; however, scientists, engineers, and oil producers have understood the significance of mudrocks since the discovery of the Burgess Shale and the relatedness of mudrocks and oil. Literature on this omnipresent rock-type has been increasing in recent years, and technology continues to allow for better analysis.


Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.063 millimetres (0.0025 in) with individual grains too small to be distinguished without a microscope. With increased pressure over time, the platy clay minerals may become aligned, with the appearance of fissility or parallel layering. This finely bedded material that splits readily into thin layers is called shale, as distinct from mudstone. The lack of fissility or layering in mudstone may be due to either original texture or the disruption of layering by burrowing organisms in the sediment prior to lithification. Mud rocks such as mudstone and shale account for some 65% of all sedimentary rocks. Mudstone looks like hardened clay and, depending upon the circumstances under which it was formed, it may show cracks or fissures, like a sun-baked clay deposit.Mudstone can be separated into these categories:

Siltstone — more than half of the composition is silt-sized particles.

Claystone — more than half of the composition is clay-sized particles.

Mudstone — hardened mud; a mix of silt and clay sized particles. Mudstone can include:

Shale — exhibits lamination or fissility.

Argillite — has undergone low-grade metamorphism.

Peyto Glacier

The Peyto Glacier is situated in the Canadian Rockies in Banff National Park, Alberta, Canada, approximately 90 km (56 mi) northwest of the town of Banff, and can be accessed from the Icefields Parkway. Peyto Glacier is an outflow glacier from the Wapta Icefield, which rests along the Continental divide. The glacier snout is subject to high melt rates from season to season and there is marked surface lowering on several parts of the glacier. Glacial silt, which is carried from the glacier by streams, ensures a turquoise appearance to Peyto Lake, a popular tourist destination. As is true for the vast majority of glaciers worldwide, Peyto Glacier has been retreating rapidly, especially since the last half of the 20th century, and has reportedly lost 70% of its mass since it was first researched (Demuth and Keller, 2006). Between 1896 and 1966, the total volume loss of the Peyto Glacier was 1088.5 x 106 m3 (Wallace, 1995). In 1987, an automatic weather station was constructed near the glacier that monitors temperature changes, radiation, and precipitation

Peyto Glacier provides the most direct access to Peyto Hut, a base of both summer and winter mountaineering.

Sediment basin

A sediment basin is a temporary pond built on a construction site to capture eroded or disturbed soil that is washed off during rain storms, and protect the water quality of a nearby stream, river, lake, or bay. The sediment-laden soil settles in the pond before the runoff is discharged. Sediment basins are typically used on construction sites of 5 acres (20,000 m2) or more, where there is sufficient room. They are often used in conjunction with erosion controls and other sediment control practices. On smaller construction sites, where a basin is not practical, sediment traps may be used.Essential sediment abundance is prevalent in the construction industry which gives insight to future endeavors.

On some construction projects, the sediment basin is cleaned out after the soil disturbance (earth-moving) phase of the project, and modified to function as a permanent stormwater management system for the completed site, either as a detention basin or a retention basin.


Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments (silt-sized particles) of other minerals, especially quartz and calcite. Shale is characterized by breaks along thin laminae or parallel layering or bedding less than one centimeter in thickness, called fissility. It is the most common sedimentary rock.

Silt, California

Silt is an unincorporated community in Kern County, California. It lies at an elevation of 2,333 feet (711 m).


Siltation or siltification is the pollution of water by particulate terrestrial clastic material, with a particle size dominated by silt or clay. It refers both to the increased concentration of suspended sediments, and to the increased accumulation (temporary or permanent) of fine sediments on bottoms where they are undesirable. Siltation is most often caused by soil erosion or sediment spill.

Sometimes siltation is called sediment pollution, although that is an undesirable term since it is ambiguous, and can also be used to refer to a chemical contamination of sediments accumulated on the bottom, or pollutants bound to sediment particles. Siltation is the preferred term for being unambigiuous, even if not entirely stringent since it also includes other particle sizes than silt.


Siltbreeze is an American independent record label based in Philadelphia. It was founded in 1989.


Siltstone is a sedimentary rock which has a grain size in the silt range, finer than sandstone and coarser than claystones.

Soil texture

Soil texture is a classification instrument used both in the field and laboratory to determine soil classes based on their physical texture. Soil texture can be determined using qualitative methods such as texture by feel, and quantitative methods such as the hydrometer method. Soil texture has agricultural applications such as determining crop suitability and to predict the response of the soil to environmental and management conditions such as drought or calcium (lime) requirements. Soil texture focuses on the particles that are less than two millimeters in diameter which include sand, silt, and clay. The USDA soil taxonomy and WRB soil classification systems use 12 textural classes whereas the UK-ADAS system uses 11. These classifications are based on the percentages of sand, silt, and clay in the soil..


Subsoil is the layer of soil under the topsoil on the surface of the ground. Like topsoil it is composed of a variable mixture of small particles such as sand, silt and/or clay, but with a much lower percentage of organic matter and humus. Below the subsoil is the substratum, which can be residual bedrock, sediments, or aeolian deposits. As it is lacking in dark humus, subsoil is usually paler in color than the overlying topsoil. It may contain the deeper roots of some plants,such as trees,but a majority of plant roots lie within the topsoil.

Clay-based subsoil has been the primary source of material for adobe, cob, rammed earth, wattle and daub, and other earthen construction methods for millennia. Coarse sand, the other ingredient in most of these materials, is also found in subsoil.

Although by no means sterile, subsoil is relatively barren in terms of soil organisms compared to humus-rich topsoil.

Unified Soil Classification System

The Unified Soil Classification System (USCS) is a soil classification system used in engineering and geology to describe the texture and grain size of a soil. The classification system can be applied to most unconsolidated materials, and is represented by a two-letter symbol. Each letter is described below (with the exception of Pt):

If the soil has 5–12% by weight of fines passing a #200 sieve (5% < P#200 < 12%), both grain size distribution and plasticity have a significant effect on the engineering properties of the soil, and dual notation may be used for the group symbol. For example, GW-GM corresponds to "well-graded gravel with silt."

If the soil has more than 15% by weight retained on a #4 sieve (R#4 > 15%), there is a significant amount of gravel, and the suffix "with gravel" may be added to the group name, but the group symbol does not change. For example, SP-SM could refer to "poorly graded SAND with silt" or "poorly graded SAND with silt and gravel."

Retaining walls
Numerical analysis

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