In Earth and planetary sciences, sediment is a solid material made of loose particles that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand and silt can be carried in suspension in river water and on reaching the sea bed deposited by sedimentation; if buried, they may eventually become sandstone and siltstone (sedimentary rocks) through lithification. Sediments are most often transported by water (fluvial processes), but also wind (aeolian processes) and glaciers. Beach sands and river channel deposits are examples of fluvial transport and deposition, though sediment also often settles out of slow-moving or standing water in lakes and oceans. Desert sand dunes and loess are examples of aeolian transport and deposition. Glacial moraine deposits and till are ice-transported sediments.
Classification
Sediment can be classified based on its grain size, grain shape, and composition.
Grain size
Sediment size is measured on a log base 2 scale, called the "Phi" scale, which classifies particles by size from "colloid" to "boulder".
Shape
The shape of particles can be defined in terms of three parameters. The form is the overall shape of the particle, with common descriptions being spherical, platy, or rodlike. The roundness is a measure of how sharp grain corners are. This varies from well-rounded grains with smooth corners and edges to poorly rounded grains with sharp corners and edges. Finally, surface texture describes small-scale features such as scratches, pits, or ridges on the surface of the grain.
Form
Form (also called sphericity) is determined by measuring the size of the particle on its major axes. William C. Krumbein proposed formulas for converting these numbers to a single measure of form, such as
ψ l = D S D I D L 2 3 {\displaystyle \psi _{l}={\sqrt[{3}]{\frac {D_{S}D_{I}}{D_{L}^{2}}}}}
where D L {\displaystyle D_{L}} , D I {\displaystyle D_{I}} , and D S {\displaystyle D_{S}} are the long, intermediate, and short axis lengths of the particle. The form ψ l {\displaystyle \psi _{l}} varies from 1 for a perfectly spherical particle to very small values for a platelike or rodlike particle. An alternate measure was proposed by Sneed and Folk:
ψ p = D S 2 D L D I 3 {\displaystyle \psi _{p}={\sqrt[{3}]{\frac {D_{S}^{2}}{D_{L}D_{I}}}}}
which, again, varies from 0 to 1 with increasing sphericity.
Roundness
Roundness describes how sharp the edges and corners of particle are. Complex mathematical formulas have been devised for its precise measurement, but these are difficult to apply, and most geologists estimate roundness from comparison charts. Common descriptive terms range from very angular to angular to subangular to subrounded to rounded to very rounded, with increasing degree of roundness.
Surface texture Surface texture describes the small-scale features of a grain, such as pits, fractures, ridges, and scratches. These are most commonly evaluated on quartz grains, because these retain their surface markings for long periods of time. Surface texture varies from polished to frosted, and can reveal the history of transport of the grain; for example, frosted grains are particularly characteristic of aeolian sediments, transported by wind. Evaluation of these features often requires the use of a scanning electron microscope.
Composition Composition of sediment can be measured in terms of:
Parent rock lithology Mineral composition Chemical make-up This leads to an ambiguity in which clay can be used as both a size-range and a composition (see clay minerals).
Sediment transport
Sediment is transported based on the strength of the flow that carries it and its own size, volume, density, and shape. Stronger flows will increase the lift and drag on the particle, causing it to rise, while larger or denser particles will be more likely to fall through the flow.
Fluvial processes
Aeolian processes: wind
Wind results in the transportation of fine sediment and the formation of sand dune fields and soils from airborne dust.
Glacial processes
Glaciers carry a wide range of sediment sizes, and deposit it in moraines.
Mass balance
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