Sediment is the matter which falls to the bottom of oceans and lakes. Compressed and gradually transformed, sediment is the basis of sedimentary rocks. The term is also used for material transported by wind or water, and deposited on the surface. In due course, this may also turn into rock. The study of how sedimentary rocks are formed is called sedimentology.
In more general use, sediment is any matter which falls to the bottom of a liquid, such as beer.
Classification
Sediment can be classified based on its grain size or 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".
| φ scale | Size range (metric) |
Size range (inches) |
Aggregate class (Wentworth) |
Other names |
|---|---|---|---|---|
| < -8 | > 256 mm | > 10.1 in | Boulder | |
| -6 to -8 | 64–256 mm | 2.5–10.1 in | Cobble | |
| -5 to -6 | 32–64 mm | 1.26–2.5 in | Very coarse gravel | Pebble |
| -4 to -5 | 16–32 mm | 0.63–1.26 in | Coarse gravel | Pebble |
| -3 to -4 | 8–16 mm | 0.31–0.63 in | Medium gravel | Pebble |
| -2 to -3 | 4–8 mm | 0.157–0.31 in | Fine gravel | Pebble |
| -1 to -2 | 2–4 mm | 0.079–0.157 in | Very fine gravel | Granule |
| 0 to -1 | 1–2 mm | 0.039–0.079 in | Very coarse sand | |
| 1 to 0 | 0.5–1 mm | 0.020–0.039 in | Coarse sand | |
| 2 to 1 | 0.25–0.5 mm | 0.010–0.020 in | Medium sand | |
| 3 to 2 | 125–250 μm | 0.0049–0.010 in | Fine sand | |
| 4 to 3 | 62.5–125 μm | 0.0025–0.0049 in | Very fine sand | |
| 8 to 4 | 3.9–62.5 μm | 0.00015–0.0025 in | Silt | Mud |
| > 8 | < 3.9 μm | < 0.00015 in | Clay | Mud |
| >10 | < 1 μm | < 0.000039 in | Colloid | Mud |
Composition
Composition of sediment can be measured in terms of:
This leads to an ambiguity in which clay can be used as both a size-range and a composition (see clay minerals).
Images for kids
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Sediment builds up on human-made breakwaters because they reduce the speed of water flow, so the stream cannot carry as much sediment load.
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Glacial transport of boulders. These boulders will be deposited as the glacier retreats.
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Modern asymmetric ripples developed in sand on the floor of the Hunter River, New South Wales, Australia. Flow direction is from right to left.
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Sinuous-crested dunes exposed at low tide in the Cornwallis River near Wolfville, Nova Scotia
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Ancient channel deposit in the Stellarton Formation (Pennsylvanian), Coalburn Pit, near Thorburn, Nova Scotia.
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Glacial sediments from Montana
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Holocene eolianite and a carbonate beach on Long Island, Bahamas.
