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.
Sediment can be classified based on its grain size or composition.
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
|< -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 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).
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Sediment Facts for Kids. Kiddle Encyclopedia.