Sediment facts for kids

Cobbles on a beach

River Rhône flowing into Lake Geneva. Sediments make the water appear brownish-grey; they are an indicator of increased water runoff, land degradation, erosion due to intensive industrialized land use, land sealing, and poor soil management.

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 in the Gulf of Mexico

Sediment off the Yucatán Peninsula

Sediment can be classified based on its grain size or composition.

Grain size

Main page: Particle size (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:

• 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).

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.