Sediment is a naturally occurring material that is broken down by processes of
weathering and
erosion, and is 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
Suspension or suspended may refer to:
Science and engineering
* Suspension (topology), in mathematics
* Suspension (dynamical systems), in mathematics
* Suspension of a ring, in mathematics
* Suspension (chemistry), small solid particles suspende ...
in river water and on reaching the sea bed deposited by
sedimentation
Sedimentation is the deposition of sediments. It takes place when particles in suspension settle out of the fluid in which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to the ...
; 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
Aeolian processes, also spelled eolian, pertain to wind activity in the study of geology and weather and specifically to the wind's ability to shape the surface of the Earth (or other planets). Winds may erode, transport, and deposit mate ...
) 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
A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate betwe ...
moraine deposits and
till
image:Geschiebemergel.JPG, Closeup of glacial till. Note that the larger grains (pebbles and gravel) in the till are completely surrounded by the matrix of finer material (silt and sand), and this characteristic, known as ''matrix support'', is d ...
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
: