Sediment
Sediment is loose solid material such as sand, silt, clay, gravel, or shell fragments that has been moved and deposited by water, wind, or ice.
Definition
Sediment is loose bits of rock, sand, mud, or shells that have been carried by water, wind, or ice and dropped somewhere new; it is the material that can turn into sedimentary rock. It is unconsolidated particulate material produced by weathering and transported by erosion, then deposited when the transporting medium loses energy. Sediment is classified by grain size from clay through silt, sand, and gravel, and by origin as clastic (rock fragments), chemical (precipitated minerals), or biogenic (shells and organic debris). Transport depends on flow velocity, grain size, and density, relationships summarized in the Hjulström curve: sand around $0.1$ to $1\ \text{mm}$ is the easiest size to erode, while clay resists erosion through cohesion yet stays suspended once mobilized. Sorting, rounding, composition, and sedimentary structures record transport distance, energy, and depositional environment. Global rivers deliver on the order of $15$ to $20$ billion tonnes of sediment to the oceans each year.
Example
The mud that settles at the bottom of a puddle after rain is sediment, and so is the sand on the beach at the Outer Banks. The Catawba River carries fine sediment into Lake Norman and Lake Wylie, where it settles and slowly fills the reservoirs, and barrier islands like Hatteras are made entirely of sand-sized sediment moved by waves and wind. Mississippi River sediment builds the delta seaward, but levees and dams now trap so much upstream that coastal Louisiana loses land as the delta subsides faster than it is replenished. Deep-sea sediment cores record climate through the oxygen-isotope ratios of foraminifera shells.
Key Insight
Sediment is rock on the move: it used to be part of a bigger rock and might become part of a new rock someday. Faster water carries bigger grains, so a flood can move boulders while still water drops even the finest clay, which is why sediment sorts itself into layers by size. Sediment is a human-scale problem, not only a geologic one. Excess sediment from construction and farming is a leading pollutant in North Carolina streams, smothering habitat and carrying attached phosphorus and pesticides.