Deposition
Deposition is the geologic process in which transported sediment settles out of water, wind, or ice and accumulates in a new location.
Definition
Deposition is when moving water, wind, or ice slows down and drops the sediment it was carrying, and the dropped sediment piles up in layers. It occurs when the energy of a transporting agent falls below what is needed to carry its load; coarse grains drop first and fine grains last, so deposits are sorted by size, and landforms built this way include deltas, alluvial fans, floodplains, sand dunes, beaches, and glacial moraines. Settling velocity for small grains follows Stokes' law, $v = \frac{2}{9}\frac{(\rho_p - \rho_f) g r^2}{\mu}$, so it scales with the square of grain radius. When flow velocity falls below the settling threshold for a given size, that fraction drops out, producing graded bedding in turbidity currents and fining-upward sequences in meandering rivers. Depositional environments are inferred from facies: sedimentary structures, grain size trends, and fossils that together indicate deltaic, fluvial, eolian, or marine settings.
Example
Where a river flows into a lake, it slows down and drops its sand and mud, building a fan-shaped delta. The sound side of the Outer Banks grows by deposition as storms wash sand over the islands, and the Mississippi Delta and Long Island, a glacial moraine, are both entirely depositional landforms. A single clay particle of $2$ micrometers takes weeks to settle through $100$ meters of still water, which is why deep-sea clays accumulate at only millimeters per thousand years. By contrast, a Mississippi flood can deposit tens of centimeters of silt on a floodplain in days.
Key Insight
Erosion takes away and deposition adds, so a beach that shrinks in one storm can be rebuilt by deposition in calmer weather. Deposition follows a size order: gravel drops near the mountains, sand along the river, and clay only in quiet lakes or the deep sea, which is why sediment layers can reveal the environment they formed in. Accommodation space, the room available below base level, controls whether deposition can happen at all. Rising sea level creates accommodation and preserves thick sequences, while falling sea level exposes deposits to erosion, which is why the stratigraphic record is full of gaps.