Deposition is the process by which sediment settles out of water or air.
Transporting agents such as rivers, waves, glaciers, wind, and ocean currents can carry particles away from their source. When the transporting medium loses enough energy, those particles are deposited. Sand may accumulate in dunes or river bars, mud can settle in lakes and floodplains, and gravel may collect where a stream slows.
Deposition builds sedimentary layers over time. The resulting deposits can later be compacted and cemented into sedimentary rock, although deposition itself is not the same as lithification. Layer thickness, grain size, sorting, and sedimentary structures can reveal the environment in which deposition occurred.
Deposition is often confused with erosion. Erosion removes and transports material, while deposition places transported material somewhere new. Weathering breaks rock down in place, and metamorphism changes existing rock through heat, pressure, or chemically active fluids. Together, these processes help reshape landscapes and preserve geological records.