An alluvial deposit is a sedimentary deposit that is formed by the accumulation of SILT, MUD, and other materials from rivers and streams. It is often found in the form of a DELTA, which is a triangular-shaped landform created by the deposition of sediment at the mouth of a river. Geest is a type of alluvial deposit that is composed of sand and gravel. Alluvial deposits are important for providing fertile soil for agriculture and are also used in the construction of roads and buildings.