In geography, longshore drift (LSD) (or littoral drift) refers to a process by which sediments move along a beach shoreline. The process arises when waves approach the shore obliquely (which in turn is determined by factors such as prevailing wind and fetch). Waves striking the shore at an angle as opposed to straight on will cause the wave swash to move up the beach at an angle. The swash moves the sediment particles (usually sand or shingle) up the beach at this angle, while the backwash brings them straight down the beach. This has the net effect of a slow movement of the particles along the shore. A net movement of water also occurs establishing a longshore current. Longshore drift is one of the principal processes in the construction of spits, bars and tombolos, and the overall sustainability of beach deposits.