When a coastline turns abruptly landwards, as at the entrance of a bay, and the long shore current does not turn with the coastline, sand and gravel accumulate in the direction of the current as a spit. The sediment load is dropped into slow-moving deep water. If the tidal or stream flow is strong enough to maintain a channel then the further growth of the spit is prevented. The tip of the spit may start to curve to landward in response to a secondary direction of wave approach. If the channel is not maintained by tidal flow then the spit will continue to grow and eventually seal off the bay to form a tombolo.
In Orkney, the spit or tombolo is termed an ayre (aire) and commonly encloses a small salt-water lagoon or oyce (uiss). Continuing sea level rise is producing landward migration of the beach, forcing the oyce inland.
Barrier Islands are related features formed by the longshore extension of spits that are subsequently broken through by storm waves. A series of disconnected islands is created such as Mirkady and Scarf Points in Deer Sound.