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.

  • The outer coast of Orkney is an outstanding location to study the erosion of hard rock coasts. Evidence of major erosion is not hard to find. The great bowl of Enegars corrie is losing its edge to the sea. The egg-shaped headland of Marwick Head is a dome half lost.

  • Amongst the many beauties of Orkney are the beaches which fringe the bays and ayres of the inner coast. These are dynamic forms, changing shape with the seasons and gradually retreating landward as sea level rises.

  • Orkney is a dissected landmass drowned by postglacial sea level rise. A drop in sea level of just 35 m would unite the archipelago into a single island. The individual islands may represent hills that formed the watershed areas of preglacial drainage basins but it is linear glacial erosion which has separated one island from another and severed Orkney from the rest of Scotland.

  • World coasts have seen sea-level variation of approximately 100 metres within the past 11,500 years through melting of the ice caps. When the ice caps melt the sea level rises globally (eustatically). The relative sea-level at any location is measured proportionate to the nearby land, which is itself subject to tectonic movement both up and down.