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.
Where the coastline changes direction abruptly, as at the opening of bay, longshore drift often continues in the same direction. The load of sand and gravel will be dropped in the deeper water, leading to the growth of a spit down drift. The spit may grow until it joins the other side of the bay and form a tombolo or ayre.
The western coasts of Orkney possess a remarkable array of boulder beaches which reflect the frequency of high seas and the availability of bedrock for quarrying. The beach crests may extend many metres above sea level and merge into angular cliff top storm deposits.
Beaches of sand and boulders would appear to be the most unlikely landform to be found facing the high-energy environment of the open ocean. The beach is far from stable - it adjusts its shape continuously in response to changing wave energy and so maintains a dynamic equilibrium with its environment.
The continuing rise in sea level and the significant tidal range on Orkney has favoured the development and persistence of salt marsh. Fine-grained sediment is trapped behind bars and ayres and gradually migrates landward, as erosion removes material from the seaward edge and shifts it inland. Orkney salt marshes are small and exist usually in areas sheltered from strong wave action.
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