Voes, Ayres and Beaches
A rise in sea level following the melting of the glaciers about 10,000 years ago was responsible for the drowned landscape of the inner coasts of Orkney and Shetland. The clearest signs of this drowning are the many long open inlets or 'voes' of the Shetland islands. These voes mark the former courses of river valleys, later hollowed out by the glaciers, before being flooded by the sea. The steeply sloping and indented character of this drowned landscape has generally hindered the formation of large, sandy beaches around Shetland. In contrast, flooding of the more gently undulating Orkney landscape has formed broad open bays, generally backed by sand dunes. Layers of peat, some containing tree trunks and roots, occur beneath the sand and shingle of some modern beaches. Some layers are submerged to depths of 8 to 9 metres in several of Shetland's sheltered voes, illustrating the extent of sea-level rise in the last 5000 to 6000 years.
Narrow spits of shingle or sand, called 'ayres', are commonly found cutting across the landward and seaward ends of shallow bays and voes. These spits may partly, or completely, cut off a sheltered stretch of water from the sea to form a shallow freshwater loch or 'oyce', which may silt up to become a stretch of fertile land. Some spits form tombolos, joining islands to offshore isles; outstanding examples are the triple tombolos joining For a Ness to Mainland Shetland at Delting and the sand-capped gravel tombolo linking St Ninians Isle to the south-western coast of Shetland. The St Ninians tombolo has existed since the Dark Ages when a church was sited on the Isle.
During the Second World War, man-made causeways were constructed and ships were scuttled to block many of the narrow tidal entrances to the eastern end of Scapa Flow, the deepwater harbour in Orkney used by the Royal Navy to protect the fleet from submarine attack. These causeways, known as the Churchill Barriers, now carry roads linking islands such as Burray and South Ronaldsay to Mainland Orkney. They have altered the currents in tidal inlets and allowed sand flats and even low dune systems to build out from the barriers or shores of the islands into the sea during the last fifty years, in a similar manner to sediment that is accumulating more slowly behind the sand and shingle ayres.