Marram, St Ninian's Ayre
Dune-beach system, Spiggie
Dune sands overlying Holocene peat
Dunes form when sand is blown from the beach and becomes stabilised by the growth of vegetation, notably marram grass. The scale and character of the dune system depends on the interaction between physical factors, such as wind and wave regime and the sand supply, and biotic controls, such as plant succession and grazing pressure. Unconsolidated sand is by its nature a highly unstable substrate and the survival of dunes depends on the ability of plants to maintain at least a partial cover on the dunes. Where dune faces have been eroded, it is often possible to observe the complexity of dune history, with buried organic layers marking periods of stability, vegetation cover and soil stability and cross-bedded sands representing period of sand deposition.
The dune systems of the middle coast of Shetland are small, isolated and highly variable in form. Individual characteristics depend on exposure, the shape of the terrain to landward and the local configuration of the coastline. Amongst the youngest of the Shetland coastal forms, the dunes are highly dynamic due to the windy and high wave energy environment of Shetland. Many are fragile forms - as shown by exposure of the prehistoric settlement of Skara Brae on Orkney during a fierce stormin the winter of 1850 and the gradual destruction of the broch at Sand Wick. Man made pressures, notably the removal of sand by quarrying from the dune system may lead to rapid and irreversible erosion.
There has been no detailed study of historic changes in these fragile systems, despite the importance of dunes on Shetland as habitat for rare plants and animals.