This report is a contribution to the Strategic Environmental Assessment (SEA7) conducted by the Department of Trade and Industry (now Department of Energy and Climate Change). The coastal and marine areas within the SEA 7 boundary are very extensive, longer and more varied than any other SEA area. Stretching westwards into the Atlantic Ocean, the region includes a very large area of relatively shallow continental shelf. There are also a few uninhabited islands to the west of the Outer Hebrides, including St. Kilda with its multiple conservation designations.

The Outer Hebrides form one of the oldest geological provinces in Europe and is mainly based on Lewisian Gneiss. A major geological fault runs parallel to the east coast (The Minch coast), whereas the western side is normally a continuation of the continental shelf. These features are prominent in the southern islands, notably the Uists. Harris and Lewis present different topographies which include the high massif with Clisham at its centre and the low peat-covered plateaux of most of Lewis to the north. The east Minch coastline is generally steeper and falls to deeper inshore waters. It is also characterised by several transverse sea lochs which resemble similar fjords on the west mainland coast.

Although the legacy of glacial processes are complex, the main effect has been to over-deepen sea lochs and inter-island straits (e.g. Sound of Harris) and deposit great masses of glacial debris, especially sands, on the shallow continental shelf to the west where, with the prevalence of strong onshore Atlantic waves and winds, vast beaches were formed in this post glacial period.

Large quantities of organic sand (crushed shells) were added to this volume; as a consequence some of the larger beach and sand dune systems in Britain are found along the west coast. These extensive blown sand systems are called machairs, and provide a unique series of environmental and ecological systems with very high conservational status at European and international levels.

The west coast of the Outer Hebrides contains many sites of archaeological interest and retains a distinctive cultural landscape as one of the last strongholds of a historical system of land tenure and working found nowhere else in Europe. This way of life is considered to be one of the prime reasons for the creation and maintenance of many of the significant conservational attributes of the Outer Hebrides.