The extensive coastline of the Outer Hebrides is a dominant feature of the natural landscape. Generally speaking, the east cost tends largely to be cliff-bound, with the deep waters of the Minch close inshore, while much of the west coast has gently sloping sandy beaches.
Immediately behind the sandy foreshore there is commonly a narrow belt of dunes separating the coastal edge from the interior of the islands. The tough Marram grass (Ammophila arenaria) which helps to stabilise the constantly shifting sands dominates the vegetation of these dunes. Also present may be Curled Dock (Rumex crispus), and Lady's Bedstraw (Galium verum), with Sea Sandwort (Honkenya peploides) towards the foreshore and Daisy (Bellis perennis) and White Clover (Trifolium repens) further inland.
Constant erosion and deposition by countless years of onshore winds and tides has created the low, sandy coastal plain known as machair which lies further inland. In the Outer Hebrides the machair areas lie along the west coast, particularly in the Uists and Benbecula where nearly 6,000 ha of machair make up about 8% of the land area.
The intimate mixing of the calcium rich sands with the acidic peat of the moorland edge has produced the "blackland", a fertile belt separating machair and moor along which many crofting villages are now located. The fertility of the machair has been enhanced by generations of crofters through rotational cropping (ploughing/planting different areas in alternate years), by fertilising the sandy soil with seaweed from the local shore, and by grazing with sheep and cattle. In this way the thin soils of the machair can be used to grow crops, yet it is protected from overcropping and overgrazing which would damage the surface covering and result in the ultimate destruction of the machair itself. The vegetation surface of the dune-and-machair areas is thin and fragile and once broken, it is liable to be quickly blown away by winter winds and tides.
The grazing mix by cattle and sheep and the fertilisation by their dung encourage a wide variety of wild flowers and in the summer months the machair is carpeted with a blaze of natural colour. In addition to many of the common flowers of the seashore and the moor, several calcium-loving plants flourish and several species of orchids are found, including many local variations.
The rich invertebrate population of the machair means that it is important for ground nesting birds such as waders, some of which nest in huge numbers, especially on the machairs of the Uists. Dunlin, Redshank, Peewit, Oystercatcher, and Ringed plover nest here, with Purple sandpiper, Turnstone, and various Terns and Gulls nesting in close proximity. These coasts are very important feeding grounds for a whole range of species and their inland breeding sites are richly fertilised by their guano.