A large proportion of the inland areas of the Outer Hebrides consists of moorland, characterised by a blanketing accumulation of nutrient poor peat. This is especially the case in Lewis, where it has been estimated there is a covering of peat nearly 595 km2 and averaging a depth of 1.5 m.
Deposits of peat are formed when the amount of water entering the soil exceeds the amount of water which escapes. This high water content in the soil prevents dead vegetation decomposing, resulting in the plant remains being preserved and slowly forming into accumulations of peat. Drainage of the moor varies from area to area with the result that peat tends to accumulate faster in rock depressions and slower on knolls, giving rise to the distinctive undulating appearance of much of northern Lewis. Some wetter parts of the moor may result in boggy areas and an intricate scattering of shallow peat lochans called pool systems.
The peatland hosts a wide variety of breeding birds such as Red Grouse, Golden Plover, Greenshank, Dunlin, and Skylark as well as many migrant species such as swans and various geese. Large inland colonies of Gulls (Herring, Great Black-backed, Lesser Black-backed), Terns (Arctic and Common), and Skuas (Great and Arctic) also breed in the Outer Hebrides and their phosphate-rich droppings result in lush green areas spread around their breeding sites. Red Deer and Hare are also present in some peatland areas, but they are generally more limited in numbers.
The peatland has long been used by crofters as rough grazing for sheep and cattle. Most of the peatland of the Outer Hebrides is divided up into common grazing areas which are allocated to individual crofting townships and managed by an elected local committee. In many areas, some of the crofters have opted to fence-off part of the moor, then spread sand (to reduce the acidity), fertiliser, and grass seed, to create more pasture for their livestock. This may be done individually or as a group of crofters. In a few places where the moor is not under crofting tenure, the local estate may manage it for grouse shooting. Strips of vegetation are burned regularly every few years in order to encourage new growth to feed the grouse. The shooting rights are then sold to visiting hunters.
In addition to the utilisation of the moor for grazing, each croft, and many of the non-crofting families, have access to a small area of the moor for cutting peat. This is largely done manually using a spade-shaped implement, and once dried the peat is transported home for use in domestic cooking and heating. The surface skin of vegetation is replaced behind the cut areas to enable plant growth (and peat formation) to continue. In recent years some mechanical peat cutting has started, both by individuals and as small-scale commercial enterprise, but still solely for domestic use. The rough topography and uneven depth of peat does not permit large-scale commercial peat extraction in the Outer Hebrides.