Nicolson's Leap on the east coast of South Uist
. In the background are Beinn Mhòr at left, and Hecla on the right.
The islands form an archipelago whose major islands are Lewis and Harris, North Uist, Benbecula, South Uist, and Barra. Lewis and Harris has an area of 217,898 hectares (841 sq mi) and is the largest island in Scotland and the third largest in the British Isles, after Great Britain and Ireland. It incorporates Lewis in the north and Harris in the south, both of which are frequently referred to as individual islands, although they are connected by land. The island does not have a single name in either English or Gaelic, and is referred to as "Lewis and Harris", "Lewis with Harris", "Harris with Lewis" etc.
The largest islands are deeply indented by arms of the sea such as Loch Ròg, Loch Seaforth and Loch nam Madadh. There are also more than 7,500 freshwater lochs in the Outer Hebrides, about 24% of the total for the whole of Scotland. North and South Uist and Lewis in particular have landscapes with a high percentage of fresh water and a maze and complexity of loch shapes. Harris has fewer large bodies of water but has innumerable small lochans. Loch Langavat on Lewis is 11 kilometres (7 mi) long, and has several large islands in its midst, including Eilean Mòr. Although Loch Suaineabhal has only 25% of Loch Langavat's surface area, it has a mean depth of 33 metres (108 ft) and is the most voluminous on the island. Of Loch Sgadabhagh on North Uist it has been said that "there is probably no other loch in Britain which approaches Loch Scadavay in irregularity and complexity of outline." Loch Bì is South Uist's largest loch and at 8 kilometres (5 mi) long it all but cuts the island in two.
Much of the western coastline of the islands is machair, a fertile low-lying dune pastureland. Lewis is comparatively flat, and largely consists of treeless moors of blanket peat. The highest eminence is Mealisval at 574 m (1,883 ft) in the south west. Most of Harris is mountainous, with large areas of exposed rock and Clisham, the archipelago's only Corbett, reaches 799 m (2,621 ft) in height. North and South Uist and Benbecula (sometimes collectively referred to as The Uists) have sandy beaches and wide cultivated areas of machair to the west and virtually uninhabited mountainous areas to the east. The highest peak here is Beinn Mhòr at 620 metres (2,034 ft). The Uists and their immediate outliers have a combined area of 74,540 hectares (288 sq mi). This includes the Uists themselves and the islands linked to them by causeways and bridges. Barra is 5,875 hectares (23 sq mi) in extent and has a rugged interior, surrounded by machair and extensive beaches.
The scenic qualities of the islands are reflected in the fact that three of Scotland's forty national scenic areas (NSAs) are located here. The national scenic areas are defined so as to identify areas of exceptional scenery and to ensure its protection from inappropriate development, and are considered to represent the type of scenic beauty "popularly associated with Scotland and for which it is renowned". The three NSA within the Outer Hebridies are:
Flora and fauna
Much of the archipelago is a protected habitat, including both the islands and the surrounding waters. There are 53 Sites of Special Scientific Interest of which the largest are Loch an Duin, North Uist (15,100 hectares (37,000 acres)) and North Harris (12,700 hectares (31,000 acres)). South Uist is considered the best place in the UK for the aquatic plant Slender Naiad, which is a European Protected Species.
There has been considerable controversy over hedgehogs on the Uists. Hedgehogs are not native to the islands, but were introduced in the 1970s to reduce garden pests. Their spread posed a threat to the eggs of ground nesting wading birds. In 2003 Scottish Natural Heritage undertook culls of hedgehogs in the area, but these were halted in 2007; trapped animals are now relocated to the mainland.
Nationally important populations of breeding waders are present in the Outer Hebrides, including common redshank, dunlin, lapwing and ringed plover. The islands also provide a habitat for other important species such as corncrake, hen harrier, golden eagle and otter. Offshore, basking shark and various species of whale and dolphin can often be seen, and the remoter islands' seabird populations are of international significance. St Kilda has 60,000 northern gannets, amounting to 24% of the world population; 49,000 breeding pairs of Leach's petrel, up to 90% of the European population; and 136,000 pairs of puffin and 67,000 northern fulmar pairs, about 30% and 13% of the respective UK totals. Mingulay is an important breeding ground for razorbills, with 9,514 pairs, 6.3% of the European population.
The bumblebee Bombus jonellus var. hebridensis is endemic to the Hebrides and there are local variants of the dark green fritillary and green-veined white butterflies. The St Kilda wren is a subspecies of wren whose range is confined to the islands whose name it bears.