The rocks of Skye represent an immense range of geological processes and around 1000 million years of Scotland's history – encompassing snapshots of some of Scotland's most spectacular geology. The Sleat Peninsula contains some of the oldest rocks in Europe, with the Trotternish Ridge hosting the most spectacular landslides in Britain.
The oldest rocks on Skye resulted from the deposition of sand, gravel and mud in rivers around 1000 million years ago – forming red Torridonian sandstones. Between 550 and 450 million years ago, the area was part of a shallow ocean fringed by white beach sand (burrowed by early worms) and covered with carbonate mud which eventually became Cambrian and Ordovician sandstones and limestones. There are also sedimentary rocks representative of all three geological periods in the Mesozoic Era (the Triassic, Jurassic and Cretaceous periods). These were deposited either by rivers or in shallow marine conditions between 245 and 90 million years ago and all contain a range of fossils including dinosaur remains and spectacular ammonites. The remains of ancient volcanic systems on Skye, which include the largest expanse of plateau basalt lavas in the UK, and igneous intrusions, date from an intense period of volcanic activity along the western margin of Scotland. This represents the time when NW Europe was being split from North America, with the formation of the North Atlantic. The roots of some of these volcanoes are represented by the Cuillin which has two main ridges: the magnificent Black Cuillin, which is primarily composed of the rocks gabbro and peridotite. They contrast with the red-coloured granite of the Red Hills to the east. These rocks formed between 59 and 55 million years ago (during the Palaeogene period). The emplacement of the intrusive complexes beneath the volcanoes resulted in the contact (thermal) metamorphism of the country rocks – both earlier lavas and sedimentary rocks. This has produced hornfelses, metalimestones and skarns.
The final form of the Skye owes much to the work of ice and frost and is one of the most outstanding areas in Britain for erosional landforms associated with glaciers (formed during the last Ice Age, between 26,000 and 13,000 years ago). It includes unquestionably the finest assemblage of aretes, corries, rock steps, ice-moulded topography and roche moutonnees, together with the 'textbook' glacial trough and basin occupied by Loch Coruisk and superb examples of ice-deposited sediments in Glen Sligachan. The ultimate veneer has resulted from frost-shattering on the oversteepened cliffs and pinnacles, so that slopes are everywhere draped with scree. The most spectacular of these is the Great Stone Shoot, which plummets for over 450m from Alisdair's summit to the tiny corrie lake of Coire Lagan. Where the Palaeogene age lavas are underlain by weaker sediments on the scarp landscapes around Skye there are spectacular large-scale landslides, which are unique in Britain. The most spectacular are the Quirang, Table, Needle, Prison, Dun Dubh and the Old Man of Storr, all forming part of the great escarpment of Trotternish. Also with the melting of the Skye glacier, the removal of the weight of the ice caused the land to slowly rise, producing classic raised beaches around the coast, and leaving Skye with its landscape of today.