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Regional Geology, Highlands

The Northern and west Highlands, to the north of the Great Glen Fault, has some of the most varied geology and spectacular scenery in Scotland.

Loch Laxford a road cut in Lewisian Gneiss.
Image provided by Scottish Natural Heritage.

In the far north-west are the Lewisian gneisses, which are nearly 3000 million years old. Over their long history, they have been heated and compressed deep beneath the Earth's surface. Thick layers of red Torridonian sandstones which form spectacular mountain landscapes, were laid down upon the gneiss by ancient river systems some 1000 million years ago.

Capping some of the Torridonian mountains are rocks, originally lime-rich mud and worm-burrowed beach sands, that were deposited during Cambrian and Ordovician times, between around 550 and 450 million years ago.

These rock sequences have been faulted and disrupted along what geologist term the Moine Thrust Belt. This zone formed 430 million years ago, when England and Scandinavia collided with Scotland, during the mountain-building event known as the Caledonian Orogeny, producing the Caledonian mountain chain.

Slioch Torridonian Sandstone infilling an ancient valley cut into Lewisian Gneiss, viewed from across Loch Maree.
Image provided by Scottish Natural Heritage.

To the east of that belt, most of the Northern Highlands is underlain by rocks known as the Moine Supergroup. These were originally sands and muds deposited in an ancient ocean, 1000 million years ago.

Since then they have been deformed and metamorphosed, most recently during the Caledonian Orogeny. Weathering and erosion of the mountains during Devonian times, between 400 and 360 million years ago, gave rise to river and lake-deposited sediment which underlies much of Caithness.

Ardnamurchan the roots of a Tertiary volcanic centre.
Image (c) Patricia MacDonald of Aerographica.

Sedimentary rocks dating from the Triassic, Jurassic and Cretaceous periods occur at the coastal margins of the region. However, the youngest rocks occur on Mull and Ardnamurchan, representing the remains of 60 million year old volcanic complexes that were erupting as continental drift split Scotland from North America, with the opening of the North Atlantic.

Geomorphologically, this is an area of great contrasts, between the ice-moulded and peat and till-covered Caithness lowlands and the heavily ice-scoured uplands and lowlands of Sutherland.

This region contains an outstanding array of mountain glacial landforms, such as valleys over-deepened by glacial erosion. Other features of glacial erosion include the many corries, rock steps, ice-moulded bedrock and roches moutonnées. The terraces at Achnasheen formed as outwash deltas into an ice-dammed lake.

A distinctive feature of the area is the occurrence of karst landforms and caves at Durness and Assynt. The latter being the longest cave system in Scotland. Some of the caves have yielded animal remains dating to the time of the last glaciation. Along the coast, spectacular fjords occur where the sea has flooded the lower reaches of ice eroded valleys. Raised shorelines and estuarine deposits well developed along the coast of Easter Ross, provide a record of past sea-level changes. Many of the lochs and peat bogs of the area contain valuable archives of past environmental changes and vegetation history, including the development of post-glacial woodland.

Gruinard Bay a landscape fashioned from ancient Lewisian Gneiss.
Image provided by Scottish Natural Heritage.