Scotland’s geology

Scotland’s spectacular scenery and the distinctive character of our land and geography derives from a long geological story stretching back three thousand million years. This geology has played a crucial role in great leaps forward in understanding how the Earth works, dating back to the 18th century and the revolutionary ideas of James Hutton.

Scotland’s geology is the result of a series of major tectonic events over time. The Earth’s continents are forever moving, splitting apart to make new ocean basins and colliding to form mountain ranges. Scotland has been caught up again and again in these processes of plate tectonics, resulting in a wide variety of rock types of different ages. This geological diversity is reflected in Scotland’s scenery, in the way that the rocks have been sculpted over millions of years to give the Highlands and Lowlands, the firths and the islands, the glens, lochs and serrated mountain ridges. The final touches were largely provided by the ice sheets and glaciers that covered Scotland during the Ice Age of the last 2.6 million years, while river coastal and slope processes continue to shape the landscape today.

In the beginning …

The oldest rocks in Scotland are found in the Outer Hebrides and on the coast of the Northwest Highlands. The ‘Lewisian Gneiss’ is ancient, highly deformed metamorphic rock that takes its name from the island of Lewis. These rocks were formed deep in the Earth’s crust, and they record an unimaginable history of tectonic change: volcanic events, mountain building, deep burial and slow erosion. This rock first reached the surface more than 1000 million years ago, and then was buried again as new sedimentary rock formed on top.

Find out more at The 51 Best Places to see Scotland’s Geology: Scourie Bay and Laxford | Luskentyre, Harris | Isle of Iona

Caledonian Orogeny – a big crash

The most important event in Scotland’s geological story is a complex continental collision associated with the closure of the Iapetus Ocean. This is called the ‘Caledonian Orogeny’ and involved the collision of the northern continent of Laurentia (including North America and the oldest rocks of Scotland), the smaller Avalonia continent (England, Wales, parts of Ireland and Atlantic Canada) and the continent of Baltica to form a huge mountain chain. Before the continents collided, erosion of existing landmasses dumped large amounts of sediment into the Iapetus Ocean and surrounding basins. These sedimentary rocks were crushed, contorted and metamorphosed in various phases as the ocean closed and the continents came together, forming the hard rock of most of the Scottish Highlands and Southern Uplands.

As the Caledonian Orogeny drew to a close 400 million years ago, melting of rock beneath the mountains created magma that rose upwards to form granite, and sometimes erupted in large volcanoes like Glen Coe. One of the final acts of this great continental collision was the ‘Moine Thrust’, where a large block of metamorphic rock was moved westwards, so that it ended up on top of the relatively undisturbed sedimentary rocks of the Northwest Highlands.

Find out more at The 51 Best Places to see Scotland’s Geology:

Moine metamorphic rocks (north of the Great Glen) at Loch Monar | Corrieshalloch Gorge

Dalradian metamorphic rocks (south of the Great Glen) at Luing and the Atlantic Islands | Schiehallion | Islay

Ocean-floor rocks of the Southern Uplands at Loch Skeen and the Grey Mare’s Tail | Back Bay, Monreith

Granite and volcanic rocks at Eshaness Coast, Shetland | Glen Coe | Cairngorms

Evidence of collision and fault movement at Unst, Shetland | Knockan Crag | Balmaha, Loch Lomond

After the mountain building – erosion and rifting

Scotland’s geological story over the last 400 million years has been quieter and simpler. Most of Scotland remained above the level of the sea, an eroding continent that slowly drifted northwards across the Equator. The Caledonian mountain range eroded quickly and episodes of continental stretching created sinking, low-lying areas where new sedimentary rock could form. This includes the Old Red Sandstone and the coal fields of central Scotland, and later the oil- and gas-bearing rocks of the North Sea. In the Central Belt, volcanic activity created upland areas including the Campsies and Arthur’s Seat.

Find out more at The 51 Best Places to see Scotland’s Geology:

Devonian and Carboniferous sedimentary rocks at North-West Hoy, Orkney | The Black Isle | Laich Sandstones, Elgin | Fossil Grove, Glasgow

Volcanic rocks at East Neuk of Fife | Holyrood Park, Edinburgh | Eildon Hills

The final stretch – opening of the North Atlantic

Scotland’s final episode of rock creation was a dramatic period of volcanic activity centred on the west coast. Slow subsidence prior to the volcanic activity allowed sediment to accumulate, including Jurassic sedimentary rocks containing dinosaur fossils. The volcanic activity started after the dinosaur extinction, and involved the creation of a thick pile of lava flows and large central volcanoes.

Find out more at The 51 Best Places to see Scotland’s Geology: Trotternish, Skye | Cuillin Hills, Skye | Isle of Eigg | Isle of Staffa

The story continues …

In the last 50 million years Scotland has been in a stable tectonic situation at the edge of the Atlantic, but the geological story doesn’t stop there. Over this long period, weathering and erosion have picked out the contrasts between different rock types to create today’s landscape. During the Quaternary Ice Age (the last 2.6 million years), the entire country has been covered by an ice sheet many times, and the moving glaciers and their meltwater have helped to shape Scotland as we see it today. Although the scale and pace of change have slowed since the last glaciers melted 11,500 years ago, the landscape continues to be modified today by river, coastal and slope processes. And as James Hutton said, we can foresee “a different state that must follow in time, from the continued operation of that which actually is in nature”.

Find out more at The 51 Best Places to see Scotland’s Geology: St Ninian’s Tombolo, Shetland | Luskentyre, Harris | Parallel Roads of Glen Roy | Corrie Fee, Glen Clova | Flanders Moss NNR | Loch Skeen and the Grey Mare’s Tail