The Cairngorms: A subarctic landscape carved out of granite

Stand among the ancient weather-beaten granite rocks and ponder the passing of geological time

The long glacially eroded trough of the Lairig Ghru follows one of several NNE-trending lines of weakness that cut across the Cairngorms Pluton. The glacially steepened valley sides are now covered by scree and active debris flows. Image © Jim Barton.

Believe it or not, the Cairngorm Mountains exist because of events that happened thousands of miles away in the southern hemisphere between 500 and 400 million years ago. At that time, continents surrounding the ancient Iapetus Ocean were drawn together on a collision course as the ocean floor sank, or ‘subducted’, beneath the land-masses to be recycled at depth. As the ocean disappeared, the continents kept on moving, pushing some of the rocks at their margins upwards to form high mountain chains, and some downwards into regions where they were heated and squashed, creating local large-scale melting. This melting created magma that was less dense than its surroundings, and so the molten rock began to rise back towards the Earth’s surface. The Cairngorm Mountains are made of granite that crystallised very slowly when a large plume of this magma rose into cooler surroundings and solidified a few kilometres below the existing land surface.

A period of rapid erosion wore away the older rocks that originally lay above the granite, exposing the underlying rocks that we see today. Since granite is a very tough rock that resists the processes of weathering and erosion, only the very top of it has been worn away over the last 400 million years.

Although the mountain landscape has changed relatively little since the granite was ‘unroofed’, much of the detail you see in today’s landscape is a direct product of the current Ice Age. The last major ice-sheet melted ‘only’ 15,000 years ago, and, at various periods during the previous two million years or so, glaciers flowed in what are now river valleys and mountain passes, such as the Lairig Ghru. These valleys were deepened and straightened, leaving steep craggy sides now covered in rock slides. Occasionally, this deepening resulted in the headwaters of some rivers (such as the Don) being ‘captured’ and diverted into other river systems. Deepening of the main valleys also caused tributary valleys to be left stranded at higher levels so that their streams now tumble over waterfalls and through gorges where they join the main valleys.

Perhaps the most distinctive landscape features of the Cairngorms are the precipitous corries, most numerous along north and east-facing slopes, which were scooped out during the Ice Age. They were formed as ice built up year after year until it began to flow downslope to feed valley glaciers. The rock behind and under the ice in the corries was dragged away with the flowing ice. This happened many times as the temperatures fluctuated during the Ice Age and the glaciers came and went repeatedly.

Away from the valleys and corries, the high summits of the Cairngorms were hardly eroded at all during the Ice Age. Typically, they are covered by large areas of loose granite blocks and boulders, prised from the bedrock below by the action of water freezing and thawing within natural cracks. Exposed to extreme weather over very long periods of time, these granite blocks have developed rounded profiles. So too do the hard cores of bedrock that have resisted being broken up and now stand above the surrounding high plateaux as tors. These come in many different sizes and are a very distinctive feature of many summit areas, especially around Ben Avon.

The Cairngorm Mountains are of especial interest to those who study landforms because of the widespread development of periglacial and other extreme weather features on a scale not seen anywhere else in Britain. Periglacial features were produced during the Ice Age by the freezing and thawing of ground adjacent to, but not permanently covered by ice and snow. Since then, the combined action of severe frosts and powerful winds has been responsible for some of the unique landforms found only in the highest parts of the Cairngorms. The range of features is surprising and includes various geometric arrangements of stones, small hollows, and low banks that are the edges of lobes of soil and stones that flowed downhill during freeze/thaw conditions.

There are many other mountains in Scotland made of similar granite, but the scale of the Cairngorms high plateau wilderness and its tundra-like appearance make it unique. The plants, animals and birds that live there also form a unique community, one that is very fragile and of great interest to study in these times of recognisably changing climate.

Text contributed by Peter Craig

Find out more
1. Cairngorms – Landscapes in Stone by Alan McKirdy [Birlinn 2017] is a very well illustrated and readable introduction to all aspects of the geological history of the area.
2. Geological structure and landscape of the Cairngorm Mountains is a professional study pulling together recent research findings. The summary and glossary sections are particularly useful.
3. The Cairngorms Learning Zone is a straightforward introduction to all aspects of Cairngorms natural history.
4. The Cairngorms National Park main website is an excellent starting point when seeking general information about the Park.
5. Cairngorm Landscapes website (Adrian Hall, 2016).

3. This particularly large tor (Clach Choutsaich) on Ben Avon shows large scale block jointing typical of granite. Note also the characteristic rounded surfaces that are generated by the long-term weathering action on granite of wind, rain and frost. Image © Kevin Pollock.

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