Geology at Loch Lomond and the Trossachs National Park

Ruth Crosbie (UK)

The Loch Lomond and the Trossachs National Park has a unique and very visible geological character. This, and the geomorphological processes that have taken place in the area have been fundamental in shaping the outstanding landscape and scenery of the park.

Rolling, relatively low-lying farmland along the southern margins of the park is underlain by Silurian to Carboniferous sedimentary rocks. North of the Highland Boundary Fault, this rolling country gives way to increasingly mountainous land, underlain by more ancient metamorphosed rocks. Many of the visible landforms represent the actions of glacial processes. Classic ‘U’-shaped valleys, such as the north Loch Lomond basin and Strathfillan, were carved by glacial ice. Other features, such as drumlins near Tyndrum and the rolling landscapes south of the Highland Boundary Fault, are the result of sediments deposited by melting glaciers. Such contrasts in the geology and landforms are reflected in similar marked contrasts in land-use patterns.

Geological Structure

The park contains a wealth of geological and geomorphological features, including some of national and international importance. The Highland Boundary Fault, which separates the Highlands from the Scottish Midland Valley, is well known. Within the park, the fault runs from Arden through Balmaha, Aberfoyle and Loch Venachar, and its line is clearly visible through the islands of southern Loch Lomond. Although less well known, other features include the complex, large-scale geological structure of the rocks in the Highlands that arose from the great Caledonian mountain building events between 500 and 400mya. These can be seen in the Aberfoyle area.

Glaciation

Like the rest of Scotland, the area has been repeatedly glaciated during the last two million years, with these glaciations giving rise to the deep glens and steep mountains that characterise the mountainous parts of the park. Loch Lomond (about 190m deep) and Loch Katrine (about 150m deep) are just two of a series of elongated, deep-water lochs occupying glacially deepened troughs radiating out from the Western Highlands. Most notably, the Ben Arthur area (also known as the ‘Arrochar Alps’), in the Western Highlands, was the centre of a resurgence in glacial conditions that began about 12,500 years ago and lasted 1,000 years. This stadial period, known locally as the ‘Loch Lomond Readvance’, resulted in a major valley glacier in the Loch Lomond trough. This glacier left terminal moraines that now separate the Loch Lomond trough from the Firth of Clyde.

After the end of the Pleistocene, sea levels rose and inundated low-lying ground, flooding the Forth Valley as far as Aberfoyle and briefly turning Loch Lomond into a sea loch (approximately 7,200 to 5,450 years ago). This flooding deposited marine silts and clays, and gave rise to the very flat, carse (that is, a fertile lowland next to a river) landscape that adjoins the south-eastern margin of the park.

Postglacial landslips are a feature of the northern area of the park. The majority of these are small and most are now stable. However, some landslips have been active in historical times and a few remain so, most notably in Hell’s Glen in Cowal and on some other steep slopes. Most recently in Glen Ogle and the Rest and Be Thankfull.

Erosion

Erosive forces in the montane and submontane zones remain important today, providing habitats for some upland plant communities of national and European conservation importance. These include the alpine pioneer flush communities and scree communities on Ben Lui and other upland Sites of Special Scientific Interest (SSSI) and Special Areas of Conservation. Erosion and deposition of sediments are also important in creating features of conservation importance on lower ground. The continued transportation of sediment in river channels is essential to the maintenance of the gravel spawning beds of salmon, trout and lampreys. For example, the actively migrating river channel of the Endrick Water SSSI below Drymen Bridge, the marshes and flood plain mires of the Endrick Mouth SSSI, the SSSI of Black Water marshes at Loch Venachar and Loch Lubnaig marshes are important sites that depend on continuous geomorphological activity.

Minerals

There is a history of industrial use of rock and other geological resources in the park, the Aberfoyle slate quarries being one of the most important examples. Quarrying of limestone at Dounans above Aberfoyle is of historic importance. Sandstone was also extracted for building stone in several quarries along the southern fringes of the park. The remains of lead and zinc mining and bare, poisoned ground from processing lead ore can still be clearly seen close to Tyndrum. There has also been more recent local, piecemeal quarrying in places, mainly for hard rock for forestry tracks. The recently constructed, but currently mothballed, gold mine at Cononish, near Tyndrum, represents a more contemporary economic interest in the same ore-bearing fault line that gave rise to the earlier mines. The Crom Allt SSSI is an important and undisturbed part of the same fault system where other, less valuable minerals can be observed.

Information for this article was taken from the State of the Park Report 2005. The report can be found at http://www.lochlomond-trossachs.org/park/default.asp?p=109.