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.
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.