Loch Lomond has the largest surface area of any freshwater loch or lake in Britain, and measures about 5 miles across at its widest point and about 24 miles long. It is crossed by the Highland Boundary Fault Line, a geological feature which splits the landscape quite markedly in two.
To the south east of the Line the landscape remains gently undulating as in the central lowlands, but to the north west, the Highlands raise up dramatically.
Along the line of the fault there are 4 islands in Loch Lomond. They are Inchmurrin, Inchcailloch, Torrinch and Creinch, and seen on a map or from above, the line across the loch is clear.
The Highland Boundary Fault line extends from Lochranza on the Isle of Arran in the west, across the Isle of Bute and the Cowal and Rosneath peninsulas before reaching Loch Lomond. It then crosses Aberfoyle, Callander and Crieff before reaching Stonehaven in the east.
The Highland Boundary Fault Line was caused by a plate tectonic collision that took place around 520 to 400 million years ago. The actual cause of such movements of the surface of the earth is as yet unknown, although several theories exist.
A complimentary fault, the Southern Uplands Fault, forms the southern boundary for the Central Lowlands.
The Highland Boundary Fault Trail is a waymarked four mile walk around the Queen Elizabeth Forest Park from the David Marshall Lodge near Aberfoyle. Visitors can take the opportunity to learn about the geological feature at the same time as experiencing the wonderful scenery of the region.
