A significant geological discontinuity, the Highland Boundary Fault traverses Scotland from Arran (North Ayrshire) to Stonehaven (Aberdeenshire) separating two distinctly different physiographic regions; namely the Highlands from the Midland Valley. To the north and west lie hard Pre-Cambrian and Cambrian metamorphic rocks of the Dalradian group and to the the south and east softer, sedimentary rocks of the Devonian and Carboniferous periods, principally the Old Red Sandstone.
The fault runs southwest-northeast from Lochranza on Arran, across the Firth of Clyde, via Helensburgh, Loch Lomond, Aberfoyle, the Menteith Hills to Callander, Comrie and Crieff. It then forms the northern boundary of the Vale of Strathmore before reaching the east coast at Garron Point, 2 miles (3 km) northeast of Stonehaven. Active during the Caledonian mountain-building episode, a plate tectonic collision which took place from Mid Ordovician to Mid Devonian periods (520 to 400 million years ago), the Highland Boundary Fault allowed the Midland Valley to descend as a major rift or graben by as much as 4000m (13,123 feet). This earlier vertical movement was later replaced by horizontal shear. A complementary fault, the Southern Uplands Fault, forms the southern boundary of the Midland Valley.