Highland Boundary Fault

Highland Boundary Fault from Conic Hill
©2022 Gazetteer for Scotland

Highland Boundary Fault from Conic Hill

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 south and east are softer sedimentary rocks of the Devonian and Carboniferous geological periods, principally the Old Red Sandstone. On the ground, the fault can be identified by marked topographic change; contrasting the rugged and barren Grampian Mountains from the rolling hills to the southeast which support particularly agriculturally-productive land.

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. Although its interpretation is not straightforward, the Highland Boundary Fault appears to have been active during a series of phases beginning at the time of the Caledonian mountain-building episode, a plate tectonic collision which took place from Mid Ordovician to Mid Devonian geological periods (480 to 400 million years ago). It seems to have begun predominantly with strike-slip motion, as the continents collided, that pinched slivers of ocean floor (known as ophiolites) which today appear along the fault as the Highland Border Complex. Later, between the Mid Devonian and Early Carboniferous periods (400 to 340 million years ago), this horizontal movement was replaced by vertical movement, allowing the Midland Valley to descend as a major rift or graben by as much as 4000m (13,123 feet). Sediments were deposited in this rift as the valley descended.

A complementary fault, the Southern Uplands Fault, forms the southern boundary of the Midland Valley.

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