Edge of the Highlands
A very old fracture in the Earth's crust marks the edge of the Highland rocks, where they have been brought against the younger, less altered rocks of the Midland Valley. Here, too, over 400 million years ago, an ancient ocean was squeezed and buried, and only thin slices of its rocks remain along the crack. This feature is known as the Highland Boundary Fault; it stretches from Stonehaven to Kintyre, crossing both Arran and Bute.
Where the fault crosses Bute, erosion has picked out the softer fractured rocks and formed a valley. The ends are flooded to form Rothsay Bay and Scalpsie Bay, and in between is mostly filled by Loch Fad (long loch) and Loch Quien. Either side of this feature the landscape shows a marked contrast: to the north the typically bare craggy ‘Highland’ scenery; to the south the more gentle, fertile scenery of softer sedimentary and volcanic rocks.
In Arran the granite has exploited the weak zone of the Highland Boundary Fault plane and partly consumed it. As a result, the fault does not form a feature but can be followed by the junction of Highland and Midland Valley rocks both inland from Sannox and across the island just south of the granite.