Highland Boundary Fault

The Highland Boundary Fault is a major fault zone that traverses Scotland from Arran and Helensburgh on the west coast to Stonehaven in the east. It separates two distinctly different physiographic and geological terrains: the Highlands from the Lowlands, and in most places it is recognisable as a change in topography. Where rivers cross the fault, they often pass through gorges, and the associated waterfalls can be a barrier to salmon migration.

The fault is believed to have formed in conjunction with the Strathmore Syncline to the south-east during the Acadian orogeny in a transpressive regime that caused the uplift of the Grampian block and a small sinistral movement on the Highland Boundary Fault.

One of the earliest and most prominent references to the Highland Boundary Fault was by George Barrow in 1912, ʻOn the Geology of Lower Dee-side and the Southern Highland Borderʼ, which highlights the nature of the rocks accompanying the Highland Border and describes the outlining mineral zones associated with metamorphism. In the same publication, Barrow also outlines the ʻHighland Faultʼ and the areas where he believes there are planes of overthrust. Barrowʼs description of the structural nature of the rocks along the Highland Border suggests that rocks along both ends of the fault plane are indistinguishable from one another, with no brecciation.

Aligned southwest to northeast from Lochranza on Arran, the Highland Boundary Fault bisects Bute and crosses the southeastern parts of the Cowal and Rosneath Peninsulas, as it passes up the Firth of Clyde. It comes ashore near Helensburgh, then continues through Loch Lomond. The loch islands of Inchmurrin, Creinch, Torrinch, and Inchcailloch all form part of the Highland Boundary Fault. From Loch Lomond the Highland Boundary Fault continues to Aberfoyle, then Callander, Comrie and Crieff. It then forms the northern boundary of Strathmore and reaches the North Sea immediately north of Stonehaven near the ruined Chapel of St. Mary and St. Nathalan. Aeromagnetic maps of Great Britain and Northern Ireland show that the Highland Boundary Fault can be traced from Ireland to the region of Greenock. The Highland Boundary Fault in these areas are seen to be dividing a northerly low area from a southerly high area. In 1970, Hall and Dagley identified the Highland Boundary Fault as coincident with a regional magnetic feature dividing a string of negative anomalies in the north from positive ones in the south. On discovering this, Hall and Dagley concluded that the observed trend, which followed the length of the Dalradian trough transition from positive to negative anomaliesʼ. This linear feature of magnetic anomalies has since been referred to as the Fair Head-Clew Bay line.

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