thumb|View along the Highland Boundary Fault from [[Conic Hill – the topographic ridge is mainly due to the presence of Devonian age conglomerates on the southwestern side of the fault and can be traced across Loch Lomond on the islands of Inchcailloch, Torrinch, Creinch and Inchmurrin, to Ben Bowie on the western shore]]
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 different geological terranes that give rise to two distinct physiographic terrains: the Highlands and the Lowlands. In most places, it is recognisable as a change in topography. Where rivers cross the fault, they often pass through gorges; 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.
Discovery
thumb|Map published in 1912 by [[George Barrow (geologist)|George Barrow 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.
Extent of fault
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 lie on 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. In these areas, the Highland Boundary Fault is seen to be dividing a northerly low area from a southerly high area. The fault is often considered a terrane boundary: the Midland Valley terrane lies to the south whilst the Southern Highlands or Grampian terrane lies to the north
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.
Features
thumb|Altered [[serpentinite and associated sediments of the Highland Border Complex caught up in the fault zone – exposed on Druim nam Buraich, near Balmaha]]
thumb|Lower [[Devonian sandstones and conglomerates of the Inchmurrin Member dipping steeply to the southeast close to the Highland Boundary Fault trace near Balmaha]]
Tectonic controversy
At present, it is believed that the Highland Boundary Fault was active during two main orogenic events associated with the Caledonian orogeny: the Grampian orogeny in the Early Ordovician and the Acadian orogeny in the Middle Devonian.), a weakly metamorphosed sedimentary sequence of sandstones, lavas, limestones, mudstones and conglomerates. These make up a zone which is found discontinuously along the line of the fault and which is up to in width.
Dalradian Supergroup
The Dalradian Supergroup consists of metasedimentary rocks which underwent polyphase deformation and metamorphism during the Precambrian and early Paleozoic.
Evidence for the Acadian displacement event is based on the geochemical study of detrital garnets in the Lower Old Red Sandstone on the Northern limb of the Strathmore Basin. These garnets were linked to those in isolated Dalradian sediments in the northwest, providing evidence for post-Early Devonian (Acadian) movement to be only few tens of kilometers.