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Loch Lomond - Highland Boundary Fault

The Highland Boundary Fault (HBF) which traverses Scotland from Stonehaven to Arran, is in most places only identifiable by the change in topography - from highlands to the northwest and lowlands to the southeast. However, Loch Lomond is one of the sites where the fault can be distinguished more readily, in particular at Conic Hill and on the islands of Inchcailloch, Torrinch, Creinch and Inchmurrin. It is also a site where the Highland Border Complex, a thin sliver of unmetamorphosed sediments that lies between the Dalradian highlands and Devonian lowlands, is well exposed.

The rocks to the northwest of the HBF are Dalradian metasediments of the Southern Highland Group. These are deep marine deposits metamorphosed to schists, phyllites and slates during the Caledonian Orogeny. The HBF came about as the Highland and Midland Valley crustal blocks came together during the orogeny. The last major movements at the fault took place in the Silurian and Lower Devonian times. Overall, the Dalradian Supergroup underwent four main phases of deformation during this orogenic activity.

To the southwest of the HBF lie the steeply dipping Lower Old Red Sandstone conglomerates and sandstones of the Garvock & Strathmore Groups. These were deposited in alluvial fans along the Highland Border; conglomerates are common owing to the active erosion of the highlands during the arid Old Red Sandstone environment. These sediments unconformably overlie the Highland Border Complex. Upper Old Red Sanstone lithologies are also found overstepping the Dalradian rocks north of the HBF.

The Highland Border Complex is a suite of rocks exposed in a series of lenses along the HBF. Composed of serpentinites, cherts, sandstones and breccias (other lithologies exist at other sites), the suite represents an ophiolite and associated marine sediments, obducted during the Caledonian Orogeny and containing no Dalradian source material. The Complex is believed to have been deposited in early/pre-Arenig to Caradoc (approximately 480 - 450Ma) and partly deformed by the overthrusting of the Dalradian block by the end of the Silurian, i.e. prior to deposition of the Lower Old Red Sandstone sequences.