Great Glen Fault

The Great Glen is a glen that truly deserves its name. Over 100km long, the glen runs from Inverness on the Moray Firth to Fort William at the head of Loch Linnhe. It cuts the Scottish Highlands into two and is also the location of Britain’s deepest freshwater loch – the famous Loch Ness. Geologically, the Great Glen Fault (GGF) is the site of a large strike-slip fault that follows the path of the glen, splitting the Highlands into the Grampian Highlands (southeast) and the Northern Highlands (northwest). Beyond Fort William, the fault continues down the sea loch of Loch Linnhe and out towards Ireland, slicing through the southeasterly edge of the Island of Mull.

Loch Lochy and the Caledonian Canal from the air - the Caledonian Canal between Fort William and Inverness marks the line of the Great Glen Fault. © BGS/NERC.

The rocks of the Northern Highlands belong predominantly to the metamorphic Moine supergroup and the rocks of the Grampian Highlands belong predominantly to the metamorphic Dalradian supergroup. Whether or not the fault marks the boundary between two distinct terranes is as yet undetermined. The GGF is a sinistral strike-slip fault. Its displacement is unknown for certain but is considered to be in the region of tens to a few hundred kilometres. Other, more minor faults run sub-parallel to the GGF in the Highlands, but their displacements are much less.

It has been proposed that the GGF was active in the Neoproterozoic, at over 1 Ga, however this is not yet known for certain. However, the fault was definitely active by the Devonian, at the same time as the Highland Boundary Fault. Rejuvenation of the fault occurred in Middle Jurassic, probably associated with major basaltic volcanic activity that was taking place in the eastern part of the Moray Firth basin at that time.

Further reading:

Stephenson, D. & Gould, D. 1995. British Regional Geology. The Grampian Highlands. 4th edn. British Geological Survey (Her Majesty’s Stationery Office, London).

Trewin, N. H. (ed.) 2002. The Geology of Scotland. The Geological Society, London.