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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.
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:
Craig, G.Y. 1991. Geology of Scotland. 3rd edn. The Geological Society,
London.
Stephenson, D. & Gould, D. 1995. British Regional Geology. The Grampian
Highlands. 4th edn. British Geological Survey (Her Majesty's Stationery
Office, London).
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