Chris Simpson gave a talk summarising the main features of the geology of the Northwest Highlands of Scotland based on a 2022 guided tour with Chris Darmon and Colin Schofield.
The area is famous in geological circles because of the "Highlands Controversy" which divided 19th Century geologists – how could it be that old rocks should be found lying on top of younger rocks? The British Geological Survey sent two geologists to conduct an in-depth survey of the whole area and settle the controversy once and for all. Messrs Peach and Horne did just that. They found that older rocks were indeed emplaced overlying younger rocks as the result of large-scale thrust faults. Crustal compression during the Caledonian Orogeny had pushed millions of tons of rock over many miles to produce the Moine Thrust in the Northwest Highlands (and the Outer Islands Thrust Fault in the Outer Hebrides).
Chris also showed pictures of many of the other geological delights to be found in the Northwest Highlands.
(Summary and photos by Chris Simpson)
The Moine Thrust as seen at Knockan Crag. The dark layer at the top of the hillside is Moine schists which are thrust over pulverised Durness Limestones.
Close-up photograph showing contact between the Moine schists and Durness Limestone
An information board about the Glencoul Thrust.
The Glencoul Thrust as seen on a cloudy day.
The chance sighting of a vogesite sill alongside the A837 a few miles South of Inchnadamph.
The vogesite sill intrudes the Ghrudaidh Formation – dolostones forming the base of the Durness limestones.
Vogesite is a plagioclase-rich lampropyre
The “multi-coloured rock stop”. Three ages of rock intermingled. Gneiss around 2.8Ba, basaltic dykes around 2.3Ba and pegmatitic granites around 1.75Ba
A close-up photograph of the pegmatitic granite -- Ignore the rock drill marks made when this road cutting was made. Nobody knew about this conjunction of rocks until the road was dug!
Achmelvich. Banded gneiss with olivine nodules.
The presence of these nodules suggests that the gneiss originated from a piece of oceanic crust