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Sub-Torridonian unconformity on the Assynt Road

Sutherland, Scotland

Sub-Torridonian Unconformity

Other unconformities in Britain and Ireland may be more famous (such as Siccar Point) but few have the clear simplicity of that on the road section on the north shore of Loch Assynt.


The landscape of western Assynt is dominated by two units – the Lewisian gneiss, that forms the crustal foundations of NW Scotland, and the sandstones of the Torridonian Group. 

The Lewisian forms the low-lying lumpy terrain of low hills and lochans while the Torridonian forms the hills of Quinag, Suilven, Canisp and others. The Torridonian strata are remarkable – still essentially flat-lying and apparently undisturbed despite the long history of tectonic upheaval experienced elsewhere in the Scottish Highlands. 

The geological contact between the Lewisian and Torridonian is dramatically and accessibly displayed in road cuttings near the shore of Loch Assynt. This unconformity represents a time-gap approaching 1.5 billion years of geological time. 

The eastern section displays a gently undulating contact, a segment of the ancient land surface upon which the Torridon strata accumulated. The section of low road cuts to the west are characterised by a weathered profile of Lewisian gneisses beneath the unconformity, which potentially gives insights into the chemical weathering processes more than a billion years ago. 

The outcrops are amongst the most important in British geology for opening the eyes of students to the abyss of geological time and are visited by many hundreds each year. They lie within the NW Highlands Geopark. Regrettably in 2013-14 some of the best parts of the outcrop were vandalised by research scientists through indiscriminate rock coring.

Text: Professor Rob Butler

100 Great Geosites

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Further Reading

A.D. Stewart, 2002. The Later Proterozoic Torridonian Rocks of Scotland: their sedimentation, geochemistry and origin. Geological Society Memoir n. 24, 130 pp.

Images (Top to Bottom):

  • Proterozoic sandstones above Archaean gneisses © Rob Butler
  • The Torridonian Basal Unconformity © Anne Burgess (Source Creative Commons Attribution-ShareAlike 2.0 license.