Sutherland, Scotland

The cuttings excavated in the 1980s as part of the A838 road improvements between Laxford Bridge and Rhiconich, in NW Scotland, provide dramatic exposures of gneisses that give a glimpse of processes that operate in the middle continental crust during mountain building.

Consequently the cuttings are much used by student groups studying tectonic processes – especially one site termed the “multi-coloured rock stop” on the interpretative panel in the adjacent layby.

The cutting contains various types of metamorphosed igneous rocks that are now various forms of gneiss. Their igneous precursors include tomalites and dolerites – now recrystallized into grey gneisses and (black) amphibolites. Crossing these are various different granitic intrusions, including coarse-grained pegmatites. 

All the units have been sheared. The grey gneises and amphibolites are smeared together so that any original intrusive discordances are obliterated. The granitic bodies are less deformed and show complex jogs that chart the development of spaces into which this magma has been emplaced during that later part of the shearing. The effect of granites in the shear zone can be seen along the cutting – where gneisses have folded adjacent to the presumably more rigid granitic intrusions (so-called flanking folds). 

The age of shear deformation at Laxford can be established by radiometric dating of the granitic intrusions. These have yielded ages of c 1855 million years. However, the grey gneisses are much older (2700-2800 million years). Collectively these outcrops chart the youngest part of the history of the Lewisian complex in NW Scotland while these rocks were still in the middle crust. Subsequently these units were uplifted to close to the earth’s surface so that they were overlain by the Torridonian strata – some 1.2 billion years ago. 

These outcrops are part of the NW Highlands Geopark.

Text: Professor Rob Butler