Geology of Durness

These Precambrian rocks were formerly sandstones formed within a tidal environment and show some evidence for cross, lenticular and wedge-shaped bedding. Water was likely shallow with ‘desiccation-cracks’ indicative of some surface exposure. These sediments likely then formed a sandstone before being metamorphosed into psammites (into green-schist facies).

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These are dark grey to golden coloured, gently to tightly folded altered sandstones, often light green in places. They are very fine grained to crystalline (recrystallised) and as a result are very hard / resistant. Composition is largely quartz and feldspar, although muscovite and biotite are abundant in grey-coloured bands. Some dark-grey to black calc-silicate layers are seen, as are mylonitic pale-green beds (increasingly lime-green hornblende/chlorite-rich further northwards). The quartzofeldspathic beds show the coarsest beds, up to coarse crystalline in places.

Bedding is visible throughout the shore-sections, commonly on average c. 0.75m thick with the parallel banding resembling fine laminae, seen as different coloured bands. These often fluctuate on a centimetre scale, indicating slight lithology changes from quartz-rich to more brown, pelitic rich laminae.

 

Preserved sedimentary features include small-scale cross-lamination (north to north-westerly palaeocurrents) and rare desiccation cracks on upper weathered surfaces. Wedge-shaped beds are also seen, as are small-scale lenticular bedding of cream-beige sediment. In highly fractured regions, deep-red staining is abundant, possibly indicating Fe-rich minerals within the rocks. Interbedded tan-orange coloured beds at Cnoc nan Sgliat also indicate oxidation of particular beds, interbedded with dark to pale grey laminae.