Geology of Durness

These orthogneisses are part of the Lewisian Gneiss that have been altered over time by the Moine Thrust thrusting (or effectively pushing) these gneisses on top of the now underlying Oystershell Rock (as seen at Faraid Head). At Sango Bay however, these appear side by side due to the effects of large faults that have uplifted these rocks that were once over 1km below their present day location.

This is a light coloured, medium-coarse banded gneiss, mainly pink/peach in colour but with bands of red, cream, pale green, black and colourless minerals. It is a very hard rock, composed mainly of quartz but contains abundant feldspar and platy biotite/muscovite crystals. Green bands are rich in amphibole (hornblende) which also dominate interlaying bright-green amphibolite horizons (often friable due to softer minerals) which are common throughout, although they are much less dominant. Two main gneiss types are found; a hornblende dominated type and a normally coarser, pale quartzofeldspathic dominated type, both continuously alternating on a metre to centimetre-scale.

Some specimens show a very deep pink-red colour, some resembling granitoid rocks, indicating a possible igneous origin before metamorphism. Some weathered rocks show smooth, well rounded and pitted surfaces where softer platy minerals have been eroded. Structurally, there is a well defined lineation parallel to banding and is seen much better in mica / amphibole-rich layers, where elongate platy crystals are seen.