Geology of Durness

These are metamorphic rocks that when weathered show features that make them look like they are made of oystershells. They are heavy, highly folded rocks formed from a process known as 'mylonitization' where they have been stretched due to other gneisses being pushed on top of them due to Moine Thrust movements. These once looked very similar to the Lewisian Gneiss seen in the area but have been so highly deformed that they have formed a 'mylonitic phyllonite'.

This rock-type consists of two main types. These vary in colour from grey-brown (semi-transparent in extremely fine crystalline parts) to dark green/grey, although the latter dominates in thick sequences and has a slight golden tint in some layers. Although coarse crystalline, quartzofeldspathic layers are occasionally seen, this is mainly a very hard and fine crystalline unit, composed mainly of quartz and some feldspar, with additional muscovite and biotite micas.

Chlorite is abundant in the dark green type, one of the elongate minerals helping to show a well developed lineation. This plus strong crenulation and cleavages help give it an ‘oystershell’ appearance on weathered surfaces. Small centimetre-wide, elongate lenses of quartz are abundant throughout the dark-green ‘oystershell-type’ and are commonly peach-pink stained.