Geology of Durness

Mafic boudins within the Lewisian Gneiss. Boudin is the French word for sausage, pronounced boo-dan. It is also a geologic term for a sausage-shaped body of rock that differs from the surrounding material. Typically, boudins are found in series of elongate bodies that may be detatched from each other (as if the original material was pulled apart, like toffee), or connected by narrow, pinched connections. They are usually interpreted to represent tectonic stretching of relatively rigid, competent rocks interlayered with rocks that are less resistant and capable of flowing into the "neck" portions of the boudins.

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These rocks belong to the Lewisian Gneiss. They are highly folded, metamorphic rocks and are coarse, highly crystalline banded gneisses composed of quartz, feldspar and mica (both biotite and muscovite). They are generally very dark to light grey in colour, although some bands give the rocks a pinkish look from afar. Centimetre thick, discontinuous and differing coloured bands reflect the gneissose texture where the different minerals have been grouped into bands of similar mineralogies. The coarser and shiny, biotite-mica rich bands have therefore formed the darker bands and the quartofeldspathic bands have formed light grey to pink coloured bands depending on the varying redness of the feldspars.