The 'Multi-coloured Rock Stop', Loch na Fiacail, Sutherland

A rock cutting on the A838 near Laxford Bridge offers superb exposures of the cross-cutting relationships and deformational history of the Lewisian gneiss, 'Scourie' dykes and later Laxfordian granite pegmatite sheet-like intrusions.

There is an information board in the lay-by opposite with diagrams and a concise explanation...



...but more details are given in Goodenough and Krabbendam (2011), below.

"Grey quartzo-feldspathic gneisses show a well-developed gneissosity, dipping gently south. Dark-green to black amphibolite-bearing basic sheets also dip gently south and are approximately parallel to the gneissosity. However, detailed examination... shows that the basic sheets have cross-cutting relationships with the gneiss. The basic sheets are probably Scourie Dykes ... but have been deformed into sub-parallelism to the gneissosity by later deformation. Both the gneisses and the basic sheets are cut by a swarm of pegmatitic granite sheets, some of which show boudinage. One of the granite sheets has been dated at c. 1855 Ma."
(Goodenough, K. M. & Krabbendam, M. (eds.), 2011. A geological excursion guide to the north-west Highlands of Scotland, British Geological Survey, Edinburgh. p.205)