 Along the western coastal margin it is characterised by Lewisian gneiss, the oldest rock in Scotland.Liathach, Beinn Alligin, Suilven, Cùl Mòr, Cùl Beag, and Quinag are just some of the impressive rock islands of the significantly younger rich brown-coloured Torridonian sandstone which rests on the gneiss.Some of the peaks, such as Beinn Eighe and Canisp, are topped with later light grey or white Cambrian quartzite.