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Where the Ross of Mull granite has few joints, it weathers into Tors, often separated by boggy ground where the rock is more densely jointed and the pink to red feldspars (which give the Ross outcrops their characteristic colour) decay to clay minerals. At the coast, the tors form skerries, though this one is a tidal skerry, easily reached on foot except at high tide. Here the clay is easily carried way by the strong tides in the Sound of Iona, leaving sandy beaches made up of the more resistant quartz grains. Towards the far side of the Sound is a major structural change where the Moine thrust marks a change to far more ancient rocks of the foreland of Lewisian gneiss and Torridonian sandstone.