The Cambrian to lower Ordovician rocks consist of two groups, the Ardvreck Group and the Durness Group. The Ardvreck Group lies above an angular unconformity over various parts of the Torridon Group and locally over the Lewisian. It is a sequence of mainly quartz arenites. The lowermost part of the Eriboll Formation, the Basal Quartzite Member, is often pebbly at its base. The overlying Pipe Rock Member is a distinctive quartz arenite with many white weathering skolithos trace fossils that act as strain markers in areas of more ductile deformation. The uppermost two parts of the Ardvreck Group form the An t-Sron Formation, with the dolomitic Fucoid Beds Member being overlain by the quartz arenites of the Salterella Grit Member. The succeeding Durness Group consists mainly of dolomites, with some limestone and chert.