The Skiddaw Group are the oldest rocks known from the Lake District. They are mainly Ordovician in age, from possibly Cambrian up to Llanvirn (upper middle Ordovician). The sequence, which is up to about 5 km in thickness, consist mainly of mudstones and siltstones, with lesser amounts of sandstone. Following deformation and low-grade metamorphism during the Acadian Orogeny, they now have a well-developed slaty cleavage, giving rise to their common name, the Skiddaw Slates. The group is divided into two main areas, the northern and central fells, to either side of the WSW-ENE trending Causey Pike Fault, with distinct successions developed.