rocks and the younger fossiliferous (Cambrian) sedimentary rocks. Much of North Shropshire is a plain which is a basin of Permian and Triassic New Red Sandstone. This basin continues north into Cheshire. Faulting has occurred within the sandstones, because of basin extension during and after the infilling of the basin. Escarpments form small prominent hills within the plain. The basin is bounded on the east by the Hodnet Fault, which runs roughly from Shrewsbury to Market Drayton. East of this fault the sandstone is thinner. In the north west of the county near Oswestry are outcrops of Carboniferous Limestone and the Coal Measures. The Shropshire Hills AONB. were formed on a continental shelf, but buckled up into hills at the time of a continental collision: the Variscan Orogeny. The most famous of these hills is probably the Long Mynd, which is Precambrian in age and forms the west side of the Stretton Valley. East of Church Stretton is Wenlock Edge, a Silurian limestone escarpment. In between lies a complete succession through the late Precambrian, Cambrian, Ordovician and into the Silurian. South West of Church Stretton bordering Wales, is the very rural area of Clun Forest formed largely from Silurian age rocks. Igneous intrusions outcropping at the surface in South Shropshire are few and small, but much larger bodies are believed to exist at shallow depths, evidenced by geophysical anomalies and radon-bearing groundwater. Precambrian Late Precambrian (Neoproterozoic) rocks occur either side of the Church Stretton valley and in the vicinity of the Wrekin and Haughmond Hill, each intimately associated with the Welsh Borderland Fault System. Cambrian There are only a few limited localities in Shropshire where Cambrian rocks occur. Its portrayed extent was once greater within the county but the reassignment of the British Tremadocian to the Ordovician period, meant that areas previously mapped as being formed by rocks from that stage were re-labelled as Ordovician. The Wrekin Quartzite lies unconformably on Uriconian volcanics and is itself conformably overlain at the Ercall by the sandstones of the Lower Comley Sandstone Formation. The presence of the mineral glauconite gives these latter rocks a green-brown colour. Lower Comley shales and sandstones also occur in a small and largely fault-bound area at Lilleshall alongside mudstones and siltstones of the Dolgellau Formation. Sandstones and mudstones of the Upper Comley Sandstone Formation unconformably overlie the Lower Comley strata. The fossiliferous Lower Comley Limestones were the first