Burf and Clee Burf though the summits themselves are formed by an igneous intrusion of dolerite. Generally the Upper Coal Measures are separated from the lower and middle Coal Measures by an unconformity in the English Midlands, though historically it was erroneously thought to be a faulted contact and known as the Symon Fault. Strata underlying the unconformity have typically been folded by Variscan earth movements. Many of the coalfields are bounded by faults associated with this mountain-building episode. Permo-Trias East-west directed crustal tension initiated during the Permian period led to the development of the Cheshire Basin as one of a series of linked sedimentary basins stretching north from the English Channel through the English Midlands to the Irish Sea. This rift system continued to operate throughout the Triassic period and into the Jurassic, with sedimentary rocks from each of these periods being found at outcrop across the northern half of Shropshire. The southern part of the basin lies across north Shropshire and is continuous in the east with the smaller Stafford Basin, parts of which extend into eastern Shropshire. The southeastern margin of the deep Wem-Audlem Sub-basin, at the heart of the southern part of the Cheshire Basin is defined within Shropshire by the northeast-southwest trending Wem Fault. Further southeast is the sub-parallel Hodnet Fault which forms the outer boundary of the larger basin with the area between the two faults hosting a thinner Permo-Triassic sediment sequence and termed the Ternhill Terrace. Within Britain, the rocks of the Permian and Triassic periods are often lumped together as the Permo-Triassic and, typified as they are by desert sandstones, also labelled the New Red Sandstone. In Shropshire they extend across the North Shropshire Plain, itself a southerly continuation of the Cheshire Plain where similar rocks are to be found. Much of this flat area is covered by more recent deposits but sandstone hills are prominent at Ruyton, Nesscliffe, Myddle, Grinshill and Hawkstone. Rocks from these periods also underlie the plains in the east of the county. The names of the various sequences have undergone a number of changes. The Enville member of the Salop Formation (formerly the Enville Beds) comprises a sequence of Carboniferous to Permian age sandstones, conglomerates and breccias up to 110m thick which occur in the east. To the west are sporadic outcrops of the Alberbury Breccia (a.k.a. Cardeston Stone). These are both unconformably overlain by the Bridgnorth