Sandstone, formerly thought to be Triassic in age, best exposed in the cliffs of the town which has lent them its name. These are aeolian 'millet seed' sandstones which originated as desert dunes. During the Early Triassic epoch, a river system flowed from the south bringing sand and gravel (rounded quartz pebbles) which formed what were once known as the Bunter Pebble Beds, later the Kidderminster Conglomerate and to the north, the Chester Pebble Beds but which are now known (along with a host of other locally named rocks) as the Chester Formation. The Chester Formation is overlain by the Helsby Sandstone Formation, a name which encompasses the Grinshill and Ryton sandstones. Jurassic The Permo-Triassic age rocks are overlain by Jurassic deposits in the area around Wem and Prees. These are assigned to the Lias Group, the lower parts of which are late Triassic in age. Though largely concealed, outcrops of Dyrham Formation sandstones and mudstones near Prees are of Pliensbachian age and hence wholly Jurassic. Igneous rocks There are Neoproterozoic age intrusions at the Wrekin and Haughmond Hill and again in the South Shropshire Hills. A sill of microgabbro (dolerite) was intruded into the Pennine Coal Measures rocks at Kinlet during the Bolsovian stage of the Carboniferous. It forms a part of the Clee Hill swarm. A NW-SE aligned dyke of Palaeogene age cuts the sandstone at Clive. Sills intrude the late Carboniferous country rocks of both Brown Clee and Titterstone Clee, the latter are known to be of Westphalian age. Structure Almost all of the geological structures within the county are aligned broadly northeast-southwest following the Caledonoid trend established during the Caledonian Orogeny.The Church Stretton Fault Zone is a major one whilst the Wem Fault Zone and Hodnet Fault faults partly define the southeastern extent of Cheshire Basin, the Shropshire portion of which is sometimes referred to as the North Shropshire Basin. The Jurassic outcrop is preserved within the shallow Prees Syncline, the axis of which parallels the Wem fault to its northwest. The Carboniferous outcrops in the Clee Hills are preserved along the axes of the Brown Clee and Titterstone Clee synclines which lie to either side of the Ludlow Anticline. Quaternary Glacial legacy Shropshire was doubtless affected by glacial ice during the Anglian glaciation but evidence for that does not remain. The county was subjected to incursions by glacial ice both of Welsh origin and of