The geology of Wales is complex and varied; its study has been of considerable historical significance in the development of geology as a science. All geological periods from the Cryogenian (late Precambrian) to the Jurassic are represented at outcrop, whilst younger sedimentary rocks occur beneath the seas immediately off the Welsh coast. The effects of two mountain-building episodes have left their mark in the faulting and folding of much of the Palaeozoic rock sequence. Superficial deposits and landforms created during the present Quaternary period by water and ice are also plentiful and contribute to a remarkably diverse landscape of mountains, hills and coastal plains. Wales' modern character derives in substantial part from the exploitation of its diverse mineral wealth; slate in Snowdonia, coal in the South Wales Valleys and metal ores in Anglesey and mid Wales, to name but three. Wales' geology influences farming practices and building stone choices but also planning of developments which must take into account ground stability and liability to flooding - geohazards which an appreciation of the geology can help deal with. History of geological study South Wales has a written record of geological interest going back to the 12th century when Giraldus Cambrensis noted pyritous shales near Newport. George Owen in 1603 correctly identified the stratigraphic relationship between the Carboniferous Limestone and the Coal Measures. Some of the first published representations of fossils were those of fossil plants taken from the coal measures near Neath (Gibson late 17th century). In the mid-19th century, two prominent geologists, Roderick Murchison and Adam Sedgwick used their studies of the geology of Wales to establish certain principles of stratigraphy and palaeontology. They did fundamental work on the Old Red Sandstone but are remembered more for their work on the lower Palaeozoic sequence. It was Sedgwick who established the Cambrian system and Murchison first described the Silurian, naming it for the ancient Silures tribe which occupied mid Wales. An overlap between the two systems as mapped led eventually to protracted dispute between the two erstwhile collaborators. After their deaths, Charles Lapworth erected the Ordovician system (again named for an ancient tribe of northwest Wales, the Ordovices), to account for the sequence of rocks at the heart of the controversy. More recently two locations in mid Wales have been selected to globally define stages of the Silurian period. Cefn-cerig Road near Cefn-cerig Farm, Llandovery, is the location of the Global