North Wales has a very diverse and complex geology with Precambrian schists along the Menai Strait and the great Cambrian dome behind Harlech and underlying much of western Snowdonia. In the Ordovician period much volcanism deposited a range of minerals and rocks over the north western parts of Gwynedd whilst to the east of the River Conwy lies a large area of upland rolling hills underlain by the Silurian mudstones and grits comprising the Denbigh and Migneint Moors. To the east, around Llangollen, to the north on Halkyn Mountain and the Great Orme and in eastern Anglesey are beds of limestone from which metals have been mined since pre-Roman times. Added to all this are the complexities posed by Parys Mountain and the outcrops of unusual minerals such as Jasper and Mona Marble which make the area of special interest to geologists.