Morfa Harlech and Morfa Dyffryn in Gwynedd and at Pendine and Pembrey Burrows in Carmarthenshire and at Merthyr-mawr Warren and Kenfig Burrows in Glamorgan. See also Coastline of Wales Karstic landforms Within the limestone areas of Wales, there have arisen karstic landscapes during the postglacial period, though elements of these were initiated during and even before successive ice ages. Limestone pavements are best developed along the 'north crop' within the Brecon Beacons National Park and there are numerous sinkholes and shakeholes, together with sinks, sections of dry valley and resurgences. Cave development is extensive and includes systems such as Ogof Ffynnon Ddu (Britain's deepest cave), Dan-yr-ogof (partly showcave) and Wales' most extensive system Ogof Draenen. Economic geology Metals Humans have mined metals and metal ores in Wales for millennia. There are Bronze Age copper workings on the Great Orme near Llandudno and at Parys Mountain on Anglesey. Gold has been obtained since pre-Roman times at places like Dolaucothi. Lead and zinc were intensively mined in the Cwmystwyth area of mid Wales and a lead mine operated at Minera near Wrexham from the Middle Ages until the early twentieth century. In Victorian times the Sygun Copper Mine was opened near Beddgelert in Snowdonia. Ironstone is a component of the Lower Coal Measures rock sequence and where it outcrops along the northern edge of the South Wales Coalfield, it was extensively worked for the production of iron and was important in the initiation of the Industrial Revolution in South Wales. Slate The slate industry of Snowdonia was once of world importance. Purple and green slates of Cambrian age were worked at vast quarries on the flanks of Snowdon and at Bethesda, Dinorwig, Corris and Blaenau Ffestiniog. Building stone The abundance of hard rock in Wales means that it has found use in building since the earliest times. The 'bluestones' (of Ordovician dolerite) which form the lintels of Stonehenge were sourced in the Preseli Hills of Pembrokeshire around 2500 BC. Bronze Age and Iron Age peoples made extensive use of local materials in erecting a variety of cairns, standing stones and defensive works as manifest in Wales' many hill forts. Wales' many fine cathedrals, abbeys and castles have used a variety of stones in their construction; Caerbwdi Sandstone in St David's Cathedral, Old Red Sandstone at Llanthony Priory and Tintern Abbey, 'Blue Pennant' sandstone in Caerphilly Castle and Sudbrook Sandstone at Caldicot Castle