Lead mines were opened in Mwyndy (between Llantrisant and Groesfaen) in the "Parke of Cloune" in May 1531 CE. By August 1531 iron ore was being mined there. Iron smelting began nearby during the summer of 1532, with one bloomery furnace site at Mwyndy, and (probably) another smaller smelting site at Rhiwsaeson, employing 4 blowers/hewars, with three blowers operating the bellows at any one time. They produced up to two blooms, of 50 kg per bloom per day, from 150 kg of ore, taking 6 to 7 hours each to produce. Charcoal to heat the furnaces was probably derived locally, from "Cloune Park" — mention is made by Rice Lewis in his 'A breviat of Glamorgan' (1595 and 1600) of the destruction of the forests of Garth Maelog and Allt Griffith because of the ironworks. The iron mines were leased by the crown in 1539, granting rights to mine and make iron and to build water powered ironworks within three miles of Cloune Park for 21 years. There is no evidence to suggest that either iron mining or smelting continued in the area after the 16th century. The name of the house built on or near the ironworks probably derived from the words "mwyn" (), and "dŷ" (soft mutation of "tŷ") (). And from that, the area name of Mwyndy. The mine was either reopened, or dug out near the original workings, about 1859. Over one million tons of iron ore (goethite) were mined at Mwyndy before the mine closed in 1884. The resulting lake feeds into Nant Mwyndy. The Afon Clun marks the southern edge of the South Wales Coalfield. There are several former collieries to the north of the river. Many pits were begun in the 1860s in Beddau, Ty'n-y-nant and Gelynog pits being the most important, prior to which the area was mostly farmland. Cwm Colliery was sunk by the Great Western Colliery Company Limited in 1909 and this marked a rapid expansion of the population. In 1923 the Powell Duffryn Steam Coal Company sunk the Ynysmaerdy Colliery at Llantrisant, also known as the New Duffryn and Llantrisant Colliery, it had three shafts, employing 216 men.