Cwmafan was a smoking industrial cauldron in the first half of the 19th century: rolling mills, foundries and the notorious copper works, built between 1835 and 1838, that pumped out noxious fumes from its smelting furnaces. Eventually something was done to try and alleviate the pollution.
The manager of the copper works, William Brunton, constructed a stone culvert from the works up the side of Mynydd Foel. It was 15 foot wide, 11 foot high and a mile long. At its mouth, on the summit, he built a 30 foot buttressed stack to direct the fumes upwards.
Brief Historical Surveys of Taibach and Cwmavon, William and Cyril Rees (1972)