The Blaenavon Ironworks, completed in 1789, was the most advanced in Wales in its day, using coke rather than charcoal for smelting with a steam engine to blow the furnace.

The coal and iron ore were extracted from mines at various levels in the north east side of the Afon Lwyd valley.

[2] The Pwll Du limestone quarry, Blaenavon, lay on the north eastern lip of the South Wales Coalfield, on the upland bluff that overlooks Abergavenny.

[4] When plans were made to build the Garnddyrys[a] iron forge in Cwm Llanwenarth on the west branch of the Blorenge hill, they included replacing a steeply sloped tramroad from Blaenavon by a shallow gradient tramroad along an extension of the earlier tunnel through the hill to Pwll Du.

[5] The tramroad ran past Garnddyrus forge, down the northern side of the Blorenge hill to Llanfoist Wharf on the Brecknock and Abergavenny Canal.

[6] The tunnel, high enough for horse drawn trams, runs through the Mynydd Garn Fawr in a sweeping curve.

[10] The Pwll Du end of the tunnel was a stone lined trench, later covered, with two linked portals in a Y branch.

[4] The double incline ran up and over the hill above the tunnel, with a steam engine at the summit to haul up the loaded trains.

In 1933–34 a team of fifteen to twenty men from the Blaenavon Company drove a heading that found a workable 2 to 3 feet (0.61 to 0.91 m) coal seam.

There was a bedding collapse about 40 metres (130 ft) into the Garnddyrys branch, perhaps from the same cause, but possibly due to open cast workings above the tunnel in the 1940s.

[6] The exploration team sank a shaft into the water-filled tunnel beyond the roof fall at the Pwll Du end.

In November 2000 UNESCO designated the Blaenavon Industrial Landscape as a World Heritage Site, including the town, ironworks, Big Pit Museum and Hill's tramroad with its Pwll Du tram tunnel.