The Clydach Gorge (also known as Cwm Clydach) is a steep-sided valley in south-east Wales down which the River Clydach flows to the River Usk. It runs for 5.6 kilometres (3.5 mi) from the vicinity of Brynmawr in Blaenau Gwent eastwards and northeastwards to Gilwern in Monmouthshire. The Gorge was one of the first locations in the region to be industrialised though it still retains its natural environment. It has long been an important transport corridor between Abergavenny and the lowlands of Monmouthshire and the northeastern quarter of the South Wales Coalfield. It is now exploited by the A465 Heads of the Valleys trunk road which runs between Abergavenny and Swansea and which serves the Heads of the Valleys sub-region.