Tags: Site Of Special Scientific Interest.
Cwm Twrch is the valley of the Afon Twrch a tributary of the River Tawe in South Wales. It forms the border between the county of Powys (anciently Brecknockshire) in the east and Carmarthenshire and Neath Port Talbot (formerly part of Glamorganshire) in the west. It was once heavily industrialised in its lower reaches. In contrast its upper reaches penetrate the heart of the Black Mountain some of the wildest country in South Wales.