Cwm Twrch SSSI is located on and around the Afon Twrch approximately 0.9 miles (1.4 km) north-east of Ystradowen, and covers 2.4 hectares (5.9 acres).
The site is notable for its geology, where river erosion provides the best exposes the Amman Marine Band, a sedimentary layer showing a diverse fauna, and which marks the Westphalian A – Westphalian B boundary in this Carboniferous rock strata.
