Cwm Graig-Ddu is a steep-sided valley just south of the village of Garth, Powys. The head of the valley is just within the Mynydd Epynt military range known as SENTA, the Sennybridge Training Area, which the Ministry of Defence took over in 1940. From here there is a panormaic view across the Irfon Valley to the southern flanks of the Cambrian Hills.