Black Mountain is a summit in Powys, Wales. It forms part of the Black Mountains range. Its elevation reaches 703 m (2306 feet) and is the only Marilyn to fall just on the Welsh-English frontier, straddling the regions of Powys and Herefordshire. The ridge is crossed by the Offa's Dyke Path. A rockier path leads to the peak from near the former youth hostel in the Vale of Ewyas to the west. The terrainis peaty and normally very wet, so its not recommended in bad weather to deviate the path. The mountain involves sandstones and mudstones of the Senni Formation of the Old Red Sandstone, which is of Devonian age.