The Hatterrall Ridge (sometimes spelled Hatterall) is a ridge in the Black Mountains forming the border between Powys and Monmouthshire in Wales and Herefordshire in England. The ridge is about 10 miles (16 km) long, and is followed by the Offa's Dyke Path. On the west side of the ridge is the Vale of Ewyas, and on the east side is the Olchon Valley and Black Hill. The western side of the ridge falls within the Brecon Beacons National Park. Much of the northern part of the ridge is a broad whaleback, but it narrows down considerably further south, and especially near to Llanthony Priory in the Vale of Ewyas to the west of the mountain. There is a large and prominent landslip on the eastern side of the mountain here, known as Black Darren, where a large slice of the rocks has fallen away fr