Corn Du, pronounced Corn Dee meaning ‘Black Horn’, is the second highest mountain in South Wales at 873 metres (2,864 ft) and is situated in the Brecon Beacons National Park.
The summit is marked by a well structured Bronze Age cairn with a central burial cist.
The views from the peak are panoramic and very extensive.
The summit is flat and anvil shaped. It is formed from Devonian age sandstones from the Upper Old Red Sandstone, known as the Plateau Beds. The rock strata are clearly visible at the edge of the escarpment, where they form a resistant edge to the cliff. The same strata are visible all along the escarpment facing north, as well as the rearward edge facing south-east and lying behind the main peak.