The Black Cuillin, low mountains mainly composed of basalt and gabbro, include 12 peaks the Scots call 'Munros'. This is some of the most dramatic and challenging mountain terrain in Scotland. The ascent of Sgùrr a' Ghreadaidh is one of the longest rock climbs in Britain and the Inaccessible Pinnacle is the only peak in Scotland that requires technical climbing skills to reach the summit. A full crossing of the Cuillin ridge may take 15–20 hours to complete.