Sugar Loaf (Wales) - Geology

Geology

It is a popular misconception that Sugar Loaf is an 'extinct volcano'; an idea born perhaps from the striking resemblance of its conical outline, particularly when seen from the east, to that of a classic volcano. It is however composed entirely of sedimentary rocks. In common with the rest of the Black Mountains, the hill is formed from Old Red Sandstone which was laid down during the early Devonian period. Its lower slopes (up to around 1000 ft / 300m) are composed of mudstones and sandstones assigned to the Senni Beds Formation whilst its upper reaches are composed of the more sandstone-rich sequence known as the Brownstones Formation. The very summit of Sugar Loaf is formed from sandstones of the Quartz Conglomerate Group which are of late Devonian age. There are a number of landslips on its flanks which are believed to date from early post-glacial times. The former Usk Valley glacier divided to north and south of it as it travelled eastwards, though the mountain itself is largely free from glacial till.