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 largely during the early part of the
Devonian Period. Its lower slopes (up to around ) are composed of
mudstones and
sandstones assigned to the Senni 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. == Land use ==