Its summit is crowned by a trig point and a mast.

The hill lies at the southeastern extremity of the South Wales Coalfield where the varied Carboniferous Period rock strata of the coalfield margin are tilted steeply northwestwards into the coalfield basin.

The sequence which outcrops on Mynydd Machen in northeast-southwest aligned bands is this (oldest at base): Warwickshire Group South Wales Coal Measures Group Marros Group Pembroke Limestone Group The summit of the hill is formed from the sandstone of the ‘Brithdir Member’ of the Pennant Sandstone Formation.

A large quarry towards the south of the hill works the dolomitic limestone of the Pembroke Limestone Group.

The upper slopes of the hill and most of the woodland which clothes its slopes are mapped as open access under the Countryside and Rights of Way Act 2000 and thereby open to access on foot by the public.