The distinctive shape of this Old Red Sandstone hill comprises a long ridge oriented roughly north-west/south-east. The upper slopes of the hill are composed of Devonian period sandstones, assigned to the Senni Formation. They overlie weaker mudstones of the St Maughans Formation - a situation which has contributed to the instability of the hill's steep flanks, resulting in small landslips at various points on the ridge. There are numerous other landslips of a similar nature on the nearby hills, especially that at Black Darren and Red Darren at the edge of the Hatterall Ridge to the south west, where the great size of the slippage dominates the side of the mountain. There is a yet larger landslip on the Skirrid, which is visible on the hill from a great distance as a deep notch in the landscape. Such landslips were probably formed at the end of the last Ice age when meltwater undermined the strata. There are many rock tables on the ridge of the Black Hill, where structural details such as rock jointing, can be seen more closely.