the distinctive shape of this old red sandstone hill comprises a long ridge oriented nearly north–south, with a jagged western side resulting from ice age landslips . the upper slopes of the hill are composed of devonian age sandstones assigned to the senni formation (formerly known as the "senni beds"). these overlie weaker mudstones of the st maughans formation - a situation which has contributed to the instability of the hill's steep flanks, resulting in a very large landslip at the northern end of the mountain, although the british geological survey map of the area (abergavenny sheet) shows the landslide extending along the whole of the west side of the mountain. there are numerous other landslips of a similar nature on the nearby hills, although that on the skirrid is perhaps the most well known owing to its visibility from several directions. some of the landslipped areas contain numerous jumbled blocks of sandstone, one of which is known as the "devil's table". the ridge is very similar in its rocky edge to that found on the black hill to the north in herefordshire.