The reserve forms part of an impressive limestone escarpment that outcrops along the north eastern edge of Mynydd Llangatwg. It is one of the largest upland limestone cliffs in South Wales.
The limestone rock rich in nutrients gives rise to soils that support a wide variety of plants, some rare. Of particular note are several species of rare whitebeam growing on the cliffs, one of which is only known from this locality. Some 50 species of bird have been recorded breeding here. The limestone itself is peppered with caves, and one of them Agen Allwedd is an important winter roost for lesser horseshoe bats.