Old Castle Down is a Site of Special Scientific Interest between St Brides Major and Ewenny in the Vale of Glamorgan, south Wales.
Soft-leaved sedge and pale heath-violet are themselves features of interest and a number of other plants occur here which are rare in Wales.
These include bristle-bent, a grass which is found in South-West England but in only a few places in South Wales.
The site and the surrounding area provides habitat for the high brown fritillary, a butterfly which is rare and declining nationally.
Cracks and fissures within these rocks were infilled by sediments in Jurassic times (about 195 million years ago).