In 1947 this reddish rock was found to contain small bones of some of the world's earliest true mammals. It consists of material preserved in fissures in the Carboniferous Limestone, and is particularly important as these fragile bones are very rarely preserved elsewhere. At the end of the Triassic, the rocks forming the surface of most of South Wales were the much the same as those exposed today. The Vale of Glamorgan was being increasingly covered by a shallow sea, with the Carboniferous limestone hills forming several small islands. The remains of the plants and animals which lived here were washed into caves and fissures in the limestone. The islands were finally drowned in the early Jurassic and the whole area was covered with more recent sedimentary rocks, most of which have now been eroded away. The Triassic fissures are only exposed in certain quarries today, but occasionally they are found to contain fossilised bones and teeth of small dinosaurs, various reptiles and early mammals, along with fossil plant remains. Morganucodon and Kuehneotherium, two of the world's earliest mammals, were tiny, shrew-like animals, probably no more than 3 centimetres long. They lived at the same time as the earliest dinosaurs. They were probably insect eaters, and may have been nocturnal.

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