A rock unit that is recognisable due to a distinctive lithology or fossil assemblage. Related Articles: Shining hill in the Arizona desertFifteenth century magico-medicinal mineralsExcursion to the South Devon coast led by Prof John CW Cope (National Museum Wales)Farming for fossils in MoroccoGeology and fossils of the Spilsby Sandstone Formation of Nettleton, Lincolnshire, UK« Back to Glossary Index