In the Iron Age and during the Roman occupation, ornaments and other objects were made from the black bituminous shale—known as blackstone or 'Kimmeridge coal'—that occurs in layers within the Kimmeridge clay that covers most of Kimmeridge parish. Armlets were manufactured from the shale using a lathe, which produced waste in the form of hard black discs; these have been discovered at several sites, and were thought by 18th-century antiquaries to be coins and therefore called 'coal money'. The Romans also used the shale as fuel for boiling sea water to produce salt.