 The Triassic rocks consist of red marls , sandstones , breccias and conglomerates which spread over the older rocks . The Dolomitic Conglomerate is an old shingle beach of Keuper Marl age . The Rhaetic Beds are full of fossils due to invasion of the Jurassic Sea . The Lias consists of clays and limestones , the latter being quarried and are famous for their fossils . Blue Lias was burnt locally to provide a source of lime for making lime mortar . It is still used as a decorative building stone . Blue Lias is believed to have been quarried on the Polden Hills as early as the 15th century and was quarried in Puriton from the early 19th century until 1973 , when the local cement works closed . 
