A Derbyshire screwstone (Mississippian) from the beach at Overstrand, Norfolk, eastern England

A distinctive chert erratic pebble found on the beach at Overstrand, north Norfolk, eastern England, is a Derbyshire screwstone. Such cherts are typical of the Mississippian (Lower Carboniferous) limestones of the southern Pennines (White Peak), over 200 km to the northwest. It was most probably transported by fluvial or glacial action during the Pleistocene and was recently disinterred by coastal erosion. The most diagnostic feature of screwstone cherts are the included mouldic crinoid ossicles, particularly columnals. Columnals of the monobathrid camerate crinoid Megistocrinus? globosus? (Phillips) are described from this screwstone; these have a circular outline, central pentagonal lumen and a raised perilumen. The uncertainty of the identification is due not only to the indifferent preservation of the columnals, but also to our poor knowledge of the morphology of the stems of all the nominal crinoids from the Mississippian of the White Peak.