Affiliation: | 1 British Geological Survey, Keyworth, Nottingham NG12 5GG, UK 2 British Geological Survey, Keyworth, Nottingham NG12 5GG, UK 3 Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, UK 4 Department of Geology, National Museum of Wales, Cardiff CF10 3NP, UK |