1. School of Geography, Geology, and Environment, William Smith Building, Keele University, Staffordshire, ST5 5BG, UK
2. School of Geography, Geology, and Environment, William Smith Building, Keele University, Staffordshire, ST5 5BG, UK; and now at School of Engineering, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
3. British Geological Survey, Keyworth, Nottingham, NG12 5GG