1. Department of Geography, Durham University, Lower Mountjoy, South Road, Durham DH1 3LE, UK
2. Department of Civil Engineering, University of Bristol, Queens Building, University Walk, Bristol BS8 1TR, UK
3. CNRS, ENS de Lyon, 15 parvis René Descartes, BP 700069342, Lyon Cedex 07, France
4. Department of Earth Sciences, Science Laboratories, Durham University, Lower Mountjoy, South Road, Durham DH1 3LE, UK