1 Department of Civil, Environment & Geomatic Engineering, Faculty of Engineering Science, University College London, Gower Street, London WC1E 6BT, UK.
2 Department of Geology, University of Nairobi, University of Nairobi, P.O Box 30197, Nairobi, Kenya.
3 Chair of Human Ecology, Department of Earth Sciences, University College London, 2 Taviton Street, London WC1H 0BT, UK.