1 MOE Laboratory for Earth Surface Processes, Department of Geography, College of Urban and Environmental Sciences, Peking University, Beijing, 100871, China. jfzhang@pku.edu.cn r.w.dennell@exeter.ac.uk.
2 Department of Archaeology, University of Exeter, Exeter, UK. jfzhang@pku.edu.cn r.w.dennell@exeter.ac.uk.