evidence: Allen, P.A., 1997, Earth Surface Processes. Blackwell Science, 404 p. Bloom, A.L., 1998, Geomorphology; A Systematic Analysis of Late Cenozoic Landforms, Third Edition. Prentice Hall, 482 p. (especially Chapter 6) Easterbrook, D.J., 1999, Surface Processes and Landforms, Second Edition. Prentice Hall, 546 p. (especially Chapter 15)