Planation surfaces in Northern Ethiopia M. Coltorti a , F. Dramis b, ⁎ , C.D. Ollier c a Department of Earth Sciences, University of Siena, Via di Laterina 8, 53100 Siena, Italy b Department of Geological Sciences, “Roma Tre” University, Largo S. Leonardo Murialdo 1, 00146 Rome, Italy c School of Earth and Geographical Sciences, University of Western Australia, Nedlands WA 6009, Australia Received 7 November 2006; received in revised form 19 November 2006; accepted 19 December 2006 Available online 22 December 2006 Abstract Planation surfaces are an old-fashioned topic in geomorphology, but they are nevertheless important where they make up much of the landscape. Northern Ethiopia is largely a stepped topography, caused by differential erosion. Exhumation of old planation surfaces that were preserved under sedimentary or volcanic cover is an important process in landscape evolution. The oldest planation surface is of early Palaeozoic age (PS1); the second is Late Triassic (PS2); and the third is of Early Cretaceous age (PS3). The Oligocene Trap Volcanics buried a surface (PS4) of early Tertiary age, which is now widely exposed by erosion as a surface that, where flat enough, is an exhumed planation surface. The surfaces do not relate to the supposed Africa-wide pediplain sequence of King [King, L.C., 1975. Planation surfaces upon highlands. Z. Geomorph. NF 20 (2), 133–148.], either in mode of formation and age. Although the region is tropical, there is scarce evidence of deep weathering and few indications that the surfaces could be regarded as etchplains. These surfaces indicate that eastern Africa underwent long episodes of tectonic quiescence during which erosion processes were able to planate the surface at altitudes not too far from sea level. Only after the onset of rifting processes, uplift became active and transformed a vast lowland plain into the present Ethiopian highlands, largely exceeding 2500 m a.s.l. Some hypotheses and speculations on the genesis of these surfaces are considered here. © 2007 Elsevier B.V. All rights reserved. Keywords: Planation surface; Exhumation; Long-term geomorphic evolution; Ethiopia 1. Introduction Planation surfaces, once a basic concern of geomor- phologists, have been largely neglected in recent years, and some even deny their existence (Smailes, 1960; Hack, 1973). But they are real features and it is impossible to make sense of planated landscapes without recognising and trying to understand them. There is increasing debate on the mechanism of formation of planation surfaces, and they play an important role in deciphering the origin of mountains (Ollier and Pain, 2000) and landscape evolution on passive continental margins (e.g. Peulvast and Sales, 2005). Here we briefly describe the landscape of northern Ethiopia, which is dominated by planation surfaces. They do not fit into the schemes of pediplain or etchplain, and seem to follow, at least in part, the classical peneplanation model of Davis (1899). In any case, they reveal significant geomorphological details and provide data for an up-to-date discussion and Geomorphology 89 (2007) 287 – 296 www.elsevier.com/locate/geomorph ⁎ Corresponding author. E-mail addresses: coltorti@unisi.it (M. Coltorti), dramis@uniroma3.it (F. Dramis), cliffol@cyllene.uwa.edu.au (C.D. Ollier). 0169-555X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2006.12.007