Early Quaternary landslides of the Sahara and their significance for geomorphic and climatic history

The central Sahara may have the highest concentration of fossil landslides, most likely of early Pleistocene age. These developed after an initial arid phase had caused oversteepening of stable sigmoidal slopes formed under a humid climate during the Neogene. The landslides post-date dissection of Late-Tertiary pediments and are older than all fluvial terraces, lake sediments and aeolian corrasion and deposition features of the central Sahara. The climate must have been considerably wetter than semi-arid, as even tiny sandstone plateaus and inselbergs could catch enough rain to have their underlying clay-and siltstones deeply wetted and made plastic and unstable. Landslides occurred in practically all places in the Sahara, depending on the presence of soft rocks, especially with swelling clay minerals. Along hundreds of kilometres of escarpment landslide fringes in places up to 3 km wide are morphologically significant features of the Sahara. They stand for the only phase of slope retreat in the history of escarpment formation from an initial etchplain since the early Neogene.