Geology of Middle Stone Age Archaeological Sites in the Main Ethiopian Rift Valley

On a ridge west of Lake Ziway, northernmost of the Galla Lakes, rich Middle Stone Age sites are present in paleosols of the late Pleistocene Gademotta Formation. The underlying rocks are of alkali rhyolite and tuff, the eruption of which commenced over 1 m.y. ago and was climaxed by caldera collapse and explosive ejection of pumiceous tephra. Lower Gademotta sediments consist of tuffaceous laharic mudstone interbedded with thick paleosols developed on unsorted colluvial volcanic detritus. Artifacts are lacking. A paleosol in the middle of the Gademotta Formation contains obsidian tools of Levallois-Mousterian technology, the earliest Middle Stone Age artifacts thus far discovered in the area. This soil was covered by a crystal-rich volcanic ash, which has been dated at ∼ 181,000 yr B.P. Above the ash, in turn, are three superposed paleosols, each with Middle Stone Age artifacts intercalated with water-laid sandstone, and two additional ash beds. Late in the sedimentary history of the Gademotta Formation it was eroded by intermittent streams forming deep gullies which subsequently were aggraded. Perhaps the cutting and filling were related to the fluctuations of an ancient lake whose level may have reached more than 100 m above present Lake Ziway (1,636 m). The consistent occurrence in this region of Middle Stone Age sites — > 35,000 to > 100,000 yr B.P., elevations ≧ 150 m above Lake Ziway — suggests geographic control of human settlement by high lake stands during late Pleistocene time. Topographically lower volcanic hills, which probably formed contemporaneously with Gademotta Ridge, appear not to have been inhabited until the Late Stone Age (< 15,000 yr B.P.) when the lake level was substantially lower.