Abstract The central Kenya Rift extends from about 1"N to0010'N and is bounded by the Elgeyo fault scarp in the west and the Laikipia fault scarp 70 km to the east (Figure1). Within this semi-arid region, there are a series of geographically and temporally distinct fault-bounded basins with fluvial and lacustrine sediments that have developed and been destroyed because of combined tectonic, volcanic, and climatic controls. These Neogene basins have tended to shift eastward with time as rift extension has progressed (Chapman and Brooke,1978; Chapman et al., 1978). Their sediments include the Miocene Kimwarer and Tambach Formations along the Eigeyo border-fault (Murray-Hughes, 1933; Shackleton,1951; Lippard, 1972; Ego, 1994; Renaut et al.,1999), several Miocene-Pliocene successions exposedin the Tugen Hills (King and Chapman, 1972; Chapmanet al., 1978; Hill et al., 1985; Hill, 1995), and the Pleistocene Kapthurin Formation and modern Lake Baringo in the eastern axial part of the rift (Figure 1)(Tallon, 1978; Renaut et al., this volume). The Tugen Hills, also known as Kamasia, is a largenorth-south trending tilt block that dips westward and rises to more than 1000 m above the rift floor. Pleistocenefaulting and erosion have exposed a sequence of more than 3000 m (Hill et al., 1986) of interbeddedfluvial and lacustrine sedimentary rocks and lavas,principally phonolites, trachytes, and basalts (Figures1, 2). Lacustrine sedimentary rocks, includingdiatomaceous shales, silts, and weakly lithified siltstones, are found in the Miocene Ngorora and Lukeinoformations and the Pliocene-Pleistocene Chemeron Formation (Figure 1). To date, few detailed mineralogical and paleolimnological studies of these sediment shave been undertaken.