In the lower Omo valley (Ethiopia), north of Lake Turkana (formerly Lake Rudolf) a long succession of fossiliferous sediments and pyroclastics is exposed, the age of which has been determined by conventional K/Ar and paleomagnetic methods to fall between 0.8 m.y. and 3.2 m.y. Still older sediments and pyroclastics occur at the northernmost margin of the basin and have ages both greater and just less than 4 m.y. Another succession of fossiliferous sediments and pyroclastics is exposed over a large area in Kenya east of Lake Turkana. This succession has also been subjected to K/Ar and paleomagnetic analyses, which have yielded ages between about 1.2 and 4.5 m.y. (Fig. 30:1).
Large collections of vertebrate fossils have been recovered from both areas. In the analysis of these assemblages, various workers have suggested that the geophysical age determinations of one or both of these successions is incorrect. In particular, there has been substantial antechamber speculation about whether or not the age of 2.61 m.y. assigned to the KBS Tuff in the East Lake Turkana area by Fitch & Miller (1970, 1976) is correct. That tuff separates the Lower Member from the Upper Member of the Koobi Fora Formation. This controversy arose not from the morphology of the now well-publicized ER-1470 skull (Leakey 1973a, b)—recovered 38 m below the KBS Tuff in Karari Area 131—as some participants in the discussion have suggested.It arose instead from an apparent discrepancy between mammalian taxa derived from pre-KBS Tuff sediments when compared with those taxa from Member
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