The Yatta Plateau is an extraordinary feature consisting of a phonolite lava flow that extends for some 300 km, but is generally only several kilometres wide, the lava capping a ridge which it nowhere appears to overflow. It commences near Ol Doinyo Sapuk, to the northeast of Nairobi, and extends southeastwards across Precambrian basement rocks. The exceptional nature of the flow is perhaps best appreciated on the 1:1,000,000 geological map (1987) with greater detail being available on four ‘Quarter degree sheets’ (1:125,000) (Dodson, 1953; Fairburn, 1963; Sanders, 1963b; Walsh, 1963). The flow is generally about 20 m thick towards the edges but drilling indicates it may be more towards the centre. There have been a number of theories proposed to account for the geometry of the flow (see discussion in Walsh, 1963 and Fairburn, 1963) the main ones being that the flow was confined to a river valley and that the lavas were formerly more extensive; there are, however, problems with these and other hypotheses. It is classified as a member of the Kapiti phonolite suite (see Nairobi area, No. 085-00-062) and contains aligned phenocrysts of anorthoclase, up to 8 cm in length, nepheline (up to 1 cm) and microphenocrysts of olivine, sometimes rimmed by aenigmatite, in a matrix of alkali feldspar, nepheline, aenigmatite, katophorite, aegirine and analcime.