Volcanic rocks cover about 70% of the Kajiado area (Matheson, 1966) those in the west lying within the Gregory Rift valley while those that occupy the northern half of the remaining area form plateaux and plains that are continuous with those around Nairobi (No. 085-00-062). Precambrian basement rocks occur elsewhere. The oldest volcanic rocks are the Kapiti lavas that extend over much of the east of the area and, with those occupying the adjacent North Machakos-Thika area (No. 085-00-065), they form the extensive Kapiti plains. A smaller area occurs within and close to the rift west of Kaijado. The Kapiti phonolites are overlain by the Upper Athi tuffs which are trachytic, sometimes welded and also extensively represented in the North Machakos-Thika area (Fairburn, 1963). These in turn are overlain in the north of the area by the Mbagathi trachytes which contain a sodic amphibole, aegirine, aenigmatite and a little nepheline. The succession is then continued by rocks of the Olorgesailie series that are assumed to have originated in the volcano of that name, which is situated in the Magadi area (No. 085-00-067) to the west (Baker, 1958). These rocks comprise biotite phonolite and phonolitic nephelinite lavas together with various trachytes, basalts and agglomerates. Unlike the type Olorgesailie nephelinite that in the Kajiado area contains appreciable feldspar in the groundmass and as phenocrysts together with phenocrysts of nepheline, microphenocrysts of aegirine-augite and titanite, and groundmass sanidine, nepheline, aenigmatite and aegirine. Immediately east of the rift are extensive areas of agglomerate that form the small hills of Ol Doinyo Narok and Ol Doinyo Arau, that may be vent areas, which grade northwards into tuffs that continue to the Nairobi area. The agglomerate blocks consist principally of trachyte and phonolite. In the northern part of the rift there occur tephrites, phonolites and trachytes of the Ol Esayeiti volcano, which are briefly described elsewhere (No. 085-00-064). This part of the volcanic succession was then completed by outpouring of the Ol Keju Nero olivine basalts that form a continuous north-south belt within the rift valley. They are overlain by peralkaline trachytes that occupy much of the rift valley and are continuous with the trachytes which are the most extensive rock type of the Magadi area. They contain aegirine, aenigmatite and a sodic amphibole.
BAKER, B.H. 1958. Geology of the Magadi area. Report, Geological Survey of Kenya, 42: 1-81.FAIRBURN, W.A. 1963. Geology of the North Machakos-Thika area. Report, Geological Survey of Kenya, 59: 1-43.MATHESON, F.J. 1966. Geology of the Kajiado area. Report, Geological Survey of Kenya, 70: 1-51.