The western half of this quarter degree sheet area consists of Tertiary lavas, pyroclastic rocks and sediments and the eastern half of basement gneisses and schists (Fairburn, 1963). There is one area of volcanic rocks in the east which is a narrow strip of phonolite lava which forms the extraordinary Yatta Plateau, which extends for some 300 km to the southeast. This occurrence is described separately (No. 085-00-066). The volcanic rocks in the west represent a continuation of those of the Nairobi area and, as in that area, the oldest are the Kapiti phonolite lavas. These, apart from the Yatta Plateau, are confined to the southwest of the area, comprise flows generally up to 30 m in thickness and contain alkali feldspar, euhedral nepheline, sodic pyroxene, katophorite and aenigmatite. In the north of the area around Thika olivine basalts overlie the phonolite. The sequence then continues with widespread tuffs, some of which are welded. They are trachytic in composition commonly containing crystals of alkali feldspar and more rarely aegirine. The welded tuff includes large, flattened fragments of obsidian. In the southwest the tuffs are overlain by the Nairobi phonolite which is a continuation of the much more extensive outcrop of the Nairobi area (No. 085-00-062). The youngest rocks are olivine basalts that extend over a small area south of Thika.
FAIRBURN, W.A. 1963. Geology of the North Machakos-Thika area. Report, Geological Survey of Kenya, 59: 1-43.SAGGERSON, E.P. 1991. Geology of the Nairobi area. Report, Geological Survey of Kenya, 98: 1-91.