These two quarter degree areas (Shackleton, 1945; Thompson, 1964) encompass the whole of the Aberdare Range, that reaches heights of over 4000 m and lies to the east and parallel with the eastern margin of the East African Rift, which occupies the western part of the area. The whole area is covered by volcanic rocks those in the northeast being the most westerly part of the Mount Kenya suite (No. 085-00-058). The oldest rocks, the Simbara Series, are basaltic lavas and agglomerates which are overlain unconformably by the Sattima Series of trachytes and phonolites, that occur mainly in the west of the area within the rift valley, and these in turn by the Laikipian basaltic lavas. The trachytes are generally feldsparphyric usually with aegirine-augite; some varieties contain aenigmatite and a sodic amphibole. Rare phonolites intercalated with the trachytes include altered nepheline, aenigmatite and aegirine (Thompson, 1964). There are also pyroclastic rocks in the succession which are mainly trachytic, some of them being welded, and they are considered to derive from the Longonot volcano (No. 085-00-056. Much of the rift valley floor is covered by further deposits of pyroclastic rocks, some of them welded, which are again essentially trachytic.