Apart from a small area of Precambrian gneisses and quartzites in the southwest corner of the quarter degree sheet (Williams, 1991) the whole of the Mau area is covered by volcanic rocks. The succession comprises the Mau phonolites at the base, overlying Precambrian basement rocks, overlain by the Mau tuffs and these in turn by the Mau ashes. The ashes blanket the whole region the underlying rocks only outcropping along the numerous, deeply incised river valleys. The phonolite lavas represent the most southwesterly part of exceptionally extensive Miocene flood phonolites (also known as the ‘plateau phonolites’) that cover a huge area in central and southern Kenya (Williams, 1991). They probably originate from fissures in the Nakuru area (No. 085-00-051) to the northeast where they attain a thickness of some 1000 m. They are probably about 300 m thick in the Mau area, though this cannot be seen directly. They contain sanidine phenocrysts up to 2 cm in length, rather fewer and smaller ones of nepheline and, in some areas, biotite, in a matrix of sanidine laths, aegirine and aegirine-augite, katophorite, aenigmatite and analcime. One small area of biotite-olivine melanephelinite (ankaratrite) has been identified. The Mau tuffs consist of upper and lower welded units separated by agglomerates and lapilli tuffs, the total thickness over the whole area being about 200-250 m. Both the lower and upper welded tuffs are glassy peralkaline rocks of flattened pumice with various combinations of crystals of sanidine, aegirine, a range of types of sodic amphibole, aenigmatite and rarely biotite. Williams (1991) gives whole rock analyses of phonolites and a melenephelinite.