An area of about 900 square miles (2300 km² ) at the junction of the Kenya and Kavirondo Rift Valleys was studied and a map of it on a scale of 1 : 50,000 is presented. The fonmations present are alkaline lavas and tuffs ranging in age from 12 m.y. b.p. to recent and can be divided into a basanite to phonolite series older than 7 m.y. and a basalt to trachyte series younger than 7 m.y. The fonna.tions are grouped into four assemblages, each .consisting of rocks derived fr.om sources in about the same area. A series of trachytio ash flows about 4m.y. old, the Eldama Ravine Tuff; and two trachyte volcanoes, Londiani of 3 m.y. b.p. and Kilornbe of 2 m.y. b.p., together with their associated syenite bombs are described in detail. The structure of the area is dominated by the Equator and Mau Monoclines which form the western margin of the Kenya Rift Valley. Faults are relatively unimportant but show three distinct trends which can be related to structures in the basement. Chemical analysis was carried out on about 200 rocks, particularly concentrating on the Eldama Ravine Tuff and the Londiani and Kilambe Trachytes. This, with the petrography, showed that the. rocks within the ba.sa·ni te to phonolite series and the basalt to trachyte series are related in general but not in detail by fractional crystallisation. It is also shown that in the trachytic rocks Na, Fe, Y and the Lanthanides are very mobile.

Two sets of cores were recovered from the southern Kenya Rift (Koora and Magadi basins) through the Hominid Sites and Paleolakes Drilling Project and the Olorgesailie Drilling Project. These contain a detailed environmental Quaternary history with records of up to ~1 million years. This period correlates with much of the Olorgesailie Formation record of 1.2 Ma in the Olorgesailie Basin. The Magadi cores reached trachyte at ~ 194 and 133 m with this project focussed on the longer core, MAG14-2A, which includes limestone, zeolitic, laminated and massive clay and silt, massive mud, chert, trona, gravel and sand. The Koora Core (OLO12-1A) extended to depths of 166.14 m and contains laminated and massive diatomites, fine to coarse sands; lime and siliciclastic muds with pumice-rich gravels. The two cores are particularly important because they provide environmental records that help to fill erosional gaps in the history of the Olorgesailie Basin, which includes important evidence for changing hominin cultures and evolution. The high-resolution lacustrine-terrestrial stratigraphies of the two basins have shown how landscapes were transformed because of complex interactions between tectonic and climatic processes. Volcanism also had a significant impact, partially damming lakes at Olorgesailie. Diatoms are present in much of the Koora Basin sequence and large parts of the Magadi sediments. These are dominated by a variety of planktonic Aulacoseira, Cyclotella and Thalassiosira taxa in both basins. Species comprising these genera and other planktonic, benthonic and epiphytic taxa preserve a detailed record of lakes that fluctuated in depth, extent and chemistry. The data document the presence of freshwater and saline lakes as well as wetlands. Diatom transfer functions from the Koora and Magadi basins indicate that these water bodies fluctuated widely in conductivity between ~200 to >20,000 µs cm−1, with pH changing between about 7.5 and 11.5. The palaeolakes also periodically exceeded diatom tolerance limits and intermittently dried out. Pollen are generally lacking in the Koora basin sediments, but deposits in the Magadi core contain common pollen that document a wide range of habitats, including forests, woodlands and grasslands that could have supported the presence of hominins and their activities in the region. Fungal spore data support pollen inferences and indicate periods when large mammals might have been common. The microfossil record shows that there was a broad trend towards more arid conditions in the southern Kenya Rift after about 510 Ka, interrupted by periodic wetter conditions. A major episode of desiccation developed between about 450 Ka to 400 Ka that partially correlates with a period of mammal extinctions and a change from Acheulean to Middle Stone Age toolkits in the Olorgesailie Basin, suggesting that these changes might have been related to environmental conditions at that time.

A detailed study was made of the composition and distribution of modern and fossil animal and plant traces around saline, alkaline lakes in tectonically active, closed lake-basins. Modern and Pleistocene traces that were examined in lake basins of the Kenya Rift Valley (Lakes Bogoria, Magadi, and Nasikie Engida) were compared directly with fossil traces from the Eocene Lake Gosiute in the Green River Formation of Wyoming, U.S.A., which had a similar hydrochemistry. Analysis of lithofacies and the stratigraphic packaging of the sediments hosting biogenic structures was undertaken so that their vertical and lateral distribution could be used to interpret lake histories and to help to develop depositional models of enigmatic sedimentary successions. A focus was given to the application of the results for paleoecology and stratigraphy, and a model for predicting the position of different trace associations in vertical successions and in different parts of saline, alkaline lake basins has been developed. Evidence from the Kenyan lakes and Eocene Lake Gosiute shows that (1) sedimentary environments are diverse in underfilled basins, and frequent lake-level fluctuations strongly impact the distribution of sedimentary environments suitable for the production and preservation of biogenic structures; (2) the distribution of biogenic structures in underfilled basins is related to the geomorphological and structural setting, tectonic activity, catchment lithology, the basin margin or basin centre location, climate, and salinity and alkalinity, together with other finer-scale environmental and biological controls; (3) because saline environments are restrictive, sites of relatively dilute inflow (springs, rivers and deltas, ephemeral streams) provide oasis-like habitats for animals and plants, and contribute to the increased diversity and laterally variable distribution of saline-lake trace assemblages; and (4) the vertical distribution of trace fossils in a stratigraphic succession reflects changing environments through time; important stratigraphic surfaces, usually formed during periods of lake-level fall, can be recognized from the overprinting patterns of traces produced under different conditions.

A detailed study was made of the composition and distribution of modern and fossil animal and plant traces around saline, alkaline lakes in tectonically active, closed lake-basins. Modern and Pleistocene traces that were examined in lake basins of the Kenya Rift Valley (Lakes Bogoria, Magadi, and Nasikie Engida) were compared directly with fossil traces from the Eocene Lake Gosiute in the Green River Formation of Wyoming, U.S.A., which had a similar hydrochemistry. Analysis of lithofacies and the stratigraphic packaging of the sediments hosting biogenic structures was undertaken so that their vertical and lateral distribution could be used to interpret lake histories and to help to develop depositional models of enigmatic sedimentary successions. A focus was given to the application of the results for paleoecology and stratigraphy, and a model for predicting the position of different trace associations in vertical successions and in different parts of saline, alkaline lake basins has been developed. Evidence from the Kenyan lakes and Eocene Lake Gosiute shows that (1) sedimentary environments are diverse in underfilled basins, and frequent lake-level fluctuations strongly impact the distribution of sedimentary environments suitable for the production and preservation of biogenic structures; (2) the distribution of biogenic structures in underfilled basins is related to the geomorphological and structural setting, tectonic activity, catchment lithology, the basin margin or basin centre location, climate, and salinity and alkalinity, together with other finer-scale environmental and biological controls; (3) because saline environments are restrictive, sites of relatively dilute inflow (springs, rivers and deltas, ephemeral streams) provide oasis-like habitats for animals and plants, and contribute to the increased diversity and laterally variable distribution of saline-lake trace assemblages; and (4) the vertical distribution of trace fossils in a stratigraphic succession reflects changing environments through time; important stratigraphic surfaces, usually formed during periods of lake-level fall, can be recognized from the overprinting patterns of traces produced under different conditions.