The Kamasia Range (or Tugen Hills) is an uplifted, faulted and arched block lying between the Elgeyo Escarpment, which forms the western boundary of the Kenya Rift, and the axial Baringo-Suguta trough between 0°15'N and l°30'N in the northern part of the rift. Metamorphic basement rocks are exposed in the central part of the Kamasia area where they are overlain by 3000 m of Miocene lavas and sediments ranging in age from 16 Ma to 7 Ma. This is the thickest sequence of rocks of this age exposed in the Kenya Rift. Downwarping occurred from the beginning of this period, but the first major fault movements occurred at about 7 Ma (late Miocene) forming the Elgeyo and Kamasia fault-scarps. Later volcanicity was mainly confined to the area east of the Kamasia, although extensive flood trachyte lavas covered practically the whole area east of the Elgeyo at one time. A second period of major fault movement occurred in the late Pliocene-Pleistocene (2-0.5 Ma) uplifting the Kamasia Range, forming the western dip-slope and downfaulting the axial graben of the rift to the ea,:;t.The overall structure of the Kamasia is a broad arch cut by large faults, with maximum displacement of about 4000 m, just to the west of its axis, but in detail the structures are complex. The numerous faults are normal dip-slip type and show an en echelon and obliquely intersecting pattern with dominant directions 000-040 ° and 330-340 ° . Folding on axes perpendicular to the fault planes produced half-domes and basins indicating a secondary compressive stress directed along the rift. It is proposed that the majority of the faults developed as vertical fractures in horizontal rock sequences and were then rotated by continued extension and up-arching to produce complex tilted fault-block systems both synthetic and antithetic to the main rift structure.
The material discussed in this paper was obtained during 1966 from the region to the west of Lake Baringo. Excavations were conducted in two areas about a mile apart, on either side of the Kapthurin River. The positions of these excavations are to be found on the 1: 50,000 Survey of Kenya map, Sheet 90/4 of Series Y731, Edition I-D.O.S. The first is located at ZR 314/637 and the second at ZR 316/621. Fig. 1 is based on a small section of Sheets 30/4 and 31/3 and is reproduced by permission of the Director of Surveys, Nairobi. A hominid mandible and a living site were discovered in the first area whilst in the second a factory site was found. These sites lie in the Kapthurin Beds of the eastern area of the Kamasian Hills. The beds are possibly upper Middle Pleistocene and are divided from the underlying Chemeron beds which are of Plio/Pleistocene age.The geology of the area was first described by J. W. Gregory (1921). It was subsequently studied and re-assessed by V. E. Fuchs (1939 and 1950) and by McCall, Baker and Walsh (1967). The most recent study has been made by Mr J. E. Martyn of Bedford College, London, who was engaged in geological mapping of the area west of Lake Baringo in 1966 and 1967. A report on the Kapthurin Beds by J. E. Martyn follows this introduction.During the course of mapping, J. E. Martyn and his assistants found a number of fossils in the Chemeron beds. He showed these to Mr Jonathan Leakey who noticed an incomplete hominid temporal bone amongst them.
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