This area extends westwards from the floor of the Gregory Rift, includes the Kamasia Hills, and the western wall of the rift, marked by the impressive Elgeyo Escarpment which rises over 1500 m above the valley floor, to the western rift shoulder. Walsh (1969) describes the southern half of the area (Eldama Ravine-Kabarnet area) in some detail and there is a more recent map of the same area on a scale of 1:125,000, published by the ‘East African Geological Research Unit’ (EAGRU) (Truckle, 1978), on which the igneous rocks have been subdivided in greater detail; unfortunately, there is no accompanying account. The Geological Survey of Kenya has not published a Report covering the northern half of the area but it is the subject of a detailed stratigraphical and structural paper by Chapman et al. (1978). Apart from a relatively small outcrop of basement rocks in the west along the foot of the Elgeyo Escarpment and recent superficial deposits in the Kerio Valley, the whole area is blanketed by volcanic rocks. The EAGRU map groups the volcanic rocks by area and the sequences are linked to numerous age determinations. The most detailed account of the volcanic rocks, and this only for the southern half of the area, is by Walsh (1969), on which the following description is based. The base of the volcanic sequence is represented by basalts and these, together with the overlying Uasin Gishu phonolites, which have an underlying tuff sequence, are the most extensive series of the area. The phonolites are variably alkali feldspar- and nepheline-phyric with groundmass aegirine-augite and aegirine, aenigmatite occurs in most rocks and less commonly riebeckite, a brown amphibole and biotite. Along the western margin of the Eldama Ravine-Kabarnet sheet are phonolites and nephelinitic phonolites that derive from the Tinderet volcano (No. 085-00-047) in the Kericho area (No. 085-00-046) to the southwest. The upward succession continues with the Eldama Ravine tuffs and sediments followed by olivine basalts, a sequence of tuffs and silts, then the Kabarnet trachytes, which are extensively developed around the town of Kabarnet in the north of the area. They comprise alkali feldspar, aegirine-augite, biotite, aenigmatite and a little riebeckite; quartz occurs rarely. The Lake Hannington phonolites cover much of the eastern part of the area and are continuous with the type sequence in the Baringo-Laikipia area (No. 085-00-032) to the east. The volcanic succession is completed by several series of tuffs. The stratigraphy of the northern half of the area is outlined by Chapman et al. (1978)
CHAPMAN, G.R., LIPPARD, S.J. and MARTYN, J.E. 1978. The stratigraphy and structure of the Kamasia Range, Kenya Rift Valley. Journal of the Geological Society, London, 135: 265-81.TRUCKLE, P.H. (compiler) 1978). Geological map of the Eldama Ravine - Kabarnet area (Scale 1:125,000). Directorate of Overseas Surveys, Ordnance Survey, East African Geological Research Unit, Kenya Rift Valley ProjectWALSH, J. 1969. Geology of the Eldama Ravine-Kabarnet area. Report, Geological Survey of Kenya, 83: 1-48.