The Suswa ‘quarter degree area’ (Randel and Johnson, 1991) is dominated by the Suswa volcano which occurs in the northeastern part and is described separately (No. 085-00-061). The low western wall of the rift lies in the western part of the area, and the southern part is heavily faulted to give horst and trough structures. The whole area is covered by volcanic rocks which, excluding those of the Suswa centre, include trachyte lavas and Pleistocene ashes that mantle much of the northwest and which probably derive from either the Suswa or Longonot volcanoes. In the southwest are a group of Tertiary basalts and pyroclastic sediments. The trachytes are feldsparphyric rocks with a groundmass of alkali feldspar, augite, aegirine, aenigmatite, arfvedsonite, katophorite and rare quartz.