Lothagam, located west of Lake Turkana in northern Kenya, is an uplifted fault block comprising a gently westward-dipping sequence of volcanic and sedimentary rocks. The lower part of the sequence, lavas and coarse volcaniclastic sediments of the Nabwal Arangan beds, was deposited mainly between 14 Ma and 12 Ma (Mid-Miocene), although the uppermost basalt has a K-Ar age of 9.1 Ma. The overlying fluvial sediments of the lower Nawata Formation have yielded ages on five tuffaceous horizons ranging from 7.4 ±0.1 to 6.5 ±0.1 Ma, Late Miocene. A tuffaceous horizon in the overlying Apak Member of the Nachukui Formation, yields an age of 4.22 ± 0.03 Ma; 40Ar-39Ar age spectra on the succeeding Lothagam Basalt indicate an age of 4.20 ± 0.03 Ma for its eruption. Much of the rich faunal assemblage from the Nawata Formation derives from the tightly dated lower intervals. Two hominoid teeth from higher in the formation can only be constrained to lie between 6.5 and 5 Ma old. The hominoid mandible, KNM-LT 329, from the lower Apak Member is older than 4.2 Ma and younger than 5.0 Ma.