The Pleistocene sediments of Peninj were described systematically by Isaac in a stratigraphic sequence he called the Peninj Group. The reference for them was provided by the badland outcrops in the river delta, which covered an area of some 1 km2 (the area known as Type Section). At the base of the sequence are the Sambu and Hajaro lavas, bracketed between 2.5 and 2 m.a, both of them deposited before the existence of the River Peninj. Above the Sambu and Hajaro volcanic materials lay two major Formations. The Humbu Formation (45 m thick), at the bottom, is made largely of sandstone and alluvial deposits such as silt and clay, which intercalate with a complete sequence of tufaceous materials. At the same time, the Humbu Formation is divided into three members: the Basal Sands and Clays (in which the Paranthropus boisei jawbone was found); the Main Tuff, a thick volcanic member; and the Upper Sands and Clays (which include the main archaeological sites). On top of the Humbu materials, the Moinik Formation (40 m thick) is largely made up of lacustrine sediments and is devoid of archaeological occurrences.


The chronological context of the palaeoanthropological evidence at Peninj is not without difficulty. In 1967, Isaac and Curtis reported an age of between 1.6 and 1.4 M.a for the archaeological sites located in the Humbu Formation. This chronological framework has been roughly corroborated by further datings. Other studies have reported a sensibly younger date, which would situate the archaeological sites at around 1 ma. However, recent correlations between the volcanic tuffs of Olduvai Bed II and the Humbu Formation have shown that the volcanic materials of Peninj are severely altered, which is a plausible explanation for the chronological discrepancies encountered. So far, it remains difficult to overcome the dating problems at Peninj. However, the technological traits observed in the Acheulean sites are coherent with the 1.6-14 m.a age initially provided by Isaac.