The palaeoanthropological evidence contained in the Upper Sands and Clays of the Humbu Formation appears distributed in the three main outcrops identified around the modern Peninj River: Type Section, Southern Escarpment (in an area named Lepolosi by the maasai people), and Northern Escarpment (or Noolchalai). The archaeological sites scattered unevenly within the Upper Sands and Clays unit appear at two slightly different depositional moments.


The oldest is represented by a palaeosoil composed of sands and clays sandwiched between two volcanic tuffs (T1-T2). This layer is very rich in lithic and faunal remains which were deposited in the context of an alluvial fan in which shallow channels drained towards the lake. This horizon (known as the Type Section complex) is particularly well preserved in the Type Section, and was the subject of intense research within the project led by Domínguez-Rodrigo. In this palaeosoil, more than 50 sites have been recorded. The lithic collections here are devoid of LCTs and were initially labelled as Oldowan. However, a number of technical traits indicate that the lithic materials discarded in this location were part of the regional spatial network that operated at Lake Natron during the Acheulean. Faunal remains are abundant in the Type Section Complex. The predominance of bovids, along with equids, elephants or suids, indicates an open savannah environment with scattered bushes.

The youngest of these depositional horizons (bracketed between tuffs T4 and T5) is associated with a slightly different landscape context (relatively large, high-energy fluvial channels over the floodplain), and crops out particularly well preserved in the North and South Escarpments. Here is where the Early Acheulean sites discovered by Isaac (RHS and MHS) and re-excavated by Dominguez-Rodrigo are located. Faunal remains are scarce and badly preserved in this horizon, while typical Acheulean LCTs have been recovered in high numbers. The research project undertaken by the University of Valladolid (and sponsored by the Institute of Cultural Heritage of Spain) is primarily focused on the second and slightly younger horizon. Since 2007, a new round of archaeological work is in progress. Our fieldwork program includes the re-excavations of the classical Early Acheulean sites, new geological, geoarchaeological, and contextual studies, intensive survey around the outcrops, archaeological excavations in others sites, and a new dating program.