The Bouri Formation is a sequence of sedimentary deposits that is the source of australopithecine and Homo (that is, hominin) fossils, artifacts, and bones of large mammals with cut marks from butchery with tools by early hominins. It is located in the Middle Awash Valley, in Ethiopia, East Africa, and is a part of the Afar Depression that has provided rich human fossil sites such as Gona and Hadar.
The Bouri Formation stretches down much of the length and breadth of the Bouri "peninsula", which projects across the dry bed of the Afar Depression. The Formation is sufficiently eroded to expose three geological members or layers: the Hatayae, the Dakanihylo, and the Herto. Human remains with signs of having been prepared for burial have been found in the Upper Herto layers.
Geology
The Bouri "peninsula" is a geological fault-raised horst that diverts the Awash River, forming a partial dam and creating Lake Yardi; and it contains the Bouri Formation. The peninsula is about 4 km wide by 10 km long, and lies in a NNW-SSE direction in the Quaternary-period rift zone in the southern part of the Afar Depression (Afar Triangle).
The Bouri Formation is a sediment area some 80 m thick that stretches down much of the length and breath of the peninsula. The formation consists of three geological units, called members, from which fossils and artifacts of different periods of human evolution have been excavated. They are: the lowest member, named Hatayae, or the Hata, dated to 2.5 million years ago (mya)—and in which fossils of Australopithecus garhi were found; the Dakanihylo, or Daka, member, one mya—fossils of Homo erectus were found; and the Herto member, Lower layer at 260 kya, and Upper layers at 160 to 154 kya—fossils of Homo sapiens idaltu were found.
The area is important because active tectonics in the Afar Depression created varying habitats for early hominins across the most recent few million years known informally as the Plio-Pleistocene. These habitats, laid down in sedimentary rocks, have since been uplifted, which allowed their erosion over time and their accessibility today to paleoanthropologists. Volcanic eruptions have left volcanic tuff layers that enable accurate dating of the sedimentary deposits via Argon–argon dating.
Hatayae
The Hatayae layer is 40 m thick at its base and is composed of variegated silt clay and paleosols, zeolitic and bentonitic tuffs, carbonates that are pedogenic, sandstone with bivalve and gastropod shells, and mudstone. It was deposited in a floodplain alongside river delta channels and a shallow fluctuating lake dated to around 2.5 mya.
In the Hatayae member have been found the remains of Australopithecus garhi, the most complete specimen of which is BOU-VP-12/130. According to Asfaw and White, et al. (1999), that species is "..descended from Australopithecus afarensis and is a candidate ancestor for early Homo."
Excavations in general have failed to disclose large numbers of stone tools. The likely explanation is the lack of raw materials on lake margins, which, in turn, is likely due to the lack of streams strong enough to carry hand-sized pebbles and the absence of nearby outcrops of basalt.
In spite of rarity, some isolated and widely scattered cores and flakes of Mode I technology have been found. The excavators (de Heinzelin, et al., 2002) report: "..our surveys and excavations have demonstrated that early hominids [hominins] were actively using stone tools on the Pliocene-Hata landscape"; and: "It is not currently possible to positively identify the creators of the earliest stone tools here or at Gona, even though A. garhi is currently the only recognized hominid [hominin] taxon recovered from Hata sediments."
Existence of stone tools is also evidenced by bones of large mammals—such as alcelaphinae (Wildebeest related bovids) and Hipparion, an extinct genus of three-toed horse—that show butchery cut marks by hominins including those made in removing an animal's tongue. "These are the earliest documented percussion marks made by hominids [hominins], who were presumably processing these bones for contained fatty marrow"; and, "These are the earliest documented cut marks made by hominids [hominins]." As concluded by the excavators, the evidence from the site shows "..a major function of the earliest known tools was meat and marrow processing of large carcasses." This pattern of butchery processing of large prey was maintained by hominins well into the Pliocene.