Uganda - the Occidental Rift

The Uganda Palaeontology Expedition (co-directed by M Pickford, B Senut & D. Hadoto) was officially created 1986, after a first reconnaissance made in june-july 1985 at the invitation of the Uganda Geological Survey and Mines Department at Entebbe. The main aim of the expedition was to identify Neogene sediments (aged from 20 to 2 million years) in Uganda which could yield information on the modification of environments and climates related to human evolution.

In 8 years, the team spent 22 months in the Albertine Rift, during which over 100 km2 of sediments were mapped and the stratigraphy established based on the 4 kms thick sediments which accumulated in the Rift. These deposits yielded more than 5 000 fossils (snails, crabs, gastropods, insect traces, fruits, leaves, wood, pigs, carnivores, antelopes, elephants, primates and the oldest man in Uganda) which serve as a base of the biostratigraphy. Several volcanic ashes were discovered (which was new to science in Uganda) and permitted to give an age to the different deposits and to make correlations with other sediments from Kenya, Ethiopia and the Gulf of Aden. The tectonic activity of the area was studied as well as its impact on the environments of Uganda. Three main periods were identified:1- around 15 million years ago, Western Uganda was almost flat, very dry, semi-arid; 2- around 10 to 8 million years, following a first tectonic movement, a lake formed called Obweruka and the climate became more humid and by 6 million years, a tropical forest was established in the area; 3- by 2,6 million years, the margins of the lift uplifted so much that it formed a rain shadow which resulted in a local establishment of wooded savannah, conditions which are still seen today. This is when two lakes were formed: lake Obweruka divided into Albert and Edward. Following the uplift of the Roof of Africa (Ruwenzoris), the climate became drier and drier in the East. All these events can be witnessed in the fossil fauna and flora from Western Uganda. In the Nyabusosi Formation (lower Pleistocene), there are exceptional concentrations of Oldowan artefacts. At NY 18, an important site was identified; it yielded more than 600 lithics older than 1,5 Ma (the age of the tuff which overlies the implementiferous sediments. This is the oldest archaeological site known on this part of the Western Rift.

Thus, the Albertine Rift appears to be a key area for understanding the evolution of apes and man and the behaviour of our oldest relatives.