Environmental changes resilience and collapse of the settlement patterns settlements decay in the lower Sourou Valley (Burkina Faso)

About twenty prehistoric sites have been found on the banks of the Débé River, a tributary of the Souoru River in NW Burkina Faso, near the boundary with Mali. These sites consist of surface scatters of hundreds of flint artefacts and polished stone tools (mainly little adzes), associated with abundant, roughly worked discarded materials, which suggest in situ production. The most investigated area is the site of Kourani 5, in which 6 explorative pits were opened; some hand boreholes were realized along the Débé and Sourou to understand the relationships existing between alluvial evolution and ancient population. The artefacts could be dated to a time range within the II millennium BC (within a “Neolithic”, or “Ceramic Late Stone Age”, or “Later Stone Age” phase, according to different authors). A preliminary classification is proposed here on the basis of a number of key reference sequences and known cultural facies from Burkina Faso and the wider Sub-Saharan West Africa. The Débé riverbed is at present occupied by stagnant ponds, seasonally fed by rain water. The sedimentary facies of the channel deposits indicate that it was formerly interested by flowing water inside its 1-2 m high scarps. This probably happened in the relatively wet, middle Holocene climatic setting, before the shift to drier climatic conditions which have affected the whole West Africa since the 2nd – 1st millennium BC. During the prehistoric occupation, the flint-bearing layers of the bedrock were outcropping along the river scarps and could be easily quarried. The other raw materials worked in the area, such as quartz and green stones, are not present along the lower stretch of Débé and were thus imported. The Débé channel was later filled in by silty-clay sediments of colluvial origin, in a semi-arid environment similar to the present one, characterized by increasingly enhanced erosion due to modern anthropic activities. The presence of accessible flint outcrops and the use of exotic raw material support the hypothesis that the Sourou valley could have developed an organized territorial system and become part of a medium to long-range exchange network.