The floor of a rift valley is prone to periodic floods that carry in fine silt--the sedimentary matter responsible for fossil formation.
Due to the delicate nature of fossils, a hardening chemical is dripped onto every fossil before it is removed from the soil.
Chimpanzees are capable of using rocks as tools to crack nuts for eating. But they don't appear to use sharp-edged tools.
African rift valleys were formed by the separation of tectonic plates. Water flows down to the valley floors, creating rivers and lakes.
Prehistoric stone tools are classified into six broad technological modes by the level of sophistication and method of fabrication.