Olorgesailie, Archaeological site in Great Rift Valley, Kenya

Olorgesailie is a geological formation and group of Lower Paleolithic sites on the floor of the Eastern Rift Valley, 67 km southwest of Nairobi. Sediments left by a former lake cover about 130 km2, and erosion has exposed layers full of Acheulean hand axes and animal bones. Mount Suswa and Mount Longonot contributed the ash that helped preserve the sites.

The site was discovered in 1919 by geologist John Gregory and holds stone tools spanning from roughly 1.2 million to 490,000 years ago. These tool cultures demonstrate how early human technology evolved and changed across hundreds of thousands of years.

The name Olorgesailie comes from the surrounding plains and the local Maasai word for the area. Excavations in the 1940s were carried out with the help of paroled Italian prisoners of war, a detail often left out of accounts. Today the site is managed with the National Museums of Kenya and the local Oldonyo Nyokie community, whose land borders the dig sites.

The location lies roughly 67 kilometers southwest of Nairobi along the Lake Magadi road and is reachable by car. Guided walks help visitors understand the excavation areas and view the collections displayed in the museum buildings.

The 2003 find, catalogued KNM-OL 45500, was a frontal bone of a skull aged 900,000 to 970,000 years, probably Homo erectus. It lay in the same sediment layer as two hand axes and several flakes, near dense deposits of axes. German sources note that its assignment to Homo erectus is disputed, as it shares few features with the type specimens.