The Kapthurin Formation is a series of Middle Pleistocene sediments associated with the East African Rift Valley.

Part of the East African Rift System, it is also an important archaeological site in the study of early humans who occupied the area and left stone tools and animal bones behind.

[1][3] Clastic sediments, tuffs, and carbonate beds, in the Kapthurin give information on past river and lake environments.

Additionally, intercalated tuffs and extrusive igneous rocks associated with Rift Valley volcanic activity have allowed for multiple argon–argon dating studies.

Generally, clastic sediments dominate the formation, but evidence of volcanic activity from tuffs and rocks representing lava flows are found throughout.

Notably, in certain Kapthurin formation outcrops, the Grey Tuff overlies hominin remains, making it an important bed for relative dating.

With the Grey Tuff, Argon-Argon dating brackets several sites associated with hominin activity around the time Homo Sapiens evolved.

Still, the fact that Middle Stone Age tools have been found beneath the Pumiceous Trachytic Tuff member confirms this industry's appearance in this region of Africa prior to any other location.

Coarsely-bedded conglomerates record evidence of flash floods, and the Bedded Tuff might represent ash fall into a lake environment, creating thin, well-defined layers.

Fossil evidence and associated elevated strontium levels in the 2nd and 3rd carbonate beds shows how the later lakes sustained more life and had a consistent freshwater source.

This could have been a spring source in either the Tugen Hills to the East, or a paleoclimatic change leading to increasing rainfall, per current hypotheses.

Lastly, each of the tufa three beds, from bottom to top, progresses from a spongy texture to a dense crystalline cap.

"[6] From this interstratification, archaeologists conclude that there was a gradual transition from Acheulean to Middle Stone Age technology than began roughly 280,000 years ago.

[13][14] The departure from the long-standing Acheulean industry may also link to environmental changes in the freshwater spring environment that existed more than 500,000 years ago near present day Lake Baringo as well.