Geologic context The Kapthurin is on the floor of the basin of a
half-graben that forms the Kenya Rift. This is one of two half-grabens in the Eastern portion of the East Africa Rift Valley. Because of nearby North-South striking
normal faults that form this half-graben, the Kapthurin and other sedimentary formations are on a fault block tiled to the West. The formation contains
lacustrine,
fluvial, and volcanic rocks (specifically
basalts and
trachytes). Generally, clastic sediments dominate the formation, but evidence of volcanic activity from tuffs and rocks representing lava flows are found throughout.
Stratigraphy The Kapthurin overlies the Chemeron Formation, dated to roughly 1.57 million years ago,
unconformably. The bulk Kapthurin formation has been dated to the Middle Pleistocene based on fossil evidence. Dating of the Kapthurin formation's members is described in the following section. Localized faulting is common in this outcrop, and the stratigraphy described here is not representative of every Kapthurin outcrop.
Notable tuff deposits The Kapthurin preserves information from volcanic eruptions in consolidated ash, of tuff. While tuff deposits vary with outcrop location and there are smaller tuff beds in members primarily categorized as silt or gravel, The first of the three tufa carbonate layers represents a shallow lake environment fed by groundwater through cracked rocks, as evidenced by their high Magnesium content and interpretations of high water temperature via
oxygen isotopes. Subsequent carbonate beds represent deeper, open lake environments. 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. Each tufa bed is also overlain by either a thin
clay layer or
paleosol. Alongside a heavy oxygen isotope signature in the paleosols that suggests high evaporation, this change in texture indicates cyclical changes in water level. At its maximum, the lake would have covered about . While the changing environment would have impacted early hominin movements and resource exploitation, more research is required to understand the relationship between water level, resource availability, and hominin activity. == Archaeology ==