Stratigraphy of Beds I through IV, Olduvai Gorge, Tanganyika.
Bed I at Olduvai Gorge is a conformable sequence of lava flows and varied sedimentary deposits that extend upward from a welded tuff overlying the Precambrian basement to the top of a widespread marker bed. Bed II is a sequence of lacustrine clays and laterally equivalent fluvial, eolian, and pyroclastic deposits. Bed III is made up of alluvial deposits and a laterally equivalent assemblage of fluvial, lacustrine, and eolian beds. Bed IV can be subdivided into a lower unit of fluvial clays, sandstones, and conglomerates, and an upper unit of eolian tuffs. The climate was relatively dry throughout much of the time that these beds were deposited, and semidesert or desert conditions may have prevailed at least twice. Tectonic movement seems to have taken place between the deposition of Beds III and IV.
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