Olduvai Gorge 

Olduvai Gorge is a valley at the western margin of the Eastern Rift Valley in northern Tanzania. The valley is cut in the Serengeti Plain, which extends about 110 km northeast toward Lake Victoria, 260 km distant. The plain has an elevation of 1,360 to 1,520 m in the vicinity of the gorge, and it slopes gently eastward and is offset in a series of steps. Several hills and insel-bergs of metamorphic rock break the even sur­face of the plain in the vicinity of the gorge, and extensive highlands of metamorphic rock bound the plain about 20 km to the north. The main branch of the gorge, known as the Main Gorge, has its origin in lakes Masek and Kdutu, and it flows 46 km eastward to empty into Olbalbal, a depression within a fault graben. The divide be­tween drainage flowing toward Lake Victoria and toward Olbalbal lies 10 to 15 km west of Lake Ndutu. The graben with Olbalbal is bor­dered on the west and south by several large volcanoes, collectively termed the volcanic high­lands.

The Main Olduvai Gorge is broad, shallow, and under­lain by a basement complex of metamorphic rocks in the western 20 km of its course. It deepens abruptly in a series of rapids and falls known as Granite Falls. These falls represent the western margin of the Olduvai basin, which is filled with Pleistocene sedimentary deposits termed the Olduvai Beds. In the eastern 26 km of its course, the gorge is steep-sided, 46 to 90 m deep, and 0.5 to 1.5 km wide. It has an average gradient of 7.8 m/km (1:128), which is about the same as the average gradient of the plain (1:137). About 9 km upstream from its mouth, the Main Olduvai Gorge is joined by a smaller, southern branch, named the Side Gorge, which originates on the western slopes of Lemagrut volcano.

The Olduvai basin appears to have originated in growth of the volcanic highlands on an east­ward-sloping surface of metamorphic basement rocks. Lavas and ignimbrites both underlie and interfinger with the basin sediments. Eruptive rocks beneath the Olduvai Beds presumably thin from east to west, and a single welded tuff, the Naabi Ignimbritc, separates the Olduvai Beds from metamorphic basement rocks near the western margin of the basin.

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