Gombe Stream National Park

The Park lies on the western arm of Africa's Great Rift Valley, a 5600-kilometer crack in the earth's crust that was caused by the East African tectonic plate slowly drifting eastward away from the rest of the continent. Here, the trough has been filled by water to create Lake Tanganyika, the world's second deepest lake, down to 1,400 meters deep. Formed about 20 million years ago, it is the oldest of the Rift Valley lakes.

The lake is fed by drainage from Zaire, Burundi and Zambia as well as from Tanzania. The largest of these rivers is the Malagarasi, which drains all of northwestern Tanzania into the lake about 40 kms south of Kigoma. The lake has an outlet at Kalemie in Zaire, only 100 kms across the lake from the inlet, and this is the source of the Lualaba River. It is likely that before the Rift Valley was formed, the Malagarasi and Lualaba were one continuous river flowing westward to the Atlantic.

Having such a large outlet, the lake's level is fairly stable and does not rise or fall more than about 1 meter. Wave action has carved out a beach ledge all along the shore of the park, the shore varying from sand to shingle to rock depending on the degree of exposure.

The underlying rocks of Gombe, as in most of Africa, are ancient gneisses, with overlying quartzites on the upper slopes. In some places, younger sandstones have been downfaulted to form red cliffs along the lake shore. Along the beach you can see examples of these rocks, particularly whitish quartzite and red sandstone.

All these rocks erode to form thin, stony, sandy soils, and only in the valley bottoms are there pockets of deeper, richer alluvial soils.