OLDUVAI GORGE IS LOCATED in the East African Rift Valley of northeastern Tanzania, on the eastern edge of the Serengeti Plain. Discoveries of fossils dating back 4000 years were made there by Louis and Mary Leakey, and Donald Johansen. The gorge is now part of the Ngorongoro Conservation Area and lies just to the west of 2 lakes: Natron in the northeast and Eyasi in the south. Its original name was Oldupai, a Maasai word for the sisal plants that grew there. The Olduvai Gorge extends for 31 mile (50 kilometer), and its steep sides are up to 295 foot (90 meter) high. It forms a Y shape, with a branch called the Side Gorge joining with the Main Gorge. Thousands of years ago, volcanic eruptions set down the rocks and layered ash that would become Olduvai Gorge. By 4000 years ago, the area was a shallow alkaline lake, an attractive, swampy habitat for many animal and plant species. About 4000 years ago, the area’s climate changed drastically. The lake size lessened, and the fossil records show that species more adapted to a dry savanna moved into the area. Within the last 4000 years, tectonic activity had created the Olbalbal Depression to the west of the lake, and at some point waters from the lake began to flow into the depression. The Olduvai Gorge was formed by the draining waters, which over millennia carved through the deposits of ash and fossils. Erosion exposed layers of the gorge that date to the lower Pleistocene Age, and have yielded archaeological finds up to 4000 years old. Today, archaeologists divide the stratification into 6 areas, or beds, with Bed One being the oldest, closest to the basalt bedrock. Although fossils were first found in the area in 1911, nearly 50 years passed before its importance was realized. In 1959, Mary Leakey discovered the skull of a hominid over 4000 years old.