Olduvai Gorge is located in the eastern Serengeti Plains in northern Tanzania. It's often called the "Cradle of Mankind" because of the large number of hominid specimens and artifacts that have come out of it. Olduvai Gorge is a steep sided ravine 30 miles long and 295 ft. deep. The main beds are in Fossil Lake Basin about 16 miles in diameter. There are 7 beds in Olduvai, Bed I, Bed II, Bed III, Bed IV, the Masek Beds, the Ndutu Beds, and the Baisiusiu Beds.
Bed I is dated at 2.1 million years old and is 197 feet thick. It's formed of volcanic flows, ash, and other sediments. The upper part of the bed contains fauna and artifacts from the Oldowan industry, the oldest stone tool industry. Hominid fossils include Paranthropus boisei and Homo habilis.
Bed II is 66-98 feet thick and is 1.15-1.7 million years old. There are two main divisions of rock, upper and lower, that are separated by an erosional break. The lower part is similar to Bed I. The upper part was fault shift that reduced the ancient lake sizes. This is where the Acheulian industry starts. Homo habilis, Homo erectus, and Paranthropus boisei are all found here.
Bed III and Bed IV have a maximum thickness of about 98 feet and consists of sediments from the streams that fed Olduvai Lake. They range from 1.15 million to 600 thousand years. 
- Olduvai Gorge. Retrieved on 2008-05-03.