The East African Rift Valley

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Ten million years ago, a huge wedge of land, stretching over 4,000 miles (6,400 km) from Jordan and the Red Sea to southeast Africa, subsided to create a depression known as the Great Rift Valley. Plateau lands rise gradually toward the valley rim, and then drop up to 3,000 feet (900 m), at a series of fault lines in the rocky crust of the earth. In places in the East African part of the Rift stretching over 4,000 miles (6,400 km) from Jordan and the Red Sea to southeast Africa, subsided to create a depression known as the Great Rift Valley. Plateau lands rise gradually toward the valley rim, and then drop up to 3,000 feet (900 m), at a series of fault lines in the rocky crust of the earth. In places in the East African part of the Rift Valley, such as Tanzania, the forces of erosion have obscured the effects of this massive earth movement, but elsewhere there are high cliffs bordering fantastic broad valleys, like lost worlds, between 30 and 40 miles (48-64 km) wide. Distinct ecosystems developed, the sheltered lands of the depression often providing conditions in which a wealth of wildlife could flourish. While tropical forests thrived to the west, the rain-shadow area created by the Rift Valley cliffs evolved into dry grassland, the savannah plains and the heart of East Africa’s safari land.
Much of the valley is made up of a series of troughs and swells along the fault lines. The troughs are around 25 miles (40 km) wide, and along the western branch of the rift have filled with fjordlike lakes, whose depths often plunge below sea level. They include Lake Tanganyika-the second deepest freshwater lake in the world after Lake Baikal-which covers 12,700 square miles (32,900 sq km). Along its shores are fine beaches and abundant wildlife.
In the eastern branch of the East African Rift Valley, which runs through Ethiopia, Kenya, and Tanzania, shallow soda lakes like Lake Natron have formed. Their alkaline waters are low in fish, but the algae and other tiny organisms attract gawky flamingos in the thousands, startling pink against mirror-dazzling water and relentless blue sky. Along this eastern branch, too, are mighty volcanic uplifts where molten rock has burst through the thinned continental crust into volcanoes such as 0I Doinyo Lengai-”Mountains of God” to the local Masai people-Mount Kilimanjaro, and Mount Kenya. As you climb Mount Kilimanjaro-at 19,340 feet (5,895 m) Africa’s highest mountain-you pass through the world’s vegetation zones, from desert through tropical forests, to permanent snows.
West of Ol Doinyo Lengai is the Serengeti Plain in Tanzania, whose dusty soils are probably a fallout of volcanic ash. The endless grasslands that flourish here are the home of the African elephant, and grazed by giraffe, zebra, wildebeest, impala, and eland, as well as their predators-lions, leopards, and hyenas. Every year, in July and August, more than a million wildebeest move north from Tanzania’s Serengeti National Park across the Mara River and into Kenya in search of the fresh pastures created by the seasonal rains; the ungainly beasts, accompanied by the more graceful zebras, move en masse often at a gallop and in single file. When the rains come again in November, they head back south to Tanzania and the cycle begins all over again. Although migrations occur in other parts of Africa, nowhere else are the numbers so prolific or the sight so easy to see. It is possible to rent a trip in a hot air balloon to view the spectacle.
The grasslands that formed as a result of the earth’s great rift also proved more suitable than the surrounding tropical forests for an upright, two-legged, sharp-eyed species to evolve. In a small place called Olduvai in northern Tanzania, not far from the town of Arusha, Dr. Louis B. Leakey and his wife Mary had been searching intermittently for the best part of 20 years when in 1959 they found part of a hominoid skull, which they called Zinjanthropus (Australopithecus boisei). The skull fragments were dated at 1.8 million years old, making it part of the oldest discovered hominoid at that time.
In 1979, Mary made another important discovery at Laetoli, some 40 miles (64 km) away, of footprints in a riverbed made by a man, woman, and child. Subsequent investigation dated these footprints at 3.5 million years. Since they were made by hominoids that walked upright, the discovery pushed the dawn of humankind much further back in time than previously thought. Today

In 1979, Mary made another important discovery at Laetoli, some 40 miles (64 km) away, of footprints in a riverbed made by a man, woman, and child. Subsequent investigation dated these footprints at 3.5 million years. Since they were made by hominoids that walked upright, the discovery pushed the dawn of humankind much further back in time than previously thought. Today you can see where the Leakeys excavated in a hole, now roofed over with a pipelike opening. Here the layers of soil and rock mark the journey back in time as you descend into the earth, an awe-inspiring experience. At the top of the gorge, there is a sense of overwhelming peace as you sit and reflect on the fact that you could be sitting on the very spot where Eve, the “mother” of all human beings, once sat admiring the same landscape several million years ago.