how the sahara desert was formed

The Sahara is the most well known, and largest desert on earth-a vast, forbidding world of barren mountains and parched plateaus, gravel plains and endless seas of sand-stretches more than 3,000 miles across the whole of northern Africa, from the Atlantic Ocean to the Red Sea. It is a land of violent contrasts and extreme conditions, where life is difficult at best. Yet some 10,000 years ago, the region we now call the Sahara-a name derived from the Arabic word for desert- was in a green and fertile phase in which rain-swollen rivers flowed from tree-covered mountains, and hippopotamuses wallowed in lakes and lagoons.

On the vast North African grasslands, leopards stalked ostriches and gazelles; elephants, rhinoceroses, and giraffes found plentiful vegetation to eat. People also thrived there, at first hunting game, later raising enormous herds of cattle. And for thousands of years, these hunters and herdsmen of the Sahara recorded their lives on cliff walls and cave roofs. Prehistoric rock engravings and paintings have been preserved throughout the region, but they are most abundantand most beautiful-in the remote and rugged heart of the desert, among the fantastic rock formations of the Tassili n'Ajjer plateaus of southern Algeria.

How did this verdant land teeming with wildlife turn into one of the driest and most inhospitable places on the planet? The Tassili n'Ajjer picture galleries illustrate a part of the story, but only the most recent part, for the drying out of the Sahara was a disaster that took eons to unfold.

Today's Sahara is, in fact, the result of hundreds of millions of years of geologic and climatic change. Traces of ancient glaciers have been found in rocks throughout the Tassili n'Ajjer plateaus-evidence of a remote past when parts of the Sahara were buried under an ice cap. The enormous sea that covered most of the area during the Carboniferous period, about 300 million years ago, eventually gave way to luxuriant tropical landscapes inhabited by dinosaurs. Early in the Cretaceous period, some 130 million years ago, the dinosaurs disappeared from the Saharan region, and large parts of what is now desert lay underwater once more. In time, the Cretaceous seas were themselves replaced by a moist environment of marshes and lakes-and never again would so much of North Africa be submerged.

Over the last 2.5 million years, climatic changes comparable to those that caused alternating ice ages and periods of interglacial warming in Europe also influenced rainfall and weather patterns in North Africa. Dry phases, when desert conditions prevailed and spread, alternated with humid periods, when great networks of rivers and lakes supported large animal and human populations. Radar images taken by the space shuttle Columbia in 1981 over the hot sands of southern Egypt and northern Sudan revealed the buried remains of Old Stone Age campsites. The radar also revealed underground traces of rivers and streams that more than 200,000 years ago carved out valleys as wide as that of the Nile River.

Fossil finds in the Libyan Desert, now one of the hottest and most barren stretches of the Sahara, show that during one of its wet phases the region was home to Stegotetrabeledon syrticus, a giant mastodon much bigger than a modem elephant and equipped with four massive eight-foot-long tusks. The vegetation in this now searing wasteland must have been abundant indeed to satisfy the needs of such a huge and voracious plant eater. In the course of its evolution, the Sahara has been both larger and considerably smaller than it is today. Fifteen thousand years ago an expanding desert created sand dunes some 300 miles south of its present boundary. On the other hand, perhaps as recently as 7,000 years ago, Lake Chad covered more than 10 times the area it now does and reached 400 miles north of its present shores into what is now desert terrain. Farther to the west, the Niger River once flowed into Lake Arawan, a body of water that today is little more than a sea of sand in central Mali.

But despite the recurrences of moist periods, the Sahara ultimately could not escape the consequences of its geography and climate. Continental drift has placed North Africa in the earth's equatorial arid belt, where-except under extraordinary circumstances, such as the ebb and flow of an ice age-constant high atmospheric pressure produces air that is simply too dry for clouds and rain to form. The peculiar nature of the ancient Sahara's drainage system also played a crucial role in creating the desert. Instead of flowing into the sea, Saharan waterways ran from the mountains into closed basins in the lowlands. During wet periods these powerful rivers and streams shaped and eroded the mountains and deposited the resulting debris in the basins, gradually filling them. The result of this process was the formation of the Saharan gravel plains, known as regs. During dry phases the wind took over the work of erosion, sifting out grains of sand and heaping them up in undulating sand seas, called ergs, at the outer edges of the desert, beyond the gravel plains.

The most recent of the Sahara's wet phases began 12,000 to 10,000 years ago, toward the end of the last glacial period. The oldest of the Tassili n'Ajjer rock engravings and paintings, dating from the sixth millennium B.C. or earlier, depict hunters of a Negroid race, who wore animal skins and masks and stalked their prey with clubs, lances, and boomerangs. Though crude, the art of these early Saharan hunter-gatherers reveals a rich, game-filled landscape, very similar to the savannas of sub-Saharan East Africa today. But the art also chronicles the gradual drying out of the area. By about 4000 B.C. the Sahara could no longer provide enough food for herds of elephants and other large game, which either died out or drifted south in search of better grazing lands. The hunter-gatherers were replaced by nomads from the east-possibly from the Upper Nile Valley-who kept huge herds of cattle and flocks of sheep and goats. These pastoral people were also the skilled artists who created some of the most spectacular of the Tassili n'Ajjer rock paintings.