| Grains of sand, deriving from ancient river or lake alluvial deposits accumulated in ground recesses and sifted by thousands of years of wind and storms, pile up in front of obstacles and thus create dunes. These cover nearly one-third of the Sahara, and the highest, in linear form, can attain a height of almost 1.000 feet (300 meters). Barchans are mobile, crescent-shaped dunes that move in the direction of the prevailing wind at rates as high as 33 feet (10 meters) per year, sometimes even covering infrastructures such as this road in the Nile Valley. Deserts have existed throughout the history of our planet, constantly evolving for hundreds of millions of years in response to climatic changes and continental drift. Twenty thousand years ago forest and prairie covered the mountains in the center of the Sahara; cave paintings have been discovered there that depict elephants, rhinoceros and giraffes, testifying to their presence in this region about 8.000 years ago. Human activity, notably the over exploitation of the vegetation of the semi-arid area bordering the deserts, also plays a role in desertification.
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