Although the formidable Sahara appears to be stable, the surface sand is in constant motion. Great dunes creep slowly across the desert, sometimes in large V-shaped groups like earthbound flocks of geese. Very few people would expect to find beauty in this bleak sea of sand near the northern border of the Sahara, one of the hottest, driest places on earth. When the sun is high in the cloudless sky, Saharan temperatures may soar far above 100° F (38° C). Rains come for only a few hours a yea.l and in some years not at all.
Yet for all its harshness, the Great Western Erg has an unquestionable beauty. The erg (from an Arabic word meaning “great sand dune area”) is typical of the picture most people conjure up when they think of a desert—a seemingly endless sea of gigantic golden sand dunes rolling dramatically toward the horizon. Although this and other ergs symbolize the Sahara, however, they actually cover only about one-fifth of its surface: much of the rest is covered by vast, monotonous rock and gravel plains.
The Great Western Erg, one of the largest in the Sahara, lies in a natural depression in northern Algeria. Bounded on the north by the Atlas Mountains and on the south by the Tademalt Plateau, it covers an area of more than 30,000 square miles (78,000 square kilometers).
And everywhere there is sand, driven by the wind into an array of patterns that range from surface ripple marks to towering dunes as much as 400 feet (120 meters) high. In some places the dunes form long curving ridges, while elsewhere the sand has accumulated into seemingly endless straight, parallel dunes separated by broad open troughs. There are highly mobile, crescent-shaped dunes known as barchans, as well as large, complex clusters of dunes that have been sculpted by the wind into huge merging pyramids.
Wind, the force that modeled all these intricate forms, sometimes rages across the Sahara with furious intensity. Especially violent storms at times whirl up thick clouds of airborne sand and dust that blot out the light of the sun. Many Saharan legends tell of caravans and even oases disappearing in such storms without a trace. Nor are their effects strictly local. The storm winds sometimes carry Saharan dust all the way across the Mediterranean into Europe. After a particularly violent sandstorm in Algeria in 1947, red desert dust, mixed with snow, turned parts of the Swiss Alps pink.
But it is the ever-present, less violent trade winds sweeping across the desert that do the major work of reshaping the dunes. Frequently they stir up a knee-deep layer of windbome sand that moves across the surface in a golden haze. Above this layer of mobile sand grains the air is almost entirely free of sand.
The sand does not actually become fully airborne. Most of it moves by being bounced along on the ground by the wind. The smaller and lighter the grains of sand, the higher and farther they bounce. Grains of sand that are too heavy to leave the ground are bumped along as they are hit by the bouncing sand grains. Thus the surface patterns on the dunes are constantly being shifted, and the dunes themselves are moved forward by the winds—in the case of barchans, as far as 100 feet (30 meters) per year.
Where did all the sand come from in the first place? It was not carried in on the wind from distadt places, nor was it, as was once believed, provided by some ancient Saharan sea that long ago dried up. In fact the sand for the most part was washed down by rivers from the nearby Atlas Mountains.
Over the past 2 to 3 million years the Saharan climate has alternated between wet and dry periods. During each wet period, streams and rivers from the mountains extended their courses farther and farther into the desert, where they deposited their loads of eroded sand and other kinds of sediment. During dry periods the wind took ove4 modeling characteristic desert landforms.
Ever since the last wet period ended about 4,000 years ago, the Saharan climate has remained very dry. Active streams no longer penetrate the region, and development of the desert has resumed—even while our ancestors watched Basins that once were filled with water slowly dried out. Where cattle had once grazed, only a few spiny shrubs and hardy grasses were able to survive. And even today, as the climate remains dry, the desert continues to advance. Anna loves traveling, lives in Prague and is looking forward to visiting Africa at some point.