People who live far from the Sahara Desert get their vision of it from movies. In Lawrence of Arabia or Raiders of the Lost Ark, the Sahara is a vast expanse of sand dunes. You may be surprised to find out that only about a quarter of the Sahara Desert is covered in sand! The vast majority of the Sahara is a hard surface, made up of scrubland, mountains, valleys, or salt flats.
The places where sand dunes form in the Sahara are called ergs. What they have in common with each other is that they are located downwind of places where the Sahara once had water in lakes and rivers. When the lakes dried up, the sediment became pulverized sand, and drifted with the wind over millennia. You can see maps of where these ancient lakes were and compare them to the ergs of today. Read about the formation of sand dunes, and what might happen to the Sahara in the future at Real Clear Science.
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