On my first space flight I was surprised to find that the Sahara is not a uniform of sea of sand. A thin coat of oxidized  rock called desert varnish produces amazingly varied colors amid the imposing mountains. Whirling winds sculpt rows of dunes at Kahala Tabelbala, Algeria.When the winds come predominantly from one direction, the sand forms into knife edged ridges, such as those above, at Erg Bouharet in Algeria.