Where does the sand in a desert come from?

From Lawrence of Arabia to Star Wars via tales of intrepid adventurers traversing lonely sand-swept landscapes, deserts have always had a powerful pull on the popular imagination. But if a desert is full of sand, where did all that sand come from in the first place? That is what CrowdScience listener Andy wants to know, so presenter Caroline Steel heads off into the dunes to find out.She begins by finding out what a desert is anyway and whether it is always sandy, as well as tracing the flow of material across the huge, ever-shifting sand seas of the Sahara.From deserts fed by sand from mountains thousands of kilometres away, to dunes migrating across the entire continent of Africa, Caroline discovers how sand has just the right properties to be carried along by the wind. She also explores how the sand in every desert has a unique fingerprint, and finds out how fish bones in the Sahara tell the story of its lush, green past.Contributors: Dr Jo Nield, University of Southampton Dr Andreas Baas, Kings College London Dr Andrea Zerboni, University of MilanPresented by Caroline Steel Produced by Ben Motley for the BBC World Service(Photo: The Sahara desert near Timbuktu, Mali Credit: Jeff Overs)