City-swallowing Sand Dunes

At this Science at NASA site, you'll learn about the physics of sand movement and the research done to understand mechanisms of dune migration. The physics and the landforms are interesting because granular materials like sand show properties of both solids and fluids, including saltation, sheet flow, and avalanches. This site provides a summary of the physics involved along with photographs of sand dunes on Mars, close-ups of sand particles, and a sand dune advancing on a town.