Sandy deserts are the least common kind of desert, but the most famous, for they contain sand dunes.

Sand dunes are ridges of sand that have been pushed up into beautiful curving shapes by the wind. All sand dunes rest on a rocky desert floor. In this way they are like sand on a beach by the sea, just a thin layer of sand on top of rock.

The sand has been washed or blown from nearby mountains. The original material was made of dust as well as sand, but the dust can be carried far away by the wind. But because the sand is heavier, the wind can only roll it along, or sometimes make it lift off the ground for a short distance.

Sand dunes have a steep side and a more gentle side. The gentle side faces the wind. Sand is pushed up this side, just as you might push a barrel up a ramp. The sand reaches the top and then falls over the other side. In this way, sand is moved from the gentle to the steep side and so the dune keeps shifting its position.

Sand dunes make all kinds of patterns. They can grow into sand mountains over a hundred metres high, but most are just a few tens of metres.

Sand dunes are found in deserts and also by the sea. Coastal dunes get their sand by winds blowing sand off the beach at low tide.