So apparently the geoblogosphere has gone "yeah sand dunes!!" in the last week (check them out here, here, here, here, here, here, here, here, and here!). I'll contribute a brief introduction to White Sands National Monument, located just west of Alamogordo, NM. White Sands contains numerous sand dunes, and as you can see in the pics below, they are quite tall and excellent for jumping off of! Geologically these are interesting because they aren't made of quartz, but rather are made of gypsum (CaSO4 * 2H2O). Therefore, the sand is soft, not abrasive, and cool to the feet even when the sun has been beating down on it all summer long. It is located in a restricted basin, and there is very little annual rainfall, which would serve to slowly dissolve the gypsum. This location is at: 32° 49.225' N, 106° 16.404' W. Take a look in google maps - there are hundreds of dunes of all shapes & sizes. The area totals about 20 miles wide!