Sand Hills, Nebraska, from space, c 2000

Sand Hills, Nebraska, from space, c 2000. The Sand hills seen in this simulated natural-colour image from NASA's Terra spacecraft cover about a quarter of the state of Nebraska. These ancient sand dunes are from the Pleistocene Epoch (the geologic time period spanning about 1.8 million to about 10,000 years ago) and are made of sediment eroded from the Rocky Mountains by the monumental Pleistocene glaciers, washed out into the plains, and now mostly stabilized by grassland vegetation. Covering an area of about 60,000 square kilometers (33,333 square miles) in western Nebraska, the Sand Hills are the largest sand dune formation in America. ©SSPL/NASA