Sunrise over Badlands National Park in South Dakota reveals a landscape of grassy prairies and rugged rock formations. The rock layers testify to tens of millions of years of deposition – when tiny grains of sediments such as sand, silt and clay are cemented together into rock. Different environments – sea, tropical land and open woodland with meandering rivers – caused different sediments to accumulate here at different times. About half a million years ago, the process reversed itself and now water and wind erode the rocks, leaving behind this jagged silhouette and its embedded fossil treasures. Photo by National Park Service.