Rock layers in Badlands National Park preserve fossils that record a dramatic cooling and drying of global climate over millions of years. The oldest layers, the Pierre Shale, began forming 75 million years ago when a shallow sea covered this area, and they contain marine fossils from when the area was underwater, although dinosaurs lived during this time they did not live here. After the sea drained away, black ocean mud was exposed to surface weathering and its upper layers developed into a yellow soil called Yellow Mounds. Sediments carried by streams and rivers flowing from the Black Hills created the Chadron Formation in a hot and humid forest flood plain. Open woodlands preserved in the Brule Formation were drier and cooler than those in the Chadron Formation, and in some areas water was hard to find. The thick Rockyford Ash, a volcanic ash layer, forms the bottom layer of the Sharps Formation, whose sediments preserve evidence of a cool and dry shrubland. Fossils in these younger layers include ammonite Placenticeras, alligator Alligator, titanothere Brontops, three-toed horse Mesohippus, nimravid Nimravus, oreodont Leptauchenia, and dog Hesperocyon. The most recent layers reveal animals and plants from an open shrubland, and today erosion strips away rock layers and exposes new fossils within these bands of time.