Nebraska, extends into the southeastern corner of South Dakota. Layers deposited during the Pleistocene epoch, starting around two million years ago, cover most of eastern South Dakota. These are the youngest rock and sediment layers in the state, the product of several successive periods of glaciation which deposited a large amount of rocks and soil, known as till, over the area. The Great Plains cover most of the western two-thirds of South Dakota. West of the Missouri River the landscape becomes more arid and rugged, consisting of rolling hills, plains, ravines, and steep flat-topped hills called buttes. In the south, east of the Black Hills, lie the South Dakota Badlands. Erosion from the Black Hills, marine skeletons which fell to the bottom of a large shallow sea that once covered the area, and volcanic material all contribute to the geology of this area. The Black Hills are in the southwestern part of South Dakota and extend into Wyoming. This range of low mountains covers , with peaks that rise from 2,000 to 4,000 feet (600 to 1,200 m) above their bases. The Black Hills are the location of Black Elk Peak (7,242 ft or 2,207 m above sea level), the highest point in South Dakota and also the highest point in the United States east of the Rocky Mountains. Two-billion-year-old Precambrian formations, the oldest rocks in the state, form the central core of the Black Hills. Formations from the Paleozoic Era form the outer ring of the Black Hills; these were created between roughly 540 and 250 million years ago. This area features rocks such as limestone, which were deposited here when the area formed the shoreline of an ancient inland sea. Ecology Much of South Dakota (except for the Black Hills area) is dominated by a temperate grassland biome. Although grasses and crops cover most of this region, deciduous trees such as cottonwoods, elms, and willows are common near rivers and in shelter belts. Mammals in this area include bison, deer, pronghorn, coyotes, and prairie dogs. The state bird, the ring-necked pheasant, has adapted well to the area after being introduced from China. Growing populations of bald eagles are spread throughout the state, especially near the Missouri River. Rivers and lakes of the grasslands support populations of walleye, carp, pike, bass, and other species. The Missouri River also contains the pre-historic paddlefish. Due to a higher elevation and level