About 60 million years ago, streams carried eroded materials eastward from the young Rocky Mountains and deposited them on a vast lowland -- today's Great Plains. During the warm, rainy periods that followed, dense vegetation grew, fell into swamp areas, and was later buried by new layers of sediments. Eventually this plant material turned into lignite coal. Some plantlife became petrified; today considerable amounts of petrified wood are exposed in the badlands. Bentonite, the blue-gray layer of clay , may be traced to ash from ancient volcanoes far to the west. But even as sediments were being deposited, streams were starting to cut down through the soft strata and to sculpt the infinite variety of buttes, tablelands, and valleys that made up the badlands we know today.
Though at first glance this landscape appears inhospitable and barren, it is home to a great variety of creatures and plants. Rainfall, scanty though it is, nourishes the grasses that cover the lands. And when the wildflowers bloom in bright profusion, they add their vibrant colors to the reds, browns, and greens of the earth and grasses. At home here, too, are over 180 species of birds, many of them songbirds. Both mule deer and white-tail deer inhabit the park. The white-tails prefer the river woodlands, and the mule deer like the more broken country and the uplands. Prairie dogs, historically a staple food source for many predators, live in "towns" in the grasslands. Through careful management some animals that nearly become extinct are once again living here. Bison and elk, for example, were reintroduced in 1956 and 1985 respectively.