The Powder River Basin is a geologic structural basin in southeast Montana and northeast Wyoming, about east to west and north to south, known for its extensive coal reserves. The former hunting grounds of the Oglala Lakota, the area is very sparsely populated and is known for its rolling grasslands and semiarid climate. The basin is both a topographic drainage and geologic structural basin, drained by the Powder River, Cheyenne River, Tongue River, Bighorn River, Little Missouri River, Platte River, and their tributaries. The major cities in the area include Gillette and Sheridan, Wyoming and Miles City, Montana. In 2007, the region produced 436 million short tons (396 million tonnes) of coal, more than twice the production of second-place West Virginia, and more than the entire Appalachian region. The Powder River Basin is the largest coal-producing region in the United States. The region includes the Black Thunder Coal Mine, the most productive in the United States, and North Antelope Rochelle Mine, the second most productive. In recent years, the region has become a major producer of natural gas, both conventional natural gas and coal-bed methane. Geologic history The Powder River Basin contains a section of Phanerozoic rocks up to thick, from Cambrian to Holocene. Cretaceous The thickest section of the Powder River Basin is composed of Cretaceous rocks, an overall regressive sequence of mostly marine shales and sandstones deposited in the Western Interior Seaway. Tertiary The coal beds of the region began to form about 60 million years ago when the land began rising from a shallow sea. The rise of the Black Hills uplift on the east and the Hartville uplift on the southeast side of the basin created the present outline of the Powder River Basin. When the coal beds were forming, the climate in the area was subtropical, averaging about of rainfall a year. For some 25 million years, the basin floor was covered with lakes and swamps. Because of the large area of the swamps, the organic material accumulated into peat bogs instead of being washed to the sea. Periodically the layers of peat were covered with sediments washed in from nearby mountains. Eventually the climate became drier and cooler. The basin filled with sediment and buried the peat under thousands of feet, compressing the layers of peat and forming coal. Over the last several million years, much of the overlying sediment has eroded away, leaving