About 65 million years ago, not long after the dinosaur extinction, the Fort Union Formation was deposited as rivers from mountains to the west and south carried abundant sediment across the subtropical, swampy coastal plain of eastern Montana toward a shallow inland sea to the east. As the rivers shifted, swamp vegetation and peat were buried beneath thick deposits of sand, silt, and clay, and over time the vegetation compacted into coal while the sediment cemented into sandstone, siltstone, and mudstone. Eastern Montana has an estimated 200 billion tons of coal formed from these swamps, much of it classified in northeastern Montana as low-energy lignite and mostly buried beneath soft rock of the Fort Union Formation, though it is sometimes exposed in hills and road cuts. Scattered red rock in the Fort Union Formation is clinker, created when lightning, wildfires, or spontaneous combustion ignites coal and the intense heat bakes adjacent sedimentary layers into dense, hard red rock. Northeastern Montana is part of the Williston Basin, a slightly irregular round depression centered in North Dakota that slowly subsided over hundreds of millions of years and accumulated sediment now up to 16,000 feet deep. The basin became a major oil and natural gas province because it contains organic-rich deposits such as black shale to generate oil and gas, porous reservoir rocks such as sandstone where they accumulated, and geologic traps formed by faults, folds, and lateral changes in the rocks that prevent them from migrating away. Oil and natural gas exploration has continued in the Williston Basin, and in 2008 the Montana Board of Oil and Gas Conservation reported about 34 million barrels of oil production there and about 1900 active producing wells. The Fort Union Formation takes its name from Fort Union, an American Fur Company trading post near the confluence of the Missouri and Yellowstone rivers that operated from 1828 to 1867 and is now a National Historic Site.