The Hell Creek Formation is an intensively-studied division of Upper Cretaceous to lower Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana. The formation spans about 1.3 million years, terminating at the Cretaceous–Paleogene boundary, dated to 65.5 million years ago. The Hell Creek Formation occurs in Montana and portions of North Dakota, South Dakota, and Wyoming. In Montana, the Hell Creek Formation overlies the Fox Hills Formation and is the uppermost formation of the Cretaceous period. "Pompey's Pillar" at the Pompeys Pillar National Monument is a small isolated section of the Hell Creek Formation. It is a series of fresh and brackish-water clays, mudstones, and sandstones deposited during the Maastrichtian, the last part of the Cretaceous period, by fluvial activity in fluctuating river channels and deltas and very occasional peaty swamp deposits along the low-lying eastern continental margin fronting the late Cretaceous Western Interior Seaway. The climate was mild, and the presence of crocodilians suggests a sub-tropical climate, with no prolonged annual cold. The famous iridium-enriched Cretaceous–Paleogene boundary, which separates the Cretaceous from the Cenozoic, occurs as a discontinuous but distinct thin marker bedding within the Formation, near its uppermost strata. Wikipedia [ - ]

The Hell Creek Formation is an intensively-studied division of Upper Cretaceous to lower... [ + ]