What is Hell Creek Formation?
{{Infobox rockunit | name = Hell Creek Formation | image = Hell Creek.jpg | caption = Exposure in the badlands near [[Fort Peck Reservoir]] | type = [[Geological formation]] | age = [[Upper Cretaceous]] 66.8&ndash;66 mya | period = Danian | prilithology = [[Claystone]], [[mudstone]] | otherlithology = [[Sandstone]] | namedfor = Hell Creek, [[Jordan, Montana]] | namedby = | region = [[Montana]], [[North Dakota]],<br> [[South Dakota]], [[Wyoming]] | coordinates = | unitof = | subunits = | underlies = [[Fort Union Formation]] | overlies = [[Fox Hills Formation]] | thickness = | extent = | area = | map = | map_caption = }} [[File:Hell Creek and Lance formations.png|thumb|230px|Map of the Hell Creek and Lance formations in western [[North America]]]] The '''Hell Creek Formation''' is a [[formation (geology)|series of rocks]] where [[fossil]] [[dinosaurs]] are found. The [[strata]] are mostly [[Upper Cretaceous]] and some lower [[Paleocene]] rocks in North America. The name comes from Hell Creek, near Jordan, [[Montana]]. The formation includes portions of Montana, [[North Dakota]], [[South Dakota]], and [[Wyoming]]. It is a series of fresh and [[wikt:brakish|brackish]]-water [[clay]]s, [[mudstone]]s, and [[sandstone]]s deposited during the end of the Cretaceous period and the beginning of the [[Palaeogene]]. It was laid down in river channels and [[river delta|deltas]], and occasional peaty [[swamp]] deposits. along the low-lying eastern margin of the [[Western Interior Seaway]]. The [[climate]] was mild, and the presence of [[crocodilia]]ns suggests a [[sub-tropical]] climate, with no prolonged annual cold. The famous [[iridium]]-enriched [[Cretaceous–Palaeogene boundary]], which separates the Cretaceous from the [[Cenozoic]], is a distinct thin bedding. It is above and occasionally within the formation, near its boundary with the overlying Fort Union Formation. The