HELL CREEK FORMATION

division of Upper Cretaceous rocks in North America (the Cretaceous Period lasted from 114 to 66.4 million years ago). Named for exposures studied on Hell Creek, near Jordan, Mont., it occurs in eastern Montana and portions of North Dakota, South Dakota, and Wyoming. In Montana, the Hell Creek overlies the Lennep Formation and is the uppermost formation of the Cretaceous Period. The Hell Creek consists of about 175 metres (575 feet) of grayish sandstones and shales with interbedded lignites. It was deposited as coastal-plain sediments during the withdrawal of the shallow Cretaceous seas that covered much of the interior of western North America. Fossils in the Hell Creek include the remains of plants and dinosaurs such as Tyrannosaurus and Triceratops and many small Cretaceous mammals, including the oldest known primates.