division of rocks in the western United States dating to the end of the Cretaceous Period approximately 66 million years ago and named for exposures studied near Lance Creek, Niobrara county, Wyoming (see Niobrara Limestone). Varying in thickness from about 90 metres (300 feet) in North Dakota to almost 600 metres (2,000 feet) in parts of Wyoming, the Lance Formation consists of grayish sandy shales (shale), light-coloured sandstones (sandstone), and thin lignite beds. This formation is well known for its Late Cretaceous fossils, which include plants, dinosaurs, and mammals. The duck-billed dinosaur Trachodon, the great carnivore Tyrannosaurus (tyrannosaur), the herbivores Triceratops and Ankylosaurus, pterosaurs (pterosaur), birds, and mammals (including marsupials (marsupial)) have been found in the Lance. The formation also contains examples of spectacular fossil preservation, including a so-called dinosaur “mummy,” a complete duck-billed dinosaur skeleton surrounded by skin impressions.