The Sundance Formation is a Jurassic rock unit spread across western North America, formed about 168 to 157 million years ago in the shallow inland Sundance Sea. Up to 100 metres thick, it preserves marine shale, sandy shale, sandstone, and limestone that record shifting coastal and offshore environments. In the regional sequence it lies beneath the Morrison Formation and above the older Gypsum Springs Formation, separated from the latter by a disconformity. It is especially known for marine fossils such as belemnites, oysters, crinoids, echinoids, gastropods, ostracods, and foraminifera, as well as dinosaur footprints at Red Gulch near Shell, Wyoming, showing dinosaurs visited the sea's margins.