The black shale rocks in this area are muddy sediments deposited by the last ocean to exist in Montana, and the Bearpaw Shale contains fossils of sea-going creatures that lived and died some 70 million years ago. Twenty foot long swimming reptiles like Mososaurus and Tylosaurus fed on fishes and ammonites, relatives of squids and octopi, and the remains of the gigantic coiled ammonites called Placentaceras and the straight shelled ammonites called Baculites are often found in these shale deposits. This inland ocean extended north to south from the Gulf of Mexico to the Arctic Ocean and divided North America into two subcontinents. Dinosaurs roamed the lands, and alligators and turtles inhabited the streams and rivers. One of the largest animals to live in this area was a gigantic forty foot long crocodile called Deinosuchus that lived in the coastal waters of the inland seaway, and the first remains of this animal were found near here. In the early 1900s, geologists learned that anticlines, large wrinkles in the rock strata, were good places to drill for oil, and they also realized that certain parts beneath structures called domes were better than others. This location stands on part of the Mosby Dome of the Cat Creek Anticline.