Bearpaw Shale and the Inland Ocean
The black shale rocks seen in this area represent the muddy sediments deposited by the last ocean to exist in Montana. The shale, known by geologists as 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. 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. But, 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. The first remains of this animal were found near here. In the early 1900s, geologists learned that geological structures called anticlines, a kind of large wrinkle in the rock strata, were good places to drill for oil. Geologists also realized that certain parts of anticlines were better than others, and that the good spots were beneath structures they called domes. Here at the Mosby rest area you are standing on a part of the Mosby Dome, of the Cat Creek Anticline.
Marker is on Montana Route 200 1.4 miles east of Garfield/Petroleum County Line, on the left when traveling east.
Courtesy hmdb.org
