PIERRE SHALE

division of Upper Cretaceous rocks in the United States (the Cretaceous Period lasted from 144 to 66.4 million years ago). Named for exposures studied near old Fort Pierre, S.D., the Pierre Shale occurs in South Dakota, Colorado, Minnesota, New Mexico, Wyoming, and Nebraska. The Pierre consists of about 600 m (2,000 feet) of dark gray shale, some sandstone, and many layers of bentonite (altered volcanic-ash falls that look and feel much like soapy clays). In some regions the Pierre Shale may be as little as 200 m thick. The fossil Cretaceous sea turtle Archelon, the largest known turtle species that ever lived, has been found in South Dakota.