In addition to the characteristic dark shales, this formation includes some limy material and sandstone beds, both of which are fossiliferous in places. Two types of marine mollusks are characteristic: Inoceramus, generic name for several species of clam-like bivalves readily identified by concentric elevations which produce a rippled effect on the shell surfaces; and Baculites, cephalopods with straight, chambered shells which often break at the suture lines, where the fossil is weakened by the chamber walls. Small oyster shells are fairly common also. The formation is to be found some distance to the east of the prominent hogback where it weathers into smooth surfaces in the form of broad valleys and flats, with rounded contours on the few elevations that may be present. It forms a soft, flaky soil when dry, is a sticky “gumbo” when wet. The clay is generally of a rather dark grayish color when freshly exposed but it takes on a rusty appearance after weathering. At various levels there are numerous iron-cemented concretions, many of which contain fossil shells.
Marine Beds of the Benton Formation, Northeastern Colorado
A stream channel has cut deeply into the formation, uncovering and partly destroying a plesiosaur skeleton which was found at the level where the men are standing.
Plesiosaur Bones in Place
Benton formation. Surface rubble has been cleared away, and several vertebrae are partially uncovered in the area at the right of the hammer.
Niobrara.
The formation contains fossils rather similar to those of the Pierre. Shark’s teeth have been found in some of the lower beds. Limestone is a prominent feature, often forming a well defined ridge near the foot of the eastern slope of the main hogback. The limestones commonly have a chalky character.