[Illustration: 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.] [Illustration: 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. _Benton._ The formation is not especially productive in this region. Marine shells are numerous in some localities, and bones of marine reptiles have been found at various places. As usually seen, it is almost entirely composed of impure clay shales, very dark, brownish-gray to almost black, and commonly interbedded with thin patches of white bentonite, yellow ochre, gypsum, and limestone. _Dakota._ This formation produces the high hogback which is usually present some distance east of the Red Rocks. There are generally two or three layers of massive, light-colored sandstone separated by clays which are used extensively in the making of bricks and pottery. Leaf impressions and some fish scales are found in the clays and occasionally in the sandstone. The hogback is a good marker from which to locate other formations, because of its prominence in the foothills landscape. _Morrison._ Good dinosaur material has been taken from the Canon City and Morrison districts. The formation is to be found on the lower west slope of the Dakota hogback. It consists of continental deposits of the stream and lake types. There is considerable sandstone in this formation and a little limestone is to be found here and there, but the most characteristic feature is in the shales. When freshly exposed, the shales are delicately tinted with gray, green, and maroon, a bronze-green being rather prominent. This formation is highly variable in character, with much of the clay often buried under the valley floor. In addition to the bones of reptiles, there are plant fossils, usually of poor quality, and fresh-water gastropods more or less abundant in some localities. _Lykins._