The Ralston Creek Formation
Ralston Creek Formation (160 - 150 M.Y.A.) - Interstate 70 roadcut through Dinosaur Ridge (Dakota Hogback) at the Morrison exit west of Denver.
Rivers and lakes, mud and sand...
By 160 million years ago, the shallow seas which created the Lykins limestone had retreated, although, not far. Eastern Colorado turned into a freshwater lagoon. For some ten million years evaporation produced layers of sandstone, shale, and gypsum in a large floodplain, known as the Ralston Creek Formation.
(1) Andrew M. Taylor, Ph.D., "Guide to the Geology of Colorado," Cataract Lode Mining Company, (Golden, CO 1999), p. 66.