As the sea deepened in eastern Colorado, finer sediments were deposited. These included the black muds of the Benton Shale, and the Niobrara Limestone, a shallow-water deposit containing abundant shells of clams (Inoceramus and Ostrea) and [ammonites] and tiny one-celled animals called [Foraminiferida]. Above the Benton and Niobrara Formations lie the fine gray muds of the Pierre Shale. Several thousand feet thick, the Pierre contains occasional beautifully preserved ammonite shells as well as bones from [fossil] fish and swimming reptiles.
Cretaceous rocks in Colorado are rich in [fossil] pelecypods. Each of the fossils illustrated above may grow to a much larger size than shown.
Shales of the Laramie Formation contain many recognizable plant [fossils].
The rocks deposited in western Colorado at this time are markedly different from those deposited in eastern Colorado. In the east, deposits are fine and very limy, containing abundant shells and little in the way of coarse debris. In the west, sandstones of the [Mesa] Verde Formation dominate, and coal beds suggest marshy or swampy conditions inshore from the ancient ocean. This is just the pattern we would expect from a low-lying region bordering a shallow sea, a region similar perhaps to the southeastern Atlantic and Gulf coasts of the United States today.
Toward the end of the Cretaceous Period, the sea receded from Colorado. Beaches and bars of the retreating sea left a sandstone layer which now outcrops prominently east of the Front Range as the Fox Hills Sandstone. Above lie interbedded sands and coals, the Laramie Formation. The presence of coal above beach sands shows that the coal swamps moved eastward as the sea retreated.
The exact age of the shoreline deposits and coal beds varies from place to place in such a way as to indicate that the sea withdrew slowly and irregularly. In general the shore moved eastward, but there are localities such as North Park where deposition lasted much longer than elsewhere. In some places no real beach was formed at the ancient strand line.
In western Colorado, the end of Cretaceous time is marked by coarser beds, indicating an increased rate of uplift in Utah. [Conglomerates] were deposited in the beds of the McDermott Formation, now visible along the Animas River south of Durango.