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. CENOZOIC ERA It is characteristic of earth history that the younger the rocks are, the more we know about them. This is because younger rocks lie near the surface, have not been disturbed as much by mountain building processes as have older rocks, and have not been affected as strongly by repeated erosion. Many of the events of the Cenozoic Era are documented in detail in the geology of Colorado, and these events have intimately influenced the scenery as we see it today. The Cenozoic is the Age of Mammals. How it happened that mammals triumphed over reptiles is one of the mysteries of geology. Some scientists think that climatic changes—dropping temperatures and increases in rainfall—swung the balance in favor of the warm-blooded mammals. Others believe that cosmic ray bombardment during some unusual astronomical event may have destroyed many surface-living dinosaurs, while small burrowing mammals (as well as many small reptiles) were able to survive. Still others maintain that the superior intelligence and regulated body temperatures of mammals enabled them to win out in the battle for survival without the aid of climatic or cosmic change.