Mesozoic the Phanerozoic Eon, preceded by the Paleozoic ("ancient life") and succeeded by the Cenozoic ("new life"). The era is subdivided into three major periods: the Triassic, Jurassic, and Cretaceous, which are further subdivided into a number of epochs and stages. The era began in the wake of the Permian–Triassic extinction event, the largest well-documented mass extinction in Earth's history, and ended with the Cretaceous–Paleogene extinction event, another mass extinction whose victims included the non-avian dinosaurs. The Mesozoic was a time of significant tectonic, climate and evolutionary activity. The era witnessed the gradual rifting of the supercontinent Pangaea into separate landmasses
Rocky Mountain National Park creating the ancestral Rocky Mountains. Fountain Formation was deposited during the Pennsylvanian period of the Paleozoic era, 290–296 million years ago. Over the next 150 million years, the mountains uplifted, continued to erode, and covered themselves in their own sediment. Wind, gravity, rainwater, snow, and glacial ice eroded the granite mountains over geologic time scales. The Ancestral Rockies were eventually buried under subsequent strata. The Pierre Shale formation was deposited during the Paleogene and Cretaceous periods about 70 million years ago. The region was covered by a deep seathe Cretaceous Western Interior Seawaywhich deposited massive amounts of shale on the