Seventy million years ago (MYA), as the Cretaceous Sea was retreating to the southeast, future Colorado was a relatively flat layer cake of Mesozoic and Paleozoic sediments, overlying the Precambrian basement rocks. Then, about 65 MYA, pressure within the North American craton caused the basement rocks to crumple upward, producing the Rocky Mountain chain. Over time, as rivers drained both sides of the mountain range, the overlying sedimentary rocks were eroded away, aprons of debris were spread to the east and west and the numerous streams carved valleys and canyons throughout the mountains and adjacent plains.
During the Eocene, some 50 MYA, great inland lakes covered the Utah-Wyoming-Colorado tristate region and would deposit the famous Green River shale (now uplifted as the Roan Plateau). Near the end of the Eocene, about 38 MYA, volcanic activity developed in southwest Colorado, initiating the formation of the massive San Juan Mountains. About the same time, grass was evolving on the High Plains, east of the Rockies; more tolerant of drought, wind, wildfire and the trampling of Oligocene megafauna (including bison), grasslands replaced woodlands across the vast plains of North America. During the Miocene, some 25 MYA, an uplift of the Mountain West added 5000 feet to the region's elevation and augmented the erosive power of its numerous streams. Then, about 4 MYA, the Sierra Nevada cut off Pacific moisture to much of the intermountain region, producing deserts and semi-arid landscapes. Finally, the cold, wet climate of the Pleistocene (2-0.1 MYA) further increased erosion via mountain glaciers and torrents of meltwater.
Today, the eastern third of Colorado is part of the High Plains Province of North America. Underlain with Cretaceous and Tertiary deposits, it was once covered by a vast, shortgrass prairie. The Rocky Mountain Province stretches through the central longitudes of Colorado, veering to the southwest to take in the San Juans; the component ranges are separated by four major "parks," including the San Luis Valley which developed as the Rio Grande Rift began to open, some 10 MYA. The western third of Colorado is part of the Colorado Plateau, a land of high mesas and tablelands, separated by rugged, rock-walled canyons; the highest mesas, protected by a veneer of volcanic basalt, are Grand and Battlement Mesas, near Grand Junction. Mesozoic rock exposures dominate this province, producing spectacular scenery and a wealth of dinosaur fossils.
