At the beginning of the Miocene Period, some 25 million years ago (MYA), the Colorado River and its tributaries were cutting their way through a layer cake of Tertiary and Cretaceous sediments (top to bottom) in western Colorado; colorful Jurassic and Triassic rocks lay beneath these sediments, not yet uncovered by the river.
Then, as the Miocene-Pliocene Uplift began, a block of Precambrian granite began to rise; 100 miles long (NW-SE) and 25 miles wide, this block caused the overlying Mesozoic and Tertiary sediments of western Colorado to warp across it, forming the Uncompahgre Plateau. Eventually rising more than 6000 feet above the surrounding terrain, the plateau has also been whittled by erosion, removing its Tertiary veneer and exposing layers of Cretaceous, Jurassic, Triassic, and Precambrian Rock (top to bottom); all Paleozoic sediments had been eroded from the top of the Precambrian block during the Pennsylvanian and Permian Periods, some 300-225 MYA, producing the unconformity that we see today (i.e. Mesozoic sediments lying directly on Precambrian rock).
At the north end of the Uncompahgre Plateau, stream erosion has produced a scenic wonderland of Mesozoic redrocks, including massive walls of Wingate Sandstone (late Triassic-early Jurassic in age) and, higher up, pink cliffs of Jurassic Entrada Sandstone, separated by the Kayenta formation. The Jurassic Morrison formation, famous for dinosaur fossils, lies above the Entrada Sandstone and, at the top, lies Cretaceous Dakota Sandstone. Younger Mancos Shale, which surfaces throughout the Grand Valley, Mesaverde Sandstone, which forms the Book Cliffs north of Grand Junction, and Tertiary sediments that compose the bulk Grand Mesa and the Roan Plateau, have eroded from the northern end of the Uncompahgre Plateau. Today, Colorado National Monument, west of Grand Junction, protects this spectacular product of deposition, uplift and erosion, allowing visitors to wind upward through the Mesozoic Era. Stopping to gaze across the Grand Valley, one looks down at rocks that are younger than those on which they stand.
For more on the Uncompahgre Plateau, see: Unaweep Canyon Mystery
