Harboring some of the most spectacular scenery on the planet, the Colorado Plateau stretches across western Colorado, northwestern New Mexico, northern Arizona and the southeastern half of Utah. Composed of a layer cake of sedimentary rocks, with the oldest at the bottom and the youngest at the top, this Province offers a geologic history that is relatively easy to read. The rock layers are horizontal and disturbed only by erosion throughout most of the Plateau; there is very little folding and faulting in comparison to other regions of our country. Only laccolithic mountains (which formed as magma within the sediments) and true volcanic peaks, disturb the geometry; the La Sal Mountains, east of Moab, Utah, are laccoliths while the San Francisco Mountains, north of Flagstaff are volcanic.
For the most part, the rock layers range in age from the late Paleozoic to the early Cenozoic Eras; Mesozoic sediments dominate across much of the Plateau. Older rocks can be found in the deeper canyons; the Grand Canyon, for example, exposes rock from the Precambrian Era (at its base) to the Permian Period (at its rim). The youngest rocks of the Plateau are found along its northern and western borders where early Tertiary sediments comprise the Roan and Wasatch Plateaus, respectively. Well known Mesozoic formations include the arches of Arches National Park (Jurassic), the Cliffhouse Sandstone of Mesa Verde (Cretaceous), the massive Wingate Sandtone walls of the Colorado National Monument (Jurassic) and the Petrified Forest of northern Arizona (Triassic).
Much of the erosion, which has sculpted the spectacular canyons and rock monuments, has occured since the Miocene Period, when the entire region was lifted 5000 feet and the rivers of the Colorado watershed were energized; a cooler, wetter climate during the Pleistocene also augmented the erosive power of these streams. Today, caught within the rain shadows of the Sierra Nevada and Rocky Mountains, the Plateau is a mosaic of arid and semiarid ecosystems; only the highest elevations support forests.
