West of Glenwood Springs, Colorado, the Colorado River passes through the Grand Hogback and enters the Colorado Plateau, a vast geophysical province that covers western Colorado, eastern Utah, northern Arizona and northwest New Mexico. Characterized by horizontal layers of bedrock, increasing in age from top to bottom, the Plateau is a scenic landscape of mesas, buttes and canyons, all sculpted by the Colorado River and its numerous tributaries; from its entry point in western Colorado to its exit west of the Grand Canyon, the River has cut through 550 million years of geologic history (early Tertiary, Mesozoic and Paleozoic), not to mention part of the ancient Precambrian basement that underlies these sediments.
Just west of the Grand Hogback, which marks the western edge of the Rocky Mountain Province, the Colorado flows across Tertiary terrain; to its north is the Roan Plateau, capped by Eocene sediments (50 million years old) and, to its south, is the massive bulk of Battlement Mesa, protected from erosion by a layer of Tertiary basalt. Further along, the River enters the Mesozoic, cutting through the Cretaceous sandstone of Debeque Canyon and then running atop the Cretaceous shale of the Grand Valley; the Book Cliffs, also Cretaceous in age and exposed below the younger Roan Plateau, run along the north edge of the Valley, extending into Utah. The River curves around the northern flank of the Uncompaghre Plateau and angles southwestward, carving through the scenic Jurassic and Triassic redrocks of Utah's Canyon Country.
Reinforced by flow from the Green River, the Colorado begins to dig into Paleozoic sediments at the entrance to Glen Canyon, now flooded by a massive reservoir; this Permian sandstone is only visible along the eastern backwaters of the lake, which is bordered by high walls of Jurassic Navajo sandstone further downstream. Beyond Glen Canyon, in northern Arizona, the Jurassic sediments disappear and the landscape is composed primarily of Triassic and late Paleozoic (Pennsylvanian and Permian) rocks; these layers, 200-300 million years old, are well exposed in Marble Canyon. Having received additional flow from the San Juan and Little Colorado Rivers, the Colorado enters the Grand Canyon, which it carved from a stack of Paleozoic sediments, one mile thick; Permian Kaibab limestone caps the Canyon walls while, at their base, the River is cutting through Precambrian Vishnu Schist, more than 1.7 billion years old.
