For over 5 million years the Colorado River has been carving the Grand Canyon through the Kaibab Plateau, a broad upwarping (outlined by snow) within the larger Colorado Plateau. At its widest, the canyon spans 29 kilometers and is 1.8 kilometers deep, although even this is dwarfed by the Valles Marineris canyon system on Mars (slide #24). The walls of the Grand Canyon are commonly likened to the pages of Earth’s history, and comprise one of the best exposed continuous vertical sections through the Earth’s crust. The youngest rocks are the 250-million-year-old Kaibab limestone formation along the canyon rim. The oldest rocks, at bottom, are the Precambrian Vishnu schists, highly deformed metamorphic rocks up to 2 billion years old. The step-like morphology of the canyon walls is due to differential erosion of sedimentary and volcanic rock layers of varying resistance. The deep narrow inner gorge is carved through the very resistant Precambrian metamorphic rock. The narrow finger-like canyons along the rim may have formed by groundwater sapping, a process that may be occurring on Mars (see slides #24 and #26).
60 images 60-83-4, 60-83-6.