The Great Unconformity at the lip of the inner gorge of the Grand Canyon, Colorado Plateau. The Cambrian Tapeats Sandstone in the foreground overlies the Proterozoic Vishnu Schist (black rocks) and Proterozoic Zoraster Granite,, a gap in time of about one billion years. Younger Paleozoic rocks rise up in the rear, the cliffs of the Mississippian Redwall Limestone being particularly prominent.