Phantom Ranch Tour - the Tonto Platform & Inner Gorge

This zoomed-in view is looking east from the South Kaibab Trail toward the Tonto Platform (in the foreground) and Inner Gorge. On the opposite side of the Inner Gorge, the Precambrian-age (Proterozoic) Bass Formation forms a flat-lying ledge along the top of the early Proterozoic Vishnu Basement Rocks (mostly granitic rocks and schist) that make up the Inner Gorge. This contact or boundary is called the Greatest Angular Unconformity. The older rocks were brought to the surface by uplift and erosion before the younger Bass Formation was deposited--all during the early history of the earth. Another unconformity, the Great Unconformity, lies above the above the cliff-forming Bass Formation, juxtaposing the Bass and the softer, slope- and bench-forming strata of the Paleozoic (Cambrian) of the Tonto Group (Tapeats Sandstone and Bright Angel Shale) (Billingsley, 2000; Billingsley and others 1997). All unconformities mark intervals of erosion and (or) non-deposition.