Grand Canyon by John Wesley Powell (1875)

The image on the right is a stratigraphic section of Grand Canyon by John Wesley Powell (1875). “A” is the metamorphic basement complex (Early Proterozoic Vishnu Group), with igneous intrusives labeled “a”; “B” is the Grand Canyon Supergroup (Middle and Late Proterozoic); “C” indicates the Paleozoic strata; “x” and “y” delineate the major unconformable contacts. The image on the left is a recent photograph of Grand Canyon from Walhalla Plateau, with the red line showing the Great Unconformity that was first noted by Powell (credit: Annie Scott/USGS).