Great Unconformity
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The Great Unconformity is the unconformity observed by John Wesley Powell in the Grand Canyon in 1869. It is an exceptional example of relatively young sedimentary rock strata overlying much older sed
Geologic stratigraphic column of strata exposed in and near the Grand Canyon
Powell's Unconformity viewed from Lipan Point on the South Rim. Rocks of the Unkar Group of the Grand Canyon Supergroup are truncated at the base of the Tonto Group.
Powell's Unconformity seen from Hopi Point on the South Rim. Steeply foliated and veined schists of the Vishnu Basement Rocks truncated at the base of the Tonto Group.
An exposure of Powell's Great Unconformity, west of Montezuma, New Mexico
Vishnu Basement Rocks
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The Vishnu Basement Rocks is the name recommended for all Early Proterozoic crystalline rocks exposed in the Grand Canyon region. They form the crystalline basement rocks that underlie the Bass Limest
Great Unconformity
…from the underlying faulted-and-tilted sedimentary rocks of the Grand Canyon Supergroup and vertically foliated metamorphic and igneous rocks of the Vishnu Basement Rocks. The unconformity between the Tonto Group and the Vishnu Basement Rocks is a nonconformity. The break between the Tonto Group and the Grand Canyon…
The Vishnu Basement Rocks were deposited as mafic and felsic volcanic rocks and sediments but were later metamorphosed and intruded by igneous rock.