What does the Vishnu Schist tell you about the history of the Grand Canyon prior to the formation of the canyon itself?

These rocks were part of the oceanic trench.

Explanation:

The Vishnu schist is a part of the Vishu complex of exposed basement rocks and is originally part of the deep ocean trench and was substantially overloaded by the sediments of the Brahma schist some 1.75 billion years ago. Being metamorphic in nature the oldest rocks of the canyon have granite and suggest they have been subjected to intense heat and pressure.

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