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| This outcrop of Vishnu Basement Rocks along the North Kaibab Trail near Phantom Ranch displays good examples of boudinage. Boudinage are common geologic structures in strongly deformed (metamorphosed) sedimentary and igneous rocks, in which original continuous competent layers or beds between less competent layers have been stretched, thinned, and broken at regular intervals into bodies resembling boudins (French for sausages). In this rock, softer layers of biotite schist are crushed and stretched relative to the harder, more competent layers of granitic rock. Rocks exposed in the Inner Gorge included the historically mapable units called Brahma Schist, Rama Schist, and Zoroaster Granite. These ancient rocks have been interpreted as an island-arc assemblage of rocks that were accreted onto the ancient Laurentian continent landmass between 1,750 to 1,663 million years ago (Ilg and others, 1996). Human for scale. |
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