The Bright Angel Shale is one of five formations making up the Cambrian Tonto Group, exposed in the Grand Canyon of northern Arizona as well as southeast California, southern Nevada and southcentral Utah, dating to the Middle Cambrian, roughly 507 to 502 million years ago. It consists mainly of greenish, micaceous, fissile shale and siltstone bearing illite, chlorite and kaolinite, with red-brown, hematite-bearing sandstone and siltstone, isolated limestone beds and glauconite-rich layers, ranging from 57 to 450 feet thick and thickest in the western Grand Canyon; its cross-stratification, laminations and wavy bedding record tidal and shallow marine environments. It is the most fossiliferous of the five Tonto Group formations, yielding at least 15 trilobite species along with brachiopods, hyoliths and trace fossils including arthropod tracks and worm burrows.
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