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Boyce Thompson Looking east from the Bright Angel trail, several of the upper rock units of the Colorado Plateau are visible. At the top, pale limestones of the Kaibab formation form cliffs. Beneath it, shales and mudstones of the Toroweap Formation slope down to the shear sandstone cliffs of the Coconino Formation. Beneath this, the mudstones of the of the Hermit Shale and Supai Group slope down to the bottom of the picture.