The Cambrian Bright Angel Shale is the middle layer of the three member Tonto Group geologic feature. The 3-rock Tonto section famously sits upon the Great Unconformity because of the highly resistant cliffs of the base layer, vertical Tapeats Sandstone cliffs. The Bright Angel Shale is easily identified for two reasons. Its soft-greenish color stands out against the browns, reds, and whites of neighboring rock units. And secondly for its slope-forming character against mostly cliff-forming resistant rocks. The units of the Tonto Group: