Unconformity means missing rock layers. The Great Unconformity can be touched at river mile 121. Black Tail Canyon is a secretive place. Created by shade, the dark moody narrow walls of Tapeats sandstone and Vishnu Shist mingle with each other at the contact zone. The story that is told is that swift waters broke apart bits of the Shist and then settled in with the sand. There are true signs of this action in the contact layer between the 560 million year old Tapeats sandstone and the 4.5 billion year old Vishnu Shist. Go there, but be silent as the spirit world of the ancient ones lingers still. Play a flute, or a guitar, or sing a song, but walk with slow quiet steps. Feel the depth.