Our group hiked as far as they could down the Bright Angel trail in the course of an hour before turning back. Despite icy conditions, many hikers made it past the base of the Coconino Formation and into the Hermit Shale. This photograph shows the contact between these two rock units. To get an idea of where this lies with respect to the canyon as a whole, look at the opposite rim, in which the Coconino is visible as a bold pale band near the top. This was Dr. Merck's geographic low point of the hike. His physiological low point came as he made his way back up the Coconino.