Abstract: This map was modified from a map originally included as part of Steve Skotnicki's Ph.D. dissertation at Arizona State University. Purpose: Paul Knauth and his students have studied the cherts within the erosion surface at the top of the Mescal Limestone for many years. Along with Bob Horodyski, Paul co-discovered what may be the oldest known terrestrial microfossils entombed within the chert (Horodyski and Knauth, 1994) and subsequently enlisted Steve to decipher the stratigraphy and petrology of these cherts. The goal was to try to unravel the sequence of events that led to their formation, in an effort to better understand life on land in the Precambrian. As a result Paul helped to support this research during parts of two summers through a NASA Exobiology Grant #NAG5-4060.