The Pinal Schist is one of the more enigmatic rock formations in southern Arizona. It formed in the Paleoproterozoic as part of a forearc complex. It's largely composed of quartz-sericite schists, meta-wackes, metavolcanic rocks, and granitoid and gneissic rocks, with minor metachert, meta-conglomerate, and carbonate rocks.
For more on Pinal, see Arend Meijer research paper, 'The Pinal Schist of southern Arizona: A Paleoproterozoic forearc complex with evidence of spreading ridge–trench interaction at ca. 1.65 Ga and a Proterozoic arc obduction event. https://tinyurl.com/PinalSchist-AM
Photographer: Mike Conway
Photo Date: April 2016