Phosphoria Formation chert

Phosphoria Formation chert comes from a Permian-age rock unit of marine chert, phosphate rock, carbonate rock, mudstone and sandstone that runs across northwestern Wyoming, southwestern Montana and southeastern Idaho. In Wyoming the best-known sources are the Black Mountain and East Spring Creek quarries in the Big Horn Basin, representatives of the high-grade Shell Creek Phosphoria chert. This chert is fine grained and high quality, with a distinctive blood-red colour. The Black Mountain Archaeological District holds two large quarry areas for it, along with campsites and rockshelters used from early in the Paleoindian period, perhaps 11,500 years ago, through the Late Prehistoric. The formation as a whole changes character across the area: chert, mudstone and phosphorite to the southwest, and carbonate rock and sandstone to the northeast.

Permian Phosphoria Formation: in northwestern Wyoming five units (A to E); units A, C and E are chiefly chert, carbonate rock and sandstone, units B and D mudstone, phosphorite and dark carbonate rock. The chert-mudstone-phosphorite facies is thickest in the southwest and grades to carbonate rock and sandstone in the northeast. The Shell Creek sources are in the Big Horn Basin.