The rocks of the Phosphoria formation in northwestern Wyoming fall into five stratigraphic units, which, from oldest to youngest, are provisionally called the A, B, C, D, and E units. Units A, C, and E are composed dominantly of chert, tarbonate rock, and sandstone, whereas units B and D are composed dominantly of mudstone, phosphorite, and dark carbonate rock. Units C and D are continuous over all of northwestern Wyoming, whereas unit A grades into unit B and unit E grades into unit D to the southwest. Both units A and B pinch out to the north. With the possible exception of unit A, these units can be recognized in southwestern Montana. Units 3, C, and D are equivalent to the phosphatic.shale, Rex, and upper shale members of the Phosphoria formation in southeastern Idaho. The formation as a whole exhibits a facies change from dominantly chert, mudstone, and phosphorite in the southwestern part of northwestern Wyoming to dominantly carbonate rock and sandstone to the northeastern part. The facies change by thinning and pinching out of units and the in.ergradation of units. The rocks of the formation in the area of the report are cyclically deposited. The rock record of a single ideal cycle consists of, from base to top, carbonate rock, chert, phosphorite, chert, and carbonate rock. Sandstone and glauconite in general are found in the carbonate phases of the cycle whereas mudstone, pyrite, and organic matter are found in the chert and phosphorite phases of the cycle. An eastward transition from phosphorite to chest and chert to carbonate rock is analagous to the upper half of the cycle, i. e., phosphorite, chert, and carbonate rock in ascending order. The phosphorite phase of the cycle is represented by units B and D, whereas the other phases are represented by units A, C, and E. Two complete cycles are found in southwestern Wyoming. These stratigraphic relations represent two transgressions and regressions of areally zoned environments, The phosphorite phase is the most transgressive whereas the carbonate rock phase is the most regressive. The physical chemical environments in which the sediments accumulated probably varied between an environment with a pH of above 7.8 and an Eh of greater than zero for the carbonate rock phase of the cycle and an environment with a pH of less than 7.8 and an Eh of less than about-0.25 for the phosphorite phase of the cycle. These environments probably were effected by cold water upwelling on a shelf to the east. The transgressive and regressive migrations of the shelf probably were tectonically controlled.