Four elongate tongues of the Shedhorn Sandstone of Permian age extend southeastward from the Yellowstone Park area into the Gros Ventre, Hoback, and Wyoming Range area. These tongues represent barrier-island complexes that built southwestward in the Phosphoria sea, creating a lagoon on their northeast side. The lagoonal facies includes algal laminated and ostracodal limestone and dolomite and a greenish-gray shale. The barrier island facies consists of crossbedded quartz sandstone which contains abraded thick-shelled fossils. The marine facies southwest of the barrier island facies includes chert, quartz sandstone, calcarenite containing a siliceous sponge, brachiopod, and bryozoan fauna, and phosphorite containing a brachiopod and fish fauna. Major phosphate deposition w s restricted to the marine environment seaward of the barrier islands, and was absent in the lagoonal environment.

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