A new model is proposed for the depositional environment of the Green River Formation of Wyoming, which is based on a combination of lake and playa environments. We propose that oil shales and trona beds accumulated in shallow lakes which were fringed by large playa flats. In these playa flats alkaline brines evolved through evaporation and precipitation of calcium carbonate and protodolomite in the capillary zone near the ground-water table. Dolomitic mudstones, marlstones, and calcareous and siliciclastic sandstones were the products of occasional floods on the playa; chief evidence consists of the assemblage of sedimentary structures found in these rocks, indicating frequent exposure to air, strong shallow-water currents, transport, and resedimentation. The playa-lake model leads to a more consistent picture of the hydrology, brine evolution, mineral formation, and sedimentation in the Green River basins than the previously accepted stratified lake concept.