The Muddy sandstones in the western Wind River Basin were deposited in a thin, but well-developed delta system during Early Cretaceous Abian time. Open marine Skull Creek shales grade upward into the lower marine Muddy sandstones (Fig. 1).The Muddy sandstones in the western Wind River Basin were deposited in a thin, but well-developed delta system during Early Cretaceous Albian time. Open marine Skull Creek shales grade upward into the lower marine Muddy sandstones (Fig. 1). The depositional surface was raised to sea level by deposition of 65 feet of prodelta sediments and delta-front sandstones at the northwest seaward edge of the- delta. Progradation of delta plain deposits then covered the be delta-front sandstones. Rapid subsidence allowed preservation of mOBt Muddy sedimentary rocks in the northwest part of the study area, but a lesser rate of subsidence or slight uplift to the southeast allowed progressively deeper erosion, resulting in two intraformational unconformities. The lower marine deposits were eroded deeply by rivers of the delta plain and mOBt of these lower marine sedimentary rocks southeast of Lander were removed (D of Fig. 21. The delta plain was successively covered by two main facies: 11 fluvial channel sandstones and 2) flood plain deposits. The Fluvial sandstones, deposited in a northwest-flowing river system, terminate to the northwest (e of Fig. 2). The overlying flood plain deposits are truncated southeastward (upstream) (F of Fig. 2) by erosion at the base of the overlying transgressive beach deposits. Progressive onlapping by beach and nearshore deposits of the middle marine Muddy sequence subsequently buried the older delta system (Fig. l). The uniformity at the base of the beach deposits cuts progressively deeper into underlying delta plain deposits to the southeast, where reworked beach Bandstones were deposited directly on the fluvial sandstones of the delta plain. A second, or upper, marine sandstone and shale sequence buried the delta to the southeast. Northwest of line H (Fig. 2) the sandstones grade seaward into marine shales.