Two previously undescribed sandstone beds in the Lower Cretaceous Mowry Shale in north-central Wyoming form elongate, retrogradational bodies, meters thick and tens of kilometers in length, that are encased in shale. The sandstones were deposited under different depth and oxygen conditions than were the shales. The sands are extensively bioturbated and contain wave-induced features (oscillation ripples and hummocky stratification), whereas the surrounding muds contain few biogenic structures and lack evidence of wave motion. Mowry sandstone units are interpreted to have been deposited by storms in shallow (10 to 30 m), oxygenated water. The shales that encompass these sandstones were deposited largely from turbid bottom flows in dysaerobic and anaerobic water. A drop in sea level is postulated to have promoted deposition of each sandstone bed on sections of the shelf where muds had accumulated previously below wave base in oxygen-poor water. During low water level this region was exposed to well-oxygenated water and to the effects of storms. Subsequent sea level rise shifted the locus of sand deposition, wave base, and the intersection of oxygen boundaries with the seafloor in a shoreward direction. The retrogradational geometry of each sand body is explained by the shoreward migration of sand deposition during sea level rise.