An important transgressive-regressive-transgressive sequence is recorded in the Albian strata of the Western Interior Cretaceous basin. A widespread marine shale, mapped throughout the basin, is known in different areas as the Skull Creek, Kiowa, Thermopolis, and Joli Fou (Canada) shales (Figs. 2.5 and 6.1). Equivalent strata in western Wyoming in the basin-margin area are generally included in the Dakota Group or Bear River formation. The shale deposits, which accumulated during a high stand of the Albian Sea, are correlated over large areas, either by contained faunas or by stratal continuity. The shales, generally 100 to 200 ft thick, represent the first widespread transgression of the Cretaceous sea into the United States portion of the Western Interior basin. Overlying regressive sandstone units, named the Muddy, J, or Viking (Canada) sandstones, are widespread and productive of petroleum in stratigraphic or structural traps. Generally less than 100 ft thick, these sandstones were deposited in a range of environments from freshwater to marine. They are generally regarded as deposits related to a lowering and rising of sea level. The following transgression is recorded by the widespread marine Mowry Formation and other highstand deposits. When the history of these strata is related to radiometric dates from associated bentonite beds, the sequence spans the time interval of approximately 96 to 98 m.y.b.p. The major event correlates with the worldwide sea-level drop 97 m.y.b.p., reported by Vail, Mitchum, and Thompson (1977) and Hancock, (1975).