The Upper Cretaceous Frontier Formation, along the western margin of the western interior foreland basin, is composed of clastic sediments that were deposited during the initial Late Cretaceous transgressive-regressive phases of the Western Interior Seaway across Wyoming. The lithofacies belonging to the Frontier Formation are subdivided into three distinct units which are, in ascending order: the lower Frontier unit, characterized by bioturbated sandstone, mudstone, bentonitic mudstone, and chert-pebble conglomerate; the middle Frontier unit, a thick sequence of sandstone, carbonaceous mudstone, bentonites, and chert-andesite-granite-quartizite pebble conglomerate; and the upper Frontier unit which incorporates thick beds of sandstone, siltstone, and bioturbated mudstone. Lateral as well as vertical variation in lithofacies is pronounced and sediment-fill is asymmetric. Facies patterns, hydrodynamic structures, ichnofaunal assemblages, and the geometry of the individual units strongly suggest sedimentation at a fluctuating shoreline with intermittent influxes of terrigenous components into a rapidly subsiding foreland basin;Temporal variations in sandstone composition and clast lithology within the Frontier indicate an evolving northwesterly source terrain comprised of Precambrian crystalline rocks, a Paleozoic sedimentary sequence in a back-arc fold-and-thrust belt, and volcanic rocks of a late Cretaceous continental-arc. Sedimentation within the foreland basin was influenced by both intra- and extrabasinal conditions. Geochemical data for the bentonites indicate that the volcanism in the northerly source area changed in character with time and point to an increase in the degree of partial melting and a decrease in crustal involvement with time.