Paleoenvironmental interpretations based on sedimentological and micropaleontological analyses indicate that the Late Cretaceous (Campanian) Foremost Formation is transitional between the underlying marine Pakowki Formation and the overlying nonmarine Oldman Formation. The sediments of the Foremost Formation in the study area were deposited in marginal-marine shoreline environments during the northeast withdrawal of the Pakowki sea. The regression was punctuated by numerous incursions of the Pakowki sea, resulting in cyclically alternating lithologies. Five depositional cycles were recognized in the paralic sediments of the Foremost Formation. Each complete cycle generally exhibits the following sequence of environments from the base upward: 1) offshore (beach) transition, 2) barrier island-shoreface and foreshore subenvironments, 3) lagoon, salt marsh, 4) fresh-water marsh. However, complete cycles were rarely preserved; most commonly only the shoreface subenvironment of the barrier-bar and lagoonal deposits are present. The Foremost Formation does not exhibit deltaic or fluvial-channel characteristics in the study area. Paleocurrent and petrographic analyses indicate that principal source areas for the Foremost and Oldman sediments were located to the southwest in Montana. The clastics were re-distributed from the northeast-trending contemporaneous deltas and river mouths by marine processes, principally longshore currents. Accumulation of sand in narrow elongate belts facilitated the development of a landward lagoonal-marsh system.