The post-Oxfordian Morrison Formation is exposed along the margins of the Bighorn and Powder River basins of Wyoming and Montana and is present in the subsurface throughout both basins. The Morrison Formation lies conformably on the marine glauconitic litharenite of the uppermost Sundance (or Swift) Formation. The upper contact is disconformable wherever the Morrison Formation is directly overlain by the chert-pebble conglomerate unit of the basal Cloverly Formation. In most places the contact between the Morrison and the overlying Cloverly formations is a paraconformity;In the area studied, the Morrison Formation consists mainly of sandstone, siltstone, and variegated mudstone with occasional limestone. Studies have revealed different environments of deposition contrary to the earlier belief of a uniform fluvial environment. The sediments were deposited in a complex of eolian, transitional marine and fluvial environments. Along the southern part of the Bighorn Basin, the lower Morrison is represented by isolated lenses of thick massive to crossbedded quartzarenites of eolian origin. They represent the final phase of coastal deposition along the retreating margin of the Sundance Sea. Equivalent facies in the north-central part of the basin are olive green silty mudstones and fine grained well sorted, bioturbated quartzwackes of intertidal origin. The upper part of the Morrison Formation consists of sediments of fluvial origin. It is represented by an interbedded claystone and siltstone lithofacies and a channel sandstone lithofacies association. The interbedded siltstone and claystone lithofacies is of overbank origin and dominates the fluvial section of the Morrison Formation;Stratigraphic analysis suggests that positive structural elements were active within the central and north central parts of Wyoming during the Late Jurassic. Morrison Formation is thin and less sandy along paleotopographic highs and sandier along paleotopographic lows, which suggests an active structural control of facies distribution during deposition. The location of these positive elements coincides with the present day Laramide uplifts.