The last major invasion of the sea in the northern Rocky Mountains is known from the distribution of the Upper Cretaceous Lewis Shale. Electric log correlation of key marine beds within the Lewis Shale provides reference horizons that facilitate the measurement of the subsequent structural deformation and accompanying terrestrial deposition during the Laramide orogeny.

Structural subsidence of the basin was contemporaneous with the accumulation of paludal, lacustrine, and fluvial deposits observed in 4,000 feet of the Upper Cretaceous Lance Formation. Post-Cretaceous erosion leveled the margins of the basin. Isopachous maps of the Lance interval, from the unconformity to the key beds, reveal areas of local uplift that are coincident in several places with hydrocarbon accumulations in the underlying Mesaverde Formation along the east flank of the Rock Springs uplift.

Periodic subsidence continued during the Paleocene and was accompanied by large scale normal and reverse faulting along the northern margin. Fort Union arkosic conglomerate, sandstone, and silty mudstone, derived from adjacent source areas, accumulated in and around the embryonic basin. Three lithologic facies that define the detritus related to each period of structural change are recognized. Lateral expansion of the basin is revealed by the onlapping relationship of the "basal" conglomerate and the distribution of the associated basin facies. The stratigraphy of the "basal" conglomerate is poorly understood from fossil evidence.

More than 6,000 feet of Paleocene Fort Union, and Eocene Battle Springs, Wasatch, and Green River strata in the subsurface have been correlated with the surface section recently described by George Pipiringos of the United States Geological Survey.

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