The Bear River Formation of the western Wyoming Overthrust Belt is of Early Cretaceous (Albian) age and equivalent to a portion of the “Rusty Beds”, the Thermopolis Shale, Muddy Sandstone, and Shell Creek Shale. East of the surface trace of the Absaroka Thrust Fault, the formation can be subdivided into seven informal units for the purpose of description and environmental classification. These units and the environments they are interpreted to represent are: (1) interbedded mudstone, marlstone and siltstone — coastal plain; (2) dark gray shale — offshore marine; (3) interbedded shale and siltstone — offshore-shoreface transition; (4) low-angle cross-stratified sandstone — lower shoreface; (5) trough cross-stratified sandstone — upper shoreface; (6) interbedded siltstone, fossiliferous limestone and carbonaceous shale — coastal swamp and lagoon; (7) dark gray shale — offshore marine. Units 2 and 7 represent transgressive deposits, whereas the stratigraphic sequence observed in units 2 through 6 is evidence of a gradual regression (shoreline progradation).

Bear River sandstones are typically texturally submature fine-grained chert arenites. Silica, calcite and kaolinite are the most abundant cements and virtually eliminate porosity in the outcrop samples. The diagenetic chronology is (1) precipitation of early calcite cement; (2) formation of silica overgrowths; (3) precipitation of pore-filling kaolinite; (4) precipitation of late calcite cement; (5) replacement of calcite by dolomite, and (6) precipitation of late quartz cement.