In mountainous parts of the study unit, precipitation varies strongly with elevation. The Montana Valley and Foothill Prairies ecoregion contains the remaining 3 percent of the study unit. The ecoregions correspond in part to the structural geologic framework of uplifts and sedimentary basins in the study unit. Rocks of Paleozoic age, and to a lesser extent Cretaceous age, are exposed along the eastern flank of the Wind River Range. Throughout much of the Bighorn Mountains, the crystalline rocks of Precambrian age remain buried beneath carbonate rocks of Paleozoic age.
Rhyolites predominate among rock types in the Yellowstone Plateau, but basalts also occur.
The Wyoming Basin ecoregion spans the Bighorn and Wind River basins and the Owl Creek and Bridger Mountains that separate the two basins. The Powder River and Williston Basins are in the northeastern part of the study unit. The structural basins are surrounded on the flanks of the adjacent uplifts by folded and faulted rocks of Cretaceous and Paleozoic age. Rock units of Cretaceous age crop out in about 23 percent of the study unit, and usually are tilted and often beveled by erosion. Sedimentary rocks of Tertiary age unconformably overlie the eroded Cretaceous surface in most of the plains and basins of the study unit.