The formation name Willwood is proposed for variegated shales and hornblende-bearing sand stones conformably overlying the Polecat Bench formation near the center of the Big Horn Basin Wyoming, and truncating folds along the western margin of the area. Willwood sediments were deposited in a forested lowland, under warm-temperate conditions. Sinking of the structural basin was slower than Willwood deposition for upper Willwood deposits spread laterally beyond the lower part of the formation. Pre-Cambrian pebbles in both Polecat Bench and Willwood conglomerates indicate erosion of the pre-Cambrian cores of the uplifts surrounding the Big Horn Basin during late Paleocene and early Eocene time. Locally, the Willwood yields Clark Fork (Late Paleocene) mammals. Above the Clark Fork beds Willwood deposits contain mammals characteristic of the Early Eocene (Wasatchian).
The Tatman formation (early Bridgerian) lies conformably upon the Willwood and below the basic breccia capping Squaw Buttes. It consists of about 870 feet of alternating fine-grained sandstone and laminated brown carbonaceous shale, swamp and lake deposits which accumulated under conditions analogous to those that produced the shore facies of the Green River formation.
Late Cretaceous and early Cenozoic orogenic movements effected no pronounced change in the topography or in the climate of the Big Horn Basin.