Upper Cretaceous rocks described and correlated in this report are the Steele Shale, Mesa verde Group (consisting of the Rock River l!'ormation (new name), Haystack Mountains)formation (new name), Allen Ridge Formation, Pine Ridge Sandstone, and Almond Formation), Lewis Shale, E'ox Hills Formation, Medieine Bow ll..,ormation, and the lower part of the Ferris Formation. '.rhe lower Tertiary rocks described consist of the upper part of the Ferris ll..,ormation of Paleocene age and the overlying Hanna ll..,ormation of late Paleocene and Eocene age. 'l'he oldest unit Jnvestigated, the Steele Shale, consists of 2,300-8,800 feet of dark-gray marine shale that contains limestone concretions, beds of bentonite as much as '5 feet thick, and• thin layers of siltstone ami very fine grained sandstone. It becomes sandy upward nnd grades into the overlying Mesaverde Group. 'The Mesaverde Group, in the Rawlins-Medicine Bow area, consists of the following formations, from oldest to youngest: Haystack Mountains Formation, Allen Ridge Formation, Pine Ridge Sandstone, and Almond Formation. The Haystack Mountains Formation comprises a sequence of mostly shallow-water marine sandstone and shale that thins southwestwardly across southcentral 'Vyoming from 2,550 to 850 feet. It contains three persistent ridge-forming sandstone members interpreted as beach and barrier-bar deposits. The oldest member, Tapers Ranch Sandstone (new name) , is 270 feet thick •and forms the base of the formation. The second member, O'Brien :Spring Sandstone (new name) is 220 feet thick and lies 1,350 feet below the top of the formation. The Hatfield Sandstone Member, as much as 167 feet thick, lies 450 feet •below the top. Nonmarine sandstone, shale, and carbonaceous beds largely mal{e up the Allen Ridge Formation, but some sandstone and shale of brackish-water and marine origin are included. The Allen Ridge is conformable with the underlying Haystack Mountains Formation. It is as much as 1,500 feet thick. It intertongues with the Rock River Formation, and in the Laramie Basin area, it is replaced by that formation. Dlsconformably overlying the Allen Ridge Formation is the Pine Ridge Sandstone, a white t