Several episodes of Laramide deformation occurred in ranges bordering the Hoback Basin. Deformation, beginning in the west, provided sediment for the Hoback formation. The Hoback formation was overriden by post-Wasatchian thrusting. The middle Eocene? Pass Peak conglomerate overlapped the thrusts; was involved in later faulting. Interpretations of the complex regional structure are reviewed.

The Hoback formation is over 15,000 feet thick. A type locality for the formation is designated and the beds there are described. Other sections are combined with the type section in a composite columnar section. Fossil localities are listed. The coal, carbonaceous shale, and abundant plant fragments; drab coloration of the beds; and irregular bedding, indicate fluviatile deposition in a forested, humid, floodplain environment. An early Tiffanian arboreal and forest mammalian faunule (from the Dell Creek Quarry) 9000 + feet above the base of the formation supports this conclusion. The lower two-thirds of the formation are Paleocene in age.

Vertebrates from the Hoback formation are described. Their significance is discussed. New species are described for each of the mammalian genera: Ptilodus?, Plesiadapis, Carpodaptes, Didymictis, Haplaletes.

Coryphodon and Probathyopsis successor are reported from a higher locality. Early development of sexual dimorphism in the Uintatheriidae is recognized.

An early Wasatchian mammalian faunule is described from a still higher horizon.

The Hoback formation is partly equivalent in age to the Almy formation but resembles Paleocene and earliest Eocene deposits in the Jackson Hole Area lithologically. Deposition probably began during early Paleocene time, continuing without major interruption into Wasatchian time.

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