The Beaver Divide is a high escarpment of Tertiary nonmarine strata in central Wyoming separating the Sweetwater Plateau from the relatively low-lying Wind River Basin. The Wagon Bed Formation of middle and late Eocene age and the Beaver Divide Conglomerate Member of the White River Formation-this member being of early Oligocene (Chadronian) age--are exposed near the top of the Beaver Divide in Fremont County, Wyo., about 23 miles southeast of Riverton. The upper part of the Wagon Bed Formation, dated as late Eocene (Uintan) age by Van Houten on the basis of vertebrate fossils, contains a highly silicified fresh-water limestone stratigraphically high in the section with a poorly preserved diatom assemblage. The Beaver Divide Conglomerate Member contains large blocks of similar white limestone, some of which have an assemblage of diatoms the same as that of the Wagon Bed Formation but showing excellent preservation. Both the Wagon Bed limestone and the limestone blocks from the Beaver Divide Conglomerate Member have virtually the same assemblages of nonmarine gastropods. A study of the local geology indicates that a marked erosional topography formed on the Wagon Bed strata before deposition of the Beaver Divide sediments. The Beaver Divide limestone blocks were derived from nearby exposures of the Wagon Bed Formation. The diatom assemblage under consideration was originally deposited in the Wagon Bed limestone and is, therefore, late Eocene rather than early Oligocene in age. This represents the earliest known assemblage of nonmarine diatoms from North America.

The diatom assemblage of the Wagon Bed Formation consists of 27 new species, two species known previously from the upper Miocene of France, and five species still found in living assemblages. One new genus containing two species also occurs in the deposit. The Wagon Bed diatom assemblage is strikingly distinct from later Tertiary fresh-water assemblages in that many diatom genera common to the younger deposits are totally lacking in this Eocene formation. These genera may be absent because of a highly specialized paleoecology during deposition of the Wagon Bed limestone, but the meager evidence provided by the few still-living species suggests nothing more specific than deposition in a temperate circumneutral lake.