Lower Triassic formations of southeastern Idaho and adjacent areas were deposited along the western margin of an ancient geosyncline (miogeosyncline). Along the eastern margin of this geosyncline (miogeosyncline) the marine formations intertongue with red-bed formations. Several distinct facies maintain rather constant geographic relationships, which are to a degree controlled by rate of subsidence and depth of water, and which reflect the amount of silt- and sand-sized detritus brought into the geosyncline. The major facies type of this geosynclinal area is an intergrading complex of siltstone and limestone, gray and buff and with abundant shell fragments. In southwestern Montana and northern Utah the detrital constituents are mostly of fine-sand size. Two tongues of black shale, siltstone, and limestone indicate periods of greater local subsidence on an otherwise uniform shelf, wherein euxinic, quiet-water conditions prevailed. The gray limestones are all derived from recrystallization of shell debris. The present distribution of the fossil faunas is largely due to factors of preservation. The abundant shell fragments in the gray-bufi siltstone-limestone-sandstone facies and the integradations suggest active bottom currents. Environmental conditions of well-oxygenated bottoms and shallow waters with active bottom currents supported an abundant fauna, rich in numbers of individuals but poor in numbers of species. Pelecypods and brachiopods (including Lingula) are common in this facies, ammonoids are generally uncommon. The fauna of the black facies includes abundant cephalopods and few species of pelecypods and fish. The fossils are well preserved only in limestone concretions or lenses. The relatively impoverished nature of Lower Triassic faunas is the result of wholesale extinctions in the late Paleozoic and not of unfavorable environments.