Much of the Great Smoky Mountains, which span the boundary between Tennessee and North Carolina, is formed of the Ocoee series, of later Precambrian age. This is a body of terrigenous clastic sedimentary rocks, which has minor intercalations of limestone and dolomite but no volcanic components or known fossils. The series is probably 30,000 feet or more thick. It lies unconformably on a basement of earlier Precambrian granitic and gneissic rocks, and on the northwest side of the mountains it is overlain by the Cochran formation, or basal unit of the Chilhowee group, which is of Cambrian and Precambrian(?) age. South of the mountains it is overlain by rocks of the Murphy marble belt; here, the top of the Ocoee is placed tentatively at the base of the Nantahala slate. The Ocoee series is divisible into three broad units of regional extent and contrasting lithologic character, which are herewith designated groups and named the Snowbird group, the Great Smoky group, and the Walden Creek group. The groups consist of local intergrading and intertonguing formations and have complex stratigraphic and structural relations. The Ocoee series is split by major thrust faults into three sequences, a southern, central, and northern, none of which contains more than two groups of the series. The lowest group, the Snowbird, is best developed in the central sequence where it is more than 13,000 feet thick; it is much thinner in the southern sequence. In both sequences it lies unconformably on granites and gneisses. Toward the east it is largely sandstone, but this is interbedded with and intertongues westward into finer-grained sandstone, siltstone, and argillaceous rocks. These differences permit the Snowbird group to be divided, in ascending order, into the Wading Branch formation, Longarm quartzite, Roaring Fork sandstone, and Pigeon siltstone. Toward the west the Snowbird is represented by more strongly metamorphosed rocks termed the Metcalf phyllite. Above the Snowbird group in the southern sequence is the Great Smoky group, more than 25,000 feet thick. The lower part, termed the Elkmont sandstone, is fine-grained sandstone. The middle part, termed the Thunderhead sandstone, is poorly sorted, coarse, feldspathic sandstone in graded beds. The upper part, termed the Anakeesta formation, includes many beds of dark argillaceous and silty rocks. South of the Great Smoky Mountains the Great Smoky group is overlain by the rocks of the Murphy marble belt. The Snowbird group in the central sequence is overlain in a few places by coarser-grained rocks. These somewhat resemble those of the Great Smoky group, but their stratigraphic relations to it are not demonstrable, so that they are left as unclassified parts of the Ocoee series. In one area such rocks are named the Rich Butt sandstone, in another the Cades sandstone. In the northern sequence the Walden Creek group, about 8000 feet thick, forms a varied assemblage of argillaceous and silty rocks and quartz-pebble conglomerate, with locally prominent quartzite, limestone, and dolomite. The Walden Creek group is divided, in ascending order, into the Licklog formation, Shields formation, Wilhite formation, and Sandsuck formation; the Wilhite is in turn divided into the Dixon Mountain member and Yellow Breeches member. The Walden Creek is overlain by the Chilhowee group of Cambrian and Precambrian(?) age but is in fault contact with the Snowbird group on the southeast; it is not in contact with the Great Smoky group. Northeast of the Great Smoky Mountains, the Walden Creek group is reported to overlie the Snowbird group as part of a sedimentary sequence from the basement rocks upward through the Chilhowee group into the overlying Paleozoic. © 1958, The Geological Society of America, Inc.