New geologic mapping and application of stratigraphic details previously unknown in Belt rocks of Middle Proterozoic age in the northwestern corner of Montana has led to the definition of a previously unrecognized thrust belt. The zone, herein named the Libby thrust belt, is a complex of thrust and high-angle normal and reverse faults about 15 mi wide and 100 mi long that extends southerly from the Canadian border through Libby, Montana, to the Hope fault zone. Dominating the bedrock of the area is the Belt Supergroup, a sequence of low-grade metamorphic strata at least 50,000 ft thick that accumulated as siliciclastic and carbonate sediments along the continental margin about 1,400-900 m.y. ago. The lowest exposed strata (Prichard Formation) contain turbidites and black pyritic argillites representing early deposits in a rift zone. The base of the Prichard is not exposed. Above the Prichard is the Ravalli Group, a sequence of quartzites, siltites, and argillites showing shallow-water features and that suggest the rate of basin filling approximated the rate of basin subsidence. The Ravalli Group includes in ascending order the Burke, Revett, St. Regis, Spokane, and Empire Formations. Overlying the Ravalli Group is the middle Belt carbonate, an informal grouping which includes the stratigraphically equivalent and laterally interfingering Wallace and Helena Formations. The Wallace is characterized by carbonate-bearing siliciclastics; whereas, the Helena is characterized by cyclic deposits of calcite-and dolomite-cemented silt beds that alternate with dolomite beds. Overlying the middle Belt carbonate is the Missoula Group, a sequence of red and green siliciclastic beds, some carbonate, and the Purcell Lava. Formations of the Missoula Group in ascending order include the Snowslip, Shepard, and Mount Shields Formations, the Bonner Quartzite, and the McNamara and Libby Formations. Most formations reflect shallow-water deposits of braided stream, mud flat, and shallow-shelf environments. The top of the Missoula Group is eroded and is overlain disconformably at a few places by the Flathead Quartzite of early Middle Cambrian age. Paleozoic sedimentary rocks are sparse in the area and are confined mostly to the Libby thrust belt. They include, in ascending order, the Middle Cambrian Flathead Quartzite, the Wolsey Shale, the Middle and Upper Cambrian dolomite of Fishtrap Creek, and one small outcrop area of unnamed Ordovician quartz arenite and dolomite beds (mapped with Cambrian dolomite).