Thick successions of quartzose and argillaceous metasediments in the northern part of the Southern Snake Range and the central Schell Creek Mountains of eastern Nevada and the southeastern part of the Deep Creek Range of adjacent Utah are assigned to the late Precambrian and are here named the McCoy Creek Group. This group is overlain concordantly but with sharp boundary by Lower Cambrian Prospect Mountain Quartzite (restricted). The lower part contains minor marble and, in the Deep Creek Range, tillitic schists. The base is not exposed. Regional metamorphism ranges from very low to medium grade and is considered Mesozoic in age. It is not associated with the formation of any migmatites or gneisses in the 3 areas described.