Geologic mapping in the South Fork Basin of the western Uinta Mountains, Utah, has yielded data which permit a more accurate interpretation of the position of the pre-Cambrian-Paleozoic contact. Two formations (Ophir and Pine Valley) heretofore assigned to the Cambrian, and thought to be conformable, are separated by a 20° angle of discordance. The plane of the unconformity is proposed as the proper position for the pre-Cambrian contact. Rocks earlier mapped as the Ophir(?) formation of Cambrian age lie beneath the unconformity and are assigned to the pre-Cambrian, and a new name proposed. Other lithic units have been recognized as distinctive formations in the upper pre-Cambrian sequence, and names and correlations proposed. The oldest formation overlying the unconf rmity is Cambrian and is correlated with the Tintic quartzite of adjacent regions.

The Madison formation (Lower Mississippian) was deposited upon an erosional surface composed largely of pre-Cambrian strata, partly of Cambrian strata, and locally of fossiliferous strata of Devonian age. Accordingly, a pre-Madison period of uplift and erosion is postulated.