Individual detrital monazite grains from the Bonner Formation of the Belt Supergroup in southwestern Montana have been analyzed using U-Pb and Sm-Nd isotopes. Combined with single-grain U-Pb analyses of detrital zircon, these data place constraints on the age and isotopic characteristics of the source area, previously inferred to be to the west and south of the Belt basin. U-Pb ages from eight single zircon grains range from 1.70 to 1.86 Ga; two grains were ca. 2.6 Ga. U-Pb ages from five monazite grains range from 1.64 to 1.79 Ga. Sm-Nd analyses of these same monazite grains yield ϵNd at the time of crystallization of +0.7 to -14.5. These data suggest that the source area for this part of the Bonner Formation was an Early Proterozoic magmatic (orogenic?) belt with some reworked Archean material. It is postulated that this source area may have been the western edge of Archean crust (Wyoming craton) that was involved in younger magmatism, possibly continuous with rocks of similar age in southcentral Montana, the subsurface of Alberta, or the southwestern United States.