The Milligen Formation at the type area in the Wood River Valley of south-central Idaho has been dated as Devonian rather than Mississippian. For this reason the McGowan Creek Formation was defined for use in east-central Idaho for Lower Mississippian rocks that formerly were called Milligen. In the area between the Wood River Valley and the region where McGowan Creek rocks are present, the lowest formation in the Mississippian Copper Basin Group also has been assigned to the Milligen. Because these rocks differ in age and lithology from the Devonian Milligen and Mississippian McGowan Creek rocks, a revision in nomenclature is required. The Little Copper Formation is proposed as the new name for these rocks. The type section, the basal part of the type section for the Cop er Basin Group, is above Little Copper Creek about 11 km (7 mi) north of the old Muldoon townsite in Blaine County, Idaho. This section formerly was proposed as a reference section for the Milligen Formation.

The Little Copper consists of 1,120 + m (3,700 ft) of thin to medium-bedded, dark-gray argillite with some interbeds of dark quartzite, granule-size conglomerate, and a few limestone lenses. The stratigraphic base of the formation is not exposed and the top is gradational with the overlying Drummond Mine Limestone of late Kinderhookian age. The Little Copper is of Early Mississippian (Kinderhookian) age, and it grades eastward into the lower part of the McGowan Creek Formation. The stratigraphic relation of the Little Copper to the Devonian Milligen is not known, although these older rocks are in thrust contact with various formations of the Copper Basin Group at several localities within the Pioneer Mountains.