The southern Absaroka Mountains are composed of layered rocks ranging in age from middle Eocene to Oligocene. Most of the rocks, which reach a maximum thickness of approximately 5000 ft, are volcanic sediments of andesitic composition. Locally, they interfinger with andesite flows and flow breccias. Hague's nomenclature, Early and Late Basic Breccias, is considered inappropriate for these volcanic sedimentary units. Later nomenclature, such as Aycross and Tepee Trail, etc., suggested as correlative with Hague's nomenclature is not considered completely compatible from a mapping standpoint. It is suggested that the terms Pitchfork and Wiggins formations respectively replace the Early and Late Basic Breccia terminology. The 2 formations, then, are completely correlative on a mappable basis with those of Hague's that have been in use for more than 60 years.