Utah’s Humbug Formation is a fossil-bearing geologic unit from the Mississippian, specifically the Meramecian age of the Carboniferous. Known as one of the state’s documented fossil-bearing stratigraphic units, it consists of interlayered limestone, orthoquartzitic sandstone, and dolomite. Its thickness changes greatly across Utah, exceeding 2,100 feet on Stansbury Island but thinning farther east and south to an average of about 600 feet. Fossils reported from the formation include bryozoans, connecting it to the marine life that lived during the Mississippian.