Decorah Shale  search
BTChronostratigraphic Units
 .Phanerozoic
 ..Paleozoic
 ...Ordovician
 ....Middle Ordovician
 .....Decorah Shale
RTIowa
 Missouri
 Illinois
 Wisconsin
 Minnesota
 Millbrig Bentonite Bed
SNW Illinois, NE Iowa, S Minnesota, N Missouri and SW Wisconsin. Added to Thesaurus in 1989.