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BTChronostratigraphic Units
 .Phanerozoic
 ..Paleozoic
 ...Ordovician
 ....Middle Ordovician
 .....Glenwood Shale
RTIowa
 Illinois
 Ohio
 Wisconsin
 Minnesota
 Michigan
 Platteville Formation
SNIn Platteville Formation. NE Iowa, W Illinois, Michigan, S Minnesota, Ohio and S Wisconsin. Added to Thesaurus in 1989.