The Dunleith Formation in Decorah, IA, represents thick, resistant layers of Middle Ordovician dolomite and limestone, forming high bluffs above the Decorah Shale, rich in fossils like echinoderms, brachiopods, and bryozoans, and often featuring distinctive nodular chert (flint) and volcanic ash layers (bentonites), crucial for understanding ancient shallow marine environments and impact structures like the Decorah Crater.