The Maquoketa formation in the outcrop area has been assigned a maximum thickness of 257 feet by earlier surface workers. Recent subsurface study in the same area reveals a thickness of 300-320 feet of Maquoketa sediments. Until now the correlation of subsurface units to recognized surface members has been hampered by this discrepancy. The recognition and identification of a new unit in the Elgin member of the Maquoketa formation, and a greater measured thickness of the Isotelus zone has resolved this difference
