The Maquoketa shale is the upper division of the Ordovician of the upper Mississippi Valley. It lies conformably on the subjacent Galena dolomite and is apparently unconformable with the overlying massive dolomitic beds of the Niagaran. In thickness, the formation varies from 160 to 200 feet or more. In common with the other Paleozoic beds of Iowa, it dips gently to the southwest and outcrops in a belt eight to ten miles wide in the northeastern part of the state; this belt is typically developed in Dubuque County and extends to the northwest into adjacent counties and to the southeast into Jackson County and across Mississippi River into Illinois.
Proceedings of the Iowa Academy of Science
©1914 Iowa Academy of Science, Inc.
Thomas, A. O.
"A New Section of the Railway Cut Near Graf, Iowa,"
Proceedings of the Iowa Academy of Science, 21(1), 225-229.
Available at: https://scholarworks.uni.edu/pias/vol21/iss1/34