Outcrops of the Maquoketa shale are limited to the eastern portions of Jackson County. Their position is controlled chiefly by the gentle regional dip to the southwest which in the north brings the basal beds and the underlying Galena formation to the surface and to the south in Clinton County causes the uppermost Maquoketa beds to disappear under the Silurian. This dip also allows the Maquoketa outcrops to extend progressively farther upstream in the beds of the northern east-flowing creeks.
Proceedings of the Iowa Academy of Science
©1924 Iowa Academy of Science, Inc.
Ladd, H. S. and Shale, Maquoketa
"Notes on the Geology of Jackson County, Iowa,"
Proceedings of the Iowa Academy of Science, 31(1), 341-345.
Available at: https://scholarworks.uni.edu/pias/vol31/iss1/109