Type designation:

Type locality: The term “Ironton Member” (Thwaites, 1923, p. 550) was first applied to exposures of several feet of coarse-grained sandstone, basalmost in the Franconia Formation, near Ironton, Sauk County, Wisconsin (Droste and Patton, 1986).

History of usage:

Most later use of the name in the upper Midwest has been at formation rank (Droste and Patton, 1986).


The Ironton Sandstone was identified (Becker, Hreha, and Dawson, 1978) in the subsurface of northwestern Indiana as overlying the Galesville Sandstone and underlying the Franconia Formation. There it consists of medium- to coarse-grained sandstone and some interbedded dolomitic sandstone (Droste and Patton, 1986). Eastward and southward these sandstones become more dolomitic and shaly, so that nomenclaturally the Ironton is arbitrarily cut off in central northern and western Indiana and replaced by the term "Davis Formation" (Droste and Patton, 1986). In several counties in northwestern Indiana the Ironton can be distinguished from the underlying Galesville Sandstone, but in other counties where the name "Davis Formation" is not used the two formations cannot be recognized separately and are mapped together as a single unit (Droste and Patton, 1986).

The Ironton and Galesville together range in thickness from about 100 ft (30 m) at the position of arbitrary cutoff with the Davis Formation to more than 200 ft (61 m) in the northwest corner of Indiana (Droste and Patton, 1986). Where the Ironton is distinguishable from the Galesville, the formation is about 100 ft (30 m) thick (Droste and Patton, 1986).


The Ironton Sandstone is recognized by the one name in Indiana, Illinois, and Wisconsin, and it has stratigraphic equivalency with part of the Davis Formation east and south of the cutoff line in northwestern Indiana (Droste and Patton, 1986). The Ironton is also equivalent to the Kerbel Formation in Ohio as recognized by Janssens (1973) and the lower part of the Franconia Formation in the Lower Peninsula of Michigan (Droste and Shaver, 1983; Shaver, 1984; Droste and Patton, 1986).

Regional Indiana usage:

Illinois Basin Margin (COSUNA 12)
Supergroup: Potsdam Supergroup
Group: Munising Group
Formation: Ironton Sandstone
Kankakee Arch (COSUNA 14)
Supergroup: Potsdam Supergroup
Group: Munising Group
Formation: Ironton Sandstone
Michigan Basin (COSUNA 15)
Supergroup: Potsdam Supergroup
Group: Munising Group
Formation: Ironton Sandstone

Misc/Abandoned Names:


Geologic Map Unit Designation:


Note: Hansen (1991, p. 52) in Suggestions to authors of the reports of the United States Geological Survey noted that letter symbols for map units are considered to be unique to each geologic map and that adjacent maps do not necessarily need to use the same symbols for the same map unit. Therefore, map unit abbreviations in the Indiana Geologic Names Information System should be regarded simply as recommendations.

COSUNA areas and regional terminology

Names for geologic units vary across Indiana. The Midwestern Basin and Arches Region COSUNA chart (Shaver, 1984) was developed to strategically document such variations in terminology. The geologic map (below left) is derived from this chart and provides an index to the five defined COSUNA regions in Indiana. The regions are generally based on regional bedrock outcrop patterns and major structural features in Indiana. (Click the maps below to view more detailed maps of COSUNA regions and major structural features in Indiana.)

COSUNA areas and numbers that approximate regional bedrock outcrop patterns and major structural featuers in Indiana.

Major tectonic features that affect bedrock geology in Indiana.


Becker, L. E., Hreha, A. J., and Dawson, T. A., 1978, Pre-Knox (Cambrian) stratigraphy in Indiana: Indiana Geological Survey Bulletin 57, 72 p.

Droste, J. B., and Patton, J. B., 1986, Ironton Sandstone, in Shaver, R. H., Ault, C. H., Burger, A. M., Carr, D. D., Droste, J. B., Eggert, D. L., Gray, H. H., Harper, Denver, Hasenmueller, N. R., Hasenmueller, W. A., Horowitz, A. S., Hutchison, H. C., Keith, B. D., Keller, S. J., Patton, J. B., Rexroad, C. B., and Wier, C. E., Compendium of Paleozoic rock-unit stratigraphy in Indiana–a revision: Indiana Geological Survey Bulletin 59, p. 63.

Droste, J. B., and Shaver, R. H., 1983, Atlas of early and middle Paleozoic paleogeography of the southern Great Lakes area: Indiana Geological Survey Special Report 32, 32 p.

Hansen, W. R., 1991, Suggestions to authors of the reports of the United States Geological Survey (7th ed.): Washington, D.C., U.S. Geological Survey, 289 p.

Janssens, Adriaan, 1973, Stratigraphy of the Cambrian and Lower Ordovician rocks in Ohio: Ohio Geological Survey Bulletin 64, 197 p.

Shaver, R. H., coordinator, 1984, Midwestern basin and arches region–correlation of stratigraphic units in North America (COSUNA): American Association of Petroleum Geologists Correlation Chart Series.

Thwaites, F. T., 1923, Paleozoic rocks found in deep wells in Wisconsin and northern Illinois: Journal of Geology, v. 31, p. 529-555.

For additional information contact:

Nancy Hasenmueller (
Date last revised: April 5, 2017