Type designation:

Type area: The St. Clair Limestone was named by Penrose (1891, p. 102-103) for exposures at St. Clair Springs 8 miles (12.9 km) northeast of Batesville, Independence County, Arkansas (Droste and Shaver, 1986).

History of usage:

Lowenstam (1949, p. 13) applied the name to the crinoidal limestones of early Niagaran age that extend discontinuously from the type area through western Tennessee and into the Illinois Basin parts of Kentucky and Illinois (Droste and Shaver, 1986). Becker (1974, fig. 8 and p. 16) adopted the name for use in the subsurface and in an area of eight-county size in southwestern Indiana (Droste and Shaver, 1986). The term has had further definition through the work of Becker and Droste (1978) and Droste and Shaver (1980 and 1987), including what amounts to a vertical cutoff boundary with rocks of the Salamonie Dolomite and the Louisville Limestone (Droste and Shaver, 1986).


In southwestern Indiana the St. Clair Limestone is a grayish-red to intense ferruginous-red and yellowish-gray fine- to medium-grained limestone (Droste and Shaver, 1986). The basic matrix color is mostly whitish to yellowish, but the other colors are expressed partly as beds and partly as splotches and mottlings down to the size of individual carbonate grains (Droste and Shaver, 1986).

The St. Clair ranges from 30 to 90 ft (9 to 27 m) in thickness and averages about 60 ft (18 m) in thickness in the subsurface of southwestern Indiana (Droste and Shaver, 1986).


The St. Clair rests unconformably on the brown cherty rocks of the Sexton Creek Limestone, whereas the upper contact with the Moccasin Springs Formation generally involves an upward transition through several feet of interbedded pure limestones and argillaceous limestones (Droste and Shaver, 1986). The formational contact is usually placed at the base of the transitional zone (Droste and Shaver, 1986).


As proposed by Droste and Shaver (1980, p. 569, and 1985, fig. 1), the upper part of the St. Clair may be diachronous between its basin position and farthest shelfward position (Droste and Shaver, 1986). Therefore, parts of the St. Clair may correlate variably with other formations, the Salamonie Dolomite, the Waldron Shale, the Louisville Limestone, and the lower parts of the Moccasin Springs and Pleasant Mills Formations of Indiana; with most of the Joliet Formation (except the lower part of the Brandon Bridge Member), the Sugar Run Formation, and the lower part of the Racine Formation in northeastern Illinois; and with the Osgood Formation upward through the Lego Limestone of western Kentucky (Droste and Shaver, 1986).

Much of this correlation is based on physical and geophysical tracing, as direct faunal ages for the St. Clair of Indiana are lacking, but ages from early into late Niagaran and late Llandoverian into early Ludlovian in the American and global standards respectively have been assigned (Shaver, 1984; Droste and Shaver, 1986).

Regional Indiana usage:

Illinois Basin (COSUNA 11)
Supergroup: none
Group: Bainbridge Group
Formation: St. Clair Limestone

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., 1974, Silurian and Devonian rocks in Indiana southwest of the Cincinnati Arch: Indiana Geological Survey Bulletin 50, 83 p.

Becker, L. E., and Droste, J. B., 1978, Late Silurian and Early Devonian sedimentologic history of southwestern Indiana: Indiana Geological Survey Occasional Paper 24, 14 p.

Droste, J. B., and Shaver, R. H., 1980, Recognition of buried Silurian reefs in southwestern Indiana: Journal of Geology, v. 88, p. 567-587.

Droste, J. B., and Shaver, R. H., 1985, Comparative stratigraphic framework for Silurian reefs–Michigan Basin to the surrounding platforms, in Cercone, K. R., ed., Symposium on the Silurian and Ordovician of the Michigan Basin: Michigan Basin Geological Society Special Paper 4, p. 73-93.

Droste, J. B., and Shaver, R. H., 1986, St. Clair Limestone, 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. 125.

Droste, J. B., and Shaver, R. H., 1987, Upper Silurian and Lower Devonian stratigraphy of the central Illinois Basin: Indiana Geological Survey Special Report 39, 29 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.

Lowenstam, H. A., 1949, Niagaran reefs in Illinois and their relation to oil accumulation: Illinois State Geological Survey Report of Investigations 145, 36 p.

Penrose, R. A. F., Jr., 1891, The Batesville region of Arkansas: Arkansas Geological Survey Annual Report 1890, v. 1, p. 33-300.

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.

For additional information contact:

Nancy Hasenmueller (
Date last revised: February 12, 2015