Type designation:

Type area: The name “Clear Creek Limestone” was first applied to a few hundred feet of cherty carbonate rocks exposed in Jackson, Union, and Alexander Counties, southwestern Illinois, by Worthen (1866, p. 126-129) (Droste and Shaver, 1986). The name was taken from Clear Creek in Union County (Droste and Shaver, 1986).

History of usage:

Reassignment: The lower part of this sequence was assigned to the Bailey Limestone (E. O. Ulrich as published by Buckley and Beuhler, 1904, p. 110) (Droste and Shaver, 1986). The Bailey at that time was considered to be Devonian in age (Droste and Shaver, 1986).

Revised: The term "Clear Creek Chert" was used in 1908 (Savage, p. 431-443) for the restricted unit, and in 1920 (Savage, p. 169-178) the Clear Creek Chert of southwestern Illinois was further defined as overlying the Backbone Limestone (Droste and Shaver, 1986).


The Clear Creek Chert consists of light-yellowish-gray to whitish fine- to medium-grained limestone that is cherty and somewhat glauconitic (Droste and Shaver, 1986). The thickness of the Clear Creek ranges from an erosional zero along its northeastern limits to 300 ft (92 m) at the tristate corner of Illinois, Indiana, and Kentucky (Droste and Shaver, 1987, fig. 8).

Distribution: The distribution of the Clear Creek Chert in the Illinois Basin was delineated by Collinson and others (1967) and, more specifically for the southwestern Indiana subsurface area, by Becker (1974, p. 28-29), Becker and Droste (1978, p. 5), and Droste and Shaver (1987) (Droste and Shaver, 1986).


The Clear Creek in the subsurface of Indiana (Becker, 1974, fig. 11D; Droste and Shaver, 1987, fig. 8) almost everywhere overlies the Backbone Limestone in an apparently conformable and onlapping relationship (Droste and Shaver, 1986). The contact is placed at the change from finer grained and somewhat cherty carbonate rocks above to coarser grained and purer carbonate rocks below (Droste and Shaver, 1986). Only in a small part of southwestern Posey County in its Indiana distribution does the Clear Creek rest directly on the Grassy Knob Chert; this contact is placed at the level within a 10-ft (3-m) gradational interval where chert becomes a major component downward (Droste and Shaver, 1986). The Clear Creek is overlain both conformably (farthest southwest) and unconformably by the Jeffersonville Limestone (Middle Devonian), including the Dutch Creek Sandstone Member (Droste and Shaver, 1986). Where it is conformable, the contact is placed amid transitional lithology (Droste and Shaver, 1986).


The Clear Creek of Indiana extends into the Clear Creek of Illinois and Kentucky (Droste and Shaver, 1986). In the interpretation of an onlapping Backbone-Clear Creek relationship, the Clear Creek is not wholly isochronous and has some age equivalency with the Backbone itself (Droste and Shaver, 1986).

Indicator fossils from the Clear Creek of Indiana have not been described, but an abundant fauna in exposures in southwestern Illinois suggested a late Siegenian and Emsian (late Ulsterian) age to Collinson and others (1967) but only an Emsian age to Boucot and Johnson (1968, p. B3) (Droste and Shaver, 1986).

Regional Indiana usage:

Illinois Basin (COSUNA 11)
Supergroup: none
Group: New Harmony Group
Formation: Clear Creek Chert

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.

Boucot, A. J., and Johnson, J. G., 1968, Stratigraphy and paleontology of the Bois Blanc Formation in New York: U.S. Geological Survey Professional Paper 584-B, p. B1-B27.

Buckley, E. R., and Buehler, H. A., 1904, Quarrying industry of Missouri: Missouri Bureau of Geology and Mines, 2nd Series, v. 2, 371 p.

Collinson, Charles, James, G. W., Swann, D. H., Becker, L. E., Carlson, M. P., Dorheim, F. H., and Koenig, J. W., 1967, Devonian of north-central region, United States, in International symposium on the Devonian System: Alberta Society of Petroleum Geologists, v. 1, p. 933-971.

Droste, J. B., and Shaver, R. H., 1986, Clear Creek Chert, 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. 28-29.

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.

Savage, T. E., 1908, On the lower Paleozoic stratigraphy of southwestern Illinois: American Journal of Science, v. 25, p. 431-443.

Savage, T. E., 1920, Devonian formations of Illinois: American Journal of Science, v. 49, p. 169-182.

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.

Worthen, A. H., 1866, Geology of Illinois: Illinois State Geological Survey, v. 1, 152 p.

For additional information contact:

Nancy Hasenmueller (
Date last revised: June 22, 2017