Type designation:

Type locality: The Backbone Limestone was named for the ridge called the Devil's Backbone Ridge along the Mississippi River north of Grand Tower, Jackson County, Illinois (Savage, 1920, p. 173; Droste and Shaver, 1986). The type section is in a quarry in the SE¼SW¼SW¼ sec. 24, T. 10 S., R. 4 W. (Droste and Shaver, 1986).

History of usage:

Extended: The Backbone Limestone was recognized in the subsurface of southwestern Indiana by Collinson and others (1967) (Droste and Shaver, 1986).

Areal extent: Becker (1974, fig. 11C) mapped the areal extent of the Backbone in southwestern Indiana.

Redifinition: Becker and Droste (1978, fig. 2 and p. 4-5) recognized that Backbone-like deposits were in an interfingered relationship with Grassy Knob-like rocks and in a complementary thickness, partial-facies relationship with Clear Creek-like deposits (Droste and Shaver, 1986). They also recognized that the clean coarse carbonate rocks at the top of what had been called the Bailey Limestone more appropriately belonged in the lowermost Backbone (Droste and Shaver, 1986).

In effect, therefore, an arbitrary vertical cutoff was defined between the two parts of the Grassy Knob facies relationship and placed along the farthest basinward extension of the lowest tongue of Backbone-like lithology (Droste and Shaver, 1986). (See Becker and Droste, 1978, fig. 5, sec. GG'; Droste and Shaver, 1987, fig. 7.) This cutoff placed in the Backbone most of the cherty rocks that had been called the Grassy Knob in Indiana (Droste and Shaver, 1986). Droste and Shaver (1987, fig. 7) also recognized the same cutoff boundary as being almost wholly beyond the state boundary.


The Backbone Limestone is characteristically made up of light-colored medium- to coarse-grained, rather pure bioclastic limestone (Droste and Shaver, 1986). The Backbone also has two prominent intervals of drab cherty dolomitic limestone and dolomitic chert that are overlain and underlain by coarse-grained Backbone limestones (Droste and Shaver, 1986). Some glauconite is present.

Distribution: In the subsurface of southwestern Indiana, the Backbone thickens southwestward from an erosional zero to a north-south elongate area of maximum thickness in the westernmost counties south of Vigo County (Droste and Shaver, 1986). The Backbone also thickens southward along this area, so that the thickest deposits may reach 600 ft (183 m) in Posey County. Westward from this area thinning occurs basinward (Droste and Shaver, 1986).


The Backbone Limestone conformably overlies the fine-grained carbonate rocks of the Bailey Limestone (Silurian) in the deeper part of the Illinois Basin and probably conformably and unconformably overlies reef and nonreef rocks of the Wabash Formation (Silurian) toward the basin margin (Droste and Shaver, 1986). Similarly, the Backbone has conformable and unconformable relations with overlying rocks, the Clear Creek Chert (Lower Devonian) and the Jeffersonville Limestone (Middle Devonian) respectively (Droste and Shaver, 1986). The boundary with the Clear Creek is placed where light-colored fine- to medium-grained, somewhat cherty rocks (Clear Creek) become dominant over the coarser and purer carbonate rocks below (Backbone) (Droste and Shaver, 1986).


The Backbone of Indiana extends to rocks of the same name in the Illinois and Kentucky parts of the Illinois Basin, but because of varying definitions in the Illinois Basin simple statements on correlation cannot be made (Droste and Shaver, 1986). In the Indiana definition, lower Backbone rocks correlate (as facies) with the Grassy Knob of extreme southwestern Indiana and adjacent Illinois, and upper Backbone rocks probably have a partial complementary age relationship with lower Clear Creek rocks of Indiana and Illinois (Droste and Shaver, 1986).

The lowermost Backbone beds, common to Indiana and Illinois (often called the upper Bailey in Illinois), have yielded the conodont Icriodus woschmidti (Collinson and others, 1967, p. 940), which indicates an earliest Devonian (Gedinnian) age (early Ulsterian, American standard) (Droste and Shaver, 1986). Other Lower Devonian fossils found in Illinois indicate ages as young as Emsian (late Ulsterian) for the Backbone of Indiana and correlative Illinois rocks. (See Collinson and Atherton, 1975, p. 109-112, and Shaver, 1984.)

Regional Indiana usage:

Illinois Basin (COSUNA 11)
Supergroup: none
Group: New Harmony Group
Formation: Backbone Limestone

Misc/Abandoned Names:

Arthur Limestone

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.

Collinson, Charles, and Atherton, Elwood, 1975, Devonian System, in Willman, H. B., Atherton, Elwood, Buschbach, T. C., Collinson, Charles, Frye, J. C., Hopkins, M. E., Lineback, J. A., and Simon, J. A., Handbook of Illinois stratigraphy: Illinois State Geological Survey Bulletin 95, p. 104-123.

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, Backbone 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. 7-8.

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., 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.

For additional information contact:

Nancy Hasenmueller (
Date last revised: June 19, 2017