Type designation:

Type locality: The Trenton Limestone was apparently named by Vanuxem (1838, p. 257) for rocks that were exposed at Trenton Falls in Oneida County, New York, and that were about 100 ft (30 m) thick (Gray, 1970; Keith, 1986).

History of usage:

In New York: There is some confusion about what rocks were included in the Trenton by Vanuxem in his 1838 and 1842 reports, but they probably consisted of interbedded dark limestone and shale and a light-gray massive crinoidal limestone (Keith, 1986). In the type area the Trenton is now given group status and is subdivided into several formations (Ross and others, 1982; Keith, 1986).

In Indiana: The first published use of the name “Trenton” in Indiana was probably by Phinney (1891), who included rocks presently assigned to the Black River and Ancell Groups except for the St. Peter Sandstone (Keith, 1986). Phinney was correct, however, in equating only the upper part of the Trenton Limestone of Indiana with the Galena Dolomite of Wisconsin and Illinois, although the exact nature of his correlation is vague (Keith, 1986). Use of the name “Trenton” continued in this broad sense in Indiana until Gutstadt (1958) separated the Black River Limestone from the Trenton Limestone (Keith, 1986).


The Trenton consists of limestone that becomes increasingly dolomitic in northern Indiana, and in places it is completely dolomitized. The Trenton is tan to light tannish gray to medium tannish gray (Keith, 1986). The color variation in the limestone is due to the variation in the content of skeletal grains versus micrite; the darker color correlates with the higher micrite content (Keith, 1986). In the dolostone, the size of the crystals appears to be the controlling factor; the more coarsely crystalline phases are lighter colored (Keith, 1986).

The Trenton Limestone is everywhere in the subsurface of Indiana except for far southeastern Indiana as noted below (Keith, 1986). Its only exposure, in the chaotic structure quarried near Kentland in Newton County, was referred by Gutschick (1983) to the Galena Dolomite (Keith, 1986). The Trenton has a maximum thickness of 265 ft (81 m) in Steuben County in northeastern Indiana, and it thins to zero thickness in far southeastern Indiana through what is believed (although not well understood) to be a geographically progressive facies change with the Kope Formation, which is replaced farther southeastward by the Lexington Limestone through a similar facies change (Gray, 1972; Droste and Shaver, 1983; and Keith, 1985; Keith, 1986). This narrow area of dual facies change extends northeastward from Spencer and Perry Counties to eastern Fayette County (Keith, 1985; Keith, 1986).


The contact of the Trenton Limestone with the underlying Plattin Formation of the Black River Group (Droste, Abdulkareem, and Patton, 1982) appears conformable where it is seen in cores, although a distinct lithologic change occurs between the units (Keith, 1986). The Trenton generally underlies the shales of the Maquoketa Group (Gutstadt, 1958, 1958; Gray, 1972), but the relationship with the overlying Maquoketa is not wholly clear (Keith, 1986). The contact in all places where it has been observed appears to represent a discontinuity (hardground) due to a period of probably prolonged submarine exposure (Keith, 1986). A major unconformity and subaerial erosion as suggested by Rooney (1966) do not apply (Keith, 1986). Regionally, the contact between these two units becomes younger and rises stratigraphically to the west because the locus of Maquoketa shale deposition was extended progressively westward from the Appalachian Basin during Late Ordovician time (Keith, 1986). This westward extension therefore accounted for the eventual burying of the Trenton and equivalent carbonate rocks (Keith, 1986).


The Trenton Limestone of Indiana has been variably correlated approximately to exactly with rocks of the same name in Michigan and Ohio; with a section made up of the Lexington Limestone, the Point Pleasant Formation, and the lower part of the Kope Formation in southwestern Ohio; with the Kimmswick and Lexington Limestones and the lower part of the Maquoketa Shale of central and western Kentucky; with the Galena Group of Illinois and with the Lexington Limestone and the lower part of the Kope Formation of southeastern Indiana (Keith, 1986). Probably no part of the Trenton of Indiana, however, is as young as the Dubuque Formation in the upper part of the Galena Group of Illinois (Keith, 1986). See Twenhofel and others, 1954; Gutstadt, 1958; Willman and Buschbach, 1975; Willman and Kolata, 1978; Droste and Shaver, 1983; Gutschick, 1983; and Shaver, 1984 (Keith, 1986).

On the basis of both conodont studies (Waterman, 1975; Sweet, 1979) and physical relations with the lower Kope shale of the type area of the Edenian Stage, the Trenton of Indiana is believed to range in age from Rocklandian (Champlainian Epoch) into Edenian (Cincinnatian Epoch) (Keith, 1986).

Regional Indiana usage:

Illinois Basin (COSUNA 11)
Supergroup: none
Group: none
Formation: Trenton Limestone
Illinois Basin Margin (COSUNA 12)
Supergroup: none
Group: none
Formation: Trenton Limestone
Cincinnati Arch (COSUNA 13)
Supergroup: none
Group: none
Formation: Trenton Limestone
Kankakee Arch (COSUNA 14)
Supergroup: none
Group: none
Formation: Trenton Limestone
Michigan Basin (COSUNA 15)
Supergroup: none
Group: none
Formation: Trenton 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.

See also:

Lexington Limestone


Droste, J. B., Abdulkareem, T. F., and Patton, J. B., 1982, Stratigraphy of the Ancell and Black River Groups (Ordovician) in Indiana: Indiana Geological Survey Occasional Paper 36, 15 p.

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.

Gray, H. H., 1970, Trenton Limestone, in Shaver, R. H., Burger, A. M., Gates, G. R., Gray, H. H., Hutchison, H. C., Keller, S. J., Patton, J. B., Rexroad, C. B., Smith, N. M., Wayne, W. J., and Wier, C. E., Compendium of rock-unit stratigraphy in Indiana: Indiana Geological Survey Bulletin 43, p. 179-180.

Gray, H. H., 1972, Lithostratigraphy of the Maquoketa Group (Ordovician) in Indiana: Indiana Geological Survey Special Report 7, 31 p.

Gutschick, R. C., 1983, Geology of the Kentland Dome structurally complex anomaly, northwestern Indiana (Field Trip 15), in Shaver, R. H., and Sunderman, J. A., eds., Field trips in midwestern geology: Bloomington, Indiana, Geological Society of America, Indiana Geological Survey, and Indiana University Department of Geology, v. 1., p. 105-138.

Gutstadt, A. M., 1958, Cambrian and Ordovician stratigraphy and oil and gas possibilities in Indiana: Indiana Geological Survey Bulletin 14, 103 p.

Gutstadt, A. M., 1958, Upper Ordovician stratigraphy of the eastern interior region: American Association of Petroleum Geologists Bulletin, v. 42, p. 513-547.

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.

Keith, B. D., 1985, Map of Indiana showing thickness, extent, and oil and gas fields of Trenton and Lexington Limestones: Indiana Geological Survey Miscellaneous Map 45.

Keith, B. D., 1986, Trenton 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. 157-158.

Phinney, A. J., 1891, The natural gas field of Indiana: U.S. Geological Survey Annual Report 11, pt. 1, p. 579-742.

Rooney, L. F., 1966, Evidence of unconformity at top of Trenton Limestone in Indiana and adjacent states: American Association of Petroleum Geologists Bulletin, v. 50, p. 533-546.

Ross, R. J., Jr., Adler, F. J., Amsden, T. W., Bergstrom, Douglas, Bergstrom, S. M., Carter, Claire, Churkin, Michael, Cressman, E. A., Derby, J. R., Dutro, J. T., Jr., Ethington, R. L., Finney, S. C., Fisher, D. W., Fisher, J. H., Harris, A. G., Hintze, L. F., Ketner, K. B., Kolata, D. L., Landing, Ed, Neuman, R. B., Sweet, W. C., Pojeta, John, Jr., Potter, A. W., Rader, E. K., Repetski, J. E., Shaver, R. H., Thompson, T. L., and Webers, G. F., 1982, The Ordovician System in the United States–Correlation chart and explanatory notes: International Union Geological Sciences Publication 12, 73 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.

Sweet, W. C., 1979, Conodonts and conodont biostratigraphy of post-Tyrone Ordovician rocks of the Cincinnatian region, in Pojeta, John, Jr., ed., Contributions to the Ordovician paleontology of Kentucky and nearby states: U.S. Geological Survey Professional Paper 1066-G, 26 p.

Twenhofel, W. H., Bridge, Josiah, Cloud, P. E., Jr., Cooper, B. N., Cooper, G. A., Cumings, E. R., Cullison, J. S., Dunbar, C. O., Kay, Marshall, Liberty, B. A., McFarlan, A. C., Rodgers, John, Whittington, H. B., Wilson, A. E., and Wilson, C. W., Jr., 1954, Correlation of the Ordovician formations of North America: Geological Society of America Bulletin, v. 65, p. 247-298.

Vanuxem, Lardner, 1838, Second annual report of the geological survey of the third district of the State of New York: New York Geological Survey Annual Report 2, p. 253-286.

Vanuxem, Lardner, 1842, Geology of New York–Pt. 3, Comprising the survey of the third geological district: Albany, W. & A. White & J. Visscher, 306 p.

Waterman, A. S., 1975, Conodont biostratigraphy, paleontology, and paleoecology of the Trenton and Lexington Limestones in southeastern Indiana: Bloomington, Indiana University, master's thesis, 60 p.

Willman, H. B., and Buschbach, T. C., 1975, Ordovician 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. 47-87.

Willman, H. B., and Kolata, D. R., 1978, The Platteville and Galena Groups in northern Illinois: Illinois State Geological Survey Circular 502, 75 p.

For additional information contact:

Nancy Hasenmueller (
Date last revised: October 31, 2014