Type designation:

Type section: The Muldraugh Formation was named by Stockdale (1939, p. 72, 200-201) for a sequence of argillaceous cherty carbonate rocks exposed along a secondary road that crosses the Muldraugh Escarpment south of Phillipsburg, Marion County, Kentucky (Burger, 1970; Rexroad, 1986).

History of usage:

The Muldraugh Formation and the Ramp Creek Formation are laterally equivalent parts of a single depositional unit lying between the clastic sediments of the Borden Group below and the Harrodsburg Limestone above, but in Indiana and Kentucky the name Muldraugh was limited to the thick carbonate sequence south and west of the Borden delta front by Nicoll and Rexroad (1975) (Rexroad, 1986). The term "Muldraugh Formation" was introduced to Indiana by Smith (1965) and it was assigned three members, in ascending order, the Floyds Knob Limestone Member, the Edwardsville Member, and the Ramp Creek Limestone Member (Burger, 1970; Rexroad, 1986).


The Muldraugh Formation is characterized by local and regional variability in thickness and lithology and ranges from about 40 ft (12 m) to more than 400 ft (122 m) in thickness (Rexroad, 1986). The major lithologies are limestone, very fine grained dolostone, and silty dolostone; there are also lesser amounts of shale and siltstone and mixtures of the two (Rexroad, 1986). The limestone and the silty dolostone in places contain abundant crinoid and bryozoan fragments (Rexroad, 1986). Cherty and siliceous intervals are common, and geodes are abundant in the shale and silty dolostone facies (Rexroad, 1986).

Distribution: In Indiana, the Muldraugh Formation is exposed only in southernmost Harrison County, but it is in the subsurface southwest of the Borden delta front, which trends roughly northwestward from Harrison County (Rexroad, 1986).


The formation is separated from the Borden Group (Formation in Kentucky) by a depositional hiatus and is overlain conformably by the Harrodsburg Limestone (Rexroad, 1986).


Nicoll and Rexroad (1975, p. 16, 17) correlated the uppermost part of the Muldraugh Formation with the lowermost part of the Warsaw Shale of the Mississippi Valley region on the basis of conodonts of the Taphrognathus varians-Apatognathus Assemblage Zone (Rexroad, 1986). They found conodonts indicative of the Gnathodus texanus-Taphrognathus Assemblage Zone in the rest and correlated it with the upper part of the Keokuk Limestone of the Mississippi Valley (Rexroad, 1986).

Regional Indiana usage:

Illinois Basin (COSUNA 11)
Supergroup: none
Group: Sanders Group
Formation: Muldraugh Formation
Illinois Basin Margin (COSUNA 12)
Supergroup: none
Group: Sanders Group
Formation: Muldraugh Formation

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:

Ramp Creek Formation


Burger, A. M., 1970, Muldraugh Formation, 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. 114-115.

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.

Nicoll, R. S., and Rexroad, C. B., 1975, Stratigraphy and conodont paleontology of the Sanders Group (Mississippian) in Indiana and adjacent Kentucky: Indiana Geological Survey Bulletin 51, 33 p.

Rexroad, C. B., 1986, Muldraugh Formation, 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. 97-98.

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.

Smith, N. M., 1965, The Sanders Group and subjacent Muldraugh Formation (Mississippian) in Indiana: Indiana Geological Survey Report of Progress 29, 20 p.

Stockdale, P. B., 1939, Lower Mississippian rocks of east-central interior (U.S.): Geological Society of America Special Paper 22, 248 p.

For additional information contact:

Nancy Hasenmueller (
Date last revised: March 21, 2017