Type designation:

Type section: The name “Ramp Creek Member” was given by Stockdale (1929, p. 240) to a part of his Lower Harrodsburg Limestone (Rexroad, 1986). The type section is along a stream in the NW¼NW¼ sec. 35, T. 8 N., R. 1 W., Monroe County, Indiana, but the formation is now very poorly exposed there (Rexroad, 1986).

Reference sections: Three reference sections of the Ramp Creek Formation and the overlying Harrodsburg Limestone within 5 miles (8 km) of the type Ramp Creek are: (1) the road cut on Indiana Highway 37 on the north bluff of Clear Creek about 1 mile (1.6 km) northeast of Harrodsburg, mostly in the SW¼SW¼ sec. 20, T. 7 N., R. 1 W.; (2) the cut on former Indiana Highway 37 less than 0.25 miles (0.4 km) west of the location noted immediately above and along a north-south line through the center of the SE¼ sec. 20, T. 7 N., R. 1 W.; and (3) the road cut along the dam-access road at Monroe Lake in the NE¼SE¼ sec. 28, T. 7 N., R. 1 W. (Nicoll and Rexroad, 1975) (Rexroad, 1986).

History of usage:

The name was modified by Smith (1965) to "Ramp Creek Limestone Member" and assigned to the Muldraugh Formation (Rexroad, 1986). Still later, this unit was elevated to formation status by Nicoll and Rexroad (1975) and assigned as the lowest unit in the Sanders Group (Rexroad, 1986).

Nicoll and Rexroad (1975, p. 5-8) recognized that the Ramp Creek and Muldraugh Formations are parts of a single depositional unit (Rexroad, 1986). They retained both names, however, and limited the name Ramp Creek to sediments deposited on the topset surface of the Borden delta (Rexroad, 1986). (See "Borden Group.") The Ramp Creek and Muldraugh units had also been equated by Lineback (1966), who used the older name Ramp Creek Member for the unit, which he considered to be the lower part of his Ullin Limestone (Rexroad, 1986).


The Ramp Creek Formation is dominantly a carbonate unit consisting of interbedded very fine grained dolostone and limestone but containing small amounts of siltstone and shale (Rexroad, 1986). Chert is common to abundant, and geodes are numerous, especially in the dolostone (Rexroad, 1986). The limestone is generally coarse bioclastic calcarenite and calcirudite (Rexroad, 1986). Although the formation is known to range from 16 to 34 ft (5 to 10 m) in thickness, it is characterized by a relatively uniform thickness of about 20 to 25 ft (6 to 8 m) and tends to be even bedded (Rexroad, 1986).

Distribution: The Ramp Creek has been recognized on outcrop in northern Putnam County in west-central Indiana and can be traced from there to southern Harrison County on the Ohio River, where abrupt thickening and facies changes mark its passage into the Muldraugh Formation (Rexroad, 1986). In the subsurface it extends to the southwest from the outcrop to the margin of the Borden deltaic platform, where it merges with the Muldraugh, which is the off-delta basin facies (Rexroad, 1986).


The boundary between the Ramp Creek and the underlying Borden Group is marked by a sharp lithologic break and in most places by a zone of glauconite at the top of the Borden that probably represent a brief hiatus in sedimentation (Rexroad, 1986). The Harrodsburg Limestone conformably overlies the Ramp Creek (Rexroad, 1986).


Nicoll and Rexroad (1975, p. 16, 17) correlated the uppermost part of the Ramp Creek Formation with the lowermost part of the Warsaw Shale of the Mississippi Valley on the basis of the presence 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 of the formation and correlated it with the upper part of the Keokuk Limestone of the Mississippi Valley (Rexroad, 1986).

In 1991, Hirt reported the presence of two diagnostic brachiopod species in the Ramp Creek Formation and Harrodsburg Limestone in south-central Indiana that indicate the Osagean-Meramecian Series boundary lies in the lower part of the Ramp Creek. Marginirugus magnus, an Osagean index fossil occurs in the lower part of the Ramp Creek Formation. Warsawia lateralis, a Meramecian index fossil also occurs in the lower part of the Ramp Creek Formation and in the lower part of the overlying Harrodsburg Limestone.

Regional Indiana usage:

Illinois Basin Margin (COSUNA 12)
Supergroup: none
Group: Sanders Group
Formation: Ramp Creek Formation
Cincinnati Arch (COSUNA 13)
Supergroup: none
Group: Sanders Group
Formation: Ramp Creek 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:

Muldraugh Formation


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.

Hirt, D. S., 1991, Mississippian brachiopod biostratigraphy and the Osagean-Meramecian boundary in south-central Indiana: Journal of Paleontology, v. 65, p. 912-916.

Lineback, J. A., 1966, Deep-water sediments adjacent to the Borden Siltstone (Mississippian) delta in southern Illinois: Illinois State Geological Survey Circular 401, 48 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, Ramp Creek 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. 121-122.

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., 1929, Stratigraphic units of the Harrodsburg Limestone: Indiana Academy of Science Proceedings for 1928, v. 38, p. 233-242.

For additional information contact:

Nancy Hasenmueller (
Date last revised: April 13, 2017