Type section and use of name: The name Alum Cave was first used by Logan (1930, p. 168) in a columnar section for an indefinite interval of rock above the Springfield Coal Member in Sullivan County, Ind. Logan's term was modified to "Alum Cave Limestone" by Wanless (1939, table 2) and restricted to the limestone above the coal, although Culbertson (1932) had called the same limestone the Arthur Limestone. This limestone was designated as the Alum Cave Limestone Member in 1950 by Wier and assigned, as the uppermost member, to the Petersburg Formation. It was later assigned, as the lowermost member, to the Dugger Formation by Wier (1961, 1965) in unpublished manuscripts and by Wier and Gray (1961) on the Indianapolis 1/ X 2/ Regional Geologic Map. A type section was designated (Wier, 1961, 1965) near the site of the former town of Alum Cave, Sullivan County, Ind., in the NW¼NE¼ sec. 25, T. 9 N., R. 8 W.
Description: Typically a medium- to blue-gray limestone that is fine grained, argillaceous, locally sandy, and fossiliferous, the Alum Cave in most places is separated into two beds by a few inches of shale. The thickness of the limestone ranges from 0.1 to 11.8 feet (0.03 to 3.6 m); the average thickness is 2.8 feet (0.8 m) (Wier, 1965). It generally contains a fauna rich in crinoid columnals, brachiopods, gastropods, and pelecypods. Fusulinids and ostracods are present in places; trilobites are rare.
The Alum Cave Limestone Member lies 1 to 30 feet (0.3 to 9 m) above the Springfield Coal Member but is generally less than 6 feet (1.8 m) above this coal (Wise, 1961, 1965). The thickest exposures of the Alum Cave are in Sullivan and Greene Counties. It is not found in the subsurface in parts of western Sullivan County and Vigo County and cannot be identified in outcrop in northern Vigo County and Vermillion County. South of Sullivan County the limestone is thin and nodular. It can be traced as gray to brown calcareous fossiliferous shale containing limestone nodules as far south as Pike and Warrick Counties.
Correlation: The calcareous fossiliferous shale extends into Kentucky, where it is called Pennywinkle Rock (presumably a variant of periwinkle) by the miners because of the abundant, poorly preserved gastropods in the shale (Wier, 1961, 1965). The Alum Cave Limestone Member is correlated with the St. David Limestone Member of the Carbondale Formation in Illinois.