Type section: What is now-called the Anvil Rock Sandstone Member was first noted by Owen (1856, p. 45), who recorded the name Anvil Rock for a natural feature of sandstone shaped like an Anvil near Dekoven, Union County, Ky. (2,750 feet from the east line and 1,100 feet from the south line, N-17, Carter coordinates). This sandstone was mapped in Illinois, Kentucky, and Indiana by Hopkins (1958). Unpublished detailed mapping in Posey County by Curtis H. Ault and in Gibson County by Donald L. Eggert (oral communication, 1985) shows that the sandstone is there as channel fill and also as a thin widespread sheet in many places. The name Anvil Rock Sandstone Member, therefore, is here adopted for use in Indiana where it has been mapped south of Sullivan County and is assigned to the Dugger Formation.
The interval cored from 538 to 561 feet (164 to 171 m) in Indiana Geological Survey drill hole (SDH) 285, SE¼NE¼SE¼ sec. 35, T. 5 S., R. 14 W., Posey County, is here designated as an Indiana reference section for the Anvil Rock Sandstone Member.
Description: In SDH 285 the Anvil Rock is a siltstone stone and very fine grained to fine-grained sandstone that is gray to light gray and crossbedded in part and exhibits small black flecks. The sheet phase contains beds of silty mudstone and shale in SDH 285 and in other places. Only a few cores of the sandstone are available in Posey and Gibson Counties, and detailed petrography of the sandstone is not well known for the subsurface in Indiana. The Anvil Rock sandstone is coarser grained in some channel fills described in Illinois than is stated above.
The sheet phase of the Anvil Rock sandstone underlies a gray shale below the Universal Limestone Member and overlies a gray shale above the Hymera Coal and Providence Limestone Members, all of the Dugger Formation. In Posey County channels filled with Anvil Rock sandstone have been cut through underlying members of the Dugger to within about 30 feet of the Springfield Coal Member (Petersburg Formation). The Anvil Rock channel-fill sandstone and the overlying sheet sandstone together exceed 100 feet (30 m) in thickness in places.
A major channel cut through the Herrin Coal Member (Dugger Formation) and filled mostly with Anvil Rock sandstone trends generally northeast-southwest in northwestern Posey County, western Gibson County, and southwestern Knox County (Hopkins, 1958). A second channel filled with Anvil Rock has been mapped m eastern Posey County and extends southward into Kentucky and probably connects with the channel noted first above.
Available information indicates no contemporaneity of deposition of the Anvil Rock with the Herrin coal, which does not appear to thicken or split as it approaches the sandstone-filled channels in Indiana.
Correlation: Part of the Anvil Rock Sandstone Member occupies the same stratigraphic position as the Bridge Junction Sandstone Member mapped by Friedman (in preparation) in parts of Vigo County, Ind. No direct correlation between the sandstones, however, has yet been shown, although they may well be the same sandstone.