Mississippian System

Type and reference sections and history of name: The Mount Pleasant Sandstone Member was named for Mount Pleasant in northeastern Perry County, Ind. The name was initially applied at formational rank (Malott, 1925, p. 112), then was later suppressed in favor of the Illinois term Degonia Sandstone (Malott, 1931, p. 222). Definition of the latter unit (Swann, 1963, p. 41; Willman and others, 1975, p. 162) is not entirely clear and is at variance with Malott's original usage. (See discussion of boundary problems under "Tar Springs Formation.") For these reasons, Malott's original name and concept were reintroduced for outcrop use by Gray (1978, p. 12). A section described by Malott (1925, p. 123-124) near Mount Pleasant was specified as the type section, and a reference section near Tobinsport in southernmost Perry County (Malott, 1925, p. 117-118) was also designated, where the Mount Pleasant Sandstone Member reaches its maximum thickness of 18 feet (5 m) and lies 12 feet (4 m) below the Negli Creek Limestone Member that marks the top of the Tobinsport Formation.

Description: In Malott's initial understanding, the Mount Pleasant in places was confused with basal sandstone of the Mansfield Formation (Morrowan), which in many places lies directly on the Mount Pleasant or appears to take its place stratigraphically as a result of pre-Mansfield erosion (Gray, 1978, p. 12). At the type section, Malott (1925, p. 123) identified 23 feet (7 m) of Mount Pleasant sandstone, but only the lower 14 feet (4 m) is now so assigned, and at the Shank Hill locality (Malott, 1925, p. 118-119) the entire ledge (35 feet, 11 m) is now considered to belong to the Mansfield. As presently understood, the Mount Pleasant Sandstone Member is a ledge- and waterfall-forming sandstone, thinly stratified to cross stratified, quartzitic and in places calcareous, and 3 to 18 feet (1 to 5 m) thick. It is widely present from the type locality southward to the Ohio River but is not known north of Perry County. It lies 55 to 85 feet (17 to 26 m) above the base of the Tobinsport Formation and about 230 feet (70 m) above the Glen Dean Limestone.

Correlation: As judged by its position within the section, the Mount Pleasant Sandstone Member is probably stratigraphically equivalent to some part of the Degonia Sandstone of the standard Chesterian section. The internal stratigraphy of the Degonia is not well understood, however, and there is at present no way to determine what part of that formation is represented by the Mount Pleasant. The term Mount Pleasant has not been applied in the subsurface.