Type locality and history of name in Indiana: The name Cypress Sandstone was first used by Engelmann (1863, p. 189-190) for massive sandstone exposures in the bluffs along Cypress Creek, Union County, Ill. As redescribed by Swann (1963, p. 35) and Willman and others (1975, p. 155), the Cypress is a complex of sandstone bodies totaling 100 feet (30 m) or more in thickness. The Cypress Sandstone is a unit in the standard Chesterian section.
Following usage established by Malott (1919), the name Cypress Sandstone was for many years incorrectly applied in Indiana to the somewhat younger unit now called the Big Clifty Formation (Swann and Atherton, 1948; Gray and others, 1957, p. 6). The Cypress Formation is equivalent to the Elwren Formation of Indiana outcrop terminology.
Description: In Indiana the term Cypress Formation is now applied only in the subsurface. The formation consists of white fine- to medium-grained sandstone, gray siltstone, and shale and extends from Parke and Owen Counties southwestward. According to Sullivan (1972, p. 20), the Cypress is recognizable in only about 40 percent of its potential area of distribution because over wide areas the underlying Reelsville Limestone is absent nevertheless, it reaches a maximum thickness of 125 feet (38 m) and is "the most prolific oil-bearing [stratigraphic] unit in the Illinois Basin" Sullivan, 1972, p. 20).
The Cypress is underlain conformably by the Reelsville Limestone where the Reelsville is present, but where that formation is absent, in and adjacent to an area called the West Baden clastic belt (see under "Elwren Formation"), the position of the base of the Cypress cannot be determined because of the lithologic similarity of the Cypress to the underlying Sample Formation. The Cypress Formation is overlain conformably by the Beech Creek Limestone or disconformably by the Mansfield Formation (Morrowan).