Type locality, reference section, and description: The Elwren Sandstone was named by Malott (1919, p. 11) for a series of exposures "in the cuts of the Illinois Central Railway" near Elwren, Monroe County, lnd. The name was later changed to Elwren Formation because the unit is not dominantly sandstone but includes much shale and siltstone (Gray and Perry, 1956, p. 1005).
No type section was ever specified for this formation. A section at Rays Cave (Malott, 1952, p. 73-76) in sec. 13, T. 7 N., R. 4 W., Greene County, about 8 miles (13 km) southwest of Elwren, is here designated as a principal reference section. At this locality the formation is 46 feet (14 m) thick and consists of gray shale and flaggy sandstone in about equal parts. (This section is also the type section for the overlying Beech Creek Limestone.)
The Elwren Formation includes thin-bedded fine-grained sandstone, cross bedded sandstone, and green-gray and red-brown shale and mudstone. It ranges from 20 to 60 feet (6 to 18 m) in thickness (Gray and others, 1957, pl. 2) and crops out from southern Putnam County southward to the Ohio River. The Elwren equivalent in the subsurface is the Cypress Formation, which is recognized from southern Parke County southwestward.
The Elwren is overlain conformably by the Beech Creek Limestone (Stephensport Group) or disconformably by the Mansfield Formation (Morrowan). It overlies the Reelsville Limestone conformably except in a few places where a sandstone body in the Elwren rests disconformably on the Reelsville or on the underlying Sample Formation (Malott, 1952, p. 14). The hypothesis of a regional disconformity at this horizon was rejected, however, by Gray and Perry (1956), who considered this relationship to be a local aspect of what later became known as the West Baden clastic belt.
West Baden clastic belt: The West Baden clastic belt, a branching linear area 2 to 8 miles (3 to 12 km) wide and more than 120 miles (200 km) long, is defined by the absence of the marker limestone formations that serve to subdivide the West Baden Group. The clastic belt extends down dip from Owen and Greene Counties through the southwest corner of Indiana and into Kentucky (Sullivan, 1972). Within the clastic belt the generally inseparable clastic formations of the West Baden Group in places aggregate more than 200 feet (60 m) in thickness. In a few localities the lateral transition from limestone to sandstone is abrupt, but more commonly the transition is gradual and is accompanied by a thickening of the limestone and an increasing content of noncarbonate material in the limestone toward the clastic belt. A band of silty to sandy shale a mile or two wide commonly separates each of the limestones from the axial sandstone body. In some places calcareous zones and even fossiliferous zones in the sandstone mark the stratigraphic position of the limestone beds.
The clastic belt marks the locus of virtually continuous clastic sedimentation that took place contemporaneously with the alternating clastic and carbonate rock units that elsewhere typify the West Baden Group. The clastic belt is therefore visualized as a distributary of Swann's (1963) Michigan River system. The limestone formations are regarded as having formed at times when the rate of clastic sedimentation was regionally diminished, so that clastic sedimentation was limited to the distributary belt the clastic formations represent times when clastic influx was more rapid and clastic sediments were dispersed throughout the Illinois Basin. The geometry and lithologic content of the clastic belt have been traced by Sullivan (1972), and additional paleogeographic interpretations were presented by Hrabar and Potter (1969).
Correlation: Malott (1931, p. 222 1952, p. 14) considered the Elwren Formation to be equivalent to the middle part of the Paint Creek Formation of Illinois, even though he was aware that in subsurface usage it was commonly called the Cypress Sand, but Swann and Atherton (1948), among others, demonstrated equivalence of the Elwren to the type Cypress Sandstone of southern Illinois as well as to the Cypress of subsurface usage. The Cypress is a unit in the standard Chesterian section (Swann, 1963, p. 35; Willman and others, 1975, p. 155).
Concept of naming formations in middle and lower parts of the Chesterian Series: In contrast to his manner of naming units in the upper part of the Chesterian (see under "Tar Springs Formation"), Malott (1919), in naming formations in the middle and lower parts of the Chesterian Series, recognized from the first that it was necessary to name entire units of sandstone and shale between the more readily identifiable limestone formations. Although he indicated that the Elwren Formation "... consists of one or more members of sandstone and frequently considerable thicknesses of shale..." (Malott, 1919, p. 11), he named the formation the Elwren Sandstone. Each of the clastic units in the middle and lower parts of the Chesterian Series was likewise designated a sandstone, even though there was never any doubt that each of these units included much shale. The paradox was sometimes avoided by referring the shales to so-called "sandstone horizons" (Malott, 1919), and at other times by contradiction: "The Sample [Sandstone] below the Reelsville is mostly shale" (Malott and Esarey, 1940, p. 7). The sandstones crop out more conspicuously than their actual abundance would suggest the Elwren, for example, is approximately two-thirds shale, siltstone, and mudstone and only one-third sandstone (Gray, Jenkins, and Weidman, 1960, p. 45). Consequently, the clastic units are more appropriately designated formations in line with the suggestion of Gray and Perry (1956), and ambiguity regarding their lithologic content and proper limits (compare boundary problems in the upper Chester cited above) is to some extent avoided.