Elwood Atherton, Charles Collinson, and Jerry A. Lineback
The Rocher Member of the Salem Limestone (Baxter, 1960a, p. 27-29), the uppermost member, is named for Prairie du Rocher, Randolph County.
The type section of the Rocher Member is in the Mississippi River bluffs above a large spring 1 mile northwest of the town of Prairie du Rocher, where the member is 69.3 feet thick.
Extent and Thickness
The Rocher thins northward to 10-15 feet at Valmeyer, Monroe County.
The Rocher is mined for high-calcium limestone just north of the type section. The limestone varies from slightly oolitic to oolite. It occurs in thick, cliff-forming beds, broken by an occasional thin bed.
It is composed largely of marine fossils, predominantly Foraminifera, but it also includes crinoid and bryozoan fragments, brachiopods, corals, and echinoids. Endothyrids and calcareous spheres with calcite centers, questionable fossils classified as Calcisphaera, are abundant (Baxter, 1960b). The microfauna is similar to that of the Chalfin and the lower part of the St. Louis.
BAXTER, J. W., 1960a, Salem Limestone in southwestern Illinois: Illinois State Geological Survey Circular 284, 32p.
BAXTER, J. W., 1960b, Calcisphaera from the Salem (Mississippian) Limestone in southwestern Illinois: Journal of Paleontology, v. 34, p. 1153-1157; Illinois State Geological Survey Reprint 1961-B.
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