H. B. Willman and T. C. Buschbach
The Elm Member of the Nachusa Formation (Templeton and Willman, 1963, p. 89) is named for Elm Street in Dixon, Lee County, just southwest of the type section.
The type section of the Elm Member is part of the Nachusa Formation type section (a quarry on the east edge of Dixon (SE SE SW 33, 22N-9E) where it is 2.8 feet thick.
The Elm Member is equivalent to the Glenburnie Member, a similar shaly unit, in the Chaumont Formation in New York.
Extent and Thickness
The Elm Member is widely present, but it is absent in subsurface in western Illinois. It is 1-4.5 feet thick in the northern outcrop area, is 7 feet thick in Calhoun County and near Ste. Genevieve, Missouri, and it thickens southward to 15 feet at Cape Girardeau.
The Elm Member consists of argillaceous, gray, cherty dolomite or limestone and is mainly nonfucoidal. It is thin bedded and has thin shale partings. It commonly weathers to a smooth surface, which distinguishes it from the members above and below that have pitted surfaces.
TEMPLETON, J. S., and H. B. WILLMAN, 1963, Champlainian Series (Middle Ordovician) in Illinois: Illinois State Geological Survey Bulletin 89, 260 p.
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