H. B. Willman and Elwood Atherton
The Offerman Member of the Kankakee Formation (Willman, 1973, p. 17), which overlies the Drummond Member, is named for Offerman School, three-fourths of a mile southeast of the type section.
The type section of the Offerman Member is located in the same exposure as the Drummond Member, in a railroad cut on the southeast side of the Des Plaines River Valley, 2 miles southwest of Joliet, Will County (NW SE SW 30, 35N-10E).
Extent and Thickness
In the type section the Offerman Member is only 2.5 feet thick, a minimum thickness, but it more commonly is about 3 feet thick along the Kankakee River and 6-11 feet thick in quarries at Joliet, Elmhurst, and Hillside.
The Offerman Member differs from the underlying and overlying members in being more argillaceous, less vesicular, and thinner bedded, and in having smoother bedding surfaces and a lighter colored weathered surface.
WILLMAN, H. B., 1973, Rock stratigraphy of the Silurian System in northeastern and northwestern Illinois: Illinois State Geological Survey Circular 479, 55 p.
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