Historical:Glencoe Member

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Handbook of Illinois Stratigraphy
Series Bulletin 95
Author H. B. Willman, Elwood Atherton, T. C. Buschbach, Charles Collinson, John C. Frye, M. E. Hopkins, Jerry A. Lineback, Jack A. Simon
Date 1975
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Lithostratigraphy: Ottawa Limestone Megagroup >>Galena Group >>Decorah Subgroup >>Spechts Ferry Formation >>Glencoe Member
Chronostratigraphy: Paleozoic Erathem >>Ordovician System >>Champlainian Series >>Trentonian Stage
Allostratigraphy: Tippecanoe Sequence


H. B. Willman and T. C. Buschbach

Name Origin

The Glencoe Member of the Spechts Ferry Formation (Templeton and Willman, 1963, p. 110), which overlies the Castlewood Member, is named for Glencoe, St. Louis County, Missouri, which is 3 miles west of the type section.

Type Section

The type section of the Glencoe Member is the same as that for the Castlewood Member (near Castlewood, St. Louis County, Missouri (NE SE SE 21, 44N-4E)), where the Glencoe is 5.3 feet thick.

Extent and Thickness

The Glencoe Member is commonly 5-8 feet thick in the area near the Mississippi River, but it thins to the east.


The most persistent and thickest Ordovician bentonite, commonly 1-3 inches thick and locally as much as 8 inches thick, occurs in the lower part of the Glencoe Member, interbedded with green, gray, or brown shale. Locally the bentonite is altered to a distinctive, hard, pink bed, which is almost entirely potash feldspar. The Glencoe is largely green shale, but it contains beds of calcarenite, greenish gray argillaceous limestone, and dark purplish gray coarse-grained limestone.


Some beds of the Glencoe Member are a coquina of Pionodema subaequafa. In the Upper Mississippi Valley, the trilobite Isotelus gigas (fig. O-5) is common in an argillaceous limestone bed near the base.


TEMPLETON, J. S., and H. B. WILLMAN, 1963, Champlainian Series (Middle Ordovician) in Illinois: Illinois State Geological Survey Bulletin 89, 260 p.

ISGS Codes

Stratigraphic Code Geo Unit Designation