Historical:Strawbridge 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 >>Platteville Group >>Plattin Subgroup >>Quimbys Mill Formation >>Strawbridge Member
Chronostratigraphy: Paleozoic Erathem >>Ordovician System >>Champlainian Series >>Blackriveran Stage
Allostratigraphy: Tippecanoe Sequence


H. B. Willman and T. C. Buschbach

Name Origin

The Strawbridge Member at the top of the Quimbys Mill Formation (Templeton and Willman, 1963, p. 92) is named for Strawbridge, Lafayette County, Wisconsin, 3 miles southwest of the type section.

Type Section

The type section of the Strawbridge Member is located in the same type section as Quimbys Mill (a small quarry near Etna, Lafayette County, Wisconsin (SE SE 11, 1N-1E)) where it is 7.8 feet thick.

Extent and Thickness

The Strawbridge Member is about 7 feet thick in the northern outcrop area in Illinois and 10-15 feet thick in the southern outcrop area.


The Strawbridge Member is similar in lithology to the Shullsburg Member but is less argillaceous, generally thicker bedded, and has fewer and thinner shale partings. It is partly cherty, especially at the top, in northern Illinois.


The Strawbridge Member contains numerous fucoids, which are scarce in the older members of the formation and much smaller than those in the underlying Nachusa Formation.


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