Medusa Member

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Lithostratigraphy: Ottawa Limestone Megagroup >>Platteville Group >>Pecatonica Formation >>Medusa Member
Chronostratigraphy: Paleozoic Erathem >>Ordovician System >>Champlainian Series >>Blackriveran Stage
Allostratigraphy: Tippecanoe Sequence

Primary source

Willman, H. B., Elwood Atherton, T. C. Buschbach, Charles Collinson, John C. Frye, M. E. Hopkins, Jerry A. Lineback, and Jack A. Simon, 1975, Handbook of Illinois Stratigraphy: Illinois State Geological Survey Bulletin 95, 261 p.

Contributing author(s)

H. B. Willman and T. C. Buschbach


Original description

The Medusa Member of the Pecatonica Formation (Templeton and Willman, 1963, p. 77).


Named for the Medusa Cement Company plant, 2 miles northeast of Dixon, Lee County, which is 1.5 miles south of the type section.

Other names


Type section

Type location

The type section of the Medusa Member is in a quarry on the east side of the Rock River Valley (SW SW NW 22, 22N-9E), where the member is 5.2 feet thick.

Type author(s)

Type status

Reference section

Reference location

Reference author(s)

Reference status

Stratigraphic relationships

Extent and thickness

In the northern outcrop area the Medusa Member is commonly 5-7 feet thick, but it thickens to 20 feet in extreme southern Illinois.


In the outcrop area the Medusa Member consists of dolomite or of limestone heavily mottled with dolomite. It is slightly argillaceous, very fucoidal, and thick bedded to massive. South of Ogle County it contains chert nodules.




Well log characteristics


Age and correlation

Environments of deposition

Economic importance



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