H. B. Willman and T. C. Buschbach
The Oglesby Member of the Pecatonica Formation (Templeton and Willman, 1963, p. 77) is named for Oglesby, La Salle County.
The type section of the Oglesby Member is in Matthiessen State Park east of Oglesby (NE NE NE 31, 33N-3E), where the member is 12 feet thick.
Extent and Thickness
The Oglesby Member is absent in Illinois north of the type locality, but it is as much as 15 feet thick in southern Illinois.
The Oglesby Member is pure, light gray, white, or buff calcarenite that is laminated, rarely conglomeratic, and locally cross bedded. It is medium to coarse grained in the type locality but fine grained elsewhere. A strong iron-stained corrosion surface occurs on the top and is equivalent to a similar surface on the top of the Medusa Member in northern Illinois, where the Oglesby is absent.
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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