H. B. Willman and Elwood Atherton
The Markgraf Member of the Joliet Formation (Willman, 1973, p. 21), which overlies the the Brandon Bridge Member, is named for the abandoned Markgraf quarry on the north side of the Des Plaines River, on the southwest side of Joliet, Will County (SW SW 16, 35N-10E).
Extent and Thickness
At the Markgraf quarry the member is 22.1 feet thick. It thins southward and is only 12.3 feet thick where exposed along the Kankakee River at the mouth of Rock Creek Canyon (SW SE SW 32, 32N-11E).
The Markgraf Member is light gray, nearly white, medium-bedded dolomite. Along the Des Plaines River the member consists of three approximately equal units-- a basal very silty dolomite, a middle moderately silty and argillaceous dolomite with bands of chert nodules, and an upper slightly argillaceous dolomite that also contains bands of chert nodules. Along the Kankakee River, where the member is relatively thin, the dolomite all resembles the upper unit along the Des Plaines Valley.
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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