H. B. Willman and T. C. Buschbach
The Scales Shale (Templeton and Willman, 1963, p. 135, in which it was called the Scales Formation), the "lower shale" formation in the Maquoketa Group in most of Illinois, is named for Scales Mound, Jo Daviess County.
The type section of the Scales Shale Formation is a railroad cut on the west side of the town (SW NE SW 26, 29N-2E). Only the lower 30 feet, overlying the Dubuque Formation, is exposed in the type section, but the upper 18 feet is exposed 5 miles east in another railroad cut (SW SW SW 15, 29N-3E).
The Scales Shale has been called "the lower brown shale member" (Willman and Payne, 1942), "the lower shale zone" (DuBois, 1945), or the Scales Formation (Templeton and Willman, 1963).
Extent and Thickness
The Scales Shale is extensive but is exposed only in northwestern Illinois and in small areas near West Point Landing in Calhoun County, Valmeyer in Monroe County, and Thebes in Alexander County. The formation is generally 75-100 feet thick, but it ranges from 50-150 feet thick.
The lower part is generally dark gray to dark brown shale, which is differentiated as the Elgin Shale Member. At the top of the Scales is a gray shale- the Clermont Shale Member- that contains beds of argillaceous limestone. In the southwestern part of Illinois, brown sandstone and siltstone at the base of the Scales is differentiated as the Thebes Sandstone Member, and the shale overlying it is the Orchard Creek Shale Member.
DUBOIS, E. P., 1945, I. Subsurface relations of the Maquoketa and "Trenton" Formations in Illinois: Illinois State Geological Survey Report of Investigations 105, p. 7-33.
TEMPLETON, J. S., and H. B. WILLMAN, 1963, Champlainian Series (Middle Ordovician) in Illinois: Illinois State Geological Survey Bulletin 89, 260 p.
WILLMAN, H. B., and J. N. PAYNE, 1942, Geology and mineral resources of the Marseilles, Ottawa, and Streator Quadrangles: Illinois State Geological Survey Bulletin 66, 388 p.
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