H. B. Willman and T. C. Buschbach
The Wall Member of the Dunleith Formation (Templeton and Willman, 1963, p. 123) overlies the Sherwood Member and is named for Wall Street in East Dubuque, Jo Daviess County, near the type section.
The type section of the Wall Member is part of the Dunleith Formation type section (in an exposure in the Mississippi River bluffs on the north side of East Dubuque (SE 19, 29N-2W))(fig. O-2B), where the Wall is 9.7 feet thick.
Extent and Thickness
The Wall Member is commonly 10-12 feet thick.
The Wall Member is less argillaceous than the Sherwood below and more argillaceous than the Wyota above. The argillaceous zone at the top is not prominent, but the upper 1-3 feet commonly has more argillaceous mottling and thin, wavy shale partings than the beds below. A widespread bentonite bed as much as 4 inches thick occurs near the middle of the member. Chert nodules are common, and the beds above the bentonite are more cherty than those below.
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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