Historical:Smithboro Till Member
H. B. Willman and John C. Frye
The Smithboro Till Member of the Glasford Formation was informally named the Smithboro till by Jacobs and Lineback (1969, p. 9) in the south-central Illinois region. It was named for Smithboro, Bond County, 5 miles west of the type locality.
The type section is the Mulberry Grove Section (Jacobs and Lineback, 1969, p. 21), in borrow pits along Interstate Highway 70, SW SW Sec. 31, T. 6 N., R. 1 W., Fayette County.
At the type section it is overlain by the Mulberry Grove Silt. As the Smithboro is the lowest till member of the Glasford Formation in south-central Illinois, it may be equivalent to the Kellerville Till Member, which is the lowest till unit of the Glasford in western Illinois. However, the tills have slightly different compositions, and separate names seem desirable until they are more directly traced between the two regions.
The till is more silty than the Vandalia Till above and is higher in expandable clay mineral content. Its character and composition have been described by Jacobs and Lineback (1969).
Age and correlation
The Smithboro Till is in the Liman Substage of the Illinoian Stage. Till fabric studies (Lineback, in press) suggest that the glacier advanced from the north, which indicates a source in the Lake Michigan Lobe.
JACOBS, A. M., and J. A. LINEBACK, 1969, Glacial geology of the Vandalia, Illinois, region: Illinois State Geological Survey Circular 442, 24 p.
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