Historical:Lake Forest Member

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Handbook of Illinois Stratigraphy
Series Bulletin 95
Author H. B. Willman, Elwood Atherton, T. C. Buschbach, Charles Collinson, John C. Frye, M. E. Hopkins, Jerry A. Lineback, Jack A. Simon
Date 1975
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Lithostratigraphy: Lake Michigan Formation >>Lake Forest Member
Chronostratigraphy: Cenozoic Erathem >>Quaternary System >>Holocene Series


John C. Frye and H. B. Willman

Name Origin

The Lake Forest Member of the Lake Michigan Formation (Lineback et al., 1970, p. 8) is named for Lake Forest, Lake County.

Type Section

The type section is the interval 16-35.5 cm in core 143, the same core as the type of the South Haven Member.

Extent and Thickness

It is 0-4 feet thick. It is present on the western and southern sides of southern Lake Michigan but is absent from the central and eastern areas.


The member is dark gray silty clay that usually contains many thin beds of black clay. The black beds contain more organic matter than the enclosing deposits and have been radiocarbon dated at 6920 ± 200 radiocarbon years B.P. (sample ISGS-33) and 7050 ± 200 radiocarbon years B.P. (sample ISGS-36). The member is generally siltier than the Winnetka and Sheboygan Members below and less sandy and more compact than the Waukegan Member above.


LINEBACK, J. A., N. J. AYER, and D. L. GROSS, 1970, Studies of Lake Michigan bottom sediments- No. 3. Stratigraphy of unconsolidated sediments in the southern part of Lake Michigan: Illinois State Geological Survey Environmental Geology Notes 35, 35 p.

ISGS Codes

Stratigraphic Code Geo Unit Designation