Historical:Winnetka 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 >>Winnetka Member
Chronostratigraphy: Cenozoic Erathem >>Quaternary System >>Holocene Series


John C. Frye and H. B. Willman

Name Origin

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

Type Section

The type section is the interval 35.5- 112 cm in core 143, the same core as the type of the South Haven Member.

Extent and Thickness

It is 1-6 feet thick.

Stratigraphic Position

It rests on the South Haven and Sheboygan Members and the Equality Formation, and it is overlain by the Lake Forest and Waukegan Members. The Winnetka is absent where the Wedron or Equality Formations are near the sediment surface.


The member consists of brownish gray clay with a few interspersed black beds. It becomes somewhat sandy near the shore, but generally it is finer grained than the overlying members.


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