H. B. Willman and Elwood Atherton
The Elwood Formation (Willman, 1973, p. 14), which overlies the Wilhelmi Formation in northeastern and north-central Illinois (fig. S-3), is named for the town of Elwood, Will County.
The type section, which is 5 miles north of the town of Elwood, lies in a ravine 3 miles southwest of Joliet, on the southeast side of the Des Plaines River Valley (NW NW SE 36, 35N-9E).
In some areas, particularly in subsurface, very cherty strata previously correlated with the Kankakee Dolomite probably belong to the Elwood Formation. It is lithologically similar to the Blanding Formation in northwestern Illinois and to strata included in the Mayville Formation in Wisconsin.
Extent and Thickness
The Elwood Formation is 27 feet thick in the type section. Although widely present in the Joliet area, it is erratic in occurrence elsewhere. It is exposed locally along the Fox River Valley north of Aurora (SE SW 3, 38N-8E) and has been traced westward in subsurface as far as Peru, La Salle County.
The Elwood Formation appears to be conformable to the Wilhelmi below and the Kankakee above.
The Elwood, which previously was referred to as the upper cherty zone of the Edgewood Formation, is a slightly argillaceous, light brownish gray, thin- to medium-bedded, fine-grained dolomite that contains layers of dense white chert as much as 4 inches thick. Chert forms 40-50 percent of the upper part of the formation but is less abundant downward. The abundance of chert decreases laterally, and in places equivalent beds may not be separable from the Wilhelmi Formation.
WILLMAN, H. B., 1973, Rock stratigraphy of the Silurian System in northeastern and northwestern Illinois: Illinois State Geological Survey Circular 479, 55 p.
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