Grassy Knob Chert

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Lithostratigraphy: Hunton Limestone Megagroup >>Grassy Knob Chert
Chronostratigraphy: Paleozoic Erathem >>Devonian System >>Lower Devonian Series
Allostratigraphy: Tippecanoe Sequence

Primary source

Willman, H. B., Elwood Atherton, T. C. Buschbach, Charles Collinson, John C. Frye, M. E. Hopkins, Jerry A. Lineback, and Jack A. Simon, 1975, Handbook of Illinois Stratigraphy: Illinois State Geological Survey Bulletin 95, 261 p.

Contributing author(s)

Charles Collinson and Elwood Atherton


Original description

The Grassy Knob Chert (Savage, 1925b, p. 139-144).


Named for Grassy Knob, a prominent high area on the Mississippi River bluffs 4 miles east of Grand Tower, Jackson County.

Other names


Type section

Type location

The type section of the Grassy Knob Chert is in the west face of the knob (SW SW 27, 10S-3W).

Type author(s)

Type status

Reference section

Reference location

Reference author(s)

Reference status

Stratigraphic relationships

The Grassy Knob is gradational to the Bailey Limestone below. It is more sharply differentiated from the Backbone Limestone above, although no unconformity is evident. In the southern part of the Devonian outcrop area, where the Backbone is absent, the Grassy Knob is not readily separated from the Clear Creek Chert.

Extent and thickness

The formation is exposed in and near the Mississippi River bluffs in Jackson, Union, and Alexander Counties and occurs in subsurface through the deep part of the Illinois Basin (Collinson et al., 1967a). It is about 200 feet thick in the outcrop area but thickens to about 300 feet in the basin to the east.


The Grassy Knob differs from the Bailey Formation below in being lighter colored and in containing more thick beds of solid chert, especially in the middle part of the formation (Weller, 1940), which is well exposed 2 miles southeast of Reynoldsville, Union County (NW cor. 32, 13S-2W).




Well log characteristics


Fossils are rare in the Grassy Knob Chert.

Age and correlation

Environments of deposition

Economic importance



COLLINSON, CHARLES, L. E. BECKER, G. W. JAMES, J. W. KOENIG, and D. H. SWANN, 1967a, Illinois Basin, in International symposium on the Devonian System: Alberta Society of Petroleum Geologists, v. 1, p. 940-962; Illinois State Geological Survey Reprint 1968-G.
SAVAGE, T. E., 1925b, Oriskany rocks in Illinois: American Journal of Science, v. 10, p. 139-144.
WELLER, J. M., 1940, Geology and oil possibilities of extreme southern Illinois: Illinois State Geological Survey Report of Investigations 71, 71 p.

ISGS Codes

Stratigraphic Code Geo Unit Designation