Named the Grassy Knob chert for Grassy Knob, 3.5 mi southeast of Grand Tower, Jackson Co., southwestern IL. Consists of more than 125 ft of chert in irregular rough layers 6 to 30 in. thick, with many cavities containing poorly preserved fossils, overlain by 12 to 15 ft of hard, siliceous and sandy limestone in layers 3 to 8 in. thick grading downward into bands of chert containing particles of glauconite. Total thickness is 150 ft or more. Unconformably overlies Bailey limestone and conformably underlies Little Saline limestone or unconformably underlies Clear Creek chert. The Grassy Knob is of Early Devonian age.

Source: GNU records (USGS DDS-6; Reston GNULEX).

Lower Devonian Grassy Knob Chert overlies the Bailey Limestone and consists almost completely of chert. Thickness in the study area is about 200 ft and about 300 ft in the subsurface to the east. Massive chert beds exposed in the roadcut on the county road 3.8 km west of Tamms are probably in the middle part of the Grassy Knob Chert. Underlies Clear Creek Chert.

Source: GNU records (USGS DDS-6; Reston GNULEX).

Grassy Knob Chert mapped in IL. Consists of yellowish-white, thick to thinly bedded, very poorly fossiliferous chert. Grades into underlying Bailey Limestone. Age is Early Devonian.

Source: GNU records (USGS DDS-6; Reston GNULEX).