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Outcrops in central and Southern Illinois, near Dongola Illinois downstream from Big Creek, along Clear Creek near Cobden IL, and in the eastern Shawnee Hills. Can be found as nodules or as bedded chert. The round nodular chert frequently is banded with alternating dark and light bands. Historically the term Dongola was used to describe the banded chert, while Cobden was used for the unbanded chert. Since the two types were found to be from the same outcrop, they are now lumped together.
Has historically been called Dongola, Dongola series, Anna, Illinois Hornstone, St. Louis Ball chert, and Cobden banded. Looks very much like Harrison county chert, Indiana and similar to Bayport chert, Michigan and some St. Louis Green chert.
Color is from a medium to dark grey and can have lighter grays or whites alternating in the darker grays. Often has concentric or horizontal banding and at times may contain quartz geode inclusions.
Fine grained, waxy to vitreous, very good knappability. Not typically heat treated.
Has historically been called Dongola, Dongola series, Anna, Illinois Hornstone, St. Louis Ball chert, and Cobden banded. Looks very much like Harrison county chert, Indiana and similar to Bayport chert, Michigan and some St. Louis Green chert.
Color is from a medium to dark grey and can have lighter grays or whites alternating in the darker grays. Often has concentric or horizontal banding and at times may contain quartz geode inclusions.
Fine grained, waxy to vitreous, very good knappability. Not typically heat treated.