Cobden/Dongola

Cobden/Dongola has historically been called Dongola, Dongola Series, Anna, Hornstone, St. Louis Ball Chert, and Cobden (Morrow 1988, Hofman 1989).  Cobden/Dongola usually has a weathered buff colored cortex with a smooth fine grained interior.  The interior generally is a blue-grey color, but it can also be found as blue-black (Spielbauer 1984) and tan (Morrow 1988).       This round nodular chert frequently is banded with alternating dark and light bands.  The term Dongola was historically used to describe the banded chert, while Cobden was used for the unbanded chert.  The two types are now lumped together (Spielbauer 1984).  Cobden/Dongola outcrops near Dongola Illinois downstream from Big Creek and along Clear Creek near Cobden Illinois (Morrow 1988).

The photos show the typical Banding in Dongola chert.  These bands are generally circular occuring around the center of the nodual.

In contrast to the Dongola chert above this Cobden chert is homogonous in color.

The cortex can be seen on the upper part of this chert.

Types

Bibliography

Timeline