Keokuk Geodes


One of the best-known occurrences of geodes in the world is an area surrounding the community of Keokuk, Iowa. It is located near the three-state intersection of Illinois, Iowa and Missouri, with geodes of this locality being found in all three states. The geodes formed in the limestones and dolomites of the Mississippian-age Warsaw Formation. Most of these geodes are a few centimeters across and have outer layers of white to gray to blue-gray chalcedony with interiors lined by tiny quartz crystals. Most of the geodes found here have weathered free of their carbonate host rock and are now in the local soils and stream sediments. A few of these geodes contain interesting crystals of ankerite, aragonite, calcite, dolomite, goethite, gypsum, kaolinite, marcasite, millerite, pyrite, sphalerite and other minerals. A few have been found with liquid petroleum inside.

Keokuk geode
Keokuk geode from Lee County, Iowa.

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