The word "geode" is derived from Latin meaning "earthlike," a reference to their rounded shape. Most Iowa geodes are roughly spherical, often lumpy or cauliflower-like in external form, with diameters typically ranging between about two and six inches. However, specimens up to 30 inches are known. The most prized geodes have hollow interiors, although many geodes are solid objects in which crystal growth has filled most or all of the interior volume. Although the distinction may seem subtle, it is important to contrast geodes with other crystal-lined cavities or "vugs." Geodes differ from vugs in possessing an outer mineral layer which is more resistant to weathering than the host rock. As such, complete geodes commonly weather out of rock exposures and accumulate in stream bottoms. Crystal-lined vugs would not weather in such a manner.
"Keokuk geodes" can be found in specific stream drainages and excavations in parts of southeastern Iowa (especially in Lee, Henry, and Van Buren Counties), including the area near Geode State Park. Muds deposited in a shallow sea about 340 million years ago were primarily calcium carbonate and clay, and were subsequently lithified to form the shales, shaley dolomites, and limestones that we see today. Fresh geodes can be dug out of exposures of the lower Warsaw Formation, where they are concentrated in certain layers. Where water and streamflow have eroded these strata, concentrations of geodes may accumulate in stream channels.
Collecting "Keokuk" geodes can be both fun and educational. Once you've located a geode-bearing stream course or outcrop, all that's required is a little patience and a good heavy hammer. A sharp blow with a hammer is usually sufficient to crack open individual geodes, exposing their crystalline interiors to daylight for the first time. Most geode-collecting localities are on private land, and permission must ALWAYS be secured before entering.
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GEOLOGY OF IOWA
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COLUMBUS JUNCTION AREA LIMESTONE MINING
