Iowa's Hidden Treasures

Exploring Keokuk Geodes: How They're Made & What's Inside

- RICHARD GROSS

What is a geode? Simply put, it's a rock with a hollow inside that's often lined with crystals, mineral deposits or both.

WHAT IS A KEOKUK GEODE?

While most are familiar with geodes, they may not be familiar with the term, Keokuk geode.

"Although geodes of different kinds are found all over the world, the greatest abundance and variety are found in the outcropping of the Mississippian lower Keokuk bed within a 70-mile radius of the intersection of the Mississippi and Des Moines Rivers," according to the Keokuk area Convention and Tourism Bureau. "The large number of mineral inclusions and the exquisite crystal groups that they form have made Keokuk geodes the most beautiful and the most sought after of all geodes by museums and collectors the world over."

The Bureau says that Keokuk geodes are most commonly about three to four inches across and most are found in the tristate border area of Iowa, Illinois and Missouri in all directions from Keokuk, Iowa which is considered the "Geode Capital of the World."

Keokuk geodes are mostly collected in the strata of the lower Warsaw Formation and in the creeks and rivers that erode material from this formation. The lower Warsaw Formation is roughly 340 million years old and contains shales, limestones and dolomites.

HOW WERE KEOKUK GEODES FORMED?

The question of how Keokuk geodes were formed has haunted geologists and may still not be completely answered. Two ideas lead the way.

One is that they formed because of the transformation of anhydrite nodules abundant on the ancient shallow sea floor. The second is that the remains of dead ancient ocean invertebrates provided the hollow site necessary for geode formation.

WHERE & HOW TO COLLECT