By Michael Swanger
Talk about standing the test of time. One of Iowa’s five state symbols turns 50 this year in the archeological blink of an eye. You could say it was millions of years in the making.
In 1967, the Iowa General Assembly designated the geode as the official state rock. Our elected leaders chose it over limestone and fossil coral in an attempt to boost tourism because Iowa was and continues to be renowned worldwide for its large number of geodes, the majority of which are beautiful and some of which are rare. But like anyone who has gazed at one, I suspect that lawmakers were spellbound by the bling of their sparkling lining of mineral crystals that feature an array of colors such as white, pink, gray, blue, yellow and orange.
Most Iowans have seen a geode, or two, whether they have searched for them outdoors in rock exposures or stream bottoms, or visited a gift shop that sells all-things-Iowa. For those who have not witnessed firsthand their natural beauty, they might be underwhelmed by their exterior at first glance. The Iowa Department of Natural Resources (DNR) describes them as being “roughly spherical, often lumpy or cauliflower-like in form, with diameters typically ranging between two and six inches.” But as the old saying goes, “it’s what’s on the inside that counts.”
The most prized geodes are those with hollow interiors that typically contain quartz, a silicon dioxide in which the primary mineral is ordinary sand. But there is nothing ordinary about colorful quartz crystals — from transparent to white — that occupy the interior walls of many Iowa geodes. Each crystal is unique in its size, pointed shape and color. Other minerals found in geodes, such as dolomite and pyrite, or fool’s gold, offer equally compelling beauty.
That kind of beauty didn’t occur overnight. Most Iowa geodes are found in the lower exposures of the Warsaw Formation, which the DNR describes as “a widespread rock unit of Mississippian age (340 million years old) composed of shales, shaley dolomites and limestones.” That dates back to a time when Iowa was part of the ocean floor and the muds deposited from it lithified to form the rocks we see today.
In more recent history, before the geode officially became Iowa’s rock 50 years ago, collectors flocked to southeastern Iowa with their rock hammers to find an abundance of them in a 35-mile radius of Keokuk. The region, famous for its “Keokuk geodes,” includes Lee, Henry and Van Buren counties, as well as Geode State Park, which is located near the small towns of Danville and New London. Unfortunately, few geodes can be found today at the state park except for a display at its office. Fortunately, however, collecting them there is no longer permitted in an effort to preserve what remains for future generations of Iowans and tourists to enjoy.
Though geodes are often thought of as decorative, they have also been utilized in early architecture. Before the advent of cement block and poured concrete construction, rocks were important building materials in Iowa, especially when quarrying was more commonplace during the late 19th and early 20th centuries.
The Mason City area, for example, is known for its good specimens of geodes among collectors. Another northern Iowa site and popular tourist attraction, the Grotto of the Redemption in West Bend, incorporates a variety of rocks encrusted over a concrete framework, including geodes.
To this day, the exact origin of Iowa’s geodes has puzzled geologists whose hypotheses share a few general truths. But there is no arguing that geodes have proven to be one of Iowa’s most enduring symbols and one of nature’s treasures.