Keokuk geodes

Geodes turn up all over the world, but Keokuk's tourism office puts the greatest abundance and variety in the outcrops of the Mississippian lower Keokuk beds, within about 70 miles of the point where the Des Moines River meets the Mississippi. Most of them come from within 35 miles of Keokuk, in the corner where Iowa, Missouri and Illinois meet, and the geode is Iowa's state rock.

The University of Iowa's Pentacrest Museums date them to the lower Warsaw Formation, about 340 million years old. They weather out of limestone walls along creeks and tributaries, mostly streams that drain to the Mississippi; on the Missouri side some come from creeks running to the Des Moines. From outside they are drab, round stones. Each has a shell of limestone or dolomite and chalcedony, a form of silica, and inside there is almost always quartz. More than 17 minerals have been identified in Keokuk geodes, among them calcite, dolomite, agate and pyrite, along with iron rust and even oil. When a geode cracks open in a creek, water can lay new minerals over the quartz crystals, like hard-water stains in a sink. The most common size is three to four inches across, and some reach two feet and larger. Fossils of the same age lie in the same ground: brachiopods, corals, gastropods and crinoids.

Museums and collectors around the world seek them out for their mineral inclusions and the crystal groups they form. One family's collection, shown at the university's Macbride Hall in 2021, was gathered within 40 miles of Keokuk, at Keokuk and Sandusky in Iowa, Warsaw, Hamilton and Quincy in Illinois, and Wayland and Alexandria in Missouri. They split small stones with a hammer, wearing safety goggles, and large ones with a cast-iron soil-pipe cutter. In Keokuk the Bevard Collection of tri-state geodes is on display at the Keokuk Area Convention and Tourism office at 428 Main Street, and the town holds an annual Geode Family Fest.

Geode State Park in Henry County, 1,640 acres along the Skunk River that joined the Iowa park system in 1937, is named for the state rock, though few geodes can be found in it and removing them from a state park is illegal. A display at the park office makes up for it. Lake Geode, 187 acres, was built in 1950 and is known for largemouth bass, bluegill and crappie.