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Iowa Rocks and Minerals

There are rocks and minerals to be found in Iowa, but the variety is low.

Iowa is mostly rolling prairie land deposited by glacial activity over sedimentary rock. In the northeast corner, where there was no glacial activity, there are cliffs and hills made of more rugged materials.

Seventy-four million years ago, a large meteorite crashed into what is now southeast Pocahontas County creating the Manson crater. Although glaciation has erased all surface evidence of the impact, the bedrock associated with this impact is unique in Iowa. Four hundred and seventy million years ago, a meteorite strike created the Decorah crater estimated to be 3.5 miles in diameter. It is now covered by Winneshiek Shale. There is no surface evidence of the impact, as the Winneshiek Shale is more than 50 feet below the bottom of the Upper Iowa River.

Iowa was a significant coal producer, particularly the Des Moines River valley from Coalville south and the greater City of Des Moines area.

Geodes erode from the sedimentary rock are found in southeast Iowa, centered around the town of Keokuk, and geodes were named the official state rock in 1967. These geodes can be lined with one or more minerals including quartz, amethyst, calcite, chalcedony, marcasite, and pyrite. Geodes are sometimes found in the area of Geode State Park in Henry County, but not so much in the park and it is illegal to collect them there anyway.

Calcite often forms in large crystals, especially in southeastern Iowa. Bell Mill Park near Burlington is known for its large black calcite crystals.

Agates, jaspers, petrified wood, and quartz crystals are fairly common finds in Iowa. Lake Superior Agates (blue and white) are found across the state thanks to glacial activity, making almost any gravel bed, river, stream or rock quarry in the state a potential rockhounding site.

Mussels in the Mississippi River use to be prolific in pearls. And you can find brachiopods, corals, trilobites, and crinoids in limestone outcrops, particularly in Devonian Fossil Gorge State Park near Coralville Lake in eastern Iowa. The fossil gorge is essentially in the flow path of the auxiliary spillway of the dam that forms Coralville Lake. High flow has washed away the soil, exposing the sedimentary rock beneath, with lots of great fossils.

Before venturing out-do some research on where and what you can collect. Remember you must have permission to collect on private property.

See my page on Rockhounding Rules for general information on the rules of collecting rocks on various lands.