Site Geology

Iowa is underlain by sedimentary bedrock, and layers of Devonian limestone bedrock of the Cedar Valley Group are well displayed in the Gorge. These sedimentary rocks formed from accumulated layers of mud, sand, or organic matter that were later compacted and hardened. The limestone is composed mainly of calcium carbonate derived from the skeletal hard parts of sea-dwelling animals and plants. In the Gorge, this limestone was deposited in a shallow sea that covered much of the Midwest during the Devonian Period about 382 to 389 million years ago. Devonian limestone strata reach thicknesses of up to 175 feet in the Coralville Lake area, and these rocks and their fossils record an ancient history of tropical seas in Iowa. Fossils in the limestone layers at the Gorge and around the Lake help geologists interpret the environmental conditions that existed when the sediments were deposited. Fossil corals and other animals in Iowa's Devonian limestone layers most closely resemble those that live today in the shallow tropical waters of the Bahamas and other parts of the Caribbean Sea where lime sediments are accumulating. Together, the fossils and sedimentary rocks provide evidence that Iowa was once covered by warm, shallow seas, and other geologic evidence indicates that Iowa occupied the southern tropics during the Devonian before the slow northward movement of the North American continent carried it to its present northern temperate latitude.

Iowa · United States