Hidden by Earth, Revealed by Water: Exploring the Devonian Fossil Gorge at Coralville Lake
Just a short drive from Iowa City lies one of the Midwest’s most unique geological treasures. Managed by the U.S. Army Corps of Engineers, Rock Island District, the Devonian Fossil Gorge at Coralville Lake preserves a 375-million-year-old ocean floor.
During the Devonian Period, the region that is now a landlocked Midwestern state was positioned south of the equator and covered by a warm, shallow tropical sea. Today, managing this site is a unique part of the USACE environmental stewardship mission, offering visitors a rare opportunity to step back in time.
The gorge was not uncovered by careful paleontological excavation, but rather through the sheer force of nature during critical flood risk management operations. The site was unexpectedly revealed during the devastating floods of 1993.
Heavy rains forced Coralville Lake to overtop its emergency spillway for the first time. The spillway operated as designed to protect downstream communities, channeling a massive amount of water, up to 17,000 cubic feet per second for 28 days, down the hillside.
This powerful surge of water destroyed a local road and campground, washing away up to 15 feet of soil and eroding several layers of limestone from the area below the spillway. When floodwaters receded, a remarkably preserved section of limestone bedrock was exposed, and ancient marine fossils were revealed.
“When the water finally stopped flowing over the spillway, rangers went down to assess the scour and infrastructure damage. What they found was a preserved prehistoric ocean floor, completely stripped clean of topsoil. It was a stark reminder of the sheer power of water, and an incredible discovery for the public,” said Doug Vogel, manager at Coralville Lake.
The gorge captured people’s imagination. When the site initially opened to the public on Labor Day weekend in 1993, it drew tens of thousands of curious visitors. Recognizing the immense educational value of the site, the USACE Rock Island District partnered with a local citizen committee that raised nearly $500,000 to protect and enhance the area.
By 2001, the site was officially dedicated as an educational park, outfitted with an entry plaza, accessible walkways, and 20 guided discovery points.
However, nature was not done shaping the landscape. In 2008, another massive flood required use of the emergency spillway, and a good portion of the educational park was washed away.
Following the 2008 floods, the Rock Island District led rehabilitation efforts to adapt the park to the newly exposed space. Sidewalks were rebuilt and markers were reinstalled to highlight the various points of interest.
Today, visitors to the Devonian Fossil Gorge can walk across the prehistoric sea floor. Because these fossils predate the dinosaurs by nearly 200 million years, there are no dinosaur bones on-site.
Instead, the limestone is rich with incredible evidence of ancient marine life. Visitors can see preserved crinoid stems, brachiopods, and dense patches of colonial corals like the honeycomb-shaped Favosites and the six-sided Hexagonaria.
“Our mission here goes beyond just maintaining the grounds; it is about preserving a 375-million-year-old snapshot of Earth’s history. We love seeing visitors point out a fossilized coral under their feet, but we rely heavily on the public to help us protect the site by leaving every fossil exactly where they found it,” said Vogel.
Things to know before visiting: • The park is free to enter. The entry plaza and upper walkways are fully accessible, though the uneven bedrock of the gorge floor itself requires careful navigation. • As with all federal lands managed by USACE, regulations strictly prohibit the removal of or damage to any natural features. Violators are subject to fines, so visitors are urged to take only photographs and leave the fossils intact for future generations to study and enjoy.
Learn more by visiting: https://www.mvr.usace.army.mil/coralvillelake/recreation/devonian-fossil-gorge/.
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