Geotimes
On Exhibit
Flood Exposes a Fossil Treasure
Brian F. Glenister

The Iowa River is one of the south-eastern flowing streams that drain the eastern half of Iowa to join the Mississippi River along the state's eastern boundary. Record floods in the early 1930s prompted Congress to establish the Flood Control Act of 1938. In an attempt to reduce flooding of the Mississippi River, the U.S. Army Corps of Engineers was authorized to construct several dams on tributary rivers, including the Iowa River.

The river itself flows through an area named as long ago as 1886, when the naturalist Louis Agassiz traveled to the end of the rail line and, before lecturing on his theory of multiple glaciations, first drew attention to "the ancient coral reefs on the Iowa City area."

The Coralville Dam was finished in 1959. Situated several miles north of Iowa City, it stretches 1,400 feet long and 100 feet high. It includes an emergency spillway with its lip 31 feet below the top of the dam to allow release of any flow that exceeds the capacity of the outlet gates.

Lake levels approached the spillway lip several times, and finally exceeded it by as much as 5 feet during the heavy, continuous rains of 1993. Maximum lake intake at that time reached 41,000 cubic feet of water per second. For 28 days, as much as 17,000 cubic feet of water per second flowed down the spillway, obliterating the road and campground at its end.

Fifteen feet of unconsolidated river-bottom silt and sand were rapidly eroded, exposing the Middle Devonian Cedar Valley Group limestones below. Up to 5 feet of limestone was then eroded near the end of the spillway, and blocks weighing as much as two or three tons were carried hundreds of feet downstream.
When the flood abated, the Devonian Fossil Gorge was revealed: a succession of 375-million-year-old bedding planes with diverse and abundant fossils standing out in relief. The rocks and fossils were well known, but the Devonian Fossil Gorge offers a particularly unique exposure.

The gorge is an invaluable resource for communicating the methodology of science. Exposed is evidence that the Midwest was once a shallow, tropical sea that was south of the equator and teamed with life - as is evidence for concepts such as geologic time and plate tectonics.

Soon after flooding exposed the gorge, 16,000 people were counted visiting the site over the Labor Day weekend of 1993. Since then, more than 750,000 people have visited, commonly combining the gorge visit with a tour of adjacent exhibits a the Coralville Dam Visitor Center and the Museum of Natural History's Iowa Hall exhibit on the University of Iowa campus.

Recognizing the educational, recreational, and tourist appeal of the gorge, a local committee, mostly retirees who live around Coralville Lake, worked closely with the Corps of Engineers to develop the site. Donations from local industry, especially limestone producers, as well as from individuals and state and local government bodies, have totaled $400,000.

Construction and landscaping at the gorge was completed for a June 23 formal dedication, and the committee has handed the gorge's management over to the Corps of Engineers. About 350 people attended the opening ceremony, and extensive media coverage has subsequently increased the number of visitors substantially.

Interpretive exhibits line the entry plaza, an open observation platform shaped as a hexagon to reflect the symmetry of the abundant colonial coral Hexagonaria. Bordering the plaza and bounded by the bedding planes are monoliths of Silurian Anamosa Stone dolomite that are 6 feet wide, 18 inches thick and up to 15 feet high. Handicap-accessible walkways lead from the entry plaza to one overlooking the gorge, and down to the Biostrome Plaza near the gorge floor. Lining the Biostrome walkway are 14 massive slabs taken from the Cedar Valley Formation in adjacent quarries and arranged in original stratigraphic succession. Construction of pathways within the gorge has been avoided, but visitors can view 20 "discovery points" marked by numbered hexagonal metal plates.


Glenister is a professor emeritus in the University of Iowa's Department of Geoscience. Email

Glenister and five neighbors from the Iowa City area are directors of the gorge committee, aided by professional advisors.


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