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Devonian Fossil Gorge in the Coralville Dam Emergency Spillway

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Devonian Fossil Gorge in the Coralville Dam Emergency Spillway T he Coralville Dam’s emergency spillway serves as a safety valve to release excess water DEVONIAN and protect the dam if Coralville Lake levels rise too high. The spillway FOSSIL GORGE has served that func- tion twice in its history, The University of Iowa’s diorama at Iowa Hall shows a Devonian coral reef. The diverse marine life once in 1993 and again associated with this reef contains life forms recognized today as fossils in Iowa’s limestone deposits. in 2008. During the 1993 event, the flood waters swept away a campground cal latitudes just south of the equator, and the De- and picnic facilities and first exposed the rocks of vonian sea covered most of what is now the gorge. It was then given the name “Devonian the United States. Fossil Gorge.” The flooding in 2008 widened the FRONT COVER gorge and swept away loose rocks and vegetation Composed of the fossilized remains of shells, dis- Detail of a diorama in Iowa Hall on the University of Iowa campus, showing an undersea view of corals, crinoids, to expose additional rocks and fossils. integrated algae (mud), and other parts of animals a cephalopod, and other Devonian animals as they would have looked 375 million years ago. living in the shallow tropical seas that covered the The limestone rocks exposed at the Devonian Fos- region during the Devonian period, this limestone Photo courtesy of the University of Iowa Museum of Natural History sil Gorge are of middle Devonian age (about 375 was deposited across most of Iowa. Since then, it million years old) and include the Rapid and Solon has been eroded away from northeastern Iowa and members of the Little Cedar Formation. During this buried by younger rocks in the west and southwest time, Iowa and much of North America lay at tropi- parts of the state. The rock exposures at Devonian Fossil Gorge are unique because they in the have bedding planes that show the sea floor in layers much as it was de- Johnson County visitor center Coralville dam IOWA posited. Most rock exposures in Iowa, road cuts for example, are vertical cuts DEVONIAN through the original bedding planes. Emergency FOSSIL GORGE Iowa Geological Survey 300 Trowbridge Hall Visit the Coralville Dam’s Visitor Center, oper- Iowa City, Iowa 52242 ated by the U.S. Army Corps of Engineers, for www.iowageologicalsurvey.org Spillway N additional information, to view flood videos, Educational Materials: EM-42 (2010) and to see fossils and other displays. Johnson county, iowa www.mvr.usace.army.mil/coralville/default.htm Phone: 319-338-3543 ext. 6300 This view of Iowa Hall’s Crinoids are animals that lived Bryozoans, like corals, STRUCTURES on view diorama shows a Dunkleoste- attached to the sea floor by live attached to the sea at the Devonian Fossil Gorge us, a giant arthrodire fish that arms grew up to 30 feet in length a long stalk made of small disk- floor. They are colonies Karst describes the dissolution of and was the largest animal on shaped plates. A calyx (head) was at- of hundreds of animals calyx earth 375 million years ago. tached to the top of the stalk and long that have branching forms limestone by groundwater moving A portion of the skull of one of these creatures was found in the feather-like arms attached to the head and can be identified by their along fractures. Many karst features Devonian Fossil Gorge and can stalk collected food. They usually fall to tiny pores. are seen at the Devonian Fossil be seen at the Coralville Dam’s pieces after death (crinoid calyx and Visitor Center. Gorge, including dissolution along stem pieces, bedding planes (below) and along below), although FOSSILS of many types of animals are found in the lime- vertical fractures (right). complete ani- stone rocks exposed at Devonian Fossil Gorge. The most mals have been common are described here. found at the Corals found at the Gorge lived in a shallow, Devonian Fossil Gorge (photo clear, warm tropical sea environment. The two hold-fast principle types are colonial (that lived in con- right). nected colonies) and solitary (single animals). Placoderms were up primitive fish (below) down Solitary partly covered by bony Hexagonaria coral plates that gave them an armored appear- “horn coral” ance. This fossil jaw (above) belonged to a 2 to 3 Faults are fractures along which rocks have ft-long placoderm. moved. There are many faults present at the De- Colonial Trilobites are arthropods that lived on the sea vonian Fossil Gorge (seen as linear fractures in corals the rock) including one that moved about 10 feet Worms left burrows floor. They have three main body sections: head, thorax, and tail. The most common at the Gorge (above). Linear scratches in muddy sediments (below). slickenside include Phacops (below left) and Greenops (below (slickensides) indicate fault lines right). Sometimes only the tails are preserved. movement (right). Cephalopods (illustration below) were the shelled relatives of today’s squids and related ani- Favosites head mals. Their shells were segmented (chambered) and cone-shaped Folds are the bending of rocks produced by thorax (photo right) tectonic forces. The folds at the Devonian Fossil Brachiopods are shelled and usually only Gorge were probably produced about 310 million animals that lived attached tail portions are years ago. to the sea floor. Typical species preserved at the fold include Spinatrypas (left), atryp- Gorge. ids (upper right), and orthospiri- Stromatoporoid sponge Calcisponge fers (lower right). Dunkleosteus photo courtesy of the University of Iowa Museum of Natural History. Hexagonaria photo These by M.A. Stainbrook, 1940, Journal of Paleontology. Favosites and Stromatoporoid photos by Shirley Trier, 1993 University of Iowa thesis. Trilobite images by Marv Houg. Bryozoan illustration by E.O. Ulrich, sponges 1890, Illinois Geological Survey. All other non-credited visuals by Iowa Dept. of Natural Resources. had hard skeletons..
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