Devonian Fossil Gorge
A Unique Glimpse into Iowa’s Geologic Past
 The Iowa River is one of several south-eastern flowing streams that drain the eastern one-half of Iowa to join the Mississippi River along the state’s eastern boundary. Record floods that swept the nation in the early 1930’s 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. Construction of the Coralville Dam began in 1949 at a location several miles upstream from Iowa City. Naming of the area goes back to 1866 when famous naturalist Professor Louis Agassiz traveled to the end of the rail line, and then before lecturing on his theory of multiple glaciations first drew attention to “the ancient coral reefs of the Iowa City area”.
Construction of the Coralville Dam, 1,400 feet long and reaching a height of 100 feet, was completed in 1959. An Emergency Spillway with its lip 31 feet below the top of the Dam was connected to the Dam to allow release of any flow that exceeds the capacity of the Dam’s outlet gates. Lake levels approached the Spillway lip
When the flood abated, the eroded Gorge surface revealed a succession of 375 million year old bedding planes with diverse and abundant fossils commonly standing out in relief.
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several times, and finally exceeded it by as much as five 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 the 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 where carried hundreds of feet downstream.
When the flood abated, the eroded Gorge surface revealed a succession of 375 million year old bedding planes with diverse and abundant fossils commonly standing out in relief. Devonian Fossil Gorge soon emerged as the name for the feature. When first admitted to the Gorge site, over the Labor Day Weekend of 1993, 16,000 visitors were counted entering from one of the two available entrances. Since then there have been more than three quarters of a million visitors, commonly combining the Gorge visit with those to related adjacent exhibits at the Coralville Dam Visitor Center and the Museum of Natural History’s Iowa Hall exhibit on The University of Iowa campus.
Recognition of the regional public interest in the educational, recreational and tourist appeal of the Gorge motivated a local committee, working closely with the U.S. Army Corps of Engineers, to develop a plan for development of the site. Donations from local industry, especially the limestone producers, as well as private individuals and State and local government bodies totaled over $500,000, and construction was completed for the June 23 2001 dedication. Interpretive exhibits line the Entry Plaza, an open observation platform whose hexagonal outline reflects the symmetry of the abundant colonial coral
Hexagonaria. Bedding plane bounded monoliths of Silurian Anamosa Stone dolomite that border the Plaza are 6 feet wide, 18 inches thick, and up to 15 feet high. Handicap-accessible walkways lead from the Entry Plaza to an adjacent Overlook Plaza, and down to the Biostrome Plaza near the Gorge floor. The Biostrome walkway is lined by a succession of 16 massive Cedar Valley slabs, taken from adjacent quarries, arranged in original stratigraphic succession. Construction of pathways within the Gorge has been avoided, but 20 “discovery points” are marked by numbered hexagonal metal plates: maps and explanatory brochures are provided.
A 15-minute video covering Coralville Dam construction, the Flood of ’93, Gorge rocks and fossils and their modern Caribbean analogs is available for distribution by the U.S. Army Corps of Engineers to students and other interested groups. Additionally, there are two professional guidebooks that deal with Gorge geology and related topics: Geological Society of Iowa Guidebook 60 (1994, 67 p.) from the Iowa Geological Survey Bureau, and the 62nd Annual Tri-State Geological Field Conference Guidebook (1999, 82 p.) from the University of Iowa Department of
Geoscience. An article similar to the present statement was published in the August 2001 issue of
Geotimes.
Devonian Fossil Gorge is an invaluable resource for communication of the methodology of science. Evidence that the Midwest was once a shallow tropical sea, south of the Equator and teaming with life, can be presented convincingly, as can concepts such as geologic time and plate tectonics.
Devonian Fossil Gorge is at the end of the Emergency Spillway at Coralville Dam, 2850 Prairie du Chien Road, N.E., Iowa City, Iowa 52240. See
www.coralvillelake.org
for more information.
Above article written by Dr. Brian
Glenister, Professor Emeritus, University of Iowa
Photo courtesy, north-liberty.com
Devonian Fossil Gorge, Inc. (DFG, Inc.), was formed by a group of area residents in 1996 to “…preserve and interpret this unique and recently discovered Iowa geological phenomenon, and to make accessible to everyone its enormous educational and recreational potential by designing and constructing an exciting facility to promote and preserve a rarely observed 375 million-year-old resource.”
DFG, Inc. raised approximately $400,000 in private donations to fund the construction of visitor’s facilities at the gorge. These facilities provide safe access to the site, and will help preserve the fossils, as well as educate the public about the geology of the area and the events leading up to their exposure.
Among the features at the visitor’s center are a plaza, educational kiosks, handicapped-accessible walkways, fossil displays, and educational materials.
In addition to financial support provided by the Iowa Groundwater
Association, several IGWA members have served on the DFG, Inc. Task Force as Project Advisors. Construction on the project was completed in the Spring of 2001, and Grand Opening took place on Saturday, June 23, 2001. The Army Corps of Engineers has assumed operation of the facility following completion of construction activities.
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