“Bite-Size Bits of Local, National, and Global History”
“Bite-Size Bits of Local, National, and Global History”
Near Coralville in Johnson County, Iowa — The American Midwest (Upper Plains)

Site Geology

Site Geology Marker image. Click for full size.
Photographed By Connor Olson, July 25, 2020
1. Site Geology Marker

Iowa is underlain by bedrock layers composed of sedimentary rocks; layers of Devonian limestone bedrock of the Cedar Valley Group are well displayed in the Gorge. Sedimentary rocks were formed from accumulated layers of mud, sand or organic matter that subsequently were compacted and hardened. Limestone is a sedimentary rock composed mainly of calcium carbonate (lime) derived from the skeletal hard parts of sea-dwelling animals and plants. The limestone in the Gorge 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 up to 175 feet in the Coralville Lake area, and, like the pages of a book, these rock and their contained fossils record an ancient history of tropical seas in Iowa.

The great variety of fossils seen in the limestone layers at the Gorge and around the Lake provide information used by geologists to interpret the environmental conditions during the time the sediments were deposited. Fossil corals and other animals that are seen in the lowa Devonian limestone layers are most
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like those that flourish today in the shallow tropical waters of the Bahamas and other areas throughout the Caribbean Sea where lime sediments are accumulating. The fossils and sedimentary rocks seen in the Gorge provide evidence that Iowa also was once covered by warm, shallow seas. Other geologic evidence indicates that Iowa occupied a position in the southern tropics during the Devonian, reaching its present position in northern temperate latitudes by the slow and inexorable northward movement of the North American continent.

Hexagonaria, a common fossil coral at the Gorge
Iowa's Bedrock Map courtesy of the lowa Geological and Water Survey

Erected by US Corps of Engineers, Department of Defense.
Topics. This historical marker is listed in these topic lists: EnvironmentPaleontology.
Location. 41° 43.31′ N, 91° 31.935′ W. Marker is near Coralville, Iowa, in Johnson County. Marker is on Unnnamed Road near Prairie du Chien Road, on the right when traveling south. Touch for map. Marker is at or near this postal address: 2850 Prairie du Chien Road, Iowa City IA 52240, United States of America. Touch for directions.
Other nearby markers. At least 8 other markers are within 4 miles of this marker, measured as the crow flies. Devonian Fossils (here, next to this marker); View from the Overlook Plaza (here, next to this marker);
Edge of Site Geology Marker at far left image. Click for full size.
Photographed By Connor Olson, July 25, 2020
2. Edge of Site Geology Marker at far left
The Coralville Dam (a few steps from this marker); An Ancient Sea (a few steps from this marker); Samuel Jordan Kirkwood (approx. 3.9 miles away); Old Brick (approx. 4 miles away); Mormon Handcart Brigade Camp (approx. 4.1 miles away); St. Mary's Church (approx. 4.1 miles away). Touch for a list and map of all markers in Coralville.
Credits. This page was last revised on November 13, 2021. It was originally submitted on November 12, 2021, by Connor Olson of Kewaskum, Wisconsin. This page has been viewed 78 times since then and 2 times this year. Photos:   1, 2. submitted on November 12, 2021, by Connor Olson of Kewaskum, Wisconsin. • J. Makali Bruton was the editor who published this page.

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Jun. 6, 2023