Site Geology

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