Essay Sample on Coralville Lake and Devonian Fossil Gorge

The Coralville Lake is an artificial lake located in Johnson County, Iowa, that formed as a result of the construction of Coralville Dam between 1949 and 1958 on Iowa River upstream of Coralville City (Jones, "The Legacy of the Flood of '93"). This project was a means to curb the widespread flooding in the United States in the early 1930s caused by the Mississippi River. In 1993, the historic floods surged over the Coralville Lake emergency spillway, eroding a huge channel into the underlying rock deposits, exposing the Devonian Fossil Gorge (Jones, "The Legacy of the Flood of '93"). The gorge further expanded by the floods of 2008 that topped the water of the emergency spillway. This made the gorge the star scenery and attraction site in Iowa (Jones, "The Legacy of the Flood of '93"). This exposure, further, provided a unique chance for Iowa's geologic past observation by geologists and the public at large. This is the beginning of the formation of Coralville Lake and the Devonian Fossil Gorge, the beautiful scenery to behold in Iowa.

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Iowa sits in one of the seven continental plates that have shifted at a slow rate throughout time. This movement shaped not only the land masses but also the ocean floor. When the continental plates move and push towards each other mountains form (Anderson 34). When they slide on each other massive earthquakes, occur, and when they have pulled apart, rifts arise. These three processes cause ocean water to move from one area to another. The plate tectonic movement is responsible for moving Iowa all over the earth. It is important to note that Iowa experiences tropical climates and artic temperatures. Over time it has been submerged beneath the ocean and forced to emerge once again (Anderson 47). This movement caused the rocks to be broken down into parts through the process of erosion and then carried away by the wind, water, and ice. The degraded rocks were then available for the formation of new stones. All these took place over a long period.

The earth is about 4.5 billion years old. Igneous rocks are the oldest Iowa rocks found deep beneath the ground. These rocks are about 2.9 billion years old. The age is estimated using radioactive dating. The rocks contain small amounts of radioactive elements that breakdown at a predictable rate; therefore, aiding geologists to measure the age of one stone relative to one of known age. The bedrock shelves and rock slabs of Devonian Fossil Gorge comprises of limestone. The rocks developed as lime-rich deposits piling up on the ocean floors approximately 375 million years ago during the Devonian period (Anderson 56). Some of the limestones are fine-grained as a result of deposition by calm, quiet waters. These beautiful limestones are composed of limy mud. In other areas, the grains are coarser, mainly made of broken shells concentrated by ocean floor currents. Calcium carbonate, the most abundant mineral in limestone, originated from various sea creatures ranging from small planktonic animals, through corals to large organisms with individual shells (Anderson 64). The cracks present in the limestone surface extend hundreds of feet vertically and across many miles horizontally. These cracks resulted from the warping of the earth's crust. The pressure further caused the cracks on the brittle sedimentary strata in the geological past. These fractures served and still serve as the underground water channels. Limestone slowly dissolved as a result of contact with underground subterranean water (Anderson 78). The exposed rock surfaces by the 1993 and 2008 floods displayed small scale karst features, found inside the eastern Iowa limestone formations. The scouring action of gushing water formed these depicted channels which are smooth-sided. Over time the canals were expanded into rounded potholes, scoured smoothly by the agitating work of cobbles.

As earlier stated the floods of 1993 formed the Devonian Fossil Gorge. During this process the water level in the dam rose to 4.5 feet pouring into the spillway, causing a massive torrent which swept away the pavement, vegetation, sediment, and the weakly cemented bedrock downhill. It further tore the solid limestone bedrock in some other places. During the 2008 floods, the raging floodwaters tore a layer of bedrock outstandingly widening the gorge. The immense power of the waters picked up and moved massive tons of limestone rocks downhill. Ten thousand years ago, during the last ice age, the normal flow of Iowa River was likely more substantial (Anderson 137). During this age, a tremendous amount of water from the Des Moines Lobe glacier gushed through the Iowa River valley, forming a deep narrow valley into the bedrock. Thick layers of silt were then deposited broad valley south of the river (Kemmis et al. 83). Devonian Fossil Gorge is a flushed exposure, with sizeable areas of single rock exposure. This stretch of exposed rocks gives an interesting look into Central Iowa's geologic past. A close observation into the rocks discloses a lot of fossils lodged into the rock surface. These fossils range from brachiopods, crinoids to coral heads. For fossils to form two things are of the essence: hard parts such as shell, bones, or even tooth; and rapid burial by water-borne sediment and mineralization. Devonian Fossil Gorge is home to numerous invertebrate fossils that include: foraminifera, bryozoan, brachiopod, graptolite, among others (Kemmis et al. 114). For instance, foraminifera were one-celled protists that secreted a calcite shell.

The most peculiar corals were of the genus hexagonaria. These contain polyps of about 0.5 inches packed hexagonally across into arrays. Coral heads can measure approximately a foot across. Fossil crinoids are also common in this gorge (Monson 201). The most common crinoid fossil in the canyon is crinoid stem fragments. These resemble sticks of flat washers. A couple of large stems have been exposed measuring up to 0.5 inches in diameter and over a foot long. Fossils of crinoid heads are scarce (Webby 30). Brachiopod fossils resemble clamshells and are moderately common in the Devonian Fossil Gorge. These are small clam-like animals with shell overhangs. A hinge or a muscle can hold these shells. Bryozoan fossils are common but are not easily recognized (Webby 34). They mirror exquisite mesh patterns arranged over the surface.

Some fascinating landforms include caves measuring as large as a foot in diameter. These were dissolved out of the limestone by the action of slightly acidic groundwater before being exposed by surging floodwaters (Anderson 189). This process of chemical weathering has brought fossils into sharp visualization. These fossils would be difficult to observe if the simple breaking of the rock only exposed them. Chemical weathering involves the weakening and ultimate disintegration of the foundations by the chemical reactions. These reactions include hydrolysis, oxidation, and carbonation. The tropical conditions and high moisture content in Iowa have accelerated this process.

In conclusion, the Coralville Lake and Devonian Fossil Gorge are beautiful scenery that describes the geologic foundation of Iowa and are worth visiting and exploring.

Works Cited

Anderson, Wayne. "Geology of Iowa: over two billion years of change". Iowa State University Press, 1983.

Jones, Douglas W. "Devonian Fossil Gorge, The Legacy of the Flood of '93". http://homepage.divms.uiowa.edu/~jones/natural/devonian.html. Accessed 20 Sep. 2019.

Kemmis, Timothy J., George R. Hallberg, and Alan J. Lutenegger. "Depositional environments of glacial sediments and landforms on the Des Moines Lobe, Iowa". Des Moines, IA: Iowa Geological Survey, 1981.

Monson, Charles. "Louis Agassiz and the fossil reefs of Iowa." Earth Sciences History 31.2 (2012): 193-209.

Webby, B. D. "Middle-upper Devonian paleogeography of north Devon and west Somerset, England." Paleogeography, Palaeoclimatology, Palaeoecology 2 (1966): 27-46.