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< Flood of 1993

Red ball iconFlood of 1993 Uncovers Devonian Sea Floor


aerial photo  
Floodwaters from the Des Moines River (background) turn Interstate 80 into a causeway connecting remaining segments of dry land in this July 12, 1993, aerial view north of Des Moines.
Photo by Steve Kalkhoff, USGS.


aerial photo  
The Mississippi River lapped at the City of Keokuk on July 7, 1993, as it flooded the municipal water pollution control plant (foreground).
Photo by Robert Buchmiller, USGS.


Cascading floodwaters gnaw away at the Lower Cottonwood Campground below the emergency spillway at the U.S. Army Corps of Engineers' Coralville Lake in Johnson County on July 7, 1993.
Photo by U.S. Army Corps of Engineers.


The Flood of 1993 was the costliest, most devastating flood in U.S. history according to the U.S. Geological Survey. Floodwaters covered as many as 23 million acres of agricultural and urban lands in the Upper Midwest for weeks. The unusual duration and magnitude of this event was triggered by a wet-weather pattern that had persisted since early in the year, followed by a series of intense rainstorms in late June and July. Iowa found itself in the center of the catastrophic flooding that resulted.

Among the effects of the 1993 summer floods in Iowa was the overflow of the emergency spillway at the U.S. Army Corps of Engineers' Coralville Lake, an event which lasted 28 days. The floodwaters ripped out a road and campground, and scoured away as much as 15 feet of glacial-age sediments to expose a remarkable expanse of 375-million-year old (Devonian) fossiliferous bedrock. Thousands of visitors followed in the flood's wake. The broad horizontal surfaces of limestone have provided the public with an opportunity to walk across an ancient sea floor and to see a clearly visible picture of bygone life that thrived in the tropical waters that once covered interior regions of North America.


E. Arthur Bettis III

Looking back toward the spillway, steep banks along the flood-scoured channel, reveal layers of glacial-age deposits. Broad exposures of fossiliferous limestone form the floor.
Photo by Ray Anderson.

A stream-lined mound of glacial-age materials is all that remains of deposits that once covered the limestone.
Photo by Ray Anderson.

While much attention is given to the bedrock geology and fossils seen along the floor of the gorge, there are layered deposits revealed along the steep banks of the flood-cut channel that provide a glimpse into the history of the Iowa River valley. Recognized by different colors and textures, these deposits record major periods of accumulation over the last few hundred-thousand years.

The oldest Quaternary deposit, located immediately above the bedrock, is the most mysterious with regard to age and origin. This dark brown, "greasy" sediment fills irregular hollows in the rock surface and extends upward for a few feet. The deposit contains a few small igneous pebbles (erratics), a clue that it accumulated sometime after the first glaciers invaded Iowa and brought igneous rocks from the north. The deposit contains no pollen; it is not organic-rich, even though it is dark-colored; and it is unlike any other Quaternary deposit in eastern Iowa. The isolated "mound" in the center of the gorge is composed mostly of this material. Even the mound's existence presents a question for many visitors. It was protected from the full eroding force of the floodwaters by a pile of large rocks that lodged just upstream during the overflow event.

A thin gravel rests on the unknown deposit, indicating that erosion preceded accumulation of overlying materials. Two later episodes of stream activity are recorded above the gravel. The older unit, reddish-brown silts and clays, formed sometime during the warm (interglacial) period between about 30,000 and 200,000 years ago. These silts and clays are cut out farther down along the gorge exposures and are replaced by a still younger stream deposit that accumulated during the coldest part of the last glacial period, between 22,000 and 16,000 years ago. Tan wind-blown silt (loess) that accumulated between 21,000 and 12,000 years ago, when the Iowa River carried glacial meltwater from north-central Iowa, mantles both of these preserved stream deposits.

This chapter of the region's geologic history will not remain in view as long as the more resistant limestone. Slumping of the softer materials will eventually cover the diagnostic contacts, and vegetation will further obscure the slopes -- perennial problems for those who study Iowa's Ice-Age record.


Brian J. Witzke

Fossil Fossil
Exceptional specimens of crinoid fossils were found. Though plant-like in form, crinoids are actually animals related to starfish. They lived attached to the sea floor by long flexible stems. Their arms, radiating outward from the head, moved with the currents and filtered the sea water for food particles.
Left photo by Brian Glenister. Right photo by Brian Witzke.

A fossil colonial coral.
Photo by Jean Prior.

The fossil shells and skeletons of sea-dwelling animals are seen in abundance in the broad expanses of limestone -- the closer one looks, the more one sees. These fossils are the remains of a multitude of creatures that inhabited a warm tropical seaway covering Iowa about 375 million years ago, during the Devonian period. Most of the fossils seen are from animals that inhabited the sea bottom and filtered small food particles from the water. Brachiopod (clam-like) shells are among the most conspicuous, and a great variety of species can be seen. Some limestone beds are crowded with their shells. Crinoids are also present. Known as "sea lilies," and plant-like in their form, crinoids are actually animals related to starfish. Most were fragmented into pieces by scavengers and bottom currents before fossilization, and their stem segments are abundant. However, some exceptional specimens at the site were buried intact, displaying their stem, head, and arms as they would have appeared in life.

Fossil corals are the namesake for the City of Coralville, and it is certainly fitting that spectacular accumulations of fossil corals are so well displayed at the spillway. Some beds show densely packed concentrations of hemispherical and disc-shaped colonial corals, while horn-shaped solitary corals abound on other surfaces. Bryozoans ("moss animals") are smaller colonial organisms that are seen as small twig-like or lacy fossils in many of the limestone beds. Delicate sponges and more massive sponge-like stromatoporoids also occur. Fossils of trilobites, pillbug-like scavengers that crawled across the sea bottom, can be recognized by visitors who carefully scrutinize the limestone beds. Other creatures burrowed through the soft bottom sediments, and, although these animals lacked hard parts necessary for fossilization, their actions are preserved as tracks and burrows in the now lithified limestone.

Primitive fish swam through the waters of the Devonian seas, and, although rare, their fossil bones and teeth have been found at the spillway. The most notable specimen, now removed for display at the Corps of Engineers' Visitor Center, is part of a large bony head-plate of an armored fish (arthrodire). This creature, a predatory giant of its time, reached lengths of 8 to 10 feet.

The fossils at the Coralville spillway have attracted phenomenal interest, and deservedly so. The floodwaters uncovered a window on the past, and visitors can gain first-hand knowledge of the fascinating creatures that lived in the tropical seas that once covered our state.


Bill J. Bunker

A flood-ravaged road, once traveled by visitors to Coralville Lake, ends abruptly in a series of bedrock ledges that take visitors on an unexpected journey into Iowa's geologic past.
Photo by Brian Glenister.


The flood-carved gorge and its rocks and fossils provide visitors with a unique opportunity to learn more about Iowa's earth history.
Photo by Ray Anderson.

Exposures of Devonian limestone in eastern Iowa are usually limited to vertical quarry faces, roadcuts, and stream cutbanks. The new spillway exposures provide a rare opportunity to see broad horizontal surfaces, many containing spectacularly abundant marine fossils. Although the exposures are of great interest to Iowa geologists, public interest in the rocks, fossils, and flood-carved gorge has been tremendous. The influx of visitors, a broad spectrum of people including young and old, began almost immediately after the overflow ceased. The numbers increased dramatically following nationwide newspaper accounts (also in Europe, Japan, and Australia) as well as broadcasts by Paul Harvey, the ABC Nightly News, and CNN. Conveniently located a few miles north of Interstate 80 in Johnson County, more than 250,000 visitors from throughout the United States and abroad have made the trek across the fossiliferous exposures, and others continue to visit the site each week. Numerous school groups from eastern Iowa have taken advantage of this learning opportunity, and the site has become an outdoor science laboratory for many. The Iowa Geological Survey prepared hand-out materials to assist visitors with interpreting the rocks, fossils, and glacial deposits. Most visitors comment that they would like to see the site remain available for viewing as it is today. In February of 1994, following a contest sponsored by the Corps of Engineers, the locality was named Devonian Fossil Gorge.


Adapted from Iowa Geology 1994, No. 19, Iowa Department of Natural Resources