Flood of 1993 Uncovers Devonian Sea Floor
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.
THE ICE-AGE RECORD
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.
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.
Adapted from Iowa Geology 1994, No. 19, Iowa Department of Natural Resources