The Flood of 1993 Reveals Fossils

In 1993 floodwaters covered 23 million acres of land in the upper Midwest for weeks. Iowa was in the center of it. According to the U.S. Geological Survey, this flood caused more damage and cost more money than any other flood in U.S. history.

Floodwaters Uncover Fossils

The city of Coralville is named for the fossil corals easily seen at the Coralville Lake spillway. Disc and half-ball shaped corals are packed together in large colonies. Individual horn-shaped corals are also visible. During the flood water flowed over the emergency spillway at the lake for 28 days. The floodwaters ripped out a road and campground. They also washed away sediments 15 feet deep. When the flooding was over, a large flat stretch of bedrock filled with other types of fossils had been uncovered.

Iowa’s Ancient Seabed

About 375 million years ago, a warm tropical seaway covered Iowa. The newly uncovered limestone gives visitors a picture of this former ocean floor.

Most of the fossils are clam-like shells. There are also many crinoids (ancient animals related to starfish). Although it is more common to find pieces of crinoid stems, some were buried intact. Their stems, heads and arms appear as they would have in life. It is easy to see how they got the nickname “sea lilies.”

Other fossil finds include:

  • Small “moss animals” that lived in colonies. These fossils appear lacy or like small twigs.
  • Delicate sponges and more massive sponge-like creatures.
  • Pillbug-like scavengers called trilobites that crawled across the sea bottom.
  • Tracks and burrows of soft-bodied creatures that tunneled through the sediments.

Primitive fish also swam through the waters. Their fossils are rare, but bones and teeth have been found at the spillway. Part of a large bony head-plate of an armored fish was found. It is on display at the Corps of Engineers' Visitor Center in Coralville. This creature, a predatory giant of its time, reached lengths of 8 to 10 feet.

While the flood of 1993 caused great damage, it also had an unexpected positive effect. The uncovered bedrock at Coralville Lake is a rare chance to see broad horizontal layers of former seabed. These surfaces provide a chance to study fossils that can reveal mysteries of the past.

Used with permission from the Iowa Department of Natural Resources Geological Survey. Adapted from their Web site and original article in Iowa Geology 1994, No. 19, Iowa Department of Natural Resources.



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