Paleontology of Ohio Coral Reef (Falls of the Ohio) National Natural Landmark

By Justin Tweet, NPS Paleontology Program

Approaching the entrance to the Interpretive Center of Falls of the Ohio State Park, with the Lewis & Clark statue in the foreground and fossil beds visible in the left background.

NPS / Leo Acosta

Over the millennia that people have traveled on the Ohio River, there has been only one major navigational hazard: a series of rapids about 3 miles (5 kilometers) long known as the Falls of the Ohio. Louisville, Kentucky and Clarksville, Indiana grew at this feature, now largely submerged behind the McAlpine Locks and Dam. It was here on October 15, 1803 that Meriwether Lewis met William Clark and the two began to organize their famous expedition; today the Lewis & Clark National Historic Trail recognizes the site for its historic importance. The natural history of the Falls goes back much farther, though.

The fossil beds at Falls of the Ohio State Park with the Fourteenth Street Bridge and the Louisville skyline in the background.

NPS / Leo Acosta

“Nowhere on earth are fossiliferous rocks of Middle Devonian age as well exposed and as easily accessed as at the Falls of the Ohio… The species may be found elsewhere, but nowhere is their vertical and lateral context in the rock layers as easily observed. The vast area of the Falls and the predominantly bedding-plane exposures allow the observation of millions of individual fossil specimens. The fossil communities at the Falls are preserved virtually in situ, allowing rare insights into the paleoecological interactions of the residents of Devonian seas” (Hendricks et al. 2005: 11)

Approximately 390 million years ago, the area now occupied by the Falls of the Ohio was beneath a shallow continental sea well south of the Equator. Precipitation of calcium carbonate (lime) out of the water produced beds of limestone now known as the Jeffersonville Limestone, named for Jeffersonville, Indiana on the north side of the Falls. In the area of the Falls it is about 35 feet (11 meters) thick. Life flourished in the Jeffersonville sea, including corals, sponges, and other marine invertebrates, whose fossils have been preserved in great numbers. The name “coral reef” is a simplification: the animals here did not produce true wave-resistant reefs like we see today in the Tropics but mound-like features called bioherms, more like a coral bank; many of the bioherm producers were sponges instead of corals; and bioherms were formed during only part of the overall deposition. In fact, five different stacked intervals can be seen in the rocks of the Falls and nearby, each with their own assemblages of animals, reflecting changing conditions over time.

An outcrop of the Jeffersonville Limestone at Falls of the Ohio State Park.

NPS / Leo Acosta

In the lowest zone of the Jeffersonville Limestone, fossils of corals and sponges are very abundant, and they lived beneath the depth of wave activity until near the end of the zone. The sponges represent an extinct group, the stromatoporoids, that had unusually strong frameworks compared to most sponges, which disintegrate upon death. The many corals found here also represent extinct groups (modern corals are not known until the Triassic, 250 million years ago). Sponge fossils are more abundant than corals in the second zone, especially small branching sponges and mat-like encrusting sponges, and the setting was probably an extensive lagoon with shallower water than the previous zone. The third zone has abundant fossils of brachiopods (“lamp shells”), crinoids (“sea lilies”), and microfossils of green algae. Snail shells up to fist-size coated with encrusting sponges are also known from this zone, which had much shallower water. Sometimes the seafloor was exposed to the air and mudcracks formed. The sea deepened during deposition of the fourth zone, which is dominated by fossils of net-like bryozoans (“moss animals”) and brachiopods, with the return of corals. The uppermost zone is similar but with particularly abundant large chunky brachiopods called Paraspirifer. This zone is not visible at the Falls but is present in nearby river bluffs. After the Jeffersonville Limestone was deposited, other sedimentary strata accumulated on top of it, and it would not be visible here until very recently in geologic time. Glacial activity rearranged the courses of rivers in the region, and the future Ohio River cut down through the area that happened to include the ancient coral bank.

A giant colonial coral (note water bottle in upper left).

Photo courtesy of Falls of the Ohio Foundation.

Fossils of many different organisms have been found here. The exact number is not certain, because many of the names coined over the years represent animals that had already been named, but the total is in the hundreds, making this site one of the most productive in the world for new fossil species. The fossils represent many groups. Apart from those mentioned above, there are also bivalves and extinct bivalve-like rostroconchs, ostracodes (“seed shrimp”), trilobites, blastoids (extinct cousins of crinoids), and bony fishes such as the predatory armored arthrodires. Fish fossils are concentrated in thin beds. Paleontologists began describing fossils from the Falls in the 1820s, and over the past 200 years fossils from this site have been distributed to museums across the world.

A partial trilobite in the field.

Photo courtesy of Falls of the Ohio Foundation

The area of the Falls is much different from its appearance when Lewis met Clark, with the addition of dams and locks in the Ohio River to allow passage and control flooding, and the growth of cities along the banks. Today the responsibility for the Falls is divided between the U.S. Army Corps of Engineers and the Indiana Department of Natural Resources. The fossil beds were designated as a National Natural Landmark in 1966, among the earliest National Natural Landmarks (the program was established in 1962). Approximately 1,400 acres of Indiana and Kentucky at the falls were set aside in 1982 as the Falls of the Ohio National Wildlife Conservation Area under the management of the Corps. In 1990, the Indiana DNR designated the fossil beds as Falls of the Ohio State Park, and the park was opened to the public in 1994.

A small part of the diversity of ancient life known from the Falls of the Ohio is illustrated in this coloring page. Click image to download coloring page.

NPS coloring page courtesy of Benji Paysnoe

The park includes trails to the fossil beds, which are most accessible during natural periods of low water and during August to October when the Corps has closed upstream floodgates. It also has an interpretive center with extensive displays of the fossils. The park is supported by the Falls of the Ohio Foundation. While fossil collecting is prohibited on the fossil beds and riverbank, park staff encourage visitors to explore and discover the many different types of fossils that can be found on the ancient sea bottom. Collecting piles (with rock and fossils from quarries) are the only places where rocks may be removed at the Falls of the Ohio. They are located by the parking lot behind the interpretive center.

Visitors explore the fossil beds at Falls of the Ohio State Park

NPS / Leo Acosta

Administered by the National Park Service, the National Natural Landmarks Program recognizes and supports the voluntary conservation of exemplary biological and geological sites that illustrate the nation’s natural heritage, including those that contain significant paleontological resources. National natural landmarks are designated by the Secretary of the Interior and are owned by a variety of public and private stewards. To date, significant paleontological resources have contributed, in whole or in part, to the designation of 58 National Natural Landmark sites across the country, and paleontological resources are known at many others as well. These landmarks represent the range of geologic history from very early marine life 510 million years ago to more recent (40,000 years ago) fossil mammals.

Ohio Coral Reef (Falls of the Ohio) National Natural Landmark is one of 58 National Natural Landmarks designated primarily for paleontological resources.

Photo courtesy of Falls of the Ohio Foundation

Further Reading

Greb, S. F., R. T. Hendricks, and D. R. Chesnut, Jr. 1993. Fossil beds of the Falls of the Ohio. Kentucky Geological Survey, Lexington, Kentucky. Series XI, Special Publication 19.

Hendricks, R. T., D. J. Phelps, F. R. Ettensohn, G. Weems, and J. Lundquist. 2005. Silurian and Devonian geology and paleontology at the Falls of the Ohio, Kentucky/Indiana. Field trip guidebook for the 42nd Annual Meeting of the American Institute of Professional Geologists.

Powell, R. L. 1970, updated 1999. Geology of the Falls of the Ohio River. Indiana Geological Survey, Bloomington, Indiana. Circular 10.