In this video from KET, stroll along the Falls of the Ohio River, one of the largest exposed fossil beds in the world, and imagine what this part of the Midwest looked like during the Devonian Period 387 million years ago. Fossil evidence of corals and other marine organisms indicate the region was located in the tropics and was covered by a warm, shallow sea.
Quick examination of exposed fossils at the Falls of the Ohio River near Louisville, Kentucky reveals that local geography and climate were much different during the Devonian Period than they are today. The fossils are of organisms from a coral reef, and they are made of limestone, a type of sedimentary rock that only forms in marine environments. Limestone and coral indicate that this land must have been under salt water when the organisms lived, died, and were buried in sediment.
Additionally, coral grows poorly in cool water, so it never forms reefs more than 27 degrees in latitude from the equator. This suggests that the land where the Falls of the Ohio is now, at 38 degrees north of the equator, is further north than it was when the reef formed. Other evidence suggests that the land was actually about 20 degrees south of the equator when the reef grew, so the land has moved 4,000 miles northward since the reef formed!
More environmental information can be gleaned from a closer examination of the fossil bed. Many of the coral fossils are upside down, and this tells us that perhaps strong currents, associated with hurricane conditions, tossed the heavy corals before they were buried in sediment. If we accept this explanation, we observe that waves this strong only occur in shallow water, so the warm sea that covered what is now the Falls of the Ohio would have been shallow.
Hundreds of fossilized species have been identified at the Falls, which indicates that this sea must have had a rich variety of marine life! Many of the fossils bear similarities to animals that live in the ocean today.
During the millions of years between the formation of the ancient coral reefs and today, many slow changes occurred. Calcium carbonate precipitated out of the water, steadily burying layer after layer of skeletons on the sea floor. Newer layers above compressed older layers below, gradually cementing sediment into limestone rock. Continents moved, the sea receded, and climate changed. Deposition and erosion alternated as the dominant crust-shaping activity in the region. Several ice ages brought glaciers stretching from Canada to Indiana, just north of the Falls. Between ice ages, the glaciers retreated and the resulting melt water carved out the Ohio River Valley. Erosion eventually removed an estimated 100 feet of glacial outwash deposits and 20 feet of limestone, finally exposing the fossil bed ledges that now form the Falls of the Ohio.
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