OHIO — While there is no ancient ocean beneath Ohio today, the state’s bedrock preserves evidence of a long-lost ocean that helped shape eastern North America hundreds of millions of years ago.
The ancient Iapetus Ocean formed between about 600 million and 540 million years ago as the supercontinent Rodinia broke apart. At the time, what is now Ohio sat along the stable edge of the ancient North American continent, known as Laurentia.
According to the Ohio Department of Natural Resources, the opening of the Iapetus Ocean created tectonic forces that stretched the Earth’s crust, leading to the formation of large fault-bounded basins. One of those features, known as the Rome Trough, developed south and east of present-day Ohio through parts of Kentucky, West Virginia and Pennsylvania, where thick layers of sediment accumulated. The Iapetus Ocean itself lay farther east of Ohio rather than directly beneath the state.
Over millions of years, tectonic plates collided and the Iapetus Ocean gradually closed. Those continental collisions eventually formed the Appalachian Mountains, and nearly all of the ocean’s crust was destroyed or buried deep within the Earth’s interior.
Although the Iapetus Ocean disappeared long ago, Ohio’s geologic history continued to be shaped by water. Beginning roughly 450 million years ago during the Ordovician Period, much of the state was covered by a warm, shallow tropical sea. Marine animals such as trilobites, brachiopods, corals and crinoids flourished in those waters, leaving behind the fossil-rich limestone and shale that make Ohio one of the nation’s best locations for Ordovician fossils.
Today, Ohio’s sedimentary rocks provide a record of these ancient environments. Geologists use those rock layers to reconstruct the state’s history, showing that while an intact ancient ocean does not exist beneath Ohio, the rocks beneath the state preserve evidence of both the nearby Iapetus Ocean and the shallow seas that later covered much of the region.
