The Cincinnati Arch
By: Cameron Schwalbach
Cincinnati and the surrounding region are known around the world for their abundant and well-preserved Ordovician-age fossils. The rocks beneath the city and its surrounding area preserve evidence of an ancient marine ecosystem that once inhabited this region nearly 450 million years ago. For nearly 200 years, paleontologists have used these fossils and the rocks in which they are preserved as a unique natural laboratory to examine important aspects of Earth’s history such as evolution, biotic invasion, climate change, sea level fluctuations, and plate tectonics during the Ordovician Period. So how is it that fossils from an ocean that was around nearly half-a-billion years ago can be found in the middle of the North American continent? The answer lies in the formation of the Cincinnati Arch.
Discovered by John Locke during his work with the First Geological Survey of Ohio in 1839, the Cincinnati Arch is a large geologic feature serendipitously centered on the city of Cincinnati. It is a broad structural uplift with the Illinois Basin to the west, the Michigan Basin to the northwest, and the Appalachian Basin to the east and southeast (Figure 1).
Features such as this form inland during mountain building events called orogenies, wherein enormous regions of rock are thrust together through the tectonic movements of crustal plates, piled up, and subsequently bent and folded as they are put under great pressure. As a mountain belt forms (in this case, the Taconic Mountains to the east), its immense weight pushes down the Earth’s crust beneath it, forming a basin (Figure 2). On the other side of this basin, the crust is bent upwards forming the opposite of a basin – an arch. Over millenia, the top of this arch is eroded away by rivers and glaciers, leaving the oldest rocks exposed at the center of the arch and the youngest rocks on the flanks (Figure 3). In the case of the Cincinnati Arch, the oldest exposed rocks date from the Ordovician Period and the youngest from the Permian Period.
So that explains how the Cincinnati Arch was formed, but why are there fossils from an ocean in Cincinnati? To answer this, we must go back to when the Arch was first beginning to form. When the sediments that formed the Ordovician rocks beneath Cincinnati were deposited, the region was beneath a shallow subtropical sea in the southern hemisphere (Figure 4). This shallow marine ecosystem had environments that ranged from peritidal (around the shoreline) to deep offshore (about 150 feet deep), each with a unique composition of sediments and population of organisms. The Cincinnati area was subjected to frequent hurricanes that brought in muddy sediments shed from the building of the Taconic Mountains and interfingered them with carbonates produced by local organisms. The organisms living on or near the seafloor during these events were buried by the sediments, and if the conditions were right, went on to become fossils. The resulting layers of fossiliferous mudstone and limestone can be seen in roadcuts and creek outcrops throughout the region today. Stay tuned to learn more about the geology and paleontology of the Cincinnati region.
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