Glaciers carved out Ohio’s unique and specialized habitats

There are some great success stories when it comes to species that had disappeared from Ohio but have been reintroduced or migrated back on their own. When biologists talk about these species — including deer, turkey, geese, otters and bobcats — they often say they are doing best in the unglaciated areas of the state. 

So, why do the southern and southeastern parts of the state have some of the largest populations of these once-absent species? And why are they called “unglaciated”? 

Hot times

First, let’s trace the history of Ohio to when it was too balmy for glaciers. At the beginning of the Cambrian Period, more than 540 million years ago, the part of the North American continent that is now Ohio was south of the equator. 

Well, technically the equator was in a different place, and the area that is now Ohio was mostly covered with shallow seas, said Erika Danielson, a geologist with the ODNR Division of Geological Survey, in a video called Ohio Rock Talks Episode 2. 

Sea creatures began to develop hard body parts, like shells, instead of being all soft and squishy. Corals began to appear, similar to those in the modern-day Caribbean. 

“The water was warm, the air was clear and sunny, so a lot of organisms were living on the seafloor,” Danielson said. 

Fossil evidence of these creatures can be found in rocks that date from between 450 and 300 million years ago. Carbonate rocks, like limestone, have the most fossils, although shales and sandstone are also good for fossil records, she said. 

Mark Peter, a paleontologist with the ODNR Geological Survey, showed some of these fossils on the video. Some had two shells like modern clams or oysters, he said. They sat in one place and opened their shells a bit to filter food out of the water. 

Other sea creatures were a bit more exciting, like the giant sea scorpion whose guitar-sized fossil was found near Cincinnati. 

“They had pincers like a horseshoe crab to tear flesh from their prey,” Peter said. “The females may have eaten the males after mating. They were kind of scary.” 

But a lot can happen in 150 million years. The seas rose and fell, and the North American continental plate had some violent collisions with others, he said. The continental crashes eventually formed the Acadian Mountains, not too far from where the Appalachians are today. 

Big guys

A lot of sediment eroded from the mountains, making the sea bottoms muddy and dark. Yet the top layers of the seas were still clear. And there swam some enormous creatures like the Dunkleosteus, an armored, fish-like creature that could grow bigger than any great white shark. 

It didn’t have teeth, but “exposed sharp bone in the lower jaw that rubs against that in the upper jaw and sharpens itself, like scissors,” Peter said. The Dunk’s bite was so strong that it could eat other armored fish, sharks and even shell-covered creatures. 

“Ohio is world-famous for fossils,” which are most plentiful in the glaciated areas around Cincinnati and Dayton, he said. Yet the largest and most complete fossils of the Dunkleosteus, which was named Ohio’s State Fossil Fish in 2020, were found in the black shale of Northern Ohio, also a glaciated area. 

When Interstate 71 was built in the 1960s, the Cleveland Museum of Natural History made use of the Federal Highway Act so museum staff could collect fossils in cooperation with road crews. The result was one of the premier collections of Dunks and other Late Devonian fish in the world. 

Plate tectonics

Fast forward a few hundred million years. It’s the beginning of the Pleistocene Ice Age, when glaciers flowing down from the north will advance and retreat many times, at one point covering two-thirds of what is now Ohio. 

Exactly how did we get from a climate that would rival the Bahamas to sheets of ice more than a mile thick? Plate tectonics played a big part, said Dr. Joseph Hannibal, Emeritus Curator of Paleontology for CMNH, who was an exploration geologist in the oil and gas industry before his 37-year career at the museum. 

The North American plate had moved northward, and glaciers started to build up because of cyclical climate change. There were also changes in Earth’s orbit, the angle at which Earth’s axis was tilted, and the direction the axis was pointing. 

“Our North Star is not the one the ancient Egyptians saw,” he said. Scientists used to think there were four major glaciations in what is now Ohio, but now they know there were many smaller advances and retreats starting about 2 million years ago. 

Ohio was a hilly area that drained in different directions, and the glaciers blocked that water flow and changed it, Hannibal said. In the eastern half of the state, the glaciers only made it a little south of Akron while in the west, they went all the way down to Cincinnati, he said. 

But the rugged, hilly areas in the South and Southeast, all part of the Appalachian Plateau, remained untouched. That is why those areas are “unglaciated.” 

Remnants of glaciers

According to an ODNR web page on the subject, the ice age in Ohio ended about 12,000 years ago when the Laurentide Ice Sheet retreated, leaving lakes of meltwater behind that were precursors of Lake Erie. 

The wood preserved in glacial sediment shows that spruce, fir, cedar and hemlock — now found in more northern environments, like Canada — were common in Ohio during the ice age. After it ended, warming temperatures brought more deciduous trees, like the oak and elm we have today. 

During the glacial periods, huge mammals called megafauna roamed Ohio, including the mammoth, mastodon, musk ox, caribou, peccary, short-faced bear, elk-moose, ground sloth and giant beaver. Tool marks on mammoth and mastodon skeletons show humans were also living in Ohio during those periods. 

Another ice age page, this one by Ohio History Central, says the glaciers helped the humans and led to Ohio’s prosperity. 

As they flattened the hilly terrain in northern, western and central Ohio, the glaciers left deposits that created the rich soil suitable for agriculture. Sand and gravel deposits furnished building material and created underground aquifers, while glacial clay could be used to make bricks and ceramics. Finally, the flattened hills made more places for humans to live and work. 

Meanwhile, the hills, rugged terrain and thin, unproductive soil in the unglaciated areas resulted in more trees, fewer humans, and not much agriculture. The different elevations — low, high and everything in between — make for a great variety of habitats, Hannibal said. 

These habitats are connected by streams and natural corridors, allowing animals to move and find others of their species, as in mating. There’s also greater access to plants or other animals they need for food. 

“Everything is connected,” he said. And that is why species that constitute “comeback kids” not only survive but thrive in the unglaciated parts of the state.