Ohio's Ice Age glaciers

At a glance

Three ice ages, not one

Ohio has essentially no preserved rock record between the early Permian, about 280 million years ago, and the Pleistocene. Then, starting roughly 2.6 million years ago, the climate cooled enough that snow piling up in northern Canada stopped melting each summer, compacted into ice, and began spreading outward under its own weight — the Laurentide Ice Sheet. The likely trigger is Milankovitch cycles: small, periodic changes in the tilt, wobble and orbital path of the Earth that, when they line up, reduce the solar radiation reaching the Northern Hemisphere enough to tip the climate into glaciation, according to ODNR's Educational Leaflet No. 7, "The Ice Age in Ohio."

At least three separate ice advances reached Ohio. The earliest, once called the Nebraskan and Kansan stages and now lumped together as "pre-Illinoian," is poorly known here — deeply weathered remnants turn up only near Cincinnati and in a few isolated exposures in northeastern Ohio. The Illinoian glaciation began about 300,000 years ago, after a warm interglacial that may have exceeded today's climate, and pushed ice as far as northern Kentucky — farther south than any later ice reached. The Wisconsinan glaciation, the most recent and the one whose deposits still shape most of the state's surface, began forming in Canada about 70,000 years ago; ice reached northern Ohio by about 28,000 years ago, peaked in Ohio between about 25,000 and 22,000 years ago, and was gone from the state again by about 17,000 years ago.

Estimates put that ice at about one mile thick over what is now the Lake Erie basin, thinning to roughly 1,000 feet over central Ohio. It didn't move as one slab: the last ice sheet to cross Ohio, the Huron-Erie Lobe, split into five sublobes named for modern water bodies in their paths — Miami, Scioto, Killbuck, Cuyahoga and Grand River — each with its own thickness, flow direction and timing, evidenced by intermingled moraines and radiocarbon dates that don't line up between sublobes, according to ODNR GeoFacts No. 33.

West of the Appalachian Plateau, the biggest obstacle in the ice's path was the Bellefontaine Outlier, a glacial-sediment-covered upland of old shale and limestone bedrock in Logan and Champaign Counties. It split the Miami Sublobe from the Scioto Sublobe and, being the highest ground in the state, was never worn down flat; ice flowed around it instead. Campbell Hill, the actual summit of the Outlier at 1,549.09 feet in Bellefontaine, is Ohio's highest point precisely because resistant bedrock there resisted the ice that flattened almost everything around it — and the hill itself is mapped as a glacial moraine sitting on that bedrock high, not a freestanding mountain the ice failed to bury, according to Wikipedia's entry on Campbell Hill).

Wisconsinan ice covered roughly the northern, central and western two-thirds of Ohio; a line running from around Cincinnati northeast through Chillicothe and on toward the Pennsylvania line marks its farthest reach. South and east of that boundary lies the Unglaciated Allegheny Plateau, the hilliest, most rugged country in Ohio — it was never smoothed by ice, so streams have had the whole Ice Age and the time since to cut it into steep, narrow-crested hills. The Hocking Hills, where erosion-resistant sandstone has been undercut into recess caves, gorges and waterfalls like Ash Cave and Old Man's Cave, are its best-known face, and the Wayne National Forest — the first and only national forest in Ohio, 244,265 acres in federal ownership as of 2018 — protects a large piece of the same unglaciated hill country.

The river the ice buried

Before any glacier arrived, western and southern Ohio drained through the Teays River, a stream comparable in size to the modern Ohio River, with headwaters near Blowing Rock in western North Carolina. It flowed north across Virginia and West Virginia, entered Ohio near Portsmouth, ran north to Chillicothe, and in the classic interpretation swung west across central Ohio into Indiana and Illinois to join the ancestral Mississippi, according to ODNR GeoFacts No. 10. Meanwhile the northeastern third of the state drained the other way, through a vanished stream called the Erigan River flowing along the axis of the modern Lake Erie basin toward the ancestral St. Lawrence — neither the Great Lakes nor the Ohio River existed yet. Some geologists instead read the deep buried valley traced across western Ohio as an ice-front meltwater channel from the earliest glacier rather than the Teays' true course; the question isn't fully settled, because both systems were repeatedly re-carved and buried by later ice, and ODNR has continued revising the mapped course of the buried valley as recently as 2023.

The earliest glacier dammed the Teays where it crossed the ice front, backing up a lake named Lake Tight — after Denison University geologist William George Tight — that covered about 7,000 square miles of southern Ohio, West Virginia and Kentucky and lasted more than 6,500 years, judging by seasonal layers in the lake-bottom clay (the Minford clay, still mined today for brick). When Lake Tight's water finally rose high enough to spill across old drainage divides, it cut new channels below the Teays' old bed — a stage geologists call Deep Stage — and that event, later reworked by subsequent glaciations, was the beginning of the modern Ohio River drainage system. An abandoned segment of the preglacial Teays valley is still visible today near Wheelersburg in Scioto County, about 1.5 miles wide and 150 feet higher than the modern Ohio River channel beside it. North of the glacial boundary the whole system is buried under glacial drift up to 500 feet thick in places, traced only from water-well logs.

Moraines, kettles, and the landscape you can still see

More than half of Ohio's surface is covered by moraines — ridges and blankets of unsorted rock debris called till, dumped directly by the ice. End moraines mark where an ice lobe's edge held still for a while; recessional moraines formed the same way farther back as the ice retreated in stages; flatter ground moraine was plastered down where the ice was retreating quickly rather than stalling. Two named moraines from that retreat still carry their own names on the map: the Defiance Moraine, an asymmetrical ridge two to five miles wide, deposited about 18,000 years ago and traceable from Defiance east across ten counties nearly to the Pymatuning Reservoir; and the Union Moraine, the outermost ring left by the Huron-Erie Lobe, running from east of Bellefontaine through Darke County and Union City into Muncie, Indiana.

Where meltwater ran across or under the stagnant ice, it built kames — mounds of layered sand and gravel dumped into holes and crevasses — and eskers, long snaking ridges marking the former course of a meltwater tunnel; one well-known esker is exposed in a sand and gravel pit north of Circleville, Pickaway County. When isolated blocks of glacier ice got buried in sediment and later melted, they left water-filled kettle depressions; Stage's Pond, a State Nature Preserve near Ashville in Pickaway County, formed exactly that way. Moraine till also carries glacial erratics — boulders of igneous and metamorphic rock, foreign to Ohio's own sedimentary bedrock, dragged down from as far as Canada; multiple Wisconsinan-age till beds full of these erratics are exposed along Rocky Fork Creek near Gahanna, Franklin County.

On the old limestone bedrock of Kelleys Island in Lake Erie, ice — or water forced beneath it — carved a set of giant parallel grooves discovered during 19th-century limestone quarrying and now protected at Glacial Grooves Geological Preserve. ODNR's current figures for the surviving "megagroove" put it at about 427 feet long, 33 feet wide and up to 10 feet deep; it once ran roughly twice that distance before quarrying destroyed the rest. Dating of the till that partly filled the grooves shows they are at least 17,000 years old, a minimum age for the carving, most likely by advancing rather than retreating Wisconsinan ice. How exactly the grooves formed is still argued: some geologists hold that hard rock "tools" frozen into the base of slow-moving ice gouged the softer limestone, while others argue that pressurized, sediment-laden meltwater under the ice did the carving instead, pointing to overhanging lips and right-angle bends that are hard to explain with ice alone, according to ODNR GeoFacts No. 36.

A lake in six acts, and a swamp that slowed an army

As the Huron-Erie Lobe pulled back north, its meltwater filled the basin it had carved, but the lake didn't simply fill to today's level and stop. It went through a whole sequence of higher, ice-dammed "proglacial" lakes as successive lower outlets opened up. The earliest and highest was Lake Maumee, which reached about 800 feet above sea level around 17,500 calendar years ago and drained west across Indiana into the Wabash River, since the lower, eastern outlet through Ontario was still blocked by ice. As the ice kept retreating and lower outlets opened, the lake dropped through Lake Arkona, then Lake Whittlesey, then Lake Warren and several further short-lived stages, before settling into early and then modern Lake Erie. Each stillstand left a beach — a long, low, sandy ridge marking where waves worked a shoreline for a while before the water dropped again — and those old beach ridges still show up today as low rises across northwestern Ohio. Modern Lake Erie is thought to have reached roughly its present level around 4,500 years ago.

The Great Black Swamp filled the flat, poorly drained floor of the old Lake Maumee basin across roughly eighteen counties of northwest Ohio, northeast Indiana and southeast Michigan. Settlers drained it between 1859 and 1885 into some of the most productive farmland in the country, eliminating more than 90 percent of Ohio's original wetlands in the process, with 60 percent of that statewide loss concentrated in the old Black Swamp counties alone.

The water Ohioans drink today

Meltwater pouring off the retreating ice carried huge volumes of sand and gravel — outwash — far beyond the ice front, filling the valleys of rivers like the Hocking, the Great and Little Miami, the Muskingum, the Ohio and the Scioto. Because those valleys were often cut into bedrock long before the ice arrived, in some cases by the Teays system itself, and then backfilled with clean, coarse glacial sand and gravel, they became Ohio's most productive aquifers — buried-valley aquifers that today's cities depend on without most residents ever knowing the valley is there. The clearest example is the Great Miami Buried Valley Aquifer beneath the Dayton area, designated a federal Sole Source Aquifer in 1988; it stores an estimated 1.5 trillion gallons and is the sole source of drinking water for roughly 400,000 people in the city of Dayton alone, with a wider estimate putting the number served across the whole Great Miami River basin at about 1.6 million.

The stakes of that buried topography are concrete: a bedrock-topography map from Madison County shows two neighboring landowners on what looks like the same flat field can have wildly different luck drilling a well, because 350 feet of buried relief between a bedrock upland and a buried valley lies hidden under only about 40 feet of surface relief — one homeowner, over the bedrock upland, drills a shallow well straight into water-bearing bedrock close to the surface, while the next, over the buried valley, drills deep into its non-water-bearing glacial drift and comes up dry.

Glacial retreat also left behind a scatter of "glacial relict" plant and animal communities that have hung on in Ohio for thousands of years, stranded far south of the boreal forest they belong to. Cedar Bog, a fen set between two end moraines in Champaign County's Mad River valley, still hosts boreal holdovers like bog birch and northern white cedar on roughly 450 of an original 7,000 acres; the state purchased it in 1942 and it became a National Natural Landmark in 1967. Brown's Lake Bog in Wayne County and the Cooperrider-Kent Bog in Portage County, which supports the largest, southernmost stand of tamarack in the continental United States, are similar holdovers from when Ohio's climate matched today's Canada.

As the ice sat over or just north of Ohio, the immediate climate to the south supported spruce-fir-tamarack-hemlock forest and tundra-edge vegetation like Canada's today, not the oak-hickory forest of later, warmer Ohio. That transition — spruce forest collapsing and being replaced by oak-dominated deciduous forest — was substantially complete by about 11,500 years ago, right around the time a wave of large Pleistocene mammals disappeared from Ohio for good; people had already been living in Ohio for well over a thousand years by then (Ohio's first people).

The glaciers left Ohio an economic legacy as well as a scenic one. They are why Ohio's northwestern and western farmland is flat and rich — till plains and lake-bed clays weather into some of the best row-crop soil in the country — while the unglaciated southeast stayed thin-soiled hill country, good for forest and little else. Glacial sand and gravel is a real industry in its own right: more than 50 million tons are mined annually from roughly 300 operations across more than 60 Ohio counties for roads, concrete and asphalt, and Ohio traditionally ranks among the top five sand-and-gravel-producing states. Streams draining the glaciated part of the state still carry a few flakes of placer gold of presumed Canadian origin, and six diamonds have turned up in Ohio's glacial deposits over the years.

Sources

• ODNR Educational Leaflet No. 7, "The Ice Age in Ohio" (rev. 1997)
• ODNR GeoFacts No. 33, "Ice Age In Ohio — Evidence Below Our Feet" (Norris, 2019)
• ODNR GeoFacts No. 36, "The Glacial Grooves of Kelleys Island, Ohio" (Peter, 2022)
• ODNR GeoFacts No. 10, "The Teays River" (Hansen, 1995)
• ODNR GeoFacts No. 1, "Bedrock Topography of Ohio" (Swinford, rev. 2001)
• ODNR Open-File Report 2022-2, "Buried Teays Valley in Western Ohio and Eastern Indiana" (2023 revision)
• City of Dayton, "Great Miami Buried Valley Aquifer"
• Groundwater Consortium, "The Great Miami Buried Valley Aquifer"
• Wikipedia, "Campbell Hill (Ohio)")
• Wikipedia, "Unglaciated Allegheny Plateau"
• Wikipedia, "Wayne National Forest"
• Wikipedia, "Defiance Moraine"
• Wikipedia, "Union Moraine"
• Wikipedia, "Lake Maumee"
• Wikipedia, "Lake Whittlesey"
• Wikipedia, "Lake Warren"
• Wikipedia, "Early Lake Erie"
• Wikipedia, "Great Black Swamp"
• Wikipedia, "Cedar Bog"
• Wikipedia, "Glacial erratic"