Evidence of Ohio's Glaciers
Glaciers are powerful geologic forces. They weather and erode vast areas of land and leave behind various landforms, such as moraines and kettle lakes; a variety of sediments; and boulders of various sizes. Read further to explore how glaciers create these landforms and the glacial materials that comprise them.
The following main types of glacial sediments are found all over Ohio and have complex relationships to each other. They were not only being deposited simultaneously during the advance and retreat of a glacier across Ohio, but there were at least three distinct periods of glaciation in Ohio during the Quaternary Period. Click here to learn more about these glacial periods.
Unlike other forces of erosion such as wind and water, glaciers are bad at sorting materials. Sediments directly deposited by glacial ice are made of a combination of clay, silt, sand, pebbles, and even boulders. This type of deposit is called glacial till, or simply till. Till is the unsorted sediment created when ice picks up, transports, and directly deposits the sediments in another location. Till is commonly found at the surface in the glaciated portion of Ohio and is the source material for the most productive agricultural soils in western Ohio. Till deposits typically do not display any bedding or layering that can be found in other geologic deposits or rocks. Geologists can also study how elongated pebbles align in a till deposit to better understand the direction that the glacier was flowing when the till was deposited.
As glaciers melt, water is produced that can also transport, and further sort, glacial materials through the creation of meltwater river systems. The material deposited in these meltwater rivers is commonly referred to as outwash. Outwash is typically composed of sand and gravel, which can exhibit layering and grading, further evidence that the material was deposited by water and not directly by the glacier. As it carries outwash away from a glacier, meltwater can flow through pre-established bedrock valleys, depositing the material and burying the valley. This results in a flat surface where a valley once was and is referred to as a buried valley. Buried valleys are typically the most productive ground water aquifers in the state and provide drinking water to millions of Ohioans.
Clay & Silt
When glacial meltwater doesn’t have a large pre-existing bedrock valley to wash away material, lakes can form between bedrock ridges and the front of the glacier. Within these lakes, fine-grained materials like clay and silt settle out to the bottom and are deposited in layers. As the glaciers retreat, water from the lake spills out into new pathways, sometimes drastically eroding away the landscape. (Learn more about glacial lake outburst floods here).
The most famous glacial lake in Ohio is our Great Lake, Lake Erie. Lake Erie formed as glacial ice advanced across easily erodible bedrock, such as the Ohio Shale, carving the deep basin that the lake fills today. However, most glacial lakes around Ohio were much smaller and are now completely drained as they were dammed directly by the glacial ice. These smaller glacial lakes are recognized now only by the flat, abandoned lake plain left behind after glacial retreat.
Under the right conditions, some of the material sorted by water in lakes and meltwater streams can be picked up by wind and transported onto nearby hillsides. Silt-sized particles entrained by wind and transported through the air are called loess. A thin mantle of loess covers most glacial deposits in Ohio. But loess tends to be thicker closer to its source, usually a river valley or dry lakebed. When sediment is too coarse to be picked up and moved out of the source area by wind, it can be shaped in sand dunes, like those preserved in the Oak Openings region near Toledo, Ohio. Loess is the best sorted of all glacially derived sediments because wind has the lowest strength relative to water and ice. Loess deposits are typically comprised of silt, with minor amounts of clay, which could be produced from weathering of silt and not primary deposition.
Many of the hills, valleys, and ridges across the glaciated two-thirds of Ohio have been shaped by glaciers. These features are called glacial landforms, and they help geologists understand how glaciers moved across the landscape.
There are two main categories of glacial landforms: erosional landforms and depositional landforms. Erosional landforms are formed when the action of glacial movement carves or scours the pre-existing landscape or substrate. Depositional landforms are formed when the action of glacial movement constructs or builds a new landform on top of the existing landscape with sediment entrained below, within, or on top of the ice.
Examples from both categories of landforms were left behind in Ohio by Quaternary-age glaciers. However, some glacial landforms found in other parts of the world are not found in Ohio today.
Erosional Glacial Landforms
Erosional glacial landforms are relatively rare in Ohio. These landforms, like cirques and u-shaped valleys, tend to be mainly preserved in mountain glaciers and would not be expected to be widespread during continental glaciation. However, a few specific examples of erosional glacial landforms remain preserved in Ohio.
The most famous of these erosional landforms are the glacial grooves at Kelley’s Island (see photo at top of the page). The grooves were carved into the Columbus Limestone beneath an overriding glacier. The traditional explanation of the formation of the grooves was that hard, igneous boulders being dragged by the glacier scoured the softer limestone bedrock. An alternative hypothesis states that highly pressurized water squeezed in between the ice and the limestone bedrock created the grooved channel. Scientists who study the physics and model the movement of glaciers can create computer models to help understand which theory is most plausible. Other examples of grooves and striations are preserved within the relatively soft limestone bedrock of northwestern Ohio. The elongated direction of these grooves and striations gives clues to the direction that the glacier was travelling.
In central Ohio, some knobs of Black Hand Sandstone were shaped by glaciers to form roches moutonnées (sheepback). These glacial landforms have a smooth gentle slope upstream of ice flow and a plucked slope on the downstream slope. One such example is partially preserved in the Columbus Metro Park Chestnut Ridge, near Canal Winchester, Ohio.
Diagram showing formation of Chestnut Ridge, Fairfield County, an example of a roche moutonnées
Some modern lakes can trace their histories directly back to glacial times. In Pickaway County, Stage’s Pond is a small modern lake that was created when an ancient lake on top of a glacier catastrophically failed, flushing water and sediment through a vertical shaft in the ice and out the bottom of the glacier. This process creates a moulin in the glacier, and the water that flows through the moulin erodes away the land beneath the glacier leaving behind a moulin scar. Stage’s Pond now occupies the moulin scar left behind by the moulin.
Stage's Pond State Nature Preserve, Pickaway County, Ohio
Depositional Glacial Landforms
The majority of glacial landforms in Ohio are depositional landforms. Ohio has many examples of landforms, including moraines, valley-train outwash/outwash plains, lake plains, kames, and eskers.
The two types of moraines, ground moraines and end moraines, are the primary landforms found at the surface of Ohio today. Ground moraines are flattened plains composed of glacial till deposited beneath a glacier. Ground moraines are well preserved in western central Ohio, where the underlying bedrock and pre-glacial landscape also was relatively flat. End moraines are looping ridges composed of complex deposits of till, sand, and gravel that represent places where glaciers stopped moving across the landscape but continued to internally move sediment in front of the glacier. There are numerous examples of moraines throughout Ohio, but one end moraine is particularly visible to those traveling between Cincinnati and Columbus. This end moraine, the Reesville Moraine, can be seen at mile marker 58 (SR-72 exit, Clinton/Greene County line).
Glacial outwash deposited through a vast, flat area and a network of braided streams is called an outwash plain. Outwash plains are depositional landforms of varying scale. They are found all over Ohio and are typically associated with an end moraine. Valley-train outwash is a depositional glacial landform where glacial outwash is routed between a valley enclosed on two sides by bedrock walls. Most of the outwash from the Scioto Sublobe during the Wisconsinan glaciation was funneled through the Scioto River Valley and eventually into the Ohio River Valley as valley-train outwash. When enough valley-train outwash builds up in the valley to fill the entire valley, the landform becomes a buried valley. Buried valleys can be undetectable from the surface. This is the case for the Teays River Valley in west-central Ohio, where depths to bedrock can decrease greatly over the 2-mile wide valley, while surface elevations show no changes in relief.
Lake plains are a depositional landform that are not unique to glacial environments. Lakes form when inland basins allow the enclosed catchment of surface and/or groundwater. Climatic or environmental conditions can change draining lakes and leave behind only a flattened plain of sediments previously deposited in the lake. In glacial environments, lake plains can be created by removing/melting an ice or sediment dam that was retaining water. Many glacial lakes have spillways that can alter pre-established drainage routes after the ice/sediment dams fail. Large lake plains related to precursors to Lake Erie are preserved across northwestern Ohio.
Kames are hills of water-sorted sand and gravel that were deposited into holes or basins in glaciers. When the ice melts away, the sediment is left behind in the form of an irregularly shaped hill. Kames are relatively common throughout Ohio, but the most dramatic examples of kames can be found between the Killbuck and Grand River Sublobes around Summit and Portage Counties.
Eskers are sinuous ridges of sand and gravel that were deposited in drainage networks below the glacier. This landform features positive topographic relief because channels were developed within the glacier. As ice began to melt away, sediments were dropped behind in place, standing taller than surrounding areas on the landscape. Sediments in eskers can sometimes share similar properties with river deposits, including crossbedding and grading. A well-known example of an esker is the Circleville Esker. Sand and gravel from the Circleville Esker was mined in the late 1800s for construction material, which helped build the town of Circleville, Ohio. Eskers remain an important source of sand and gravel resources to this day.
Geologists use three main categories to classify rocks at the most basic level: igneous rocks, metamorphic rocks, and sedimentary rocks. Igneous rocks are formed as molten material (lava/magma) cools and solidifies. How long it takes this material to cool, along with the chemical composition of the melt, will determine the properties of the rock. Sedimentary rocks are first deposited as loose sediment derived from older rocks before being cemented together and lithified or hardened. These rocks often contain layering (bedding) and fossils. Metamorphic rocks are formed by applying heat and/or pressure to sedimentary and igneous rocks. Ohio bedrock is composed entirely of sedimentary rocks at the surface. So why is it possible to find igneous and metamorphic boulders along the surface of Ohio?
Glaciers are the reason we have igneous and metamorphic boulders in parts of Ohio. As glaciers advanced southward across Canada, they eroded igneous and metamorphic bedrock. These boulders were carried by the ice into Ohio. The boulders can be transported hundreds of miles by being trapped within glacial ice until conditions of the ice flow change, or the ice melts, and the boulder is deposited in a new location. These boulders, known as erratics, are surrounded by other glacial sediments, such as till and outwash, indicating they were deposited at the same time. Occasionally the erratic boulders are clustered together in a narrow band called a boulder belt.
Examples of glacial erratics
Glacial erratics can help geologists determine the path that a glacier travelled. Certain rocks that are rare or have characteristics that are localized to a specific location can be used to trace where the glacier came from. For example, the Gowganda Tillite or Gowganda Conglomerate is a formation from the Huronian Super Group, which outcrops in Ontario, Canada, along the northern shore of Lake Huron. Gowganda Tillites are relatively common glacial erratics across the glaciated portion of Ohio, and they indicate that Quaternary glaciers must have travelled across that source region in Ontario before advancing into Ohio. Matching other examples of distinctive glacial erratics can provide more information about the exact path glaciers took as they advanced into Ohio. Tracing glacier pathways can be important to find important resources, like gold, silver, and diamonds, which are found in the bedrock of Canada and have been spread across the region by glaciers.