At the greatest extent, glaciers covered about 2/3rds of Ohio. As glaciers extended down, they scraped the surface of the Earth, collecting debris and rock as they went. In the reserve, as the glaciers began melting, they left deposits of unsorted sediment in their wake.
The rock that was deposited during the Quaternary period in Ohio came from melting glaciers which filled valleys with sand, clay, silt, gravel, and boulders. These deposits are still commonly found in Ohio to this day, making up much of the upper layers of our rock and dirt.