The entire area of Northwest Ohio and Southeast Michigan was covered by glacial ice during the Late Wisconsin Period thousands of years ago. The glaciers deposited clay till and leveled any valleys or ridges, making the region very flat. The clay also tended to hold water at or near the surface. This flat, poorly drained area was called the Lake Plains and formed the basis for the Great Black Swamp south of the Maumee River. Glacial lake deposits of clay are up to 100 meters thick over Paleozoic bedrock creating poorly drained mineral soils which characterize the Lake Plains.
When the glacier melted back from Michigan and Ohio about 15,000 years ago, an ice sheet blocked the eastern outlet of what is now Lake Erie. The melt waters rose and covered northwest Ohio and southeastern Michigan in a sequence of lakes and sandy shorelines. When the glacier melted, meltwater streams deposited sand into the shallow pro-glacial lakes. Typically sand in these channels is only one to three meters thick. The sand belt extends for approximately 120 miles from northeast of Napoleon, Ohio to west of Detroit, Michigan, oriented towards the northeast.
When Lake Warren receded, the sand was blown into dunes many feet thick which rested on top of a nearly impervious layer of clay till. Water flowed readily through the sand making the dunes dry and desert-like on top, whereas the soil adjacent to the swales are poorly drained. The thick layer of sand over the clay till plays an important role in water availability to plants and animals in the Oak Openings.
Interested in more about the geology of the area and what makes the Lakeplain Oak Openings different that the Oak Openings in other parts of southern Michigan and northwest Ohio? Check out these sites:
Sources: Living in the Oak Openings: A Homeowners’s Guide to one of the World’s Last Great Places, The US EPA Region 5 Great Lakes Ecosystem report, and Cormer et. al A Survey of Lakeplain Prairie in Michigan. 1995.