Articles

Ohio's glacial past created every inch of today's terrain

Timothy A. Snyder
Ohio Division of Natural Areas and Preserves
Urbana Daily Citizen Sunday Extra, May 5, 2002

Everyone knows that Ohio was glaciated sometime in the past, but just how that event shaped the present face of the state may be less well known. You would be hoard-pressed to find a square foot of ground in Ohio that escaped the influence of the ice.

What was not covered by the glacier suffered rock-cracking fluctuations of temperature, the wasting effects of violent freeze-thaw cycles and complex interactions of erosion and deposition resulting from floods of meltwater. Those parts of the state actually covered by ice were affected even more dramatically.

The first principle of continental glaciation is that ice moves, but only in one direction. Accumulating ice in the Hudson Bay region of Canada flowed outward, just as water poured from a bucket mounds for an instant on the ground then runs off in every direction. And just as that water cannot flow uphill back toward or into the bucket, so the continental ice sheets could not flow back into Canada.

But eventually the ice "retreated." All the books say so. After all, it's not here anymore. In this case, retreat means simply to melt away.

Since the advancing front of a glacier will usually be thinner than the body of ice farther back, it will melt away more rapidly.

As long as the glacier is moving forward faster than its front edge is melting away, the ice front will advance.

When melting gets the upper hand, the front edge of the glacier will appear to move backward - to retreat.

Another tendency of water, whether hared or soft, is its habit of flowing toward areas of least resistance, which usually means lower ground.

For the Hudson Bay ice, that meant a southward flow as far as the Adirondack Highlands of New York. Blocked by the mountains, it then flowed west up the present St. Lawrence River valley, gouging out the basins of the Great Lakes along the way.

Once it passed beyond the confining hills of Pennsylvania and eastern Ohio, the advancing ice was able to spread south across the broad, level plains of western Ohio and Indiana.

It came as a massive rounded lobe, an amoeba-foot of ice hundreds, if not thousands, of feet thick. We call this section of t he great glacier the Erie Lobe after the lake it formed.

As the Erie Lobe ground its way southward, it found only two obstructions.

One was the Bellefontaine Outlier, an island of Devonian-aged bedrock which for some reason had never eroded away and stood a hundred feet higher than the plain of Silurian dolomite that surrounded it. The outlier may not have been very high, but it was enough to split the advancing lobe of ice into two sublobes: the Scioto Lobe to the east and the Miami Lobe to the west. As the thicker ice behind the glacial front advanced, it was able to override the outlier, but its influence remained.

The other obstacle to the ice was, in the end, more formidable: the sun. eventually the climate warmed and the glacier could not advance fast enough to replace the ice melting away at its front. Although the ice still advanced, its front stalled and then began to retreat. There would be other minor advances during the long dance between fire and ice, but there would be far more retreats. Slowly the melting ice uncovered the face of Ohio, a face now far different in appearance from what it had been before.

The ice not only took away as it gouged and scraped its way across the Midwest; it also gave back. All the intermingled ground-up rock and old soils and broken forests it had gathered up in its advance were left behind as it retreated like the spoils of a vanquished army. Ohio owes its agricultural wealth and much of its topography to what the glacier left behind. But that's another story.