Most of us recognize the
escarpment’s most notable formation, Niagara Falls. But the escarpment extends far
beyond this magnificent sight. It actually begins east of Rochester, New York, near Union
Hill, and stretches to Wisconsin. Along the way, it sometimes disappears into farmland or
sinks beneath lakes.
Four geological formations contributed to the making of today’s
escarpment. The first began approximately 430 million years ago, during the geological
period called Upper Ordovician. At that time, the Appalachian Mountains on the eastern
coast of the U.S. were as high as the Rocky Mountains. As these mountains eroded, ancient
rivers carried the sediment westward into a delta region where Lake Ontario and Lake Erie
exist today. This sediment hardened into red shale and sandstone forming the
escarpment’s base.
The second formation that contributed to the escarpment was a giant
shallow sea that lay in a depression of the earth’s crust, the center of which is
located in the state of Michigan. The sea slowly filled and flooded the river delta
region. Over millions of years the ocean’s plant and animal life died, mixing with
ocean minerals. Over time, the ocean waters rose and fell depositing varying types and
amounts of sediment into distinct layers, such as soft shale, sandstone, limestone, and
dolostone.
Then the third geological change came about. Changes in the
earth’s crust caused the Michigan Basin to rise, slowly draining the waters. Over
millions of years, erosion began removing the softer shale underlying the more resistant
dolostone. As the softer material eroded away, large chunks of dolostone broke off
creating cliffs. Today, erosion continues to be the most important factor shaping the
escarpment. Water and wave action have created dramatic results, such as cliffs along the
shores of Georgian Bay on the Bruce Peninsula.
The fourth and last change occurred in the most recent of geologic
times. At least four glaciers buried the area in sheets of ice one to two miles (2 to 3
km) thick. Each glacier left its mark by widening valleys, scraping rock layers off the
top of the escarpment, and depositing mounds of rock below the cliffs. The last glacier,
known as the Wisconsin, retreated 12,000 years ago.