Geology of the Niagara Escarpment

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.