A Geological and Scenic Treasure
The fossiliferous limestone cliffs of the Bruce Peninsula are a small part of an extensive geological phenomenon known as barrier reefs.
Formed mainly by colonial tabulate corals during the Silurian Period between 438 and 408 million years BP (before present), barrier reef structures completely enclose the Michigan Basin region of North America.
While present in the subsurface of Ohio, Indiana, Wisconsin and Michigan, barrier reef limestone forms cliffs all along the Niagara Escarpment from Niagara Falls to Tobermory (and beyond to the Georgian Bay islands including Manitoulin) in the Province of Ontario.
The thickness of the main reef limestone varies between 100 and 500 meters. In the Bruce Peninsula, the cliff-forming limestone is generally named the Cabot Head Formation.
Before Silurian time, reefs were small, pinnacle shaped, and were built by corals and algae, or algae alone. During the Silurian, the colonial tabulate corals began massive reef structures which, of themselves, created a new marine environment, the back reef lagoon. In these lagoons, rapid evolution of many new marine organisms occurred. Extensive biocommunities thrived in the back reef lagoon hosting trilobites, brachiopods, crinoids, bryozoa, and other corals. Periodically, the lagoons dried up, forming thick deposits of salt, gypsum and anhydrite as well as converting some limestone into dolomite.
During Silurian time, the earth’s first land plants (the Psilopsids) appeared. There were no land animals. Later, during the Devonian Period, 408 to 360 million years BP, amphibians first evolved to populate the earth’s surface above the sea.
It is worth knowing that during the Silurian, the North American continent was smaller than today, and was situated with a different geographic orientation. What is now north was then east. And while the Bruce Peninsula is now 45° north of the equator, it was then 10° south of the equator. The barrier reefs were thriving in warm, shallow, tropical seas.
And on the tectonic scene, the Northern Appalachian Mountains were being built by the Taconic Orogeny as Baltica (Scandinavia and western Russia) collided with ancestral North America (Laurentia), closing the Iapetus Ocean (the ancestral pre-Atlantic Ocean).
The film Jurassic Park, premised cloning certain dinosaurs such as Tyrannosaurs Rex which lived during the Jurassic Period about 175 million years BP. The limestone cliffs along the Niagara Escarpment are more than 233 million years older and predate any animal on the earth’s land surface.
Hal Noltimier
Emeritus Professor of Geological Sciences
Ohio State University
