The Death and Life of the Great Lakes by Dan Egan:
Niagara Falls are what made the Great Lakes unique in the natural world. The falls are the most famous 1,100 yards of a 650-mile-long ridge of sedimentary rock arcing from western New York, into the province of Ontario, and down into Wisconsin. This escarpment is the rim of a 400-million-year-old seabed that cradled a shallow, tropical ocean that once sloshed across what is today the middle of North America. At about 170 feet high, the falls that tumble over the Niagara escarpment near present-day Buffalo, New York, are nowhere near the world’s tallest or even largest by volume. But they were among the most ecologically important because they created an impassable barrier for fish and other aquatic life trying to migrate upstream from Lake Ontario into the other four Great Lakes.