Niagara Escarpment, prominent ridge in North America that extends in a semicontinuous broad arc across the Great Lakes region. Geologically, the Niagara Escarpment forms the steep outer rim of the Michigan Basin, a saucer-shaped geologic structure underlying the Great Lakes. The escarpment, or scarp, is composed of a thick layer of dolomite, a type of rock that resists erosion. Thus, it stands in high relief to the surrounding terrain, extending (with breaks) more than 1,500 km (more than 900 mi) from Wisconsin to New York. Along the western shore of Lake Michigan, the scarp forms Wisconsin's Door Peninsula and Michigan's Garden Peninsula, which separate the lake from Green Bay. To the northeast it forms the islands and peninsulas that separate Lake Huron from Georgian Bay. Near Hamilton, Ontario, the escarpment bends east, crossing between Lake Erie and Lake Ontario. Niagara Falls crosses it at the border between the United States and Canada. The escarpment then continues east into New York, where it is cut by the Finger Lakes.

Several waterfalls occur where rivers cross the escarpment. The Niagara River drops 99 m (326 ft) within 55 km (34 mi), from its source at Lake Erie to its mouth into Lake Ontario. More than one-half of this drop occurs at Niagara Falls, which consists of the Horseshoe Falls in Canada and the American Falls in the United States. Three cataracts are created when the Genesee River traverses the escarpment, including the 28-m (92-ft) High Falls in Rochester, New York.

The rapid change in the elevation of rivers along the escarpment provides great potential for the development of hydropower. Waterpower use began at Niagara Falls in 1875. By 1881 a hydroelectric generator was operating, and by 1896 a tunnel beneath the falls was producing electricity to light the streets of Buffalo, New York. So much water was being used for waterpower that by 1950 an international treaty was passed to require minimum flows over the falls.

In 1990 the United Nations Educational, Scientific and Cultural Organization (UNESCO) designated the Niagara Escarpment an International Biosphere Reserve in order to protect and monitor its ecosystem. It is one of over 300 Biosphere Reserves throughout the world.