The Niagara Escarpment
Queenston Heights is part of the Niagara Escarpment, a height of land which extends 725 kilometres across Ontario from Niagara Falls to Manitoulin Island. Over 430 million years ago, a shallow tropical sea covered most of central North America. Sediments and coral reef on the seabed were compressed into dolomite, a hard type of limestone which was more resistant to erosion then the bedrock of the adjacent lands after the water retreated. The cliffs of the escarpment are the exposed floor of the ancient sea. The escarpment's rugged terrain, home to a wide variety of plants and wildlife, forms a natural corridor through both urban and rural areas. In 1990, the United Nations designated the Niagara Escarpment a World Biosphere Reserve.
Plaque via Alan L. Brown's site Ontario Plaques. Full page here.
French side, via @EricDHoover :
