cuesta (kwĕsˈtə) [key], asymmetric ridge characterized by a short, steep escarpment on one side, and a long, gentle slope on the other. The steep side exposes the edge of erosion-resistant rock layers that form the cuestas. They are usually formed by erosion in plains areas underlain by gently dipping sedimentary rock layers. Cuestas have a more gentle dip than similar structures called hogbacks. Along the U.S. Atlantic and Gulf coastal plains are found a series of low subdued cuestas composed of seaward-dipping and poorly cemented Cretaceous and Tertiary sandstones, while the intervening lowlands are underlain by impermeable clays. These conditions produce ideal structures for artesian water supply systems, which have been extensively tapped by coastal cities. A well-known example of a cuesta is the Niagara cuesta that runs westward across W New York State and Ontario, then swings northward, forming the peninsula between Lake Huron and Georgian Bay, and finally curves southward, forming the Door Peninsula between Green Bay and Lake Michigan. Following withdrawal of the last Pleistocene ice sheet about 10,000 years ago, Niagara Falls first formed where the Niagara River crosses the Niagara cuesta at Lewiston, N.Y., and Queenston, Ont. Since then, the falls have migrated nearly 7 mi (11 km) southward as a result of undercutting and rockfall, leaving the steep-walled Niagara Gorge to mark its path.