Adirondack Mountains (ădˌərŏnˈdăk) [key], mountain mass, NE N.Y., between the St. Lawrence valley in the north and the Mohawk valley in the south; rising to 5,344 ft (1,629 m) at Mt. Marcy, the highest point in the state. Geologically a southern extension of the Canadian Shield, the Adirondacks are sometimes mistakenly included in the Appalachian system. Chiefly metamorphic in composition, they were formed as granite and other rocks intruded upward, doming the earth's surface; later faulting and surface erosion, particularly by glaciers, resulted in a rugged topography, with 46 peaks over 4,000 ft (1,220 m), scenic gorges, waterfalls, streams, and ponds. In the 1980s many Adirondack lakes were found to be unable to support life because of acid rain pollution. The Hudson, Ausable, and Black rivers rise in the Adirondacks. The region contains many resorts, including the famous "great camps"; most of it has been set aside as Adirondack Park, the largest (9,375 sq mi/24,281 sq km, roughly 40% public and 60% private land) U.S. park outside Alaska. Lake Placid and Lake George are major centers. After intensive 19th-century lumbering, the industry has gradually declined. Mines in the Adirondacks have produced iron ore, titanium, vanadium, and talc. The Adirondack Museum, in Blue Mountain Lake, and the Wild Center–Natural History Museum of the Adirondacks, in Tupper Lake, focus on the human and natural histories of the region, respectively.