Thousand Islands, a group of more than 1,800 islands and 3,000 shoals in the St. Lawrence River, E of Lake Ontario, N N.Y. and S Ont., stretching c.50 mi (80 km) along the U.S.-Canada line. Most of the islands are in Canada; Wolfe Island, Ont. (48 sq mi/124 sq km), is the largest. The islands are part of a belt of metamorphic rock connecting the Adirondack Mts. and the Canadian Shield; they were formed at the end of the Ice Age, when the St. Lawrence River became the chief outlet of the Great Lakes. The forested region is a popular summer resort; many of the islands are privately owned, and some have castles built by former wealthy owners. There are numerous parks on the islands, including Canada's St. Lawrence Islands National Park. The five-span Thousand Islands Bridge and highway (7 mi/11 km long; opened 1938) between the New York and Ontario mainlands crosses several islands and channels.