Lakes are Minnesota's hallmark resources, central to recreation, fish and wildlife habitat, tourism, and the state's history. Most Minnesota lakes formed during the last ice age about 10,000 to 20,000 years ago, when glaciers created depressions from stranded blocks, rock debris dams that made long finger-lakes, and chains of lakes along glacier margins where wet sediment collected and settled into uneven terrain. For Native Americans, lakes and rivers provided food and transportation, and before European settlement they were the lifeblood of native culture, supplying fish, game, wild rice, and other plant foods. Lakes and streams later supported the fur trade and provided plentiful fish and game to settlers and loggers. Nearly all of Minnesota was once covered by glacial ice during the Great Ice Age. In the early 1900s, small rustic summer homes and typical fishermen's cabins began appearing on lakeshores in north-central Minnesota, but after WWII lakeshore homes and family-owned resorts increased dramatically, later giving way to large resort and conference centers and spacious year-round homes. Increased year-round use has harmed water quality, aquatic and shoreline habitat, and recreational enjoyment, though some natural shoreline remains and other portions have been preserved in parks, wildlife refuges, and research areas. Central Minnesota's lakes and rivers also support wildlife such as the great blue heron, the largest heron in North America, a blue-gray and white bird about 4 feet tall with a 7-foot wingspan that nests in high treetops, usually lives alone except during breeding, and feeds mostly on small non-game fish, aquatic insects, frogs, and small mammals, sometimes spearing fish with its bill and swallowing prey alive.