Duluth, Minnesota Lakes Waterway and the Saint Lawrence Seaway. Duluth forms a metropolitan area with neighboring Superior, Wisconsin, called the Twin Ports. The cities share the Duluth–Superior harbor and together are the Great Lakes' largest port, transporting coal, iron ore (taconite), and grain. A tourist destination for the Midwest, Duluth features the United States' only all-freshwater aquarium, the Great Lakes Aquarium; the Aerial Lift Bridge, which spans the Duluth Ship Canal into the Duluth–Superior Harbor; and Minnesota Point (known locally as Park Point), the world's longest freshwater baymouth bar, spanning . The city is also the starting point for vehicle trips along
Duluth, Minnesota sandy core. As the last glacier retreated, meltwaters filled the lake to as high as 500 feet above the current level; the Skyline Parkway roughly follows one of the highest levels of the ancient Lake Superior, Glacial Lake Duluth. The sandstone that buried the igneous rocks of the rift is exposed near Fond du Lac. At one time a large number of quarries produced the stone, sold as Fond du Lac or Lake Superior brownstone. It was widely used in Duluth buildings and also shipped to Minneapolis, Chicago, and Milwaukee, where it was also used extensively. The weathered sandstone forms