Great Lakes Waterway River between Huron and Erie. Usually, one or more U.S. Coast Guard icebreakers help keep the water passage open for part of the fall and early winter, although shipping usually ceases for two to three months thereafter. The Great Lakes Waterway is co-administered by the governments of Canada and the United States of America. The waterway allows passage from the Atlantic Ocean to the inland port of Duluth on Lake Superior, a distance of and to Chicago, on Lake Michigan, at . Together with the Saint Lawrence Seaway, the Waterway allows both ocean-going vessels and the ore, grain and coal-bearing
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