 As the last glacier retreated, meltwaters filled the lake to as high as 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.