Editing Duluth, Minnesota (section)

===Geological history===
[[File:Large anorthosite xenolith in ophitic ilmenite gabbro (anorthosite series, Duluth Complex, Mesoproterozoic, 1099 Ma; Keene Creek East Skyline Parkway roadcut, Duluth, Minnesota, USA) 1 (22039818790).jpg|thumb|[[Anorthosite]] [[xenolith]] in Duluth, part of the Duluth Complex]]

The geology of Duluth demonstrates the [[Midcontinent Rift]], formed as the North American continent began to split apart about 1.1 billion years ago. As the [[Mantle (geology)|Earth's crust]] thinned, [[magma]] rose toward the surface. These intrusions formed a {{convert|16|km|abbr=on}}-thick [[Sill (geology)|sill]], primarily of [[gabbro]], which is known as the [[Duluth Complex]].<ref name="Duluth Streams">{{Cite web |last=Miller, Jim |title=An Introduction to the Geology of the North Shore |url=http://www.lakesuperiorstreams.org/understanding/geology.html |url-status=dead |archive-url=https://web.archive.org/web/20150708182221/http://www.lakesuperiorstreams.org/understanding/geology.html |archive-date=July 8, 2015 |access-date=June 28, 2015 |publisher=Duluth Streams}}</ref>

The creation of the Lake Superior basin reflects the erosive power of [[Laurentide Ice Sheet|continental glaciers]] that advanced and retreated over Minnesota several times in the past 2 million years. The mile-thick ice sheets easily eroded the [[sandstone]] that filled the axis of the rift valley but encountered more resistance from the [[igneous rock]]s forming the flanks of the rift, now the margins of the lake basin. As the last glacier retreated, meltwaters filled the lake as high as {{convert|500|ft|m}} above the current level; the [[Skyline Parkway]] roughly follows one of the highest levels of the ancient Lake Superior, [[Glacial Lake Duluth]].<ref name="Duluth Streams" /> The sandstone that buried the igneous rocks of the rift is exposed near [[Fond du Lac (Duluth)|Fond du Lac]]. At one time, a large number of quarries produced the stone, after which it was sold as Fond du Lac or Lake Superior [[brownstone]]; such stone was widely used in Duluth buildings and also shipped to Minneapolis, Chicago, and Milwaukee. The weathered sandstone forms the sandy lake bottom and shores of Park Point.<ref name="Duluth Streams" />