Lake Duluth was a

proglacial lake In geology, a proglacial lake is a lake formed either by the damming action of a moraine during the retreat of a melting glacier, a glacial ice dam, or by meltwater trapped against an ice sheet due to isostatic depression of the crust around t ...

that formed in the

Lake Superior Lake Superior is the largest freshwater lake in the world by surface areaThe Caspian Sea is the largest lake, but is saline, not freshwater. Lake Michigan–Huron has a larger combined surface area than Superior, but is normally considered tw ...

drainage basin A drainage basin is an area of land in which all flowing surface water converges to a single point, such as a river mouth, or flows into another body of water, such as a lake or ocean. A basin is separated from adjacent basins by a perimeter, ...

as the Laurentide Ice Sheet retreated."Post-Valders Lake Stages in the Lake Superior Basin"
, i

by N. King Huber, USGS Geological Survey Professional Paper 754-A The oldest existing shorelines were formed after retreat from the Greatlakean advance (previously called the Valders), sometime around 11,000 years B.P. Lake Duluth formed at the western end of the Lake Superior basin. Lake Duluth overflowed south through outlets in Minnesota and Wisconsin at an elevation of around 331 m above sea level.

Predecessor lakes

As the Superior lobe of the Laurentian Ice Sheet melted, water became trapped along the ice margin and formed several independent lakes. The modern water basins in which these lakes stood have been used for their names.

Lake Nemadji A lake is often a naturally occurring, relatively large and fixed body of water on or near the Earth's surface. It is localized in a basin or interconnected basins surrounded by dry land. Lakes lie completely on land and are separate from t ...

occupied the headwaters of the

Nemadji River The Nemadji River is a river rising in Pine County, Minnesota, United States, which flows through Carlton County, Minnesota, and Douglas County, Wisconsin, to Lake Superior. The river is long measured from its source in Maheu Lake in Pine Coun ...

, Lake Brule occupied part of the

Brule River The Brule River is a U.S. Geological Survey. National Hydrography Dataset high-resolution flowline dataThe National Map, accessed December 19, 2011 river in the U.S. states of Michigan and Wisconsin. Nearly, almost all of the course forms a po ...

basin, and Lake Ontonagon was in the

Ontonagon River The Ontonagon River ( ) is a river flowing into Lake Superior at the village of Ontonagon, on the western Upper Peninsula of Michigan in the United States. The main stem of the river is long''The American Rivers Outstanding Rivers List'', Secon ...

basin.Professional Paper 154—A, Moriaines and Shore Lines of Lake Superior Basin: Frank Leverett; United States Government Printing Office, Washington; February 9, 1929; (Pages 1-72) These lakes are all west of the

Keweenaw Peninsula The Keweenaw Peninsula (, ) is a peninsula of the U.S. state of Michigan. Part of the greater landmass of the Upper Peninsula of Michigan, Upper Peninsula, the Keweenaw Peninsula projects about northeasterly into Lake Superior, forming Keweena ...

. To the east, the ice melt drained into Green Bay or

Lake Michigan Lake Michigan ( ) is one of the five Great Lakes of North America. It is the second-largest of the Great Lakes by volume () and depth () after Lake Superior and the third-largest by surface area (), after Lake Superior and Lake Huron. To the ...

through channels crossing the western part of the

upper peninsula of Michigan The Upper Peninsula of Michigan—also known as Upper Michigan or colloquially the U.P. or Yoop—is the northern and more elevated of the two major landmasses that make up the U.S. state of Michigan; it is separated from the Lower Peninsula of ...

. Lake Duluth extended a few miles farther east than the Keweenaw Peninsula, to the border of the

Huron Mountains The Huron Mountains are located in the Upper Peninsula of Michigan, Upper Peninsula of the U.S. state of Michigan, mostly in Marquette County, Michigan, Marquette County, and extending into Baraga County, Michigan, Baraga County, overlooking Lake ...

, east of

Keweenaw Bay Keweenaw Bay is an arm of Lake Superior in North America. It is located adjacent to the Upper Peninsula of the U.S. state of Michigan, to the southeast of the Keweenaw Peninsula. Keweenaw Bay is 22 miles long and 12 miles wide at the mouth. The he ...

. In the early part of the recession of the ice front Lake Duluth, with an outlet from the

Valley through the St. Croix River Valley, was present in the western part of the Lake Superior Basin. A little later the ice border retreated sufficiently to allow the small independent lakes to become a part of Lake Duluth or to be drained by the lowering of the water level, for in general the water level was lowered as these lakes merged with Lake Duluth. Lake St. Louis was one of the bordering lakes present when the Superior ice lobe was at its full extent. It stood in the part of the St. Louis River drainage basin northwest of the border of the Superior lobe. Lake Nemadji was an outlet channel leading west from the west end of the Lake Superior Basin to Moose Lake, where it joined the outlet of Lake St. Louis. Lake Brule was a small lake in the Brule River basin south of the copper range. It has its outlet to the St. Croix Valley. This was before Lake Duluth gained the St. Croix as its outlet. Lake Ashland occupied several townships in northwestern

Ashland County, Wisconsin Ashland County is a county located in the U.S. state of Wisconsin. As of the 2020 census, the population was 16,027. Its county seat is Ashland. The county was formed on March 27, 1860, from La Pointe County. The county partly overlaps with ...

. It extended west in

Bayfield County Bayfield County is a county located in the U.S. state of Wisconsin. As of the 2020 census, its population is 16,220. Its county seat is Washburn. The county was created in 1845 and organized in 1850. The Red Cliff Band of Lake Superior Chipp ...

to the east slope of the Bayfield Peninsula. Lake Ontonagon occupied much of the Ontonagon basin, south of the copper range. The ice border remained along the copper range for a long period during its existence.

Limits of the lake

Lake Lundy with Lake Chicago and Lake Duluth 1913

As the ice lobe retreated northwards, the independent lakes listed above joined together, forming a single lake at the level of the St. Croix Valley outlet. It is probable that several small glacial lakes in succession, Lakes Nemadji, Ashland, and Ontonagon, merged at lower levels with that of Lake Brule. Lake Nemadji and Lake Ashland dropped less than . Lake Ontonagon dropped nearly . Its area is now land. Several narrow bays of Lake Duluth, extended up each of the tributaries of

south of the copper range. The ice lobe retreated eastward in the Superior basin, taking a long period of time to completely leave the basin. During the process, the outlet continued to deepen and the beaches around the lake continued to drop. Thus, the higher beaches in the eastern portion of the basin are extension of middle and lower beaches in the western basin. The eastern limits of Lake Duluth on the south shore have been found to be at the

, in

Marquette County, Michigan Marquette County ( ) is a County (United States), county located in the Upper Peninsula of the U.S. state of Michigan. As of the 2020 United States census, 2020 census, the population was 66,017. It is the most populous county in the Upper Peni ...

. Once the ice receded to the east of the mountains, new outlets opened across the

Upper Peninsula of Michigan The Upper Peninsula of Michigan—also known as Upper Michigan or colloquially the U.P. or Yoop—is the northern and more elevated of the two major landmasses that make up the U.S. state of Michigan; it is separated from the Lower Peninsula of ...

, carrying the melt water into the Lake Michigan basin. Lake Duluth was lowered as lower lines drainage along the ice front. Eventually the water level reached that of

Lake Algonquin Lake Algonquin was a prehistoric proglacial lake that existed in east-central North America at the time of the last ice age. Parts of the former lake are now Lake Huron, Georgian Bay, Lake Superior, Lake Michigan, Lake Nipigon, and Lake Nipissing ...

. The body of water occupied the western part of the

basin as well as the basins of

and

Lake Huron Lake Huron ( ) is one of the five Great Lakes of North America. It is shared on the north and east by the Canadian province of Ontario and on the south and west by the U.S. state of Michigan. The name of the lake is derived from early French ex ...

and finally the eastern part of the Lake Superior basin. The eastern limits of Lake Duluth on the north coast of Lake Superior have not been determined. It is known, however, that the lake extended at least to the

Kaministiquia River The Kaministiquia River () is a river which flows into western Lake Superior at the city of Thunder Bay, Ontario. ''Kaministiquia'' () is an Ojibwe word meaning "where a stream flows in island" due to two large islands (McKellar and Mission) at ...

basin west of

Fort William, Ontario Fort William was a city in Ontario, Canada, located on the Kaministiquia River, at its entrance to Lake Superior. Incorporated as a town in 1892 and as a city in 1907, it was amalgamated with Port Arthur and the townships of Neebing and McIntyre ...

. It is probable that there was a protrusion of the ice into Keweenaw Bay at the time of greatest expansion of Lake Duluth, and the east end of the Keweenaw Peninsula may have been beneath the ice down to the time the Lake Duluth waters were drained eastward.

References

{{DEFAULTSORT:Duluth, Lake Geology of Minnesota Geology of Wisconsin Geology of Michigan Former lakes of North America Proglacial lakes Glacial lakes of the United States Glacial lakes of Canada

Predecessor lakes

Limits of the lake

See also

References