The Glacial Landscape of Minnesota
Quaternary Geology ES 767
For the last two million years the present location of Minnesota has been sculpted by glacial ice. Details of previous glaciations have been largely covered by the most recent Wisconsin glaciation. The current landscape of Minnesota was greatly influenced by these most recent glaciations and the corresponding melting of ice. Over the last 75,000 years, the Wisconsin glaciation has brought four prominent ice lobes into the state from the Laurentide Ice Sheet of Canada. The paths of the four glacial lobes can be tracked and dated by their specific lithologies and landforms they create. The sculpting ice and flowing water left Minnesota with a history that can be told by analyzing the various glacial hills, lakes, and rivers valleys.
The Wisconsin Glaciation
Starting about 75,000 years ago, the Wisconsin glaciation in Minnesota was dominated by four lobes that extended from the Laurentide Ice Sheet of Canada. Over the course of about 65,000 years, these lobes advanced and retreated several times leaving clues within Minnesota’s landscape as to where their greatest position was and which direction the lobes were traveling. Due to differences in the lithologies of the landforms throughout the state, the ice lobe or lobes that deposited the landform can be identified.
Early Wisconsin glacial advances of these lobes tend to be speculative due to the fact that these advances are covered by younger advances of the same or different lobes. Many of these advances generally date older than 40,000 years (Ojakangas & Matsch, 1982, p. 105). Early glacial advances came from three of the four lobes that dominated Minnesota glaciations: The Wadena, Rainy, and Superior lobes. The advances of the glacial lobes within the last 30,000 years are much less speculative. Evidence of these advances is left as surface deposits within Minnesota’s landscape. During the late Wisconsin glaciation, the Des Moines lobe was the dominant lobe, with the Superior, Rainy, and Wadena lobes having much weaker advances. Due to this fact, the majority of Minnesota is covered by the glacial till of the Des Moines lobe.
One of the last glacial advances in Minnesota during the Wisconsin age. This image shows the ice coverage of the Des Moines lobe and its sub-lobes. Map created by the Minnesota Geological Survey.
For many years the origin of the Wadena lobe was thought to be Manitoba. Evidence now suggests that the Wadena lobe likely originated much further east than originally thought. The evidence cites the carbonates and the omars contained in the Wadena till was likely from the Hudson Bay area (Ojakangas, 2009, p. 23). The bedrock contained in the Wadena till is limestone and granite. This leaves the till with a buff to yellowish-brown color. The Wadena lobe and the Des Moines lobe originate from a similar direction. The great difference between the two is the Wadena till does not contain large amounts of Crataceous shale, while the Des Moines till does (Ojakangas & Matsch, 1982, p. 104).
Three very prominent landforms deposited by the Wadena lobe were the Alexandria and Itasca moraines and the Wadena drumlin field. All three features can be found in central Minnesota. The Alexandria moraine would later be covered by the Des Moines lobe. The Wadena drumlins number up to about 1,200 tear-shaped hills near present day Wadena, Minnesota.
The origin of the Rainy lobe is from the northeast. The bedrock incorporated in the Rainy till is much more diverse than that of the Wadena till and contains rocks from various locations in Ontario and northeast Minnesota. The Rainy till contains granite, gabbro, basalt, red sandstone, iron formation, slate, and greenstone. This combination of bedrock makes for a red to brown till. The till of this lobe is very similar to that of the Superior lobe. The difference between the till is the Rainy till tends to be sandier and more course while the Superior till contains much more clay (Ojakangas & Matsch, 1982, p. 104). The Rainy lobe was responsible for creating the Toimi and Pierz drumlins in northeast and east-central Minnesota and also part of the St. Croix moraine in central Minnesota.
The foreground contains a gravel pit, while the background is Powder Ridge Ski Area in Kimball, Minnesota. The ski hill is part of the St. Croix moraine in Central Minnesota. Photo date 10/11; by Mark Krippner
The origin of the Superior lobe is quite similar to the Rainy lobe from the northeast. The bedrock contained in the Superior lobe ice was a mixture of granite, gabbro, basalt, red sandstone, iron formation, and slate. The till tends to be more red and finer grained than the Rainy lobe till (Ojakangas & Matsch, 1982, p. 104). The red till contains more clay than the Rainy lobe, which likely came from scouring the bottom of Glacial Lake Duluth during various advancements and retreats of the Superior lobe (Ojakangas, 2009 p.50).
Superior lobe till at a gravel pit along the Merden esker in Collegeville, Minnesota. The center of the photo contains a fresh exposure of the red, fine-grained till. Photo date 10/11; by Mark Krippner
Two large moraines were deposited from the advancement of the Superior lobe. The earliest moraine system developed from the Superior was the St. Croix moraine of central Minnesota. The St. Croix moraine was likely formed in the mid-Wisconsin glaciation. The Superior lobe was also responsible for depositing the Mille Lacs moraine of east-central Minnesota. This advance occurred in the late Wisconsin and was one of the last advances of the glaciation.
Des Moines Lobe
The Des Moines lobe is one of the more studied and most understood lobes from the Wisconsin glaciation due to the fact that its glacial sediment is found near the surface of most of the state. The origin of the Des Moines lobe is similar to that of the Wadena lobe. The lobe likely originated west of the Wadena lobe due to the Crataceous shale found in its till. The bedrock contained in the till is a mixture of limestone, granite, and Crataceous shale. The till of the Des Moines lobe takes a buff to yellowish brown color. The till contains few boulders and tends to be fine textured.
Des Moines till atop Salisbury hill near Henderson, Minnesota. Photo date 6/11; by Lee Schmidt, used with permission
The Des Moines lobe reached its maximum position in present day Des Moines, Iowa about 14,000 years ago. The ice at its maximum covered about 75% of the state. The lobe itself had two well known sub-lobes in the north and central part of the state. Both of these sub-lobes traveled in an easterly direction following lower elevations on the Minnesota landscape. The northern sub-lobe was the St. Louis sub-lobe, covering most of northern Minnesota. The St. Louis sub-lobe was responsible for depositing the Culver moraine. The sub-lobe in the central part of the state was the Grantsburg sub-lobe, which is known to have deposited the Pine City moraine in east-central Minnesota (Ojakangas, 2009, p. 65-66).
Being one of the last lobes to cover the state of Minnesota, the Des Moines lobe left some glacial sediment on other previously deposited moraines such as the St. Croix and Alexandria moraines. The Des Moines lobe is responsible for depositing the Bemis and Altamont moraines of central and southern Minnesota.
Due to an increase in air temperature, Minnesota’s ice started to melt. Throughout the course of about 5,000 years Minnesota was prone to glacial lakes. Seven prominent glacial lakes were formed and drained between 9,000-14,000 years ago. These lakes were usually dammed by a combination of moraines and glacial ice.
Two of the first glacial lakes to form were Glacial Lakes Grantsburg and Duluth about 14,000 years ago. Glacial Lake Duluth formed in the Lake Superior basin after the retreat of the Superior lobe. Glacial Lake Grantsburg formed in front of the retreating Grantsburg sub-lobe in east-central Minnesota. The water from Glacial Lake Grantsburg would eventually drain into Glacial Lake Duluth. Glacial Lakes Aitkin and Upham formed 1,000 years later in north-central Minnesota from the St. Louis sub-lobe (Ojakangas, 2009, p.39). During times of large ice melt, these lakes were often connected. Glacial Lake Upham received a similar fate to Glacial Lake Grantsburg in that it drained into Glacial Lake Duluth. Glacial Lake Aitkin emptied to the south via the Mississippi River. Two glacial lakes formed from the retreating of the Des Moines lobe. Glacial Lake Minnesota formed in south-central Minnesota near present day Mankato. Glacial Lake Benson formed along the present day Minnesota-South Dakota border. Both Glacial Lake Benson and Minnesota were located in what is now the Minnesota River Valley.
Fine-grained sediment in Blue Earth County from Glacial Lake Minnesota. Photo by Carrie Jennings, used with permission
These giant potholes of Interstate State Park in Taylor’s Falls, Minnesota were created by boulders caught in swirling currents in the St. Croix River. Photo date 6/11; by Lee Schmidt, used with permission
The largest of the glacial lakes, Glacial Lake Agassiz, resided in parts of northwest Minnesota, northeast North Dakota, Ontario, and Manitoba. Glacial Lake Agassiz formed about 11,700 years ago after the retreat of the Des Moines and James lobes (Ojakangas, 2009, p. 191). The ever-changing lake used several outlets to drain the high volume of water including the Glacial River Warren. Glacial River Warren occupied the present day Minnesota River Valley and drained Lake Agassiz to the south. In some Minnesota River Valley areas, Glacial River Warren carved a stream bed nearly eight kilometers wide and 100 meters deep.
A photo of flood protection in Henderson, Minnesota. This photo shows the concrete flood gates from atop the man-made ridge. The small town is located in the Minnesota River valley and experiences seasonal flooding nearly every year. Photo date 6/11; by Lee Schmidt, used with permission
Evidence of these glacial lakes exists today in the fertile soils of the lake bottoms, beach and shoreline deposits, and flat plains (Bluemle, 2007). The Red River Valley, home to the former Glacial Lake Agassiz, is one location where the fine grained lake bottom sediment allows for some of the more fertile soil in Minnesota. Four major Lake Agassiz beaches have been identified in Minnesota. The oldest beach, dating back 12,000 years, is named the Herman beach and found near the town of Herman while the youngest dates back to 9,500 years and is named Campbell and found near the town of Campbell (Ojakangas, 2009, p. 193). The Red River Valley, situated on the lake bed of Agassiz, is extremely flat. This leads to flooding nearly every spring along the Red River.
The only areas of Minnesota that escaped the most recent glaciations were the extreme southeast and southwest corners of the state. The southwest corner of the state is home to the Coteau des Prairies. The Coteau des Prairies is a high plateau which the Des Moines lobe of Minnesota and the James lobe of the Dakotas missed due to the plateau’s high elevation. The southeast corner of the state is home to the Mississippi River bluffs. Much like what was happening on the southwest side of the state, the elevation of the bluffs shielded the southeast from the Des Moines lobe.
A view from atop the Red Wing overlook in Red Wing, Minnesota. Red Wing, part of the driftless area, did not escape large amounts of melt-water traveling down the Mississippi River. The water created this deep valley and left the area much lower than the surrounding bluffs. Photo date 6/11; by Lee Schmidt, used with permission
Bluemle, J. (2007). Glacial Lake Agassiz. Retrieved October 1, 2011 from https://www.dmr.nd.gov/ndgs/ndnotes/Agassiz/Lake%20Agassiz.asp
Jennings, C. (2011). Cenozoic glaciation of Minnesota. Powerpoint lecture presented at Hamline University
Lusardi, B.A. (1997). Minnesota at a glance: quaternary glacial geology. Retrieved October 1, 2011 from http://www.d.umn.edu/~pmorton/4110/notes/001_Mn_Quaternary.pdf
Ojakangas, R. (2009). Roadside geology of Minnesota: Mountain Press Publishing Company.
Ojakangas, R., Matsch, C. (1982). Minnesota’s geology: University of Minnesota Press.