Wisconsin glaciation

{{Short description|Glaciation in North America during the Last Glacial Period}} {{Distinguish|Weichselian glaciation}} [[File:Iceage north-glacial hg.png|thumb|upright=1.35|Maximum glaciation of the Northern Hemisphere (black) during the Quaternary climatic cycles]] The '''Wisconsin glaciation''', also called the '''Wisconsin glacial episode''', was the [[most recent glacial period]] of the North American ice sheet complex, peaking more than 20,000 years ago. This advance included the [[Cordilleran Ice Sheet]], which nucleated in the northern [[North American Cordillera]]; the [[Innuitian ice sheet]], which extended across the [[Canadian Arctic Archipelago]]; the [[Greenland ice sheet]]; and the massive [[Laurentide Ice Sheet]],<ref name="Harris1997" /> which covered the high latitudes of central and eastern North America. This advance was synchronous with global glaciation during the last glacial period, including the North American [[Glacier#Classification_by_size,_shape_and_behavior|alpine glacier]] advance, known as the [[Pinedale glaciation]]. The Wisconsin glaciation extended from about 75,000 to 11,000 years ago, between the [[Sangamonian Stage]] and the current interglacial, the [[Holocene]]. The maximum ice extent occurred about 25,000–21,000 years ago during the [[last glacial maximum]], also known as the ''Late Wisconsin'' in North America. [[File:Global sea levels during the last Ice Age.jpg|thumb|The [[Last Glacial Period]] caused a much lower global sea level|upright=1.3]] This glaciation radically altered the geography north of the [[Ohio River]], creating the [[Great Lakes]]. At the height of the Wisconsin Episode glaciation, the [[ice sheet]] covered most of [[Canada]], the [[Upper Midwest]], and [[New England]], as well as parts of [[Idaho]], [[Montana]], and [[Washington (state)|Washington]]. On [[Kelleys Island]] in [[Lake Erie]], northern [[New Jersey]] and in [[New York City]]'s [[Central Park]],<ref>SERC Media; Glacial Grooves, Central Park; Image 14884 is a 208 by 173 pixel JPEG; Uploaded: Apr5 09; Wayne Powell, CUNY Brooklyn College; http://serc.carleton.edu/details/images/14884.html</ref> the [[Glacial striation|grooves left in rock]] by these glaciers can be easily observed. In southwestern [[Saskatchewan]] and southeastern [[Alberta]] a suture zone between the Laurentide and Cordilleran ice sheets formed the [[Cypress Hills (Canada)|Cypress Hills]], North America's northernmost point that remained south of the continental ice sheets. During much of the glaciation, sea level was low enough to permit land animals, including [[humans]], to occupy [[Beringia]] (the [[Bering Land Bridge]]) and move between [[North America]] and [[Siberia]]. As the glaciers retreated, glacial lakes were breached in great [[glacial lake outburst flood]]s such as the [[Kankakee Torrent]], which reshaped the landscape south of modern Chicago as far as the Ohio and Mississippi Rivers.

==Timeline== Two related movements have been termed Wisconsin: Early Wisconsin and Late Wisconsin.<ref name=Sherzer>Chapter II. Glacial History of the Huron-Erie Basin; Geological Report on Wayne County; W.H. Sherzer; Michigan Geological and Biological Survey, Publication 12, Geological Series 9; Lansing, Michigan; Wynkoop Hallenbeck Crawford Co., State Printers; 1913</ref>{{rp|40–75}} The Early Wisconsin was the bigger of the two and extended farther west and south. It retreated an unknown distance before halting. During this period of quiet, the glacial deposits were eroded and weathered. This first Wisconsin period erased all the [[Illinoian]] glacial topography that its glaciers extended over.<ref name=Sherzer/> The Late Wisconsin ice sheet extended more towards the west than the earlier movements. This may have been due to changes in the accumulation center of the ice sheet, topographic changes introduced by the Early phase or by pressure changes in the ice mass in the north.<ref name=Sherzer/> {|class="wikitable" |- ! colspan="2" style=" background: #efefef;" | Table I |- ! colspan="2" style="background: #ffdead;" | Table VII – Estimated Age of Glacial Episodes (Leverett) <ref name=Sherzer/>{{rp|74}} |- ! Age ! Years before Present (YBP) |- | Culmination of Late Wisconsin | align="right"| 50,000 |- | Culmination of Early Wisconsin | align="right"| 100,000 |- | Beginning of Wisconsin | align="right"| 150,000 |- | Culmination of [[Illinoian]] | align="right"| 300,000 |- | Beginning of Illinoian | align="right"| 350,000 |- | Culmination of [[Pre-Illinoian]], i.e., old Nebraskan<ref name=Attig>Attig, John W.; Mickelson, David M. (1999). Glacial processes past and present. Boulder, Colo: The Geological Society of America, Inc. {{ISBN|0-8137-2337-X}}</ref><ref>Boellstorff, J (1978). "Chronology of some Late Cenozoic deposits from the central United States and the Ice Ages"(pdf). Transactions of the Nebraska Academy of Science 6: 35–49. Retrieved 2012-04-04.</ref> | align="right"| 550,000 |- | Beginning of Pre-Illinoian | align="right"| 1,200,000 |}

==Continental ice sheets==

===Laurentide ice sheet=== {{main|Laurentide Ice Sheet}} {| class="wikitable" |- ! colspan="6" style=" background: #efefef;" | Table III [[Laurentide Ice Sheet]] |- ! colspan="6" style="background: #ffdead;" | Glacial lobes and sublobes of the southern Laurentide Ice Sheet during the late Wisconsin Glaciation.<ref>The southern Laurentide Ice Sheet; Cavid M. Mickelson and Patrick M. Colgan; Department of Geology, University of Wisconsin Madison; Madison, Wisconsin; 2003</ref> |- align="center" ! Major Lobes | colspan="4" | Minor Lobes |- | Des Moines | Grantsburg | St. Louis | Rainey | |- | Lake Superior<ref name=Hill>Geological Framework and Glaciation of the Eastern Area; Christopher L. Hill; pg 82-98</ref> | Wadena | Chippewa<ref name=Hill/> | Wisconsin Valley<ref name=Hill/> | Langlade<ref name=Hill/> |- | Green Bay<ref name=Hill/> |- | rowspan="2"|Lake Michigan<ref name=Hill/> | Delavan | Harvard-Princeton | Peoria | Decatur |- | colspan="3"|Minor lobes: Milwaukee, Two Rivers; Straits of Mackinac | |- | Saginaw |- | Lake Huron<ref name=Hill/> | East White<ref name=Hill/> | Miami<ref name=Hill/> | Scioto<ref name=Hill/> | |- | Lake Erie<ref name=Hill/> |- | Lake Ontario<ref name=Hill/> | Lake Champlain<ref name=Hill/> | Hudson River<ref name=Hill/> | | |- | unnamed lobe in Quebec – New England | Connecticut Valley<ref name=Hill/> | Buzzards Bay<ref name=Hill/> | Cape Cod<ref name=Hill/> | Georges Bank<ref name=Hill/> |} <!-- ===Labrador=== The Labrador ice dome centered east of Hudson Bay. Expanding towards the southwest, it reached into the eastern edge of [[Manitoba]] and across the [[Great Lakes]] to the [[Ohio River]], upwards of {{convert|1600|mi|km}} from its source. Its eastern lobes covered [[New England]] and reached south to [[Cape Cod]] and [[Long Island, New York]].<ref name=C&S3>Chamberlin, Thomas C. and Salisbury, Rollin T., Geology, 3 Vols. 1906, Vol III., pp. 330-333.</ref>

===Keewatin=== The Keewatin ice dome began west of Hudson Bay in the Canadian Territory of [[District of Keewatin|Keewatin]]. The ice moved south some {{convert|1500|mi|km}} into Kansas and Missouri. To the west, it reached {{convert|1000|mi|km}} to the foothills of the [[Rocky Mountains]].<ref name=C&S3/> --> ===Cordilleran ice sheet=== {{main|Cordilleran ice sheet}} The Cordilleran Ice Sheet has left remnants throughout the [[Northern Rocky Mountains]], covering [[British Columbia]] and reaching into northern [[Washington (state)|Washington]] State and [[Montana]]. The Cordilleran ice sheet has more of an Alpine style of many glaciers merged into a whole. The striations made by the ice field in moving over the bedrock show that it moved principally to the west through the passes of the coast range.<ref name=C&S3>Chamberlin, Thomas C. and Salisbury, Rollin T., Geology, 3 Vols. 1906, Vol III., pp. 330-333.</ref>

===Innuitian ice sheet=== {{main|Innuitian ice sheet}} The Innuitian ice sheet was centered on the [[Queen Elizabeth Islands]].

==Formation of proglacial and prehistoric lakes== [[File:Glacial lakes.jpg|thumb|right|upright=1.5|A diagram of the formation of the Great Lakes]] {{see also|List of prehistoric lakes}} Whenever the ice sheet melted from the north at a [[moraine]], water would begin to pond in the divide between a moraine and the ice front. The ice would act as a dam as water could not drain through the ice sheet, which in the Wisconsin period covered most of the proglacial river valleys. Numerous small, isolated water bodies formed between the moraine and the ice front. As the ice sheet would continue to melt and recede northward, these ponds combined into [[proglacial lake]]s. In areas without an available outlet, the water levels would either continue to rise until reaching one or more low spots along the rim of a moraine, or the ice sheet would retreat, opening access to a lower portion of the moraine.<ref name=Sherzer/>{{rp|40–99}} Multiple outlets could form through low spots too until one would become dominant after erosion lowered both the outlet and lake surface.

===Meltwater=== [[File:Champlain Sea 1.png|thumb|upright=1.5|The [[Champlain Sea]] - The best evidence of this former sea is the vast clay plain deposited along the [[Ottawa River|Ottawa]] and [[St. Lawrence River|St. Lawrence Rivers]].<ref>Chapman, L.J. and D.F. Putnam. 1984. ''The Physiography of Southern Ontario''. Third edition. Ontario Geological Survey, Special Volume No.2. Government of Ontario, Toronto.</ref> ]]Ice melt and rainfall carried large quantities of [[clay]], [[sand]], and [[gravel]] from the ice mass. Clays could be moved long distances by moving water, while sand and gravel could not. Thus, sand and gravel [[landforms]] developed along the sides and front of the ice sheet;<ref name=Sherzer/> elongated accumulations of this material are known as [[kame]]s. Mounds along the frontal edge of the ice are called [[moraines]]. Wherever a subglacial tunnel began infilling, long winding formations known as [[eskers]] would form. The sweeping plain of sand and gravel beyond the ice margin and a [[terminal moraine]] is called an [[outwash plain]] .<ref name=Sherzer/> The materials left under the glacier when it melts back is called the [[ground moraine]] or [[till plain]].<ref name=Sherzer/> Till is highly permeable and creates a large ground reserve for water. This formation is highly desirable for human economic development as a source of water.<ref name=Sherzer/>

==Stages of the Wisconsin episode== {| class="wikitable" |- ! colspan="4" style=" background: #efefef;" | Table IV |- ! colspan="4" style="background: #ffdead;" | Maxima of the Wisconsin ice sheets<ref>Correlation of Wisconsin Glacial Maxima; Ernst Antevs; ca 1943; The Quaternary Geology of Southeastern Wisconsin with a Chapter on the Older Rock Formations, Professional Paper 106; William C. Alden; United States Geological Survey; Government Printing Office, Washington, D.C.; 1918</ref> |- align="center" ! Western Ice ! Eastern Ice ! Estimated years before present ! Position of ice border |- | Mankato | Valders | align="center"|25,000 | Northern Washington, Idaho, and Montana to the Continental Divide – north of Edmonton – 65 miles east of Edmonton – northwest corner of North Dakota – Des Moines – west end of Lake Superior – Milwaukee – Port Huron – Buffalo – Schuylerville – St. Johnsbury. |- | rowspan="2" | (Great reduction of ice) | Cary | align="center"|27,500 | Minneapolis – north Wisconsin – south of Chicago – Central Ohio – 50 miles south of Buffalo – Binghamton - Northampton |- | Tazewell | align="center"|40,000 | Rockford, Ill. – Peoria – south of Indianapolis – north of Cincinnati – northwestern Pennsylvania – central Long Island |- | Iowan | No known ice | align="center"|65,500 | Northern Washington, Idaho, and Montana – northwest North Dakota – east central Iowa - Minneapolis |}

==Role in human migration== Prehistoric human migration was likely greatly influenced by this last glacial period, as during much of the Wisconsin era, the formation of a [[land bridge]] known as [[Beringia]] across the [[Bering Strait]] is believed to have allowed human occupation of this area which provided potential access for some of the first humans to move between North America and [[Siberia]] in [[Asia]] (see [[Settlement of the Americas]]). Other human migration routes also opened during interglacial periods in both [[Europe]] and Asia.<ref name="Harm2005" />

==Flora and fauna== North American flora and fauna species were distributed quite differently during the Wisconsin era, due to altered temperatures, surface water distribution, and in some cases coverage of earth surface by glaciers. A number of scientific studies have been conducted to determine species distribution, particularly during the Late Wisconsin and early to mid-Holocene. An example of findings is from the investigation of flora species using [[pollen core]] samples in present-day California. Here in the [[Waterman Hills]] researchers found that ''[[Juniperus osteosperma]]'' and ''[[Pinus monophylla]]'' were early to mid-Holocene dominant trees, while ''[[Monardella arizonica]]'' has been a continuously present understory plant. ''[[Celtis reticulata]]'' is an example of a plant present in the early Holocene following Wisconsin glacial retreat, a species no longer present at the [[Waterman Mountains]] site.<ref name="Hogan2009" />

==See also== {{Portal|Geology|North America}}   * {{Annotated link |Driftless Area}} * {{Annotated link |Geology of Ontario}} * {{Annotated link |Glacial history of Minnesota}} * {{Annotated link |Glacial Lake Iroquois}} * {{Annotated link |Ice Age Trail}} * {{Annotated link |Ice core}} * {{Annotated link |Last Glacial Period}} * {{Annotated link |Retreat of glaciers since 1850}} * {{Annotated link |Timeline of glaciation}} * {{Annotated link |Younger Dryas}}

=== Pleistocene historic names ===  {| class="wikitable" |+ Historical names of the "four major" glacials in four regions. ! Region ! Glacial 1 ! Glacial 2 ! Glacial 3 ! Glacial 4 |- | '''Alps''' | [[Gunz glaciation|Günz]] | [[Mindel glaciation|Mindel]] | [[Riss glaciation|Riss]] | [[Würm glaciation|Würm]] |- | '''North Europe''' | [[Eburonian]] | [[Elsterian]] | [[Saalian]] | [[Weichselian]] |- | '''British Isles''' | [[Beestonian]] | [[Anglian Stage|Anglian]] | [[Wolstonian]] | [[Devensian]] |- | '''Midwest U.S.''' | [[Pre-Illinoian|Nebraskan]] | [[Kansan glaciation|Kansan]] | [[Illinoian]] | [[Wisconsinan]] |} {| class="wikitable" |+ Historical names of interglacials. ! Region ! Interglacial 1 ! Interglacial 2 ! Interglacial 3 |- | '''Alps''' | [[Cromerian Stage|Günz-Mindel]] | [[Hoxnian Stage|Mindel-Riss]] | [[Riss-Würm]] |- | '''North Europe''' | Waalian | Holsteinian | [[Eemian]] |- | '''British Isles''' | [[Cromerian]] | [[Hoxnian]] | [[Eemian Stage|Ipswichian]] |- | '''Midwest U.S.''' | [[Aftonian]] | [[Yarmouthian Interglacial (Stage)|Yarmouthian]] | [[Sangamonian]] |}

==References==

{{Reflist|refs= <ref name="Harm2005">Harm J. De Blij. 2005. ''Why geography matters: three challenges facing America'', 308 pages, p.69</ref> <ref name="Harris1997">Ann G. Harris, Esther Tuttle, Sherwood D. Tuttle. 1997. ''Geology of National Parks: Fifth Edition'', Iowa, Kendall/Hunt Publishing {{ISBN|0-7872-5353-7}}</ref> <ref name="Hogan2009">[http://www.globaltwitcher.com/artspec_information.asp?thingid=90792 C. Michael Hogan, 2009. ''Elephant Tree: Bursera microphylla'', GlobalTwitcher.com, ed. N. Stromberg]</ref> }}

==External links and references==

* [https://www.britannica.com/science/Wisconsin-Glacial-Stage Wisconsin Glacial Stage] at the [[Encyclopædia Britannica]]

{{DEFAULTSORT:Wisconsin Glaciation}} [[Category:Glaciology of Canada]] [[Category:Glaciology of the United States]] [[Category:Ice ages]] [[Category:Pleistocene geology]] [[Category:Pleistocene United States]] [[Category:Quaternary Canada]] [[Category:Quaternary Colorado]] [[Category:Quaternary Michigan]] [[Category:Quaternary Minnesota]] [[Category:Quaternary Montana]] [[Category:Quaternary Wisconsin]] [[Category:Quaternary Wyoming]] [[Category:Last Glacial Period]]