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Laurentide ice sheet
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==Description== [[File:Retrait des glaces en Amérique.svg|thumb|Timeline of glacier retreat in North America]] This ice sheet was the primary feature of the [[Pleistocene]] epoch in North America, commonly referred to as the [[Quaternary glaciation|ice age]]. During the [[Pre-Illinoian|Pre-Illinoian Stage]], the Laurentide Ice Sheet extended as far south as the [[Missouri River|Missouri]] and [[Ohio River]] valleys. It was up to {{convert|2|mi|km|abbr=on}} thick in [[Nunavik]], [[Quebec]], [[Canada]], but much thinner at its edges, where [[nunatak]]s were common in hilly areas. It created much of the surface geology of southern Canada and the northern United States, leaving behind glacially scoured valleys, [[moraines]], [[eskers]] and [[glacial till]]. It also caused many changes to the shape, size, and drainage of the Great Lakes. As but one of many examples, near the end of the last ice age, [[Glacial Lake Iroquois|Lake Iroquois]] extended well beyond the boundaries of present-day [[Lake Ontario]], and drained down the Hudson River into the Atlantic Ocean.<ref>Flint, R.F. 1971. Glacial and Quaternary Geology. Wiley and Sons, NY. p. 892.</ref> Its cycles of growth and melting were a decisive influence on global [[climate]] during its existence. This is because it served to divert the [[jet stream]] southward, which would otherwise flow from the relatively warm [[Pacific Ocean]] through [[Montana]] and [[Minnesota]]. That gave the [[Southwestern United States]], otherwise a desert, abundant rainfall during ice ages, in extreme contrast to most other parts of the world which became exceedingly dry, though the effect of ice sheets in [[Europe]] had an analogous effect on the rainfall in [[Afghanistan]], parts of [[Iran]], possibly western [[Pakistan]] in winter, as well as [[North Africa]]. [[File:Sea Ice off Baffin Island.jpg|thumb|right|200px|The [[Barnes Ice Cap]], containing remnants of the Laurentide Ice Sheet.]] Its melting also caused major disruptions to the global climate cycle, because the huge influx of low-[[salinity]] water into the [[Arctic Ocean]] via the [[Mackenzie River]]<ref name="Murton_Bateman">{{cite journal|last=Murton|first=J.B.|author2=Bateman, M.D. |author3=Dallimore, S.R |author4=Teller, J.T. |author5=Yang, Z. |year=2010|title=Identification of Younger Dryas outburst flood path from Lake Agassiz to the Arctic Ocean|journal=Nature|volume=464|pages=740–743|doi=10.1038/nature08954|pmid=20360738|issue=7289|bibcode = 2010Natur.464..740M |s2cid=4425933}}</ref> is believed to have disrupted the formation of [[North Atlantic Deep Water]], the very saline, cold, deep water that flows from the [[Greenland Sea]]. That interrupted the [[thermohaline circulation]], creating the brief [[Younger Dryas]] cold epoch and a temporary re-advance of the ice sheet,<ref name="Broeckery_Denton">{{cite journal |last=Broecker |first=W.S. |author2=Denton, G.H. |year=1989 |title=The role of ocean-atmosphere reorganizations in glacial cycles |journal=Geochimica et Cosmochimica Acta |volume=53 |pages=2465–2501 |doi=10.1016/0016-7037(89)90123-3 |issue=10 |bibcode=1989GeCoA..53.2465B}}</ref> which did not retreat from [[Nunavik]] until 6,500 years ago. After the end of the Younger Dryas, the Laurentide Ice Sheet retreated rapidly to the north, becoming limited to only the Canadian Shield until even it became deglaciated.<ref>{{cite journal |last1=Margold |first1=Marin |last2=Stokes |first2=Chris R. |last3=Clark |first3=Chris D. |date=1 June 2018 |title=Reconciling records of ice streaming and ice margin retreat to produce a palaeogeographic reconstruction of the deglaciation of the Laurentide Ice Sheet |journal=[[Quaternary Science Reviews]] |volume=189 |pages=1–30 |doi=10.1016/j.quascirev.2018.03.013 |bibcode=2018QSRv..189....1M |s2cid=53511921 |doi-access=free |url=https://durham-repository.worktribe.com/file/1333266/1/Accepted%20Journal%20Article }}</ref> The ultimate collapse of the Laurentide Ice Sheet is also suspected to have influenced European agriculture indirectly through the rise of global sea levels. Canada's oldest ice is in remnants of the LIS: the [[Barnes Ice Cap]] and the [[Penny Ice Cap]].
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