TheCordilleran ice sheet was a majorice sheet that periodically covered large parts ofNorth America during glacial periods over the last ~2.6 million years.
The ice sheet covered up to 1,500,000 km2 (580,000 sq mi) at theLast Glacial Maximum[2] and probably more than that in some previous periods, when it may have extended into the northeast extremity ofOregon and the Salmon River Mountains in Idaho. It is probable, though, that its northern margin also migrated south due to the influence ofstarvation caused by very low levels of precipitation.[citation needed]
At its western end it is currently understood that several small glacialrefugia existed during the last glacial maximum below presentsea level in the now-submergedHecate Strait and on theBrooks Peninsula in northernVancouver Island. However, evidence of ice-free refugia above present sea level north of theOlympic Peninsula has been refuted by genetic and geological studies since the middle 1990s.
Unlike the Laurentide Ice Sheet, which may have taken as many as eleven thousand years to fully melt,[3] the Cordilleran ice sheet melted very quickly, probably in four thousand years or less.[4] This rapid melting causedfloods such as the overflow ofLake Missoula and shaped the topography of the fertileInland Empire ofEastern Washington.[5] Further north, the Cordilleran is responsible for a large number ofglacial landforms scattered across the west ofCanada.[6] The rate of thawing has also played a significant role in research surrounding early human migration into the American continents.
The rapid retreat of the Cordilleran ice sheet is a focus of study byglaciologists seeking to understand the difference in patterns of melting in marine-terminating glaciers, glaciers whose margin extends into open water without seafloor contact, and land-terminating glaciers, with a land or seafloor margin, as scientists believe the western marine-terminating margin retreated much faster than its southern, land-terminating front.[7] This rapid retreat resulted in noticeably fewer glacial landforms in the west of the Cordilleran's maximum extent compared to the south and east, though the exact mechanisms behind this disparity are unknown. Some glacial landforms are still present though: the well-characterized landscape of coastal Washington State contains glacial troughs, some glacial lakes, and an extensive outwash plain.[8] Many of the southern and eastern landforms fall near the northern reaches of theAmerican Cordillera, the mountain ranges which geologists believe to be the region from which the Cordilleran first grew, and, after its sudden retreat and ultimate collapse, where it terminated.[9]
The timing of the retreat of the Cordilleran bears significance not just to glaciologists, but toanthropologists interested inthe migration of early humans into the Americas. In particular, the collapse of the western front of the Cordilleran ice sheet has been proposed as one route through which early humans could have migrated after crossing theBeringian Land Bridge during theLast Glacial Maximum.[10] This serves as an alternative to the Ice Free Corridor previously posited to have allowed for migration amid the retreat of the eastern front of the Cordilleran ice sheet and the western front of the Laurentide ice sheet.[11] The Ice Free Corridor is a subject of debate among anthropologists in recent years. Recent studies have provoked skepticism, with areas of discussion including the lack of evidence of sufficient flora in the area to support megafaunal migration,[12] to radiometric dating placing the emergence of a corridor through the centralCanadian Shield too late to account for theearliest known human sites south of the glaciers.[13]
^Fulton, R. J. & Prest, V. K. (1987). Introduction: The Laurentide Ice Sheet and its Significance. Géographie physique et Quaternaire, 41 (2), 181–186
^abcMartin Margold; Krister N. Jansson; Johan Kleman; Arjen P. Stroeven; John J. Clague (18 February 2013). "Retreat pattern of the Cordilleran Ice Sheet in central British Columbia at the end of the last glaciation reconstructed from glacial meltwater landforms".Boreas.42 (4). Wiley Online Library:830–847.Bibcode:2013Borea..42..830M.doi:10.1111/bor.12007.
^Dyke, Arthur S. (2004), "An outline of North American deglaciation with emphasis on central and northern Canada",Quaternary Glaciations-Extent and Chronology - Part II: North America, Developments in Quaternary Sciences, vol. 2, Elsevier, pp. 373–424,doi:10.1016/s1571-0866(04)80209-4,ISBN978-0-444-51592-6
^Flannery, Tim F. (2002).The eternal frontier: an ecological history of North America and its peoples (2. [print.] ed.). New York, NY: Grove Press.ISBN978-0-8021-3888-0.
Hidy, A. J., Gosse, J. C., Froese, D. G., Bond, J. D., and Rood, D. H. (2013).A latest Pliocene age for the earliest and most extensive Cordilleran Ice Sheet in northwestern Canada.Quaternary Science Reviews 61:77-84.[1]
Brown, A. S., and H. Nasmith. 1962.The glaciation of the Queen Charlotte Islands.Canadian Field-Naturalist 76:209–219.
Byun, S. A., B. F. Koop, and T. E. Reimchen. 1997.North American black bear mtDNA phylogeography: implications for morphology and the Haida Gwaii glacial refugium controversy.Evolution 51:1647–1653.
Richard B. Waitt, Jr., and Robert M. Thorson, 1983.The Cordilleran Ice Sheet in Washington, Idaho, and Montana. IN: H.E. Wright, Jr., (ed.), 1983,Late-Quaternary Environments of the United States, Volume 1: The Late Pleistocene (Stephen C. Porter (ed.)): University of Minnesota Press, 407p., Chapter 3, p.53-70.Abstract
Holder, K., Montgomerie, R., and V.L. Friesen. 1999.A test of the glacial refugium hypothesis using patterns of mitochondrial and nuclear DNA sequence variation in the rock ptarmingan (Lagopus mutus).Evolution 53(6):1936–1950.[2]
Warner, B.G., Mathewes, R.W., and J.J. Clague. 1982.Ice-free conditions on the Queen Charlotte Islands, British Columbia, at the height of late Wisconsin glaciation.Science 218(4573):675–6770[3]
