The Cordilleran ice sheet was a major ice sheet that covered, during glacial periods of the Quaternary, a large area of North America. This included the following areas:

• Western Montana
• The Idaho Panhandle
• Northern Washington state down to about Seattle and Spokane, Washington
• All of British Columbia
• The southwestern third or so of Yukon territory
• All of the Alaska Panhandle
• South Central Alaska
• The Alaska Peninsula
• Almost all of the continental shelf north of the Strait of Juan de Fuca

The ice sheet covered up to two and a half million square kilometres at the Last Glacial Maximum and probably more than that in some previous periods, when it may have extended into the northeast extremity of Oregon and the Salmon River Mountains in Idaho. It is probable, though, that its northern margin also migrated south due to the influence of starvation caused by very low levels of precipitation.

At its eastern end the Cordilleran ice sheet merged with the Laurentide ice sheet at the Continental Divide, forming an area of ice that contained one and a half times as much water as the Antarctic ice sheet does today. At its western end it is believed nowadays that several small glacial refugia existed during the last glacial maximum below present sea level in now-submerged Hecate Strait and on the Brooks Peninsula in northern Vancouver Island. However, evidence of ice-free refugia above present sea level north of the Olympic Peninsula has been refuted by genetic and geological studies since the middle 1990s. The ice sheet faded north of the Alaska Range because the climate was too dry to form glaciers.

Unlike the Laurentide ice sheet, which is believed to have taken as much as eleven thousand years to fully melt, it is believed the Cordilleran ice sheet, except for areas that remain glaciated today, melted very quickly, probably in four thousand years or less. This rapid melting caused such floods as the overflow of Lake Missoula and shaped the topography of the extremely fertile Inland Empire of Eastern Washington.

Sea levels during glaciation

Because of the weight of the ice, the mainland of northwest North America was so depressed that sea levels at the Last Glacial Maximum were over a hundred metres higher than they are today (measured by the level of bedrock).

However, on the western edge at the Queen Charlotte Islands, the lower thickness of the ice sheet meant that sea levels were as much as 170 metres lower than they are today, forming a large lake in the deepest parts of the strait. This was because the thickness of the centre of the ice sheet actually served to push upwards areas at the edge of the continental shelf which, even though glaciated, were displaced and lifted by the pressing of the crust further inland. The effect of this during deglaciation was that sea levels on the edge of the ice sheet, which naturally deglaciated first, initially rose due to an increase in the volume of water, but later fell due to rebound after deglaciation. Some underwater features along the Pacific Northwest were exposed because of the lower sea levels, including Bowie Seamount west of the Queen Charlotte Islands which has been interpreted as an active volcanic island throughout the last ice age.

These effects are important because they have been used to explain how migrants to North America from Beringia were able to travel southward during the deglaciation process due purely to the exposure of submerged land between the mainland and numerous continental islands. They are also important for understanding the direction evolution has taken since the ice retreated.

Even today, the region is notable for its rapid changes in sea level, which, however, have little effect on most of the coast due to the numerous fjords.

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References

• Gwaii Haanas National Park Reserve and Haida Heritage Site
• Clarke, T.E., D.B. Levin, D.H. Kavanaugh and T.E. Reimchen. 2001. Rapid Evolution in the Nebria Gregaria Group (Coleoptera: Carabidae) and the Paleogeography of the Queen Charlotte Islands. Evolution 51:1408–1418
• 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. [117928]
• 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. [117929]