Cordilleras Ice Sheet
The Cordilleras Ice Sheet was a major ice sheet that periodically covered large parts of North America during the cold ages of the last 2.6 million years or so. This included the following areas:
- West Montana
- today's Idaho Panhandle
- today's northern Washington southwards to around Olympia and Spokane
- all of present-day British Columbia
- the southwestern third of today's Yukon territory
- the entire Alaska Panhandle
- what is now Southcentral Alaska
- the Alaska Peninsula
- almost the entire continental shelf north of the Strait of Juan de Fuca
The ice sheet covered up to 2.5 million km² in the last ice age and thus probably more than in the previous glaciation periods when it reached into the northeastern foothills of today's Oregon and the Salmon River Mountains in Idaho . It is also possible that its northern limit also shifted southwards, as a result of the influence of so-called starvation, the retreat of glaciers due to low rainfall.
At its eastern border, the Cordilleras Ice Sheet joined the Laurentide Ice Sheet on the continental divide and thus formed an ice zone that contained one and a half times as much water as the Antarctic Ice Sheet today. For its western border, it has recently been assumed that several small glacier refuges existed below today's sea level during the maximum last glaciation in what is now the submarine Hecate Strait and on the Brooks Peninsula in the north of Vancouver Island . However, the assumption of ice-free refuges above today's sea level north of the Olympic Peninsula has been refuted by genetic and geological studies since the mid-1990s. The ice sheet streaked north of the Alaska Range because the climate was too dry for glaciers to form .
Unlike the Laurentide Ice Sheet, which is believed to have taken more than eleven thousand years to melt, the Cordilleras Ice Sheet, with the exception of the areas that are still glaciated today, is believed to melt very quickly, possibly in 4,000 years or less. This rapid melting led to floods like the overflow of Lake Missoula and shaped the topography of the extremely fertile Inland Empire of east Washington .
Sea level during the icing
Due to the weight of the ice, the North American mainland was compressed so much that the sea level during the last glaciation maximum was more than a hundred meters higher than today (measured by the level of the rock in front of it ).
However, the west coast of Haida Gwaii (formerly known as Queen Charlotte Islands) suggests that because of the thinner ice sheet at its edge, sea level was up to 170 meters below today's level and formed a lake at the deepest point of Hecate Strait. This happened because of the greater thickness of the ice sheet in the center, which pushed the areas at the edges of the continental shelf upwards in a glacial protrusion. The effect of this during the "de-glaciation" was that the sea levels at the edges of the ice sheet, which naturally melted first, initially rose as a result of the increase in water volume, but later fell again after the melting ended. Some underwater objects across the Pacific Northwest came to the surface due to low sea levels, including the Bowie Seamount west of Haida Gwaii, which is believed to be an active volcano during the last Ice Age.
These effects are significant because they are used to explain how immigrants from Beringia to North America were able to migrate south during the glacier retreat, solely due to the emergence of submerged areas between the mainlands and numerous continental islands. They are also important in understanding the direction of evolution since the receding ice.
Even today the region is known for the rapid changes in sea level, which however have no influence on most of the coast because of the many fjords .
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- Hidy, AJ, Gosse, JC, Froese, DG, Bond, JD, and Rood, DH (2013). A latest Pliocene age for the earliest and most extensive Cordilleran Ice Sheet in northwestern Canada . Quaternary Science Reviews 61: 77-84.
- Gwaii Haanas National Park Reserve and Haida Heritage Site - Parks Canada website for Gwaii Haana National Park
- Clarke, TE, DB Levin, DH Kavanaugh and TE Reimchen. 2001. Rapid Evolution in the Nebria Gregaria Group ( Coleoptera : Carabidae ) and the Paleogeography of the Queen Charlotte Islands. Evolution 51: 1408-1418.
- Brown, AS, and H. Nasmith. 1962. The glaciation of the Queen Charlotte Islands . Canadian Field-Naturalist 76: 209-219.
- Byun, SA, BF Koop, and TE 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: HE 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, pp. 53-70. Abstract
- Holder, K., Montgomerie, R., and VL 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.
- Warner, BG, Mathewes, RW, and JJ Clague. 1982. Ice-free conditions on the Queen Charlotte Islands, British Columbia, at the height of late Wisconsin glaciation . Science 218 (4573): 675-677.