Cordilleran Ice Sheet
Encyclopedia
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:
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
.
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 much greater thickness of the centre of the ice sheet served to push upwards areas at the edge of the continental shelf in a glacial forebulge
. 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.
Ice sheet
An ice sheet is a mass of glacier ice that covers surrounding terrain and is greater than 50,000 km² , thus also known as continental glacier...
that covered, during glacial periods of the Quaternary
Quaternary
The Quaternary Period is the most recent of the three periods of the Cenozoic Era in the geologic time scale of the ICS. It follows the Neogene Period, spanning 2.588 ± 0.005 million years ago to the present...
, a large area of North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
. This included the following areas:
- Western MontanaWestern MontanaWestern Montana is the western region of the state of Montana, United States. Although there is no firm definition, Western Montana is roughly considered by some the western third of the state.-Geography, Biomes and Climate:...
- The Idaho PanhandleIdaho PanhandleThe Idaho Panhandle is the northern region of the U.S. State of Idaho that encompasses the ten northernmost counties of Benewah, Bonner, Boundary, Clearwater, Idaho, Kootenai, Latah, Lewis, Nez Perce, Shoshone. Residents of the panhandle refer to the region as North Idaho...
- Northern Washington state down to about OlympiaOlympia, WashingtonOlympia is the capital city of the U.S. state of Washington and the county seat of Thurston County. It was incorporated on January 28, 1859. The population was 46,478 at the 2010 census...
and Spokane, WashingtonSpokane, WashingtonSpokane is a city located in the Northwestern United States in the state of Washington. It is the largest city of Spokane County of which it is also the county seat, and the metropolitan center of the Inland Northwest region... - All of British ColumbiaBritish ColumbiaBritish Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...
- The southwestern third or so of Yukon TerritoryYukonYukon is the westernmost and smallest of Canada's three federal territories. It was named after the Yukon River. The word Yukon means "Great River" in Gwich’in....
- All of the Alaska PanhandleAlaska PanhandleSoutheast Alaska, sometimes referred to as the Alaska Panhandle, is the southeastern portion of the U.S. state of Alaska, which lies west of the northern half of the Canadian province of British Columbia. The majority of Southeast Alaska's area is part of the Tongass National Forest, the United...
- South Central AlaskaSouth Central AlaskaSouthcentral Alaska is the portion of the U.S. state of Alaska consisting of the shorelines and uplands of the central Gulf of Alaska. Most of the population of the state lives in this region, concentrated in and around the city of Anchorage....
- The Alaska PeninsulaAlaska PeninsulaThe Alaska Peninsula is a peninsula extending about to the southwest from the mainland of Alaska and ending in the Aleutian Islands. The peninsula separates the Pacific Ocean from Bristol Bay, an arm of the Bering Sea....
- Almost all of the continental shelfContinental shelfThe continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...
north of the Strait of Juan de FucaStrait of Juan de FucaThe Strait of Juan de Fuca is a large body of water about long that is the Salish Sea outlet to the Pacific Ocean...
The ice sheet covered up to two and a half million square kilometres at the Last Glacial Maximum
Last Glacial Maximum
The Last Glacial Maximum refers to a period in the Earth's climate history when ice sheets were at their maximum extension, between 26,500 and 19,000–20,000 years ago, marking the peak of the last glacial period. During this time, vast ice sheets covered much of North America, northern Europe and...
and probably more than that in some previous periods, when it may have extended into the northeast extremity of Oregon
Oregon
Oregon is a state in the Pacific Northwest region of the United States. It is located on the Pacific coast, with Washington to the north, California to the south, Nevada on the southeast and Idaho to the east. The Columbia and Snake rivers delineate much of Oregon's northern and eastern...
and the Salmon River Mountains in Idaho. It is probable, though, that its northern margin also migrated south due to the influence of starvation
Starvation (glaciology)
In glaciology, starvation occurs when a glacier retreats, not because of temperature increases, but due to precipitation so low that the ice flow downward into the zone of ablation exceeds the replenishment from snowfall...
caused by very low levels of precipitation.
At its eastern end the Cordilleran ice sheet merged with the Laurentide ice sheet
Laurentide ice sheet
The Laurentide Ice Sheet was a massive sheet of ice that covered hundreds of thousands of square miles, including most of Canada and a large portion of the northern United States, multiple times during Quaternary glacial epochs. It last covered most of northern North America between c. 95,000 and...
at the Continental Divide
Continental Divide
The Continental Divide of the Americas, or merely the Continental Gulf of Division or Great Divide, is the name given to the principal, and largely mountainous, hydrological divide of the Americas that separates the watersheds that drain into the Pacific Ocean from those river systems that drain...
, forming an area of ice that contained one and a half times as much water as the Antarctic ice sheet
Antarctic ice sheet
The Antarctic ice sheet is one of the two polar ice caps of the Earth. It covers about 98% of the Antarctic continent and is the largest single mass of ice on Earth. It covers an area of almost 14 million square km and contains 30 million cubic km of ice...
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
Sea level
Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...
in now-submerged Hecate Strait
Hecate Strait
Hecate Strait is a wide but shallow strait between the Haida Gwaii and the mainland of British Columbia, Canada. It merges with Queen Charlotte Sound to the south and Dixon Entrance to the north...
and on the Brooks Peninsula in northern Vancouver Island
Vancouver Island
Vancouver Island is a large island in British Columbia, Canada. It is one of several North American locations named after George Vancouver, the British Royal Navy officer who explored the Pacific Northwest coast of North America between 1791 and 1794...
. However, evidence of ice-free refugia above present sea level north of the Olympic Peninsula
Olympic Peninsula
The Olympic Peninsula is the large arm of land in western Washington state of the USA, that lies across Puget Sound from Seattle. It is bounded on the west by the Pacific Ocean, the north by the Strait of Juan de Fuca, and the east by Puget Sound. Cape Alava, the westernmost point in the contiguous...
has been refuted by genetic and geological studies since the middle 1990s. The ice sheet faded north of the Alaska Range
Alaska Range
The Alaska Range is a relatively narrow, 650-km-long mountain range in the southcentral region of the U.S. state of Alaska, from Lake Clark at its southwest end to the White River in Canada's Yukon Territory in the southeast...
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
Inland Empire (Pacific Northwest)
thumb|The Inland Empire regionThe Inland Northwest, or Inland Empire, is a region in the Pacific Northwest centered on Spokane, Washington, including the surrounding Columbia River basin and all of North Idaho....
of Eastern Washington
Eastern Washington
Eastern Washington is the portion of the U.S. state of Washington east of the Cascade Range. The region contains the city of Spokane , the Tri-Cities, the Columbia River and the Grand Coulee Dam, the Hanford Nuclear Reservation and the fertile farmlands of the Yakima Valley and the...
.
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 MaximumLast Glacial Maximum
The Last Glacial Maximum refers to a period in the Earth's climate history when ice sheets were at their maximum extension, between 26,500 and 19,000–20,000 years ago, marking the peak of the last glacial period. During this time, vast ice sheets covered much of North America, northern Europe and...
were over a hundred metres higher than they are today (measured by the level of bedrock
Bedrock
In stratigraphy, bedrock is the native consolidated rock underlying the surface of a terrestrial planet, usually the Earth. Above the bedrock is usually an area of broken and weathered unconsolidated rock in the basal subsoil...
).
However, on the western edge at the Queen Charlotte Islands
Queen Charlotte Islands
Haida Gwaii , formerly the Queen Charlotte Islands, is an archipelago on the North Coast of British Columbia, Canada. Haida Gwaii consists of two main islands: Graham Island in the north, and Moresby Island in the south, along with approximately 150 smaller islands with a total landmass of...
, 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
Lake
A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...
in the deepest parts of the strait. This was because the much greater thickness of the centre of the ice sheet served to push upwards areas at the edge of the continental shelf in a glacial forebulge
Forebulge
In geology, a forebulge is a flexural bulge in front of a load on the lithosphere. This load causes the lithosphere to flex by depressing the plate beneath it. Because of the flexural rigidity of the lithosphere, the area around the load is uplifted by a height that is 4% of that of the depression...
. 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
Pacific Northwest
The Pacific Northwest is a region in northwestern North America, bounded by the Pacific Ocean to the west and, loosely, by the Rocky Mountains on the east. Definitions of the region vary and there is no commonly agreed upon boundary, even among Pacific Northwesterners. A common concept of the...
were exposed because of the lower sea levels, including Bowie Seamount
Bowie Seamount
Bowie Seamount is a large submarine volcano in the northeastern Pacific Ocean, located west of Haida Gwaii, British Columbia, Canada.The seamount is named after William Bowie of the Coast & Geodetic Survey....
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
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
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.