Cordilleran Ice Sheet

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 Olympia 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.