Vashon Glaciation

The Vashon Glaciation or Vashon Stade was the most recent period of very cold climate in which glaciers existed at elevations at and near sea level in the western part of present-day Washington state. It occurred during a colder than present part of the current ice age.

The Vashon Glaciation lasted from about 19,000 - 16,000 BP (Before Present - present defined as January 1, 1950 for this scale). The Cordilleran Ice Sheet was an ice sheet that covered present-day southern Alaska and parts of western Canada. During the Vashon Glaciation, the Cordilleran Ice Sheet advanced into the Puget Sound region.

Pollen data collected from Battle Ground Lake in the southern Puget Sound Region shows that from 20,000 - 16,000 BP, annual temperatures in the area were about 6 ± 1 °C (10.8 ± 1.8 °F) colder than in present times (present times as of 1990), and precipitation was around 1 meter (39.4 inches) less. The Battle Ground area averaged 52.14 inches (132.44 cm) of precipitation per year for the period of 1961–1990. A meter less precipitation means that during period of 20,000 - 16,000 BP, the average precipitation would have only been around 24.5% of what it was in the near present 1961–1990 period.

The Laurentide Ice Sheet had a major effect on the climate. It was an ice sheet covering much of Canada, and parts of the northern United States in the Midwest and east. The Rocky Mountains separated the Laurentide Ice Sheet from the Cordilleran Ice Sheet. The Laurentide Ice Sheet had a cooling effect on the middle latitudes. This caused the jet stream over North America to split in two. The southern branch was pushed further south than it is in present times meaning that the storm tracks were missing the Pacific Northwest most of the time. Because of this, Southern and Central California had wetter climates than in present times.

Average annual temperatures in the lowlands of Western Washington were above 0 °C (32 °F). This means that there was more summer thawing than there was winter freezing. This would seem to be a climate too warm to support glaciers, but the ice was pushing in from the north faster than it could melt.

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