Fraser Glaciation  
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21,000 B.P. - 10,000 B.P.
 
The Fraser glaciation lasted about 10,000 years and consisted of 3 stades (periods of ice expansion) and 2 interstades (ice recession). The Fraser ice advance in the Olympics began with expansion of alpine glaciers--the Evans Creek stade. Although poorly dated on the Olympic Peninsula, at maximum advance--about 21,000-19,000 B.P.(before present)--glaciers extended down west-side valleys. Glaciers in east-side drainages were smaller and were restricted to upper valley areas or headwalls. The Evans Creek advance coincided with the beginnings of an enormous ice buildup in the mountains of British Columbia.    

Alpine glaciers retreated to undetermined positions up valleys following the Evans Creek stade. This brief interstade was followed by advance of Cordilleran ice sheet from British Columbia into the Puget Sound area--the Vashon stade. The ice reached its maximum extent around 15,000 B.P., splitting into the Juan de Fuca and Puget ice lobes as it encountered the Olympic Mountains. Ice at the northeast corner of the Olympics was at least 3,800 feet thick at maximum advance. The Vashon ice produced glacial lakes behind massive ice dams that formed in the northern and northeastern river valleys. The ice sheet apparently did not contact the remaining alpine glaciers. The spatial and temporal relations between ice sheets and alpine glaciers have important implications for the biogeography of endemic taxa; suitable habitat for alpine plants evidently persisted in or near the Olympic Mountains during both alpine and ice sheet advances of the Fraser glaciation.      

The Vashon advance was short-lived; by 13,600 B.P., the two lobes had receded into a single lobe located in the northern Puget lowlands. A minor readvance (the Sumas stade) occurred about 11,500 B.P., but the extent and climate significance of this stade has been questioned. The Fraser glaciation ended about 10,000 B.P. when major climatic changes occurred.    

(Source: Mountain Goats in Olympic National Park: Biology and Management of Introduced Species, Scientific Monograph NPS/NROLYM/NRSM-94/25)