PLEISTOCENE GLACIATION OF WESTERN WASHINGTON

The oldest Pleistocene glaciation in western Washington, the Orting, occurred between 1.6 and 2.4 m.y. The overlying interglacial Alderton Fm. and Stuck Drift lack datable material, but the Puyallup Formation is K-Ar dated at 1.69 ± 0.11 m.y. and 1.64 ± 0.13 m.y. The Lake Tapps tephra in the Salmon Springs Drift has been fission-track dated at 1.06 ± 0.11 m.y. Clay beneath Double Bluff Drift yielded TL-dates of 289 ± 74 ka and 291 ± 86 ka. Double Bluff glaciomarine drift is TL dated at 177 ± 38 ka and amino acid ages range from 111 to 178 ka at the type locality and 150 to 250 ka elsewhere. Interglacial Whidbey sediments give TL-dates of 102 ± 38 ka, 106 ± 17 ka, 142 ± 10 ka, and 151 ± 43 ka and marine sediments amino acid dates of 97 ± 35 ka, 96 ± 35 ka, and 107 ± 9 ka. The mean age of amino acid dates of the Possession Glaciation is 80 ± 22 ka. Vashon ice passed through Seattle after 14,500 ¹⁴C yrs and reached Olympia shortly thereafter. Between 14,000and 13,500 ¹⁴C yrs B.P., the Vashon glacier underwent rapid, large-scale back-wasting and down-wasting and had retreated from Seattle and the western Strait of Juan de Fuca by 13,500 ¹⁴C yrs B.P. By 13,000 ¹⁴C yrs B.P., marine water entered the Puget Lowland and floated the remaining ice, depositing Everson glaciomarine drift over the region between 13,000 and 11,500 ¹⁴C yrs B.P. During the Sumas Stade between 11,500 and 10,000 ¹⁴C yrs B.P., the Cordilleran Ice Sheet fluctuated during four phases.