ADVANCE AND RETREAT OF THE LATE PLEISTOCENE CORDILLERAN ICE SHEET IN THE PUGET LOWLAND

Radiocarbon dates beneath Vashon till limit the timing of the advance of the late Pleistocene Cordilleran Ice Sheet into the Puget Lowland. As ice moved southward, meltwater built a broad outwash plain in front of the advancing ice. Wood and peat in outwash beneath Vashon till in the central Puget Lowland have yielded dates of 18,265, 18,000, 17,350, 17,250, 17,000, and 16,510 ¹⁴C yrs B.P., indicating that Vashon ice had reached 115 km south of the Canadian border by then. However, mixed wood dates from deposits just north the border ranging from 16,000 to 20,000 ¹⁴C yrs B.P. have led some to suggest that Vashon ice didn’t cross the border until after 16 ka ¹⁴C yrs B.P. These dates are anomalous because older wood lies above younger wood and wood with dates of 16-20 ka occur together. The reason for the incongruous dates remains uncertain, but the variability is almost certainly due to reworking of wood of mixed ages. No Vashon till occurs in this section so the deposit may not even be pre-Vashon. The conclusion that Vashon ice didn’t cross the border until after 16,000 ¹⁴C yrs B.P. would require throwing out six 16-18,000 ¹⁴C dates on wood and peat in outwash far to the south.

Vashon ice passed through Seattle after 14,500 ¹⁴C yrs B.P. and reached its maximum near Olympia soon after. Between 14,000 and 13,500 ka 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. Marine water then entered the Puget Lowland and floated the remaining ice, depositing Everson glaciomarine drift over the entire central and northern lowland. 150 ¹⁴C dates indicate that deposition of Everson gmd in the central and northern Puget Lowland continued until 11,500 ¹⁴C yrs B.P. The lowland emerged from the sea ~11,500 ¹⁴C yr B.P. During the Sumas Stade, the CIS advanced and retreated at least three times, building prominent moraines 11,500 to 11,400, 10,980 to 10,250, and 10,250 to ~10,000 ¹⁴C yr B.P.