ADVANCE AND RETREAT OF THE LATE PLEISTOCENE CORDILLERAN ICE SHEET

Radiocarbon dates of 16-20,000 ¹⁴C yrs B.P. beneath laminated mud near the Canadian border have been used to suggest that the Cordilleran Ice Sheet (CIS) crossed the border after 16,000 ¹⁴C yrs B.P. However, these sediments are not known to be overlain by Vashon till and are anomalously young because the CIS was building an outwash plain in front of its advancing margin ~115 km to the south 18,000 ± 400 ¹⁴C yrs B.P. ~150 km to the south at 16,510 ± 320, 15,450 ± 450, and 15,350 ± 210 ¹⁴C yrs B.P., and in Seattle ~14.5–16,000 ¹⁴C yrs B.P. If the CIS didn’t cross the border until after 16,000 ¹⁴C yrs B.P., it could hardly have been depositing outwash as far south as Seattle at 16,000 ¹⁴C yrs B.P. Which set of dates is correct remains uncertain. 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 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 central and northern lowland. 140 ¹⁴C dates indicate that deposition of Everson gmd in the central and northern Puget Lowland continued from 13,000 to 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 yrs B.P.