During the late Pleistocene the Vashon glacier, a lobe of Cordilleran ice which at its maximum was 5300–7000 feet thick in northern Washington, occupied most of the Puget Lowland. During a late stage in the recession of the glacier when the ice was no more than a few hundred feet thick marine waters entered the area, floating the ice. Organisms living on the sea floor were incorporated in glaciomarine drift deposited beneath the floating ice. Radiocarbon dates from shells in the drift indicate an age of 11,660 ± 350 years (W-996).
Several hundred feet of emergence followed, during which fluvial and lacustrine sediments were deposited. A radiocarbon date from wood at the base of these sediments indicates deposition had begun by 11,640 ± 275 years ago (W-940).
A readvance of ice into northern Washington from British Columbia coincided with submergence of the lowland. Marine water and floating ice again covered the area depositing a second glaciomarine drift in places now at least 400 feet, and perhaps as high as 600 feet above sea level. Radiocarbon dates of 11,800 ± 400 (I-1037) and 10,370 ± 300 (I-1035) were obtained from wood in the deposits of two localities.
Emergence and deposition of till and outwash occurred near the Canadian border about 11,000 years ago.
Radiocarbon dates and stratigraphic relationships suggest that 350 feet of emergence, 500–700 feet of submergence, and emergence of 500–700 feet occurred in a period of only 1000–2000 years. These changes in relative sea level during such a short period may have resulted from a combination of two opposed tendencies, isostatic uplift of the land due to glacial unloading, and custatic rise of sea level, superimposed on tectonic movement.