The Puget Lowland and San Juan Islands, lying in the Puget Trough between the Cascade Range and the Olympic Mountains, were in the path of Pleistocene ice sheets which flowed southward from Canada.
Floodplain, silt, sand, and peat of the Whidbey Formation were deposited prior to 40,000 years ago, during an interglaciation believed to be equivalent to the Puyallup and Sangamon Interglaciations. Possession Drift, which overlies sediments of the Whidbey Formation, is radiocarbon dated at 34,000 years B.P. in its upper part and is limited by a radiocarbon age of >39,900 in its lower part. The Possession is correlated with late phases of the Zyriansk Glaciation in Russia and the early Würm in Europe.
Peat lying on Possession Drift yielded radio-carbon dates between 27,200 and 22,700 ± 550, representing the Olympia Interglaciation which is correlated with the Karginsk Interglaciation of Russia, the Paudorf Interval of Europe and the Farmdalian of the midwestern United States.
Esperance Sand, deposited by meltwater streams in front of advancing ice during the Fraser Glaciation and subsequently overridden during the Vashon Stade, was radiocarbon dated at 18,000 ± 400 years. During deglaciation, the ice sheet thinned until it floated in marine water and poorly sorted, fossiliferous glaciomarine drift was deposited in the Everson Interstade. Shell-bearing sediment at altitudes up to 400 feet above present sea level and unfossiliferous glaciomarine drift up to 600 feet indicate that relative sea level at that time was 500 to 700 feet higher. Eighteen radio-carbon dates from marine shells in Everson glaciomarine drift vary from 10,370 ± 300 to 13,010 ± 170 years. The Everson Interstade ended with lowering of relative sea level to approximately its present position and the disappearance of floating ice. The following Sumas Stade ended shortly before 9,920 ± 760 years ago.