Marine sedimentary deposits associated with relative sea level changes and the waning late Wisconsinian Cordilleran ice sheet are widely exposed above present sea level in the central Puget lowland. Some of these deposits, commonly referred to as glaciomarine drift, are traditionally interpreted as having been deposited as a rain of sediment from floating ice in water depths greater than 70 m. Deposits previously included in this drift include till-like diamictons, pebbly silt, massive well-sorted silt, and laminated silt and sand. Other glacial-marine or marine deposits include massive to crudely stratified sand and various mixtures of sand and gravel.

Recent studies indicate that all these deposits are broadly contemporaneous (14,000 to 12,000 ¹⁴C yr B.P.) and that they represent a wide variety of depositional environments, many of which involved no glacier ice. In addition to marine environments receiving debris from floating ice, depositional settings may also include: (1) marine beach; (2) tidal flat and

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estuary; (3) shallow (less than 10 m) subtidal marine; (4) deeper (greater than 10 m) marine; (5) marine-glaciofluvial delta; and (6) marine ice-contact glaciofluvial.

Microfossil and macrofossil evidence constrains the interpretation of water depth, sedimentation rate, and possibly water temperature and salinity for many of these environments. Radiocarbon dating of shells preserved in glacial-marine and associated deposits indicates that the retreat of continental ice was rapid. Age differences of approximately 1,000 ¹⁴C yr between shallow-marine deposits at similar altitudes suggest differential isostatic rebound, perhaps related to the regional pattern of ice retreat or to inferred crustal structure.

We propose a deglaciation model that includes: (1) a rapidly retreating continental-ice margin that calved in open marine waters along an irregular isostatically active coastline; (2) rapid eustatic changes; and (3) large sediment volumes transported into marine waters by rivers draining the recently deglaciated landscape.

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