Proglacial lake shorelines in the Puget Sound area were distorted by the isostatic response associated with recession of the Cordilleran ice sheet about 18,000-13,500 yr ago. The shorelines, which are highest near the east-west midpoint of the elongate lowland, are upwarped northward, with a regression slope of 0.85 ± 0.04 m km-1 departures from the trend may have been caused by Holocene tectonic warping. Calibrating this lacustrine record with data from the Everson marine limit permits the actual (rather than relative) uplift to be estimated and contoured. The high degree of isostatic equilibrium attained (at least 50%) during the brief loading hemicycle suggests that the mantle beneath this part of the Cascadia subduction zone has anomalously low viscosity and strength. Crustal strain associated with the loading/unloading cycle in the south-central lowland appears to he correlated with the pattern of Quaternary faulting and seismicity.