VASHON (LATE FRASER) AGE CLIMATIC 'HYSTERESIS', NORTHWEST WASHINGTON

High-resolution lidar topography of the Skagit valley, northwest Washington, shows that global cooling, then warming, during the last glacial maximum were accompanied by significant, unreversed, changes in the regional spatial pattern of precipitation and snow accumulation.

Near the town of Concrete, the Skagit River and tributaries are incised into a flat-topped valley-floor deposit more than 150m thick. Lower valley walls are decorated with numerous landslide amphitheaters; the physiography is similar to that of the central Puget Lowland, where such features are diagnostic of coarsening-upwards advance outwash strata (lake clays and silts overlain by sand). Heller and Dethier (Northwest Science, 1981) confirm the presence of early Vashon-age lacustrine clay and silt overlain by sandy outwash in the Concrete area. An early-Vashon lake in this area requires an ice dam at the lower, western, end of the Skagit valley; i.e., at the beginning of Vashion glaciation, ice (with an original source in British Columbia to the north) advanced from the lowland east into the North Cascades. A similar history has long been known from Cascade Range-front valleys farther south.

Stoss-and-lee features near Lyman Pass at an elevation 600 m above the Skagit valley floor, streams on the south side of Mt Josephine that follow former ice margins, a well-developed terminal moraine and associated outwash flat near Hamilton, a fragment of terminal moraine north of Birdsview, the ice-marginal course of Grandy Creek (former path of the Baker River), the ice-marginal course of lower Finney Creek, and a low-elevation kettled outwash flat near Rockport collectively demonstrate that, for a protracted period at the end of Vashon glaciation, ice in the Skagit valley flowed west from a source in the North Cascades.

The final stages of glaciation did not mimic the initial stages. Climate during the Vashon stade did not simply change to glacial conditions and then back to non-glacial conditions.