RE-EVALUATION OF THE SIGNIFICANCE OF THE GLACIAL STRATIGRAPHY OF SW BC
Recent stratigraphic work in Coast Mountain watersheds north of the Fraser Lowland, drill core data in the Metro Vancouver area, and lidar imagery have led us to question the climatic significance of these stades and interstades. Specifically, work in Chehalis Valley in the eastern Fraser Lowland indicates that it was not directly affected by Coquitlam ice; at ca. 20 ka, a glacier in the eastern Fraser Lowland blocked the mouth of Chehalis Valley, impounding a lake. Importantly, this blockage occurred at least 1.5 ka later than the Coquitlam advance in western Fraser lowland. The organic-rich sediments associated with the Chehalis flooding event are identical to those of the later Port Moody Interstade, indicating that glaciers were advancing into an established subalpine forest in both areas. Drill holes in the western Fraser Lowland confirm the presence of the Coquitlam and Vashon stades, separated by Port Moody, but also indicate the presence of marine sediments, implying significant glacio-isostatic depression. Another study concludes that at least 85 m of glacio-isostatic depression of the westernmost Fraser Lowland had been achieved by 26 ka. We speculate that Coquitlam glacier retreat from the western Fraser Lowland was caused by glacio-isostatic depression in tandem with eustatic sea rise at ca. 18.5 ka. In this scenario, the Port Moody interstade records establishment of subalpine fir forest in recently deglaciated areas, driven more by glacio-isostatic adjustments of the crust than climate.