REASSESSING A PROPOSED LARGE LATE-GLACIAL ALPINE ADVANCE NEAR MT. BAKER, WA

We combined recent LiDAR data and field investigations to reevaluate the evidence for a large late-glacial (post-Vashon) alpine glacier advance proposed for the Nooksack River drainage centered on Mt. Baker, northwest Washington (Kovanen and Easterbrook, 2001). Major evidence for this event includes four putative latero-frontal moraines identified in the North and Middle forks of the Nooksack River and near Lake Whatcom. Our reassessment is motivated by the recent availability of high-resolution bare-earth LiDAR altimetry in the area, in addition to the fact that the magnitude of this proposed advance is anomalously large compared to other late-glacial alpine glacier advances in the region.

In the North Fork Nooksack, LiDAR imagery shows the “Kendall moraine” to be a nearly north-south trending streamlined hill that matches the shape, size and orientation of numerous other nearby fluted hills. A roadcut through the “Kendall moraine” exposes highly compacted till. Cobbles in the till are dominated by lithologies associated with bedrock exposed north of the site (e.g., crinoid-rich limestone). We found no cobbles of Mt. Baker Andesite, previously reported as major evidence that the till originated from a west-flowing valley glacier. Our data indicate that this “terminal moraine” is instead a subglacial flute deposited by southward flow of the Cordilleran Ice sheet during the Vashon Stade. The “Maple Falls moraine,” directly up valley of the Kendall site, also appears to be a subglacial deposit of the Vashon Stade. In the Middle Fork Nooksack valley, the primary evidence for the alpine advance, a putative “lateral moraine,” instead appears to be post-glacial river incision into ice-stagnation deposits in the valley. The primary evidence for a glacier originating in the South Fork Nooksack basin, a “moraine” along the southeast shore of Lk. Whatcom, is morphologically indistinguishable from numerous other north-south subglacial flutes in the area. We also did not find erratics or till cobbles requiring a source to the east (e.g., dunite). In summary, our results indicate that deposits previously interpreted as subaerial terminal and lateral moraines from a late-glacial alpine glacier advance at Mt. Baker are instead either subglacial flutes or ice-stagnation deposits related to late phases of the Vashon stade.