LATEST PLEISTOCENE LANDFORMS NEAR FERNDALE, WHATCOM COUNTY, WA IMAGED BY HIGH-RESOLUTION LIDAR SURVEY

A lidar survey of the Lummi Reservation and vicinity illuminates late-glacial stratigraphy that has been argued to indicate synchronous climate change across the northern hemisphere and extreme local tectonism. The standard history of the region includes: (1) glacioisostatic depression leading to relative sea level (RSL) in excess of 100 m above modern sea level about 13 (¹⁴C) ka, after retreat of the Vashon ice sheet, and subsequent falling RSL; (2) Everson glaciomarine deposition between 13 and 11 ka, with RSL possibly fluctuating by tens of meters; and (3) subsequent Sumas glaciation, ending by 10 ka, marked by subaerial deposition of moraines and outwash. The new data show that Everson and Sumas deposits are in large part synchronous and that field observations have not reliably separated submarine from subaerial deposits.

Lidar topography discloses a clear distinction between submarine (Everson) and subaerial (Sumas) surfaces. Everson surfaces are smooth and marked by relict shorelines formed during falling RSL. In contrast, Sumas surfaces are irregular, locally steeper, lack shorelines, and in this area are truncated by lower-elevation shorelines. Everson surfaces cover all of Point Francis, most of the Lummi Peninsula, and much of the Lake Terrell upland up to 90 m elevation. Sumas glacial surfaces include an older (S0) moraine, east of Lake Terrell, that coexisted with RSLs of ~90 to ~70 m, and an eastern, younger moraine (SI of Kovanen and Easterbrook, 2002) that also extends southwest along the N side of the Nooksack delta and is associated with RSL of ~30m. The SI moraine is probably coeval with a moraine fragment on the N end of the Lummi Peninsula that is also associated with ~30m RSL. SI ice extended to saltwater on both sides of the Lummi Peninsula. Behind S0 and SI moraines alike is extensive lumpy ground formed by subaerial deposition, probably from melting ice. The subjacent deposits as well as the moraines were previously mapped, on lithologic criteria, as Everson glaciomarine drift.

The Sumas ice sheet appears to have been mantled with marine drift. Did Sumas ice advance across Everson marine drift and incorporate its substrate? Or did debris-rich latest Vashon ice spread and melt until it was below sea level, thence to be covered with marine deposits and rebound isostatically as the Sumas ice sheet?