A SPECULATIVE HISTORY OF THE SUMAS ICE ADVANCE, FRASER LOWLAND, SOUTHWEST BRITISH COLUMBIA AND NORTHWEST WASHINGTON

Published radiocarbon ages and topography, new lidar topography, and limited field observations suggest that Sumas ice reached its maximum extent after 11,400 BP (14C years) and latest Pleistocene relative sea level (RSL) fell monotonically.

Late Vashon and Sumas-age ice issued mostly from the Harrison Lake valley, not the Fraser gorge as earlier thought, as: 1) Late Vashon-age Fraser meltwater appears to have been routed into the upper Skagit drainage via the Nicolum-Sumallo and Silverhope-Klesilkwa valleys. Such routing requires an ice-surface-high west of Hope, B.C. 2) Sumas-age Fraser meltwater was diverted south past Cultus Lake. Such diversion requires that the ice surface slope south across the Fraser valley. 3) Major Sumas ice advance was post-11,400 BP and the lower Fraser gorge was ice-free as of 11,400 BP (375 m elevation) and 11,100 BP (225 m elevation).

Shortly after 11,400 BP, Sumas ice blocked the Nooksack valley southeast of Everson and reached the sea at Bellingham Bay, Lummi Bay, Birch Bay, and Drayton Harbor. This advance approximates Kovanen's Sumas I. Coeval RSL was at ~58 m west of Langley and ~35 m near Marietta. Wide preservation of subaerial melt-out topography indicates that later RSL was never higher and thus the Bradner (75 m elevation) and Deming (50 m elevation) locales could not have experienced marine conditions after 11,400 BP.

Over the next thousand years (or less), Squalicum Creek spillway and delta were established, with a coeval strandflat that extended from Lynden to Ferndale and Blaine; Tenmile Creek spillway formed; outburst floods(?) distributed Cultus Lake-sourced gravels from Cedarville almost to Ferndale; and then a stable Fraser outflow via Everson was established. Subsequent collapse of late Sumas ice in the Matsqui-Sumas-Agassiz prairie allowed the Fraser to drain west past Mission and formed an early Holocene lake. Onset of Fraser delta sedimentation at ~9,000 BP followed initial filling of this lake.