TILL-FABRIC ANALYSES OF NE-SW TRENDING FLUTES ON NORTHERN WHIDBEY ISLAND: EVIDENCE OF READVANCE OF THE CORDILLERAN ICE SHEET DURING THE LATEST VASHON STADE OF THE FRASER GLACIATION

NE-SW trending flutes were first recognized by R.A. Haugerud using LIDAR topography and inferred to represent a directional change of ice flow late during the last glaciation.

Till-fabric and stratigraphic analyses of these NE-SW trending flutes, as well as stratigraphically lower Vashon till units indicate that the Cordilleran ice sheet (CIS) likely readvanced over north Whidbey Island following its initial retreat. Two hypotheses have been proposed to explain the change in ice flow direction from predominantly north-south during the early to late stages of the Vashon advance, to the NE-SW flow direction during the latest episode of readvance of Vashon ice over northern Whidbey Island:

1. The ice front in the Strait of Juan de Fuca retreated earlier (through rapid calving) than that in the Puget Lowland, inducing a change in ice divide and local flow direction over northern Whidbey Island as originally proposed by R.M. Thorson in 1980.

2. During the retreat and readvance of the CIS over northern Whidbey Island ice flow direction in this region was controlled by extensive alpine glaciation from the North Cascades similar to that suggested by D. Easterbrook and D. Kovenan for the Nooksak drainage further to north.

Evidence supporting and/or contradicting these two hypotheses will be presented in this paper.