GEOLOGIC CONSTRAINTS ON THE GEOMETRY OF THE CORDILLERAN ICE SHEET IN SKAGIT VALLEY DURING MARINE ISOTOPE STAGE 2

Erratics, upper elevations of ice-scoured bedrock, and moraines limit the geometry of the last Cordilleran Ice Sheet (CIS) in the North Cascade Mountains. Evidence was collected in Skagit valley, an interlobate zone between the Puget and Okanagan lobes of the CIS during marine isotope stage 2. Flow into and out of the watershed included multiple flow vectors. Pebble lithology, erratic, and striae evidence record flow east up the lower part of the valley from the Puget Lobe, which impounded lakes and built a massive moraine-outwash complex at the mouth of Baker valley. The Okanagon Lobe flowed south into the upper Skagit valley as an ice stream and diverged near Jack Mountain to the west down Skagit valley and to the southeast toward Lake Chelan. Geomorphic and stratigraphic data indicate that the two lobes met near Rockport, where the Okanagon Lobe turned south up the Sauk valley. At its maximum extent the ice sheet spilled over high divides and through cols from the northwest and northeast, and had a surface near 2100 m near the international boundary that sloped to 1800 m near the mouth of the Baker and terminated at 500m in the upper Sauk basin. CIS glaciation occurred in a span of about 3,000 years, and concluded with frontal retreat of the ice sheet as the lower valley was flooded by the Skagit Marine Embayment.