Late Pleistocene, post-Vashon, alpine glaciation of the Nooksack drainage,North Cascades, Washington

During the maximum late Wisconsin glaciation ca. 15 000 C-14 yr B.P., the C ordilleran Ice Sheet overwhelmed the Nooksack drainage of the northwestern Cascades, leaving only peaks higher than 2000 m above the ice surface. Ice- sheet flow over the Nooksack drainage and the adjacent Puget Lowland was es sentially north-south. Rapid deglaciation between 14 500 and 12 500 C-14 yr B.P. resulted in dropping of the ice-sheet surface below ridge crests in t he Nooksack drainage, and glacial activity thereafter became topographicall y controlled. Long valley glaciers in the upper Nooksack Valley were no lon ger connected to the Cordilleran Ice Sheet, and the source area changed fro m the main ice sheet to Mount Baker, Mount Shuksan, and the Twin Sisters Ra nge. At that time, the margin of the remnants of the Cordilleran Ice Sheet was 30 km to the northwest, separated from the Nooksack Valley glaciers by several ridges 1200 m higher than the surface of the ice sheet. Moraines were built in all three forks of the Nooksack drainage, 25-45 km d own-valley from their sources, The Middle Fork glacier stagnated 12 300 C-1 4 yr B.P. and deposited ice-contact drift that was later overridden when ic e readvanced over ice-contact drift and deposited a prominent, 2-km-long, l ateral moraine, Logs in a lateral moraine in the upper Middle Fork were dat ed at 10 680 +/- 70 and 10 500 +/- 70 C-14 yr B.P. The North Fork glacier, which originated at a large cirque on Mount Shuksan and was fed by glaciers from Mount Baker, extended to Kendall, where two moraines were deposited. Outwash from the younger moraine in the North Fork valley overlies glacioma rine drift, dated as 11 910 +/- 80 C-14 yr B.P., and contains charcoal laye rs dated at 10 603 +/- 69 and 10 788 +/- 77 C-14 yr B.P. The South Fork gla cier at its maximum was joined by ice from the Middle Fork and extended dow n-valley to Lake Whatcom and to Cranberry Lake, It retreated from its termi nal position slightly before ca, 12 700 C-14 yr B.P.