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

Abstract

During the maximum late Wisconsin glaciation ca. 15 000 14C yr B.P., the Cordilleran 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 essentially north-south. Rapid deglaciation between 14 500 and 12 500 14C yr B.P. resulted in dropping of the ice-sheet surface below ridge crests in the Nooksack drainage, and glacial activity thereafter became topographically controlled. Long valley glaciers in the upper Nooksack Valley were no longer connected to the Cordilleran Ice Sheet, and the source area changed from the main ice sheet to Mount Baker, Mount Shuksan, and the Twin Sisters Range. 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 downvalley from their sources. The Middle Fork glacier stagnated 12 300 14C yr B.P. and deposited ice-contact drift that was later overridden when ice readvanced over ice-contact drift and deposited a prominent, 2-km-long, lateral moraine. Logs in a lateral moraine in the upper Middle Fork were dated at 10 680 ± 70 and 10 500 ± 70 14C 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 glaciomarine drift, dated as 11 910 ± 80 14C yr B.P., and contains charcoal layers dated at 10 603 ± 69 and 10 788 ± 77 14C yr B.P. The South Fork glacier at its maximum was joined by ice from the Middle Fork and extended downvalley to Lake Whatcom and to Cranberry Lake. It retreated from its terminal position slightly before ca. 12 700 14C yr B.P.