Early Fraser glacial history of the Skagit Valley, Washington

The primary objective of this two-day field trip is to examine sediments from the Evans Creek stade of the early Fraser Glaciation at four key sections along the Skagit River near Concrete and the shoreline of Ross Lake. These sediments provide important new information on the timing and extent of alpine glacier advances during the Evans Creek stade (early Fraser Glaciation). In lower Skagit valley at Cedar Grove, glacial drift overlies an organic bed that yielded a radiocarbon age of 25,040 (super 14) C yr B.P.; this age is a maximum limiting date for the Evans Creek stade. Three radiocarbon ages within 400 years of 24,000 (super 14) C yr B.P. record damming of upper Skagit valley by the Big Beaver alpine glacier. The ice dam created glacial Lake Skymo, which persisted until at least 18,020 (super 14) C yr B.P., suggesting that Cascade glaciers remained at advanced positions throughout most of the Evans Creek stade. However, growth of a forest on early Evans Creek drift at Cedar Grove 20,730 (super 14) C yr B.P. requires at least some recession of the Baker valley glacier. An increase in the number of lowland and montane macrofossils in glacial Lake Skymo sediments after 20,770 (super 14) C yr B.P. is consistent with a mid-Evans Creek stade warm interval. Sometime after 20,730 (super 14) C yr B.P., the Baker valley glacier overrode the forest bed and deposited till at Cedar Grove. The advance dammed Skagit River and created glacial Lake Concrete, which persisted until about 16,400 (super 14) C yr B.P.