MID-HOLOCENE GLACIAL ACTIVITY IN BRITISH COLUMBIA AND WASHINGTON

Pre-Neoglacial Holocene glacial history in the Coast Range of British Columbia has been moderately well characterized through study of lateral moraine stratigraphy, dating of in-situ and detrital wood in glacier forefields, and analysis of annually resolved lake sediments in glacierized watersheds. Evidence from the Washington Cascades suggests that mid-Holocene glacial history there is similar to that of British Columbia.

The term “Garibaldi Phase” (GP) was applied in the mid-1980's to an expansion of alpine glacier ice in the interval 6000-5000 14C yr BP, recorded at a few glaciers in the southern Coast Range of British Columbia. Evidence included stumps sheared by glacial overrunning. More recently, evidence for advancing ice between 6400 and 5100 14C yr BP has been found at Warren, Stave, Sphinx, Overlord, and Lava glaciers in Garibaldi Provincial Park north of Vancouver, B.C. These glaciers overran forests up to 800 m downvalley from present glacier toes, but well upstream of maximum Holocene limits, between 6400 and 5800 14C yr BP. Evidence for GP expansion also comes from lake-sediment proxies in the southern Coast Range, and from a few glaciers farther north in the Coast Range. The GP is regarded as marking the onset of Neoglaciation in the area.

Over 35 years ago Dan Miller reported on trees that were sheared by an advance of South Cascade Glacier in Washington state about 5000 to 4700 14C yr BP. We have likewise documented a mid-Holocene glacier advance in the Cascades based on the age of an unconformity in the left-lateral moraine of Easton Glacier on Mt. Baker. In a thin zone between older and younger tills, a mat of logs dated at ca. 5300-5200 14C yr BP overlies deformed peaty sediments containing Baker Set OP, Mazama, and Baker Set SC tephras (5740, 6800, and 8850 14C yr BP, respectively). The wood mat likely was emplaced by the advancing ice that deposited the till above it. The lack of early/mid-Holocene glacial sedimentation, which has analogs at Bugaboo Glacier in the Purcell Mountains of British Columbia and Stutfield Glacier in the Canadian Rockies, is interpreted to mean that ice was less extensive at that time than during the later Holocene. The evidence from both the Cascades and the Coast Range implies that glaciers advanced in the mid-Holocene after a prolonged period of minimal ice cover.