REVISED LATE PLEISTOCENE/EARLY HOLOCENE GLACIAL HISTORY AT MT. RAINIER, WASHINGTON

Interpreted timing of late Pleistocene/early Holocene glacial advances in the North American Cordillera is in a state of flux. Some work has identified a single, Younger-Dryas-aged advance and other work suggests pre-YD and/or post-YD advances in various ranges. In the Cascade Range, Mt. Baker shows evidence of a single, YD advance. At nearby Mt. Rainier, original work by Crandall, some with Miller, indicated a single advance older than 8.75 ka (recorded by “McNeeley Drift” overlain by R tephra), but later work by Heine suggested two advances, one pre-YD and one post-YD. To investigate these contradictory conclusions we remapped glacial deposits in cirques and high valleys on Mt. Rainier using lidar, Google Earth imagery, and field work; we also cored lake sediments in Tipsoo Lake, downstream of a McNeeley moraine.

Comparison of our mapping and that of Crandell to that presented by Heine shows significant differences. Of the 17 McNeeley moraines identified by Heine, our analysis corroborates only a single deposit: the Tipsoo Lake moraine. In most basins where Heine mapped McNeeley moraines, we find no evidence at all for such deposits. In a few cases, we identified McNeeley deposits at the heads of valleys where Heine mapped moraines, but none at locations identified by him. In these cases, Heine’s putative moraine crests lie 0.5 - 1.0 km downvalley from our deposits and reach elevations that are about 200 m lower. We did not find evidence of two advances. Given the errors in Heine’s mapping, ages he ascribed to McNeeley moraines provide no insight into the actual history. The absence of sets of paired late-glacial cirque moraines in the area support Crandell and Miller’s original notion of a single McNeeley advance. The only age constraint on the advance remains a minimum age provided by R tephra; presently the age of R tephra is estimated from previous work to have an age of 10 ka.

In the Tipsoo Lake core R tephra occurs at 108-114 cm depth. A macrofossil at 88 cm depth dates to 10.2-9.9 ka. A bulk sediment age also at 88 cm dates to 11.1-10.6 ka, suggesting the bulk date is ~800 years too old. The same differential, if applied to a bulk sediment age at 98 cm of 12.6-12.2 ka, would suggest that R tephra is ~1600 years older than previously thought, and that the McNeeley advance, whose correlative sediments lie below R, is of YD age.