Abstract

Paleomagnetic secular variation and a hiatus defined by two tephra layers confirm that tens of floods from Glacial Lake Missoula, Montana, entered Washington's Yakima and Walla Walla Valleys during the last glaciation. In these valleys, the field evidence for hiatuses between floods is commonly subtle. However, paleomagnetic remanence directions from waterlaid silt beds in three sections of rhythmically bedded flood deposits at Zillah, Touchet, and Burlingame Canyon display consistent secular variation that correlates serially both within and between sections. The secular variation may further correlate with paleomagnetic data from Fish Lake, Oregon, and Mono Lake, California, for the interval 12,000–17,000 14C yr B.P. Deposits of two successive floods are separated by two tephras derived from Mount St. Helens, Washington. The tephras differ in age by decades, indicating that a period at least this long separated two successive floods. The beds produced by these two floods are similar to all of the 40 beds in the slack-water sediment sequence, suggesting that the sequence is a product of tens of floods spanning a period of perhaps a few thousand years.

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