| Two classes of field evidence firmly establish that late Wisconsin glacial Lake Missoula drained periodically as scores of colossal jökulhlaups (glacier-outburst floods). (1) More than 40 successive, flood-laid, sand-to-silt graded rhythmites accumulated in back-flooded valleys in southern Washington. Hiatuses are indicated between flood-laid rhythmites by loess and volcanic ash beds. Disconformities and nonflood sediment between rhythmites are generally scant because precipitation was modest, slopes gentle, and time between floods was short. (2) In several newly analyzed deposits of Pleistocene glacial lakes in northern Idaho and Washington, lake beds comprising 20 to 55 varves (average = 30-40) overlie each successive bed of Missoula-flood sediment. These and many other lines of evidence are hostile to the notion that any two successive major rhythmites were deposited by one flood; they dispel the notion that the prodigious floods numbered only a few. -- Waitt, 1985 |
