General Observations and interpretations:  When Dr. Alt and I first visited the Ninemile Creek roadcut in late 1969, rather than finding a single thick sequence of glacial lake varves we discovered a much more complex and far more interesting situation. What we saw was about 35-40 small-scale cycles up to a meter or more thick (Alt and Chambers, 1970). The rhythmically bedded deposits show a very distinctive pattern of alternating light and dark-colored layering at the outcrop scale.  In detail, a typical Lake Missoula rhythmite has a light-colored base of very fine-grained sand and silt grading upward into darker-toned rhythmically laminated silt and clay couplets that are interpreted to be glacial lake varves. The varves typically thin upward and are unconformable with overlying sand-silt unit of the next rhythmite (Chambers, 1971, 1984, 1989). The sand-silt beds are interpreted to be deposited by density underflows sourced by sediment-ladened streams flowing into a shallow, but rising lake level. As the lake level continued to rise, varves were deposited in deeper parts of the lake basins (Chambers 1971, 1984, 1989; Hanson, et al., 2012).

 

The schematic below is a stratigraphic log for the Ninemile Creek section. The general trend shown in this measured section is thinner rhythmites containing fewer varves at the base of the section, thicker units with more varves upwards, and then thinner rhythmites with fewer varves near the top. These same observations were made later by Curry (1977) and Hanson, et al. (2012). Curry (1977) interpreted this variation as indicating that the early and late lakes were relatively short-lived in comparison to the middle age of Lake Missoula. He also concluded at first the frequency of flood releases decreased, then the magnitude of each release increased, while the opposite trend existed later in the lake history. Waitt (1980) and Chambers (1984, 1989) explained these inferred relations to thickening and thinning of successive ice dams and the capacity to impound greater or lesser water volumes during the advancing and wanning phases of the Cordilleran Ice Sheet.

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Evidence for repeated exposure and lake level fluctuations is documented at the Ninemile Creek, Jocko Valley and Camas Prairie pages.