Chambers (1984) presented a schematic illustrating his proposed conceptual model for the deposition of Glacial Lake Missoula rhythmites. The 3-D block diagram has a vertical scale in meters to ten's of meter with a horizontal scale of tens of kilometers. Time lines (T0 to T7) are indicated on the left of the diagram with hypothetical standlines (S1 to S3) shown as dashed lines. The Sand-Silt facies (Silt subfacies on the diagram) was deposited during the filling stage of the lake. Sedimentary structures within this facies suggest deposition by sediment laden streams flowing into a rising lake-level. Because the sediment-water mixture is much denser than the lake water, it most likely entered the lake as a hyperpycnal plume (underflow; see the insert by Ashley, 1975). The sheet-like nature of the Sand-Silt facies suggests deposition by multiple, coalescing density underflows. Hanson, et. al. (2012) also came to the same conclusion based on their work at Ninemile Creek and the Rail Line sections. T4 represents a partial or complete lake drainage, followed by desiccation and weathering of the varves on an exposed lake surface. At that time small streams traversed the lake floor, eroding the Silt-Clay facies (varves) depositing the Pebble-Gavel facies as a channel lag. It is most probable that periglacial conditions prevailed creating frost cracks in the exposed lake sediments.

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Like Ashley (1975), Chambers (1971, 1984) found three varve types:

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• Group I: Clay thickness great than silt thickness.

• Group II: Clay and silt thicknesses approximately equal

• Group III: Clay thickness less than silt thickness

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Ashley (1975) suggested that there is a chronological relationship between the three groups as well as a direct correlation to their proximity to the sediment source. Reviewing the diagram above, the relationship is shown schematically. At a given location, once the lake-level was deep enough for varve formation, Group III varves formed first and as the lake continued to deepen, Group II varves formed, followed by Group III varves as the shoreline continued to migrate east, in the Missoula Valley basin, for example. Such a chronological sequence makes sense because as long as the ice-dam had integrity, the lake continued to fill and the sediment was dispersed over an expanding lake floor.

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SUMMARY

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About 40 last glacial Lakes Missoula are documented in the Ninemile roadcut, however there is no conclusive evidence that each drainage was complete or catastrophic (Chambers, 1971, 1984; Hanson, et al., 2012), but the lake did drain below an altitude of about 936-m ASL. Because the number of varves in any given sequence ranges from 9 to 58, Alt and Chambers (1970) inferred periods of several decades between lake drainages; Alt (2001) thought that the average interval between lake fillings to be about 50 years.  I agree with the conclusion of Smith (2006) that the sediments preserved at Ninemile Creek and throughout other basins are evidence of non-catastrophic drainings of glacial Lake Missoula or at the very least, the major floods occurred before deposition of the lake bottom sediment we see preserved today.