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Rock Flour Deposits on the Floor of Glacial Lake Missoula

Glacial rock flour transported by meltwater under glacier.

Rock flour transported in suspension by meltwater beneath glacier.

The Big Draw Glacial Lake Missoula, Montana.
The Big Draw (NE of Nirada, Montana) once a canyon, is now choked with glacial outwash (sand, gravel and rock flour)that was carried into Glacial Lake Missoula.

Lake Missoula Strandlines - Missoula, Montana.
Rock flour, or glacial flour, is the result of glaciers grinding against rock beneath the glacier. The clay-sized particles of rock were suspended in meltwater and transported into the Clark Fork drainage (Lake Missoula). The image above shows bedrock that has been scraped by rocks carried in glacial ice. The resulting flour is shown.

Lake Missoula Strandlines - Missoula, Montana.
A clump of consolidated rock flour in the Little Bitterroot Valley turns to powder with a gentle squeeze.

Lake Missoula Strandlines - Missoula, Montana.
Rock flour in the Mission Valley is easily stirred up by vehicles traveling the many unpaved roads. The flour assures steady business at the carwash in Ronan, MT.

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In his book, Glacial Lake Missoula and its Humongous Floods, David Alt describes glacial outwash deposits associated with Glacial Lake Missoula.

All photos by Tom Foster unless otherwise noted.*
*The 1st and 3rd image on this page were taken in the Cascade Range.

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