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A Pleistocene view of Missoula Valley. The drainage of the Clark Fork river was blocked by the continental ice sheet as it moved southward about 15,000 years ago. The lake formed by this ice dam, known as Glacial Lake Missoula, was approximately the size of modern Lake Ontario (500 cubic miles), and was up to 2000' deep. This lake drained in matter of days, releasing the greatest flood of known geologic record. Evidence in the scoured flats of eastern Washington indicate that the lake filled and drained at least 41 times.

As a reference to modern Missoula's topography an 'L' and 'M' (not to scale!) were place in their approximate locations in the scene.

For more information consult: Alt and Hyndman, "Roadside Geology of Montana". Mountain Press Publishing, Missoula, Montana, 1986.