Fig. 26. Histograms of pumice-scoria flows in Annie Creek canyon |
The fine ash overlying the pumice-scoria flows in the canyons of Castle, Annie, Sun, and Sand creeks cannot be a product of these eruptions, for it is restricted to the central parts of the valleys. For this reason, it is interpreted as the result of settling of the finer constituents that rose into the air as the avalanches swept down the valleys. The thickness of such ash locally reaches 50 feet, but is usually between 5 and 20 feet. It fell immediately or soon after the flows came to rest.
Similar in mode of origin is the fine glass dust which overlies the pumice flows near Chemult. No scoria flows reached this far from the source, and consequently there is no crystal-rich ash; on the other hand, there must have been violent turbulence at the snouts of the pumice flows, and great clouds of fine glass dust must have risen into the air, to settle slowly on the coarse deposits.
The volume of the final “ash” fall is probably of the order of 0.25 mile.
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