Less than 10 per cent of the total volume of pumice fell west of the summit of the Cascade Range, and much of this was swept away by the headwaters of the North and South Umpqua rivers. Thick banks of washed pumice occur far down the North Umpqua, and though most of it was derived from erosion of the later pumice flows, much of it represents products of the pumice fall washed from the neighboring hills. Certainly all the washed pumice found along the walls of the South Umpqua must be reworked pumice fall, for none of the later pumice flows entered the drainage basin of that river.
In the low country east of the Cascades, the thickness of the pumice varies gradually and regularly over long distances. Near the crest of the range, on the other hand, the variations are both rapid and irregular, and the isopachytes shown on the map are greatly generalized. To some extent, the variability in this high country is the result of erosion, but chiefly it seems to have been caused by swirling cross winds at the time of eruption. The influence of such disturbances on the thickness of ejecta was exemplified clearly during the eruption of Quizapu, already mentioned. Most of the pumice blown out during that eruption drifted eastward across Argentina, but instead of thinning regularly away from the source, the Quizapu pumice suddenly thickened in central Argentina and then continued to thin farther east. This local thickening was the result of complex eddies in the lee of the Sierra de Cordoba. We may reasonably imagine that turbulence of the winds among the peaks of the Cascades, by diminishing the transporting capacity of the air, likewise caused irregular dumping of ejecta.
Nevertheless, if these minor variations be ignored, the thickness of the pumice fall erupted from Mount Mazama varies with such regularity as to suggest that all of it was laid down within a short time. Had the eruptions continued for years, there would probably be far more irregular variation both in thickness and in distribution. Nowhere have any signs of erosion been noted within the pumice deposits themselves, nor are there any traces of interbedded soils. The probability is, therefore, that the whole eruptive episode was short-lived and that the explosions, if not actually continuous, followed each other at short intervals.
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