Only one thick deposit of peat was examined in the immediate vicinity of Crater Lake, namely a 6-foot section in Munson Valley, about a mile below Government Headquarters. Here the peat had accumulated in a marshy basin cut through the pumice deposits left by glowing avalanches. Unfortunately, there is no means of determining how soon after the eruptions the peat began to form. At the bottom of the section, two-thirds of the pollen was from lodgepole pine; 17 per cent was from whitebark pine; the remainder consisted of approximately equal amounts from western yellow pine, mountain hemlock, and ?noble fir. Upward, the content of the three last-named species showed little change, but whitebark pine increased while lodgepole pine diminished, until at the top the former was in excess. This succession either indicates progressive cooling of the climate and increase in precipitation, or may simply imply that lodgepole pine, having fewer soil requirements, was the pioneer invader.1
7. What may prove to be an additional clue to the age of Crater Lake has lately been obtained by J. E. Allen2 from observations in northeast Oregon. Many valleys in that region were deeply filled with alluvium during the warm, dry third of post-Pleistocene time, when the carrying power of the streams was considerably diminished. Subsequently, when the climate became cooler and moister, the rejuvenated streams cut deep channels in their alluviated floors and in their fans and exposed a layer of extremely fine, white pumice not far beneath the surface. Accordingly, the pumice must have fallen toward the close of the driest part of post-Pleistocene time, or approximately 5000 years ago. It must be admitted, however, that until more detailed studies are made, the provenance of the pumice remains in doubt. The thickness, the distribution, and the fineness of the ejecta, as well as the nature of the glass particles, accord well with the view that Mount Mazama was the source; on the other hand, the few samples thus far examined microscopically contain a larger proportion of hornblende relative to hypersthene than is usual in the pumice unquestionably erupted by Mazama.
8. A minimum age for Crater Lake is given by the trees on Wizard Island. The oldest of them is said to be approximately 800 years of age, and if, as seems probable, the last eruptions from Wizard Island killed all the pre-existing vegetation, then the final activity took place a little more than 800 years ago. Unfortunately, we cannot tell how long it took for the Wizard Island cone to grow to its present height, nor can we say how long an interval of quiet followed the formation of the caldera before volcanism was renewed.
All the criteria listed above are admittedly vague. Taken together, however, they suggest that Crater Lake is, geologically, of very recent origin. That Mount Mazama was destroyed long after the period of maximum glaciation can no longer be doubted; that the catastrophe happened toward the end of a long dry period and shortly before the present pluvial cycle seems equally certain. Tentatively, the evidence points to an age of approximately 5000 years, and to a maximum of 7000 and a minimum of 4000 years. Yet we may anticipate with confidence that continuing studies of deglaciation in the High Cascades, coupled with examination of the present and former lakes in the lowlands to the east, especially in relation to the pumice deposits and postglacial vegetation, will provide a more accurate solution of this fascinating problem.
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