5. The recency of the eruptions which brought about the collapse of Mount Mama is also attested by the fact that pumice covers the youngest moraines on the east flank of the Cascade Range, by the lack of deep weathering of the ejecta, and by the surprisingly regular manner in which the thickness of the pumice varies from place to place. Except along the banks of the major streams, the thickness has scarcely been influenced by denudation.
6. The forests on Mount Mazama, though scantier than they are today, were otherwise similar. Within the limits of the park trees were few, but below 4500 feet they grew in profusion, especially on the western side of the volcano.
Fragments of charcoal collected from the deposits of glowing avalanches in road cuts bordering the Rogue River were examined by R. A. Cockrell, of the Forestry Department, University of California. From cuts along the Diamond Lake highway, a mile or so from the Union Creek junction, charred pieces of the following trees were identified: Pinus monticola (western white pine), P. Lambertiana (sugar pine), P. albicaulis (whitebark pine), P. contorta Murrayana (lodgepole pine), P. ponderosa(western yellow pine), Abies concolor (white fir), and Pseudotsuga taxifolia (Douglas fir). From the roadside quarry near McLeod, close to the snout of the Rogue River pumice flow, carbonized limbs of white pine and Douglas fir were identified. In Beaver Marsh, beyond the east base of Mount Mazama, the only tree represented was lodgepole pine.
At all these localities, trees of precisely the same kinds are now thriving on the pumice. The inference is that when the pumice was laid down the climate was almost if not locally quite the same as that of today. To test this inference further, Waldo Glock studied the growth rings of the charcoal logs. His conclusion was that when the trees were living, rainfall came dominantly but not exclusively in the winter, as it does today. Many logs had rings indicating growth under ideal conditions, with a plentiful supply of water; it should be recalled, however, that most of the carbonized trees were growing close to the banks of rivers, where more groundwater was available than elsewhere.
The extensive and detailed studies made by H. P. Hansen, of Oregon State College, of the pollen grains in post-Pleistocene peat bogs of the Pacific Northwest provide another welcome clue to the age of Crater Lake. At four localities, he examined thick sections of peat resting on the pumice blown from Mount Mazama during its culminating eruptions. In three of these profiles, the highest proportions of lodgepole pine were found just after the yellow-pine maximum. Hence, Hansen concludes that the eruptions occurred during or soon after the dry period denoted by the yellow-pine maximum, at a time when the climate was slightly drier than it was soon thereafter and at present. If, as many geologists suppose, the warm, dry third of post-Pleistocene times was between 4000 and 7500 years ago, then on this evidence Crater Lake was formed approximately 5000 years ago.
