The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams
The Climax: Culminating Explosions of Pumice and Scoria
Detailed Description of the Individual Flows
Having enumerated the general features of the glowing avalanches, we may now pass to an account of the individual flows.
The Castle Creek Flows
The avalanches that descended the southwest flank of Mount Mazama left scarcely any trace of their passage until they reached the base of the steep slopes about 2 miles beyond the rim of the caldera. Only when the flows had reached the valley of Castle Creek, between the opposing slopes of Mount Mazama and Union Peak, was their speed sufficiently checked to permit large-scale deposition. Of all the pumice flows from Mazama, none contains such a large proportion of coarse material. Even 12 to 15 miles west of Annie Spring there are places where half the deposits consists of pumice lumps between 3 and 12 inches across.
Just as at the heads of Munson, Sun, and Kerr valleys, so at the head of Castle Creek, there is a broad transitional zone in which the amount of glacial drift gradually diminishes downstream as that of pumice and scoria increases. For more than 2 miles, the valley floor is cut through the recessional moraines which underlie the pumice. Much of the finer glacial debris was incorporated in the basal parts of the pumice flows as they scoured the surface of the moraines. Indeed, the flows churned up the surficial glacial deposits to a depth of several feet, and doubtless this admixture was accentuated by floods accompanying the flows.
The pumice flows of Castle Creek traveled 18 miles before they joined the flows of the Rogue River valley. They were prevented from spreading northward by a high ridge of lava, but southward they overflowed the gorge of Castle Creek and spilled into the valley of Union Creek, which they filled to a maximum depth of 250 feet. Pumice bombs up to 2 feet across are common, but few lithic fragments exceed 1/2 inch in average diameter. The total content of foreign fragments approximates 5 to 8 per cent. It must be emphasized, however, that there is wide variation in the coarseness of the deposits over short distances. Locally bombs more than an inch across may make up half the total volume; elsewhere fine dust is by far the principal constituent.
Though the pumice flows spread all the way down Castle Creek, the later scoria flows did not extend far beyond the park boundary, a distance of little more than 7 miles from the source. Moreover, as in other canyons, the scoria flows were confined to a central and narrow depression in the earlier pumice deposits. It is among these scoria deposits that the only “fossil fumaroles” of Castle Creek are to be found, and they are almost restricted to the part of the canyon above the junction of Castle with Whitehorse Creek. Here the dark scoria deposits have been carved into the spectacular gorge known as Llao’s Hallway, a chasm narrow enough in places to be spanned by outstretched arms, yet 200 feet deep (plate 15, figure 2). Farther downstream, the scoria deposits are characterized by curved joints that favor the development of large caves. Here also the deposits are traversed by vertical joints defining triangular and quadrangular columns from 6 to 10 feet wide and as much as 100 feet ii height.
Above the unbedded scoria lies a layer of crystal-rich ash up to 30 feet in thickness, within which is the pink zone due to oxidation of fumarolic gases that rose from the hot mass of scoria.