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 Sun Creek Flows
From Sun Notch to the lower end of Sun Meadows, the valley floor is blanketed with hummocky glacial drift practically devoid of pumice cover. Apparently the pumice and scoria flows discharged through Sun Notch began to deposit their load only when they had passed beyond the snout of the Sun Creek glacier, 1 1/2 miles below. The thin, patchy sprinkle of pumice on the moraines in this upper part of the valley is not a relic of the flows, but the remains of a later fall which settled on the ice and survived its melting.
Below Sun Meadows, the valley floor falls suddenly and is constricted. In this part of its course, Sun Creek has cut a gorge through glacial drift mixed with and covered by black scoria bombs and pumice. Downstream, the proportion of pumice and scoria increases, until at an elevation of about 5800 feet the two are approximately equal in amount, though still confusedly intermingled.
The first traces of fumarolic action occur about 4 miles south of the caldera rim. But nowhere on Sun Creek are “fossil fumaroles” so common as in Sand and Annie Creek canyons. The reason may be that the flows were more thoroughly mingled with glacial debris and traveled farther before dumping their load. This may also explain the absence of columnar structure and the incoherence of the scoria deposits in the canyon of Sun Creek.
In the lower reaches of the canyon, the amount of dark scoria diminishes, so that for the last 3 miles of its course in the park, the creek cuts through pumice alone, or has penetrated deeper into the underlying moraines. As the volume of Aria decreases, so do the signs of fumaroles, until in the pure pumice walls the traces of gas action can no longer be recognized.