Half a mile above the falls, and from there to the confluence of Annie Creek with its East Fork, the west bank shows several striated surfaces of lava overlain by coarse piles of morainic material. Northward to Munson Valley, irregular mounds of till appear at intervals at the base of the walls of both Annie Creek and its Middle Fork, and the streams repeatedly pour in small cascades over a bouldery bottom, as if for most of their course they had already cut through the overburden of pumice and were flowing on the surface of moraines. Where Pole Bridge Creek tumbles into Annie Creek it has likewise stripped off the cover of pumice, and one may walk over morainic debris from the rim of the canyon to its base.
If the reader consults the topographic map of Crater Lake National Park published by the U. S. Geological Survey, he will observe that on the divide between Annie Creek and its Middle Fork the contours are unusually sinuous. As Atwood first discovered, this irregularity is the expression of a buried kame-and-kettle topography. In other words, the surficial deposits of pumice are mantled on hummocky moraines. That this is so may easily be verified by study of the neighboring walls of the Middle Fork, where the upper and drier slopes of pumice pass down into wet banks of bouldery till.
From this point northward to Government Headquarters, the moraines are covered by only a thin sheet of pumice, and many rise above the pumice as conspicuous ridges. Even the pumice is greatly admixed with glacial debris. In the upper part of the valley the proportion of pumice gradually diminishes. From Government Headquarters to the caldera rim, the moraines stand bare and their forms seem scarcely to have been modified by erosion. These are the youngest moraines of Mount Mazama. Because they are free from even the thinnest veneer of pumice, they must have been deposited after the great explosions which led to the collapse of Mount Mazama and the formation of Crater Lake. In other words, when this catastrophe took place, the Munson Valley glacier still reached as far down as Government Headquarters, and the collapse of the top of the volcano cut off the glacier between a mile and a mile and a half above its snout.
Unlike the glaciers that flowed down Sun and Kerr valleys, the Munson Valley glacier did not excavate a deep notch in the rim. Nevertheless, if the thick moraines near the Sinnott Memorial were removed, a shallow valley half a mile wide would mark its channel.
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