The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams
The Glaciation of Mount Mazama
Evidence of Glaciation Outside the Caldera
The Munson Valley or Annie Creek Glacier
One of the largest glaciers on Mount Mazama, and certainly the last to survive, passed over the caldera rim near Crater Lake Lodge, continued down Munson Valley, and entered the broad canyon of Annie Creek. At the time of maximum glaciation, it was united with the Sun Creek glacier to the east, and all but a few points on Vidae Ridge were covered by ice. Where the snout of the Munson-Annie Creek glacier lay at the time of its greatest extent cannot be determined with precision, though probably it was at least 3 miles south of the park boundary and not far above Fort Klamath.
Inside the park, Annie Creek has cut through the pumice in many places. For 7 miles up the valley, as far as Government Headquarters, recessional moraines are exposed at intervals on the canyon floor. Wherever they appear, there are copious seepages on the canyon walls.
One of the finest lateral moraines anywhere within the park may be seen on the north wall of Annie Creek canyon a short distance inside the south boundary. For more than a mile it runs parallel to the creek as an imposing embankment, from 100 to 150 feet high. The crest stands between 500 and 600 feet above the bottom of the creek, which thereabouts coincides closely with the base of the pumice fill. How much deeper the “bedrock lavas” lie we have no means of determining, but probably not more than a few tens of feet. Accordingly, when this lateral moraine was laid down, the Annie Creek glacier, even g miles from the top of Mount Mazama, was approximately 500 feet thick.
Continuing up Annie Creek, the next exposures of till occur about 100 yards below the Main Falls. The river has here penetrated some 6 feet of morainic debris. A mile to the east, between 250 and 300 feet above the canyon bottom, till may be seen resting on the bench which marks the contact between the lavas of the Union Peak volcano and the dacites of Mount Mazama.