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
Evidences of Early Glaciation on the Caldera Walls
There is no better place to examine the alternation of lavas with glacial moraines than in the cliffs bordering Cloudcap Bay (plates 10 and 23). A short distance south of the center of the bay, coarse bouldery till and fluvioglacial gravels underlie the lowest lava flow and probably extend beneath the level of the lake. These are the oldest visible glacial deposits on Mount Mazama. When they were laid down, the volcano cannot have been more than 9000 feet high. Probably the records of earlier glaciers lie beneath the waters of Crater Lake. The thickness of the exposed deposits varies between approximately 20 and 30 feet. Resting upon them is a flow of andesite of about the same thickness, the top of which is well polished and striated. Above this flow rests a second layer of till approximately 25 feet thick, and above this a second lava flow 10 to 15 feet in thickness (plate 10). This is succeeded by a third deposit of till, about 50 feet thick, capped in turn by a third flow up to 80 feet in thickness. The upper surface of this third lava is extremely rough and shows no trace of glacial action. But above it there lies a poorly exposed layer of coarse bouldery detritus which is more likely of glacial than of pyroclastic origin. If so, then within a vertical distance of some 250 feet from the water’s edge there are four glacial layers separated by three lava flows, vivid testimony to the advance and retreat of glaciers on the side of the young volcano.
Above this alternating series of flows and tills follow ten or twelve thin sheets of andesite, on the topmost of which may be found a fifth lens of glacial material with a maximum thickness of 15 feet. This is overlain by the pink and buff pumice in which the spectacular Cottage Rock has been cut, and by the thick dacite flow from Cloudcap. On top of the latter lies a patchy deposit of bouldery till, then a layer of coarse lump pumice, and finally another till, part of the sheet which occurs almost everywhere along the caldera rim. In all, then, there are seven glacial horizons on the caldera wall at this point. None of these layers, with the exception of the highest, can be traced continuously for more than a few hundred yards. As might be expected, most of them are lenticular deposits left by narrow valley glaciers.
How quickly most of the glacial layers pinch and disappear laterally is well seen by comparing the cliff section on the north side of Cloudcap Bay with that on the south side, just described. On the north side only the topmost till and one other of the six exposed to the south can be identified with certainty, though a third layer may also be present as indicated on the panorama, plate 23. The lowest till can be traced intermittently as far as Skell Head. Occasional striae indicate the direction of ice movement to have been south 20° west, almost parallel to the shore of the lake. On the top of the small promontory north of Skell Head, 30 feet above the lake, the till rests on a striated floor of lava, the striae trending south 65° west. A quarter of a mile north of Skell Head, the till descends to the water’s edge and disappears.