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
Cleetwood Cove to Pumice Point
Throughout this stretch of the caldera wall, as the panorama, plate 26, shows, an irregular layer of glacial debris occurs at or close to the base. About midway along the stretch, the glacial deposits rest on a scratched surface of lava only 20 feet above the lake.
Higher up the caldera wall, there are other lenses of till interbedded with flows of andesite and sheets of dacite pumice, but their presence is indicated only by scattered erratics in the talus.
On Pumice Point itself, three distinct glacial beds have been recognized. These are restricted to the upper half of the caldera wall (plate 11, figure I). Separating the pumice deposits from the main series of lavas below is a glacial layer approximately 7 feet thick, composed of bouldery till overlain by glacial sands. The lava surface on which they rest is marked by striations trending south 25° west. Above the glacial deposits follow 47 feet of incoherent lump pumice and then a second layer of glacial sand, 5 feet 6 inches thick. This glacial layer cannot be traced continuously, but a short distance away, on the same horizon, a bed of bouldery till, 10 feet thick, may be seen resting on the glaciated surface of a thick lava flow intercalated in the pumice. At this point the striations trend south 18° west. Above the till, Atwood uncovered a dark soil band containing much charred vegetation. Obviously, the eruptions of Mount Mazama were interrupted long enough for plants to gain a foothold on the till before being destroyed by the next explosions. Eruptions then continued until approximately 130 to 170 feet of bedded pumice accumulated on the old soil. Thereafter the glaciers readvanced, leaving in their wake a third and nonpersistent bouldery till up to 6 feet in thickness. Finally, after the ice had once more retreated, the concluding pumice eruptions began. The evidence for this history is presented in the section, figure 8.