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
Summary of Glacial Evidences on the Caldera Walls
The walls of Crater Lake reveal a complex history of alternating volcanism and glaciation. Doubtless many other High Cascade volcanoes passed through a similar history. Only here is the record laid bare for inspection.
The record at Mount Mazama shows that even the oldest of the exposed lavas are interbedded with glacial debris and that this debris must have been-deposited before the volcano was more than 9000 feet high. Glaciers may have existed on the volcano long before this. Until the main andesitic cone had almost reached its maximum height, the glaciers were small and either had little power to erode or were periodically destroyed by falls of ash and flows of lava More they had time to carve deep valleys. That is why none of the older glacial deposits on the caldera walls, with the exception of one on the walls of Danger Bay, lie in U-shaped troughs, but are regularly interbedded with the volcanic rocks. Only when the andesitic eruptions had almost come to an end and the dacites on the south slope of the volcano had been erupted did the – maximum glaciation begin. Not until then were the great glacial valleys carved on the sides of the cone. At that time, Mount Mazama must have been all but completely mantled by ice, and long glacial tongues extended far beyond the limits of the park. After these extensive glaciers had dwindled and were confined to the upper part of the volcano, the dacites of Llao Rock, Grouse Hill, Cleetwood Cove, and Redcloud were erupted from the Northern Arc of Vents. Though the low margins of these flows were later covered by ice, the higher parts were never glaciated.
Glaciation seems to have begun later on the south side of the volcano than elsewhere, and the moraines on that side are therefore almost confined to the top few hundred feet of the walls. Yet when the period of maximum glaciation began it was on this same side of the volcano that the largest glacial canyons were carved. On the north, east, and west walls of the caldera we look in vain for U-shaped valleys comparable with the Munson, Sun, and Kerr valleys on the south. We can only infer that it was some peculiarity in the configuration of the lost, upper part of Mount Mazama which caused the largest of the glaciers to develop on the sunward-facing slopes.