Finally, it may be suggested that the coarse lump pumice which outcrops in Kerr Notch to a thickness of 50 feet and in Sun Notch to a thickness of m feet, beneath the latest glacial moraines, is also part of the same deposit that underlies the Wineglass tuff.
Summary. After the andesite and dacite eruptions from the Northern Arc of Vents had come to an end, glaciers readvanced down the slopes of Mount Mazama. The tops of the Watchman, Hillman Peak, Llao Rock, and the Cleetwood and Redcloud lava flows remained bare, but elsewhere the ice descended a short distance beyond the present rim of the caldera.
The ice then retreated, leaving most of the rim bare. Subsequently, powerful explosions took place and a thick stratum of coarse lump pumice containing abundant lithic blocks was deposited over most of the cone, but particularly on the north and east slopes. Immediately afterward, glowing avalanches swept down the northeast flank of the volcano. Where these came to rest in shallow valleys, they left a remarkable deposit of streaky, welded tuff; where they covered higher ground they left a weakly compacted buff, pink, and orange layer of pumiceous lapilli tuff. There is no means of determining how far these avalanches continued down the slopes, for, except at a point about a mile east of the Wineglass, their deposits are entirely concealed beyond the caldera rim by the products of later explosions.
Once more the glaciers advanced. Along what is now the north wall of Crater Lake they left patches of till on top of the welded tuff; along the east wall they deposited an almost continuous sheet of blocky detritus on top of the coarse lump pumice; in Sun and Kerr notches they covered the pumice with bouldery moraines.
A considerable interval of quiescence must have followed. Everywhere except in Sun, Kerr, and Munson valleys the glaciers withdrew above the present rim of the caldera. Within the magma chamber fundamental changes were going on preparatory to the climactic explosions which were to destroy the summit of Mount Mazama.
***previous*** — ***next***
