At least six of the Northern Arc of Vents are visible on the caldera walls. In a clockwise direction, these are as follows: the vents of the Watchman andesite, of the andesitic cone forming Hillman Peak, of the Llao Rock, Cleetwood, and Redcloud dacite flows, and lastly of the Sentinel Rock andesite. Possibly another vent lay close to the Wineglass. Possibly, also, such thick flows as those of the Palisades and Roundtop were erupted from vents adjacent to the main arc.
The evidence concerning the exact order of eruption of these vents is incomplete, though the andesitic vents were active before those which erupted dacite. Some of the vents, notably those of Redcloud and Llao Rock, erupted pumice before extruding lava; at other vents, the only activity was the effusion of viscous lava; at Hillman Peak, explosions and effusions alternated many times.
The age of the Northern Arc of Vents relative to that of the other parasitic eruptions on the sides of Mount Mazama is not easy to determine. No doubt the long dacite flows on the south side of the cone, described in an earlier section, are much older, for they were erupted before the period of maximum glaciation. Perhaps the eastern cluster of dacite domes was active about the same time as the Llao, Cleetwood, and Redcloud vents or a little earlier. Most of the basaltic cinder cones must also be approximately coeval, though some may have developed later.
We may now continue to a discussion of the principal vents of the Northern Arc, beginning with those which erupted andesite.
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