Obviously, a long interval of time elapsed between the eruptions of the two deposits of pumice, for a deep glacial valley was carved across the inter-andesitic pumice before the younger pumice was laid down on the valley side. Probably the older pumice was blown from the central, summit vents of Mazama; the younger pumice, on the other hand, may represent the first products of the vent from which the lava of Llao Rock was extruded.
On the west wall of the glacial valley, the succession is strikingly different from that just described. Instead of a thick series of bedded pumice deposits with lenticular flows of andesite, there are many flows of andesite, a few short and thin lenses of pumice, and at least two layers of bouldery till, the topmost of which forms the floor over which the Llao dacite spread. Both of the observed tills contain much pumiceous debris. Possibly this reworked pumice and the thin lenses of pure pumice associated with it correspond to the thick sheet of pumice on the opposite side of the glacial valley. There is, however, no pumice corresponding to that molded on the east side of the valley. Why the successions on the two sides of the valley are so different is not apparent.
This concludes the account of the inter-andesitic dacite pumice on the caldera wall. Following this period of activity, a semicircular arc of vents opened on the north flank of Mount Mazama, possibly in response to a northward enlargement of the underlying magma chamber.
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