Plate 26. Panorama of the caldera wall: Cleetwood Cove
No inter-andesitic dacite pumice occurs on the caldera walls between Redcloud Cliff and Cleetwood Cove. Beneath the Cleetwood dacite, however, pumice is again well developed. In the conspicuous white slide on the west wall of Cleetwood Cove (plate 26), the measurable thickness approximates 150 feet, and if the deposits continue beneath the talus to the edge of the lake, their total thickness must approximate 250 feet. The topmost 60 feet, immediately beneath the Cleetwood lava, consists of well bedded pumice charged with blocks of andesite up to 3 feet in diameter. By contrast, few of the pumice lumps measure more than 3 inches across, and the majority are not even 1/2 inch in diameter. Beneath this block-rich layer lies a thick accumulation of equally well stratified but finer pumice in which the content of andesite fragments is much less. A thin wedge of andesitic lava partly separates the two types of pumice, and for a depth of 6 feet the older pumice has been reddened thereby. Three other tongues of andesite are interbedded with the pumice near the base of the slide. The record of events here is therefore as follows: first, alternating eruptions of andesitic lava and fine dacite pumice; then increasingly violent explosions of pumice during which great quantities of andesitic debris were torn from the walls of the vents; and finally, the extrusion of the Cleetwood dacite flow. Had the pumice been blown from the vent which erupted the Cleetwood lava, the dips of the bedded pumice would radiate from the center of Cleetwood Cove. Actually, they conform with the dips of the interbedded andesites and suggest a common source near the summit of Mount Mazama.
On the opposite wall of Cleetwood Cove, interandesitic pumice is poorly exposed, outcropping just below the Cleetwood dacite to a thickness of only 35 feet (plate 26). Judging by the abundance of pumice in the talus below, however, there may be a thick series of similar ejecta reaching to the edge of the lake.
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Plate 11. Fig. 1. Pumice Point, north wall of the caldera. A thick pile of dacite pumice resting on andesitic and breccia. For position of glacial layers, see figure 8c. (Photograph by George Grant, National Park Service.) |
