The pre-eruption surface. Before the Cleetwood lava was extruded, there must have been in this vicinity a broad and shallow basin with a hummocky floor. Whether it was cut by ice or by glacial streams is not clear, for wherever the base of the lava is exposed it rests either on andesite or on pumice with no intervening layer of glacial material. Beyond the rim of the caldera, the slope at first was steep and then gradually flattened, the average angle for a mile being approximately 15°. Outward, also, the valley walls diminished in height and soon disappeared. Possibly a corrie glacier had scooped out the depression.
The feeder. It is not surprising that in the limited time at his disposal Diller decided that the lava on the caldera wall was a “backflow,” for the banding conforms closely to the slope and the rugged, fissured surface suggests a recent flow, frozen in its cascade toward the lake. This appearance, as we have seen, is deceptive. The ruggedness of the surface results from erosion of the lava along joints arranged parallel and perpendicular to the banding, and from the glassy nature of the lava itself.
The contact of the feeding pipe with its walls cannot be seen, though the pipe must widen upward. In general, the banding also becomes steeper in that direction. Close to the lake shore, it lies almost horizontally or dips into the wall at low angles. Higher up, the lakeward dip is generally between 30° and 40°, and where the lava escaped at the surface the banding is vertical or even overturned. In the flow proper, the dips are generally away from the lake, at low angles near the base and at increasingly high angles in the upper parts. Both in the feeder and in the flow adjacent to it, the fluxion planes strike a p proximately east-west, right across Cleetwood Cove.
There is little difference between the dacite in the feeder and that in the flow. Much of the flow consists of jet-black and dark-gray glass, whereas the feeder is composed of slightly duller and more crystalline dacite, as might be expected considering that it cooled more slowly beneath the surface.
The flow. The present area of the Cleetwood flow is a little more than a square mile. Its southern end disappeared when the caldera was formed, but it can never have extended far toward the south, since progress in that direction was impeded by the opposing slope of Mount Mazama. Assuming an average thickness of 300 feet, approximately 1/15 cubic mile of lava was erupted.
Despite its great thickness and volume, the lava was too viscous to move much more than a mile even with the assistance of a 15° slope. On all sides it ended abruptly. Further proof of extreme viscosity may be seen at the base of the flow, where there is commonly a highly brecciated layer up to 12 feet thick, composed of black glass including sporadic blocks from the underlying pumice. Above this basal part rests a layer, averaging 6 feet in thickness, characterized by intensely convoluted flow bands, indicative of viscous drag against the blocky bottom. Above this, in the main part of the flow, the lava is paler and slightly more crystalline; the banding here is no longer contorted, but lies almost horizontally below, then sweeps up to verticality and finally turns over at the surface (figure 12). The crust of the flow, like the bottom, is a glistening black obsidian. The arrangement of the flow bands is thus identical with that seen in the Llao flow.
