The Geology of Crater Lake National Park, OregonWith a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams
The Northern Arc of Vents
The Cleetwood Flow
At Cleetwood Cove, on the north wall of Crater Lake, the stratified lavas and pyroclastic rocks are suddenly interrupted by a dark, jagged tongue of dacite which descends from the rim to the water’s edge. The caldera rim, elsewhere fairly smooth, is here so broken by bristling crags of glassy lava as to be called the Rugged Crest.
Diller was much impressed, as every geologist must be, by the fact that the lava forming the tongue on the caldera wall dips lakeward, whereas that on the outer slope generally dips in the opposite direction. This, he thought, added proof to his conclusion that Crater Lake was formed by engulfment and not by explosion, for there seemed to him no escape from the inference that the Cleetwood lava was fluent when the summit of Mount Mazama collapsed. Escaping from a vent near the rim of the caldera, some of the lava moved down the outer slope, while the remainder poured slowly backward into the newly formed depression. Finding no alternative explanation of the supposed “backflow,” he was obliged to conclude that the Cleetwood lava was among the final products of Mount Mazama and the youngest of all the dacite flows.
Among the geologists who subsequently paid brief visits to Crater Lake, there were a few who found it difficult to accept Diller’s interpretation of the “backflow,” and some entertained the idea that faulting on the caldera wall might in some way offer an explanation. Allen, however, was the first to understand the relations correctly. He saw that if the “backflow” is regarded as an “upflow,” in other words as the actual feeder of the Cleetwood dacite, all difficulties disappear. The lakeward dip of the lava on the wall does not mean movement in that direction, but exactly the reverse. When the summit of Mount Mazama collapsed, the bounding fracture sliced across the inclined conduit of the Cleetwood flow.
Beneath the flow lies a thick deposit of dacite pumice, interbedded with thin tongues of andesite. The relations are identical with those at Llao Rock, where similar beds of pumice, intercalated with andesites, underlie the great flow of dacite. The low margins of both the Llao and the Cleetwood flows were covered by ice at a later date, but their higher parts remained bare. Presumably, therefore, the two lavas were erupted at about the same time, and long before the catastrophe which produced the caldera.