(The Geologist accounts for the deep canyons in the Park.)

The Pinnacles, located in the canyon of Wheeler Creek in the east part of the park, are a splendid illustration of the result of recent stream erosion and wind abrasion. These grotesque spires occur on the canyon walls and are worn out in the debris that in the geologic past was deposited in the outwash plain and valley train of the glaciers which came down off the slopes of Mount Mazama.

The valley floor resulting from the sorting activity in the melting of the ice masses is material of volcanic origin. The debris is somewhat sorted and portrays the combined glacial and water-laid characteristics. This semi-stratified rock material is termed glacio-fluvial. The marked accessibility of the Pinnacles, since they can be viewed as the visitor passes along the highway approaching the Lost Creek Ranger Station, the east entrance to the park, causes practically all who go that way to see the interesting features. Immediately the question is asked, "How did they happen to be?", "Of what are these spires composed?" They are located in the so-called "Box canyon" of Wheeler Creek just above its confluence with Sand Creek. The spectacular erosional features in other parts of the park, such as Godfrey Glen, Llao's Hallway and various other steep walled canyons are similar gorges out in the friable ash and pumice conglomerate.

It is apparent that the volcanic ejecta, ash, pumice, lapilli, bomb and larger volcanic blocks were gathered into the mass of ice during the time of glaciation. Much of the rock material has been ground into rock flour and then upon the melting of the ice the debris was sorted by the action of the water from the glacial wastage. This semi-sorted material was built up layer upon layer. The overburden consolidated it and thus the process of induration was achieved along the Wheeler Creek Canyon walls and in the very spires of the Pinnacles themselves.

The careful observer who has studied the inner rim of Mount Mazama finds evidence that the volcano was of the intermittently effusive and explosive type. The fragmental igneous material, solidified molten fragments, was hurled out and deposited on top of the previous lava flows which coursed down the mountain slopes. The old volcano in eruption thus poured out layer upon layer of lava successively on top of the explosive masses which were scattered by each intermittent episode in the history of Mount Mazama. One can see from inside the Caldera walls successive layers upon layers separated by the slaggy red to buff colored pyro-clastics or fire fragmented particles. The crust of solidified, or frozen lava, which repeatedly formed in the crater or orifice of the volcano was blown to fragments by the renewal of explosive activity and as this volcanic ejecta came to rest an agglomerate was formed. The lapilli, small cindery fragments, from the size of peas to that of oranges, along with the infinitely small particles of dust and ash were scattered over a vast area.

The semi-sorting action of the rushing waters from the wasting glacial tongues or the glacio-fluvial deposition left sloping plains made up fragmental volcanic debris which has become partially indurated. The loose friable material appearing in well defined layers in places shows the characteristics of mud flows and in others the discoloration caused by escaping gasses. The tuff deposits partake more of the characteristics of sedimentary rocks and the resulting grotesque erosional forms result from the protection which a larger volcanic fragment, possibly bombs, offers to the disintegrating effects of water wear and wind abrasion.

As you stand on the banks of Wheeler Canyon it is highly probable that your feet may be resting immediately over what will be the topmost portion of the spire of a pinnacle in future lapses of time. It is quite evident that the canyon wall will move laterally and a much wider canyon floor will eventually develop in which beautiful flower gardens will bloom.