By Carl E. Dutton, Ranger Naturalist
On the west rim of Crater Lake there are two very conspicuous peaks known as The Watchman, and Hillman Peak. The Watchman is well known to visitors at Crater Lake because of its accessibility and the presence of a fire-lookout at its summit from which sunsets are especially attractive. The jagged form of Hillman Peak, a short distance north of The Watchman, rises 1979 feet above the surface of Crater Lake. It is the highest point on the crater rim. Viewed from The Watchman, the layers of volcanic material in Hillman Peak are inclined southwestward at such a steep angle as to produce a conspicuously abnormal relationship as compared to the gently inclined layers of the (ancient) volcanic mountain which existed before the formation of the present crater. The reason for the steeply inclined layers of Hillman Peak is not entirely apparent when viewed from The Watchman.
From a point on the rim just north Hillman Peak one may obtain a rather diagrammatic east-west cross section of the crater wall below the peak. This view reveals that the upper portion of the peak is composed of layers of lava and fragmental material which are inclined westward at an angle of 35 degrees from the horizontal. Below a succession of such layers there is an area of cinder material which is well stratified, the beds dipping westward only 20 degrees. Toward the crater wall the cinder layers terminate abruptly against a mass of rock and rock fragments.
When Hillman Peak and the crater wall below the peak are studied from Wizard Island to the east, or even from the Sinnott Memorial to the southeast, the interruption of the normal volcanic sequence is very apparent. The normal succession of approximately horizontal layers of lava and fragmental material extends upward from the lake through about half of the crater wall. The layers of lava and fragmental material in the adjacent and upper half of the crater wall are interrupted below Hillman Peak by a triangular mass resembling the cross section of a cone whose apex is upward and whose sides include an angle of approximately 90 degrees. The edges of the layers of lava and fragments adjacent to the triangular mass turn up and overlap on the sides of the triangular area. Close examination revealed that the triangular mass is the same as the cinder mass described from a point on the rim just north of the peak, the view from the north presenting an east-west cross section while the view from the east or southeast presenting essentially a north-south cross section. The sketch accompanying this article shows Hillman Peak and the crater wall below the peak as seen from the Sinnott Memorial, southeast of the peak. Viewed from the east or southeast, spires of massive rock are seen in almost a central position in relation to the triangular cinder area. These masses of rock are the same ones against which the edges of the cinder material terminate as previously described.
From these observations it may be concluded that the materials and the structures are indicative of a secondary cone on the slopes of the ancient volcanic mountain which existed before the formation of the crater now occupied by Crater Lake. The overlap of the layers of lava and fragmental material on the slopes of the cinder cone show that they successively surrounded the subsidiary cone and were influenced by its position and form. At least a portion of the elevated character of Hillman Peak is caused by the accumulation of materials about a subsidiary vent on the western slope of the ancient mountain. In addition to these features, the colors of the rock in the crater wall below Hillman Peak are most likely the result of alteration produced by the escape of gases and solutions along fractures in the vicinity of the conduit as it became plugged with solidifying lava which one sees today as the spires of rock at the center of the cinder cone.
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