19 Eagle Crags Andesite Area

The Geology and Petrography of Crater Lake National Park, 1902

 PART I.

MOUNT MAZAMA.

LAVAS OF MOUNT MAZAMA.a

ANDESITES.

EAGLE CRAGS ANDESITE AREA.

The large area of andesite forming the broad divide between Anna and Sun creeks is made up of many prominent sheets. This is best seen from Dutton Cliff, where the edge of the platform appears under Vidae Peak, the highest point in the rim of the lake. Although the structure is partially obscured by steep talus slopes, enough is visible to show that the rim is made up of successive sheets of lava, which dip away from the lake approximately parallel to the present surface. On the upper surface of this platform exposures are few, excepting where the surface layer of sand and pumice is removed.

In strong contrast to the eastern edge is the opposite border of this platform in Castle Crest, which is rugged, and so sharp as to be impassable. At the foot of a precipitous wall a thousand feet high, on the north side of this serrated crest, lies Crater Lake. This is the only portion of the crest that is practically impassable. In the summit of Castle Crest the gray andesite is like that which is most common about the rim, but was found by Dr. Patton to contain a small amount of hornblende, thereby affording an approach toward hornblende-andesite, which has not yet been found about Crater Lake. On the ragged edge of Castle Crest, toward Eagle Crags, are great masses of volcanic conglomerate, such as are rarely exposed on the outer slope of the rim, but are common on the inner slope. They frequently alternate with solid sheets of andesite, and sometimes attain large dimensions, especially in the southwestern portion of the rim. They are generally composed of fragments of andesite, rich in amorphous matter and often dark or red in color. On the divide south of Dyer Rock there is much morainal matter and pumice, so that the solid lavas rarely outcrop, but where they do they are well glaciated.

Near the southern margin of the area mapped is Crater Peak, a hill of basalt resting on a platform partly of andesite and partly of dacite. The relative age of these rocks is clearly shown, and is further demonstrated by the fact that fragments of the underlying andesite were thrown out during the activity of the basaltic crater.

Great cliffs of andesite border Sun Creek Canyon in places, and the rock is well exposed also at the falls where Sun Creek, at an altitude of about 6,500 feet, leaves the broad U-shaped valley and plunges 400 feet over a series of cascades into a sharper canyon cut in dacite. This line of cliffs across the course of Sun Creek connects the andesites of the two divides east and west of that stream.

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