31 Other Flows

The Geology and Petrography of Crater Lake National Park, 1902






Toward the southwestern corner of the park there is a large area of basalt, in which there is considerable variation, and the mass may have been built up from a number of vents. It forms a bluff near the road southwest of Anna Creek from Pole Bridge Creek toward the summit. It ranges in color from gray and reddish to almost black and some is vesicular. North of Union Peak it rises to over 7,000 feet and is associated with basaltic tuff.

The divide at this point is a ridge of basaltic lapilli, indicating the proximity of a volcanic vent, although no well-defined crater was seen. The rocks are well glaciated and the original form of the cinder cone may have been greatly modified thereby. The ridge of lapilli affords a fine view of Union Peak a few miles farther south on the crest of the range. Union Peak is rugged and composed largely of andesite, which came from a vent which was more ancient than that from which the basalts issued. Upon the northern border, as elsewhere, gray (164) and reddish (166) colors are common, but on the whole the darker colors (182, 187, 163) are most abundant, and in places, especially near the Rogue River road, the rock is decidedly platy.

In the southeastern corner of the park is a hill of scoriaceous basalt with lapilli overlying dacite, and west of this, in the flatter country near the border of the park, there are bowlders and bluffs caused by streams of basalt which may have descended from Crater Lake, although the connection was not observed. The falls of Anna Creek below the forks are over basalt, but the exposure is very small. Small areas of basalt occur along the road between the falls and Pole Bridge Creek, and basalt may cover much of the country marked andesite in the southwestern portion of the park.

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