21 Sentinel Rock Andesite Area

The Geology and Petrography of Crater Lake National Park, 1902






On the lake slope of Kerr Notch is a vitreous andesite, well jointed (16), with columns 46.6 inches thick. This sheet of columnar lava dips away from the lake and is exposed near the bottom upon both sides of Sand Creek Canyon near the notch.

From Sentinel Rock eastward to the vicinity of Scott Peak is one of the largest areas of andesite, but by far the greater part of it is covered with pumice, and outcrops are few. Occasional cliffs and large fragments of andesite occur on the steep slopes east of Sand Creek Canyon, but beyond these the surface is generally covered with a layer of small pumice fragments. Scott Peak is about 1,000 feet higher than any other point in the vicinity of Crater Lake. It was once an active volcano, and among its neighbors next in size and importance to Mount Mazama. In fact, it marks the only distinct andesitic vent of this center outside of the principal one represented by Mount Mazama. Its lavas spread to the east, away from the lake, for in the Sentinel Rock section of the rim the lavas appear to have flowed westward from the Mazama center.

Scott Peak was once a well-defined crater, but it has been broken away upon the northwest, and drains into the South Fork of Bear Creek by a broadly rounded valley, which looks as if it had been cut by glacial action. The slopes of the mountain are generally covered with pumice, but here and there are fragments of andesite (80), and near the summit actual ledges (77) occur. The lava is generally reddish or gray (200), but sometimes greenish (198), as if considerably altered. Occasionally (4) it is somewhat glassy. Both Nos. 200 and 4 were loose pieces on the southwest side of the crater, but No. 198 was in place. The quaquaversal dip of the sheets of volcanic material shows that Scott Peak was a crater, and the 2 miles of country intervening between it and the rim shows its individuality. The whole aspect of the mountain is one of considerable age, and it is evident that it became extinct before the last eruption of Mount Mazama, which spread pumice everywhere. On the outside the slopes are gentle, but within the curve of the ancient crater they are very steep, and the snow lodged there is but a remnant of the glacier that once started at that point.

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