37 Wineglass Dacite Flow

The Geology and Petrography of Crater Lake National Park, 1902






The Wineglass flow is of small size and peculiar. lt lies in the next gap of the rim southeast of Round Top (Pl. VII, B), has a width of scarcely 500 feet, a length of probably a mile and a thickness of about 20 feet. It is decidedly like a tuff (114) and might well be so considered were it not for the stringers of black glass intermingled with a reddish groundmass containing fragments of other material and imparting a decided fluidal structure to the mass.

A short distance farther east there is another small sheet of this tuffaceous dacite 10 feet thick. It has the characteristic streaks of black glass which fix its identity. It occurs locally along the western edge of the Cloud Cap flow, and closely resembles much of the tuffaceous material of Redcloud Cliff. At the Wineglass slide it is immediately associated with pumiceous tuff. The hard rock of the rim is andesite, overlain by 15 feet of pumiceous tuff with 10 feet of red tuffaceous dacite. This is overlain by 30 feet of conglomerate and capped by a fine layer of pumice 25 feet thick. It is evident from this section that the fragmental dacite does not represent the final eruption, for that finds expression in the top layer of pumice.

The gap west of Round Top contains a small sheet of tuffaceous dacite like that on the other side, but is thicker. lt appears in Pl. VII, B, at the left of the Palisade crowned by Round Top. It is 50 feet thick in the middle and tapers to a thin edge on the west. On the east of Round Top it extends up the slope a short distance, for it is clearly seen above glacial striae and is undoubtedly a post-Glacial flow. In this gap, as upon the east, it overlies a sheet of pumice and underlies conglomerate into which it appears to pass by becoming more fragmental. Near the western edge of the gap it appears to be overlain by large bowlders, and where the exposure of the dacite ceases the sheet of large bowlders becomes more marked. On the geological map this area of dacite adjoins the edge of the great flow of Rugged Crest. In reality, however, it overlaps the Rugged Crest flow and is of later date.

The same tuffaceous dacite occurs farther west along the crest beyond Cleetwood Cove, where, as upon the east, it overlaps the flow of Rugged Crest. It extends nearly to Pumice Point (Pl. IX, B), where it appears between two thick masses of pumice. This peculiar tuffaceous dacite occurring along much of the northern crest of the rim all belongs to one flow, which spread as a uniformly thin sheet over that portion of the base of Mount Mazama. It is altogether unlike the other flows of dacite and appears to be intermediate between them and tuff.

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