The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams
The Northern Arc of Vents
The Grouse Hill Dome and Flow
A short distance beyond the north rim of the caldera rises the steep-sided and wooded Grouse Hill. Diller supposed that the lava of Grouse Hill was older than the Llao Rock flow, because it seemed to have suffered more from erosion, and because it seemed to be partly overlain by the same sheet of pumice that underlies the great dacite cliffs of Llao. The first of these arguments is questionable, and the second is invalid, for the pumice on top of Grouse Hill is in reality part of the same sheet that lies above the Llao flow. There is, in fact, no way of deciding which of the two flows is the older. Probably they are of about the same age.
Form and internal structure. The culminating explosions of pumice from Mount Mazama left a thick sheet of ejecta on top of Grouse Hill. Exposures of the underlying lava are therefore comparatively rare. But the sides of the hill are steep and for long stretches they are almost precipitous. Here, the lava is perfectly exposed and the attitude of the flow planes may be mapped in detail (figure 11). In general, as Allen observed,2 the banding steepens toward the summit, with the result that, in cross sections, the structure is fan-shaped. Over most of the surface the banding is either vertical or almost so, and it may even maintain this attitude to the edge of the hill. Close to the steep western margin, however, the flow planes commonly dip outward or bulge in that direction. How is this structure to be interpreted?