The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams
The Climax: Culminating Explosions of Pumice and Scoria
Detailed Description of the Individual Flows
Having enumerated the general features of the glowing avalanches, we may now pass to an account of the individual flows.
Flows down the East Slope
In the valley northeast of Roundtop and in Bear Valley there is no difficulty in distinguishing the deposits of the pumice fall from those of the flows which followed, for the former consist of pale, granular dacite and cover the valley sides, whereas the latter are confined to the valley bottoms and consist mainly of basic scoria and crystal ash. In the upper parts of both these valleys, the scoria cover is thin and patchy. Only below Cascade Spring on Bear Creek, 1 1/2 miles from the caldera rim, are thick deposits of pumice and scoria to be found. The pumice flows, as usual, traveled a much greater distance than the scoria flows, for the latter only reached the base of the volcano, whereas the pumice spread for another 10 miles to the edge of Klamath Marsh.
Glowing avalanches also poured down the canyons east of Mount Scott, uniting with the Sand Creek flows in wide-fronted floods that emptied into the marsh. The water level in the marsh may have been higher than now, for at the head of Wocus Bay there are water-worn lumps of pumice between 20 and 30 feet above the present level. Of course the water level may not have been that much higher than at present, for the discharge of tremendous volumes of pumice into the marsh must temporarily have raised the level, and the flows, entering the water at great speed, must have caused powerful waves capable of carrying pumice lumps far up the opposite shore. Much of the pumice, after entering the marsh, escaped down the Williamson River, but most of it, after floating a short time, probably sank to the bottom.