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 North Slope

On the north side of Mount Mazama there were no deep glacial canyons to concentrate the flows. The consequence was that once they had passed through the gap between Red Cone and Grouse Hill, they spread out in wide sheets. The momentum of the earlier pumice flows was enough to carry them across the basin of the Pumice Desert and over the saddle to the north, but the later scoria flows failed to escape from the desert in that direction. Consequently the surface of the Pumice Desert consists chiefly of crystal-rich scoria and ash, among which lie many bombs. One of these measures 12 by 12 by 3 feet, but most of the others are less than a yard across. Many are exceptionally rich in hornblende, and the fine ash is crowded with glistening prisms of this mineral. Despite the large size of the bombs and the proximity to the source, fragments of old lava more than 1/2 inch in diameter are notably rare. What the volume of the pumice and scoria deposits is in the Pumice Desert there is no means of determining accurately, though their thickness may well be 200 feet. The pronounced red and pink blotches revealed by road cuts through the upper part of the scoria show that fumaroles must have been abundant and long-lived in this region. A well drilled adjacent to the highway where it crosses the lowest part of the desert passed through 106 feet of smoke-gray, hornblende-rich scoria without penetrating the underlying white dacite pumice. Samples consist almost entirely of lithic fragments and discrete crystals of feldspar and hornblende. With these are much less numerous crystals of pyroxene, and a little brown glass. The content of lithic fragments increases, though not regularly, with depth. Near the bottom they are far more abundant than crystals. An interesting deduction may be drawn from these observations. Although the Pumice Desert is thickly covered by ejecta, particularly in its central part, it still has the form of a shallow basin. Hence, a deep valley must have existed there before the final eruptions of Mount Mazama began. Yet when the first glowing avalanches of dacite pumice swept across the valley they did not convert it into a flat plain. Instead they rushed across with such vehemence that they left it much as it was and continued over the divide to the north. The later avalanche of scoria and crystals lacked either the momentum or the mobility of the dacite avalanche, for little of it escaped across the northern divide. Most of it came to rest in the valley itself, and some made its way out via Desert Creek and so continued to the plateau beyond.

The earlier pumice flows raced onward to Diamond Lake. Exactly what happened when they reached the lake can only be conjectured. This much, however, seems certain: the flows passed across the lake and made their exit down Lake Creek. They followed this for 5 miles before splitting into two branches at Toolbox Meadows, one branch turning westward to empty into the valleys of Lava and Clearwater creeks, while the other continued down Lake Creek and joined the North Umpqua River. An enormous volume of coarse pumice must thus have been discharged into the North Umpqua, and much of it was washed downstream for many miles. Between 25 and 30 miles below Toketee Falls, there are banks of washed pumice including lumps 6 inches across; about 20 miles below the falls, near Panther Leap, the tops of the pumice banks lie 20 feet above the river. In many places, these washed deposits carry charred logs.

It may be argued that all the pumice north of Diamond Lake must have been washed into place by rivers, that it would have been impossible for the flows to cross a body of water 3 miles in length. Yet the flows did somehow traverse the lake, for the deposits bordering Clearwater Creek show no trace of stratification. Had they been washed thus far by streams, they would surely show bedding, and the pumice lumps would show considerable rounding. Moreover, the deposits carry logs of charred wood that lie at random, and in places they are characterized by the pink color indicative of fumarolic action. They must therefore have been hot and gas-charged when they came to rest. Not until the flows had traveled approximately 10 miles beyond Diamond Lake did they lose their own propulsive force. Presumably, when the first onrush of pumice entered the lake, the light ejecta remained on the surface, forming a blanket across which the later waves of pumice poured as if on dry land. How else could they have retained their heat and gas?

Though much of the pumice that crossed Pumice Desert poured into Diamond Lake, and much of it, as we have seen, turned westward into the valley of the Rogue, the most voluminous flows were deflected eastward through the depression between the slopes of Mount Thielsen and Timber Crater. These flows crossed a low saddle in the crest of the Cascade Range and continued for another 20 miles, as far as the site of the present village of Chemult. Their total journey from the source was therefore more than 30 miles, though the slopes down which they moved were extremely gentle. The buoyancy and mobility of the mass was such that at Beaver Marsh, 6 miles from the snout of the flow, the deposits carry many bombs of pumice up to 6 feet across, and one as much as 14 feet in maximum dimension.

In the vicinity of Chemult, near the margins of the flows, the coarse, unbedded lump pumice is overlain by fine pumice, much of which is almost pure glass dust (histogram 252, figure 21). East of Beaver Marsh, on the road to Willow Spring, fine crystal-lithic ash and pumice lie on the deposits of the flows as well as on the older pumice fall beyond. These ejecta probably represent the settling of the finer constituents which rose in swirling clouds near the margins of the flows.

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