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.

     The Rogue River Flows

The glowing avalanches that followed the Rogue River and its tributaries united near the confluence with Union Creek, and the composite flow continued for another 20 miles to a point 1 1/2 miles above the present site of the village of McLeod. The snout of this composite flow thus lies 35 miles in an air line from the former summit of Mount Mazama.

The following notes refer to the deposits found along the Rogue River from the vicinity of Diamond Lake to McLeod; the flows that ran down the tributaries of the Rogue are described in a later section. Although most of the flows of basic scoria which poured down the north slope of Mount Mazama through the depression between Llao Rock and Grouse Hill came to an end in the Pumice Desert, the earlier and more voluminous flows of pumice not only crossed the desert but surmounted the divide beyond, and some of them discharged into Diamond Lake. The bulk of the pumice then separated into two branches, one directed eastward toward the Klamath Marsh and the other westward down the valley of the Rogue. The latter branch was joined by flows which skirted the west side of Desert Ridge. As far down as the junction with National Creek, the deposits of the Rogue valley consist of buff and pale-gray dacite pumice unusually poor in lithic debris. In many places the river has cut through the deposits to the underlying basalts, as at the Upper Falls and in Hamaker Meadows. The great scarcity of charcoal among the ejecta in this region implies that this part of the Rogue River valley supported only a scant cover of trees at the time of the eruptions.

Where Copeland Creek enters the Rogue, the thickness of the pumice increases to almost 200 feet. Still farther south, the thickness continues to increase and locally approximates 300 feet. Along this stretch of its course, the river has cut through the deposits to the underlying intracanyon olivine basalts. Here also the pumice deposits are abundantly charged with burned wood. On the west bank of the river, a mile below the crossing of the Diamond Lake highway, Smith discovered at the base of the pumice an 8-foot stump of a cedar tree, 3 feet in diameter, standing upright in the position of growth. Its upper part was thoroughly charred, but the lower part, perhaps because of quick burial, was burned only a little. This is the only example known of an upright tree within the pumice flows. The rest of the charred wood occurs as scattered limbs and logs lying more or less prostrate.

Some of them measure between 2 and 3 feet across, but by far the majority are only a few inches in diameter. Hemlock, cedar, and lodgepole pine occur sparingly, but most of the wood belongs to the white and sugar-pine group. Clearly the forests, though less dense, were otherwise like those now living in these parts.

Mention should be made here of the splendid sections exposed along the Diamond Lake highway south of the Rogue River bridge. At this locality, the coarse, unbedded lump pumice is overlain by fine, well bedded pumice up to 15 feet in thickness (plate 17, figure 3). About 1/2 mile below the bridge, the deposits of the pumice flow are cut by a channel 7 feet deep and 50 feet wide, filled with stratified, granular pumice that must have been washed into place (plate 19, figure 1). Noteworthy is the much greater proportion of lithic detritus in this upper, bedded pumice (see histogram 360, figure 25). In these exposures the pink layer produced by oxidation of fumarolic gases is to be found, not at the top of the pumice flow, but between 3 and 5 feet from the top of the bedded ejecta. Presumably, therefore, the pumice showers fell on the flow while it was still giving off gas, for if they had fallen long after, the pink layer would be found at the top of the flow, as it is in the roadside sections nearer Union Creek, where the overlying stratum of bedded ejecta is absent.

Close to the confluence of the Rogue with Bybee and Castle creeks, the thickness of the pumice flow rapidly diminishes, for thereabouts the pre-existing valley was wider and the flow spread over a broader front. The nature of its finer constituents is shown in the histogram, figure 25. None of the dark scoria flows that swept down Castle Creek reached this distance, nor did any of the scoria flows that entered via National, Copeland, and Bybee creeks. As for the lithic fragments in this part of the flow, it is extremely rare to find pieces more than an inch long, though many of the bombs of pumice exceed 2 feet across.

For the next 10 miles down the Rogue River, as far as Prospect, most of the pumice flow has been removed by erosion. The presence of pumice benches 100 feet above the river clearly shows that a great volume has been carried away. Apparently the flows here spread over broad basaltic flats in a series of braided tongues. Some of these overflowed the valley of the Rogue, crossed Mill, Barr, and Red Blanket creeks, and poured into the valley of the Middle Fork (see map, figure 16), where they accumulated to a thickness of 250 feet. Near the Upper Power House on the Middle Fork, the flows coming from the north cascaded into the gorge, eddied upstream for a mile, and piled against the south bank. Checked in their rush, they then turned westward and plunged into the deep gorge of the Rogue below Prospect.

Mention may be made in passing of the thick deposits of bedded and reworked pumice along the South Fork of the Rogue. Clearly these cannot be related to the flows under discussion, for a high ridge separates the Middle from the South Fork, across which it would have been quite impossible for the flows to pass. Nor can the deposits be related to the pumice fall immediately preceding the flows, for the South Fork drains an area beyond the limits of that fall. The deposits were therefore derived by down-washing of older pumice, possibly products of the same eruptions that left the coarse ejecta in the Pumice Flat at the east base of Union Peak.

The branching tongues of the pumice flow, having diverged below Union Creek, converged again at Prospect, where they tumbled into the main canyon of the Rogue. This they followed for another 8 miles. Along this stretch of the river most of the deposits have been washed away, but there are many places where pumice benches are still preserved. Near the Prospect Power House, for example, there are benches 150 feet above the river, and at Laurelhurst, a few miles lower, there are benches 100 feet above the river. These indicate the original thickness of the flows at those points. Not until the flows reached the junction of the Rogue with Cascade Creek, where the main river leaves its basaltic gorge and enters an older and broader valley through the Western Cascades, were they able to spread out again. Here their remains form wide flats 50 to 80 feet above the river.

Suddenly the flows come to an end, 1 1/2 miles above McLeod. Fortunately a roadside quarry and deep road cuts offer excellent opportunity for study of these terminal deposits. These show, as might be expected, an extremely high content of fine pumice dust. The flows had traveled so far, and the larger pumice lumps had been so reduced in number and size by internal explosions and mutual bombardment, that much of the material was reduced to powder. Whereas half the material in the flows above Union Creek may be composed of pumice bombs more than an inch across, the proportion of such bombs at the snout of the flows rarely exceeds 10 per cent. A sample from the roadside quarry near the snout shows the following content by volume: less than 0.5 mm., 70 per cent; 0.5-1 mm., 8 per cent; 1-3 mm., 13 per cent; 3-5 mm., 3 per cent; 5-10 mm., 3 per cent; and greater than 10 mm., 3 per cent. Moreover, the content of lithic fragments is much less than in the flows nearer the source, and few exceed even 1 mm. in size. In a second sample from the same quarry, 41 per cent of the material was less than 0.125 mm. in size, and all fragments larger than 5 mm. were composed of pumice. Though lithic material made up 16.7 per cent of the fraction between 0.125 and 5 mm., the total lithic content in the deposit was only about 6 per cent. Histogram 361, figure 25, shows the nature of the finer constituents. Finally, it should be added that charcoal logs, some of them 3 feet across, are almost as plentiful at the snout of the flows as anywhere higher up the valley of the Rogue.

The choking of the Rogue River gorge by a mass of pumice up to 150 feet in thickness must have produced extensive floods. Much of the pumice must have been swept downstream immediately and more has since been removed. Accordingly, terraces of washed pumice occur far below the snout of the flows. For instance, an excavation along the roadside close to the Rogue-Elk junction shows reassorted, bedded pumice about 50 feet above the level of the river (plate 19, figure 2). Pumiceous sand and gravel border the river at least as far down as the Agate Desert, near Medford.

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