These great landslides and mudflows were followed shortly by another whose size surpassed that of anything before or since. This was the remarkable Osceola Mudflow, which streamed down the valleys of the White River and West Fork about 5,800 years ago. When these great rivers of mud joined in the White River valley, they formed an even larger mudflow which swiftly flowed downvalley for a distance of 15 miles and then spread beyond the Cascade mountain front into the Puget Sound lowland. There the mudflow submerged a total area of more than 100 square miles to depths as great as 70 feet and buried the sites of the present towns of Enumclaw and Buckley. One tongue of it even flowed into an arm of Puget Sound, south of Seattle, that has since been filled with river deposits to form the fertile valley occupied by the towns of Kent, Auburn, Sumner, and Puyallup.
The Osceola Mudflow is remarkable in that it affected areas so far from its place of origin. This long distance of travel was due to its great volume, which we estimate to have been more than half a cubic mile, and to the abundance of slippery clay in it. The clay had been formed by the alteration of rocks in the volcano by hot gases and solutions over many centuries.
The northeast flank of Mount Rainier. A remnant of the Osceola Mudflow lies at the summit of Steamboat Prow in the center. Two and one-half miles to the left is Little Tahoma Peak, from whose steep north face at least seven large masses of rock fell in 1963. Mount Adams volcano can be seen at the left, and Mount Hood, Oregon, in the far distance. (Fig. 21)
Where did the Osceola Mudflow originate on the volcano? This we must deduce from several lines of evidence. The mudflow occurred so long ago that there is no historical record, and volcanic events since that time have covered up part of the scar it left on the volcano. Remnants of the Osceola Mudflow veneer the sides and ridges of Glacier Basin, and a small amount of it is even preserved at the top of Steamboat Prow, at an altitude of 9,700 feet ([fig. 21]). This distribution tells us that the slides responsible for the mudflow originated somewhere on the volcano above Steamboat Prow. But now there is no great chasm in the side of the volcano large enough to have provided a source of the mudflow; so we must consider a former summit of the volcano itself as a possible source.
I. C. Russell, one of the first geologists to study Mount Rainier, wrote in 1896 that the present summit of the volcano consists of a small lava cone. Enclosing this cone is a broad depression whose rim is partly preserved at Gibraltar Rock, Point Success, and Liberty Cap ([fig. 11]). High points on the rim indicate that the former summit of the volcano above an altitude of about 14,000 feet was removed in some way. The destruction of the old summit, which may have reached a height of 16,000 feet, left a broad east-facing depression in the top of the volcano between Gibraltar Rock and Russell Cliff. The depression has since been mostly filled by the recent lava cone. You can see these features best from high points east of the mountain.
Our best explanation of how the former top of the volcano was removed also solves the problem of finding an adequate source of material for the Osceola Mudflow. Before 5,800 years ago, the topmost part of Mount Rainier probably consisted of rock that had been weakened by hot volcanic fumes and solutions and partly converted to clay. Then, this mass of weak rock was jostled off or pushed off by a volcanic explosion and slid down the northeast side of the volcano. One or more of these mighty avalanches of moist clay and rock resulted in the Osceola Mudflow.
Large avalanches have also occurred many times during the last 3,000 years on the west side of the volcano. Sunset Amphitheater ([fig. 11]) is part of the large scar left by them. About 2,800 years ago one of these avalanches created a mudflow in the valleys of the South Puyallup River and Tahoma Creek that was temporarily deep enough to submerge Round Pass (on the West Side Road) to a depth of nearly 400 feet. This is especially remarkable when we see that Round Pass itself is 600-700 feet above the nearby valley floors. Another deep mudflow, started by an avalanche at Sunset Amphitheater, moved down the Puyallup River valley about 600 years ago and buried the site of the present town of Orting in the Puget Sound lowland under 15 feet of mud and rock.
Table 2.—Summary of important geologic events in the history of Mount Rainier National Park
| Geologic time scale | Years ago | Geologic events in the area of the park |
|---|---|---|
| “Postglacial” | Present summit cone of Mount Rainier probably was built about 2,000 years ago. The last known pumice eruption occurred between 1820 and 1854. | |
| Glaciers started to grow and advance about 3,000 years ago. Maximum extents were reached about 1850 A.D. From then until about 1955, glaciers were receding; now they are in balance or advancing. | ||
| Huge masses of rock have slid from the volcano repeatedly during the last 10,000 years. One of these destroyed the summit of Mount Rainier and formed the Osceola Mudflow about 5,800 years ago. | ||
| 10,000 | ||
| Pleistocene (Ice Age) | Last major glaciation. | |
| 25,000 | ||
| Birth and growth of Mount Rainier volcano, and repeated glaciation. | ||
| 2-3 million | ||
| Pliocene | Uplift and erosion of the Cascade Range. | |
| 12 million | ||
| Miocene | Intrusion of granodiorite. | |
| Folding of older rocks. | ||
| Deposition of Fifes Peak and Stevens Ridge Formations. | ||
| 26 million | ||
| Oligocene | Deposition of Ohanapecosh Formation. | |
| 37-38 million | ||
| Eocene | Deposition of Puget Group. | |
| 53-54 million | ||