[Illustration: 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.