Chapter D: R. Mullineaux of the U.S. Geological Survey has studied in detail the

pumice deposits of Mount Rainier National Park. One of his first and most important discoveries was that even though some pumice layers are spread widely over the park, they were erupted from other volcanoes. Strangely enough, one layer is thicker and more widespread than any recent pumice erupted by Mount Rainier. We can clearly see that these foreign pumice layers did not come from Mount Rainier, for they thicken and coarsen southward, away from the park. The oldest was erupted by Mount Mazama volcano at the site of Crater Lake, Oregon, about 6,600 years ago; this pumice forms a yellowish-orange layer about 2 inches thick nearly everywhere in the park. The pumice has a texture like that of sandy flour, and it feels grainy when rubbed between the fingers. It is so fine grained because of the great distance to its source, 250 miles due south of Mount Rainier. Near Crater Lake this same pumice consists of large chunks and is many feet thick.

Two other foreign pumice deposits in the park were erupted by Mount St. Helens, a symmetrical young volcanic cone about 50 miles southwest of Mount Rainier. The older of the two is between 3,250 and 4,000 years old; it forms a blanket of yellow sand-sized pumice that is as much as 20 inches thick in the western part of the park. The younger pumice layer is most conspicuous at the ground surface in the eastern part of the park, where it is as much as 4 inches thick and resembles a fine white sand. It is about 450 years old.

An inconspicuous bed of pumice records the first eruption of Mount Rainier that occurred after Ice Age glaciers melted back to the slopes of the volcano. It can be found on the east side of the mountain from Grand Park south to Ohanapecosh campground (fig. 8). In roadcuts near the east end of Yakima Park (Sunrise) the pumice forms a rusty-brown bed about 4 inches thick which contains fragments as much as 2 inches across. Wood from a thin layer of peat just above the pumice was dated by its content of radioactive carbon as about 8,750 years old; thus, the pumice is even older. We call this pumice layer R for convenience; other letter symbols have been assigned to the younger layers (table 1).

TABLE 1.—_Characteristics, sources, and ages of pumice layers,
Mount Rainier National Park_
[Based on studies by D. R. Mullineaux]

Common range of
thickness in park
Pumice West East Common Color Source Approximate
layer side side range in age in 1968,
(inches) (inches) diameter or limiting
of pumice dates (years
fragments ago)
(inches)

X Absent [1] ¼-2 Light olive Mount 100-150
gray Rainier.
W 0-1 1-3 Medium White Mount St. [2]450
sand Helens.
C Absent 1-8 ¼-8 Brown Mount 2,150-2,500
Rainier.
Y 5-20 1-5 Coarse Yellow Mount St. 3,250-4,000
sand Helens.
D Absent 0-6 ¼-6 Brown Mount 5,800-6,600
Rainier.
L Absent 0-8 ¼-2 Brown Mount 5,800-6,600
Rainier.
O 1-3 1-3 Flourlike Yellowish Mount About 6,600
to fine orange Mazama.
sand
R Absent 0-5 ⅛-1 Reddish Mount 8,750-11,000?
brown Rainier.

The next two eruptions of Mount Rainier occurred between 5,800 and 6,600 years ago. Again, pumice spread over the area east of the volcano. The older pumice, which we call layer L, covers a band only a few miles wide that extends to the southeast from the volcano (fig. 8). The younger pumice, layer D, covers an area at least 10 miles wide directly east of the volcano. The distribution of both deposits shows that there were strong directional winds during the eruptions. The long, narrow pattern of layer L probably was caused by strong northwesterly winds during a short-lived eruption. The pattern of layer D was caused by winds from the west.

Some time during these eruptions, hot volcanic bombs and rock fragments were thrown out of Mount Rainier’s crater and fell onto surrounding areas of snow and ice. Wholesale melting resulted, and floods descended the east flank of the volcano carrying millions of tons of ash, newly erupted rock debris, and breadcrust bombs. Breadcrust bombs seem to be solid rock, but if you would break one open, you would find that the inside is hollow or is filled with a spongy mass of black glass. Their outer surfaces are cracked like the crust of a loaf of hard-crusted bread (fig. 9), so we call them breadcrust bombs. They originated as blobs of soft, red-hot lava which were thrown out of the volcano’s crater. As the masses arched through the air, they quickly chilled on the outside, and a hardened skin formed around the still hot and plastic core. As their outsides cooled, gas pressure in their hot interiors caused the bombs to expand slightly and their solidified outer skin to crack. When they struck the ground, many of the bombs became flattened on one side, but they were still plastic and sticky enough to remain whole.

Bombs can be found in two deposits that form the south bank of the White River about half a mile downstream from the White River campground. The deposits are mudflows caused by the mixing of hot rock debris with the water from melted snow and ice. As the mudflows moved down the valley floor they must have resembled flowing masses of wet concrete.

Mount Rainier erupted several times between about 2,500 and 2,000 years ago. During one of the first eruptions, a mass of hot ash, rock fragments, and breadcrust bombs avalanched down the side of the volcano and buried the floor of the South Puyallup River valley. Although this hot mass flowed like a wet mudflow, the temperature of the rock debris was above 600°F. Thus, if any water had been present, it would have been in the form of steam. You can see the resulting deposit in cuts along the West Side Road on both sides of the bridge across the South Puyallup River. Innumerable bombs have rolled from the cuts into the ditches beside the road. A charcoal log found in the deposit had a radiocarbon age of about 2,500 years.

Large amounts of pumice were thrown out of the volcano at the same time as the bombs or soon after. The pumice covers most of the eastern half of the park, and fragments are scattered as far southwest as Pyramid Peak and as far northwest as Spray Park. This pumice, called layer C, is especially thick and coarse at Yakima Park and Burroughs Mountain, where it lies at the ground surface (fig. 10). Here the light-brown layer is 5-6 inches thick and consists of irregularly shaped pumice fragments as much as several inches across. Mingled with the pumice fragments are fist-sized chunks of light-gray rock that probably were simultaneously thrown out of the volcano by violent explosions. Some of these angular rocks were hurled as far as Shriner Peak, 11 miles east of Mount Rainier’s summit.

The eruptive period was climaxed by the building of the volcano’s present summit cone, which is at least 1,000 feet high and 1 mile across at its base. Although dwarfed by the tremendous bulk of Mount Rainier, it is a little larger than the cone of the well-known Mexican volcano Parícutin that appeared in 1943 and erupted until 1952. Mount Rainier’s present summit cone was built within a broad depression at the top of the main volcano that had been formed nearly 4,000 years earlier (fig. 11; see p. 40). The cone consists of a series of thin black lava flows, and its top is indented by two overlapping craters. Rocks around the craters are still warm in places, and steam vents melt caves in the summit icecap. The first climbers who reached the top of the mountain, in 1870, spent the night in one of these caves, as have many benighted climbers since.

Even though the lava flows that formed the summit cone were relatively short, their eruption greatly affected some valleys at the base of the volcano. The hot lava melted snow and ice at the volcano’s summit, causing floods that rushed down the east and south sides. When the floods reached the valley floors, they picked up great quantities of loose rock debris and carried it downstream, sometimes forming mudflows. The resulting flood and mudflow deposits raised the floors of the White and Nisqually River valleys as much as 80 feet higher than they are today. These valley floors, as well as several others, then became broad wastes of bare sand and gravel that extended beyond the park boundaries. Later, the rivers cut down to their present levels, but they left remnants of the flood and mudflow deposits as terraces or benches along the sides of the valleys. You can camp on such a terrace in the Nisqually River valley at the Cougar Rock campground. The White River campground occupies a similar terrace in the White River valley.

When did Mount Rainier erupt last? The most recent pumice eruption was just a little over a century ago. However, between 1820 and 1894, observers reported at least 14 eruptions. Some of these may have been just large dust clouds, caused by rockfalls, that were mistaken for clouds of newly erupted ash. Other clouds may have been from genuine eruptions that left no recognizable deposits. D. R. Mullineaux has found that at least one eruption of that era did spread pumice over an area east of the volcano between Burroughs Mountain and Indian Bar to a distance of at least 6 miles from the crater. Pieces of the pumice, layer X, are light brownish gray and as large as 2 inches across. We find only scattered fragments of the pumice, and nowhere are they in a continuous layer. Where the X pumice is directly on top of layer C, we cannot tell them apart. The best areas for us to study the younger pumice, therefore, are glacial moraines formed within the last 150 years, because no pumice other than layer X is present on the moraines. Fortunately, R. S. Sigafoos and E. L. Hendricks of the U.S. Geological Survey have determined the ages of the moraines by counting the growth rings of trees on them. Their studies show that the pumice was erupted between about 1820 and 1854.

Captain John Frémont, an early explorer of the Oregon Territory, recorded that Mount Rainier was erupting in November 1843, but his journals give no details. Others have reported eruptions in 1820, 1846, 1854, and 1858. Pumice layer X probably was erupted during one or more of these times, but we do not know exactly when.

And will Mount Rainier erupt again? We think that it will, but we now have no sure way of predicting the time, the kind, or the scale of future eruptions.

Why Glaciers?

We frequently hear the question: “Why are there glaciers on Mount Rainier?” A glacier forms wherever snowfall repeatedly exceeds melting over a period of years. Above 6,500-7,000 feet on Mount Rainier, more than 50 feet of snow falls each winter, and not all of it melts before the next winter. The survival of this snow from one year to the next depends partly on the cooler temperatures at the higher altitudes, and perhaps also on the somewhat deeper snowfalls there.

LITTLE TAHOMA PEAK
MOUNT RAINIER
INGRAHAM GLACIER
COWLITZ GLACIER

A line that marks the limit on a mountain above which snow persists from one winter to the next is called the annual snowline, and this line on a glacier is called the firnline (fig. 12). Above the firnline, snow that falls each year packs down and changes into glacier ice as air is slowly forced out of it. This part of the glacier is its accumulation area, where more snow falls each year than is lost by melting. Below the firnline is the ablation area, where melting predominates. The firnline on Mount Rainier’s glaciers has been well above 6,500 feet in recent years. But some glaciers extend to altitudes below 5,000 feet—that is, far down into the ablation area. They do this by slowly flowing downhill. Solid ice flows by sliding on the hard bedrock under the glacier and by slipping along the innumerable surfaces within the ice crystals that make up the glacier.

The rate of flow and the rate of ablation govern the distance a glacier extends down into the ablation zone. If these rates remain fairly constant, the glacier will be in balance and its size will be about the same from year to year. But if changing weather patterns affect the rates of ablation or accumulation, or both, the glacier will either become smaller or grow larger. The change you are most likely to notice is in the position of the glacier’s terminus, which may either recede or advance, but precise measurements of the upper reaches of a glacier also show volume changes there, some of which may not affect the glacier’s terminus for many years, if ever.

Crevasses are a glacier’s most awesome features and are a constant hazard for climbers. They form where adjacent parts of a glacier are moving at different speeds. Some of Mount Rainier’s glaciers may be flowing at a speed of several thousand feet per year along their centers but at a much slower rate along their margins. This unequal rate of flow produces stresses in the ice that cause it to break. Groups of crevasses often form where the glacier flows over a steep place in its bed. The ice moves faster here, and pulls apart, and a crevasse is formed. Although a large crevasse may seem to be bottomless to the observer, most crevasses are less than 100 feet deep because ice pressure tends to close the open spaces in the ice below that depth.[3]

Ice covers 37 square miles of the park today. Individual glaciers that make up this ice blanket are placed into three groups, depending on their place of origin. Those of one group originate at the volcano’s summit and flow far down the valleys that radiate from the cone. Most of the snow that nourishes these glaciers probably falls on them at altitudes well below the summit. The largest examples of the group are the Emmons, Nisqually, and Tahoma Glaciers.

Glaciers of the second group originate on the flanks of the volcano, mostly at altitudes between 7,000 and 10,000 feet. This group is represented by the South Tahoma, Carbon, and Inter Glaciers. Glaciers of the third group are on north-facing slopes in the mountains around Mount Rainier. They are mostly at altitudes of about 6,000 feet, and they owe their existence to locations well protected from solar heat. Glaciers representing this group are the Unicorn and Pinnacle Glaciers in the Tatoosh Range, a small unnamed glacier near the west end of Burroughs Mountain, and the somewhat larger Sarvent Glaciers east of Mount Rainier.

Work Habits of Glaciers

Glaciers are extremely capable workers. Their work includes erosion, transportation, and deposition. The smoothed and grooved bedrock at Box Canyon and at many points along the trail to the ice caves near Paradise Park shows erosion of rock by glacier ice. Rock fragments carried by glaciers cut grooves in the hard bedrock and polish its surface (fig. 13). Although any one rock fragment might scrape away only 1 millimeter of rock along a single groove, the total effect is great when multiplied by countless thousands of similar fragments rasping a bedrock surface for hundreds or thousands of years. Glacier ice may also break chunks of rock loose as it overrides them and may even plow up sections of softer rocks.

Glaciers transport not only the rocks that they quarry and scrape from their beds but also, more conspicuously, those that fall onto their surfaces from nearby cliffs. These falling rocks range in size from tiny particles to individual masses that weigh tens of thousands of tons, like those that fell onto Emmons Glacier from Little Tahoma Peak in 1963. (See p. 35.)

A glacier deposits most rock debris at its terminus. The steep snout of any major glacier is a dangerous place to approach closely because rock debris almost constantly falls, rolls, and slides down the melting ice faces. Much of this debris collects at the ice margin, and if the margin stays in one place long enough a ridge-shaped end moraine of rock debris forms along the ice front (fig. 14). If such a moraine forms across the front of a glacier at its farthest advance it is called a terminal moraine. End moraines that form as the ice recedes are called recessional moraines (fig. 15). Ridges of rock debris that form along the sides of a glacier are called lateral moraines (fig. 12).

Some recent moraines of modern glaciers are only a few feet away from the present ice margin; others, formed thousands of years ago during the most recent major glaciation, are on ridgetops and valley sides or floors miles away from modern glaciers. By examining the shape and location of these moraines, we can reconstruct the size and character of past glaciers, as we will see in the next section.

Glaciers erode, transport, and deposit huge quantities of rock debris. So do their coworkers, the melt-water streams. These turbulent streams flow from tunnels beneath every glacier, and their degree of muddiness roughly shows how active the glacier is. Glaciers that move very slowly, or that are stagnant, produce relatively clear melt water because they are not actively eroding bedrock. In contrast, streams of muddy water that look like chocolate milk often come from very active or “live” glaciers. These streams carry rock debris ranging from flour-size particles to large boulders. You can sense the carrying power of this swiftly moving water on warm summer days, when large cobbles and boulders are bumping along in a stream swollen by rapid glacier melting. Although you can rarely see these boulders, you can hear their constant low thunder. Their repeated impacts on other boulders in the streambed will vibrate the nearby streambanks beneath your feet. Hikers often find that a melt-water stream safe to cross in the early morning of a warm summer day is an impassable torrent at the same spot by early afternoon.

A melt-water stream generally deposits coarse material wherever the slope of the valley floor decreases and the stream loses some of its velocity and carrying power. Only a flood may move the boulders farther downstream. However, the current carries fine material far downstream to deposit it in lakes, in Puget Sound, or in the Pacific Ocean. The Puyallup River, for example, is still very muddy where it enters Puget Sound at Tacoma, more than 40 miles from its source in the glaciers on Mount Rainier.

During the last glaciation, when glaciers were much larger than they are now, melt-water streams carrying great quantities of sand and gravel built valley floors up to levels tens of feet higher than they are today. Later, as the glaciers grew smaller, the rivers cut down into their valley floors and remnants of the sand and gravel deposits were left standing in benches or terraces along the sides of the valleys. You can see a good example of such a terrace in the Nisqually River valley beyond Ashford, which is 5 miles west of the park. You cross it on the highway that leads to the park. Cuts beneath the terrace reveal deposits of sand, cobbles, and boulders that look the same as those deposits being formed today by melt-water streams. The terrace west of Ashford was formed a little more than 15,000 years ago, when a glacier extended down the Nisqually River valley to the vicinity of Ashford.

Yesterday’s Glaciers

Mount Rainier’s great sprawling cone would seem incomplete without the glistening sheets of ice that descend its flanks. We have reason to believe that the volcano has borne glaciers ever since its origin—sometimes smaller than now, at other times vastly larger. Mount Rainier probably started to grow during the middle part of the Pleistocene Epoch, or Ice Age, which began more than 1 million years ago, but glaciers had covered this part of the mountains even before the volcano appeared. Masses of rock debris formed by ancient glaciers occur beneath lava flows from Mount Rainier on the west side of Mazama Ridge just upslope from Narada Falls, on the north side of Glacier Basin, and at a few other places in the park.

Mount Rainier may have reached its present size by about 75,000 years ago. Since that time great icefields and glaciers have formed at least three times on the slopes of the volcano and in the nearby mountains. During the first two glaciations, ice completely buried the flanks of the volcano and the surrounding mountains, except for the very highest ridges and peaks. These great ice masses slowly flowed down all the valleys that head at Mount Rainier. The glacier in the Cowlitz River valley, for example, extended 65 miles from the volcano and reached a point about 33 miles west of the community of Randle. Deposits of the younger of these two glacial episodes can be seen in cuts along the Mowich Lake Road for a distance of about 1½ miles inside the park boundary. The glacial deposits were originally more widespread, but in most of the park they have been removed by erosion or covered by the deposits of yet younger glaciers.

During the most recent major glaciation of the park, which lasted from roughly 25,000 to 10,000 years ago, ice again sheathed the slopes of the volcano, but glaciers in the nearby mountains were smaller than before. Most of the glaciers originated at the valley heads, where they gouged out countless bowl-shaped bedrock basins called cirques. Many of the basins held lakes after the glaciers disappeared. (See frontispiece.) Hikers on the trail to the Paradise ice caves cross the floor of a typical cirque at the head of Paradise Valley. From the Sunrise Visitor Center at Yakima Park you can walk a short distance to a point along the crest of the Sourdough Mountains and stand at the rim of a deep north-facing cirque. Ice originating in this cirque and in the cirques adjacent to it moved northward down the valley of Huckleberry Creek at least as far as the park’s north boundary (fig. 16).

These glaciers left most valley walls in the park covered with rock debris. Lateral moraines can be seen along the highway at and just east of Ricksecker Point (fig. 17). Other glacial deposits are especially well displayed in roadcuts along the north wall of the White River valley.

A little more than 15,000 years ago the long glaciers began to shrink and recede. By 11,000 years ago there was only about as much ice on Mount Rainier as there has been within the last century. Then, during a short period of renewed glacier growth, most glaciers expanded short distances and new ones appeared in cirques from which ice had disappeared only a short time before. In some of these cirques so much rock debris was being dislodged from surrounding cliffs by repeated freeze and thaw that a rock glacier, consisting mostly of rock fragments bound together by ice, was formed. A trail to the Huckleberry Creek valley crosses hummocky rock debris left by such a rock glacier in a cirque on the southeast side of Mount Fremont. A larger rock-glacier deposit lies about 2 miles north of Sunrise Point in an east-facing cirque between The Palisades and Hidden Lake (fig. 18).

In other cirques, where the proportion of ice to rock debris was greater, the glacier transported the debris a short distance forward and built a terminal moraine. You can see particularly good examples of moraines formed about 11,000 years ago near Tipsoo Lake, where the pond southeast of the lake is dammed by a moraine, and at Mystic Lake. The ice that formed the terminal moraine at Mystic Lake was a tongue of Carbon Glacier.

In some places the orientation or altitude of the cirque did not permit enough snow to accumulate to form a glacier but just enough to create a permanent snowbank. Rock debris that fell from the surrounding cliffs rolled down these snowbanks and formed low ridges at their toes. Such a ridge is called a protalus rampart because it is found just in front of the apron of rock fragments, called talus, that lies beneath cliffs. A trail at Sunrise Point leads to protalus ramparts along the north side of Sunrise Ridge.

During the last 10,000 years, glaciers have been very small by comparison with the great ice mantles that overwhelmed the park earlier. However, glaciers have grown larger at least twice just within the last 3,000 years. During both of these periods most glaciers were slightly larger than they are today, and ice occupied most cirques at altitudes above 6,500 feet—even some that are now free of ice. The most recent time of extensive glacier growth began at least 800 years ago, and various glaciers in the park reached their maximum size between the mid-14th century and the mid-19th century. Oddly enough, even though all the glaciers headed on Mount Rainier, they did not all attain their maximum size simultaneously. The largest terminal moraine of this most recent glacial period was built by Emmons Glacier in the White River valley (fig. 19). It is now largely forested, and cores taken from the trees with a special boring tool that does not harm the tree show ages indicating that the moraine was stable enough to permit seedlings to survive on it by the mid-17th century. A similar but smaller terminal moraine built by Cowlitz Glacier has trees on it that started to grow in the mid-14th century.

Nearly all the glaciers gradually decreased in size after the mid-18th century. Although the shrinkage was sometimes interrupted by short periods of renewed glacier growth, by 1950 the glaciers at Mount Rainier covered only about two-thirds of the area that had been buried by ice only a century before. The overall loss of volume by Rainier’s glaciers, as well as those elsewhere in the Pacific Northwest, was slowed or halted by slightly cooler temperatures and higher precipitation starting in the mid-1940’s. Volume increases in their upper reaches caused the larger glaciers to grow from year to year, and since the early 1950’s the terminuses of many glaciers have been advancing. This renewed growth of glaciers is not unique at Mount Rainier—similar changes have been observed at other glaciers in the Cascade Range and elsewhere.

Landslides and Mudflows—Past, Present, and Future

Some of the most effective means of erosion at Mount Rainier are landslides and mudflows. Erosion of this kind is sometimes spectacular. Within an interval of only minutes or a few hours huge masses of rock may fall, slide, or flow off the volcano and move far downvalley.

Large landslides have occurred at many other places in the park—one in the area northeast of Mount Rainier is so conspicuous that its source has been named Slide Mountain. The ragged scar left by another slide near Grand Park is aptly called Scarface. You cross a slide on the Mather Memorial Parkway (U.S. Highway 410) just north of Cayuse Pass. Broken and jumbled rock debris of many sizes borders both sides of the highway there. This landslide broke loose in rocks of the Ohanapecosh Formation, slid downslope to the bottom of the valley, and dammed Klickitat Creek to form Ghost Lake. Rocks have also slid downslope on the west side of Backbone Ridge and on the east side of the Ohanapecosh River valley a short distance north of Ohanapecosh campground. The slide on Backbone Ridge is still slowly moving today. Another slide moves a few inches each year on the west side of the Nisqually River valley about 1 mile northwest of the visitor center at Paradise Park. You can recognize the slide by a jagged horizontal crack 1,000 feet long at its top.

A far more spectacular variety of landslide occurs when a mass of rock drops from a cliff to form a rockfall. The largest rockfalls on Mount Rainier in historic time occurred in December 1963 on the east flank. Masses of rock hundreds of feet across fell repeatedly from the steep north face of Little Tahoma Peak onto Emmons Glacier. The rock masses shattered into dust and countless fragments, fanned out across the glacier, then avalanched down the steep ice surface at tremendous speed. When the avalanches reached the end of the glacier they shot out into space as sheets of rock debris. As these hurtling sheets settled toward the valley floor, a cushion of compressed air formed beneath them, comparable to the air cushion that momentarily buoys up a sheet of plywood that is dropped onto a flat surface. Air that was trapped beneath these speeding avalanches reduced friction and permitted one of the avalanches to move almost 2 miles beyond the end of the glacier. This avalanche passed completely over a small wooden gage house about 5 feet high on the valley floor without damaging it, then ran headlong into the north base of Goat Island Mountain where it scraped away trees and bushes. A later avalanche stopped just short of the gage house, and wind that was expelled from beneath the rock debris blasted the still-undamaged house several hundred feet forward. It now rests in the scar left by the earlier avalanche on the side of Goat Island Mountain.

At least seven rockfalls and avalanches descended from Little Tahoma Peak, separated perhaps by only minutes or hours. The reddish-gray masses of broken and pulverized rock—some spread helter-skelter, some piled in long sharp-crested ridges—now lie on the valley floor between the White River campground and Emmons Glacier (fig. 20).

The rockfalls might have been caused by a steam explosion near the base of Little Tahoma Peak. Steam jets and small explosions are not unusual phenomena at Mount Rainier, although they never have had such dramatic effects in historic time.

Incredibly larger avalanches of rock fell repeatedly from the sides of Mount Rainier during prehistoric time. One such avalanche originated near the summit of the volcano and blanketed Paradise Park and Paradise Valley with a yellowish-orange mixture of clay and rocks sometime between 5,800 and 6,600 years ago. You can see this avalanche deposit in shallow cuts along trails and roads in the Paradise area. Huge blocks of rock that came down with the avalanche are scattered in the meadows of Paradise Park between the visitor center and Panorama Point. Although the deposit is now less than 15 feet thick in most places, the mass that flowed down Paradise Valley must have been 600 feet thick, because we can find remnants of it on top of Mazama Ridge. A tongue of the wet mass flowed through a low saddle near the south end of Mazama Ridge and extended into the basin now occupied by the Reflection Lakes. You can see the yellowish-orange deposit in the first roadcut west of the lakes, where it lies on top of gray glacial deposits.

The avalanche probably was wet when it crossed the Paradise area, and the moisture in it may have already been present in the rocks in which the avalanche originated. The mass was fluid enough to move down the Paradise and Nisqually River valleys as a mudflow hundreds of feet thick, and the resulting deposits extend at least 18 miles downstream from the volcano. The volume of rock that slid off to produce the mudflow may have been as much as 100 million cubic yards—or roughly enough to cover a 1-mile-square area to a depth of 100 feet.

At about the same time as the Paradise avalanche and mudflow occurred, a tremendous rock mass also slid off the east side of the volcano in the area between Steamboat Prow and Little Tahoma Peak. This slide formed a mudflow on the floor of the White River valley that was several hundred feet deep at the north boundary of the park and that extended at least 30 miles beyond the base of the volcano. The most remarkable feature of the deposit left by this mudflow is its surface, which is dotted with scores of mounds 5-35 feet high and as much as several hundred feet in diameter. These mounds have cores of huge rocks which are similar in size to those scattered on the surface of the avalanche deposits from Little Tahoma Peak. You can see the mounds best in an area which is a few miles north of the park boundary, west of the White River, and which can be reached by the Huckleberry Creek Forest Road. The total volume of this mudflow deposit may be as much as one-fifth of a cubic mile.

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.

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.