THE GEOLOGIC STORY 

OF MOUNT RAINIER

ORTON MEMORIAL LIBRARY

THE OHIO STATE UNIVERSITY

155 S. OVAL DRIVE

GEOLOGICAL SURVEY BULLETIN I292

The Geologic Story

of 

Mount Rainier
Eunice Lake, northwest of Mount Rainier. The lake lies in a small bedrock 

basin that was scoured out by a glacier between about 15,000 and 20,000 

years ago. The rounded green slopes at the far edge of the lake are 

underlain by rock that has been smoothed and grooved by glacier ice. 

This side of Mount Rainier rises to Liberty Cap, which hides the true 

summit of the volcano.
The Geologic Story 

of Mount Rainier

By

Dwight R. Crandell

A look at the geologic past of one 

of America's most scenic volcanoes
WALTER J. HICKEL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

Library of Congress Catalog-Card No. 79-601704

U.S. GOVERNMENT PRINTING OFFICE: 1969

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 - Price 65 cents (paper cover)
contents

Page

The changing landscape of 12-60 million years ago. .... 3

Thumbnail biography of Mount Rainier............... 11

Results of recent eruptions.......................... 12

.Why glaciers?..................................... 23

Work habits of glaciers............................. 25

Yesterday's glaciers ............................... 29

Landslides and mudflows past, present, and future... 35

The volcano's future?.............................. 42

Further reading in geology......................... 43

FRONTISPIECE. Eunice Lake, northwest of Mount Rainier.

Page

FIGURE 1. Outcrop of sandstone and shale

in the Puget Group........................................ 6

2. Outcrop of welded tuff in the

Stevens Ridge Formation............................ 8

3. Granodiorite looks like granite...................... 9

4. Geological cross section of Mount Rainier.... 10

5. An old lava flow which forms Rampart Ridge 13

6. Columns of andesite at the end

of an old lava flow....................... .............. 13

7. Layers of pumice on the floor of a cirque...... 14

8. Generalized distribution of

some pumice layers...................................... 16

9. Breadcrust bomb enclosed in a

mudflow deposit............................................ 19

10. Pumice layer C, which consists of

light-brown fragments................. .............. 20

11. The recent lava cone lies in a depression...... 21
Page

FIGURE 12. Two ice streams meet to form

Cowlitz Glacier............................................ 23

13. Glacier-smoothed and grooved rock.............. 26

14. A lake lies behind an end moraine

of Flett Glacier............................................ 27

15. Recessional moraines on the valley

floor of Fryingpan Creek.......................... 28

16. Extent of glaciers between

15,000 and 25,000 years ago...................... 30

17. Lateral moraine at Ricksecker Point.......... 31

18. Rock-glacier deposit at The Palisades.......... 33

19. Hummocky end moraine in front of

Emmons Glacier.......................................... 34

20. Avalanche deposits in the

White River valley...................................... 37

21. The northeast flank of Mount Rainier.......... 39

TABLE 1. Characteristics, sources, and ages of pumice

layers, Mount Rainier National Park.......... 17

2. Summary of important geologic events in the

history of Mount Rainier National Park.... 41

VI
The Geologic Story of 

Mount Rainier

By DWIGHT R. CRANDELL

Ice-clad Mount Rainier, towering over the landscape of 

western Washington, ranks with Fuji-yama in Japan, Popocatepetl in Mexico, and Vesuvius in Italy among the great 

volcanoes of the world. At Mount Rainier, as at other inactive volcanoes, the ever-present possibility of renewed 

eruptions gives viewers a sense of anticipation, excitement, 

and apprehension not equaled by most other mountains. 

Even so, many of us cannot imagine the cataclysmic scale 

of the eruptions that were responsible for building the giant

WASHINGTON 

U

100 200 MILES
cone which now stands in silence. We accept the volcano as 

if it had always been there, and we appreciate only the 

beauty of its stark expanses of rock and ice, its flowerstrewn alpine meadows, and its bordering evergreen forests.

Mount Rainier owes its scenic beauty to many features. 

The broad cone spreads out on top of a major mountain 

range the Cascades. The volcano rises about 7,000 feet 

above its 7,000-foot foundation, and stands in solitary splendor the highest peak in the entire Cascade Range. Its rocky 

ice-mantled slopes above timberline contrast with the dense 

green forests and give Mount Rainier the appearance of 

an arctic island in a temperate sea, an island so large that 

you can see its full size and shape only from the air. The 

mountain is highly photogenic because of the contrasts it 

offers among bare rock, snowfields, blue sky, and the incomparable flower fields that color its lower slopes. Shadows 

cast by the multitude of cliffs, ridges, canyons, and pinnacles change constantly from sunrise to sunset, endlessly 

varying the texture and mood of the mountain. The face of 

the mountain also varies from day to day as its broad snowfields melt during the summer. The melting of these frozen 

reservoirs makes Mount Rainier a natural resource in a 

practical as well as in an esthetic sense, for it ensures steady 

flows of water for hydroelectric power in the region, regardless of season.

Seen from the Puget Sound country to the west, Mount 

Rainier has an unreal quality its white summit, nearly 3 

miles high, seems to float among the clouds. We share with 

the populace of the entire lowland a thrill as we watch skyward the evening's setting sun redden the volcano's western 

snowfields. When you approach the mountain in its lovely 

setting, you may find something that appeals especially to 

you the scenery, the wildlife, the glaciers, or the wildflowers. Or you may feel challenged to climb to the summit. 

Mount Rainier and its neighboring mountains have a special 

allure for a geologist because he visualizes the events some 

ordinary, some truly spectacular that made the present 

landscape. Such is the fascination of geology. A geologist 

becomes trained to see "in his mind's eye" geologic events 

of thousands or even millions of years ago.. And, most remarkable, he can "see" these events by studying rocks in a 

cliff or roadcut, or perhaps by examining earthy material 

that looks like common soil beneath pastureland many miles 

away from the volcano.
Our key to understanding the geology of Mount Rainier 

is that each geologic event can be reconstructed or imagined from the rocks formed at the time of the event. With 

this principle as our guide, we will review the geologic ancestry of this majestic volcano and learn what is behind its 

scenery.

The Changing Landscape of 

12-60 Million Years Ago

The rocks of the Cascade Range provide a record of earth 

history that started nearly 60 million years ago. Even then, 

as today, waves pounded on beaches and rivers ran to the 

sea, molding and distributing material that formed some of 

the rocks we now see in the park.

You may find it difficult to imagine the different landscape of that far distant time. There was no Mount Rainier 

nor Cascade mountain range. In fact, there was very little 

dry land in the area we call western Washington. Instead, 

this was a broad lowland of swamps, deltas, and inlets that 

bordered the Pacific Ocean. Rivers draining into this lowland from the east spread sand and clay on the lush swamp 

growth. Other plants grew on the deposits, and they were 

covered, in turn, by more sand and clay. In this way, thousands of feet of sand and clay and peat accumulated and 

were compacted into sandstone, shale, and coal. We can see 

some of the rocks formed at that time in cuts along the 

Mowich Lake Road west of the park (fig. 1). Seams of coal 

were mined at Carbonado and Wilkeson, 10 miles northwest 

of the park, during the late 19th and the early 20th centuries.

These beds of sandstone, shale, and coal make up a sequence of rocks called the Puget Group, which is 10,000 feet 

thick. Wave-ripple marks and remains of plants show that 

the rocks were formed in shallow water fairly close to sea 

level. How could the rocks have piled up to this great thickness? The coastal plain and adjacent basin must have been 

slowly sinking, and the influx of sand and clay must have 

just barely kept pace with the downward movements.

A little less than 40 million years ago, the western 

Washington landscape changed dramatically. Geologists R. S. 

Fiske, C. A. Hopson, and A. C. Waters have discovered that


OUTCROP of gray to brown sandstone and dark-gray to black coaly shale 

in the Puget Group along the Mowich Lake Road. (Fig. 1)

volcanoes then rose on the former coastal plain at the site 

of Mount Rainier National Park and became islands as the 

area sank beneath the sea. When molten rock was erupted 

underwater from the submerged flanks of these volcanoes, 

steam explosions shattered the lava into countless fragments. The resulting debris, mixed with water, flowed as 

mud across great areas of the submerged basin floor.

You can see rocks formed from these layers of volcanic 

mud and sand in cuts along the highway on the east side 

of Backbone Ridge and between Cayuse Pass and Tipsoo 

Lake. Look there for alternating beds of grayish-green sandstone and breccia, a concretelike rock in which the pebbles 

have sharp corners. These rocks are known as the Ohanapecosh Formation. Like the Puget Group, the Ohanapecosh 

Formation is at least 10,000 feet thick. Yet, nearly all of it 

accumulated in shallow water as western Washington continued to sink slowly during the volcanic eruptions.

The long-continued sinking finally ended after the Ohanapecosh volcanic activity ceased. Western Washington was 

then lifted several thousand feet above sea level, and the
Puget and Ohanapecosh rocks were slowly compressed into 

a series of broad shallow folds. Before eruptions began 

again, rivers cut valleys hundreds of feet deep, and weathering of the rocks produced thick red clayey soils similar to 

those that are forming in some areas of high rainfall and 

high temperature today. Look for the red rocks formed from 

these old soils in roadcuts as you drive along the Stevens 

Canyon road about 2 miles southeast of Box Canyon.

The next volcanic eruptions, which may have begun between 25 and 30 million years ago, differed from those of 

Ohanapecosh time. These volcanoes, somewhere beyond the 

boundaries of the park, erupted great flows of hot pumice 

that, being highly mobile, rushed down the flanks of the 

volcanoes and spread over many square miles of the adjacent 

regions. The pumice flows were "lubricated" by hot volcanic 

gas emitted from inside each pumice particle, which buffered 

it from other particles. Some hot pumice flows were 350 

feet deep. The heat still remaining in the pumice after it 

stopped flowing partly melted the particles to form a hard 

rock known as welded tuff. Repeated pumice flows buried the 

hilly landscape and eventually formed a vast volcanic plain. 

The rocks, which are mostly welded tuffs, are now the Stevens Ridge Formation, which you can see along the highway 

in Stevens Canyon 1-2 miles west of Box Canyon. You can 

recognize the welded tuff by its light-gray to white color 

and its many flattened and sharp-edged inclusions of darker 

gray pumice (fig. 2).

Another period of volcanism followed, of still a different 

kind, when lava flowed outward from broad low volcanoes. 

The flows were of two kinds: basalt, the kind now erupted 

by Hawaiian volcanoes, and andesite, the type erupted by 

Mount Rainier. Individual flows 50-500 feet thick were 

stacked on top of one another to a total depth of fully 2,500 

feet. We know these rocks as the Fifes Peak Formation. 

They form many of the cliffs and peaks in the northwestern 

part of the park. You can examine them in cuts along the 

Mowich Lake Road between Mountain Meadows and Mowich 

Lake. The time of the eruption of the Fifes Peak lavas may 

have been between 20 and 30 million years ago.

When the Fifes Peak volcanoes finally became extinct, 

this part of western Washington changed again. The rocks 

once more were uplifted and compressed into broad folds 

parallel to those formed at the end of Ohanapecosh time.
The rocks buckled and, in places, broke and shifted thousands of feet along great fractures, or faults.

About 12 million years ago one or more masses of molten 

rock, many miles across, pushed upward through the Puget

OUTCROP of light-gray welded tuff in the Stevens Ridge Formation along 

the road in Stevens Canyon. The angular dark-gray fragments in the 

welded tuff are chunks of pumice. (Fig. 2)

8
Group and younger rocks. When this molten rock cooled 

and hardened, it formed granodiorite, a close relative of 

granite. Although most of the molten rock solidified underground, some of it reached the land surface and formed 

volcanoes at a few places within the area of Mount Rainier 

National Park.

Granodiorite is probably the most attractive rock in the 

park. It is mostly white, but it contains large dark mineral 

grains that give it a "salt-and-pepper" appearance (fig. 3).

GRANODIORITE looks like granite and has a light-gray speckled appearance. The knife is about 3 inches long. (Fig. 3)
The large size of the grains is a result of the molten rock 

cooling slowly at a considerable depth below the land surface the individual minerals had a long time to grow before the "melt" solidified into rock. In contrast, the rocks 

formed from lavas that flowed onto the ground surface are 

generally fine grained because the lavas cooled too quickly 

for the mineral grains to grow appreciably.

Granodiorite underlies the White River valley, the Carbon River valley, and parts of the upper Nisqually River 

valley and the Tatoosh Range. You can see it in roadcuts 

between Longmire and Christine Falls and at several places 

along the road between White River Ranger Station and 

White River campground.

After the granodiorite solidified, the foundation of Mount 

Rainier was complete except for one other landscape change 

that preceded the birth of the volcano. Not long after the 

granodiorite was formed, the Cascade mountain range began 

to rise not rapidly, but little by little over many thousands 

of years. As the land rose, rivers cut valleys into the growing mountains so that by the time the new volcano began 

to erupt, the Cascades had already been carved into a rugged 

range of high ridges and peaks separated by deep valleys.

NORTH

Lava flows from 

Mount Rainier

Echo

Lava flows from Rock 

vent at Echo Rock

s Ridge .Formation'.'- ' 

'O'hanapecosh Formation

GEOLOGICAL CROSS SECTION of Mount Rainier and its foundation rocks 

from Mother Mountain southward to Tatoosh Range. True-scale cross

10
Deep erosion thus laid bare the rock layers in which we 

today read the geologic history of the park (fig. 4).

Thumbnail Biography of 

Mount Rainier

The life span of a volcano can be compared to that of 

an individual after his birth and a brief youth, he matures 

and grows old. The birth date of Mount Rainier is not known 

for sure, but it must have been at least several hundred 

thousand years ago. We cannot tell much about the volcano's 

complex youth because most of its earliest deposits are now 

buried under later ones. At an early age, well before the 

cone grew to its present size, thick lava, like hot tar, flowed 

repeatedly 5-15 miles down the deep canyons of the surrounding mountains. Because these lava flows resisted later 

erosion by rivers and glaciers, most of them now form 

ridgetops, as at Rampart Ridge, Burroughs Mountain, Grand 

Park, and Klapatche Ridge (figs. 5 and 6). Violent explosions occasionally threw pumice onto the slopes of the growing volcano and the surrounding mountains. As the volcano

MOUNT R A I N I E, R 

14,410 FT

Summit

SOUTH

Tatoosh 

Range

3 MILES 

J

section is nearly 17 miles long. Slightly modified from U.S. Geological 

Survey Professional Paper 444, Plate 1. (Fig. 4)
matured, the long thick flows were succeeded by thinner 

and shorter ones which, piled on top of one another, built 

the giant cone that now dominates the region. Even though 

Mount Rainier has grown old now, it has revived briefly at 

many times during the last 10,000 years or so and may 

erupt again in the future.

The events of the last 10,000 years, because they are so 

recent, in terms of geologic time, are better known than 

those of any earlier time, and we can examine this part of 

the volcano's history in some detail. We will study three 

principal subjects: eruptions because they have had widespread effects; glaciers because they are such conspicuous 

features on the mountain; and landslides because they 

have drastically changed the volcano's shape.

Results of Recent Eruptions

While hiking, you soon become aware that there is a 

large amount of pumice along the trails in Mount Rainier 

National Park. Pumice is a lightweight volcanic rock so full 

of air spaces that it will float on water. The air spaces, or 

bubbles, originated when fragments of gas-rich lava were 

explosively thrown into the air above the volcano, and the 

molten rock hardened before the gas could escape. If you 

examine pumice deposits in a trail cut, in a streambank, 

or in the roots of blown-over trees, you may also note that 

there is more than one layer (fig. 7). If you circle the volcano on the Wonderland Trail, you may notice that the 

greatest number of pumice layers are on the east side of 

the park, but the thickest single layer is on the west side. 

The explanation lies partly in the source of the pumice 

deposits, because some pumice was erupted not by Mount 

Rainier but by other volcanoes in the Cascade Range of 

Washington and Oregon and brought to the park by strong 

southerly or southwesterly winds. The layers of pumice 

thrown out by Mount Rainier within the last 10,000 years 

lie mostly on the east side of the volcano. Strong winds 

evidently swept eruption clouds to the east during the outbursts and prevented the pumice from falling west of the 

volcano. This pattern of distribution, coupled with the coarsening and thickening of the pumice toward the volcano, 

reveals that the layers were erupted by Mount Rainier.

12
AN OLD LAVA FLOW from Mount Rainier which forms Rampart Ridge west 

of the meadow at Longmire. The thick lava flowed down an old valley 

floor and cooled and solidified. Rivers then eroded new valleys along 

both sides of the flow. These new valleys, subsequently glaciated, are 

today followed by the Nisqually River and Kautz Creek. Thus, the area 

of a former valley floor is now a ridge. (Fig. 5)

W f"

COLUMNS OF DARK-GRAY ANDESiTE at the east end of an old lava 

flow from Mount Rainier. This outcrop is near the point at which the 

highway to Yakima Park crosses Yakima Creek. (Fig. 6)

13
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 southLAYERS OF PUMICE on the floor of a cirque near Paradise Park. The 

yellow bed at the bottom is layer 0, which was erupted by Mount 

Mazama volcano at the site of Crater Lake, Oregon, about 6,600 years 

ago. The yellowish-brown layer a few inches above layer 0 is layer D, 

a pumice that was erupted by Mount Rainier between 5,800 and 6,600 

years ago. The light-yellowish-brown pumice bed at the top of the outcrop is layer Y, which originated at Mount St. Helens volcano between 

3,250 and 4,000 years ago. Photograph by D. R. Mullineaux, U.S. Geological Survey. (Fig. 7)

14
ward, 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.

Mount St. Helens as it appears from Mount Rainier.

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

15
M

Layer X 

(Between 110 and 150 years old)

Layer C 

(Between 2,150 and 2,500 years old)

Layer D 

(Between 5,800. and 6,600 years old)

Layer L 

(Between 5,800 and 6.600 years old)

Layer R 

(More than 8,750 years old) Layer W (line pattern), and

Layer Y (stipple pattern)

(About 450 years old and 3,250 to

4,000 years old, respectively)

10 MILES

16
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 Common 

thickness in park range in

West 

Pumice side 

layer (inches)

X...... Absent

W.....O-1

C..... ..Absent

Y......5-20

D...... Absent

L...... .Absent

0......1-3

R...... Absent

East of pumice 

side fragments 

(inches) (inches)

1

1-3

1-8

1-5

0-6

0-8

1-3

0-5

' y*-2

Medium 

sand

V4-8

Coarse 

sand

Vi-6

V4-2

Flourlike to 

fine sand

Vs-1

Color

Light 

olive 

gray

White

Brown

Yellow

Brown

Brown

Yellowish 

orange

Reddish

brown

Source

Mount 

Rainier.

Mount 

St. Helens.

Mount 

Rainier.

Mount 

St. Helens.

Mount 

Rainier.

Mount 

Rainier.

Mount 

Mazama.

Mount 

Rainier.

Approximate age 

in 1968, or 

limiting dates 

(years ago)

100-150

2450

2,150-2,500

3,250-4,000

5,800-6,600

5,800-6,600

About 6,600

8,750-11,000?

ui y.iov yeuis Him 101 vuriau 

cation to Meyer Rubin, 1968).

<]GENERALIZED DISTRIBUTION of some pumice layers within Mount 

Rainier National Park. The pumice of layers W and Y was erupted by 

Mount St. Helens; all the other pumice originated at Mount Rainier. 

Letters represent the following localities: C, Cougar Rock campground; 

I, Ipsut Creek campground; L, Longmire; M, Mowich Lake; 0, Ohanapecosh campground; P, Paradise Park; S, summit crater; T, Tipsoo Lake; 

W, White River campground; and Y, Yakima Park. Based on studies by 

D. R. Mullineaux. (Fig. 8)

17
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

18
A LARGE BREADCRUST BOMB enclosed in a mudflow deposit that consists of a mixture of volcanic ash and rock fragments. The outcrop is 

on the south bank of the White River about half a mile downstream 

from the White River campground. (Fig. 9)

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

19
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 Paricutin

PUMICE LAYER C, which consists of light-brown fragments, lies at the 

ground surface over much of the eastern part of the park. (Fig. 10)

20
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

THE SNOW-COVERED LAVA CONE lies in a depression 1V4 miles wide at 

the summit of the volcano. The cone was probably built about 2,000 

years ago. Liberty Cap is to the left and Point Success is to the right. 

The cliffs below and to the right of Liberty Cap enclose Sunset Amphitheater. (Fig. 11)

21
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 Fremont, 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.

22
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.

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

TWO ICE STREAMS meet to form the half-mile-wide Cowlitz Glacier. One 

heads on the flank of the volcano and the other (Ingraham Glacier) at 

the summit. The firnline is a short distance above the junction of the 

glaciers. The high bare embankment at the extreme right is a lateral 

moraine that was formed about 100 years ago when the glacier was 

thicker and about \Vz miles longer. (Fig. 12)

LITTLE TAHOMA 

PEAK
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 depth1 .

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 altiJ For more information about glaciers read "Glaciers" by Robert P. 

Sharp, published in 1960 by the University of Oregon at Eugene.

24
tudes 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

25
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.

GLACIER-SMOOTHED and grooved rock along the Wonderland Trail between Indian Bar and Panhandle Gap. (Fig. 13)

26
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

A MUDDY GRAYISH-BLUE LAKE several hundred feet long lies behind 

a small horseshoe-shaped end moraine of Flett Glacier, on the northwest 

side of Mount Rainier. The glacier is mostly out of view to the left. 

(Fig. 14)

27
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.

FOUR CURVED RECESSIONAL MORAINES are spread over a distance of 

2,000 feet on the valley floor of Fryingpan Creek. They were formed 

within the last few hundred years as Fryingpan Glacier lost volume and 

shrank back toward its present position above a line of cliffs. (Fig. 15)

28
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

29
the Cowlitz River valley, for example, extended 65 miles 

from the volcano and reached a point about 33 miles west 

of the community of Handle. Deposits *of the younger of 

these two glacial episodes can be seen in cuts along the 

Mowich Lake Road for a distance of about li/2 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.

122°

10 15 MILES

EXTENT OF GLACIERS in the Cascade Range near Mount Rainier between 

about 15,000 and 25,000 years ago. Arrows show the direction of ice 

movement; solid black represents modern glaciers on Mount Rainier. 

(Fig. 16)

30
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.

LATERAL MORAINE of rock debris at Ricksecker Point. It was formed by 

Nisqually Glacier when the glacier was at least 1,000 feet thick and 

about 15 miles longer than it is today. (Fig. 17)

31
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 mid19th century. Oddly enough, even though all the glaciers

32
ROCK-GLACIER DEPOSIT (light-gray rubble beyond the brown slopes in 

the foreground) at The Palisades, which was formed about 11,000 years 

ago when the climate was colder than it is today. Rocks fell from the 

cliffs in such great quantity that a small glacier in front of the cliffs 

consisted of more rock debris than ice. The melting of the ice left a 

mass of broken rock several hundred feet thick which covers about 

80 acres. (Fig. 18)

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.

33
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

THE HUMMOCKY END MORAINE at the left still had blocks of ice buried 

in it when this picture was taken in 1954. The front of Emmons Glacier 

was near the left edge of the bare moraine in about 1900. Now the 

glacier ends 1 mile farther upvalley at the upper right. The valley floor 

and moraine were buried by an avalanche of rock debris from Little 

Tahoma Peak in 1963. (Fig. 19)

34
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

35
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 stillundamaged 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 sharpcrested 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.

36
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

AVALANCHE DEPOSITS in the White River valley. The rockfalls and 

avalanches from Little Tahoma Peak formed a mass of reddish-gray 

rock debris that contrasts with the darker gray glacial debris deposited 

by Emmons Glacier within the last century. The avalanche deposits are 

about 1,500 feet across at their widest point. (Fig. 20)

37
million cubic yards or roughly enough to cover a 1-milesquare 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.

38
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.

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)

39
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

40
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"

-10,000-

Pleistocene 

(Ice Age)

-25,000-

-2-3 millionPliocene

-12 millionMiocene

-26 million

Oligocene

-37-38 millionEocene

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.

Last major glaciation.

Birth and growth of Mount 

Rainier volcano, and repeated 

glaciation.

Uplift and erosion of the Cascade Range.

Intrusion of granodiorite.

Folding of older rocks.

Deposition of Fifes Peak and 

Stevens Ridge Formations.

Deposition of Ohanapecosh Formation.

Deposition of Puget Group.

-53-54 million41
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.

Avalanches and mudflows like those described are normal 

events at Mount Rainier and are expected to happen again 

in the future. Almost any cliff on the volcano can produce a 

large rockfall, but which cliff will collapse next, or when, 

cannot be predicted. Should the volcano again become active, 

earthquakes and volcanic explosions would trigger avalanches and mudflows that would rush down the mountain. 

Molten rock would melt snow and ice at the volcano's summit and send floods of water down the volcano's flanks. These 

indirect effects of an eruption would be much more hazardous than lava flows and pumice, if eruptions are on a scale 

similar to that of the past 10,000 years.

The Volcano's Future?

An active volcano changes continually. Repeated eruptions build the cone by piling one lava flow on top of others, 

or on top of other volcanic formations. Simultaneously, the 

combined processes of erosion wear the volcano down. The 

relative importance of the two processes one building, the 

other destroying is reflected in the volcano's shape. The 

scarred and deeply gouged sides of Rainier's cone show that 

erosion has been dominant here for a long time. Is Mount 

Rainier now doomed to continued piecemeal destruction until 

the lofty cone is reduced to a featureless mound? Will future 

eruptions of lava restore some of the volcano's bulk ? Or will 

the volcano erupt violently some day, and then collapse as 

did Mount Mazama to form the deep basin of Crater Lake ? 

The answers may not be known for centuries or they may 

appear tomorrow.

42
Further Reading in Geology

Crandell, D. R., 1969, Surficial geology of Mount Rainier 

National Park, Washington: U.S. Geological Survey 

Bulletin 1288. A geologic map that shows where glacial 

deposits, landslides, and mudflows are located in the 

park is accompanied by an illustrated nontechnical 

description of these and other surficial deposits.

Crandell, D. R., and Fahnestock, R. K., 1965, Rockfalls and 

avalanches from Little Tahoma Peak on Mount Rainier, 

Washington: U.S. Geological Survey Bulletin 1221-A, 

30 pages. A description of the seven successive landslides of December 1963 that buried the upper White 

River valley under thick deposits of rock debris.

Crandell, D. R., and Mullineaux, D. R., 1967, Volcanic hazards at Mount Rainier, Washington: U.S. Geological 

Survey Bulletin 1238, 26 pages. A discussion of Mount 

Rainier's eruptions during the last 10,000 years and 

the anticipated effects of similar future eruptions.

Fiske, R. S., Hopson, C. A., and Waters, A. C., 1964, Geologic 

map and section of Mount Rainier National Park, Washington: U.S. Geological Survey Miscellaneous Geologic 

Investigations Map 1-432, with text. A geological map 

of the park's bedrock is accompanied by a brief nontechnical discussion of the geological ^volution of the 

park as recorded by the rock formations.

Sigafoos, R. S., and Hendricks, E. L., 1961, Botanical evidence of the modern history of Nisqually Glacier, 

Washington: U.S. Geological Survey Professional Paper 

387-A, 20 pages. A description of the recent moraines 

of several glaciers and an explanation of how they are 

dated by counting the growth rings of trees growing 

on them.

43

 & U.S. GOVERNMENT PRINTING OFFICE: 1969 O 353-560
Crandeil-THE GEOLOGIC STORY OF MOUNT RAINIER-Geolo