PINE CREEK

VOLCANIC ASSEMBLAGE 

AT MOUNT ST. HELENS, 

WASHINGTON

SURVEY BULLETIN 1383-A

PINE CREEK VOLCANIC ASSEMBLAGE 

AT MOUNT ST. HELENS, WASHINGTON
The flat surface of the Pine Creek assemblage in the middle distance slopes 

away from the southeast side of Mount St. Helens. Part of the east bedrock 

valley wall of the Pine Creek valley is visible in the right foreground.
Pine Creek Volcanic Assemblage 

at Mount St. Helens, 

Washington

By DWIGHT R. CRANDELL and DONAL R. MULLINEAUX

GEOLOGY OF MOUNT ST. HELENS 

VOLCANO, WASHINGTON

GEOLOGICAL SURVEY BULLETIN 1383-A

A study of a valley-fill assemblage of 

pyroclastic-flow deposits, lahars, 

and alluvium of postglacial age 

southeast of Mount St. Helens volcano

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1973
DEPARTMENT OF THE INTERIOR 

WILLIAM P. CLARK, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

First printing 1973 

Second printing 1984

For sale by the Distribution Branch, U.S. Geological Survey, 

604 South Pickett Street, Alexandria, VA 22304
CONTENTS

Page

Abstract Al 

Introduction - 1

 - ---- "« --" ---- '"""""""' ""' ""- -T- -- - " ~^ * -'- *~"- i- -" - »" tj

-. - __ ______ _________ ___ __________________________________ « __________ 7

Description of the Pine Creek assemblage

Pre-J unit of the Pine Creek assemblage 10

Post-J, pre-Y unit of the Pine Creek assemblage 13

Post-Y unit of the Pine Creek assemblage 13

Ash-cloud deposits - 18

Volume of the Pine Creek assemblage- 20

Character of old Mount St. Helens in Pine Creek time 20

v oicdjiic iiflz£ircis~" - "" --" - - -----« « . «* ».»«.»-»»» &£*

References - 22

ILLUSTRATIONS

Pag.?

FRONTISPIECE. The flat surface of the Pine Creek assemblage slopes away 

from the southeast side of Mount St. Helens.

FIGURE 1. Index map of the Mount St. Helens region ~ A3

2. Map showing extent of the Pine Creek assemblage 4

3. Diagrammatic section showing tephra layers in set P 9

4. Section showing stratigraphic sequences of deposits in the

Pi TIP C~^Y*Gpk 3.sspinblfifirp~_ _ __ __ .- . --. . _....._..._.......... 11

5. Section showing stratigraphic sequences of deposits exposed

in the Pine Creek valley . 14

6. Photograph of pyroclastic-flow deposits 15

7. Photograph of lahars interbedded with fluvial deposits- - ^

TABLES

Page 

TABLE 1. Age and predominant Fe-Mg phenocryst content of tephra sets- A8

2. Radiocarbon dates pertinent to the age of the Pine Creek assemblage and associated tephra deposits- - - ~,~~ 8

3. Sequence of events at Mount St. Helens and in the adjacent 

region, as indicated by deposits in the area southeast of 

the volcano -----~ ~~~~ ~~~~...._ .. ._.... 21

GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

PINE CREEK VOLCANIC ASSEMBLAGE AT 

MOUNT ST. HELENS, WASHINGTON

By DWIGHT R. CRANDELL and DONAL R. MULLINEAUX

ABSTRACT

The Pine Creek volcanic assemblage consists of the deposits of hot pyroclastic 

flows and lahars that are interbedded with alluvium. The assemblage was formed 

during two or more postglacial episodes of explosive andesitic and dacitic volcanism 

at an old Mount St. Helens volcano which coincided with, but predated, the modern 

cone. The assemblage forms a sloping flat-topped to convex valley fill locally more 

than 100 meters thick and 1-4 km (kilometers) wide. The fill extends from the base of 

the modern volcano, where it emerges from under a broad apron of younger lahars, 

lava flows, and alluvium, 16 km southeastward to the Lewis River valley and thence 

70 km downstream. In the Lewis River valley, however, it is mostly concealed by 

three large reservoirs. Hot pyroclastic-flow deposits in the assemblage are found as 

far as 16 km from the volcano, lahars reached at least 55 km downvalley, and the 

Lewis River transported volcanic rock debris downstream to its confluence with the 

Columbia River. Clouds of ash accompanying the hot pyroclastic flows formed deposits 5-100 centimeters thick and 1-3 km wide which are interbedded with tephra 

along both sides of the Pine Creek assemblage as far as 11 km from the volcano. 

Radiocarbon dates indicate that the oldest Pine Creek eruptive episode occurred before about 12,000 years ago and the youngest between 3,000 and about 2,500 years 

ago.

INTRODUCTION

Mount St. Helens is a late Quaternary stratovolcano in southern 

Washington 70 km (kilometers) northeast of Portland, Oreg. (fig. 1). 

The modern cone is made up largely of olivine basalts and pyroxene andesites through which several dacite domes have been 

erupted, one of which forms the present summit (Verhoogen, 1937; 

Hopson, 1971). The modern cone overlaps an older eruptive center 

which included several dacite domes and which is characterized by 

pale-red hornblende dacite and gray hornblende-hypersthene 

andesite. Verhoogen (1937, p.268) referred to these rocks as the "old 

Mount St. Helens series." The main purpose of this report is to describe a valley-fill assemblage of pyroclastic flow deposits, lahars, 

and alluvium which was formed during two or possibly three eruptive episodes of the old Mount St. Helens of Verhoogen in late PleisAl
A2 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

tocene and Holocene time. The Pine Creek assemblage, which is the 

informal name we will use for the deposits of this valley fill and 

genetically related ash-cloud deposits, provides examples of how 

some types of volcanism can affect valleys which head at a 

volcano, and illustrates some kinds of hazards which could accompany similar volcanism in the future.

This report is the first chapter in a Geological Survey bulletin on 

various aspects of the geology and eruptive history of Mount St. 

Helens volcano. Jack H. Hyde, Tacoma Community College, 

Tacoma, Wash., who is currently working on the south and southwest sides of Mount St. Helens, has provided us with unpublished 

information concerning the stratigraphic sequence in those areas. 

Clifford A. Hopson, University of California at Santa Barbara, has 

contributed unpublished data based on his current geologic mapping of Mount St. Helens and the adjacent region.

Mount St. Helens is drained on the north by the North Fork of the 

Toutle River, on the west by the South Fork of the Toutle and by the 

Kalama River, and on the east and south by tributaries of the Lewis 

River (fig. 1). The major tributaries that drain the east and southeast sides are Smith Creek, Muddy River, and Pine Creek. Smith 

Creek occupies a bedrock valley that lies east of and predates 

Mount St. Helens (fig. 1). Relatively little volcanic rock debris has 

been transported into the upper part of the Smith Creek valley because of a bedrock interfluve between it and the volcano, and it is 

still deep and V shaped. Pine Creek occupies the next bedrock 

valley west of Smith Creek. However, the old Pine Creek valley has 

been partly filled with rock debris from Mount St. Helens, and Pine 

Creek now flows through a relatively narrow and shallow trench 

cut into the fill. In the past, the Muddy River also flowed down the 

Pine Creek valley; now, however, its upper reach is along the east 

margin of the fill in the Pine Creek valley, and then it passes 

through a low saddle in the interfluve and joins Smith Creek. Smith 

Creek was mainly responsible for the erosion of the valley that is 

now followed by the lower 14 km of the Muddy River; thus, the entire length of the valley east of the Pine Creek-Smith Creek 

interfluve will be referred to as the Smith Creek valley.

The eastward diversion of the Muddy River into the Smith Creek 

valley evidently occurred when aggradation of the Pine Creek 

valley by the Pine Creek assemblage raised the valley floor to the 

level of a low saddle in the interfluve. Ape Canyon Creek (fig. 2) also 

was once a tributary of Pine Creek but was forced to cross a relatively low point on the interfluve by a lava flow that is younger 

than the Pine Creek assemblage.

The Pine Creek assemblage is mainly a valley fill of uncon-
PINE CREEK VOLCANIC ASSEMBLAGE A3

122°

'Spirit Lake

5 10 MILES

i i I_______I

0 5 10 KILOMETERS

!,,,,! I

FIGURE 1. Index map of the Mount St. Helens region. Bedrock interfluve between Pine Creek and Smith Creek valleys is shown by chevron pattern.

solidated volcanic rock debris 1-4 km wide and in places more than 

100 m (meters) thick; its surface slopes about 70 m/km along Pine 

Creek immediately southeast of Mount St. Helens. The fill extends 

from the base of the modern volcano, where it emerges from under a 

broad apron of younger lahars, lava flows, and alluvium, along 

Pine Creek to the Smith Creek valley, where it forms Cedar Flats 

(fig. 2). Remnants of the fill extend on down the Lewis River to 

Woodland, which is near the confluence of the Lewis and Columbia 

Rivers (fig. 1). In the Lewis River valley, however, the fill is now 

largely concealed by water in three large reservoirs.

The part of the fill that is within about 15 km of the volcano consists largely of the deposits of hot pyroclastic flows. Lahars and 

coarse fluvial gravels are volumetrically less important here, but 

they predominate at greater distances from the volcano. The 

assemblage is represented only by fine-grained fluvial deposits in 

the part of the Lewis River valley that is downstream from Lake 

Merwin (fig. 1).
122"00'

46° 10'

EXPLANATION

| ] LAHARS AND ALLUVIUM

PINE CREEK 

ASSEMBLAGE: 

Mid-P ash-cloud deposit

Post-J, pre-B unitsPre-J unit

(A) LOCALITY DISCUSSED

IN TEXT

O O

s

O OO C!

2!

CO

<OKCOffi

O

FIGURE 2. Extent of the Pine Creek assemblage in the area southeast of Mount St. Helens.
PINE CREEK VOLCANIC ASSEMBLAGE A5

Lithologically similar lahars which are apparently only a few 

hundred years younger than the youngest part of the Pine Creek 

assemblage are present in some of the other valleys that head at the 

volcano namely, the North and South Forks of the Toutle River 

(Mullineaux and Crandell, 1962), the Kalama River (Hyde, 1970), 

and Swift Creek (J. H. Hyde, oral commun., 1972).

A volumetrically small part of the Pine Creek assemblage consists of beds of ash which occur in areas adjacent to the valley-fill 

deposits. These beds are thought to be the deposits of clouds of ash 

that accompanied hot pyroclastic flows, or to be the deposits of 

such flows.

TERMINOLOGY

The term "pyroclastic-flow deposit" will be used in this report to 

describe the deposit of a hot dry flow of vesicular or nonvesicular 

volcanic rock debris that is mobilized chiefly by gravity and hot air 

trapped within the debris, or by hot gases emitted by the rock 

debris, or by both. Lithic pyroclastic-flow deposits consist almost 

entirely of nonvesicular rock debris and probably were mobilized 

chiefly by air that was trapped and heated, although hot gas may 

also have been emitted by the rock fragments. Pumiceous pyroclastic-flow deposits consist largely of pumice fragments of a wide 

size range, and these may have been mobilized mostly by hot gases 

emitted by the constituent pumice particles during movement.

Tephra, a term introduced by Thorarinsson (1954, p. 1-3), was 

defined as fragmental volcanic debris which was transported from 

the crater through the air. Thus, it serves as a general term which 

does not denote properties of composition, vesicularity, or grain 

size as do most other terms for volcanic clastic material. Most 

material described as tephra in this report was erupted high into 

the air over the volcano, carried laterally primarily by wind, and 

then fell back to the land surface at a high angle. Some of these 

tephra deposits consist chiefly of vesicular material, others are 

composed mostly of nonvesicular (lithic) fragments, and still 

others contain major proportions of both kinds of particles. Near 

the vent, the deposits may have either a wide range or a limited 

range in grain size. Downwind, they form elongate lobes in which 

the deposits become progressively thinner and the particles are 

finer and better sorted with increasing distance from the volcano. 

The deposits also thin rapidly from the central axes of the lobes toward the margins.

Still another group of deposits seems to have had a composite 

origin; they could be considered tephra in part, but they seem to 

consist chiefly of fine-grained pyroclastic flows and dust from such
A6 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

flows. We have not found a term that is entirely appropriate for this 

group, although we wish to distinguish it from other tephra deposits. For the purpose of this report, therefore, we will use the term 

"ash-cloud deposit" for beds of material deposited from clouds of 

airborne ash whose origin was associated with hot pyroclastic 

flows. Some ash-cloud deposits may have been formed directly by 

sheetlike or tonguelike pyroclastic flows of mostly ash-sized 

material which flowed down the side of the volcano and across the 

landscape as broad overwhelming density currents, to some degree 

independent of topography. Others probably fell from dust clouds 

rising above hot pyroclastic flows that moved mostly along topographic depressions in contact with the underlying ground surface. The ash-cloud deposits consist chiefly of nonvesicular ash a 

few centimeters to a meter thick. They commonly display bedding 

that suggests rapid lateral transport during deposition. They may 

decrease in thickness away from the axis of the valley down which 

hot pyroclastic flows traveled, but commonly they do not decrease 

as noticeably as does most tephra in thickness and grain size with 

distance from the source vent.

Lahars are flows of rock debris, mobilized by water, which originated on the slopes of a volcano. They may carry either hot or cold 

rock debris. Lahars generally show a rude vertical size gradation 

from coarse to fine toward the top, which implies a mudflow origin 

(Mullineaux and Crandell, 1962; Crandell, 1971, p. 6-7).

Differentiation of lahars from pyroclastic-flow deposits is 

generally difficult, but a few criteria are helpful. A deposit in which 

individual rock fragments all have a preferred direction of remanent magnetism is inferred to have been formed by a pyroclastic 

flow because the ferromagnetic minerals in the fragments must 

have still been above their Curie temperatures when the deposit 

came to rest (Aramaki and Akimoto, 1957; Crandell, 1971, p. 5). 

Conversely, a coarse unsorted deposit of volcanic rock debris in 

which rock fragments have random directions of remanent magnetism is probably a lahar. A brick-red to reddish-gray color in the 

uppermost 1-5 m also suggests that a deposit was hot when it came 

to rest, and therefore is probably of pyroclastic-flow origin. Such a 

red zone at the top of ash flows has been mentioned by Smith (1960, 

p. 831). Williams (1960, p. 13) also described "glowing avalanche" 

deposits in Guatemala in which the gray color "changes to pink 

near the top, in some places, owing to sublimation of finely divided 

hematite from fumarolic vapors given off by the ejecta while they 

were still hot." One such deposit described by Williams (p. 35) has a 

pink zone 4.5 m thick and also contains fragments of carbonized 

wood. Williams (p. 67) noted that "The presence of a pink top
PINE CREEK VOLCANIC ASSEMBLAGE A7

strongly suggests an [glowing] avalanche deposit, but its absence 

has no diagnostic value."

A few deposits in the Pine Creek assemblage which have reddish tops were found to contain rock fragments having randomly 

oriented directions of remanent magnetism. These rock fragments 

may have cooled below the Curie temperature while being transported by pyroclastic flows. Other deposits, in which some rock 

fragments have a preferred orientation of remanent magnetism, 

lack reddish zones at their tops. Some of these deposits were formed 

by pyroclastic flows, but others may have been formed by lahars 

which were carrying hot rock fragments.

Units in the Pine Creek assemblage which are texturally similar 

to lahars or pyroclastic-flow deposits, but which lack features from 

which their origin can be inferred, are designated in this report as 

"flowage deposits."

TEPHRA

Mount St. Helens has erupted tephra repeatedly during the last 

few tens of thousands of years before, during, and after the formation of the Pine Creek assemblage. Thus, the assemblage can be 

separated from other deposits by its stratigraphic position relative 

to tephra, and tephra layers interbedded with the assemblage permit it to be subdivided, as well.

The tephra deposits are divided into several groups, or sets; each 

set consists of several layers that contain similar suites of Fe-Mg 

phenocrysts and that differ in Fe-Mg mineral content from the 

tephra in underlying and overlying sets. Each of the sets listed in 

table 1, except set T, contains more than one layer. The Pine Creek 

assemblage was deposited after the eruption of tephra set S and before the eruption of set B.

Tephra set S consists of several beds of coarse pumice in which 

cummingtonite and hornblende are abundant (table 1). All the 

layers in the set probably were erupted during a relatively short 

period of time, between limiting dates of about 18,000 and 12,000 

years ago (table 2). Their actual age, however, may not be close to 

either limiting date. A soil profile was formed on the unnamed 

tephra deposits after 18,000 years ago but before set S was erupted, 

and another soil profile developed on set S before set J was deposited.

Tephra set J includes two coarse pumice layers and one distinctly 

finer bed. No Fe-Mg minerals other than hypersthene and 

hornblende have been found in this set. Set J is similar in general 

appearance to set S and commonly must be distinguished by 

mineral content. The set is bracketed by radiocarbon dates of about 

12,000 and 8,000 years (table 2).
A8 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON 

TABLE 1. Age and predominant Fe-Mg phenocryst content of tephra sets

[Data mostly from Mullineaux, Hyde, and Rubin (1972)]

Set

T

W

g

p

j

o ____________________ _____ _ _____

Approximate age (years)

150-200

450 

< 2,500, >450

3,000-2,500(7) 

4,000-3,000 

< 12,000, > 8,000 

< 18,000, >12,000 

> 35,000-18,000

Predominant Fe-Mg 

phenocrysts

Hypers thene, 

hornblende, and 

augite. 

Hypersthene and 

hornblende. 

Olivine, 

augite, and 

hypersthene. 

Hypersthene and 

hornblende. 

Cummingtonite and 

hornblende. 

Hypersthene and 

hornblende. 

Cummingtonite and 

hornblende. 

C ummingtonite , 

hornblende, and 

biotite.

Tephra set Y includes 10 or more coarse pumice beds, all of which 

contain Cummingtonite and hornblende. Set Y pumice layers 

generally can be identified by their coarse grain size and young 

age. Two layers within set Y are known to be very extensive; one 

forms a northeast-trending lobe which was first described in Mount 

Rainier National Park (Crandell and others, 1962) as "ash layer 

Y"; the other extends southeast from Mount St. Helens. The Y 

tephra that is associated with the Pine Creek assemblage is mostly 

that of the southeast-trending lobe.

The age of set Y as a whole ranges from about 4,000 to 3,000 years. 

The two most extensive layers in it are closely bracketed by radiocarbon dates of 3,500 and 3,350 years at Mount St. Helens (table 2).

Tephra set P consists of several thin pumice-bearing layers 

which also include a high proportion of lithic and crystal fragTABLE 2. Radiocarbon dates pertinent to the age of the Pine Creek assemblage 

and associated tephra deposits

[All dates were determined by Meyer Rubin in the radiocarbon laboratory of the U.S. Geol. Survey]

Date (years 

before present)

1 ,740+250 - 

9'fi70-l-9Sft

2 850±250

3|350±:250 -----

^ QftO J f)cif\

11,800±300 -----

18's60i550 '

Laboratory

No.

W-2527 

W-1394 

W-2529 

W-2541 

W-2542 

W-2675 

W-2549 

W-1752 

W-2677 

W-2587 

W-2655 

W-2441 

W-2413

Kind of sample and stratigraphic 

position

Charcoal from within tephra set B. 

Wood from above or within upper part of tephra set P.

Do. 

Do. 

Charcoal from base of tephra set P. 

Charcoal from within tephra set Y. 

Do. 

Charcoal from base of tephra set Y. 

Charcoal from above or within upper part of tephra set J. 

Wood from lahar beneath tephra set J. 

Charcoal from ash beneath tephra set J. 

Charcoal from pyroclastic-flow deposit beneath tephra set S.
PINE CREEK VOLCANIC ASSEMBLAGE A9

ments. The pumice and lithic fragments contain only hypersthene 

and hornblende as Fe-Mg phenocrysts and can thereby be distinguished from tephra of set Y. Hypersthene in set P has a lower refractive index than that in set W; thus, the two sets can be distinguished even though they have a similar mineralogy.

Set P contains a coarse, poorly sorted bed of pumice and lithic 

fragments at its base and one or two finer, well-sorted beds in its 

lower part (fig. 3). Reddish- to brownish-gray sand and silt beds of 

undetermined origin generally separate the tephra layers and are 

thickest near the middle of the set. One to four fine, well-sorted 

tephra layers, similar to those in the lower part, lie above the thick 

sand and silt. The uppermost bed of the set consists of yellowishgray pumiceous lapilli and ash.

The older age limit of the set is established by a radiocarbon date 

of about 3,000 years at its base (table 2). At Mount St. Helens, the 

closest limiting date above the tephra is one of about 1,700 years in 

the overlying set B. A date of about 2,400 years was obtained at 

Mount Rainier from wood above the upper fine-grained, well-sorted 

beds of set P. Whether or not the wood is younger than the topmost 

bed of the set at Mount St. Helens, however, is not known.

Set B consists mostly of layers of olivine-bearing scoria and is 

readily distinguished by its overall brown color and abundant dark 

glass. Minor variations in mineral content allow the set to be subdivided into lower and upper units. The set is dated by its stratigraphic position relative to sets P and W and by a single radioTopmost 

layer

Upper

fine-grained

layers

EXPLANATION

TEPHRA DEPOSITS: 

Pumice lapilli size or larger

Lithic fragments lapilli size 

or larger

DEPOSITS OF UNDETERMINED ORIGIN: 

Sand

r^-~ silt

Lower

fine-grained

layers

Basal 

layer

FIGURE 3. Diagrammatic section showing tephra layers in set P.
A10 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

carbon date of about 1,700 years from thin sand and silt beds that 

lie between the lower and upper units of the set.

DESCRIPTION OF THE PINE CREEK ASSEMBLAGE

The Pine Creek volcanic assemblage originated during two or 

possibly three episodes of explosive dacitic and andesitic volcanism. The assemblage is divided into three principal units by their 

stratigraphic relation to the tephra sets, as shown below:

Tephra set B 

Post-Y unit (contemporaneous with tephra set P)

Tephra set Y 

Post-J, pre-Y unit

Tephra set J 

Pre-J unit

Tephra set S

These tephra sets extend far beyond the limits of the Pine Creek 

assemblage and are not regarded as a part of the assemblage where 

they are interbedded with it.

PRE-J UNIT OF THE PINE CREEK ASSEMBLAGE

The oldest part of the Pine Creek assemblage consists of lithic 

pyroclastic-flow deposits and lahars which are younger than 

tephra set S and older than set J. The deposits are well exposed near 

the confluence of the Muddy and Lewis Rivers and can be seen in 

scattered outcrops down the Lewis River valley in segments between the three reservoirs. The unit can be traced only a short distance up the Pine Creek valley because it is covered by younger 

units of the Pine Creek assemblage. The pre-J unit forms a fan at 

the mouth of the Pine Creek valley which spreads across the lower 

Smith Creek valley and forces the Muddy River against the east 

valley wall. The surface of the northern part of this fan is called 

Cedar Flats. A section through the fan deposits is exposed in cuts 

along an abandoned logging road 0.6 km north of the mouth of the 

Muddy River (loc. A, fig. 2). The sequence of deposits exposed at 

locality A and the sequence in an outcrop at the front of a terrace 

cut into the Cedar Flats fan about 2 km farther north (loc. B, fig. 2) 

are shown in figure 4. The pink-topped pyroclastic-flow deposit beneath tephra set J at locality A is unweathered at the top.

The deposits shown in figure 4 as of pyroclastic-flow origin are 

unsorted mixtures of fragments of nonvesicular andesite and 

dacite as large as 1 m in diameter in a compact matrix of gray silty 

sand. These deposits are 1-5.5 m thick. Units designated as lahars 

are lithologically similar to the pyroclastic-flow deposits and are 

0.5-7 m thick. Both kinds of deposits are generally bounded by
PINE CREEK VOLCANIC ASSEMBLAGE All

Altitude

(feet) 

1350

WEST EAST

1150

1000

LOCALITY B

FIGURE 4. Stratigraphic sequences of deposits in the Pine Creek assemblage 

exposed near the mouth of the Muddy River (loc. A, fig. 2) and about 2 

km farther north (loc. B, fig. 2). Stipple pattern indicates covered intervals.

nearly horizontal contacts, along some of which can be found 

lenses of silt and very fine sand 1-4 cm (centimeters) thick. The 

only deposit of fluvial origin at locality A is a single lens of pumicelapilli gravel 40 cm thick.

At locality A the upper surface of the pre-J part of the Pine Creek 

assemblage lies about 75 m above the Muddy River, and the valley 

fill seems to have temporarily blocked the Lewis River at least once. 

Remnants of a lahar backfill are found along the Lewis River 

valley for about 8 km upstream from the mouth of the Muddy River, 

and they veneer terraces of glacial outwash sand and gravel. The 

backfill extends about 5 km upvalley from the terminal position
A12 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

reached by the Lewis River glacier in late Pleistocene time (fig. 2). 

Comparison of profiles of weathering in till in the terminal moraine 

with those in till of alpine glaciers in the Mount Rainier region 

(Crandell, 1969, p. 18) suggests that the terminal moraine was 

formed during the Evans Creek Stade of the Fraser Glaciation. 

Several tephra layers of set S, which is less than 18,000 years old, lie 

between the deposits of the backfill and the outwash sand and 

gravel.

The lahar backfill deposits become finer in grain size up the 

Lewis River valley. At a point 4 km upvalley from the Muddy River, 

a lahar of the backfill is more than 3 m thick and consists of subangular rock fragments mostly 2-5 cm in diameter in a gray sand 

matrix. The deposit also contains scattered dacite rock fragments 

as large as 0.7 m across and grades to coarse sand and granule 

gravel at the top. Two kilometers farther upstream the lahar 

backfill is finer in texture and consists mostly of unstratified coarse 

sand and granule gravel.

The pre-J part of the Pine Creek assemblage has been recognized 

along the Lewis River valley between Swift Dam and Cougar and 

between Yale Dam and Speelyai Bay (fig. 1). In both of these areas 

the pre-J deposits are chiefly lahars. The top of the assemblage is 

about 60 m above the prereservoir valley floor at Cougar.

The presence of wood that is about 11,800 years old in the pre-J 

deposits at locality B limits the age of the older part of the Pine 

Creek assemblage. The wood-bearing lahar seems to be part of a 

somewhat younger fill in a canyon that was entrenched into the 

Cedar Flats fan after the units that make up the fan were deposited. The lahar and alluvium that form the younger fill evidently moved down the Smith Creek valley.

The 11,800-year date suggests that the lahar occurred during the 

Everson Interstade of the Fraser Glaciation, which lasted from 

about 13,000 to 11,000 years ago in the Fraser Lowland of northwest Washington and southwest Canada (Armstrong and others, 

1965). It has been suggested that alpine glaciers in the Olympic 

Mountains of western Washington reached their maximum extents and began to retreat before 15,000 years ago, and alpine 

glaciers in the Cascade Range probably behaved in the same way 

(Crandell, 1965, p. 345-348). The oxidized zone at the top of the outwash sand and gravel beneath the lahar of the Lewis River backfill records a time interval between deposition of the outwash and 

arrival of the lahar; this suggests that the pre-J part of the Pine 

Creek assemblage was formed not much more than 12,000 years 

ago. The near-absence of wood in that part of the assemblage may 

reflect a lack of dense forests at these altitudes in late-glacial time.
PINE CREEK VOLCANIC ASSEMBLAGE A13 

POST-J, PRE-Y UNIT OF THE PINE CREEK ASSEMBLAGE

Deposits of the Pine Creek assemblage that are younger than 

tephra set J but older than set Y have been recognized only in the 

lower valley of Pine Creek and on the Cedar Flats fan. A borrow-pit 

exposure on the north side of Pine Creek at locality C (fig. 2) shows 

layer Y on top of about 0.5 m of yellowish-brown fluvial sand and 

gravel. Beneath the gravel is a lithic pyroclastic-flow deposit more 

than 12 m thick. This deposit consists of pebble- to boulder-sized 

fragments of nonvesicular dacite in a very compact matrix of gray 

silty sand.

The surface of the Cedar Flats fan west of locality A is veneered 

by a lahar as much as 1.5 m thick which lies between tephra sets J 

and Y. The lahar is a loose unsorted mixture of subangular rock 

fragments as large as 15 cm in diameter in a matrix of silty sand. At 

some places the lahar is underlain by as much as 1 meter of fluvial 

gravel, and at other places there are two lahars between tephra sets 

J and Y.

The topographic position of the pre-Y lahar at the surface of the 

Cedar Flats fan suggests that Pine Creek did not entrench the fan 

until after set J and the lahar were deposited, even though the 

Muddy River had done so before these deposits were formed. In 

some exposures tephra set Y can be seen to overlie an oxidized zone 

as much as 0.7 m thick which is developed within a post-J lahar. 

This oxidation, which resulted from weathering, seems to represent 

a substantial time interval after deposition of the lahar but before 

tephra set Y was formed between 4,000 and 3,000 years ago, and its 

presence implies that the lahar is closer in age to set J than to set Y.

POST-Y UNIT OF THE PINE CREEK ASSEMBLAGE

The most extensive and best preserved part of the Pine Creek 

assemblage is younger than tephra set Y and older than set B. The 

post-Y sequence includes valley-fill deposits of hot pyroclastic 

flows, lahars, and streams, and ash-cloud deposits beyond the 

margins of the valley fill. Within a distance of 3 km from the base of 

the volcano the valley fill is partly covered by younger lahars (fig. 

2); elsewhere it is mantled only by tephra. The surface of the valley 

fill slopes southeastward along the course of Pine Creek at a 

gradient of about 70 m/km. At a distance of 3-7 km from the base of 

the volcano the surface of the fill is convex in cross profile, and 

farther downvalley it forms a succession of fill terraces that extends to the edge of Swift Reservoir. The part of the valley of Pine 

Creek that is below the lowest terrace (fig. 5) is thought to postdate 

the Pine Creek eruptive episode, but the higher terraces probably 

were formed during that episode and were caused by alternations of 

valley filling and cutting.
A14 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

Altitude 

(feet) SOUTH

1550 rNORTH

'-O 

APPROXIMATE SCALE

B Tephra set B 

Y Tephra set Y 

A Fluvial deposit 

C Ash-cloud deposit

EXPLANATION

LF Lithic pyroclastic-flow deposit 

PF Pumiceous pyroclastic-flow deposit 

L Lahar 

F Fkiwage deposit of unknown origin

Stipple pattern indicates covered intervals

FIGURE 5. Stratigraphic sequences of deposits exposed in the Pine Creek Valley 

near locality C (fig. 2). Horizontal scale is diagrammatic.

Pyroclastic-flow deposits form the largest part of the sequence 

from the base of the volcano downvalley for about 11 km (fig. 6). Individual lithic flow deposits are as much as 15 m thick and typically 

have pink zones 1-3 m thick at their tops. Some pumiceous pyroclastic-flow deposits are compact mixtures of pumice fragments 

and nonvesicular glassy rock fragments, and others consist of 

breadcrusted pumice blocks and lapilli in a loose ash matrix. The 

pumice in each of the pyroclastic-flow deposits examined contained hypersthene and hornblende as the predominant Fe-Mg 

minerals.

One of the best exposures of the post-Y part of the Pine Creek
PINE CREEK VOLCANIC ASSEMBLAGE A15

FIGURE 6. Pyroclastic-flow deposits exposed in the side of the trench of Pine 

Creek about 5 km southeast of the base of Mount St. Helens. The largest 

block in the center of the photograph is about 3 m in diameter.

assemblage is on the east side of Pine Creek at locality E, where 

there are at least 10 pyroclastic-flow deposits which range in thickness from 1 to 12 m. The lowest of these, which is 1.2 m thick, consists mostly of pumice but also contains many fragments of non-
A16 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

vesicular glassy rock. Charcoal in it was found to have a radiocarbon age of about 2,850 years (sample W-2542, table 2). An overlying pumiceous pyroclastic-flow deposit which is 2.7 m thick contains pumice fragments mostly less than 5 cm in diameter as well 

as angular fragments of nonvesicular glassy rock. Above it is a 

lithic pyroclastic-flow deposit 1-3 m thick which is especially conspicuous because of a reddish-gray zone, about 1 m thick, at its top. 

This deposit contains angular and subangular nonvesicular 

dacite fragments as large as 1.2 m in diameter in a loose sand 

matrix. Rock fragments tested with a fluxgate magnetometer 

showed a preferred orientation of the direction of remanent magnetism. All the wood fragments in the deposit, including large logs, 

have been entirely converted to charcoal.

Above the charcoal-bearing lithic pyroclastic flow is a similar deposit which is estimated to be about 12 m thick. The direction of 

remanent magnetism of rock fragments in it was not determined, 

but a reddish-gray zone at its top suggests that the deposit also originated as a hot pyroclastic flow. An interesting feature of the upper 

half of this deposit is the presence of long thin layers of pumice 

blocks. These do not seem to have the continuity that would suggest that they separate a succession of pyroclastic flows.

Ross and Smith (1961, p. 21) attributed alternating layers of thin 

pumice zones and typical unsorted ash-flow tuff to the overlapping 

of eddies or surges of an ash flow near its terminus. They suggested 

that lobes of a flow would have concentrations of pumice at top and 

bottom and would override one another, thus leaving a deposit that 

is crudely bedded. They also noted that incorporation of rubble by 

an ash flow from the underlying terrain may lead to concentrations of this material in lenses or linear zones within the ash flow. 

Although such lenses are most common near the base, they may 

also occur within the flow and give an impression of bedding.

Charcoal from another pyroclastic-flow deposit within the 

assemblage, which was obtained from an outcrop along a logging 

road at locality F (fig. 2), had a radiocarbon age of about 2,670 

years. Charcoal from a pyroclastic-flow deposit that is overlain 

only by set B and other tephra west of Pine Creek at locality G had a 

radiocarbon age of about 2,580 years (table 2).

Lahars form a subordinate part of the post-Y unit within a distance of about 11 km from the base of the volcano, but they predominate farther downvalley and extend beyond Swift Dam. 

Lahars were observed at several places along Pine Creek. At the 

confluence of two branches of the creek near locality E (fig. 2), at 

least 8 lahars are exposed. A roadcut 1 km farther south shows a 

very coarse lahar that contains blocks of nonvesicular dacite and
PINE CREEK VOLCANIC ASSEMBLAGE A17

andesite as much as 2 m in diameter; the deposit is more than 15m 

thick, but possibly more than one lahar is present. Another good exposure of lahars in the assemblage is in a pit near the mouth of Pine 

Creek at locality H (fig. 2). Many thin bouldery lahars can be seen 

there in both longitudinal section and cross section in the vertical 

walls of the pit (fig. 7), where they are interbedded with thin fluvial 

sand and gravel deposits. Some boulders on the floor of the pit are 

as large as 2.5 m in diameter.

Fluvial deposits are not common in the Pine Creek assemblage 

upstream from the mouth of Pine Creek, but they become progressively more abundant and finer in grain size down the Lewis River 

valley. From the mouth of that valley at Woodland upstream for 

about 8 km, gray pumice-bearing sands form a terrace that is about 

7.5 m higher than the Lewis River flood plain. Beds of pumice 

gravel were noted at a few places, and ash and lapilli of white 

pumice make up nearly all of some beds and lenses. The pumice 

contains hypersthene and hornblende and thus is mineralogically 

similar to the pumice in the pyroclastic-flow deposits farther upFlGURE 7. Lahars interbedded with fluvial deposits exposed in the walls of 

a pit near the mouth of the Muddy River (loc. H, fig. 2).
A18 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

stream as well as to pumice in tephra set P. The lithology of the 

sand deposit indicates that it is not a normal fluvial deposit of the 

Lewis River, and we infer that the sand is the downstream correlative of the post-Y part of the Pine Creek assemblage.

ASH-CLOUD DEPOSITS

Ash clouds which were formed during the accumulation of the 

post-Y part of the Pine Creek assemblage created several deposits 

5-100 cm thick over an area 1-3 km wide adjacent to the Pine Creek 

valley. The ash-cloud deposits are interbedded with pumice and 

lithic tephra of set P along both sides of the valley as far as 12 km 

southeast of the base of the volcano.

Ash-cloud deposits have been recognized in the tephra sequence 

associated with sets J and Y, and at two stratigraphic positions 

within set P, but only the deposit at one horizon within set P will be 

described because it is the thickest and most widespread ash-cloud 

deposit recognized in the area southeast of Mount St. Helens. It will 

be referred to as the mid-P ash-cloud deposit. Stratigraphic positions of all post-S ash-cloud deposits recognized within the tephra 

sequence are as follows:

Tephra set B 

Ash-cloud deposit

Topmost pumice layer of tephra set P 

Ash-cloud deposit

Upper fine beds of tephra set P 

Ash-cloud deposit (mid-P)

Middle silt and sand layers of tephra set P

Lower fine beds of tephra set P

Basal layer of tephra set P

Upper layers of tephra set Y 

Ash-cloud deposit

Lower layers of tephra set Y 

Ash-cloud deposit 

Ash-cloud deposit

Tephra set J 

Ash-cloud deposit(?) 

Ash-cloud deposit(?)

Tephra set S

The distribution of the mid-P ash-cloud deposit is shown in figure 

2. The deposit is thickest along the northeast base of Marble Mountain and on the interfluve between the Pine Creek and Smith Creek 

valleys just above the east margin of the valley-fill part of the Pine 

Creek assemblage. It decreases in thickness away from the valley 

fill and also toward the southeast, away from the volcano. The 

deposit veneers the north slope and crest of Marble Mountain as
PINE CREEK VOLCANIC ASSEMBLAGE A19

much as 2.5 km south of, and 360 m higher than, the top of the 

valley fill at the north base of Marble Mountain.

Texturally, the mid-P ash-cloud deposit is fine to coarse lithic ash 

that contains scattered pumice ash and lapilli. Angular fragments 

of nonvesicular rock as large as 2 cm in diameter are not uncommon in the deposit near the margins of the valley-fill part of the 

Pine Creek assemblage. The mid-P deposit becomes finer upslope 

from the valley fill and also away from the volcano.

The mid-P ash-cloud deposit seems to be massive in many places, 

but planar bedding is not uncommon. Foreset bedding in which 

the beds dip away from the volcano was noted at a few places.

At places where the cloud deposit is thin, it resembles tephra 

layers of set P, but the following characteristics aid in its identification:

1. Areal distribution of the ash-cloud deposit is closely related to 

the margins of the valley-fill part of the Pine Creek assemblage; its thickness decreases outward from these margins 

to the east and to the west, as well as with increasing height 

above the top of the valley fill. In contrast, the thickness and 

grain size of set P tephra sheets decrease predictably away 

from the volcano according to distance and wind directions 

at the time of the eruption. Those tephra thicknesses are governed primarily by distance from the volcano rather than 

by local topographic features or by altitude.

2. The ash-cloud deposit consists largely of sand-sized ash where 

it is as much as 1 m thick. In contrast, most tephra layers 

derived from Mount St. Helens which are more than about 

20 cm thick typically consist mainly of fragments of lapilli 

size or larger.

3. The ash-cloud deposit consists mostly of nonvesicular material, 

whereas most tephra deposits contain a significant or predominant component of vesicular fragments. 

The mid-P ash-cloud deposit probably was formed in at least two 

ways. Part of it may have resulted from fallout of fine material fi-om 

clouds of dust rising above hot pyroclastic flows moving down 

valley floors. Clouds of this kind are illustrated in accounts of the 

1902 and 1929 eruptions of MontPele'e(Lacroix, 1904; Ferret, 1937). 

Other parts probably were formed by hot dense clouds of predominantly fine grained rock debris moving across the ground surface at high velocity. Such clouds may originate from the eruption 

of finely comminuted gas-rich debris which descends one or more 

flanks of a volcano with increasing speed because of gravitational 

acceleration. Incandescent clouds of this kind, which resulted from 

the climactic eruption of Mount Lamington, Papua, on January 21,
A20 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

1951, were referred to as "ash hurricanes" by Taylor (1958, p. 45). 

Features characteristic of base-surge deposits, such as dune surface form and dune-type bedding (Moore, 1967; Fisher and Waters, 

1970), have not been recognized in the Pine Creek area.

Although some features of bedding, texture, and distribution suggest ways in which different kinds of ash-cloud deposits can be distinguished, the mid-P deposit has not yet been studied in enough detail to be differentiated.

VOLUME OF THE PINE CREEK ASSEMBLAGE

Although we cannot yet calculate the relative volumes of the preJ and post-J parts of the Pine Creek assemblage, we estimate that 

the original volume of the entire assemblage was between 3 and 4 

km3 . This amount can be compared with the volume of the present 

cone of Mount St. Helens, which is about 27 km3 if the cone is assumed to be symmetrical and smooth-sided and to have a basal diameter of 8 km and a height of 1.6 km. The volume of the assemblage 

is also less than that of tephra set Y, whose volume may be as much 

as 10 km3. Set P may have only one-tenth or one-fifth the volume of 

tephra set Y.

CHARACTER OF OLD MOUNT ST. HELENS 

IN PINE CREEK TIME

We have as yet no direct evidence concerning the size or shape of 

the old Mount St. Helens volcano from which the Pine Creek assemblage was derived. However, something of its general character 

can be inferred from the nature and distribution of the assemblage 

itself. The large volume of rock debris in the Pine Creek deposits 

suggests a derivation from dacite domes which were repeatedly extruded and then destroyed by explosions. These domes probably 

were not part of a high symmetrical volcano similar to modern St. 

Helens; rather, they may have risen through low cones of fragmental debris. Mass-flowage deposits dating from the interval 3,000 to 

2,500 years ago (table 3) have been found thus far only in the area 

southeast of the volcano. This suggests that some kind of a topographic feature prevented volcanic debris from moving into 

valleys to the north and west at that time. If St. Helens was then a 

single cone with a central dome, rather than a group of domes and 

cones, it probably had a crater open to the southeast which discharged into an "avalanche valley" at the head of Pine Creek.

The interbedding of lithic and pumiceous pyroclastic-flow 

deposits in the post-Y part of the Pine Creek assemblage shows that 

the volcano erupted these materials alternately, perhaps during a 

single short eruptive episode. The lithic deposits could have orig-
PINE CREEK VOLCANIC ASSEMBLAGE A21

TABLE 3. Sequence of events at Mount St. Helens and in the adjacent region, 

as indicated by deposits in the area southeast of the volcano

Approximate age, or

limiting dates 

Event (years ago)

Eruption of tephra set W - - 450

Eruption of tephra set B < 2,500, > 450

Eruption of tephra set P and formation of

hot pyroclastic flows and lahars -- 3,000-2,500(7) 

Eruption of tephra set Y ----- -- ----- 4,000-3,000 

Formation of hot pyroclastic flows

and lahars----------------------- < 8,000, 4,000

Eruption of tephra set J --- - - -- - < 12,000, > 8,000 

Formation of hot pyroclastic flows

and lahars < 18,000, > 8,000

Eruption of tephra set S - ---- --- < 18,000, > 12,000

Retreat of alpine glaciers 

inated from the explosive disruption of dacite domes and spines as 

they were being elevated, and the pumiceous flow deposits probably originated when explosive venting of gas-charged magma occurred.

The old Mount St. Helens of pre-J time probably also consisted of 

one or several domes and cones of fragmental debris. We do not yet 

know how widely flowage deposits of this age are distributed 

around the volcano.

The volume of the valley fills of pyroclastic flows and lahars in 

the Pine Creek assemblage may have an inverse relation to the 

volume and vesicularity of tephra erupted at about the same time. 

Thus, most of the post-Y valley-fill deposits of the assemblage were 

formed during the same eruption period that produced tephra set P, 

which is of relatively small volume and mostly of nonvesicular 

material. Likewise, pumice is virtually absent from the pre-J part of 

the assemblage, the deposition of which evidently was not accompanied by eruptions that produced recognizable tephra deposits. In 

contrast, few pyroclastic flows or lahars were formed at times when 

the volcano was erupting large volumes of highly vesicular material, like that represented by tephra set Y and perhaps also by set J.

The eruption of domes and pumiceous tephra seems to have 

characterized Mount St. Helens for a long time. Some tephra and 

pyroclastic-flow deposits, probably derived from domes, date back 

to the period between 18,000 and 36,000 years ago (Mullineaux and 

others, 1972; J. H. Hyde, written commun., 1972). In addition, 

domes (Hopson, 1971) as well as tephra and pyroclastic flows 

(Mullineaux, 1964; Hyde, 1970) have been erupted within the last 

500 years or so.
A22 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

VOLCANIC HAZARDS

The events represented by the deposits of the Pine Creek assemblage and associated tephra are not the only kinds of volcanic activity that have occurred during the last 15,000 years or so, but they 

are potentially among the most dangerous. Pyroclastic flows, ash 

clouds, and tephra falls could be a direct and immediate threat to 

human life in areas adjacent to an active vent. Pyroclastic flows 

and lahars could, in addition, move large masses of rock debris 

down valley floors within a short period of time and endanger 

persons and property at greater distances from the volcano. Movement of large volumes of fragmental rock material into valleys on 

the south side of the volcano might also displace a large volume of 

water from Swift Reservoir and perhaps endanger the dam itself.

If volcanism of this kind were to occur in the future, areas 

affected by tephra fall would be downwind from the active vent or 

vents; the extent of the endangered area would depend on the explosiveness and volume of the eruption and the strength and direction of winds. Areas affected most by lahars and pyroclastic flows 

with their associated ash clouds would be the floors and sides of 

valleys that head at or near the active vent. The greatest danger 

would exist in the area actually reached by the flows, but if floods 

were caused by reservoir displacement, the danger could be extended far downstream.

REFERENCES

Aramaki, Shigeo, and Akimoto, Syun-iti, 1957, Temperature estimation of pyroclastic deposits by natural remanent magnetism: Am. Jour. Sci., v. 255, 

no. 9, p. 619-627.

Armstrong, J. E., Crandell, D. R., Easterbrook, D. J., and Noble, J. B., 1965, 

Late Pleistocene stratigraphy and chronology in southwestern British Columbia and northwestern Washington: Geol. Soc. America Bull., v. 76, no. 

3, p. 321-330.

Crandell, D. R., 1965, The glacial history of western Washington and Oregon, 

in The Quaternary of the United States: Princeton, N. J., Princeton Univ. 

Press, p. 341-353.

____1969, Surficial geology of Mount Rainier National Park, Washington: 

U. S. Geol. Survey Bull. 1288, 41 p.

___1971, Postglacial lahars from Mount Rainier volcano, Washington: 

U.S. Geol. Survey Prof. Paper 677, 75 p.

Crandell, D. R., Mullineaux, D. R., Miller, R. D., and Rubin, Meyer, 1962, 

Pyroclastic deposits of Recent age at Mount Rainier, Washington, in Short 

papers in geology, hydrology, and topography: U.S. Geol. Survey Prof. 

Paper 450-D, p. D64-D68.

Fisher, R. V., and Waters, A. C., 1970, Base surge bed forms in maar volcanoes: 

Am. Jour. Sci., v. 268, no. 2, p. 157-180.

Hopson, C. A., 1971, Eruptive sequence at Mount St. Helens, Washington: Geol. 

Soc. America, Cordilleran Sec., Abs. with Programs, v. 3, no. 2, p. 138.
PINE CREEK VOLCANIC ASSEMBLAGE A23

Hyde, J. H., 1970, Geologic setting of Merrill Lake and evaluation of volcanic 

hazards in the Kalama River valley near Mount St. Helens, Washington: 

U.S. Geol. Survey open-file report, 17 p.

Lacroix, Alfred, 1904, La Montagne Pele'e et ses eruptions: Paris, Masson et 

Cie., 662 p.

Moore, J. G., 1967, Base surge in recent volcanic eruptions: Bull. Volcanol., 

v. 30, p. 337-363.

Mullineaux, D. R., 1964, Extensive Recent pumice lapilli and ash layers from 

Mount St. Helens volcano, southern Washington, in Abstracts for 1963: 

Geol. Soc. America Spec. Paper 76, p. 285.

Mullineaux, D. R., and Crandell, D. R., 1962, Recent lahars from Mount St. 

Helens, Washington: Geol. Soc. America Bull., v. 73, no. 7, p. 855-870.

Mullineaux, D. R., Hyde, J. H., and Rubin, Meyer, 1972, Preliminary assessment 

of upper Pleistocene and Holocene pumiceous tephra from Mount St. Helens, 

southern Washington: Geol. Soc. America, Cordilleran Sec., Abs. with Programs, v. 4, no. 3, p. 204-205.

Perret, F. A., 1937, The eruption of Mt. Pelee, 1929-1932: Carnegie Inst. Washington Pub. 458, 126 p.

Ross, C. S., and Smith, R. L., 1961, Ash-flow tuffs Their origin, geologic relations, 

and identification: U.S. Geol. Survey Prof. Paper 366, 81 p.

Smith, R. L., 1960, Ash flows: Geol. Soc. America Bull., v. 71, no. 6, p. 795- 

842.

Taylor, G. A., 1958, The 1951 eruption of Mount Lamington, Papua: Australia 

Bur. Mineral Resources, Geology and Geophysics Bull. 38, 100 p.

Thorarinsson, Sigurdur, 1954, The tephra-fall from Hekla on March 29th, 1947, 

in Einarsson, T., Kjartansson, G., and Thorarinsson, S., eds., The eruption 

of Hekla 1947-1948: Visindafelag Islendinga, pt. 2, no. 3, 68 p.

Verhoogen, Jean, 1937, Mount St. Helens a recent Cascade volcano: California 

Univ. Pubs. Geol. Sci., v. 24, no. 9, p. 263-302.

Williams, Howel, 1960, Volcanic history of the Guatemalan Highlands: California 

Univ. Pubs. Geol. Sci., v. 38, no. 1, p. 1-87.

U.S. GOVERNMENT PRINTING OFFICE: 1973 516-336/56