UPPER PLEISTOCENE

PYROCLASTIC-FLOW 

DEPOSITS ANDLAHARS

SOUTH OF MOUNT ST. HELENS VOLCANO, 

WASHINGTON

GEOTOGICAL SURVEY BULLETIN 1383-B

Upper Pleistocene Pyroclastic-Flow 

Deposits and Lahars South of 

Mount St Helens \blcano, 

Washington

By JACK H. HYDE

GEOLOGY OF MOUNT ST. HELENS 

VOLCANO, WASHINGTON

GEOLOGICAL SURVEY BULLETIN 1383-B

A study of a valley-fill assemblage 

of pyroclastic-flow deposits, lahars, 

and alluvium of late Pleistocene age 

south of Mount St. Helens volcano

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1975
UNITED STATES DEPARTMENT OF THE INTERIOR

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress Cataloging in Publication Data

Hyde, Jack H.

Upper Pleistocene pyroclastic-flow deposits and lahars south of Mount St. Helens volcano,

Washington.

(Geology of Mount St. Helens volcano, Washington) 

(Geological Survey Bulletin 1383-B) 

Bibliography: p.

Supt. of Docs, no.: I 19.3:1383-6

1. Volcanic ash, tuff, etc. Washington (State) Saint Helens, Mount. 2. Lahars Washington 

(State) Saint Helens, Mount. 3. Geology Stratigraphic Pleistocene. 4. GeologyWashington (State) Saint Helens, Mount.

I. Title. II. Series. III. Series: United States Geological Survey Bulletin 1383-B. 

QE75.B9 no. 1383-B [QE461] 557.3'08s [551.2'l] 75-619236

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

Washington, D. C. 20402 

Stock Number 024-001-02729-1
CONTENTS

Page

Abstract........................................................................................................................... Bl

Introduction.................................................................................................................... 1

Terminology................................................................................................................... 4

Tephra............................................................................................................................. 5

Glacier extents during Swift Creek time........................................................................ 6

Description of the Swift Creek assemblage.................................................................... 9

Older part of the Swift Creek assemblage............................................................... 9

Younger part of the Swift Creek assemblage......................................................... 15

Character of volcanic eruptions ..................................................................................... 18

Effect of volcanism on the Lewis River valley .............................................................. 19

References cited............................................................................................................... 20

ILLUSTRATIONS

Page

FIGURE 1. Map showing inferred original extent of the Swift Creek assemblage....... B2

2. Photograph of till of Eraser age veneered with pyroclastic-flow deposits

and tephra................................................................................................. g

3. Aerial photograph looking eastward up the Lewis River valley showing

the top of the Swift Creek assemblage...................................................... 10

4. Profile showing relationship between the surface of the Swift Creek assemblage and the Lewis River valley.............................................................. 10

5. Section showing stratigraphic sequence of deposits exposed near the

mouth of Swift Creek................................................................................ 12

6. Photograph of pyroclastic-flow deposits...................................................... 14

7. Photograph of lahars ................................................................................... 17

TABLES

TABLE 1. Selected radiocarbon dates............................................................................ B6

2. Sequence of events at Mount St. Helens...................................................... 1H

in
IV CONTENTS

COMMON MEASURES AND 

THEIR METRIC EQUIVALENTS

Common Measure Equivalent

Inch......................................................... 2.54 centimetres (cm).

Foot......................................................... 0.3048 metre (m).

Mile......................................................... 1.6093 kilometres (km).
GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

UPPER PLEISTOCENE PYROCLASTIC-FLOW

DEPOSITS AND LAHARS SOUTH OF

MOUNT ST. HELENS VOLCANO,

WASHINGTON

By JACK H. HYDE

ABSTRACT

The Swift Creek volcanic assemblage consists chiefly of deposits of pyroclastic flows, 

lahars, and alluvium which are interbedded with tephra. These deposits and the glacial drift 

of late Pleistocene age associated with them provide the best available record of geologic 

events at Mount St. Helens during the period between about 36,000 to 12,000 years ago. The 

assemblage was formed during intermittent episodes of andesitic and dacitic volcanism and 

includes some of the oldest known volcanic deposits from Mount St. Helens. The Swift 

Creek assemblage forms a valley fill that extends 14 km down the Swift Creek valley and into 

the Lewis River valley, where it is as much as 200 m thick. Pyroclastic flows reached the 

mouth of Swift Creek and lahars extended at least 24 km farther. Glaciers which originated 

on the volcano during the Eraser Glaciation were apparently small and did not extend very 

far down valleys. The small size of glaciers, together with the nonvesicular andesitic and 

dacitic rock debris in the volcanic assemblage, suggests that the old Mount St. Helens 

eruptive center consisted mostly of low domes or a succession of domes rather than a large, 

high volcano. The large volume of rock debris transported into the Lewis River valley 

during eruptions caused the river to aggrade; downcutting occurred when the influx of 

material from the volcano decreased.

INTRODUCTION

Mount St. Helens is a volcano of late Quaternary age situated near the 

west margin of the Cascade Range in southwest Washington, 70 km 

northeast of Portland, Oreg. (fig. 1). The present cone consists of lava 

flows of olivine basalt and pyroxene andesite which surround a summit 

plug of dacite. Valley fills adjacent to the volcano include pyroclasticflow deposits, lahars, and alluvium, which are locally interbedded with 

tephra and glacial drift. These deposits provide the best available record 

of geological history at an eruptive center which predated the present 

cone. This eruptive center, which is called old Mount St. Helens, was 

characterized by repeated eruptions of hornblende-hypersthene dacite and 

andesite (Verhoogen, 1937; Hopson, 1971).

The purpose of this report is to describe the stratigraphic sequence of

pyroclastic-flow deposits, lahars, tephra, and alluvium in the Swift Creek

Bl
B2 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

122° 20'

46 10

46^00'

Portland

YALE DAM

FIGURE 1. Inferred original extent of Swift Creek assemblage 

deposits near the base of Mount St. Helens. Numbers in circles
UPPER PLEISTOCENE PYROCLASTIC-FLOW DEPOSITS AND LAHARS B3

10* 122°00'

xXV^

''/,

,V

\

MOUNT 

ST. HELENS

A

9673 ft 

2949m

123 MILES

J_____I____I

I \ \ I 

0123 KILOMETRES

(pattern) upvalley from Lake Merwin. The assemblage is covered by younger volcanic 

show localities mentioned in text. Line A-A' is location of profile shown in figure 4.
B4 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

and Lewis River valleys south of Mount St. Helens and the relation of 

these materials to drift of the last major glaciation, to determine the 

eruptive and glacial history represented by the deposits, and to discuss the 

effect of these deposits on the Lewis River valley. The Swift Creek 

assemblage, which is the informal term used for the group of unconsolidated deposits in the Swift Creek and Lewis River valleys, provides the 

best known record of the eruptive history of old Mount St. Helens prior to 

about 12,000 years ago. Correlative deposits were formed in some other 

valleys heading at Mount St. Helens, but nowhere is there as extensive a 

record as in the Swift Creek and Lewis River valleys. The Pine Creek 

valley, southeast of the volcano, contains a similar depositional record of 

events between about 12,000 and 2,500 years ago (Crandell and 

Mullineaux, 1973), and deposits in the Kalama River valley southwest of 

the volcano provide details of eruptions within the last 2,500 years (Hyde, 

1970).

Mount St. Helens is drained by the Kalama River on the southwest and 

by tributaries of the Lewis River on the south that include Swift Creek and 

Pine Creek (fig. 1). Swift Creek drains most of the south side of the volcano 

and joins the Lewis River about 8 km east of the community of Cougar. 

The Lewis River heads about 55 km east of Mount St. Helens and joins the 

Columbia River at Woodland, about 55 km southwest of the volcano. 

Downstream from the mouth of Pine Creek, the Lewis River is 

impounded by three dams for the purpose of hydroelectric-power 

generation; the resulting lakes are Swift Reservoir, Yale Lake, and Lake 

Merwin.

TERMINOLOGY

The term "tephra" refers to fragmental volcanic debris originating 

from a volcanic vent and transported through the air (Thorarinsson, 

1954). Tephra may consist of (1) new magma represented by pumice or 

scoria, (2) dense glassy rock fragments, or (3) crystals, or may consist of a 

mixture of all three. Some tephra may also contain fragments derived 

from preexisting solid rock. In this report nonvesicular tephra is referred 

to by the adjective lithic. Most tephra at Mount St. Helens was erupted 

into the air and was deposited as a result of vertical or near-vertical fallout.

The term "ash-cloud deposit" is used in this report for material 

deposited from clouds of airborne ash associated with pyroclastic flows 

(Crandell and Mullineaux, 1973). These deposits consist mostly of lithic 

ash but pumice is not uncommon. The deposits range in thickness from a 

few centimetres to a metre and may decrease in grain size and thickness 

away from the vent and in directions away from the axis of the valley 

down which a pyroclastic flow traveled. Most such deposits form a single 

massive bed, but planar parallel bedding is common and "foreset" 

bedding was noted at a few localities. Ash-cloud deposits genetically 

related to pyroclastic flows resemble airfall tephra layers but may be dis-
UPPER PLEISTOCENE PYROCLASTIC-FLOW DEPOSITS AND LAHARS B5

tinguished from them by a more restricted areal distribution, by rapid 

lateral changes in thickness and grain size, and by an abundance of lithic 

fragments (Crandell and Mullineaux, 1973).

Many of the unconsolidated volcanic deposits in the Swift Creek 

assemblage resulted from the mass flowage of volcanic rock debris. Some 

deposits were formed by hot, dry pyroclastic flows and others by lahars, 

both of which are discussed below. The precise origin of some massflowage deposits cannot be determined, and these will be referred to as 

flowage deposits; this term is also used collectively for sequences which 

include deposits of both modes of origin.

A lahar is a rapid flow of rock debris mobilized by water and derived 

from a volcano (Bemmelen, 1949; Mullineaux and Crandell, 1962; 

Crandell, 1971). The term also is used to designate the resulting deposit. 

The rock debris may be either hot or cold.

Pyroclastic flows are hot, dry flows of volcanic rock debris. They owe 

their mobility to gravity as well as to the explosive force of the eruption, to 

gas-emitting particles, to air trapped and heated within the mass, or to 

some combination of these factors. The resulting deposits may consist of 

dense rock fragments, pumice fragments, or mixtures of both. Pyroclastic 

flows range from gas-rich, highly mobile flows of pumice to less mobile 

avalanches of lithic blocks and ash. Pumiceous pyroclastic flows 

generally originate from the rapid eruption of very large volumes of 

pumice; pyroclastic flows of predominantly lithic material commonly are 

caused by the sliding of accumulations of hot rock debris on the side of the 

volcano, or by the partial collapse of a volcanic dome or spine.

The deposits of pyroclastic flows generally can be differentiated, by 

evidence that they once were hot, from colluvium, landslide deposits, and 

alluvium. The presence of heat when a deposit was formed can be 

demonstrated if the constituent rock fragments possess a preferred 

orientation of a natural remanent magnetism (NRM), because the rock 

fragments must have come to rest while their ferromagnetic minerals were 

su'll above their Curie or blocking temperature (Aramaki and Akimoto, 

1957; Chadwick, 1971: Crandell, 1971). A red to reddish-gray color in the 

upper few metres of a flowage deposit has also been used as evidence that 

the deposit was hot when emplaced and was therefore probably of 

pyroclastic-flow origin (Smith, 1960; Williams, 1960). The deposits of 

lahars generally show a rude vertical size gradation from coarse at the base 

to fine near the top (Mullineaux and Crandell, 1962; Crandell, 1971). 

Deposits formed by lahars which were carrying hot rock fragments may 

show some characteristics of both lahars and pyroclastic flows.

TEPHRA

The Swift Creek assemblage can be subdivided and correlated by means 

of three principal groups, or sets, of tephra deposits. The tephra forming 

these sets were erupted before, during, and after the assemblage was
B6 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

formed. Each set consists of several layers having the same suite of Fe-Mg 

phenocrysts; overlying and underlying tephra sets contain different 

heavy-mineral suites (Mullineaux and others, 1972; 1975).

The oldest tephra set, unnamed, is distinguished by the presence of 

biotite crystals which generally can be seen with a hand lens. In the area 

south and southwest of the volcano, the set includes a lower bed 15-30 cm 

thick and an upper pair of beds which typically are separated by a layer of 

silt a few centimetres thick; this pair of beds is referred to here as "the 

couplet." A radiocarbon date of 37,600±1,000 years has been obtained on a 

sample of charcoal (table 1) from the upper part of the lowest layer of the 

set east of Mount St. Helens (D. R. Mullineaux, unpub. data, 1973). Dates 

of about 20,000 and 18,500 years, that have been determined on samples 

from a locality south of the volcano, bracket the couplet (table 1).

TABLE 1. Selected radiocarbon dates of volcanic deposits derived from Mount St. Helens

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

Date (years 

before present)

8,300± 350 ..............

11,880± 350. .............

13,130±350..............

18,560±550 ..............

20 350±500

36,000±2,000 ...........

Laboratory

No.

W-2587

W 9441

1A7 9QSS

W-2653

Kind of sample and stratigraphic 

position

Tephra set S includes several beds of pumice which contain abundant 

cummingtonite and hornblende. The age of the set is not yet precisely 

known. It is younger than 18,500 years, and a radiocarbon date of about 

13,100 years has been obtained on charcoal taken from a pyroclastic-flow 

deposit that underlies one or more ot the upper beds of the set. Other 

radiocarbon dates indicate that the entire set is more than 12,000 years old. 

Set S is the oldest tephra found on top of glacial drift formed during the 

last major glaciation of the area, and has been found interbedded with 

pyroclastic-flow deposits and lahars which postdate that glaciation.

Tephra set J consists of several beds of coarse pumice in which the only 

Fe-Mg phenocrysts are hornblende and hypersthene. A brown oxidized 

zone is commonly present in the upper part of the tephra. The age of the 

set is between 12,000 and 8,000 years.

GLACIER EXTENTS DURING SWIFT CREEK TIME

Much of the Swift Creek assemblage accumulated during the last major 

glacial episode (Fraser Glaciation) in western Washington which 

occurred between about 25,000 and 10,000 years ago (Armstrong and 

others, 1965), and glacial drift is interbedded with deposits of volcanic 

origin in some areas.

A glacier originating on the south flank of the old Mount St. Helens
UPPER PLEISTOCENE PYROCLASTIC-FLOW DEPOSITS AND LAHARS B7

volcano moved down the valley of Swift Creek. The cirque in which this 

glacier originated is apparently carved into the rocks of old Mount St. 

Helens (C. A. Hopson, oral commun., 1972) and is now mostly obscured 

by young lava flows and flowage deposits, but part of its headwall is still 

visible at an altitude of 1,200-1,500 m, about 4 km directly south of the 

summit of the volcano. End moraines formed by this glacier are crossed by 

U.S. Forest Service road N818 about 0.5 km north of its junction with road 

N83 (loc. 1, fig. 1). There, about 8 m of drift is exposed beneath 2-3 m of 

tephra and pyroclastic-flow deposits (fig. 2). These moraines evidently do 

not mark the greatest downvalley extent of the glacier, because till having 

weathering features similar to those of the moraines is exposed about 1.5 

km farther south, near the entrance to Ape Cave. Till is also exposed in the 

west valley wall of Swift Creek at locality 2 (fig. 1), where it seems to be 

part of an end moraine that was subsequently buried by several flowage 

deposits. The oldest flowage deposit is coarse and displays crude internal 

stratification, and may be either alluvium or a mudflow of glacial origin. 

Above it is a succession of pyroclastic-flow deposits (p. B15).

Glacial drift and moraines in the upper part of the Swift Creek valley 

were formed by a glacier originating on the southeast flank of Mount St. 

Helens in the area north of June Lake. Faceted spurs and lateral moraines 

containing over 60 percent andesite and dacite fragments are present on 

both sides of a valley south of June Lake, but farther south, near the base 

of Marble Mountain, the drift is covered with young flowage deposits. 

Glacial drift is not exposed in deep cuts farther downstream, and the 

glacier probably terminated near the base of Marble Mountain.

The glacial drift that is interbedded with the Swift Creek assemblage at 

Mount St. Helens probably was deposited during the Evans Creek Stade of 

the Fraser Glaciation. This correlation is based on a comparison of such 

relative weathering features as depth of oxidation and thickness of 

weathered rinds on stones in glacial drift in this area with those described 

in other areas of western Washington (Crandell, 1969, p. 18). The Evans 

Creek Stade was originally thought to have occurred between about 

25,000 and 15,000 years ago (Armstrong and others, 1965). Subsequent 

work has suggested that alpine glaciers in southwestern British Columbia 

began to advance after 20,000 years ago (Halstead, 1968), and alpine 

glaciers on the west side of the Olympic Peninsula evidently reached their 

maximum extents before about 18,800 years ago (Heusser, 1974).

Glaciers of Fraser age also developed in cirques at altitudes above 1,100 

m in the mountains adjacent to Mount St. Helens, but most were less than 

10 km in length. Small glaciers existed along the ridge at the head of 

Cougar Creek northeast of Cinnamon Peak, where till and end moraines 

are present in several cirques. Small glaciers also extended down the north 

and west sides of Cinnamon Peak and terminated in the Kalama River 

valley about 2 km north of Merrill Lake.
B8 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON
UPPER PLEISTOCENE PYROCLASTIC-FLOW DEPOSITS AND LAHARS B9

Exposures of glacial drift in the vicinity of Mount St. Helens and in the 

Lewis River valley west of the volcano show that the area was glaciated at 

least three times prior to the Fraser Glaciation and prior to the formation 

of the Swift Creek assemblage. These earlier alpine glaciers extended at 

least 50 km farther down the Lewis River valley than did the glacier of 

Fraser age. During these earlier glaciations all the area at, and adjacent to, 

the present site of Mount St. Helens, except perhaps the highest peaks, 

may have been covered by glaciers.

DESCRIPTION OF THE SWIFT CREEK ASSEMBLAGE

The Swift Creek volcanic assemblage was formed prior to the eruption 

of tephra set J, and includes some of the earliest products of the old Mount 

St. Helens eruptive center. The assemblage consists of a succession of 

pyroclastic-flow deposits, lahars, and alluvium, and is divided into two 

parts; one part is older than glacial drift of Fraser age, and the other is 

younger.

OLDER PART OF THE SWIFT CREEK ASSEMBLAGE

Before and during the early part of the Fraser Glaciation, a broad fan of 

pyroclastic-flow deposits and lahars resulting from volcanism at the old 

Mount St. Helens eruptive center spread southward and southwestward 

and formed fills in preexisting valleys that drained the south side of the 

volcano. These deposits have been covered with younger volcanic 

deposits and glacial drift in the area between the base of the volcano and 

the Lewis River valley. They can be seen, however, in a streamcut near 

June Lake (loc. 3, fig. 1), beneath till of Fraser age. There, the volcanic 

deposits include three lahars from 0.3 to more than 2 m thick, the upper 

two of which are separated by sand and fine gravel. A reddish-gray 

oxidized zone in the upper two lahars suggests that they were hot when 

they were deposited.

The older part of the Swift Creek assemblage is also exposed in a 

roadcut near McBride Lake (loc. 4, fig. 1). There, till of Fraser age is 

underlain by a pumiceous pyroclastic-flow deposit 0.7 m thick which is 

separated by a thin bed of sand from a lithic pyroclastic-flow deposit as 

much as 3 m thick. Lenticular sand and gravel as much as 2.7 m thick 

underlie this deposit, and the basal unit of the exposed sequence is a 

flowage deposit more than 8 m thick. The flowage deposit is unsorted and 

unstratified, and consists of angular fragments of andesite and dacite in a 

compact silty sand matrix. Rock fragments in the flowage deposit have 

random directions of NRM.

The fill deposits in the old Swift Creek valley extend southward and 

form high terraces on both sides of the Lewis River valley near Swift Dam 

(figs. 1, 3). The deposits originally must have formed a fan in the Lewis 

River valley, whose surface extended not only southward across the valley 

as well as downstream, but also sloped eastward, upstream. On the north
BIO GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

FIGURE 3. Aerial view eastward up the Lewis River valley. The long southward-sloping 

surface left and above Swift Darn is formed by the top of the Swift Creek assemblage.

side of the valley, the top of the fill is at an altitude of about 370 m, and on 

the south side of the valley, 1.5 km downstream from the dam, its top is 

about 70 m lower because of a 55 m/km southward slope of the fill surface 

(fig. 4). The deposits just east of the mouth of Swift Creek slope eastward. 

The Lewis River was temporarily ponded by this fan, but a fill terrace 

upstream from the fan and graded to it has not been found.

The nature of the valley-fill deposits near Swift Dam is illustrated by 

streambed outcrops, roadcuts, and borrow-pit exposures. The lowest 

deposit exposed is a lahar which crops out in the bed of the Lewis River 

between Swift Dam and the upstream end of Yale Lake; it contains wood 

fragments which have a radiocarbon age of about 36,000 years (table 1).

1 KILOMETRE

FIGURE 4. Profile showing the relationship between the surface of the Swift Creek assemblage and the Lewis River valley. Vertical exaggeration about X 2.
UPPER PLEISTOCENE PYROCLASTIC-FLOW DEPOSITS AND LAHARS Bl 1

The deposit is partly oxidized and slightly cemented by secondary iron 

oxides. The lahar contains fragments of nonvesicular andesite and dacite 

as large as 25 cm in diameter as well as fragments of pumice which has a 

biotite-cummingtonite-hornblende heavy-mineral suite. Wood fragments are abundant, and some are as large as 30 cm in diameter. The wood 

is not carbonized, and the deposit shows no evidence of having been hot 

when it was emplaced. The base of the lahar is not exposed, and it may be 

considerably below the streambed. Holes drilled beneath the valley floor 

near the centerline of Swift Dam encountered angular rock fragments, 

pumice, and wood down to an altitude of about Him, which is about 49 

m below the present river level (Chief Engineer, Pacific Power and Light 

Co., unpub. data, 1967). It is not known whether the drill holes penetrated 

a single lahar nearly 50 m thick or a series of thinner but texturally similar 

lahars. However, the presence of these deposits beneath the flood plain 

suggests that when Mount St. Helens first came into existence the Lewis 

River was flowing on a profile at least 50 m lower than at present. The 

drill holes show the base of the valley fill to be at an altitude of about 126 

m (fig. 5).

Other parts of the valley fill crop out in roadcuts and in borrow pits (fig. 

5). On the north side of the Lewis River valley, the lowest deposit exposed 

is a crudely bedded boulder gravel more than 23 m thick whose base is 

concealed. The boulders are as large as 2 m in diameter and are chiefly 

dacite and andesite derived from old Mount St. Helens. They are subangular and subround and are enclosed in a silty sand matrix. This 

deposit probably is chiefly of fluvial origin, but parts of it may have been 

formed by lahars. Overlying the gravel is a series of pyroclastic-flow 

deposits and lahars. The lowest unit of this series is an unstratified and 

unsorted coarse flowage deposit of dacite and andesite fragments 10-20 

cm in diameter in a compact matrix of silty sand. The deposit is 20 m 

thick. Inconclusive results were obtained from remanent-magnetism 

determinations on rock fragments from this deposit, and it is not known 

whether it was formed by a pyroclas tic flow or by a lahar. It is overlain by a 

pyroclas tic-flow deposit which consists almost entirely of blocks of gray 

pumice as large as 2 m in diameter and which is 23 m thick. Deposits of 

fluvial sand and gravel, lahars, and another pyroclastic flow, having a 

combined thickness of about 13m, form the upper part of the fill on the 

north side of the valley.

The coarse flowage deposit of unknown origin and the overlying 

pyroclas tic-flow deposits and lahars are also exposed along a logging 

road and in a borrow pit (loc. 6, fig. 1; measured section, p. B13) on the 

south side of the Lewis River, 1.5 km west of Swift Dam. There, a 

pyroclastic-f low deposit of blocks of dark-gray pumice, which overlies the 

coarse flowage deposit, is 12 m thick and contains charcoal that has a 

radiocarbon age of 20,350*500 years (table 1; fig. 6). The pyroclastic-flow
o

e:

so s: 2 

M fB J2 

s-r 1 I 2. 8 

C/3 O QR ^ = 

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C. oo O 

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8-8?

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of part Older 

assemblage Creek Swift 

AOOTO3O JO INIIOW 'IS SNT13H 'ONV31OA NOiONIHSVAY 
UPPER PLEISTOCENE PYROCLASTIC-FLOW DEPOSITS AND LAHARS B13

deposit is overlain by fluvial sand and gravel 1-2 m thick which contains 

rock types derived from the drainage basin of either the upper Lewis River 

or Swift Creek, as well as rocks from old Mount St. Helens. On top of the 

gravel is a thick deposit of clayey silt and sand about 2 m thick, in the 

middle of which is the tephra couplet (p. B6). A second pyroclastic-flow 

deposit of light-gray pumice blocks, 2 m thick, overlies the fine sediments 

and contains charcoal which has a radiocarbon age of 18,560*550 years 

(table 1). The deposits of clayey silt and sand, which lie between the two 

pyroclastic-flow deposits, are unstratified and probably are of eolian 

origin. They may consist of sediment that was blown from the Lewis 

River flood plain, as well as of fine-grained tephra. The clayey silt and 

sand deposits both below and above the couplet seem to be weathered. 

However, in view of the time interval of less than 2,000 years during which 

they were formed, their clayey texture may be wholly original and 

probably does not represent weathering of individual mineral grains to 

clay.

Measured section

[Locality 6 (fig. 1), south side of Lewis River valley, borrow pit beside a private logging road near Rain 

Creek, SE'/4 sec. 29, T. 7 N., R. 5 E. Color notations are those of the Munsell system (Munsell Color

Co- 1954>] Thickness

in metres

10. Sandy silt, brown (IOYR 4/4); nonsticky and nonplastic to slightly sticky

and slightly plastic, structureless.................................................................... 1

9. Proclastic-flow deposit, medium-gray; pumiceous silty sand matrix; rock fragments mostly gray pumice as large as 1 m; charcoal at base (radiocarbon 

sample W-2413: 18,560±550 yr); thickness variable, average.......................... 2

8. Tephra(P); layers of silt and sand, brown to gray, some sand-size white pumice.. .15

7. Clayey silt and sand, dark-brown (\OYR 3/3), grades downward to brown

(IOYR 4/4) and yellowish-brown (WYR 5/6)................................................ 1

6. Tephra couplet; two layers of yellow pumice and lithic fragments 5 cm thick

separated by 10 cm of silty sand..................................................................... .2

5. Clayey silt and sand, mottled brown, reddish-brown, and gray; sticky, plastic.. 1

4. Stratified sand and pebble-to-boulder gravel, light-yellowish-brown to gray; 

rock fragments of basalt, dacite, andesite, and Tertiary bedrock; some 

boulders as large as 1 m.................................................................................. 1-2

3. Pyroclastic-flow deposit, medium- to dark-gray; pumiceous silty sand matrix; 

rock fragments mostly gray pumice as large as 1.3 m; abundant charcoal 

(radiocarbon sample W-2540: 20,350*500 yr)................................................. 12

2. Sand, gray; discontinuous; as much as.............................................................. .06

1. Flowage deposit, gray; silty sand matrix, compact; rock fragments mostly

dacite and andesite, angular, 10-20 cm in diameter; unstratified.................. >2

Remnants of the older part of the Swift Creek assemblage are preserved 

along the sides of the Lewis River valley near Cougar and in an 

abandoned segment of the valley which is now occupied by Speelyai 

Creek (fig. 1). The top of the fill at Cougar is at an altitude of about 230 m 

and is about 85 m above the valley floor now drowned by Yale Lake. Only 

the uppermost part of the fill deposits is exposed there. It consists of a
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FIGURE 6. Borrow-pit exposure of pyroclas tic-flow deposits separated by fluvial sand and gravel and tephra in the older part of the Swift Creek

assemblage at locality 6. Note shovel (center of photograph) for scale.
UPPER PLEISTOCENE PYROCLASTIC-FLOW DEPOSITS AND LAHARS B15

coarse fluvial gravel made up mostly of cobbles and boulders of dark-gray 

pumice. This pumice probably was derived from one of the two 

pyroclastic-flow deposits exposed on the south side of the Lewis River 

valley near Swift Dam (measured section, units 3 and 9, p. B13).

Only coarse fluvial gravel, interbedded with tephra, is exposed above 

lake level at the upvalley end of the Speelyai Creek fill. The tephra couplet 

occurs near the top of this sequence and is overlain by coarse gravel which 

locally fills a channel cut into the tephra couplet and into the underlying 

fluvial deposits. Both gravel deposits are made up of rocks derived from 

Mount St. Helens. Fragments of dark-gray pumice were not identified 

within these gravel deposits, but a few boulders of the pumice were noted 

in a beach gravel at the base of a cliff cut by waves into the deposits. At the 

downvalley end of the Speelyai Creek fill, the top of the fill is at an altitude 

of about 130 m, and it lies 90 m above the valley floor drowned by Yale 

Lake. The basal part of the fill exposed there consists of a lahar more than 

10 m thick; it is overlain by another lahar whose thickness is 10m. The 

upper 70 m of the fill deposits consists of interbedded fluvial gravel and 

lahars.

Remnants of the older part of the Swift Creek assemblage can also be 

found along the present channel of the Lewis River between Yale Dam 

and the head of Lake Merwin. The upper surface of the fill in that area is 

at an altitude of a little less than 135 m.

YOUNGER PART OF THE SWIFT CREEK ASSEMBLAGE

The formation of the older and younger parts of the Swift Creek 

assemblage was separated by a time interval of unknown duration, during 

which alpine glaciers advanced and then retreated in the higher parts of 

the area. Recession of the glaciers was followed by an additional interval 

which is represented by a weathered zone in the uppermost 24-30 cm of 

till. After this weathering interval, volcanism at the old Mount St. Helens 

eruptive center again caused pyroclastic flows and lahars to move down 

valleys cut into the older fill. Pyroclastic-flow deposits formed at that time 

are best exposed in roadcuts along the west wall of the Swift Creek valley 

at locality 2 (fig. 1). There, two pumiceous pyroclastic-flow deposits 

overlie a f lowage deposit of unknown origin which rests directly on till of 

Fraser age. Rock fragments in the lowest pyroclastic-flow deposit consist 

of 73 percent pumice and 27 percent nonvesicular andesite and dacite, but 

23 percent of the fragments in the overlying deposit are pumice. The 

pyroclastic flows that formed these deposits probably originated in 

eruptions of gas-rich magma. Tephra set S separates these two deposits 

form a third pyroclastic-flow deposit in which pumice is absent. The 

deposit contains angular, radially jointed rock fragments 10-20 cm in 

diameter and is only about 1 m thick. In an old borrow pit on the ridge 

above the locality 2 road exposures, this same deposit is 7 m thick and 

contains blocks as large as 2 m in diameter. Rock fragments in the third
B16 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

pyroclastic-flow deposit have a preferred orientation of NRM. The 

abundance of nonvesicular rock fragments in this deposit suggests that 

the pyroclastic flow was caused by the disruption of part of a growing 

dome at the eruptive center.

Ash-cloud deposits, which were formed during the accumulation of the 

younger part of the Swift Creek assemblage, are interbedded with the 

tephra sequence on the west and south sides of the Swift Creek valley. 

West of Ape Cave they are as thick as 2 m at a height of 200 m above the 

valley floor. On the north side of Marble Mountain, they are 2 m thick 

about 100 m above the valley floor. The deposits typically are nonstratified fine- to coarse-grained lithic ash with scattered pumice 

fragments. The ash-cloud deposits probably formed from clouds of ash 

rising above hot pyroclastic flows moving down the Swift Creek valley.

The younger part of the Swift Creek assemblage in the Lewis River 

valley is represented by a few pyroclastic-flow deposits and by lahars and 

fluvial deposits derived from old Mount St. Helens. Remnants of two 

pyroclastic-flow deposits are exposed on the east valley wall near the 

mouth of Swift Creek. The lower deposit consists of an unsorted and 

unstratified silty sand containing rock fragments of dacite and andesite as 

large as 60 cm. Above it is a pyroclastic-flow deposit from which charcoal 

has yielded a radiocarbon date of 13,130*350 years (table 1). The two 

deposits are overlain by upper beds of tephra set S (D. R. Mullineaux, 

unpub. data, 1973) and rest upon units of the older part of the Swift Creek 

assemblage.

The best exposure of the younger part of the Swift Creek assemblage 

farther down the Lewis River valley is in a borrow pit at locality 5 (fig. 1), 

at the outskirts of the community of Cougar, where three lahars are 

exposed. The lowest lahar, which is more than 8 m thick, is separated 

from the other two by a lenticular deposit, less than a metre thick, of 

oxidized silt and stratified sand and gravel (fig. 7). The upper two lahars 

are each a little more than a metre thick. All the lahars consist chiefly 

of sand and granule gravel; the few pebbles and cobbles in them are 

andesite and dacite derived from old Mount St. Helens. The lowest lahar 

contains masses of stratified sand and gravel as large as Ix2m across 

that may have been transported in a frozen condition which prevented 

disaggregation during movement. A time interval during which the 

lowest lahar was exposed to subaerial weathering is suggested by oxidation in the silt which overlies it.

The top of the deposits at the pit near Cougar is at an altitude of about 

200 m and is about 20 m lower than the top of the older part of the Swift 

Creek assemblage nearby.

Four lahars are exposed in the walls of the valley of Dog Creek, about 1 

km west of Cougar but closer to the center of the valley than the borrowpit exposure. The lahars are each 1 to 2 m thick and are texturally similar 

to those at the borrow pit. Fluvial sand and gravel derived from the Lewis
UPPER PLEISTOCENE PYROCLASTIC-FLOW DEPOSITS AND LAHARS B17

FIGURE 7. Borrow-pit exposure at locality 5 of two lahars of the younger part of the 

Swift Creek assemblage. The lahars are separated by a lenticular deposit of oxidized 

silt, sand, and gravel. The lower lahar is about 3 m thick in this outcrop and shows 

a textural gradation toward the top.

River drainage basin is interbedded with lahars in the lower part of the 

outcrop, and locally derived gravel deposited by Dog Creek is interbedded 

with the lahars in the upper part of the outcrop.

Farther down the Lewis River valley, fine-grained lahars with a 

minimum total thickness of 15 m are exposed in cuts along State Highway 

503, 3 km west of Yale Dam. A discontinuous layer of tephra, not more 

than 1 cm thick, is preserved between two of the lahars here, as well as 

between the lahars that crop out along a road near the mouth of Speelyai 

Creek. The heavy-mineral suite of this tephra is similar to that of tephra 

set S.

The younger part of the Swift Creek assemblage forms a flat westsloping surface at an altitude of 120-135 m immediately west of Yale Dam. 

The surface is directly underlain by lahars which have a combined 

thickness of several metres. In gravel-pit exposures 1.3 km west of Yale
B18 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

Dam, the lahars overlie a thick deposit of oxidized silt which caps the 

older part of the Swift Creek assemblage.

The Lewis River near Yale Dam probably first occupied its present 

course when a pre-Fraser alpine glacier blocked the former main valley, 

forced the river to the south, and eroded the new channel below the level of 

the former channel. The abandoned segment of the old valley, now 

occupied by Speelyai Creek, subsequently was aggraded by deposits 

formed by lahars and floods.

CHARACTER OF VOLCANIC ERUPTIONS

The stratigraphic succession and physical character of volcanic 

deposits in the Lewis River and Swift Creek valleys show that the old 

Mount St. Helens eruptive center was characterized by explosive dacitic 

and andesitic volcanism from its beginning until the time tephra set J was 

erupted (table 2). Other studies (Hyde, 1970; Hopson, 1971; Mullineaux 

and others, 1972; Crandell and Mullineaux, 1973) have shown that no 

significant change in eruptive patterns occurred after Swift Creek time, 

except for the intermittent eruption of more mafic lava flows and tephra 

during the last 2,500 years.

TABLE 2. Sequence of events at Mount St. Helens and in the adjacent region prior 

to 12,000 years ago, as indicated by deposits in the area south of the volcano

Approximate age, or

Event limiting dates

(years ago)

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

Eruption of tephra set S and formation of hot pyroclastic flows

and lahars (younger part of the Swift Creek assemblage)............<18,000(?), >12,000

Advance and retreat of alpine glaciers.............................................<20,000(?), >18,000(?)

Eruption of tephra (unnamed set), formation of hot pyroclastic 

flows, lahars, and coarse fluvial deposits (older part of the Swift 

Creek assemblage)..........................................................................<40,000(?), 18,000

Advance and retreat of alpine glaciers.............................................<40,000(?)

The lithic and pumiceous pyroclastic-flow deposits of the Swift Creek 

assemblage probably were caused by repeated intrusion of viscous magma 

at shallow depth. The pumiceous pyroclastic-flow deposits probably 

were formed by eruptions of volatile-rich magma. These eruptions 

alternated with those of volatile-poor magma which formed domes and 

.spines whose disintegration and collapse resulted in hot pyroclastic flows 

of nonvesicular rock debris. The preferred orientation of NRM in rock 

fragments shows that many pyroclastic-flow deposits were emplaced 

above temperatures of at least 300° C, and perhaps more than 500° C, but 

below their welding temperature.

The distribution of volcanic deposits shows that an eruptive center,was 

located in the approximate position of the modern cone throughout the 

time the Swift Creek assemblage accumulated, but little is known of its
UPPER PLEISTOCENE PYROCLASTIC-FLOW DEPOSITS AND LAHARS B19

exact position, size, or shape. The distribution of cirques and glacial 

deposits suggests that the volcano was not large during Fraser time, and 

the nature of the volcanic rock debris derived from it suggests that the old 

center consisted of one or more low andesite and dacite domes. Crandell 

and Mullineaux (1973) have suggested a similar character for old Mount 

St. Helens between about 12,000 and 2,500 years ago.

EFFECT OF VOLCANISM ON THE LEWIS RIVER VALLEY

The main effect of volcanism on the Lewis River valley during Swift 

Creek time seems to have been the formation of a large fill of 

unconsolidated volcanic deposits which was continuously or intermittently trenched by the Lewis River. The volume and texture of 

material contributed to the Lewis River temporarily exceeded the ability 

of the river to transport all the rock debris downstream on the existing 

gradients, and thus the river was temporarily ponded. Periods of subsequent downcutting occurred when the influx of rock debris from the 

volcano decreased.

Before volcanism began at the old Mount St. Helens eruptive center the 

Lewis River was probably flowing on or close to bedrock near the mouth 

of Swift Creek and for a considerable distance downstream. Bedrock was 

encountered at a depth of 65 m below the valley floor at Swift Dam, and 

rock is exposed today at river level immediately downstream from Yale 

Dam and about 20 km farther downvalley, below Merwin Dam.

The character of volcanic eruptions suggests that most of the deposits 

in the Swift Creek assemblage accumulated in a very small fraction of the 

total time represented by the oldest and youngest units in the fill. Some 

deposits may represent events which occurred over a period of days or 

months, but many were formed as a result of a single volcanic event, such 

as a pyroclastic flow or lahar, whose total duration was perhaps only a 

matter of minutes. Thus, because of the short-lived nature of the events 

which resulted in the Swift Creek assemblage, it seems unlikely that 

dep _sits transported down Swift Creek into the Lewis River valley had 

more than a temporary effect on the profile of the river upstream. This 

conclusion is supported by the apparent absence of a fill which would 

have formed in the Lewis River valley upstream from the mouth of Swift 

Creek if the profile of the river had been more than temporarily changed. 

The Lewis River must have entrenched the fill at and downstream from 

Swift Creek in the intervals between volcanic events, and probably 

removed only a small part of the fill before it became reestablished on a 

gradient not unlike the one it had before the event.

The lahars and alluvium of the older part of the Swift Creek assemblage 

aggraded the abandoned segment of the Lewis River valley near Speelyai 

Creek, as well as the main part of the present valley. Lahars of the younger 

part of the Swift Creek assemblage were mostly confined to the trench cut 

by the Lewis River into the older part of the assemblage.
B20 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

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