Geological Society of America Bulletin - Oregon State … lahars...Recent Lahars from Mount St....

Geological Society of America Bulletin

doi: 10.1130/0016-7606(1962)73[855:RLFMSH]2.0.CO;2 1962;73, no. 7;855-870Geological Society of America Bulletin

D. R MULLINEAUX and D. R CRANDELL Recent Lahars from Mount St. Helens, Washington

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D. R. MULLINEAUXD. R. CRANDELL

U. S. Geological Survey, Denver, Colo.

Recent Lahars from Mount St. Helens, Washington

Abstract: Late Recent eruptions of Mount St.Helens volcano in the Cascade Range of southernWashington have caused numerous lahars; some ex-tend more than 40 miles down valleys west of thevolcano. These lahars typically consist of poorlysorted and unstratified detritus derived almostwholly from the volcano. The lahars and inter-bedded fluvial gravel commonly form valley fills;such a fill in the valley of the North Fork of theToutle River contains at least three lahars.

Lahars from Mount St. Helens are composedmainly of nonvesicular rock fragments; thus, theirgreat distance of transport is attributed to mobility

provided by water rather than by gas emitted fromparticles in the matrix. Near the volcano, some ofthe lahars contain charred wood, proving that theywere hot and resulted directly from eruptions.

Wood from within a lahar in the fill in the NorthFork of the Toutle River valley has a radiocarbonage of about 2000 years. The lahar consists of ma-terial derived from a Mount St. Helens volcanothat existed before the present cone was built; thus,the present cone must be less than 2000 years old.In addition, interpretation of soil profiles youngerthan the dated wood suggests that the modern conemay have been built within the last thousand vears.

CONTENTS

IntroductionAcknowledgmentsRecognition of laharsDescription o f lahars studied . . . .

Silver Lake lahar assemblage . . . .DescriptionOriginWeathering profiles and age . . .

Lahar at Spirit Lake Lodge . . . .Probable lahar in Kalama River valley

Discussion of hot laharsAge of Mount St. HelensReferences cited

. 855

. 857

. 857

. 859

. 859

. 859

. 861

. 862

. 862

. 864

. 866

. 866

Figure1. Index map of vicinity of lahars studied . . . . 8562. Cumulative curves illustrating grain size of lahars

at Gilmore Corners 860

3. Cumulative curves illustrating gram size of laharat Spirit Lake Lodge and lahar in KalamaRiver valley 863

4. Point diagram of azimuth and dip of north-seek-ing poles in 10 stones from Kalama Riverlahar 865

5. Index map of vicinity of Mount St. Helens . . 867

Plate Facing1. Silver Lake lahar assemblage 8582. Comparison of stones in lahar and alluvium, and

lahar near Spirit Lake Lodge 859

Table1. Lithology of stones in upper lahar at Gilmore

Corners gravel pit 861

INTRODUCTION

Mount St. Helens is a Recent volcano on thewestern flank of the Cascade Range in southernWashington (Fig. 1). Thick deposits of eruptivedetritus, including coarse and fine pyroclasticdeposits and lahars1, form much of the slopesof the volcano and extend over older rocks.Construction of the volcano has extensively de-ranged previously established westward drain-

1 Lahar is an Indonesian word that describes mud-flows and debris flows originating on volcanoes (vanBemmelen, 1949, p. 191).

age in its vicinity, and valley fills of lahars andalluvium from it extend for many miles downold valleys west of the volcano.

This paper examines criteria for the recog-nition of lahars and describes a 40-mile fill ofalluvium and lahars in the valley of the NorthFork of the Toutle River. Two shorter flow-age deposits, probably Mount St. Helens lahars,were also examined. These are similar to thosein the Toutle River valley and are especiallysignificant, as they contain charred wood thatshows they resulted directly from volcaniceruptions.

The lahars were studied as part of a U. S.

Geological Society of America Bulletin, v. 73, p. 855-870, 5 figs., 2 pis., July 1962

855



MULLINEAUX AND CRANDELL—RECENT LAHARS, WASHINGTON



INTRODUCTION 857

Geological Survey investigation of volcanicmudflows and debris flows in the CascadeRange of the Pacific Northwest. Field work inthe Toutle River valley and at Mount St.Helens was done during the summers of 1957,1958, and 1959.

Explorers and geologists who visited thePacific Northwest in the 19th century (Fre-mont, 1845, p. 193; Gibbs, 1854, p. 497) re-ported eruptive activity of Mount St. Helens.Diller (1899) discussed pieces of coniferouscharcoal collected in the Kalama River valleyby botanist Frederick Colville. Although Dillernoted that no such wood had actually been seenin tree casts in lava, he concluded that Col-ville's specimens must have come from logseroded from lava and redeposited in gravel.

Verhoogen (1937) described the volcanicstratigraphy and structure of Mount St.Helens and distinguished the hornblende-hypersthene andesites and hornblende dacitesof an "older Mount St. Helens series" from theolivine basalts and pyroxene andesites thatform most of the present cone. Verhoogen alsorecognized volcanic mudflows in deposits ofboth the older and present volcanoes. Later,Erdmann and Warren (1938) investigated dam-sites along the North Fork of the Toutle Riverand described a volcanic mudflow at the outletof Spirit Lake. Lawrence (1938, 1939, 1954)discussed the pyroclastic deposits on the northflank of Mount St. Helens.

ACKNOWLEDGMENTSWe are indebted to the Weyerhaeuser Tim-

ber Company for access to the Kalama Rivervalley and to Ralph Sheppard of the Washing-ton State Division of Forestry for guidance tothe charred logs. Dr. Donald Clark, of theForest Products Laboratory at the Universityof Washington, identified fragments of thecharred wood and supplied information re-garding the indicated conditions of burning.A. E. Roberts, U. S. Geological Survey, con-tributed background information on thegeology of the lower part of the Toutle Rivervalley. Meyer Rubin supervised the analysis ofthe radiocarbon sample in the laboratories ofthe U. S. Geological Survey, and W. E. Huffanalyzed remanent magnetism of rock samples.

We express our appreciation to the follow-ing people for helpful criticism of the contentand organization of the paper: R. W. Hay, JohnVerhoogen, and Howel Williams of the Univer-sity of California at Berkeley, J. Hoover Mackinof the University of Washington, Laurence H.

Nobles of Northwestern University, andRichard Q. Lewis, Howard A. Powers, DonaldE. Trimble, and Ray E. Wilcox of the U. S.Geological Survey.

RECOGNITION OF LAHARS

The term, lahar, includes all of the broadtextural range of debris flows and mudflows ofvolcanic origin. Sharp and Nobles (1953, p.550) suggest that the term, debris flow, be usedas a general designation for all types of rapidmass flowage involving debris of various kindsand conditions. They state that a mudflow is avariety of debris flow in which the mud, al-though not necessarily quantitatively dom-inant, gives to the mass the specific propertiesand modes of behavior that distinguish it fromflows of debris in which a very fine-grainedcomponent is lacking. Varnes (1958, p. 37) sug-gests that the term, mudflow, be reserved formaterial with at least 50 per cent sand, silt, andclay-size particles; he uses the term, debrisflow, to apply to material with a relatively highpercentage of coarse fragments. As the flowagedeposits described herein grade from a debrisflow at the base to a mudflow at the top, theterm, lahar, is used to designate any unsortedor poorly sorted deposit of volcanic debris thatmoved and was deposited as a mass and owedits mobility to water.

Lahars tend to follow pre-existing valleys andare commonly interstratified with nearly con-temporary alluvium derived from the samesource area. A deposit of two or more lahars,including interbedded alluvium where present,that is lithologically distinct and that lies in agiven valley is here referred to as a lahar as-semblage. The lahar assemblages from MountSt. Helens consist chiefly of lahars near the vol-cano but become progressively more dilutedwith alluvium downstream, as individual laharsthin and pinch out.

Lahars in the valleys heading at Mount St.Helens have several features in common: (1)they are poorly sorted, (2) unstratified, (3)have typically subangular component stones,(4) occur in valley fills interstratified withfluvial gravel, and (5) consist almost wholly ofmaterial derived from Mount St. Helens.Lahars share some of these features with de-posits of other origins, such as till, colluvium,fluvial gravel, pyroclastic deposits, and dryvolcanic avalanches.

Features that distinguish lahars from till arevertical grading from coarse material at thebase to fine material at the top (Crandell,



858 MULLINEAUX AND CRANDELL—RECENT LAHARS, WASHINGTON

1957), flat tops, restriction to valley floors, and,in some lahars, the presence of wood that wascharred in place. Vertical grading in lahars iswidespread but is relatively uncommon inlahars on steep gradients near the source vol-cano. Farther from the volcano, vertical grad-ing is probably the single most useful indica-tion of origin; where this feature is present indeposits of coarse nonvesicular material farfrom the source, we believe it is a criterion ofmass transport of the material in a flow mobi-lized by water.

The prevailing Mount St. Helens provenanceof the lahars studied distinguishes them, downvalley from the volcano, from colluvial de-posits derived locally from older rocks.

As some lahars contain a small proportion ofsilt and clay, they may readily be confusedwith poorly sorted fluvial sand and gravel.Stones in the lahars from Mount St. Helens arenoticeably more angular than those of the samerock types in fluvial gravel a comparable dis-tance from the volcano. This angularity isprobably a result of transport in a suspension,whereby stones are buffered by a viscous matrixthat greatly reduces abrasion. In contrast,stones are rapidly rounded as they roll andbounce along the stream bed during fluvialtransport.

The absence in lahars of structures character-istic of alluvium also aids in differentiating thetwo kinds of deposits. Individual lahars extendfor considerable distances with little change inthickness and without significant lateral changein grain size. These features are not typical ofbeds of fluvial gravel. Bar or cut-and-fill struc-tures typical of alluvium are absent. Gradualvertical change in grain size from coarse at thebase to fine at the top is common in lahars, butthe vertical alternation of grain sizes typical ofalluvium is absent.

The presence of large boulders in laharsfurther serves to differentiate them fromfluvial deposits. Some lahars studied containboulders whose diameter is as much as athousand times the median diameter of particlesin the enclosing matrix. These large bouldersare in an aggradational fill that is formed by alahar assemblage. Their presence demands someagent other than fluvial transport, for unlikegraded or degrading streams, aggrading streamstend to bury rather than rework the coarsestchannel material, and the resulting depositsshow a marked down-valley decrease in grainsize (Mackin, 1948, p. 505; Pettijohn, 1957, p.541). On the other hand, the ability of debris

flows and mudflows to transport very largeboulders for long distances is well known.

Where a lahar contains fragile wood that wascharred in place or surrounds stumps charredby bombs in the deposit, the fact that it washot or carrying hot material distinguishes itfrom alluvium.

The distinction of lahars from coarse, air-laid, volcanic material is difficult to make closeto the source volcano. Restriction of lahars tovalley floors, however, provides a usefulcriterion.

Differences in composition and evidences ofmobility and heat are helpful in distinguishinglahars from deposits of hot, dry, volcanicavalanches. The extreme types of hot, dry,volcanic avalanches are (1) gas-rich, highlymobile, pumiceous avalanches and flows,usually referred to as glowing avalanches, glow-ing clouds, or nuees ardentes and (2) varioustypes of gas-poor, less mobile, block-and-ashavalanches, some of which are referred to bythese same terms.

Highly mobile, glowing, pumiceous ava-lanches and flows originate in an eruption ofgas-rich magma. As the magma is expelled fromthe volcano, gas pressure causes it to froth, andas an avalanche of this material descends theslopes of the volcano, hot dense gas is emittedfrom incandescent particles of the erupted ma-terial, enveloping the particles and makingthem mobile (Ferret, 1937, p. 84, 89). Becauseof their great mobility, individual gas-rich ava-lanches have traveled down valleys to distancesof as much as 40 miles from the source volcano(Williams, 1942, p. 81). The most significantfeatures of the resulting deposits are theirlength, abundance of constituent glass shardsor vesicular glass, and evidence of very hightemperature (Marshall, 1935; Gilbert, 1938;MacGregor, 1938; Williams, 1942, 1956; Hay,1959). Deposits of glowing avalanches nearCrater Lake, Oregon, which were formed dur-ing the climactic eruptions of Mount Mazama,contain from 41 to 85 per cent vesicular glass,as compared to 10 to 30 per cent lithic frag-ments (Williams, 1942, p. 84). The crystal-rich(50 per cent), glowing avalanche deposits ofthe Soufriere of St. Vincent, B. W. I., contain36 per cent scoria and vitric ash, and only 14per cent nonvesicular rock (Hay, 1959, p. 546).Great mobility and length are characteristic ofboth lahars and glowing avalanches; however, ascarcity of vesicular glass or evidence of lowtemperatures may distinguish one from theother.



Figure 1. View of northeast side of borrow pit, Gilmore Corners, approximately atmeasured section; depositional units labeled as in measured section, except unit 4(not visible). Crude stratification, imbrication of stones in unit 2 show that it is fluvialgravel and not coarse base of overlying lahar (unit 3).

Figure 2. Upper lahar of Silver Lake assemblage in northern part of Gilmore Corners gravelpit, showing distinct vertical size gradation

SILVER LAKE LAHAR ASSEMBLAGE, MOUNT SAINT HELENS AREA, WASHINGTON

MULLINEAUX AND CRANDELL, PLATE 1Geological Society of America Bulletin, volume 73



Figure 1. Comparison of typical stones from upper lahar at Gilmore Corners (onsample sack) with gravel on bar of Toutle River near Gdmore Corners. Note meagerabrasion on edges and faces of most stones from lahar. Well-rounded stones in bargravel are lithologically like stones from lahar. Sack is 20 inches long.

Figure 2. Younger bomb-bearing lahar 50 yards east of Spirit Lake Lodge. Boulders arealmost all breadcrust scoria bombs. Man points to completely charred small log. Largeupright stump behind man, entwined in younger roots, is charred only where bombs lieagainst it.

COMPARISON OF STONES IN LAHAR AND ALLUVIUM, AND LAHAR NEAR SPIRITLAKE LODGE, MOUNT SAINT HELENS AREA, WASHINGTON

MULLINEAUX AND CRANDELL, PLATE 2Geological Society of America Bulletin, volume 73



RECOGNITION OF LAHARS 859

Block-and-ash avalanches, composed mostlyof nonvesicular rock, are generally initiated bymild gas-poor eruptions, collapse of spines andparts of domes, or slides of rapidly accumulatedpyroclastic deposits on the slopes of volcanoes.Ferret (1937) reported that some block-and-ash flows are cushioned by gas emitted fromconstituent particles and regarded them as thefeeblest type of nu'ee ardente. Avalanches ofblock and ash from accumulated hot pyro-clastic debris on the upper part of Vesuviusduring the 1906 eruption traveled a maximumof 2.5 miles from the vent and were confined tothe slopes of the volcano, although the ma-terial did contain hot gas (Ferret, 1924, p. 102).Although McTaggert (1960) suggests that themobility of both gas-rich and gas-poor ava-lanches is chiefly due to heating of entrappedcold air and is a function of temperature ratherthan gas, the descriptions of Ferret, Williams,and others indicate that block-and-ash flowscharacteristically are shorter than pumiceousflows. If so, relatively limited distance of traveland abundance of nonvesicular constituentrock are significant features of hot block-and-ash avalanches; their limited extent helps todifferentiate them from lahars, which maytravel tens of miles from the source volcano.

These criteria arc useful in recognizing laharsbut are not always diagnostic, as lahars com-monly grade into some of the other types of de-posits mentioned. For example, hot dry ava-lanches of all types may become hot lahars byaccumulation of water, generally from snow-fields or streams; hot lahars may cool as theytravel down-slope or down-valley, and thosethat flow into relatively large rivers may gradeimperceptibly into muddy streams whose de-posits are texturally indistinguishable fromthose of other muddy streams. In this paper wediscuss some lahars that are far from the vol-cano, interbedded with alluvium, and prob-ably cold where we studied them; a hot laharin which the largest fragments were hot enoughto char adjacent wood; and a deposit that washot enough at one time to convert entire logsto charcoal and may have been transitional be-tween a hot dry avalanche and a hot lahar.

DESCRIPTION OF LAHARS STUDIED

Silver Lake Lahar Assemblage

Description. A fill terrace in the ToutleRiver valley near Silver Lake consists of laharsinterbedded with fluvial sand and gravel; sim-ilar deposits underlie terraces along the whole

length of the North Fork of the Toutle Riverfrom Spirit Lake to the Cowlitz River valley(Fig. 1). Fill terraces formed of this lahar as-semblage west and north of Castle Rock indi-cate that the valley fill extended into and downthe Cowlitz River valley to a distance of atleast 40 miles downstream from Mount St.Helens.

On the north flank of Mount St. Helens, thesurface of the Silver Lake lahar assemblage hasa gradient of 300-500 feet per mile downtoward the valley of the North Fork of theToutle River and Spirit Lake. The surface ofthe assemblage in the valley just west of SpiritLake has a gradient of about 150 feet per mile,which decreases to about 40 feet per mile nearSilver Lake, 30 miles down-valley. A few ter-races in the Cowlitz River valley between themouth of the Toutle River and Castle Rockindicate a gradient there of a little less than 20feet per mile.

The full thickness of the lahar assemblage isnot exposed, because the Cowlitz and Toutlerivers have not yet cut below its base. In theCowlitz River valley, 22 feet of the depositcrops out in the wall of a gravel pit northwestof Castle Rock (Fig. 1). The part of the as-semblage exposed there consists of two lahars, 2and 10 feet thick, respectively, and fluvial sandand gravel. The thickest known section cropsout along the Toutle River near Silver Lake,where the depositional top of the assemblage is60 feet above exposures at river level, althoughthe maximum thickness exposed in any oneoutcrop is about 45 feet.

Near Gilmore Corners (Fig. 1), the top ofthe Silver Lake lahar assemblage forms awesterly sloping surface back into a broadtributary valley and is responsible for SilverLake. The lahars are well exposed along theToutle River east of Silver Lake, but the bestoutcrops are at Gilmore Corners (PI. 1, fig. 1)in a gravel pit excavated in the assemblage be-tween Silver Lake and the Toutle River. Thefollowing section was measured in the north-east side of the pit, the deepest part, in August1959.

Measured sectionLocation: SWJ^NE^ sec. 30, T. 10 N.,

R. 1 E., in northeast side ofgravel pit at Gilmore Cor-ners, Washington

5. Lahar: pebbles and cobbles in a fineto coarse sand matrix, light graywith bluish cast; unstratified; few

Feet Inches




Feet Inchesstones in basal 6-12 inches; materialcoarsest in lower middle part of de-posit, becoming finer toward top;contains oxidized zone 1-3 feet thickat top 13-15

4. Alluvium(?): lenticular silt and finesand, pale brown, oxidized; containsscattered carbonized wood particles 1-9

3. Lahar: pebbles and granules in sandmatrix, light gray with bluish cast;

permost unit of the assemblage between ToutleRiver and Silver Lake and is exposed along thehighway on the north side of Toutle River 2.5miles northeast of Gilmore Corners. The areacovered by this lahar is at least 5 square milesin the vicinity of Silver Lake alone.

The upper lahar is typical of those in theSilver Lake assemblage. It consists of unstrati-fied poorly sorted material, which, although

Size of particles,

^

Figure 2. Cumulative curves illustrating grain size of lower (1A and IB) and upper (2A and 2B) laharsat Gilmorc Corners. Curve 1A based on sample from 7 to 10 feet below top of 14-foot thick upperlahar; curve IB based on sample from 2 to 3.5 feet below top of upper lahar. Curve 2A based on samplefrom 5 to 7 feet below top of lower lahar; curve 2B based on sample from 1 to 3 feet below the top ofthe same lahar

marked gradation from coarse ma-terial at base to fine material at top;locally oxidized in upper few inches 7-8

2. Alluvium: pebble and cobble gravel,light gray, rudely stratified; overliesand locally lies within channel cutinto unit 1; contact with unit 3 in-distinct 5-14

1. Lahar: Pebbles and granules in sandmatrix, light gray with bluish cast;unstratified; grades from pebble andsand mixture at base to granule andsand mixture at top; upper 10-12inches contains weathering profileand wood fragments. Base not ex-posed 9+

The upper lahar (unit 5) in the assemblageexposed in the walls of the pit varies little inthickness. This lahar apparently forms the up-

ranging in size from clay to boulders, is mainlyof sand to pebble size (Fig. 2). Except for abasal zone of fine material several inches thick,the deposit in most places shows a gradual de-crease in grain size from bottom to top (PI. 1,fig. 2). At some places, however, the upwarddecrease in grain size may be limited to theupper half or third of the lahar or may even bemissing. This change in grain size is marked;the median size of a sample from 7 to 10 feetbelow the top of the upper lahar is 16 mm, butthe median size of a sample from 2 to 3.5 feetbelow the top at the same place is only 0.95 mm(Fig. 2). The graded top of the upper lahar isalso somewhat better sorted than the rest ofthe deposit; samples from the basal and upperparts of the lahar have coefficients of sorting of10.2 and 6.62, respectively. Similarly, the co-



DESCRIPTION OF LAHARS STUDIED 861

efficient of sorting in the lower lahar is 8.35 ina sample 5-7 feet below the top but is 4.92 ina sample 1-3 feet below the top.

Most pebbles in the lahars are subrounded orsubangular; sand grains are subangular orangular. The angularity of rock fragments inthe lahars contrasts markedly with the round-ness of pebbles of the same rock types found inmodern bars of the Toutle River near GilmoreCorners and in fluvial gravel interbedded withthe lahars (PL 2, fig. 1). The long axes ofelongate pebbles in each lahar show a preferredorientation parallel to the trend of the valleyin which the deposits lie.

Large boulders are not abundant in the laharsnear Silver Lake or farther downstream, butthey become more abundant upstream. Manyboulders of dacite and andesite from MountSt. Helens, concentrated by screening opera-tions, lie on the floor of the gravel pit nearCastle Rock. Dimensions of the largest boulderare 3.5 x 5 x 6.5 feet, and about a dozen otherboulders are only slightly smaller. At the Gil-more Corners pit, a boulder about 2 x 3.5 x 4.2feet was found in place, embedded in and ex-tending about a foot above the top of themiddle lahar. The presence of this boulder isnoteworthy, for it lies in medium to coarse sandthat makes up the graded upper part of thelahar.

Although on the whole the lahars at GilmoreCorners are unstratified, some structures oflocal extent suggest crude layering. Nearlyhorizontal, subparallel, light-brownish-grayzones, some of which split into two zones andothers of which end abruptly at pebbles, arepresent in the upper few feet of the upper laharin the northern corner of the pit. Some of thesezones are caused by iron-oxide stains in the sandmatrix of the lahar. In other zones, however,interstices of the matrix are filled with fine-grained iron oxide or iron oxide and clay. Ap-parently, the zones are not associated withvertical changes in grain size, and the stainingand filling of the interstices probably are dueto deposition from ground water.

In addition to the brownish zones, the upperpart of the middle lahar at one place in the pitwalls contains a poorly defined zone of ma-terial that is somewhat finer than the rest of thedeposit. In cross section this zone is shaped likea small channel fill, but like the rest of thelahar, the material within the "channel" is un-stratified. The finer grained material within the"channel" probably is a small part of the laharthat moved separately from and later than the

main mass. Crude layering between slightlydifferent masses of mudflow material in theNirasaki mudflow in Japan has been attributedby Mason and Foster (1956, p. 78-79) to theoverrunning or underrunning of some parts ofthe mudflow by other parts.

Stones in the Silver Lake assemblage aremostly pale-red, glassy, hornblende dacite andgray hornblende-hypersthene andesite (Ver-hoogen, 1937, p. 266; Roberts, 1958, p. 40)(Table 1); these stones are derived only fromthe older Mount St. Helens. No olivinc basaltor pyroxene andesite from the modern conewere found in the lahar assemblage.

TABLE 1. LITHOLOGY OF STONES IN UPPER LAHAR ATGILMORE CORNERS GRAVEL PIT, in the SWJ4NEJ4

SEC. 30, T. 10 N., R. 1 E.

Per centGray, glassy, hypersthcne-hornblende andesite 58Pale-red, glassy, hornblende dacite 28Sedimentary and volcanic rocks derived from

formations of Tertiary age 14

100

Although pumice pebbles and granules arescattered through the alluvium (unit 2 of themeasured section) at Gilmore Corners and areabundant in thin alluvium at the top of the as-semblage for several miles downstream fromSpirit Lake, pumice is virtually absent in thelahars themselves. Vesicular glass and glassshards are sparse in the sand fraction of thelahars, which consists almost wholly of crystalsof plagioclase, hypersthene, and hornblendeand fragments of crystal-charged devitrifiedglass derived from the groundmass of the pale-red dacite and gray andesite.

A sample of fine silt and clay from the upperlahar several miles up the North Fork of theToutle River from Gilmore Corners was identi-fied by X-ray methods by Arthur f . Gude, 3d,of the U. S. Geological Survey. The fractionsmaller than about 37 microns and larger than2 microns contains, in order of relative abun-dance, feldspar, alpha cristobalite, and quartz;the fraction smaller than 2 microns containsalpha cristobalite, feldspar, quartz, and hal-loysite.

Origin. In the Silver Lake assemblage in-terpretation of deposits as lahars is based onlithology, vertical grading, poor sorting, ab-sence of stratification typical of fluvial deposits,angularity of stones, and the presence of large



862 MULLINF.AUX AND CRANDELL—RECENT LAHARS, WASHINGTON

boulders in a fine matrix. The very small con-tent of pumice, scoria, or glass shards indicatesthat the deposits were not formed by gas-richhot avalanches; their length indicates that theyare not the products of dry, gas-poor, block-and-ash avalanches from Mount St. Helens.

The lahars in the Silver Lake assemblageprobably originated in eruptions of MountSt. Helens. The erupted material probablyfell in large masses on the upper slopes of thevolcano and then slid or flowed down-slopeas either wet or dry block-and-ash avalanches,collecting enough additional water from melt-ing snow and ice and from the North Fork ofthe Toutle River to form wet slurries thattraveled far down-valley. Downstream, thelahars presumably were progressively morediluted with water, until they became littlemore than a very muddy stream; some, in theabsence of excessive water, retained theiridentity for many tens of miles from thevolcano. During intervals between the for-mation of lahars, streams reworked the depositson the volcano and lahars on the valley floorsto form fluvial gravel found in the lahar as-ssmblage.

The lahars themselves give little evidenceas to the kind of eruption that caused them orwhether they were wet or dry high on thevolcano. Their similarity to block-and-ashflows suggests that they probably originatedfrom mild explosive eruptions or from collapseof spines or domes (Williams, 1956, p. 60).

Weathering profiles and age. Two weather-ing profiles and dated wood within the SilverLake assemblage suggest that emplacement ofthe assemblage continued until late in Recenttime. The weathering profiles are in the upperparts of the lower and upper lahars. The profilein the lower lahar consists of an oxidized zoneas much as 12 inches thick overlain by a humi-fied zone 1-2 inches thick that contains smallcarbonized wood fragments. In places, justbelow the humified zone, there is a light-brownish-gray zone about 1 inch thick that isconspicuously lighter in color than either theoxidized or unoxidized part of the lahar. Thelight-colored layer probably is a bleachedhorizon (the \z horizon of pedological termi-nology) of a podzolic soil. Where the laharhas been channeled, the weathering profilefollows the bottom of the channel, provingthat the channel was formed prior to theweathering.

The profile in the upper lahar, at the presentground surface, consists of an oxidized zone

overlain by a layer of duff 1-2 inches thick.The oxidation extends to an average depth ofabout 18 inches but locally to a depth of asmuch as 36 inches.

A zone of oxidation 4-12 inches thick occursin the silty sand alluvium that overlies themiddle lahar, and in places it extends a fewinches down into the underlying lahar. Thisoxidation may be part of a profile resultingfrom surface weathering, but it could havebeen produced by aerated ground water mov-ing along the contact between a permeablematerial above and a less permeable materialbelow.

The only direct evidence of age of the laharassemblage was obtained from wood in thelower lahar at Gilmore Corners. Most wood inthis lahar is almost completely replaced byiron oxide, but one wood fragment from itwas identified as part of a stem and branch of aconifer by Dr. Donald Clark (oral communica-tion). As the fragment was found about 1 footbelow the top of the lahar, it must have beentransported by the lahar. It has a radiocarbonage of 2030 ±240 years (W-811).

The weathering profile in the lower lahar isnearly as thick as the profile at the top of theassemblage and apparently includes a leachedhorizon; it probably required about as muchtime for its development as did the profile atthe top. If this is true, accumulation of the fillprobably continued until perhaps a thousandyears ago, thus implying that the volcano con-tinued to erupt the hornblende dacite andhornblende-hypersthene andesite of olderMount St. Helens for at least a thousand yearsafter the dated wood was incorporated. Theknown thickness of the deposits near SilverLake and the possibility that there are weath-ering profiles in unexposed parts of the fillbelow the dated wood suggest that the assem-blage may have been emplaced over a periodof several thousand years.

The flat-topped fill formed by the SilverLake assemblage in the Toutle and CowlitzRiver valleys is banked against rounded, silt-covered, fluvial terrace deposits of Pleistoceneage. A short distance north of Castle Rock, thefacial bone of a mammoth recovered from sucha terrace deposit (Roberts, 1958, p. 39) wasassigned an age of late Pleistocene.

Lahar at Spirit Lafe Lodge

A younger lahar lies within a valley cut intothe Silver Lake lahar assemblage in the valleyof the North Fork of the Toutle River near




Spirit Lake Lodge (Fig. 1), 4.5 miles from thesummit of Mount St. Helens. This laharoriginated at the summit or somewhere on thenorth slope of the volcano and moved north-ward down to the valley of the Toutle, whereits nearly unmodified flattish surface is severaltens of feet below the upper surface of theSilver Lake assemblage. The younger laharpresently forms the south bank of the ToutleRiver for about a quarter of a mile in the

ments. Red and dark-gray scoria makes up10-20 per cent of the medium sand size, butcolorless vesicular glass or pumice is rare. Bothbrown and colorless vesicular glass and glassshards are more common in the finest sand sizebut are minor constituents.

Wood fragments are abundant in the lahar,and an upright tree stump extends from theunderlying fluvial sand and gravel through thelahar. Some of the wood fragments are corn-

e of particles, mm

S

Figure 3. Cumulative curves illustrating grain size of lahar at Spirit Lake Lodge and lahar in KalamaRiver valley. Boulder- and cobble-size bombs excluded from lahar sample at Spirit Lake Lodge. Samplein Kalama River vallev taken from 1 to 4 feet above base

vicinity of Spirit Lake Lodge. The gradientof the lahar there is about 300 feet per mile.

The lahar near Spirit Lake Lodge is an un-stratified mixture of pebbles, cobbles, andboulders in a friable matrix of angular sand(Fig. 3). At the only outcrop where it isexposed in cross section, it is about 15 feetthick and shows neither a size gradation frombase to top nor any imbrication of the stones(PI. 2, fig. 2). The boulders and many of thesmaller stones are breadcrust bombs of medium-to dark-gray scoria; the surface of the depositis dotted with large boulders of the scoria, somehaving a maximum dimension of 5-6 feet.Other stones in the deposit are mostly light- ormedium-gray hornblende and pyroxene andes-ite. The matrix of the lahar is composedpredominantly of nonvesicular rock frag-

pletely converted to charcoal, but others arecharred only on the outside. Some fragmentsof charcoal are fragile twigs, and others arelong slender branches with bark still present;all are brittle. The stump that protrudes intothe lahar still has bark and is charred onlywhere large bombs rest against it; no charringwas observed where only matrix material isadjacent to the stump.

The lahar at Spirit Lake Lodge containsabundant large bombs and other fragments ofscoria, but the stump, uncharred except whereadjacent to bombs, rules out the possibility ofan incandescent gas-emitting matrix. In addi-tion, the breadcrusted exteriors of the bombsindicate that they began to solidify prior toincorporation in the deposit. The presence ofwholly charred wood, as well as only partly




charred and uncharred wood, suggests that thematrix was first very hot, but that it cooledduring transport and was not hot enough tochar wood by the time the mass reached theToutle River. Preservation of fragile branchesin the deposit suggests relatively gentle trans-port, more gentle than expected in a dryavalanche of tumbling blocks and ash. Thelow temperature of the matrix, the apparentgentle transport, and the mobility that enabledthe mass to flow on a relatively low gradientsuggest that water was the mobilizing agent,and that the mass moved as a viscous lahar.

The surface of the lahar at Spirit Lake Lodgeis covered by only a thin layer of organicmatter; large rotten logs are absent, and mostof the largest standing trees are less than 3 feetin diameter. In contrast, some trees are morethan 6 feet in diameter on the Silver Lakeassemblage nearby, and many large rotten logslie on its surface. Tree-ring counts of someof the largest trees on the lahar at Spirit LakeLodge suggest an age of about 330 years.Observation of the 1947 Kautz Creek debrisflow at Mount Rainier and a study by Dicksonand Crocker (1953) of lahars from MountShasta indicate that reseeding takes only ayear or two on the surface of lahars in environ-ments similar to that at Spirit Lake Lodge;thus, the lahar there is probably very littleolder than 330 years.

Probable Lahar in Kalama River Valley

A deposit of volcanic debris that extends10 miles down the Kalama River valley south-west of Mount St. Helens exhibits character-istics of both lahars and glowing avalanchedeposits. Although we are not certain thatwater in the fluid state was responsible for itsmobility, it seems certain that water was in-volved, and for convenience we refer to thedeposit as a lahar.

The lahar overlies fluvial sand and gravel and,10 miles from the volcano, underlies the surfaceof a rather wide flat floor in the valley thatcontrasts markedly with the narrow V-shapedvalley a few miles downstream. However, out-crops of a lava flow from Mount St. Helens,underlying or surrounded by the lahar, sug-gest that the Kalama River valley owes itsflat floor to a valley-filling lava flow or to acombination of lava flows, lahars, and alluvium.The gradient of the wide valley floor decreasesfrom 200 or 300 feet per mile near Mount St.Helens to less than 100 feet per mile near itswestern end.

The lahar is best exposed about 8 miles fromthe summit of the volcano in a narrow part ofthe Kalama River valley in sec. 33, T. 8 N.,R. 4 E. and in a deep trench cut by the riverabout a mile farther upstream. In section 33,the exposures are in road cuts and in a borrowpit (Fig. 1) near the point where the old trailfrom Merrill Lake to Mount St. Helens andthe present logging road cross the KalamaRiver. Here the lahar is 15-20 feet thick and isoverlain by local deposits of pumice gravel.

The lahar consists of an unstratified andpoorly sorted mass of pebble-and cobble-sizerock fragments in a friable sandy matrix(Fig. 3). Most fragments are angular or sub-angular, and no size gradation is apparent frombase to top in the borrow-pit exposure. Theover-all color of the deposit is light gray, butthe upper 6 feet has a faint reddish cast. Therocks are mostly light- and dark-gray andesitesand basalts, a few of which are scoriaceous.Reddish-gray andesites are a minor constituent.Most were derived from the modern MountSt. Helens volcano. Pumice gravel is abundantonly in the basal 1-2 feet of the lahar thatoverlies pumice in channels. The coarse-sandsize components are dominantly fragments ofcrystal-charged devitrified glass, and crystalsthat are found as phenocrysts in the largerstones. Scoria and pale-brown vesicular glassmake up about 10-15 per cent, and roundedwhite pumice and clear glass shards constituteanother 10-15 per cent of the coarse-sand size.In the finest sand size, about half is glass shards,clear vesicular glass, or crystals with vesicularglass clinging to them. Euhedral crystals ofplagioclase, hypersthene, and clinopyroxeneare abundant in the sand matrix.

Near the borrow pit, the lahar containsmany logs that have been converted to char-coal, and it probably was here that Colville(Diller, 1899, p. 639) found the charcoal logsin the "Kalama River gravels." Colville re-ported partly charred logs, but none were seenduring this investigation. In the borrow-pitexposure, a log of Douglas fir (Pseudotsugataxifolia) 3 feet in diameter lies in a nearlyhorizontal attitude about 10 feet below thetop of the deposit and is converted to charcoalequally from outside to inside, including well-preserved bark. The log is fragile and light inweight, and the grain of the wood is perfectlypreserved. According to Doctor Clark (oralcommunication), the condition of the charcoalindicates that the log was heated to atemperature of at least 200° C, and the ab-




sence of ash indicates that oxygen was excluded.Within 6 inches of the top and sides of the log,much of the silt and fine sand has been removedfrom the enclosing lahar, leaving an openworkgravelly sand. The remaining stones are coatedwith a soft, dark, sooty material, which ap-parently is an organic product expelledduring destructive distillation of the log. Above

temperature in the deposit, 10 stones werecollected for remanent magnetism tests, todetermine if the temperature was higher thanthe Curie point of the contained magneticminerals (Aramaki and Akimoto, 1957). Theorientation of the north-seeking poles in thestones (Fig. 4) is distinctly not random, andalthough stability tests have not been made,

Figure 4. Point diagram of azimuth and dip of north-seeking polesin 10 stones from Kalama River lahar. Two points, representingtwo different samples from same stone, are plotted for each exceptstone 2. Stone 3 may have been turned end for end when collected.

the log, spiral "pipes" of openwork gravellysand, also coated with sooty material, leadtoward the surface of the deposit. The fragilityof the log and the presence of bark suggest thatthe log was not reworked in its present statefrom an older deposit; the coating on stonesaround the log and in the "pipes" clearlyshows that the wood was converted to charcoalwhere it now lies.

In a nearby road cut, several dozen logs ofcharcoal at or near the base of the lahar aremixed with fragments of pumice. A mile and ahalf upstream from this exposure, where morethan 50 feet of the lahar is exposed in a cut-bank of the Kalama River, no wood was seen.

Because the charcoal logs indicate a high

the temperature of most of the stones testedwas probably near or above the Curie pointafter the mass came to rest. Nevertheless, thegrouping of the magnetic directions is definitelymore scattered than groupings obtained byAramaki and Akimoto on hot dry avalanches,and the directions in stones 2 and 7 vary fromthe present direction of the earth's magneticfield by more than 45°. The temperature ofmost of the larger stones was probably near orabove the Curie point when the lahar came torest, but some included stones and the mass,as a whole, probably were at a lower tempera-ture.

As the Kalama River valley lahar evidentlywas of high temperature and contains pumice




in the matrix, it somewhat resembles thedeposit of a gas-rich glowing avalanche. Thebulk of this deposit, however, is of nonvesicularrock. Moreover, the lack of lapilli-size pumicehere is a noteworthy difference between thisdeposit and deposits of gas-rich glowing ava-lanches described elsewhere (Williams, 1942;Hay, 1959). The Kalama River valley depositprobably started either as an intermediate typeof hot avalanche or as a gas-poor avalanche thatincorporated fresh, vesicular, pyroclastic ma-terial. Its mobility, shown by its length andrelatively low surface gradient, could becaused by gas-emitting particles in the matrix,heating of entrapped cold air, or water in theliquid or gaseous form. The discharge of theKalama River at the time this deposit was laiddown probably was comparable to that of thepresent river, and a large amount of watermust have been incorporated by the time themass reached the point where we studied it.Although we are not at all certain that waterwas essential to movement, a laharic origin issuggested by the length, the probability thatmuch water was incorporated, and the absenceof lapilli-size pumice. The evidence of a hightemperature indicates that any incorporatedwater probably was expelled as steam duringtransit and after the mass came to rest.

DISCUSSION OF HOT LAHARSMany features of the deposits studied at

Spirit Lake Lodge and in the Kalama Rivervalley suggest that they are the flowageproducts of hot, gas-poor, volcanic debrismobilized by water, but the presence of enoughheat to carbonize wood seems anomalous. How-ever, the nature of the carbonization of woodin the lahar at Spirit Lake Lodge suggests ananswer. Before the lahar arrived at the ToutleRiver, it probably was hot enough to convertall contained wood to charcoal, and any waterincorporated in the lahar at this stage musthave been converted to steam. The formationof steam would have dissipated heat, causedturbulence, and added considerably to themobility of the mass by decreasing internalfriction. Gradual loss of heat probably loweredthe temperature of the flow until newly in-corporated wood was only slightly charred orleft uncharred. Nevertheless, the includedbombs, because of their low heat conductivity,apparently retained a high internal heat evenafter the matrix cooled. After the mass cameto rest and the water either drained away orescaped as steam, the heat that had been re-

tained in the bombs was sufficient to charadjacent wood.

In this way, water and steam in a lahar couldmobilize the mass on a low gradient; yet, afterthe lahar came to rest, heat from included rockfragments could raise the whole mass above100° C. Kemmerling (in Stehn, 1929, p. 12)reported a temperature of 360° C in gasesstreaming from a lahar from Keloet volcano inJava shortly after it came to rest. Temperaturesof 100° C were measured in the same lahar ayear after its formation.

Hot lahars are a common product of volcanicactivity. A notable result of the 1929 eruptionof Santa Maria volcano in Guatemala was theproduction of lahars hot enough to carbonizewood (Sapper and Termer, 1930). These wereformed by the descent of glowing avalanchesinto streams; the interior of the deposits re-mained hot for 4 weeks. Extensive lahars re-sulted from the 1926 eruption of Tokachidakevolcano in Japan when a hot avalanche on thewestern slope of the volcano melted an accumu-lation of snow (Tada and Tsuya, 1927; Murai,1960). Hot lahars also have been producedduring eruptions of the Hibok Hibok volcanoin the Philippines (Alcarez, 1956, p. 33) andMt. Pelee in Martinique (Anderson and Flett,1903, p. 481).

AGE OF MOUNT ST. HELENS

From the time of early exploration in the19th century, the smooth slopes and lack ofglacial features on Mount St. Helens have beeninterpreted to mean that the volcano is young.Verhoogen (1937) used topographic evidenceand the absence of olivine basalt (the earliestrock type erupted by the present volcano) inthe terrace deposits of the North Fork of theToutle River to date it as of Recent age. Heregarded the terrace deposits as glaciofluvialgravel derived from the older Mount St.Helens during Pleistocene time.

Not only is the present cone of St. Helens ofRecent age, but the older St. Helens that pro-vided the material in the terraces apparentlyexisted until late in Recent time and may havebeen entirely of Recent age. The presence ofonly the older St. Helens is indicated at thetime the 2000-year-old wood sample wasincorporated in the Silver Lake lahar assem-blage, for neither lahars nor alluvium in theassemblage contain rocks from the presentcone. The present cone may have been formedwithin the last thousand years, for soil profilesin the assemblage above the dated wood sug-



AGE OF MOUNT ST. HELENS 867

gest an interval of perhaps as much as athousand years during which the volcano con-tinued to erupt the hornblende dacite andhornblende-hypersthene andesite of the olderSt. Helens. A critical point in these estimatesof age is Verhoogen's correlation of rocks in

in a canyon below Goat Rocks, and in thecanyon wall of the South Fork of the ToutleRiver (Fig. 5). Except in the canyon where therock may be intrusive, the old St, Helens rocksat each locality consist mostly of unconsolidatedvolcanic debris composed of pale-red horn-

Figure 5. Index map of vicinity of Mount St. Helens and Spirit Lake

the Toutle River terrace deposits (the SilverLake lahar assemblage) with rocks of the olderMount St. Helens. The northern and north-western parts of the volcano were reconnoiteredin an attempt to confirm Verhoogen's correla-tion of the two deposits.

Deposits, tentatively identified by Ver-hoogen as "old Mount St. Helens series" orwhich are lithologically indistinguishable fromold St. Helens rocks, were observed nearWindy Pass, near Timberline Campground,

blende dacite and gray hornblende-hyperstheneandesite, pumice, and ash. No basalt was foundin any old St. Helens deposit, and the con-stituents of the unconsolidated debris areindistinguishable from those in the Silver Lakelahar assemblage. The old St. Helens rocks ateach locality are overlain by thin basalt lavaflows; thus, we believe Verhoogen's stra-tigraphy and correlation are correct.

Presumed block-and-ash flow deposits makeup most of the canyon walls of the South Fork



S68 MULLINEAUX AND CRANDELL—RECENT LAHARS, WASHINGTON

of the Toutle River where it is incised in the present volcano were not separated by a longbase of Mount St. Helens. The block-and-ash time interval.flows in the lower two-thirds of the canyon wall The proportion of the present volcano thatconsist of the red and gray rocks of the older consists of olivine basalt and pyroxene andesiteMount St. Helens. These deposits are overlain is not known. The highest outcrops of "old St.by thin, basalt lava flows intercalated with Helens series" described by Verhoogen, orblock-and-ash flows containing basaltic stones found by us, are at an altitude of about 5000and, near the top of the canyon, by block- feet near Windy Pass and below Goat Rocks,and-ash flows of light- to medium-gray, dense, Because these rocks have not been found higherpyroxene andesite. No significant erosional on the cone, the upper 3000-4000 feet of theunconformity was noted between the units, volcano is probably composed of debris andsuggesting that eruptions of dacites and andes- lava flows that were erupted late in Recentites of the old St. Helens and eruptions of the time, perhaps within the past thousand years,olivine basalts and pyroxene andesites of the

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Alcarez, Arturo, 1956, Philippines, p. 32-34 in Symposium on volcanology of the Pacific with emphasison nuees ardentes: Quezon City, Philippines, Natl. Research Council of the Philippines, Univ. of thePhilippines, 8th Pacific Sci. Cong. Proc., v. 2, 573 p.

Anderson, Tempest, and Flett, J. S., 1903, Report on the eruptions of the Soufriere in St. Vincent in1902, and on a visit to Montagne Pelee, in Martinique, Part I: Royal Soc. London Philos. Trans.,ser. A, v. 200, p. 353-553

Aramaki, Shigeo, and Akimoto, Syun-iti, 1957, Temperature estimation of pyroclastic deposits bynatural remanent magnetism: Am. four. Sci., v. 255, p. 619-627

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Crandell, D. R., 1957, Some features of mudflow deposits (Abstract): Geol. Soc. America Bull., v. 68,pt. 2, p. 1821

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Gibbs, George, 1854, Report on the geology of the central part of Washington Territory: 33d Cong. 1stsess., House Doc. 129, v. 18, pt. 1, p. 494-512

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Geology, v. 67, p. 540-562Lawrence, D. B., 1938, Trees on the march: Mazama, v. 20, no. 12, p. 49-54

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no. 13, p. 41-44MacGregor, A. G., 1938, Royal Society expedition to Montserrat, B.W.I.; The volcanic history and

petrology of Montserrat: Royal Soc. London Trans., v. 229, p. 1-90Mackin, J. H., 1948, Concept of the graded river: Geol. Soc. America Bull., v. 59, p. 463-512Marshall, Patrick, 1935, Acid rocks of the Taupo-Rotorua volcanic district: Royal Soc. New Zealand

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64, p. 74-83McTaggert, K. C., 1960, The mobility of nu'ees ardentes: Am. Jour. Sci., v. 258, p. 369-382Murai, Isamu, 1960, On the mud-flows of the 1926 eruption of volcano Tokachidake, central Hokkaido,

Japan: Tokyo Univ. Earthquake Research Inst. Bull., v. 38, pt. 1, p. 55-70



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Ferret, F. A., 1924, The Vesuvius eruption of 1906: Carnegie Inst. Washington Pub. 399, 151 p.1937, The eruption of Mt. Pelee, 1929-1932: Carnegie Inst. Washington Pub. 458, 126 p.

Pettijohn, F. J., 1957, Sedimentary rocks: New York, Harper and Brothers, 2d ed., 718 p.Roberts, A. E., 1958, Geology and coal resources of the Toledo-Castle Rock district, Cowlitz and Lewis

counties, Washington: U. S. Geol. Survey Bull. 1062, 71 p.Sapper, Karl, and Termer, Franz, 1930, Der ausbruch des vulkans Santa Maria in Guatemala vom 2-4

November 1929: Zeitschr. iur vulkanologie, v. 13, pt. 2, p. 73-101Sharp, R. P., and Nobles, L. H., 1953, Mudflow of 1941 at Wrightwood, southern California: Geol. Soc.

America Bull., v. 64, p. 547-560Stehn, C. E., 1929, Keloet: 4th Pacific Sci. Cong., Java, Excursion E. 2a., pt. 1, p. 1-23Tada, Fumio, and Tsuya, Hiromichi, 1927, The eruption of the Tokachidake volcano, Hokkaido, on

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gineering practice: Highway Research Board Special Rept. 29, 232 p.Verhoogen, Jean, 1937, Mount St. Helens; a Recent Cascade volcano: California Univ., Dept. Geol. Sci.

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MANUSCRIPT RECEIVED BY THE SECRETARY OF THE SOCIETY, JULY 2, 1960PUBLICATION AUTHORIZED BY THE DIRECTOR, U. S. GEOLOGICAL SURVEY



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