Guides to some Volcanic Terranes 

in Washington, l~ho, Oregon, 

and Northern California 

David A. Johnston and Julie Donnelly-Nolan, Editors 

GEOLOOICAL SURVEY CIRCULAR 838 

1981 
UNITED STATES DEPARTMENT OF THE INTERIOR 

JAMES G. WAIT, Secretory 

Geological Survey 

H. William Menard, Director 

Free on application to Branch of Distrib ution, U.S. Geo logical Survey 604 South Pickett Street, Alexandria, VA 22304 
CONTENTS 

Guide to geologic field tri p between Lewiston , Idaho, and Kimberly , Oregon, 

emphasizing the Columbia River Basalt Group by D. A. Swanson , U. S. 

Geological Survey, Menlo Pa r k , Califor nia 94025 ; and T. L. Wright, u. s. 

Geo logical Su r vey , Reston , Virginia 2209 2 ------ ------------------------ 1 

Figure l. Index map with extent of Columbia River Basalt Group --------- 2 

2. Chart of stratigraphy , age , magnetic polarity, and chemical 

type , Columbia Ri ver Basalt Group ---------~---------------- 3 

3. Ma ps showing distribu tion , feeder dikes of four formations in 

Columbia River Basalt Grou p -------------------------------- 4 

4. Stratigra phic section near Pomeroy , Washington --------------- 4 

5. Maps showing distribution , feeder dikes of four members in two 

formations of Columbia River Basalt Group ------------- ----- 7 

6. Idealized cross section of flow in Yakima Basalt Subgroup ---- 9 

Table l. Major-element compositions of chemical t ype s , Columbia Ri ver 

Basalt Group ------- ---------- ------------------------------ 5 

2 . Trace- element compositions of chemical types , Yakima Ba salt 

Subgrou p --------------------------------------------------- 6 

Roadlog for geologic field trip between Lewiston , Idaho , and Kimberly , 

Oregon by D. A. Swanson , U. S. Geological Sur vey, Me nlo Pa r k, CA 94025; 

and T. L. Wright, U. S. Geological Survey , Reston , VA 22092 ------------- 15 

Figure 1 . Map of field trip stops --------------------------------------

Guide to geologic field trip between Kimberly and Bend, Oregon with 

emphasis on the John Day Formation by Paul T. Robinson , De pa r tment of 

Earth Sciences, University of California, Riverside , CA 92521; and Ge rald 

F. Brem , Department of Earth Sciences , California State Uni ve rsity , 

16 

Fullerton, CA 92634----------- --------------------- --------------------- 29 

Figure l. Index map of north- central Oregon with extent of John Day 

Formation -------------------------------------------------- 30 

2. Stratigraphic columns, western and eastern John Day Formation- 32 

Table l. Modal analyses of John Day rocks ----------------------------- 35 

2 . Chemical composition , fresh and altered welded tuff ---------- 37 

3. Chemical analysis, least altered John Day ash- flow tuffs ----- 38 

4. Average composition of mafic lava flows ---------------------- 38 

Roadlog for geologic field trip between Kimberly and Bend, Oregon , with 

emphasis on the John Da y Forma tion by Paul T. Robinson , Department of 

Earth Sciences, University of California , Rive r side , CA 92521 ----------- 41 

Figure 3. Map of field trip stops --------------------------------------

Central High Cascade roadside geology , by Edward M. Taylor, Department of 

Geology, Oregon State University ----------------------- --- --------------

Figure 1 . Index map ---------------------------------------------------- 2 . Diagrammatic cross section of Cascades and Deschutes Basin ---

Ill 

42 

55 

56 

58 
~Vl'Or.t~ 1 --~ 11:.-.trtU U 

Roadlog f r Centr l High C cad ge l gy , Bend , ist rs, McK nzi P nd 

S ntiam Pass , Or gon by Edw rd M. Tayl r, D p r vm nv of ology , Or gon 

State University--------------------------------------------------------- ' 9 

Figure 3. 

4. 

5. 

6. 

7. 

8. 

9. 10 . 

ll. 

12 . 

Geologic sketch map, mil 

Geologic sketch map, mile 

Geologic sketch map, mile 

Geologic sketch map, mile 

Geologic sketch map, mile 

Geologic sketch map , mil 

Geologic sketch map, mile 

Geologic sketch map, mil e 

Geologic sketch map, mile 

Geologic sketch map, mil 

0.0 vO 1 . 6 ------------------------

16 . 6 to 7 . 1 -----------------------

7 . 1 vO 34 . 5 vO 44 . 6 ---------------

44 . 6 vO 57 . 9 -----------------------

7 . 9 to 7 . 3 -----------------------

72 . 3 to 83 . -----------------------

83 . ? to 91 . to 99 . 8 ---------------

99 . 8 to 119 . 0 ----------------------

119 . 0 to 1 9 . 1 ---------------------

129 .1 to 142. 0 ---------------------

Newberry Volcano, Oregon by Norman S. MacLeod, David R. h rrod, U. 

Geological Survey, Menlo Park , CA 94025 ; Lawr nee A. Chitwood , U .. 

Forest Service, Bend, OR 97701; and .dwin H. McK e , U. S. Geological 

60 

63 

6) 

69 

71 

74 

76 

78 

80 

8 

Survey , Menlo Park , CA 94025-------------------------------------------- 8 

Figure 

Table 

l . Geologic sketch map of Newberry Vo lcano ----------------------

2. Geologic sketch map of Newberry caldera ----------------------

1 . Representative chemical analyses of Newb rry rocks -----------

Roadlog for Newberry Volcano, Oregon, by Norman S. M cLeod , David R. 

Sherrod, U. S. Geological Survey, Menlo Park, CA 94025 ; L wr ence A. 

Chitwood, U.S. Forest Service, Bend, OR 97701 ; and .dwin H. McKee, U. 

8 

89 

0 

Geological Survey, Menlo Park, CA 94025 -------------------------------- 3 

High Lava Plains, Brothers fault zone to Harney Basin , Oregon by George W. 

Walker, U. S. Geological Survey , Menlo Park, CA 94025 , and ruce ol f , 

Central Oregon Community College, Bend, Oregon -------------------------- 10) 

Figure 

Table 

l . Inde x map showing High Lava Plains and Harney asin ----------

2A , B, C. Route map with ma jor structural elements and rhyolit 

K/Ar ages ----------------------------------------------------

3. Map of basalt fields -----------------------------------------

l. Compositions of selected basalts and rhyolites ---------------

Roadlog for High Lava Plains, Brothers fault zone to Harney B sin, Oregon, 

by George W. Walker, U. S. Geological Sur vey , Menlo Park, CA 02 , a nd 

10 

0 

0 

10 

Bruce Nolf, Central Oregon Community College, Bend, Oregon -------------- 1 3 

A field trip to the maar volcanoes of the Fort Rock-Christmas La e Va ey 

basin, Oregon , by G. H. Heiken, Geosciences Divis ion, Los Alamos c ientific Laboratory , Los Alamos, NM 87545 ; R. V. isher , Departmen v o f 

Geology, University of California, Santa Barbara, CA 93106; an V. 

Peterson, State of Oregon, Department of Geology and Min ra l I ndusvr i es, 

Grants Pass, OR 97526 --------------------------------------------- ------

Figure l . Location map ------------------------------------------------- 2 . Map of Fort Rock- Christmas Lake Valley Basin , showing route of 

field trip -------------------- -----------------------------

IV 

l 9 

1 0 
CO NTENTS--Con:inued 

Figure 3. Cross sections :hrough Moffitt Butte , Klamath Co unty , Oregon - 21 

4. Photograph of maar deposits , Big Hole ------------------------ 123 

5 . Map of Hole-in-the-Ground ------------------------------------ 124 

6 . Geologic section through Hole - in- the - Ground ------------------ 125 

7 . Fort Rock tuff ring : map, photograph , cross section --------- 27 

8a . Map of Table Rock tuff ring complex ------------------ -------- 129 

8b . Stratigraphic section of sediments under Table Rock tuff ring 

complex ------------------------------------------------------ 30 

9 . Diagram of sediment-tuff - breccia contact , Table Rock tuff ring 

complex ------------------------------------------------------ 13 

10 . Sketch of cliff showing vent 8, Table Rock tuff ring complex - 132 

11. Diagram showing palagonitiza tion of tuff , Table Rock maar 

complex ---------------------------------------------------- 133 

12 . Map and cross sections of vents 4 and 5 , Table Rock tuff ring 

complex ---------------------------------------------------- 134 

13 . Sketch and map of ven t 5 , Table Rock tuff ring complex ------- 135 

14 . Stratigraphic section, Table Rock tuff cone ------------------ 136 

15 . Cross section , Table Rock ------------------------------------ 137 

16 . Sketch and map of vent 9 , Table Rock tuff ring complex ------- 138 

17 . Slump structure in tuff ring l , Table Rock, and sketches 

showing its formation -------------------------------------- 13 

Table l. Chemical analyses of basal ts, Fort Rock- Christmas Lake Val l ey 

Basin ------------------------------------------------------ 137 

Roadlog for field trip to Medicine Lake Highland by Julie M. Donne ly - olan , 

U. S. Geological Survey, MS - 18, 345 Middlefield Road, Men lo Park , CA 

94025 ; Eugene V. Ciancanelli , Cascadia Exploration Corporation , 3358 

Aposta l Road, Escond i do, CA 92025 ; John C. Eiche berger, Geologica 

Research G-6, Los Alamos Scientific Laboratory , Los Alamos, NM 87 545; 

Jon H. Fink, Geology Department, Stanford University , Stanford, CA 9 305 ; 

and Grant Heiken, Geological Researc h G- 6 , Los Al amos Scientific 

Laboratory, Los Alamos, NM 87 545 --------------------------------------- l l 

Fi gure l . Location map for field trip to Medicine Lake Highlana -~------

Ca ptain Jack 's Stronghold (The geo logic events that createa a natura l 

fortress) by Aaron C. Waters , 308 Moore Street, Santa Cruz , CA 95060 . 

l 2 

Map by Dav id Kimbrough and Aaron C. Waters ----------------------------- 151 

Fi gure 1 . Map of Captain Jack's Stronghold ----------------------------- 152 

Pre - Holocene silicic volcanism on the northern and western margins of the 

Medicine Lake Highland, California by Stan ey A. Mertzman, Department of 

Geology, Franklin and Marshall College, Lancaste r, PA 17604 ------------ 163 

Figure 

Table 

1 . Geologic map of selected units, Medicine Lake Highlana -------

l . Ana ly~ica resu ~s o -Ar a~in ----------------------------

2. Chemical ana yses of the An desite Tu ff -----------------------

v 

16 

165 

166 
Figure 

CO NTEN TS-- ontinued 

3. Chemical analyses of th la v flow equival ent of the An d sit 

Tuff and of older ilicic rocks ----------------------------

4. Moda l analys s of the And site Tuff --------------------------

5. Chemica l analyses of three Holoc ne glass flow --------------

167 

168 

168 

Surface structure of Little Glass Mountain by John H. Fink , Geology 

Department , Stanford University, Stanford CA 4305 (Pr sent addre s : 

Department of Geology , Arizona State Un i versi ty , Tempe, AZ 85281) ------ 171 

Figure 1 . Schematic cross section through 2 rhyolitic obsidi n flow ---- 171 

2 . Flow front, Little Glass Mountain: phovo , foliations , 

interpretation --------------------------------------------- 171 

3. Ma p of part of northeast lobe , Little Glass Mountain --------- 172 

4. Compressional folds in flow front, Little Glass Mountain ----- 173 

5. Diagram of diapir rise in rhyolitic obsidian flow ------------ 173 

6. Map of part of northwest lobe , Little Glass Mountain --------- 174 

7 . Map of large ridge, northwest lobe --------------------------- 175 

8. Pho to , part of northwest lobe, Little Glass Moun tain --------- 176 

Holocene plinian tephra deposits of the Med ici ne Lake Highland, California, 

by Grant Heiken, Los Alamos Scienti fie Labor a tory, Geosciences Di vis ion, 

Los Alamos, NM 87545 ------------------------ --------------------------- 177 

Figure 1. Map of deposits <1100 years old , Medi cine Lake Highland ------

2. I sopach map of Little Glass Mountain and Glass Mountain 

tephra ---------------------------- ------------------------- 3. Correlation o f Little Glass Mo untain tephra units ------------

Table 1. Age relations of Holocene volcanic deposits, Medicine Lake 

Highland ---------------------------------------------------

17 

179 

180 

178 

Mechan i sm of magma mi xing at Glass Mountain , Medicine Lake Highland 

volcano, California, by John C. Eichelberger, Geosciences Di vision , 

University of California, Los Alamos Sci en t i fie Labor a tory, Los Alamos, 

NM 87545 - ------- - - ----------------------------------- ---------- 183 

Figure l. Map of Glass Mountain sample locations and litho logic un its 183 

2. Map showing distribution of Holocene lavas, Med icine Lake 

Highland --------------------------------------------------- 184 

3. Cross section through Med icine Lake Highland volcano --------- 85 

4. Photomicrographs of basaltic xenolith in Glass Mountain 

rhyodacite, shown in reflected light, transmitted light, and 

crossed polars ------------------------------------ --------- 186 

5. Graph of density versus pressure, Glass Mountain materia ls --- 187 

6. Ske tches of probable erupti ve sequence at Glass Mountain ----- 188 

Table l. Densities of Glass Mounta in samples -------------------------- 187 

VI 
PREF ACE 

This guidebook arose out of a series of field trips held in conjunction with the 

pacific Northwest American Geophysical Union meeting held in Bend, Oregon, September 

l979 . The PNAGU meeting included special volcanology sessions planned by William I . 

~se , Jr ., Bruce A. Nolf , and David A. Johnston . Publication of the guidebook volume 

was originally planned for early 1980 by the Oregon Department of Geology ana Mineral 

Industrie s (DOG AMI) . I nevi table delays , subsequent scheduling problems , and the death 

of Dave Johnston in the May 18 eruption of Mount St . Helens led to this publication as 

a usGS Circular . 

This circular differs from typical U. S . Geological Survey compilations in that not 

all these papers have been examined by the Geologic Names Commit tee of the Survey . 

This Committee is charged with ensuring consistent usage of formational and other 

stratigraphic names in U. S . Geological Survey publications. Because many of the contributions are from workers outside the Survey , review by the Geologic Names Committee 

would have been inappropriate . Each author provided camera - ready pages , and the articles have not been edited for uniformity of style or usage . 

The contributions are generally ordered so as to describe the areas from north to 

south . Typically , the roadlog comes after the descriptive article except in the case 

of the Medicine Lake Highland articles, for which the road log is first and several 

topical contributions follow . 

I wish to thank Wes 

memorial to Dave Johns ton . 

with publication . 

Hildreth for responding so quickly to my request for a 

Also, my thanks go to Beverly Vogt of DOGAMI for her help 

Most of all, we owe special posthumous thanks to Dave Johnston for conceiving the 

field -trip ideas , overseeing the planning and arrangements for the trips and 

maintai ning his sense of humor in spite of the last- minute airline strike , and 

remindi ng us gently but firmly to produce the promised articles. 

Julie Do nnelly- Nolan 

VII 
Dav Johnston, relaxed and happy in th land h 

loved , Vall y of T n Thousand Smokes , Alaska , 

June 1978 . Pos·ng h r at th b of n, 

the rhyolite dome extruded at the clos 

great eruption of 1912 , Dav had sampl 

fumaroles atop several nearby volcanoes and had 

undertaken a study of the emplacement , compac ion, 

and welding of the 1912 ash-flow sheet . Th 

initials ar those of petrologist C. . F nn r 

and his assistant Charles Yori , who worked in th 

valley in 1919 and 1923 . (Photo by Dan Kosco . ) 

VII I 
DAVID ALEXANDER JOHNSTON 

1949-1980 

Dave Johnston, 30-year-old volcanologist with the U.S. Geological Survey, was swept 

awaY by the great directed blast of Mount St. Helens on May 18, 1980. Because Dave had 

been an organizer of the AGU meeting at Bend, an editor of its abstracts and field-trip 

guidebooks, and a champion of the Pacific Northwest, it is particularly appropriate to 

dedicate this volume to him, i n grateful remembrance of the effect that Dave's 

enthusiasm, diligence, and vitality had upon so many of us. As a working colleague and 

daily running companion, I was asked to try to summarize Dave's career and my 

impress ions of him. 

Dave Johnston was born and raised in Illinois, and it remained his custom to return 

there every Christmas season to visit his parents and sister, who survive him. At 

school and college, Dave became an outstanding scholar, runner, and photographer and, 

in 1971, he graduated from the University of Illinois with "Highest Honors and 

Distinction" in Geology. His first geologic project was on the Upper Peninsula of 

Michigan where, ten years ago, he studied the progressive metamorphism of a pile of 

Precambrian basaltic lavas and a differentiated gabbroic sill. Here was the inception 

of Dave's petrographic acuity and, here too, in an associated intrusion of gabbro and 

diorite, were the roots of Dave's first volcano. 

In 1971 and 1972, Dave assisted Pete Lipman in mapping the Platoro and Lake City 

calderas in the San Juan Mountains volcan ic field of southwestern Colorado. This 

fortunate apprenticeship so confirmed his enthusiasm for field petrology that, as a 

graduate student at the University of Wash i ngton, Dave undertook a detailed mapping, 

dating, and geochemical study of the Cimarron Volcano, an andesitic complex of 

Oligocene age in the western San Juans. His intricate reconstruction of the eruptive 

style and structural history of this center from evidence provided by its intertonguing 

pyroclastic and alluvial deposits prepared Dave very well for his big jump to the study 

of active volcanism. 

The jump came in the summer of 1975 when, with JUrgen Kienle's group from 

Fairbanks, Dave took part in a geophysical survey of Augustine Volcano, an island in 

lower Cook Inlet . When Augustine erupted explosively in early 1976, Dave rushed back 

to study the sequence and mechanisms of its eruptions and the petrology of their 

ejecta. His careful field and microprobe work demonstrated convincingly (l) that the 

pyroclastic flows had become less pumiceous with time (correlating with a change in 

emplacement mechanism); (2) that H20, Cl, and S contents of the magmas were notably 

high; and (3) that basaltic magma, although at no time actually erupted, had mixed with 

the more silicic melts and had possibly triggered their outburst. Only 25 months 

later, Dave turned in his Ph.D. thesis, not on his nearly completed Cimarron project, 

but on Augustine. This was a remarkable achievement, since both studies were 

labor-intensive and of high quality. 

Every summer Dave returned to Augustine and, in 1978 and 1979, he went to Katmai as 

well. In the Valley of Ten Thousand Smokes, Dave was taking the lead in studying 

physical aspects of the emplacement, compaction, weld ing , and degassing of the ash-flow 

sheet as part of our comprehensive restudy of the 1912 eruption. The fundamental role 

of the gas phase in volcanic processes had, by 1978, led Dave increasingly to focus his 

work upon magmatic volatile components, the main evidence for wh ich i s preserved in 

glass-vapor inclusions within phenocrysts, in quenched crystal-liquid equilibria, and 

in the fumarolic emissions of active volcanoes. Characteristically, Dave pursued all 

three lines of evidence vigorously . He became an exceptionally rigorous analyst, 

whether with the microprobe, gas chromatograph, extraction line, or mass spectrometer, 

but Dave knew better than most not to waste his time on a lousy sample. Thus, he put 

enormous energy into improving the equipment and techniques necessary to collect 

m anin ful and repr sen ative samples, especially the least fract iona ted and least 

contami nated as samples possible . Dave's agility, nerve, patience, and determination 

around th jet-like summ· fumaroles in the crater of Mt. Mageik were to me a spectacle 

of unfor ettable beauty . 

His work on vol anic ases brought Dave in 1978 to the U.S. Geological Survey, 

where he was assigned to expand our program for monitoring volcanic emissions in Alaska 

and in the Cascad s. He knew that such studies could improve our understanding of the 

nature and evolution of magma bodies, eruptive mechanisms, and contributions of 

magmatic volatiles to hydrothermal systems, but I think Dave's dearest hope was 

IX 
that systematic monitoring of fumarolic mis ion might p rmit d t ct·on of cha ng s 

charac teristica lly precursory to eruptions. Behind th i mage conv yed by his 

exc eedingly solita ry work was, i n fact, a gr eat cone rn for mak·ng a contribution to 

society. Dave wanted to formu late a general mod 1 for the behavior of magmatic 

volatiles prior to explo i ve outbursts and to de velop a co rollary rational for the 

evaluation of hazards . But Dave also held the conv iction that , as a scientist, he 

should devote time, energy, and imagination ufficient to commun icate ff cti vely to 

the non-sc ientific public the true range of potential volcanic haza rd , the geologic 

(or someti mes i nstrumenta l) reasons for our poor predictive ca pability, and some notion 

of the characteristic time scales upon wh ich volcanic behavior proceeds . Dave ' s 

concern for the societal importance of his work was nowhere more evident than i n the 

thoroughness and dedication with wh ich he r ecentl y undertook an asse sment of the 

geothermal-energy resources of the Azores and mainland Portugal . Also during his last 

year, Dave developed growing i nterests i n volcan ic contributi ns of halogens, sulfur , 

and co2 to the atmosphere and in the long-range e ffects on climate , health, a nd 

agriculture of both volcanic and anthropogenic emissions. 

Thus, it was only i n part by accident that when Mount St . Hel ns resumed its 

activ ity in March 1980, Dave Johnston was the first geologist on the mountain. From 

the earliest outbreak until the catastrophic event, Dave spent vi rtua lly the entire 

seven weeks at Mount St. He l ens, monitoring S02 emissions with a correlation 

spectrometer and coordinating the airborne sampli ng of gases and particulates. It 

deserves emphasis that Dave's work was an important contribution to a well-coordinated, 

scientificall y mult · -faceted, group effort that persuaded the authorities to resist 

public pressures to re-open the area around the volcano, thereby holding the May 18th 

death toll to a few dozen instead of thousands. Ironicall y, Dave was caught at an 

observation post thought to be relati ve ly safe, by an unusual eruptive event that was 

largely unanticipated, in magn i tude or style, except perhaps by Da ve himself. Three 

years ago, Dave had published this warning about Augustine: "High-temperature and 

high-ve locity shock waves extended far beyond the limits of discernable deposits [of 

the 1976 eruption ] . Hazard zone s defined on the basis of deposits [alone do not] 

reflect this more extensive shock wave hazard, wh ich at Augustine extends many 

kilometers offshore." No one was more aware of the danger of directed blasts, 

pyroclastic ·surges, and the shock waves that sometimes accompany explosive eruptions 

than was Dave Johnston . Dave repeatedly voiced his convictions that stud i es of 

deposits left by old eruptions provide only part of the story and that informed hazards 

assessment requires accepting the dangers of on-site monitoring of active volcanic 

processes. Dave was unaffectedly genui ne in everything he ever said or did, so he gave 

his energy and vitality to advance our understand i ng and thus our predictive capability . 

Dave Johnston is gone now, and many of us who were c l ose to him can still scarcely 

believe it. Dave was a natural scientist i n the finest sense, and to the extent that 

natural science is a collaborative effort, he was our sh i ning example, and we are all 

diminished. His infectious curiosity and enthusiasm, his unaffected concern fo r the 

opinions and feelings of other people, and his joyful spontaneity uplifted his friends 

and touched people who barely knew him. His generosity was unsurpassed, and I am 

recurrently astonished whenever I recall that Dave volun tarily carried 40 kg of rocks 

out of Katmai for me. Dave Johnston was as self-aware and capable a person as I have 

ever known, a man who would learn to do and dared to do whatever might be necessary to 

get the job done. In his final year, he began taking a night course to improve his 

rna thema tics. And, fed up with the stage fright that plagued his speaking career 

(though he gave excellent talks), Dave took the bul l by the horns and (though he 

otherwise rarely drank) solved the problem by taking a jug to the podium with him. 

But perhaps his most essential quality was the ability to dissipate cynic·sm 

spontaneously and to uplift the mood a ll around him . Dave looked for, saw, and thereby 

encouraged the best in all of us . Dave Johnston wou ld expect us to carry on the game 

without him, with all his wonderful enthusiasm. 

X 

Wes Hildreth 

July 1980 
GUIDE TO GEX:>LOGIC FIELD TRIP BETWEEN LEWISTON, IDAHO AND KIMBERLY, OREGON, 

EMPHASIZING THE COLUMBIA RIVER BASALT GROUP 

D.A. Swanson, U.S. Geological Survey, Menlo Park, Ca lifornia 94025 

and 

T.L. Wright, U.S. Geological Survey, Reston, Virginia 22092 

The Columbia River Basalt Group comprises a tholeiit i c flood-basalt province of moderate size (fig. 

1) , covering an area of about 2 x 105 km2 with an 

estimated volume of about 2 x 105 km3 (Waters, 

1962). The group is the youngest assemblage of flood 

basalt known, with an age range from about 17 to 6 

m.y. ago; most eruptions took place between about 17 

d 14 m.y. ago (Watkins and Baksi, 1974; McKee and 

others, 1977). It is the only flood basalt province 

of Phanerozoic age i n North America. 

Wide-ranging regional studies of the basalt, 

underway for the last 10 years, have been devoted 

primar ily to defi ning strati graphic and chemical 

rel ations for use in unraveling the h istory of the 

province and comparing the Columbi a River Basalt 

Group with flood basalt elsewhere. Recently, these 

studi es have been accelerated because of the need to 

know more about the stratigraphy and structure of the 

basalt as related to potenti al storage of nuclear 

vaste within the basalt pile . A geol ogic map of the 

en t ire prov i nce is under preparati on. Reconnaissance 

geolog ic maps of the basalt in most of Washington and 

northe rn Idaho have been compl eted (Swanson and 

o~ers , 1979a) and eventually wi ll be published in 

color (for example, Swanson and others, 1980 ) . 

Stud i es in 1978 and 1979 were conducted by f ull-time 

aoo temporary personne l of the U.S. Geol og ical Survey 

~der Interagency Agreement No . EW-78-I-06-1978 wi th 

the U.S . Department of Ener gy i n support of the 

Basalt Waste Isolation Program, admin istered by Rock-

~11 Han f ord Operations, Ri chland, Wash i ngton. Perronnel invo l ved i n this mapping project have been: 

J.L. Anderson, R.D. Bentl ey, G.R. Byerl y, V.E. Camp, 

J.N. Ga rdner, P.R. Hooper, D.A. Swanson, W.H. Taubeneck and T.L. Wright. 

GENERAL ASPECTS 

The Columbia Ri ver Basalt Group is character i zed 

by most features cons i dered typical of flood-basa l t 

provinces. Flows are vol uminous, typically 10-30 km3 

with a max imum volume of 700 km3, and many cover 

lar ge areas, as much as 40,000 km2. They generally 

advanced as sheetfl oods, rather than as channe li zed 

or tub -fed flows, and form thick cooling units composed of one or more flows. Eruptions took place 

fr fissure systems tens of kilometers long. Eruption rates wer high, generally greater by 2-3 orders 

of magnitude than those of other Cenozoic basalt provinces, as determined by 1) heoretical considerations based on the relations among eruption volumes, 

dimensions of lin ar vent systems, and distanc 

traveled without appreciable crystallization or cooling breaks, and 2) the absence of constructional 

shields even though viscosities calculated at constant temp rature from dry-weight chemical analyses 

are equal to or higher than those for oceanic tholeiites containing similar amounts of MgO. Small 

spatter ramparts formed along fissures but are poorly 

preserved owing to bulldozing and rafting by flows 

and to later erosion. Cinder cones are very rare. 

In these and other features, basalt of the Columbia 

River Basalt Group contrasts with that produced by 

basaltic pla i ns or oceanic volcanism (Greeley, 1977), 

as in the Snake River Plain, Iceland, and Hawaii. 

The flows cover a diverse assemblage of rocks 

ranging in age from Precambrian to Miocene. The prebasalt topography had considerable local reli~f, 1000 

m or more in places, near the margin of the Colu~ia 

Plateau. Some of the prebasalt hills today stand 

high above the plateau surface, especially in the 

Spokane area. Little is known about the natur e of 

the rocks or pre-basalt surface beneath the central 

part of the plateau. Sparse evidence from a drill 

hole 3.24 km-deep j ust west of the Pasco Basin suggests that a thick weathered or altered zone caps a 

sequence of lower Tertiary volcanic rocks of mafic 

and in termediate composi t i on at least 1970 m thick 

(Raymond and Till son, 1968 ; Newman, 1970; Jackson, 

1975; Swanson and others, 1979b) . 

STRATIGRAPHY 

Formal stratigraphic subdivision of the Columbia 

Ri ver Basa l t Group has recently been made (Swanson 

and others, 1979b) (fig. 2) . Considerable effort was 

expended in doing this, in order to provide a strong 

framework for topical studies. Some additional distinctive units have been found in Idaho and Oregon 

since the nomenclature for the Columbia River Basalt 

Group was established, but they can easily be given 

member rank and assigned to one of the three formations in the Yakima Basalt Subgroup. 

The criteria used to recognize specific stratigraphic units include megascopic and less commonly 

microscopic petrography, magnetic polarity, and chemical composition. Megascopic petrography and magnetic polarity can be determined in the field and, 

taken together in over-all stratigraphic context, are 

generally sufficient to i dentify a particular unit. 

Ambiguities commonly arise, however, and chemical 

analyses provide an i nvaluable and independent guide 

for checking and correcting field identifications. 

In fact, use of chemical analyses for correlation 

purposes is so reward ing that no study of the basalt 

requiring identifi cation of flows should be undertaken without provision for chemistry. Physical 

charac ter istics such as weathering color, size and 

shape of vesi c l es, t hickness, and type of columnar 

jointing have been used by some past workers as correlation criteria, but we have found them unreliable 

because of lateral var iability except in some local 

areas. 
CANADA 

------- UNITED STATES---- ------- ---: -

I o ~ WASHINGTON ~ ~ 

47° 

% 

~ 

~ 

\.) 

a ... 

.. 

l> .. 

....... 

4... 

....... 

l> 

~ 

Q., 

• Seattle 

OREGON 

0 150 KILOMETERS 

123° 

COLUMBIA ow Crf!'t'k 

• Warden y • Othello 

117° 

0 1 2 - '0 

~~ ' \ 

Figure 1 . Index map showing approximate outcrop extent of the Columbia River Basalt Group 

(from Waters, 1961) and locations mentioned in text. See figure 7 for details in 

southeast Washington and northeast Oregon. 

The formal stratigraphic units of the Columbia 

River Basalt Group (fig. 2) have been described in 

detail by Mackin (1961), Bingham and Grolier (1966), 

Schmincke (1967b), Swanson and others (1979b), Swanson and Wright (1978), and Caap and others (1979); 

only general statements are made here. 

Imnaha Basalt 

Outcrops of the Imnaha Basalt, the oldest formation in the group, are confined to extreme southeast 

Washington, northeast Oregon, and adjacent parts of 

Idaho, where feeder dikes are known (fig. 3A). 

Whether the Imnaha occurs farther west beneath 

younger rocks is conjectural. It covers a surface of 

ru99ed loca l relief and has an aggregate thickness of 

.or~ than 500 m. Most flows in the formation are 

2 

coarse grained and plagioclase phyric (Hooper, 1974) . 

Five cheaica types have been distingu ished (Bolden 

and Hooper, 1976; Kleck, 1976; Va llier and Hooper, 

1976; Reidel, 1978; table 1). Trace element compositions are given by Nathan and Fruchter (1974). Many 

flows conta in zeolite amygdules (Kleck, 1976), and 

smectitic alteration is widespread. The Imnaha 

Basalt conformably underlies the Grande Ronde 

Basalt,and future work may find areas in which the 

two form- ations interfinger. Most of the Imnaha has 

normal magnetic polarity, but the oldest and youngest 

flows known have reversed polarity, based on 

measurements with a portable fluxgate magnetometer. 

Two samples of the Imnaha have 87sr;86sr initial 

ratios of .7044 and .7043 (McDouga ll, 1976). 
0 

~ 

::> } 

; 

.& 

9 

.8 

"' 

~ 

..'l 

Sodd ... 

UrnohUo mbe.• 

t;;l;.~ ~~;;, m,;:--

..._.,..,l"'""''~ .... Botoh ~010 Ntte ~ 

~ ~::.~s:::.,:...--· 8o:toh of Shumol.e' \.. 

ao.o• o4 ilod9" 

beth of ~ne Movntro.n 

(

Bo.oh o4 a.,.,. ... 

Bcm.ohof ~· 

&cr,olt of l woe \..enhom 

·~ I 'Cibl,e 1 fOf ~tO ct'M':m<ol ~ 

toftOtn lw'c_ (_~Of"'d~·, 19fl J J 

J 

' IS 

13 

• 10 

( 0 / )> 8 11 

1 3 ~ 

lhe ~ ond PIC I~ G<Hge bo~ ewe- ·~ .... n to~ on < on'CJ( t 11\'t"tp.-r'OhO, ~ ptr ltm..nory 

~tO"Vt"ohgtophoe doto ~ lhot rhe ~ ~ t:'OI"C!~•e olc:H!t JhOn the Pl(turr Cor~ Sec! le •' 

lnf 0fmo1.on ., pott"'n~ rele,. to Pl.cture Goroe Bc:nolt 

Figure 2. Stratigraphic nomenclature, age, magnetic polarity, and 

cheaical type for units in the Coluabia River Basalt Group. 

N, normal aagnetic polarity; R, reversed; T, transitional. 

Subscripts refer to aagnetostratigraphic units of SWanson 

and others (1977; 1979b). Geologic time scale froa Berggren 

and Van Couvering (1974). From SWanson and others (1979b). 

Picture Gorge Basalt Grande Ronde Basalt 

Much of the Picture Gorge Basalt is apparently 

coeval vith the middle part of the Grande Ronde 

Basalt (fig. 2), as judged by the interfingering of 

the t~ formations (Cockerham and Bentley, 1973; 

Nathan and Fruchter, 1974) and reconnaissance aagnetostratigraphic work (R.D. Bentley and D.A. Svanson, unpub. data, 1977) along lower Butte Creek, Oregon (fig. 1). The Picture Gorge crops out only in 

and surrounding the John Day Basin in north-central 

Oregon (fig. 3A), vhere feeder dikes comprise the 

Monument dike svarm (Waters, 1961; Fruchter and Baldvin, 1975). Possibly the ancestral Blue Mountains 

uplift kept most flows from spreading northvard out 

of the basin. The foraation is at least 800 m thick; 

Thayer and Brown (1966) report a thickness of more 

than 1800 m at one locality, although faulting may 

have duplicated part of the section . The Picture 

Gorge contains aphyric to highly plagioclase-phyric 

fl wi h posi i n falling into a broad fi ld 

known as Pictur Gorge cheaical type (Wright and 

oth rs, 1973; tabl 1). Trac lement data are given 

by Osava and Gales (1970) and Nathan and Fruchter 

(1974). ~ of th thick r flows contain pegaatoids 

(Lindsl y and o h rs, 1971). The format i on can be 

s ubdivided into thr informal units based on field 

charac eris ics and magn ic polarity, according to 

R.D. B ntl y (Svanson and oth rs, 1979b). The Picture Gorge at its type section has an averag 87sr/ 

86sr initial ra io of .7037 (McDooga ll , 1976 ). 

3 

The Grande Ronde Basalt is the oldest foraation 

of the Yakima Basalt Subgroup (fig. 2) and the aost 

volWIIinous and areally extensive foraation in the 

entire Columbia River Basalt Group, underlying aost 

of the Coluabia Plateau (fig. 3B) vith an estiaated 

voluae of more tha.n 150,000 kJ113. Its thickness 

varies videly depending on underlying topography; the 

thickest preserved section exceeds 1000 a in drill 

holes in the Pasco Basin, and sections 50Q-700 • 

thick occur in the Blue Mountains and other uplifted 

oc deeply incised areas. Most flows in the formation 

are very sparsely plagioclase-phyric to essentially 

aphyric, although a few of the oldest flows in and 

near the Lewiston Basin contain abundant la.rge 

plagioclase phenocrysts. Major element co.positions 

fall in a broad range termed Grande Ronde cheaical 

type (table 1). Flows having different compositions 

vithin this range are interleaved throughout the section, although flows of high- Hg type rather consistently overlie flows of low~g type in the western 

part of the plateau. The Grande Ronde Basalt is subdivided into four magnetostratigraphic units on the 

basis of aagnetic polarity (Svanson and Wright, 

1976b; Svanson and others, 1979b); these units provide the only useful subdivisions of the foraation on 

a plateau-wide basis. Feeder dikes occur throughout 

the eastern half of the plateau and are apparently 

not confined to distinct swarms as formerly thought 

(Wate rs , 1961). The Grande Ronde conformably over-
0 

WA 

• n I d I 

US 4 I 

124 12 11 11 

1m n a h aSJ P i c t u r e G o r g eo B a sa 1 t 

A I 

f ee er I ke B 

11 

W anapum B asal t 

c D 

Fi gure 3. Maps showing generalized d istribution and feeder di ke s 

for the formations in the Columbia River Basa l t Group. 

From Swanson and Wr i ght (1978, fig. 3.4 ) . 

lies the Imnaha Basalt, inter tongues with t.he Picture 

Gorge Basalt, and conformably underlies and locally 

interfingers with the Wanapum Basalt. Commonly a 

thick soil and, locally, weakly lithified clastic 

sediments occur on top of the Grande Ronde; they 

indicate a significant time break, although probably 

no longer than a few tens of thousands of years judging ~rom the local interbedded relations between the 

Grande Ronde and Wanapum Basalts in southeast Washington (fig. 4). The 87sr;86sR initial rati os 

may increase slightly upsection, from values of about 

• 7046 in the older flows to about . 7052 in the 

younger flows (McDougall, 1976); this interpretation 

is tentative, as the older flows were sampled at different locations than the younger flows, so that 

variation due to lateral isotopfc heterogeneity in 

the source rocks is also possible. 

Wanapum Basalt 

The Wanapum Basalt is the most extensive formation exposed at the surface of the Columbi a Plateau 

(fig. 3C) but is much less voluminous than the Grande 

Ronde, probably having a volume of less than 10,000 

km3. On a local scale, the Wanapum conformably 

overlies the Grande Ronde, except for minor erosional 

unconformities or interbedded relations. On a 

regional scale, however, the Wanapum overlies progressively older basalt from the center toward the 

eastern margin of the plateau. Such onlap is not 

apparent along the northern and western . margins, however. These relations suggest that the plateau had 

tilted westward before Wanapum time. 

The oldest member of the Wanapum Basalt, the 

Eckler Mountain Member, occurs in the Blue Mountains 

and adjoining foothills of southeast Washington and 

northeast Oregon (Swanson and others, 1979b, 1980). 

The oldest flow in the member, the basal t of Robin4 

EXPlANATION 

C\oysrone 

Oodg en mocol rype 

Hoon-

~ MgO Grond 

Ronde chemocol type 

I I I I 

Frenchmon Sprrnos 

che m1col rype 

Fi gure 4. Schematic strati graph ic section in Benjami n Gu l ch, 3 km south of Pomeroy , 

wash i ng t on, showi ng chemi ca l types for 13 

basa l t f l ows. Note i nterbedded nat ure of 

chemical l y different f l ows. Uncor r ected 

for north d i p of about 4 degrees. From 

Swanson and others (1979b) . 
(J\ 

Table 1. Average major-element compositions for chemical types in the Columbia River Basalt Group 

(Ave rages include ana l yses available through March 1977 ; see Wr i ght and others (197 9) 

for more recent averages and further discussion of chemical types ) 

Chemical t l 2 3 4 5 6 7 8 9 10 ll 12 13 14 15 

(1J.V (44 ) (21) (68) (7) (8) (4) (13) (13) (8) (1 0) (9) (20) (4) (8) 

Oxide --- ---- ·-- _ ------· 

Si02 

Al i'3 

"FeO".Y 

MgO 

50 .99 

15 .42 

12 .24 

5.94 

51 . 14 51.18 49.53 

15 .06 14.06 16.34 

13 .04 14.11 12.38 

5.07 4.60 6.06 

9 . 31 8.59 9.15 

2 . 58 2.65 2.58 

0.91 1.19 0.93 

2.2 4 2.93 2.41 

0. 42 0.48 0.4 1 

50 .73 

17.10 

ll. 26 

50.36 

15.54 

ll. 25 

6.68 

10.67 

2.95 

0.57 

l. 56 

0.22 

51. 46 

15.39 

12.46 

4.86 

9.45 

3.29 

0.74 

l. 79 

0.33 

51.57 

13.87 

12.28 

4.44 

8.12 

3.36 

2.02 

2. 7l 

l. 39 

53.78 

14.45 

11.35 

5.25 

9.07 

2 .83 

1. 05 

l. 78 

0.28 

55. 94 

14.04 

11.77 

50.01 

17 .08 

10 .01 

7.84 

CaO 

Na20 

K20 

Ti 02 

P2o5 

MnO 

TotaL~/ 

10 . 11 

2.55 

0.53 

l. 66 

0.34 

0.22 0.22 0.22 0.20 

5 .42 

9.30 

2.45 

0.85 

2.32 

0.38 

0.19 0.20 0.23 0.24 0.19 

3.36 

6.88 

3. 14 

l. 99 

2 .27 

0.43 

0.19 

54.37 

15 .28 

9.46 

5.91 

9.79 

2.80 

0. 77 

1.17 

0.29 

0.16 

11.01 

2.44 

0.27 

1. 00 

0.19 

0.14 

100.00 99.99 100.00 100.00 100.00 100.00 100.00 100.00 100.01 100.01 100.00 99.99 

Chemica l tv·oe 17 18 19 20 21 22 23 -----:2:-::4:-- --2-5 - 26 27 28 

Ox ide 

Si02 

Al203 

"FeO"~/ 

MgO 

CaO 

Na 2o 

K20 

Ti02 

P20s 

MnO 

TotaLY 

(15) 

50.27 

13.69 

15 .04 

4.29 

8.31 

2.67 

1.16 

3.55 

0.81 

0.21 

100.00 

(55) 

50.09 

14. 31 

13 . 78 

5 . 18 

8.88 

2.57 

l. 07 

3.15 

0.78 

0. 19 

100 .00 

(13) 

54.70 

14.10 

12.63 

2. 7l 

6.14 

3.20 

2.68 

2.80 

0.88 

0. 17 

100.01 

(ll) 

54.41 

14.51 

ll. 07 

4 .51 

8.32 

2.69 

1.77 

l. 95 

0.56 

0.21 

100.00 

l/Number of ana l yses used i n computing average 

~/pea+ 0.9Fe2o3 

(6) 

50.72 

16.23 

9.64 

8.19 

10 .70 

2.22 

0.51 

( 3) 

52.12 

14.33 

ll. 64 

5.58 

9.64 

2.69 

0.87 

1.45 2.48 

0.18 0.49 

0.17 ~ 

100.01 100.00 

(2) 

49.75 

15.26 

ll. 82 

7. 10 

10.13 

2.32 

0.46 

2.42 

0.55 

0.21 

100.02 

(12) 

54.16 

13.84 

12.60 

3.91 

7. 7l 

2.66 

1. 70 

2.82 

0.41 

0.19 

100.00 

l/oifference between total and 100 is due to rounding during normalization 

Chemical types (defined by method of Wright and Hamilton, 1978) 

l. Picture Gorge (Imnaha Basalt) 3. Frenchman Springs (Imnaha Basalt) 

4. Rock Creek 

(3 0) 

51.88 

14.88 

10.55 

6.96 

10.67 

2.36 

0.64 

l. 62 

0.25 

0.17 

99.98 

(41) 

51.08 

13.54 

14.75 

4.28 

8.34 

2.45 

1. 25 

3.52 

0.59 

0.20 

100.00 

(8) 

54.46 

14.29 

11. OS 

4.85 

8.54 

2 . 75 

1. 39 

2.17 

0.35 

0.15 

1 00.00 

(12) 

47.45 

13.84 

15 . 22 

5.99 

9.71 

2 . 31 

0. 72 

3.62 

0.91 

0.23 

100.00 

7. Low Mg-Picture Gorge (Wright 

and others, 1973) 

8. Prineville (recalculated from 

Uppulur i , 1974) 

9. High Mg-Grande Ronde (one flow) 

10. Low Mg-Grande Ronde (one flow) 

11 . Very high Mg-Grande Ronde (one 

flow) 

12. Robinette Mountain 

13 . Dodge 

14. Shumaker Creek 

52 . 13 

15.41 

10.66 

5.92 

10.18 

3.00 

0.68 

l. 48 

0.35 

0.19 

100 .00 

29 

(13) 

48.73 

13.88 

14.41 

5.88 

9. 72 

2.42 

0.73 

3.30 

0.73 

0.20 

100.00 

54.80 

13 .86 

13 . 32 

2.84 

6.48 

3.18 

1. 87 

2.46 

0.93 

0.26 

100.00 

30 

(8) 

47.50 

12.50 

17 . 53 

4.41 

8.80 

2.44 

1. 23 

3.79 

1. 54 

0.27 

100.01 

18. Lolo 

19. Umat illa 

52.29 

13.21 

14.38 

4.04 

7.90 

2.67 

l. 41 

3.17 

o. 7l 

0.22 

100.00 

31 

(24) 

50.44 

14 .07 

13.78 

5 .01 

8.67 

2.79 

1. 47 

2.90 

0.66 

~ 

100.00 

20. Wi l bur Creek 

21. Asot in 

22. Slippery Creek 

23. Lewiston Orcha rds 

24. Esquatzel 

25. Pomona 

26. Elephant Mounta in 

27. Buford 

28. Basin City 

16 

(35) 

51.19 

14.07 

13.91 

4.39 

8.48 

2. 72 

l. 22 

3.13 

0.67 

0.23 

1oo-=o1 

2 . American Bar (equivalent to the 

high-Ti Picture Gorge chemi cal 

type of Wr ight and others, 1973) 

5. Fall Creek (Kleck, 1976) 15. Frenchman Springs (one flow) f9. Martindale (Ice Harbor l) 

6. High Mg-Picture Gorge (Wright and 

others, 1973) 

16. Roza 

17. Rosalia 

30. Goose Island (Ice Harbor 2) 

31. Lower Monumental 
Table 2. Av rage tra e - mpositi ns for h mi 1 types in h Yakim B s l Subgroup 

[Instrumental neutron a tivati analys s by L.J. s hwarz und r di r i n of J.J. Row 1 

.Y 

2. 10 12 13 14 15 16 17 18 ~ 

Ba 496.0 783.0 196 .o 319.0 1021.0 564.0 03.0 534.0 510.0 3t9s.0 

Co 41.3 36.9 45.6 38.1 23.2 39.4 7.5 37.7 41.1 28. o Cr 100.] 11.7 149.5 168.8 7.2 33.1 54.6 15.6 96.7 4.9 

Cs 0.7 1.4 - ]:_/ 0.9 0.95 1.2 1.0 0.7 0. ) 

Rf 3.7 5.2 1.5 2.8 6.2 4.35 4.3 5.4 4.6 10.) 

Rb 28.0 46.0 44.0 28.5 25.0 28.0 19.5 47 .o Ta 0.71 0.94 0.57 l. 41 1. 01 0.99 1.12 1.14 1.4( Th 3.5 6.1 0. 45 1.2 5.2 3.7 3.8 4.1 3.3 6. 9 

Zn 132.0 147.0 118.0 118.0 181.0 201.0 177.0 220.0 192.0 14S.o Zr 349.0 350.0 232.0 216.0 308.0 222.0 590.0 

Sc 37.04 31.20 36.26 40.34 34.0 36.39 35.45 36.69 36.44 26.) 

La 18.2 28.7 8.0 17.0 37.0 26.5 27.0 34.0 29.0 46.) 

Ce 38.0 58.0 16.0 37.0 77 .o 52.5 55.0 69.0 59.0 88.0 

Sm 5.4 7.7 3.0 5.4 11.5 7.2 7.8 8.7 10.) 

Eu 1. 68 2.18 1.04 1. 60 3.27 2.27 2.34 2.81 2.59 3. 93 

Yb 2.6 3.6 2.0 5.7 3.4 3.2 4.2 3.7 4.4 

Lu 0.50 0.60 0.37 0.52 0.81 0.63 0.61 0.73 0.63 0.6 

20 21 24 25 26 27 28 29 30 

Ba 823.0 284.0 614.0 235.0 517.0 415.0 579.0 505.0 808.0 

Co 44.5 46.4 40.6 43.4 45.1 39.6 45.0 42.0 37.6 

Cr 37.6 281.3 19.9 112.0 18.0 45.5 148.3 192.9 44.8 

Cs 0.8 1.0 0.7 0.7 

Hf 6.0 2.8 5.5 3.2 6.8 5.4 6.6 5.8 10.6 

Rb 41.0 55.0 29.5 4.3 26.0 

Ta 1.02 0.67 1.68 0.77 1. 73 1. 54 1. 82 1. 51 3.09 

Th 6.2 2.1 8.5 2.5 6.0 7.0 2.2 2.2 4.4 

zn 142.0 118.0 144.0 131.0 181.0 142.0 206.0 189.0 296.0 

Zr 345.0 200.0 445.0 256.0 328.0 505.0 

Sc 26.85 30.73 27.8 34.35 31.50 31.50 37.95 38.07 36.44 

La 43.5 16.0 38.0 17.0 36.0 33.5 46.0 42.0 77.5 

Ce 82.5 30.5 73.0 34.5 72.5 64.0 91.0 80.0 155.0 

Sm 8.4 4.3 8.5 4.7 9.6 7.4 12.9 10.8 21.3 

~u 1. 92 1. 26 2.18 1. 43 2.63 1. 86 3.54 3.03 5.68 

Yb 4.3 2.3 3.6 2. 7 4.9 4.1 4.9 4.0 8.5 3.1 

Lu 0.63 0.34 0.52 0.39 0.68 0.57 0.86 0.73 1.40 o.r 

!/chemical types: 

9. High~g Grande Ronde (one flow) 15. Frenchman Springs 20. Wilbur Creek 27. Buford 

10. Low-Mg Grande Ronde (one flow) 16. Roza 

12. Robinette Mountain 17. Rosalia 

13. Dodge 18. Lolo 

14. Shumaker Creek 19. Umatilla 

J:./Dash means not determined 

ette Mountain, is diktytaxitic and contains the 

lowest KzO and incompatible trace element concentrations of any other flow in the Yakima Basalt Subgroup (tables 1 and 2); it was erupted from a long 

fissure south of Dayton, Washington. Several flows 

and dikes of the next youngest flow--the basalt of 

Dodge, a very coarse-grained plagioclase-phyric, 

grusy-weathering unit--form excellent markers in the 

Blue Mountains. The basalt of Dodge is chemically 

similar to but much coarser and more porphyritic than 

some high-Mg flows of the Grande Ronde Basalt (tables 

1 and 2). The basalt of Shumaker Creek, the youngest 

unit in the member, is neither widespread nor easily 

recognized in the field, although its high KzO and 

PzOs are distinctive (tables 1 and 2). 

6 

21. Asotin 28. Basin City 

24. Esquatzel 29. Martindale 

25. Pomona 30. Goose Islcml 

26. Elephant Mountain 31. Lower Monu· 

mental 

The Frenchman Springs Member overlies and toea :. 

interfingers with the Eckl r Mountain Member d 

crops out widely in the central and western parts :! 

the plateau (fig. SA) . Its volume is probably 3 : 

to 5000 km3. Generally three to six flows, in 

places as many as ten, occur in any one secti oo . 

Flows were erupted from north-northwest-trending 

dikes extending through the Walla Walla area ~ 

southeast Washington (Swanson and Wright, 1978; 

son and others, 1979b, 1980). Highly porphyri tic 

flows near Soap Lake at the southern end of Gr ~d 

Coulee may have erupted along the northward exte !. 

of the known feeder system. Most flows of the 

Frenchman Springs Member contain rare to abundant 

glomerocrysts of plagioclase, although some are 
Frenchman Springs Member Roza Member 

A 

Priest Rap i ds Member Pomona Membe r 

c D 

Figu r e 5 . Maps showi ng genera~i ze d distribution and feede r dikes 

for the Frenchman Spr ings, Roza, and Priest Rapids Members 

of the Wana pum Basalt and the Pomona Member of the Saddle 

Mountains Ba salt . From Swanson and Wr i ght (1978, f ig . 3.5 ) . 

aph ric and i nd isti ngu ishable i the f ield from som 

flows of Grande Ronde Basa l . The member ha s a h1g 

PeO and Ti02 composition known as Frenchman Springs 

chemica l ype (tab les and 2) . The 87sr; 86sr 

initial ratio is about . 7053 (McDougall, 976). The 

Frenchman Spri ng s is overlain by the Roz a Membe 

The Roza Member, a highly plagioclase-phyric un it 

with a volume of abou 1500 km3, is well known, and 

readers are referred to papers b Lefebvre (1970) and 

Swanson and others (1 97 5; l97 9b) for details. The 

member was erupted from a 1near vent system mo re 

ha 165 km long in the easter par of t he platea 

!fig. SB ) . The Roza consists pr 1ncipall oft 

cooling units, a l though more thin units occur near 

v n sys e . Th compos1 10n o he Roza is 

s1 il ar o tha of h Fr nchman Spr "ngs Membe 

1 hough on he av rage sligh ly rich r in MgO 

I ab es 1 and 2) . Its 87sr; 86s i ni ial atio is 

abou . 70 4 (McDougall, 1976). 

The Pri st Rapids M mb r ov rlies he Roza Membe 

and 1s h young st b sal hroughou mos o he 

nor h rn par of the Columbia Plateau. The memb r 

oc urs as ar southw s as h Colu ia Go ge (fig . 

~). AlJ known fed r dik s are confined to the far 

as rn part of th provinc . Severa l dik s occur 

near Orofino , Idaho, and along Sla Cre k abou 16 

m e s of Fre dom, Idaho (W.H . Tauben ck , T.L. 

7 

Wr ight and D.A. Swanson, unpub. chemical data, 197 7; 

V.E. Camp i n Swanson and others, 1979a ) , and probabl 

vents are l ocated near Emi da, Idaho, and i n Pa l ouse , 

Was hington. Other vents presumably exist in northe rn 

Idaho, as intracanyon flows of the member occur far 

up the ancestra l St. Joe River valley. The estimated 

vol ume of the Pr iest Rapids is 2000-3000 km3. In 

and near Spokane, flows of t he Priest Rap ids Member 

fill va ll eys as much as 100 m deep e r oded into the 

ma in part of the Latah Formation, a sequence of finegra i ned c l astic sediments interbedded with and ove -

l y1ng the Grande Ronde Basalt. Flows of the Pr ie st 

Rap i ds i nvade sed iments o f the Lat ah in many pl ace 

nea r Spokane as well as other sediments near Orof1no 

Idaho . The Priest Rap ids Membe r contains magnet ically re versed flows of two distinctly d ifferent compositions, a very high FeO and Ti02 type (Rosa lia 

chemical typ~) and a hi gh MgO type (Lol o chemica l 

type ) (ta bles l and 2). The flows of Rosal i a chemical type are found throughout most of the extent of 

he membe r and are consistentl y o lder than those of 

Lolo chemical type , wh ich a re confined to the 

southern two-thirds of the membe r 's outcrop area. A 

few thin f ows of diffe rent compositions occur near 

vent a reas in norther n Idaho and adjacent Wash i ngton. McDougall (1976) obtained an 87sr; 86sr 

initial ratio of .7053 on a f ow of Rosa li a chemi ca l 

type near Frenchman Springs Coulee. 
Saddle Mountains Basalt 

This formation, the younges i th Columbia 

River Basalt Gro up, is about 3. 5 to m. . 1 and 

conta ins fl owc erupted sporadi c 11 ur1ng · period 

of wan 1ng volcanism, deformation, canyon cutting, and 

deve l opmen t of thick but local sedim ntar y deposi t s 

between flows. The Saddle Mounta1ns Basalt has a 

volume of only about 3000 km3, less than one percent of the total vol ume of basall, ye t conta1ns b 

far t he greatest chemica l and 1~ otop1 c d i ve rsity of 

any formation in the group. 

The Umati la Member is the oldest and on of the 

most extensive members in the forma 1on. It occurs 

in extreme southeast Washington and northwes Oregon 

(the Troy and Lewiston bas1ns and Un1ontown Plateau) 

(Pr1ce, 1977; Ross, 1978; Swanso ana o her , 1980 ) ; 

vent areas and a feeder dike occur i the Puffer 

Butte area (fig. 1; Pr i ce, 1977) . Remnants of the 

Umatill a f ill a broad shallow paleovalley leading 

fr om the Troy basin across the presen -day Blue 

Mountains i n northern Oregon to the Milton-Freewater 

area, where the flow spread out o the pa leova lley as 

a sheet flooci cover ing muc of south-ce ntral Wa shing-· 

ton (D.A. Swanson and T.L. Wrigh , unpub . map, 1978; 

wanson and o hers, 1979a ). Lava w s charnell o 

dlong some canyons eroded dur 1ng po ... -Wanapurr. im 

the western part o the Columb ta Plateau, as along 

Yak1ma Ridge (R . D. Bentley 1n Swanson and ot hers. 

l979a l . The d istri bu tion ot the Umati la prov i de s 

the earliest evidence fo: extens1ve eros1on and 

canyon-cu~~i g o the Columo1 R1ve. Basal o 

the Col umb1 a Plateau, although e ros1o wa 

suostantia i r the Columbi· Gorq beror e 

Rap1ds time (Beeson and Mer n , 197q . 

has an unusual chemical composit1on n racter . z d 

l ower contents of CaO and MgO anc hlQne r con ents o 

Na20, K20, and u~compatib l :: ac ·l ementr -:: .ar 

most other flows in the roup (t aoles an ~~- Th . 

content of Ba is 2000 ppm or more, su::icien~ a l on 

to identify the member. P. R. Roope :: loral commun .. 

1979 ) has recognized two flows of sUghtl,. differen: 

K20, Ti0 2 and Ba contents i the member . Th 

87sr; 86sr i nitial ratio o the Umat 1 a 1< htg 

abou ~ .709 2 {McDouga l, 1 976 Th urn unde 

lies the Wilbur Cr eek Member . 

The Wilber Creek Membe r an h ver l·l a Aso 1r 

Membe r , distinctly diffe ren l ow!= ou• possi D. cnen>-

i cally re l ated, were apparentl e- up ed 1 the Cl earwater embayment o f Idaho. From the r e, th~ flo ~ 

advanced down vall eys and gorges eadi ng 'rom th 

Uniontown Plateau to the centra l oa , · of hP c o l umb1 a 

Plateau; remnants of the va lle -f · l 1nq 1 ~ n c r 

east anc west of l owe r Cow :ree ~- i~ . l l nea r 

Wa rden and Othe llo , and elsewhere {Swanso anc 

others, 1980 ) . The flows c osseo h nortne r pa • 

o. the Pasc Basin (Myers anc I) tC(. .:. "1 n · rr:ovecdow a c anyon along Yak ima Fodgt pos:;iol "'!. f a !: we., t 

as Yak ima (R.D. Bentle i Swanso ana others. 

1S79a ) . The flows overli quartZltic :: av· ~ o 

extra-plateau derivation alon9 Ya tma Ridge and rae 

a westward course of the ancestr a . Columb1 Rive 

f rom the Pasco Basin to Yak1ma . Th Wil bur Cree ha c 

a ma jor element compositio simila ~ t that of th 

lntermediate-Mg Grande Ronde cnemicd 4 ype , and h 

Asoti n is similar to the basalt of Robinett• Mounta 1 r 

ftable 1); however, trace element compos "t1ons eas · ; 

disc- lmlnate the flows (tabl£ 

The Weissenfels Ridge Membe r overlies t he Asotl 

Member and contains severa l flows con ine· ma1 nl to 

thP. Lew1ston Basin and presumaol er p ec• : her . Th 

basall o Lewiston Orchards, on flow aver aging 

10-15 m thick , is spars ly plagioclas -phyric a 

contains groundmass olivin visibl witl a handn 

lens. I t 1~ r la i v ly rich in MgO nd poor i 

(tabl ) . Th over ying basal o~ Sl t pper 

consis s of s veral lows , at eas on 

conta in bundan roundm ss o liv1n . . 

compos1t1on differs from o her fl owc o 

Mounta1ns B sal . 

1977) ha a maJor lemen 

Lolo c h mica l ypc but i he 

Th Esquatz 1 Membe r (fig. 2) occur. s isolated 

remnants o an intracanyon flow along and us nor h 

of the mode r n Snak Ri ver upstream fr o Dev ils Canyo 

ISwan son and others, 1980). It apparen ly was 

er upted within the ancestral Snak dr 1nag , flowed 

down can~on, and en ered th ance stra l Columb ia River 

valley 1n the cen ral part of th plat eau. Th 

Esquatzel then flowed along Yak ima Ridg 

simila to that of the Wi bur Creek an 

Th Pomona Membe 

th :: 1· of. . cnm1n k 

ocat 

a flow 

a 

the Col umb1 a Go:ge . 

southwes Was hln ton 1 

an 

extrao rd ln a~il 

Ne lson ana other ... , 1976 

fo l low an ances ra ~ eolumota dra1na 

Pomona w· ~ 

0 th 

2 

ow d1 d Peper ite 

t h m t f 

s , an 

o':he. 

vo l 

Th · E ~phan Mountal Membe 

ably e up: _ tr. par fr o a dik 

19 78 ) tr he Tr o bas 1r of nortneas 

i i nowr a.. the Wen ah' flow o f 

Flows o f th membe r advar.ced dow t 

Ri ver .anvon as the Pomona Member 

n •• y. ea ~ The fl ows 

mouth o· •· h · :anyon n~ r 

red mucr o so t -cen~ 
wing volcanlclastic debris that had been erupted 

~ ~e cascades, carried eastward by rivers, lahars, .n . nd wi nd s, and depos1ted on the Pomona Member. 

:ecent mapping has defined the west and southwest 

argin of the member alon? a line extending approxiately southward from Yak1ma to the Horse Heaven 

plateau (Swanson and others, 1979a) . The Elephant 

~~ntain consists of several flows, all chemically 

similar , of normal and transitional magnetic polaritY· Its major element composition is similar to 

that of Rosa lia chemical type except for lower 

PiJS (table 1), but its trace element content is 

distinct. The member has a 87sr;86sr initial 

ratio of about . 7078 (McDougall , 1976; Nelson and 

others, 1976), simi l ar to that of the Pomona. 

The Buford Member (fig. 2), a single magnetically 

reversed flow 20-30 m thick, is the youngest known 

ba salt on the plateau surface of extreme southeast 

~shington and northeast Oregon, where it is confined 

and presumably was erupted . Its major-element composition shows distinctly higher light REE contents 

(table 2) . 

~e Ice Harbor Member (fig. 2), dated as about 

8.5 m.y. old (McKee and others, 1977), was erupted 

fr~ the central part of the Columbia Plateau, where 

dikes and remnants of vent areas have been recogni zed. The last previous eruptions from the central 

part of the plateau were those of the Frenchman 

~rings Member, about 6 m.y. before the Ice Harbor 

vol canism. Most flows are confined to the area of 

venting, but at least one flow spread westward to the 

Richland area and southwestward to Wallula Gap. The 

Ice Harbor Member can be subdivided into three readily mappable units of different compositions (tables 

1 and 2) . The l owest unit--the basalt of Basin 

Ci ty--contains plagioclase and olivine phenocrysts, 

as normal magnetic polarity, and is chemically distinc t. The middle unit--the basalt of Martindale--

carrie rs clots and single crystals of clinopyroxene, 

. plagi ocl ase, and oli vine and has reversed magnetic 

~larity; two related compositions characterize the 

Martindale (Helz, 1978), the dominant of which is 

isted in table l. The upper unit--the basalt of 

Goose Island--contains sparse plagioclase and mageti te phenocrysts, has normal magnetic polarity, and 

has the most Feo-r ich composition of any known flow 

in the group (and one of the most Feo-r ich compositions of any te rrestrial basalt). The three informal 

units have simil ar 87sr; 86sr initial ratio of 

aoout . 7077 (He lz, 1978). Helz (1978) has recently 

mpleted n e xhausti ve experimental and petrogenetic 

s udy of h I Harbor Member. The Ice Harbor vent 

system is boo 0 km long and has s rong aeromagetic e xprE:> ssi n (Swanson and o hers, l979c). 

The Lower Monum n al Member, about 6 m.y. old, is 

he mber in he Saddle Mounta ins Basalt. 

o h modern Snake River Canyon 

be we nyon and so i n, Wa shington, a dis-

~nce 50 km. I s sou rce as presumably 

bu has o yet been 

omposi tion is -l milar to 

li qht y higher i n 

g t REE ( abl es 

\ i gh 

1109 ' NE:> l son 

PHYSIC' L HARACTERtST CS F LOWS 

Gr nd t.tonc1e , Wan:tpum , 3nd 

~anq€' fr<Jm 1 ~'-e w en 

) 0 -40 

m. The thick flows generally record ponding i n pre -

basalt all eys , i n s tructurally controlled basins 

that developed during volcanism, or in narrow canyons 

eroded into older flows; such intracanyon flows are 

common only in the Saddle Mountains Basalt . Even the 

thinner flows generally show evidence of hav ing 

ponded. This ev1dence consists of the columnarjointed nature of the basalt (fig. 6 ) . Such columns 

can apparently form only under static cooling conditions; their development therefore implies that the 

lava had ponded. What impounded the lava can rarely 

be determined. Natural levees several meters high 

have been observed in places and probably account for 

most of the pending. El sewhere, flows could have 

pinched out against opposed topographic slopes. 

Figure 6. Cross section of flow in Yakima Basalt 

Subgroup, showing, in idealized form, 

jointing patterns and other structures . PPC, pillow palagonite (hyaloclastite) complex, present at base or 

throughout flows that entered water. 

Flows that cooled under stagnant conditions contracted and developed a characteristic jointing 

habit, shown in idealized form in figure 6. The 

terms colonnade and entablature were borrowed from 

classical architectural usage by Tomkeieff (19 40 ) . 

Columns in the colonnade are from 10 em to 5 m 1n 

diameter, averaging about l m, and can be as long 3S 

50-75 m although generally 5-10 m. Most are 

straight, but curved columns are rather common and 

generally unexplainable i n terms of simple cooling 

mode ls. Columns in the colonnade are commonly subdivided in to prismatic blocks by cross joints; the 

oarsest-grained par of a flow may be platy ow ing to 

the close spacing of cross j oints. 

The colonnade-entablature contact is relatively 

sharp, he hange commonly taking place within 1-2 

em. The contact is traceable i n many pl aces for several kilometers before other complexities obscur ~ 

it. The glass content o f the groundmass 1ncreases 

a rup ly from the col onnade to the entabl ature for 

on, 1967; Long, 1973 l he 

enta la t~r e cons 1c s i columns of smal Le r iameter . 

en eral y l ess han <;em, and less -::>ns1stem. · : :-· 

enta ti o, th n hose i n he colonnade. Co l~~ s t 

many n:a l atures re 'undled into fans, svn or~s . 

e nt s , = th er un Jsu. ly haped arranaements . 1o;t 

ol umn s in an enta I l u re are highly eqmented nv 

l!req:.~lar Jross }Oln s , so that the coltlll\T\s ar 

roadily roken 1 to f is size pieces. rhe en t a 1.·- •_; ,... •:1ener .111 . comprise s a bou 70 o..rr- pn '=' 
thickness of a flow but can make up 00 percent (one 

example that we know of) to zero percent. The upper 

part of the entablature is scoriaceous and commonly 

merges into a zone of short, wide, generally poorly 

defined co umns that some workers call the upper 

colonnade. A rubbly, clinkery zone occurs above the 

entablature of some f ows. Such a zone is absent 

from the base of flows. The origin of this rubbly 

zone is in question. The rubble is, in our experience, more common near vent areas than elsewhere; 

this observation suggests that the rubble may represent material thrown out of the vent near the end of 

eruption and mod ifi ed during movement of the flow. 

Idealized jointing patterns can be satisfactorily 

explained by existi ng theory for the cooling of 

bodies of igneous rock {Jaeger, 1961), but such patterns are seldom found in nature. Acceptable thermomechanical explanations for the typically complex 

jointing patterns, particularly in the entablature, 

are not available despite considerable descriptive 

information (Tomkeieff, 1940; Waters, 1960; Mackin, 

1961; Spry, 1962; Swanson, 1967; Schmincke, l967b; 

Long, 1978; Ryan and Sammis, 1978). Complications 

related to the mutual interference between columns 

growing inward from irregular contacts, pending of 

water on a flow surface and percolation down joint 

planes during solidification, the influence of chemical composition on tensile strengths and heat conduction, and inadequate knowledge of rock mechanics 

under high temperature-low pressure conditions are 

some of the difficulties that plague attempts at 

analysis of natural jointing habits. 

Some flows have a tiered appearance defined principally by alternating layers of vesicular and relatively nonvesicular rock rather than by joints. 

These layers may record separate gushes or thin flows 

that piled up and solidified as a single compound 

cooling unit. Tiers much more commonly occur in 

entablatures than colonnades. 

The upper surface of a flow is rarely exposed in 

plan view. Whe re seen, the surface is rather flat, 

smooth, filamented, and locally ropy--surface features characteristic of lava ponds at Kilauea. The 

surface of a flow with a rubbly upper zone is rough, 

has a relief of as much as 6 m, and otherwise appears 

unlike typical surfaces of ponded flows. 

Many flows entered water and formed pillows. 

Recent studies {Jones, 1968; Moore, 1975) have demonstrated conclusively that pillows are nothing more 

than the subaqueous equivalent of pahoehoe toes. 

Many of h pillowed flows occur near he margin of 

the plateau as it existed at the time of eruption, 

apparently because lakes were formed as flows ponded 

rivers draining from marginal highlands. Other pillowed flows are much more extensive, perhaps signifying entry into shallow lakes standing on the plateau 

surface. One such extensive flow, in the Pr iest 

Rapids Member, is pillowed throughout an area of tens 

of square kilometers in the Cheney-Palouse scabland 

southwest of Spokane. 

In places, lava deltas (Fuller, 1931; Moore and 

others, 1973) formed as lava poured into shallow 

lakes and ponded streams. The direction of dip of 

foreset "bedding" defined by elongate pillows and 

thin sheet flows in the lava deltas in~icates the 

local flow direction of the lava. Particularly good 

examples of lava deltas can be seen near Ma lden south 

of Spokane (Griggs, 1976), near the mouth of Moses 

Coulee {Fuller, 1931), and at the mouth of Sand Ho llow south of Vantage. 

10 

Other cri eria for efining flow directions 

i nc lude i nc Uned pipe v sicles, which plunge upcur~ 

rent, and bent spiracles (fig . 6), formed by steam 

blasts eneath flow, which tail out down-current 

Flow directional da a for basalts must be treated ln 

~he sa~e way as those for current-produced structu~s 

1n sed1mentary rocks--carefully . A few data in a 

small area show the local direction but say little 

about regional patterns. Nonetheless, careful 

studies by Schmincke (l967a) and on-going work by 

othPrs are succeeding i n defining patterns of lava 

advance with in the plateau. 

FEEDER DIKES, VENT SYSTEMS, AND ERUPTION 

One of the major results of recent mapping has 

been identi fication of sources for most stratigraph 'c 

units and even single flows. Feeder dikes have been 

found for flows in all formations and membe rs of the 

Colt:mbia River Basalt Group except the Wilbur Creek, 

Asotin, Esquatzel, Buford, and Lower Monumental Members of the Saddle Moun tains Basalt. The feeder 

dikes avfHage about 8 m wide but vary from a few 

centimeters to more than 60 m. They may tend to thin 

upward, but this is far from cer ain. The dikes 

generally trend north to north- northwest . They cannot be traced far along strike, in part because of 

exposur problems. Obviously related dike segments, 

offset a few meters to form an en echelon pattern, 

form systems extending tens of kilometers. Compound 

or multiple dikes, consisting of two or more pulses 

of magma re ated to the same intrusive event, are 

common, but composite dikes, containing two or more 

phases of contrasting compositions, have not been 

- reported. In other words, each fissure was used just 

once, not repeatedly. 

The chance of finding a dike connecting with a 

flow i fed is small, owing to problems of exposure. 

Nonethe less, severa l dikes have been found displaying 

such a connection. The top of most such dikes is 

rubbly, apparently consisting of slabs of crus once 

floating on a flow before it poured back into the 

fissure {for example, Plate la in Swanson and others, 

1975). In one dike lacking such rubble, the dike 

merges imperceptibly with its flow of the Frenc~n 

Springs Member {fig. l, number 6, Swanson and others, 

1975). 

Many other dikes can be inferred to correlate 

with particular flows, or at least sequences of 

flows, on the basis of chemical composition and magnetic polarity. In this way, feeders have been 

identified for most of the named stratigraphic units. 

The shape and extent of vent systems for specific 

flows or related sequences of flows can be reconstructed from the locations of feeder dikes, thick 

piles of degassed flows (presumably near their vent), 

abundant collapsed pahoehoe (Swanson, 1973), and 

accumulations of basaltic tephra (in places occurr ing 

in still recognizable spatter cones and ramparts). 

Such reconstructions show that eruptions of single 

flows or related flows took place from fissures concentrated in long, narrow vent systems on the order 

of tens of k ilometers long and several kilometers 

wide {Swanson and others, 1975, and later work). 

Vent systems for the Grande Ronde Basalt are distributed across the eastern half to two-thirds of the 

Columbia Plateau, but those for other units are more 

res tricted {Swanson and Wr ight, 1979 ). For example , 

feeder dikes for the Picture Gorge Basalt are confined within the John Day Basin and neighboring 

areas, those for he Frenchman Springs Member wi hin 
e 6o km wi de and probably more than 200 km long, 1 ~those f or the I ce Harbor Member within a zone 15 

!.. 'd and about 90 km long . On a still finer 

VI event systems f or specific flows are near ly 

sea e, as they occur along single dikes or closely · near , . ced re l ated dikes. Examples are the vent s~stem 

~ e Roza Member (probably l ess than 5 km w1de and 

~k nown to be more than 165 km long Swanson and 

5 1975; P.R. Hooper, unpub. data, 1978 ) , the 

: t of Robinette Mountain in the Eckler Mounta in 

:~ a r (a singl e dike extending at least 25 km across 

:;e sl ueMountains) , th~ basalt .of Basin City in the 

, Harbor Member (poss1bly a s1ngl e dike at least 50 

' ong ), and several chemically . distinct flows in 

•e Grande Ronde Basalt (T.L. Wnght and D.A. Swan-

~' unpub. data, 1978). 

ow edge of t he geometry of the vent systems not 

y allows pred i c tion of where vents for spec i fic 

, 0115 should occur once one such vent i s found but 

;·so further prov i des i mportant constra i nts r egarding 

·· generati on, s torage, and eruption mechan i cs. 

·sing such knowl edge, at tempts have been made to 

~~~e the rates of eruption and advance for single 

;, s. The est imates take into account the 

.. that flows, even those that advanced tens 

of kilometers from their sources, 

fJenched to a crystal-I_X>O~ s~deromel~ne glass ~hen 

tey entered water; th1s 1nd1cates l1ttle coollng 

:Jring transport and hence rapid advance, since the 

. ava apparently moved as sheet floods rather than 

• rough insulating tube systems. Applicat ion of 

r~l c models, developed in part from this observation by Shaw and Swanson (1970), to vent systems of 

i dimensi ons s uggests eruption rates of about l 

3jday pe r li near kilometer of active fissure for 

~ argest flows , s uch as those in the Roza Member, 

. abou t lo-4km3/day / km for the smaller flows 

svanson and ot hers, 1 975) . For flows of "aver age" 

'10 ume , probably s evera l t ens of cub i c kilometers, 

rates of lo-1 to l o-2km3/ day/ km may be i nferred . 

3y comparison , susta i ned rates of eruption at Kilauea 

and Mauna Loa a r e l o-3 to l o-4km3/ day/ km . 

:sing observed dike widths, theoret i ca l modeling 

s qqests tha t the high erupti on rates cou l d indeed 

ave been sustai ned by supply from depth (Shaw and 

Sllanson , 1970) . Such eruptions probab l y l asted a few 

ays. Flow rate s of 5 to 15 km/ hr down slopes of 

: 000 are ca l cu l ate d from the model, adequate to 

a ~thic k fl ows to move far with little cooling. 

~pi d e ruption ra t es do not necessarily impl y 

rapid melti ng rates in the mantle . Flows were 

Hu pted only once every ten thousand years or so in 

ryo~ area on the plateau during even t he peak o f 

'10 canic ac ivity (Grande Ronde t i me ) , as est i mated 

oounting the number of flows i n a mag ne t os t r ati-

;raphic unit of assumed duration based on compa rison 

· h seafloor magnetic anomalies o f roughly compar-

·.e age . Calcula ions s how that cont i nuous melting 

at the prese nt Hawaiian rate , lo-lkm3/yr (Swanson, 

: 2) , could accoun for t he volume o f t he Columbia 

iver Basalt Group in t he allote d t i me . Episod i c 

rap1d melting ven ( " flash melti ng" ) ca nnot be 

!xc luded but are no r equ i red . 

If melting progressed at th Rawa iian rate, then 

~~ , deep storage rese r voirs ar e required in orde r 

:D account for he 1 rge volum of si ngle lows . 

.... s contras s with th H waiian situa t ion , wher e 

!ruptions ar much more freque nt and lava "leaks" t o 

'•e surface more or less continuously . The presence 

Jf arg , deep stor g r e ser voirs, possibly in the 

.pper mantle, may be a pri nc1pal and disti nguish i ng 

: arac eristic of flood-b salt provi nc s in genera l. 

11 

CHEMICAL PETROLOGY OF THE YAKIMA BASALT SUBGROUP 

Two major geochemica l breaks occur within the 

Yak ima Basa l t Subgroup. The older break separates a 

i gh-Si02, l ow-FeO and Ti02 sequence below 

(Grande Ronde Basalt) from a relatively low-Sio2, 

hi gh-FeO and Ti02 sequence (Wanapum Basalt). This 

chemical change took place over a short time, as magnetic stratigraphy is continuous across the break, 

K-Ar ages above and below agree within analytical 

error for these rocks (+ l m.y.), and flows of different chemical types a~e locally interlayered (fig . 

4). Trace-element levels are similar and Sr isotopic 

ratios are similar and relatively low (0. 704-0.706) 

i n both sequences . Major- and trace-element 

chemistry of the Grande Ronde Basalt, which makes up 

75 percent of the volcanic pile, shows cyclic variation rather than an evolutionary trend. 

The younger geochemical break, separating the 

Wanapum and Saddl e Mountains Basalts, is marked by 

l) i ncrease in Sr isotopi c ratios to 0.708-0.715, and 

2) greater abundance of most incompatible elements 

and steeper chrondrite-normalized REE patterns in 

those flows of the Saddle Mountains Basalt whose 

major ox i de chemistry is similar to older flows. 

These changes took place over a relatively long time 

(.5-l m.y .? ) , as there is evidence of erosion and 

deposition of sediments between the Wanapum and 

Saddle Mountains Basalts . 

The Columbia River Basalt Group shows far less 

c9herent chemical variation than is typical for 

ocean i c tholeiites. Ratios of incompatible elements 

vary widely among the various formations as well as 

among flows belonging to the same formation. Enrichment factors differ for different incompatible elements in un i ts which show smooth chemical variation 

trends. For exampl e, the least magnesian flows of 

the Grande Ronde Basalt (Mg0r3 percent), have 

contents of PzD5, Hf, Ta, and LREE that are 2-3 

times hi gher t han i n the most magnesian fl ows of the 

Grande Ronde (Mgor6 percent) , whereas the enrichment of Th and K in these same flows is greater, 

about 3-4 . 

We make the following inferences as a start 

toward a model to explain the generation of magmas in 

the subgroup . 

1. We infer that magma compositions are controlled principally by partial melti ng. Enrichment 

factors in the Grande Ronde Basalt are not consistent 

with a fractionation model in which crystallization 

o f 55-70 pe r cent of stored liquid would have to take 

pl ace r epeatedly. Mixing calculations fail to balance both ma jor o xi des and incompatible trace elements usi ng any reasonable fractionating mineral 

assembl age. 

2. The scarc i ty of phenocrysts, absence of 

high-pr e ssure "megacrysts", and absence of fractionation or accumu l ation t r ends suggest that magmas 

accumul ated near the site of melting and may have 

been s upe rheat ed during transport to within a few 

k ilomete rs of the surface. 

3. we i nf er littl e or no high-leve l crustal 

storage f or most of the magma, as evidenced by l) 

absenc e o f grabe ns or calderas associated wi t h s i ngle 

fl ows of l arge volume and 2) absence of phenocrystrela ed chemica l f ractionation or accumulation trends 

e ven between phyr i c and aphyric flows of the same 

strat i gr aph i c un i t. We f ee l that pl agioc l ase phenocryst s prese nt in e flows may have formed du r i ng 
ascent of magma just prior to er uption and per haps 

even after eruption. 

4. If a partial melting mode l is accepted, me lting must be relati ve l y "wet" to yield quartz-norma -

tive magmas. Some metasomatic enrichment of trace 

elements is required to expla in the i ncoherent ratios 

of incompatible elements and possibly the h ighest 

87sr; 86sr ratios. 

5. Bulk-lava chemistry suggests that the source 

for most of the magma was r elativel y iron-rich and 

olivine-poor clinopyroxenite. The major chemical and 

stratigraphic units and many of the individual flows 

require chemica lly distinct source rocks. The overall source for magmas of the Col umbia Ri ver Basalt 

Group must be heterogeneous both in space and through 

time. 

INVASIVE FLOWS 

Weak ly consolidated sedimentary rocks, genera lly 

medium-grained sandstone to sil tstone, occur between 

many flows near the margin of the plateau and between 

some of the younger flows in local structural basins 

on the plateau. The sedimentary rocks rest depositionally on the underlying flow in some places, but 

in many other places the contact relations show that 

subjacent basalt intrudes or invades the sediment. 

Schmincke (1967c) was one of the first to recognize 

this, and recent work has demonstrated how common 

such invasive relations are. We estimate that mo re 

than half of the observed contacts between basalt and 

sedimentary rocks on the Col umb ia Plateau are invasive . 

How are such contacts interpreted? Do they si gnify "normal" intrusive re l ations in which basaltic 

magma never reached the surface before solidifying as 

in classic dikes and sills, or are they formed as 

l ava flows burrow into unconsolidated sediments 

accumu l ating on the ground surface (i nvasive flows of 

Byerly and Swanson, 1978)? Both processes produce 

similar results. 

The key to proper interpretation lies in the 

stratigraphy. If the basalt is at its proper stratigraphic position relative to overlying flows, it 

almost certainly was a flow that invaded sed imen ts at 

the ground surface. This is because thin sedimentary 

deposits, generally less than 10 m th ick on the 

plateau, are light and hence exert little con fi ning 

pressure; vesiculating magma rising and encounter i ng 

such sediments wou ld certa inly blast through rather 

than spread laterally i nto h m. 

Mapping and chemical studies have shown that, in 

every exa.mple so far found on the· pl ateau , the invasive basalt is at its expected strati graphic positi on 

relative to overlying flows and hence is in invasi ve 

flow (Schmincke, 1967c; Camp, 1976; Byerly and Swanson, 1978; D.A. Swanson, G.R. Byer ly and T. L. Wr ight , 

unpub. data, 1978). We include in this interpretation two thick sills previously interpreted conventionally, the Hammond sill of Hoyt (1 961) near 

Wenatchee, Washington and the "Wh iskey Cr eek sills" 

of Bond (1 963 ) . Work by Byerly and Swanson (1 978 and 

unpub. data) and V.E. Camp (unpub. data, 1978 ) shows 

that even these thick sill s, more than 120 m thick 

l ocally, are in proper stratigraph ic position 

relative to overlying f l ows. 

These conclusions are significant, because they 

show that invasive contacts ptovide insufficient, i n 

fact totally misleading, evidence E r he former 

12 

pres nee o magma benea h he ar a. For example , 

in vasive relations are particularly well displayed 

along the northw st margin of the plateau, where all 

other evidence negat s the former presence of magm 

b~dies~ no intrusi ve cont.cts are found h re in an; 

s1tuat1on other than o ne 1nvolving sediments, the 

invad i ng fl ows ar in heir proper stratigraphic 

position relative to other flows , and flow dir ections 

show that surface flows moved toward, not away from 

correla t ive sills. Invasive relations also demon- ' 

strat the unre iability of using sedimentary interbeds as stratigraph ic guides on the Columbi a 

Plate~u . T~e bas~l~ flows are always in their pro r 

strat1graph1c pos1t1on, but the i nterbeds , at least 

fine-grai ned ones , are commonly no t , owing to rafting 

by invasi ve flows. 

REFERENCES CITED 

Beeson, M.H., and Moran , M.R., 19 79, Columbia River 

Basalt Group stratigraphy in western Oregon: 

Or egon Geology, v. 41, p. 11-14 . 

Berggren, W.A., and van Couver ing , J.A., 1974, The 

Late Neogene : Bi ostratigraphy, geochronology, and 

pa leoclima tology of the last 15 million years in 

marine a~d continental sequences: Palaeogeography, 

Pa laeocllmato1ogy, Pa laeoecology , v. 16, p. l-216. 

Bingham, J.W., and Grolier, M.J., 1966, The Yakima 

Basalt and Ellensburg Format ion of south-central 

Washington: U.S. Geol. Survey Bu ll . 1224-G , 15 p. 

Bond, J.G., 1963, Geology of the Clearwater embayment: 

Idaho Bur. Mi nes and Geol., pamphlet 128, 83 p. 

Brown, C.E., and Thayer, T.P., 1966, Geologic map of 

the Canyon City quadrangle, northeastern Oregon: 

U.S. Geol . Survey Misc. Geol . Invest. Map I- 44 7, 

sca le 1 :250,000. 

Byer ly, Gary, and Swanson, Don, 1978, Inva sive Col~ 

bia River basalt flows along the northwestern margin 

of the Columbia Plateau, north-centr al Washington: 

Geol. Soc. Ame rica Abstracts with Programs, v. 10, 

no. 3, p. 98. 

Camp, V.E., 1976, Petrochemical stratigraphy and 

structure of he Columbia River basalt, Lewiston 

Basin area , Idaho-Washington: Washington State 

Univ., Pullman, Ph.D. Dissert., 201 p. 

Camp, V.E., Price, S.M., and Reidel, S.P., 1979, Descriptive summary of the Grande Ronde Basalt type 

section, Columbia River Basalt Group: Rockwe ll Hanford Ope rations Report RRQ-BWI-LD-15, 24 p. 

Cocke rham, R.S., and Bentley, R.D., 1973, Picture 

Gorge and Yak ima Basalt between Clarno and Butte 

Creek, Oregon: Geol. Soc . Amer ica Abstracts with 

Programs, v. 5, no. 2, p. 23. 

Fruchter , J .S., and Baldwin, S.F., 1975, Correlations 

between di kes of the Monument swarm , central 

Or o n, nd Pic ur Gorg Bas lt fl s: Geol. Soc. 

Amer ica Bull., v . 86, p. 14-516. 

Fu ller, R.E., 193 1 , The aqueou s chilling of basaltic 

lava on the Columbia River Plateau: Am. Jour. 

Sci., v . 21, p. 281- 300. 

Greeley, Rona ld , 1977, Basaltic "plains" volcanism, 1n 

Greeley, Ronald, and King, J.S., eds., Volcanism of 

the eastern Snake River Plain, Idaho , a Comparative 

Planetary Geology Gu1debook: Office of Planetary 

Geology , N.A . S. A., Washington, D.C., p. 23- 44. 

Griggs, A.B., 1976 , The olumbia River Basalt Group 1n 

the Spokane quadrangle , Was hington , Idaho, and Mon -

tana : U.S. ~eol. Sur vey Bull . 1413, 39 p . 

He 1z, R .. , 978, The e rogenesis of he Ice arbor 

Member, Colu mb1~ Pate u. Washington - A hem1 ca 

and expe rimental ·tudy· enn . State Univ. , ntver 

sity Park, Ph.D . Jts s~ rt. , 284 p. 

Helz, R. T., Wognt, . .... nd Swanson , D.A., 19 6. 

Petroqene :t· aqntfic.H.::e f chemical t ren ~ n h 

younqes un1 {aklMa ~as 1 n the Columbl~ 

Pl teau, ' ··.we sr J .. ~... '). onzales ?r.r 'l r • .-d . . 
Inter . Assoc. Volcanol. Chern. Earth's Interior, Symposium on Andean and Antarctic Volcanology, Proc., 

p 465-481. 

H~enson, G.M ., 1964, Geology and ground water of the 

umatilla River basin, Oregon: U. S. Geo . Survey 

wate r - Supply Paper 1620, 16 2 p . 

Hol den , G. S., and Hooper, . R., 1 97& , Petrology and 

hem1 stry of a Columbia River basa l section, Rocky 

can on, west-central Idaho: Geo _ . Soc. America 

aul ., v. 87, p. 215-225. 

Hcoper , P.R . , 1974, Petrol ogy and chemistry of the 

Rock Creek flow, Columb ia Rive r Basa lt, Idaho: 

Geol . soc. America Bul ., v. 85, p. 15-26. 

aoyt, c .L., 1961, The Hammond sill- an ~ntrusion in 

the Yakima Basalt near Wenatchee, Wa shington: 

Northwest Science, v. 35 , p. 58-64. 

Jackson, D.B., 1975, Description of the geoelectric 

section, Rattlesnake Hills unit 1 we ll , Washington: 

Jour. Research U.S. Geol. Survey, v. 3, p. 665-669. 

a~er, J.C., 1961, The cooling of irregularly shaped 

i gneous bodies: Am. Jour . Sc i ., v. 259, p . 721-7 34. 

Jones, J.G., 1968, Pillow lava and pahoehoe: Jour. 

Geology, v. 76, p. 485-488 . 

Kleck, W.O., 1976, Chemistry, petrog r aphy, and stratigraphy of the Columb ia Ri ver Group in the Imnaha 

Rt ve r va lley region, easter Oregon and western 

Idaho: Washington State Un1v., Pullman, Ph.D. 

01sse r t., 203 p. 

Lerebvre, R.H., 1970, Co lumb1a R1 ver basalts of the 

Gr an · Coulee area, in Gilmou r , E. H. , and Stradl ing , Dale, eds., Proc . Second Col umbia Ri ve r Basalt 

Svmposium: Cheney, Ea s ter Wash. State Colleg 

Pr ess, p. l- 38. 

L1ndsle . D. H., Sm i t , Dougl as, and Hagge rty, S . E., 

! 97 1. etrogr aphy anc minera l chem1stry of a d i fferenti ate f l ow of P1cturc Gor ge Basal near Spray, 

Oregon: Carnegie Ins i ut o Was h ington , Year book 

69 • 264-285 . 

~ng, .E., 1978, Character i zation and recognit io of 

1n~r aflow structures, Grande Ronde Basa l t: Rockwe l 

Hanford Operations In orma l ~eport RBO-BWI-LD-1 0, 

7l f- · 

Ma ckin, .H., 196 , A stra oqraphlC. section 1n th 

Ya irna Basal and th Ellensbur Formation In sout -

en ra Washington: Wash 1nato. D Mines and 

Geolog} Report Inv . lS, 4 . ~ 

M~9a:· , Ian , 1976, Geochemis ~~ a orig1n of 

o sal o the Columb a R ve r ucou , Orego ana 

Wasnington: Geo . oc . Ame ric Bu : ., v . 87, p. 

~ 7- 792. 

McKee, E.H., Swanson, u .A. , and Wr ignt, T.L., 1977 , 

Dur ation and volume ot Co l umbia Ri ver Basa lt volcanlsm, Washington, Oregon, ana Idaho: Geol. Soc. 

America Abstracts wit ~ Programs, · . 9, no. 4, p. 

4 - 46 . 

.:. G., 1975, Mechanisr tJ f torma tion of ~ll ow 

AA. . Scien is , ,. ,-_:., t •• 269- 277. 

, . G., Phi ~l ipr., .. .... . , G:: 1G , R.W. , Peterson, 

a nd Swanson, .A ., l ,~. FL of l a va 1nt o the 

sei. l 96 - 197 K i a e a Vu1.cano . Hawa iI: Geo . Soc . 

Amenca Bu ll. , v . 84 , J:-. jJi-:>4 t. . 

My r!., ' .. W. , and Pr1c , S .l'l . . 1979, Compi ation ge -

olOCJlC map o the Pascc BasH •. south-centra l WashIng on . Rockwe ll Hantor0 Opera 1ons, RHD-BWI-ST-4 , 

seal. :62, 500. 

Nathar .. Simon, and Fr uchter , .J . S , 19 74, Geochemica l 

pa l eomagne t ic str tigraph ot the Picture Gorge 

ima Basalts (Columbia R1ver Group) 1 

Geol . Soc. America Bul ., v. 85, 

N lsor, D. O., Swanson , 0 .1. . , and Wr 1ghl, T. L., 1976, 

St c) ium isotopic compos 1 ti o f 1ntracanyon flow 

o) 't , 1ma Basal , southea t Wa s n1ngton: Geo . Soc. 

Am ~ •ca Abst r act with Pro r am . , no. 3, 

1. : 9. 

13 

Newman, K.R., 1970, Palynology of interflow sediments 

from the Standard Oil Company of California RattleSnake Hi lls no. l well, Benton County, Washington, 

in Gilmour, E.H. , and Stradling, Dale, eds., Proc. 

Second Columbia River Basalt Symposium: Cheney, 

East. Wash. State College Press, p. 201-207. 

Osawa, M., and Goles, G.G., 1970, Trace-element abundances in Columbia River basalts, in Gilmour, E.H. , 

and Stradling, Dale, eds., Proc. Second Columbia 

Rive r Basalt Symposium: Cheney, East. Wash. State 

College Press, p 173-175. 

Price, S.M., 1977, An evaluation of dike- flow correlations indicated by geochemistry, Chief Joseph 

Swarm, Columbia Ri ver basalt: Idaho Univ., Moscow, 

Ph.D. Dissert., 320 p. 

Raymond, J .R., and Tillson, D.O., 1968, Evaluation of 

a thick basalt sequence in south-central Washington : 

Atomic Energy Cormnis. Research and Development Rep . 

BNWL-776, 126 p. 

Reidel, S.P., 1978, The stratigraphy and petrogenesis 

of the Grande Ronde Basalt i n the lower Salmon and 

adjacent Snake Ri ver canyons: Wash. State Univ. , 

Pullman, Ph.D. thesis, 415 p. 

Ross, M.E., 1978, Stratigraphy, structure, and petrology of Columbia River basalt in a portion of 

the Grande Ronde-Blue Mountains area of Oregon and 

Washington: Idaho Univ., Moscow , Ph.D. Di ssert., 

413 p. 

Ryan, M.P., and Sammis, C.G., 1978. Cyclic fractur 

mechanisms in coo l ing basalt: Geol. Soc. Ame rica 

Bull., v. 89, p. 1295-1308 . 

Schmincke, H.-U., 1967a, Flow directions in Columbi c 

~ive r basa lt flows and paleocurrents of interbedded 

sedimentary rocks, south-central Washington: Geo 1 • 

Rundschau, v. 56, p. 992-1020. 

1967b, Str atigraphy and petrography of four upper 

--ya£ima Basalt flows in south-central Waghington: 

Geol . Soc. America Bu ll ., v. 78, p. 1385-1422. 

l967c, Fused tuff and peperites in south-central 

Wash i ngton: Geol . Soc. America Bul l ., v. 78, p. 

319-330. 

Shaw, R.R. , and Swanson, D.A., 1970, Eruption and flow 

rates of flood basalts, in Gilmour, E.H., and 

Stradling , Dale, eds., Proc . Second Columbia River 

Basal t Symposium: Cheney, Eastern. Wash. State 

College Press, p . 271-299. 

Snavely , P.O., Jr., MacLeod, N.E., and Wagner, H.C. , 

1973, Miocene tholeiitic basalts of coastal Oregon 

and Washington and their relations to coeval basalts 

of the Columbia Plateau: Geol . Soc. America Bull., 

v. 84, p. 387-424. 

Spry, A., 1962, The origin of columnar jointing, particularly i n basalt flows: Geol. soc. Australia 

Jour., v. 8, p. 191-216. 

Swanson, O.A., 1972, Magma supply rate at Ki lauea Volcano, 1952-1971: Science, v. 175, p. 169-170 . 

1973, Pahoehoe flows from the 1969-1971 Mauna Ul u 

--er-uption , Kilauea Volcano, Hawaii: Geol. Soc. 

Ame rica Bull ., v. 83, p. 615-626. 

Swanson , D.A., and Wr ight, T.L., l976a, Guide to 

field trip between Pasco and Pullman, Washington , 

emphasizing stratigraphy, vent areas, and intracanyon flows of Yak i ma Basalt: Geol. Soc. America 

Cordilleran Sect. Mtg., Pullman, Washington, Field 

Guide no. l, 33 p. 

l976b, Magnetostrat igr aphic units in the Yakima 

--sa5alt, southeast Washington: Geol. Soc. America 

Abstracts with Programs, v. 8, no. 3, p. 413-414. 

1978, Bedrock geology of the northern Columbia 

~teau and adjacent areas, in The Channeled Scabland, Baker, V.R., and Nummedal, Dag, eds.: 

N.A.S.A. Office Space Sci., Planetary Geol. Program, 

Wash ington, D.C., p. 37-57. 

1979, Source areas and d istribution of major 

--units in the Columbia River Basalt Group: Geol. 
soc. America Abstracts with Programs, v. 11, no. 3, 

p. 131. 

swanson , D.A. , Wright, T. L., and Helz, R.T., 1975, 

Linear vent systems and estimated rates of magma 

production and eruption for the Yakima Basa lt on the 

Columbia Plateau: Am. Jour . Sc i . , v. 275, p. 877-

905. 

swanson, D.A., Wright , T.L., Camp, V.E. , Gardne r, 

J .N., He l z, R.T., Price, S.A., and Ross, M.E. , 1977, 

Reconna i ssance geologic map of the Columbia River 

Basalt Group, Pullman and Wa lla Walla quadrangles , 

Wash i ngton and adjacent Idaho: U.S. Geol. Survey 

Open-fil e Rept. 77-100, scale 1:250,000. 

swanson, D.A., Anderson, J.L., Bentley, R.D., Byerly, 

G.R., Camp, V.E., Gardner, J.N., and Wright, T.L., 

1979a, Reconnaissance geologic map of the Columbia 

River Basalt Group in eastern Washington and 

northern Idaho: U.S. Geol. Survey Open-file Rept. 

79-1363, scale 1:250,000. 

Swanson, D.A., Wright, T.L., Hooper, P.R., and Bentley, R.D., l979b, Revisions in stratigraphic nomenclature of the Columbia River Basalt Group: U.S. 

Geol. Survey Bull. 1457-G, 59 p. 

Swanson, D.A., Wright, T.L., and Zietz, Isidore, 

l979c, Aeromagnetic map and geologic interpretation 

of the west-central Columbia Plateau, Washington and 

Oregon: u.s. Geol. Survey Geophys. Invest. Map. 

G.P. 917, scale 1:250,000. 

Swanson, D.A., Wright, T.L., Camp, V.E., Gardner, 

J.N., Helz, R.T., Price, S.M., Reidel, S.P., and 

Ross, M.E., 1980, Reconnaissance geologic map of 

the Columbia River Basalt Group, Pullman and Walla 

Walla quadrangles, southeast Washington and ad jacent 

Idaho: U.S. Geol . Survey Misc. Geol . I nvest. Map. 

I-1139, scale 1:250,000, in press. 

Thayer, T.P., and Brown, C.E., 1966, Local thickening 

of basalts and l ate Tertiary silicic volcanism in 

the Canyon City quadrangle, northeastern Oregon , i n 

Geological Survey Research 1966: U.S. Geol. Survey 

Prof. Paper 550-c, p C73-C78. 

Tomkeieff, S.I., 1940, The basalt lavas of the Giant ' s 

Causeway district of northern Ireland: Bull . Volcanologique, v. 6, p. 90-143. 

Vallier, T.L., and Hooper, P.R., 1976, Geologic gu ide 

to Hells Canyon, Snake River: Geol . Soc. Ame rica, 

Cordilleran Sect. Mtg., Pullman, Wash. , Field Guide 

no. 5, 38 p. 

Walker, G.W. , l973a, Reconnaissance geologic map of 

the Pendleton quadrang l e, Oregon and Wash ington: 

U.S. Geol. Survey Misc. Geol . Invest. Map . I-727, 

scale 1:250,000. 

l973b, Contrasting compositions of the youngest 

Columbia River basalt flows in Union and Wa llowa 

Counties, northeastern Oregon: Geol . Soc. Amer ica 

Bull., v. 84, p. 425-430. 

Waters, A.C., 1960, Determining dir ecti on of flow in 

basalts: Am. Jour. Sc i . , v. 258-A, p. 350-366 . 

1961, Stratigraphic and lithologic variations 1 

~Columbia Ri ver basalt: Am. Jour. sc · ., v. 259, 

p. 583-61 . 

1962, Basa lt magma types and their tecton ic asso-

--ciation--Pacific Northwest of the United States: 

Am. Geophys. Union, Mon. 6, p 158-170. 

Watkins, N.D., and Baksi , A.K. , 1974, Magne tostratigraphy and oroclinal fold i ng of the Columbia River, 

Steens, and Owyhee basalts i n Oregon, Wash i ngton, 

and Idaho: Am. Jour. Sci., v. 274, p . 148-189. 

Wright, T.L., and Hamilton, M.S., 1978, A computerassisted graphical method for i dentificati on and 

correlation of igneous rock chemistri es: Geol ogy, 

v. 6, p 16-20. 

Wright., T.L., Grolier, M.J., and Swanson , D.A., 1973, 

Chemical vari ation related to the stratigraphy of 

the Col umbia Ri ver basalt: Geol . soc. Ame rica 

Bu ll ., v. 84, p. 371-386. 

14 

Wright, T. L. , Swanson, D.A., Helz, R.T., and Byerl 

G.R., 1979, Major oxide, trace element, and ~l asy , 

chemistry of Columbia Ri ve r basalt samples CGll e~te · between 1971 and 1977: U.S . Geol. Open-file Rept 0 

79-711. . 
ROADLOG FOR GOOLOGIC FIELD TRIP BETWEEN LEWISTON, IDAHO AND KI MBERLY, OREGON 

MILES 

(0. 4) 

0.4 

(0.95) 

1. 35 

(0. 2) 

l. 55 

(0.2) 

1. 75 

(1. 7) 

3.45 

(0.55) 

4.0 

(0. 95) 

4.95 

( 1. 1) 

6.05 

(0. 95) 

7.0 

(2. 55) 

9 . 55 

(0. 55) 

10 . 

(0.1) 

10.2 

(0. 3) 

10.5 

(2.3) 

12.8 

D.A. Swanson, U.S. Geological Survey, Menlo Park, CA 94025 

and 

T.L. Wright, U.S. Geological Survey, Reston, VA 22092 

Sacajawea Motor Lodge in Lewiston, Idaho (fig. 7). Drive west. 

Tapader a Inn. Continue west, following signs to u.s. 12. 

Turn left to west U.S. 12. 

Cross Snake River. Enter Clarkston, Washington. The Snake here is now a 

lake, impounded behind the recently completed Lower Gr anite Dam. 

Continue straight ahead on U.S. 12 toward Pomeroy and Walla Walla. 

At 12:00, the Pomona Member of the Saddl e Mountains Basalt fills ancient 

canyon of Snake River eroded into flows of the Grande Ronde Basalt (R2 

magnetostratigraphic unit). 

At 11:00, house on bluff rests on Lower Monumenta l Member, which was confined to an old Snake River canyon carved deeply into the older flows 

between about 10.5 and 6 m.y. ago. 

Roadcuts in Grande Ronde Basa l t for several miles. 

On left, colonnade of the canyon-fi lling Pomona Member. Note underlying 

white ash, a probable corre l ative of which occurs near Ice Harbor Dam 

(stop 9). 

At 1:00, steeply dipping flatirons of the Grande Ronde Basalt near core of 

the Gaging Station anticl ine. 

This route passes through the Lewiston Basin, a structurall y compl ex area 

recently studied by Camp (1976). Flows at or near river l evel along the 

road also occur near the top of the escarpment north of the river. A 

major fault follows the west- flowing river, and a faulted anticline north 

o f the river exposes the lower three magnetostratigraphic units of the 

Grande Ronde Basal t as well as the upper part of the unde r lying Imnaha 

Basa l t. The latest tectonism occurred in post-Lower Monumental time, as 

canyon-fi l ling remnants of this 6 m.y. member have been deformed (Camp, 

1976). 

Quarry on left in the Pomona Member. Good exampl e o f co l onnade and entablature jointing habits, generally well developed in i ntr acanyon flows. 

Look back at 4 :00-5 :00 and see the Gaging Station anti c line i n core of 

Lewiston Basin structure. 

Bluff on left is the Lower Monumental Member . At 2: 00, vi ew down Snake 

River of the Grande Ronde Basalt . 

Road leaves Snake Ri ver and follows Alpowa Creek . 

Road crosses approximate axis of Gaging Station anticline, rapidly dying 

out west of the Lewiston Basin. 

15 
SADDLE MOUNT A INS 

P A S C 0 

8 AS I N 

0 Heppner 

0 Ei topia 

11 7° 

\ ~ La 

Enterprise 

Grande 

10 20 40 50 MILES 

r 1 r r1 

0 10 20 30 40 50 60 KILOMETERS 

--- Route of field trip 

• 2 Stop 

Figure 1. Map showing route of field trip, scheduled stops, 

and l ocations of important geographic features. 

16 
(1. 3 ) 

14. l 

( l. 9) 

16.0 

( 1. 25 ) 

17.2 5 

( 0. 7) 

17.95 

( l. 55) 

19.5 

(0. 55) 

20.05 

( 0. 45) 

20.5 

( 0. 35) 

20.85 

(0. 2) 

21.05 

(0. 3) 

21.35 

(0.15) 

21.5 

(0.15) 

21.65 

(1. 25) 

22.9 

(0. 3) 

23.2 

( l. 5) 

24.7 

(0. 8) 

25.5 

( 0. 85) 

26.35 

(l. 55) 

27.9 

(5.45) 

33.35 

( 4. 05) 

37.4 

(0. 4) 

37.8 

Note sjgn on l eft commemor a t i ng passage of Lewis and Cl ark on their return 

trip. 

Intersection with Clay t on Gu l ch r oad. Continue on u.s. 12. 

Intersection with Howell Grade road. From here, u.s. 12 passes upsection 

across the upper two magnetostratigraphic units of the Grande Ronde Basalt 

capped by the Roza Member of the Wanapum Basalt near Alpowa Summit. An 

unknown but observant geologist-poll uter painted yellow letters on each 

flow i n the section, with L the oldest and E (Roza Member) the youngest. 

The dip of the section is only slightly less than the gradient of the road. 

Base of flow L. R2. 

Base of flow K. R2. 

Base of flow J. N2. 

Base of flow I. N2. 

Base of flow H. N2. 

Base of flow G. N2. 

Base of flow F. N2. 

Road cut on left shows thick ancient soil (saprolite) developed on flow F. 

This saprolite, marking the top of the Grande Ronde Basalt, is overlain 

the Roza Member. 

Flow E (Roza Member) exposed from here to summit. 

Alpowa Surrmit, elevation = 2785 ft. 

Intersection with Ledgerwood Road. Turn right. 

Small outcrop of Roza spatter on right (near-vent locality 13 in Swanson 

and others, 1975, table 1) . 

by 

Junction with Kirby-Mayview Road. Continue straight (northward) on pavement. 

Junction with Valentine Ridge Road. Continue straight. Valentine Ridge 

Road leads to stop 18 of Swanson and Wright (1967a), an interesting vent 

area for a flow in the Grande Ronde Basalt. 

Roadcut just beyond junction with Connell Hill Road. Thin flows of the 

Roza Member, with little or no spatter. Experience shows that, in this 

area, such thin flows are likely to be near a vent. Continue on KirbyMayv i ew Road. 

Intersecti on with Tramway Road. Turn right toward Tramway. 

Cliffs ahead expose a flow of LOlo chemical type in the Priest Rapids 

Member of the Wanapurn Basalt. 

Stop at white stile. 

STOP NO. 1. Gracefully cross stile and walk to lip of Snake River canyon 

for view of Grande Ronde flows and discussion of stratigraphy and mapping 

methods. Quarry is in a flow of Lolo chemical type in the Priest Rapids 

Member. 

17 
(0.45) 

38.25 

(0.9) 

39.15 

( 0.]) 

39.25 

(2.1) 

41.35 

(0.6) 

41.95 

( o. 55) 

42.5 

(5. 9) 

48.4 

·. ( 3. 35) 

51.75 

( 4 .1) 

55.85 

(7 .9) 

63.75 

(0.15) 

65.9 

(8.4) 

74.3 

(2.65) 

76.95 

(2. 3) 

79.25 

(2.85) 

82.1 

(l.l) 

83.2 

(2.3) 

85.'1 

Continue north on Tramway Road. Keep left at inters ction with East 

Wawawai Grade road. 

Junction with Kirby-Mayvi ew Road. Turn right. 

Junction with Wawawai Grade Road. Continue on pavement. 

Mayview. Continue straight. 

Roadcut exposing two thin flows of the Roza Member separated by welded 

spatter. Continue ahead. 

STOP NO. 2. Bus lets passengers off here and proceeds for 1.3 miles, 

turns around, and waits for passengers, who walk down road. Roadcuts show 

the Roza Member overlying thin, discontinuous silt resting on the sparsely 

plagioclase-phyric Frenchman Springs Member (2 flows). A thin saprolite 

and detrital(?) clay separate the Frenchman Springs from the Grande Ronde 

Basalt. The Frenchman Springs thickens west from here to 150 m in Devjls 

Canyon (Stop 6); it does not occur farther east. Eastward thinning of the 

Frenchman Springs Member is typical throughout southeast Washington and 

northeast Oregon and is interpreted to mean that the flows pinched out 

against a gentle westward slope. All known feeder dikes for the Frenchman 

Springs Member occur in the central part of the plateau; we will see some 

unusual examples tomorrow. 

The flows of the Grande Ronde Basalt, all in the N2 magnetostratigraphic 

unit, display a variety of primary structures, such as joints and rubbly 

tops. Many flows are thin and may have erupted locally. 

One thick and two thin feeder dikes for the Roza Member are exposed. Perhaps the spatter at mileage 41.95 was erupted from this dike. This location is along the axis of the Roza vent system. One of the vent areas 

will be viewed at the next stop. 

Return to U.S. 12 via Kirby-Mayview, Ledgerwood, and Ledgerwood Spur Roads. 

Junction with Tramway Road. Keep right, on pavement. 

Junction with North Deadman Creek road, turn right, staying on KirbyMayview Road. 

Junction with Tramway Road. Continue straight. 

Junction with Ledgerwood Road. Turn left, leaving pavement. 

Junction with U.S. 12. Cross highway, continuing on Sweeney Gulch Road. 

Junction with State Highway 1 28. Continue straight, up a gentle dipslope 

related to the Blue Mountains uplift farther south. 

Road intersection. Continue straight on 128. 

Metropolis of Peola. Bear left on main road, avoiding road to Cold 

Springs. 

Continue east on Highway 128. 

Asotin-Garfield County Line. 

At 4:00, good view of dip slopes on north flank of Blue Mounta ins. Potter 

Hill ahead. 

18 
(1.1) 

86.6 

(12.35) 

98.95 

(0.1) 

99.05 

(0.15) 

99.2 

(1.25) 

100.45 

(0.9) 

101.35 

(0.6) 

101.95 

(4.05) 

106.0 

(0.65) 

106.65 

(0.1) 

106.75 

(1.25) 

108.0 

(1 .5) 

109. 5 

(0.4) 

109.9 

(6.0) 

115.9 

(7.2) 

123.1 

STOP NO. 3. Potter Hill vent area for Roza Member. Note pumice, welded 

spatter, dikes, and primary dips related to cone. The Roza Member vent 

system is more than 165 km long and less than 15 km (probably less than 5 

km) wide. The vent system is defineq by dikes, such as those at stop 2, 

and vent areas such as this one. Big Butte, visible from here to south, 

is another vent area from which one of the latest eruptions took place. 

Turn around, and retrace route to Sweeney Gulch Road. 

Junction with Sweeney Gulch Road. Bear left, staying on Highway 128. 

Roadcut exposing a Dodge flow in the Eckler Mountain Member of the Wanapum 

Basalt. 

Base of Dodge over red saprolite developed on Grande Ronde Basalt. Roadcuts beyond expose flows in uppermost magnetostratigraphic unit (N2) of 

the Grande Ronde. 

Cross Pataha Creek at Columbia Center. Keep right across bridge. 

STOP NO. 4. Roadcut shows the basalt of Dodge overlain by dense aphyric 

flows of the Frenchman Springs Member. Walk back along road 30 m to see 

saprolite under Dodge. The basalt of Dodge is in places interleaved with 

saprolite, showing that it was erupted during the time of regional soil 

formation on the Grande Ronde Basalt. Note the coarse grain size, large 

plagioclase phenocrysts, and grusy nature of the Dodge, which make it a 

fine marker in the field. 

Turn right on pavement and continue down approximate dip slope on the Roza 

Member. 

Dirt road to right. Start of Benjamin Gulch section. This section, capped 

by the Roza Member, exposes interbedded flows of Grande Ronde, Dodge, and 

dominantly Frenchman Springs chemical types, and is the only such section 

known in Washington. This section is of great importance, as it demonstrates that eruptions of greatly different magma chemistries overlapped 

in time. Presumably this locality contains flows erupted throughout the 

time interval during which a soil was forming elsewhere in southeast Washington. 

Quarry in dense flow of the Frenchman Spri ngs Member as at Stop 4. 

The basalt of Dodge under the Frenchman Springs Member. Next few cuts reveal interlayering; exposures are good but not eyecatching. All contacts 

are exposed. 

Approximate base of section. 

Pomeroy town limit. 

Intersection with U.S. 12. Turn left (west) on highway. 

Junction with State Highway 1 26. Continue ahead on U.S. 12. For the next 

few miles, a l edge-forming flow is quite prominent, particularly in about 

3.5 miles. This f l ow, i n f o r mally called the Pataha flow, is of high-Mg 

Grande Ronde c hemical type and is near the top of the Grande Ronde section 

i n th i s area. 

Junction with State H" g hway 127. Continue left (west) on U.S. 12, toward 

Walla Walla. Type loca l ity of the basalt of Dodge is in roadcuts along 

S.H. 127, 0.95 miles northeast of junction. 

19 
(8.45) 

131 .55 

(6. 85) 

138.4 

(3.3) 

141.7 

(0. 3) 

142.0 

(1. 4) 

143.4 

(1. 65) 

145.05 

(1.1) 

146.15 

(1. 3) 

147.45 

(2.4) 

149.85 

(2.2) 

152. OS 

(2. 2) 

154.25 

(4.4) 

158.65 

Junction with State Highway 261. Turn right toward Palouse Falls State 

Park . Roadcuts in next few miles expose flows in N2 part of Grande Ronde 

Basalt. Road follows the Tucannon River . 

Junction with paved road to Little Goose Dam. Stay left , in valley. Road 

to dam is access to stops 10-14 of Swanson and Wright (1976a). For next 

mile, black cliff above road is in the Lower Monumenta l Member of the 

Saddle Mountains Basalt, which occupies a road valley eroded through the 

Wanapum Basalt into the Grande Ronde Basalt at the confluence of the Snake 

and Tucannon Rivers. Exposures of the Pomona Member in a canyon-filling 

r e lation occur nearby. 

Good exposures of tiered flow of the Grande Ronde Basalt. White ash in 

pockets on hillside is Mazama ash. 

STOP NO. 5 (optional). Origin of tiered flows. Are tiers the result of 

some process complete ly internal to a ponded flow and related to its cooling history, or does each tier record a separate pulse of lava i nto a 

gradually deepening pond? In other words, is a tiered flow a single or 

multiple-flow cooling unit? Good exposures of vesicular layers defining 

boundaries of tiers are found above the railroad tracks to left of road. 

Dark mesas to left are remnan ts of the Lower Monumenta l Member resting erosionally on the Frenchman Springs Member. Intracanyon remnants of the 

Pomona and Esquatzel a lso occur near here. The prominent bench seen above 

both the Palouse and Snake Rivers is along the contact of the Grande Ronde 

and Wanapum Basalts. Cross Snake River near mouth of Palouse River. The 

renowned archeologic site, Marmes Rock Shelter, is drowned benea th lake 

waters just up the Palouse. 

Entrance to Lyons Ferry State Park. Continue straight. 

Poorly exposed contact of the Grande Ronde Basalt and the Frenchman Spri ngs 

Member of Wanapum Basalt. 

Base of the Roza Member. 

Junction with road to right to Palouse Falls State Park. Continue 

straight ahead. The junction is just below the contact of the Priest 

Rapids and Roza Members. The road to the park crosses the Roza and the 

Frenchman Springs Members. The lip of the falls is carved into the oldest 

flow of the Frenchman Springs. 

Flow of Lolo chemical type in the Priest Rapids Member crops out just 

above road; it is the youngest flow throughout the area. Good example of 

scabland topography (Scablandia) to east (right). Beg in to cross upper 

reaches of H.U. Coulee; roadcuts expose the Roza and Frenchman Springs 

Members. 

View to left down H.U. Coulee, a scabland channel made famous by J. Harlan 

Bretz. Roadcuts beyond are the Roza and Frenchman Springs Members. 

Junction with Highway 260. Turn left toward Pasco. Highway follows dry 

Washtucna Coulee, former course of Palouse River. Erosion caused by 

catastrophic flooding (the Spokane floods) in the late Pleistocene 

diverted the river from its former course into a more direct link with the 

Snake River via Palouse Falls. Prior to the floods, the Palouse flowed 

down Washtucna Coulee to Connell and then south-southwestward to the Pasco 

area via Esquatzel Coulee. 

The highest continuous ledge along the coulee walls between here and 

Kahlotus is the Roza Member; the highest discontinuous exposures are of 

20 
(2. 85) 

161.5 

(5. 2) 

166.7 

(0.7) 

167.4 

( 4. 55) 

171.95 

(0. 65) 

172.6 

{0. 65) 

173. 25 

(1. 5) 

174.75 

the Priest Rapids Member. 

View to right of gravel bar deposited by Spokane floods. 

Junction in Kahlotus with paved road to Pasco and Lower Monumental Dam. 

Turn left. 

Junction with road to Lower Monumental Dam. Turn left down Devils 

Canyon. All roadcuts expose flows of the Frenchman Springs Member, 

totalling about 150 m thick. Contrast with the 20 m of the Frenchman 

Springs seen at stop 2. 

Look ahead at remnant of the Lower Monumental Member forming cliff overlying river gravel across river beyond dam. 

Large parking area to right opposite turn toward dam. 

STOP 00. 6. Contact of the Frenchman Springs Member of the Wanapum Basalt 

and Grande Ronde Basalt. Lowest exposed flow is the Grande Ronde. It has 

an oxidized top and is overlain by a Frenchman Springs flow with sparse, 

small plagioclase phenocrysts. The oxidized zone is an incipient 

residual soil horizon. Traced eastward, this zone gradually thickens into 

a true saprolite, which is well exposed at stops 2 and 4. As the .soil 

zone thickens, the overlying Frenchman Springs thins and finally _dies out, 

leaving the Roza Member resting directly on saprolite. This is evidence 

that the Frenchman Springs flows were erupted in a basin, presumably 

created by earlier subsidence or westward tilting of the Columbia Plateau. 

STOP 00. 7. {Optional) From stop 6, drive across _Lower Monumental Dam and 

park along road 1 mile beyond southeast end of dam, at base of the flow 

defining the Lower Monumental Member. This flow, at 6 m.y. the youngest 

flow in the Columbia River Basalt Group, extends at least as far east as 

Asotin, Washington. The Lower Monumental has not been recognized farther 

west than its type locality. The flow overlies river gravel consisting of 

basalt, metavolcanic rocks, and a very few plutonic rocks with lenses of 

quartz-feldspar-muscovite sand. This assemblage is clearly that of an 

ancestral Snake River, not a small stream heading on the plateau itself. 

Retrace route from stop 6. Looking up Devils Canyon from this point, a 

good cross-sectional view of flows filling a steep-walled gorge is exposed 

on the northeast wall of the canyon. 

Park in small turnout on right. 

STOP 00. 8. Intracanyon flows. Before Lower Monumental time, the 

ancestral Snake River apparently occupied a gorge 2-3 km north of the 

present canyon. Four different lava flows poured down this gorge, with 

renewed canyon cutting between each eruption, resulting in the compound 

lava fill exposed in cross section in the cliffs on either side of the 

road. East of the road, a "triple junction" is well exposed. Here the 12 

m.y. Pomona Member {right) filled an almost vertically wall ed gorge cut 

into the oldest known intracanyon flow in this area, about 12.5 m.y. 

{left). Note the nearly horizontal columnar jointing in the Pomona along 

the contact. After a period of erosion, which beveled the Pomona and 

older intracanyon fl ow, the 10.5 my. Elephant Mountain Member was 

erupted. It forms the uppermost flow in contact with both of these older 

intracanyon flows. West of the road, relations are more involved. Four 

flows are present here, and they meet at a common point above the railroad 

grade. The same three flows in the cliff east of the road are also here, 

with a fourth, the Esquatzel Member, between the unnamed intracanyon flow 

and the Pomona. Eastward from a point 8 km east of Devils Canyon, the 

pre-Lower Monumental canyon of the Snake follows the present canyon rather 

closely. West of there, however, the older canyon continued due west, 

meeting Esquatzel Cou l ee between Mesa and Connell. The course of this old 

canyon can be traced using detailed aeromagnetic data {Swanson and others, 

21 
(3.05) 

177.8 

(9.3) 

187.1 

(0. 75) 

187.85 

(9.4) 

197.25 

(0.65) 

197.9 

(10.65) 

208.55 

(5.05) 

213.6 

(2.75) 

216.35 

(0.3) 

216.65 

(0.4) 

217.05 

(0.2) 

217.25 

1979c), as the thick intracanyon flows of uniform polarity exhibit na rrow, 

sinuous anomalies of greater amplitude than the thin, norma l and reversed 

flows forming the wallrock. We can speculate that the Elephant Mounta in 

flow so thoroughly choked the canyon that the Snake spilled southward over 

a divide, located about 8 km east of Devils Canyon near Magallon ranch, 

into a tributary of the ancestral Columbia River leading southwest toward 

Pasco. Erosion of this divide proceeded comparatively rapidly, and the 

Snake has remained in this canyon since then. 

Continue up Devils Canyon. 

Roadcuts in the Roza Member. 

highest cut. 

Turn left at intersection, toward Pasco. 

Contact with the Priest Rapids exposed in 

Continue straight at junction with Wallace Walker road. 

Outcrop of the Priest Rapids Member below the Elephant Mountain Member in 

gully to right. 

Continue straight at intersection with Burr Canyon road. A feeder dike 

for a flow of the Frenchman Springs Member can be reached by following the 

Burr Canyon road to the lowest railroad right-of-way shown on the Lower 

Monumental Dam 7-1/2' quadrangle. The tracks have been removed from this 

right-of- way, and the route was drivable with care in 1977. From Burr 

Canyon, follow the abandoned railroad about 2.5 miles to near Abe triangulation station (Elwood 7-1/2' quad). Several small faults offset the red, 

oxidized contact of the Grande Ronde Basalt and Frenchman Springs Member 

exposed along the way. The widest plagioclase-phyric dike, evident in 

cuts, projects across the river and feeds the prominent flow exposed at 

top of dike. By climbing up hill (poor exposure) above Abe, the top of 

the dike can be seen in cuts along the relocated railroad. A shallow pool 

of columnar basalt fills the flared top of dike and merges laterally into 

an extensive porphyritic flow. 

Ghostly sand dunes to right. 

Leave loess hills and enter onto dip slope developed in the Elephant 

Mountain Member. Spokane floods stripped away the loess. 

Junction with road to Ice Harbor Dam. Turn left (south). 

Turn left, onto another paved road, just before dam. Sign points to Boat 

Landing. 

Junction with gravel road. Keep left on gravel road across wooden plank 

bridge. 

Junction of track roads. Both lead to same place. Righthand route was best in 1979. 

End of road. Bus may need to park before end. 

STOP NO. 9. Examine railroad cuts 500-1200 m upriver from end of road. 

Flows of the Pomona and Elephant Mountain Members are well exposed at 

railroad level. The basalt of Martindale in the Ice Harbor Member overlies the Elephant Mountain Member above the railroad. Examine invasive 

contact relation of the Pomona and a white vitric tuff 1200 m from end of 

road. Three dikes of Basin City chemical type (Helz and others, 1976 ) 

intrude the Elephant Mountain at locality 7 of Swanson and othe rs (1975, 

table 2). The dikes are difficult to spot. The nomally magnet i zed dikes 

occur along a positive linear aeromagnetic anomaly that extends 30 km 

south of the Snake River and 50 km nor th of the river (Swanson and others, 

1979c). They form part of a linear vent system for the 8.5 m.y. Ice 

Harbor Member described by Swanson and others (1975). 

22 
(0 . 85) 

218.1 

(1. 05) 

219.15 

(0 .15) 

219 . 3 

(1.1) 

220.4 

(1.1) 

221.5 

(0 . 2) 

221.7 

"(1. 3) 

223 .0 

(0 . 95 ) 

223 . 9 5 

(5 .3 ) 

229. 25 

(4. J) 

232.55 

( 4. 6 ) 

238 . 15 

(2. 75) 

240.9 

(0. 75) 

241. 6 5 

( l. 7) 

243.35 

( 1. 25) 

244.6 

Retrace route to intersection wi th road across dam. 

Turn left and drive across dam. Keep right on pavement at south end of 

dam. 

Turn right toward river at T-intersection. 

Bear left onto grave l road. Foll ow road toward west. Blu ffs on left 

expose the basalt of Mart i ndale overlying the Elephant Mountain Member. 

Park. 

STOP NO. 10. The craggy bluff near the parking area is the remnant of a 

tuff cone built over a vent for the basalt of Martinda l e in the Ice Harbor 

Member (locality 16 of Swanson and others, 1975, table 2). Note the 

poorly bedded nature of the cone, the palagonitic alteration of most of 

the glassy tephra, and the angular unconformity visible on the northwest 

face. A Martindale flow overlies the cone. Walk 600 m downriver along 

trail to cuts exposing the Elephant Mountain Member and the basalts of 

Martindale and Goose Island (locality 12 of Swanson and others, 1975, 

table 2). The flows occupy a shallow northwest-trending syncline, a lso 

recognizable across the river. Note the plagioclase-clinopyroxene c lots 

in the co l umnar Martinda l e flow. A dike of Goose Island chemica l type 

cuts the Martindale flow . Breccia interpreted as drainback rubble occurs 

in upper part of dike; the rubble merges into thin vesicular flows, 

overlain by a thicker flow of Goose Isl and type . At th is l oca lity, the 

upper Goose Isl and flow and the columnar Martinda le f l ow have both been 

dated by E. H. McKee, using whole rock K-Ar methods, at 8.6 ~ 0.3 m.y. 

Refer to He l z (1978) for more information on the Ice Harbor Member. 

Retrace route back to paved road. Continue straight ahead (south ) . 

Martinda le tephr a in roadcuts. 

Horse Heaven anticlinal uplift straight ahead in distance. 

Junction with road to Pasco. Turn right. Continue through Burbank. 

Junction with U.S. 395. Turn left (south). 

At 1 :00, entrance to Wa llula Gap, a water gap across the Ho r se Heaven 

up lift through wh i ch the Columb i a River flows. 

Boise Cascade plant. Hold your nose. 

To right across Columbia River is a series of f aulted, doubly plunging 

antic lines exposing the Frenchman Spr i ngs Member and younger fl ows. 

Junction with u.s. 12 . Turn l eft , toward Walla Walla. Side trip to ri ght 

for 4.2 miles i nto Wallula Gap shows excellent exposures of the thick 

Frenchman Springs Member overla i n by the Umatilla Membe r of Saddle Mountains Basa l t; the Roza and Priest Rapids Members are mi ssing. A Mart i ndale flow caps highest visible point west of river; it overlies imbricated 

Snake River gravel, compositionally similar to that at stop 7, showi ng 

that an ancestral Wa llula Gap existed at least 8.5 m.y.b.p. 

Faceted ridge spurs to right reflect rela tively young faulting along Horse 

Heaven front . 

Roadrut on right shows wh ite Mt. St . He lens . "S" tephra layer in silts of 

the Touchet beds . The t e phra is about 12,000 years old and helps to date 

some of the youngest Spokane flooding, which produced the Touchet beds. 

23 
(1. 05 

245.65 

(l.l) 

246.75 

(0 .1) 

246.85 

(1. 9) 

248.75 

(1. 4) 

250.15 

( 2 . 3) 

252 . 45 

(1. 3) 

253 . 75 

(16 .15) 

269.9 

( 0. 7) 

270.6 

( 0. 3) 

270.9 

0 

( 4. 75 ) 

4.75 

(0. 75 ) 

5.5 

(0.15) 

5.65 

(0.15) 

5.8 

(1. 2) 

7.0 

(l. 45) 

8.45 

(0. 2) 

8 . 65 

(1. 55) 

10.2 

(0. 45) 

10.65 

( 0 . 85) 

11.5 

Roadcuts in the Frenchman Springs Member. 

Flow of Martindale type in the Ice Harbor Member forms low mesa ahead. 

View left across Walla Walla River shows the thick Umatilla Member 

overlain by the basalt of Martindale and underlain by the Frenchman 

Springs Member. A small sheeted dike complex, consisting of 5 dikes of 

Goose Island (Ice Harbor Member) chemical type, cuts the Umatilla but is 

not evident from highway. 

Roadcuts for next mile in the Frenchman Springs Member. 

Approximate crest of north-south anticline forming west boundary of Walla 

Walla Basin . 

Easternmost basalt exposure (Frenchman Springs Member) in western part of 

Walla Walla basin. 

Bluffs to right are type locality of Touchet beds . From here to Walla 

Walla, roadcuts reveal Touchet beds and loess. 

Leave U.S. 12, turning right on road to Pendleton, Oregon. 

Continue straight at junction with Highway 125. 

Turn right on Highway 125. Stay on 125 through Walla Wa l la toward MiltonFreewater, Oregon. This route bypasses most motels. Trip l og i s resumed 

about 5.5 miles from downtown Wa l la Walla, at Oregon-Washington State 

line. Here, Washington Highway 125 becomes Oregon Highway 11 . 

SECOND DA 

Oregon-Washington borde r between Walla Walla, Wash. and Milton-Freewa t e r, 

Oregon. 

From left lane in Milton Freewater, turn left toward Pendleton on Highway 

11. 

Turn left off Highway 11 at bottom of hill , just beyond Clifton Mote l , 

toward upper Walla Walla River. Cross railroad tracks, keeping r i ght. 

Follow pavement to left on SE 15th Ave. 

The Frenchman Springs Member is exposed on hillside ahead. 

Keep left at intersection. 

Approximate base of the Frenchman Springs in roadcut on left. 

Bridge across Walla Walla River. Road enters the Blue Mounta i ns, an upljfted and faulted region extending from central Oregon to extreme southeast Washington. Route goes progressively downsection, as dips are about 

2-3 degrees toward the Walla Walla basin. 

Bridge across South Fork Walla Walla River. 

Turn right toward South Fork. 

Roadcut on left exposes 10 m-wide, vertical feeder dike of the Frenchman 

Springs Member. Note nearby horizontal columns. Dike cuts vesicular flow 

top in N2 unit of the Grande Ronde Basalt. 

Route follows the straight canyon of the South Fork of the Walla Walla 

River, which forms a segment of the Olympic-Wallowa lineament, a straight 

24 
( 2. 4) 

13.9 

(l. 4) 

15.3 

(2. 3) 

l7.6 

(0. 55) 

18.15 

( 0. 4) 

18.55 

( 0. 4) 

18.95 

(13. 4) 

32.35 

( 0. 95) 

33.3 

(2.1) 

35.4 

( 0. 5) 

35.9 

( 0. 55) 

36.45 

(l. 5) 

37.95 

(19. 5) 

57.45 

( 0. 2 ) 

57.65 

(0.4 ) 

58.05 

( 0. 4 ) 

58.45 

Cl . OS) 

59.5 

( 0. 2) 

59.7 

(0. 25) 

59.95 

( 0. 15) 

topographic feature reaching about 700 km from the northwest tip of washi ngton to the Wallowa Mountains i n northeast Oregon. Origin of the lineament is subject of much debate. Evidence to be seen at stop 11 shows that 

this segment of the lineament has been structural l y inactive for the last 

14-JS m.y. 

Approximate location of contact between R2 and N2 units of the Grande 

Ronde Basalt. 

Good exposure of rubbly top on a flow in the R2 unit of the Grande Ronde. 

Exposure on left of gently dipping feeder dike of the Frenchman Springs 

Member. 

Cross river. Enter Harris Park. Continue straight. 

Bus parking. 

End of road. 

STOP NO . 11. Observe the 60 m-thick, gently dipping (15-20 degrees) dike 

of the Frenchman Springs Member on both sides of river. Note that it 

crosses canyon, which defines the Olympic-Wallowa lineament in this area, 

with no obvious offset . K-Ar date on dike by Ted McKee is about 14.5 

m.y. Walk across footbridge, turn left on trail, dodge low-hanging 

branches, and walk to lower contact of columnar dike . Observe glassy 

selvedge . Continue on higher of two trails to upper glassy selvedge. 

Discuss criteria for distinguishing a flow from such a dike. What are 

conditions under which such gently dipping dikes are formed? 

Retrace route to Highway 11 in Milton-Freewater. Turn left on Highway 1 

toward Pendleton. 

Good view to left of Blue Mountains front. 

Roadcut in the Umatilla Member. 

Start of series of roadcuts in the Frenchman Springs Member. 

Fault breccia in the Frenchman Springs Member. Rock throughout roadcut is 

shattered. Undated colluvium is offset by small fault at far end of cut. 

The breccia occurs along north-trending faults. 

Junction of road to Tollgate and Elgin across Blue Mountains. Continue 

toward Pendleton, more or less along axis of the Agency syncline, with 

poorly exposed flows of the Frenchman Springs Member in places. 

Narrow bridge across Umatilla River in Pendleton. Beyond bridge, keep 

right toward Portland. 

Junction with U.S. 30. 

Original Pendleton woolen mill to right. 

Continue straight ahead through town. 

Get in l eft lane opposite Pendleton Round-Up grounds. 

Stop sign. Continue on U.S. 395 across railroad tracks. Follow sign to 

Pilot Rock. 

Turn left at blinking stop light. 

25 
60.] 

(0.1) 

60.2 

(2.45) 

63.6 

(0. 95) 

64.55 

(0 . 75) 

65.3 

( 5. 5) 

70.8 

( 2. 55) 

73.35 

( 0. 6) 

73.95 

(0 . 3) 

74.25 

( 0. 2) 

74.45 

(7. 7) 

82.15 

( 4. 55) 

36.7 

. (7 . 3) 

94.0 

(1. 5) 

95.5 

( 0. 2) 

95.7 

( l. 65) 

97.35 

(0. 25) 

97.6 

( 0. 5) 

98.1 

(0.4) 

98.5 

(1. 25) 

99.75 

(5. 9) 

105.65 

( 2. 85) 

108.5 

Turn right at stop sign. Dead ahead is cut s howing the Frenchman Springs 

Member overlying thick saprolite on the Grande Ronde Basalt. 

Continue on U.S. 395 toward Pilot Rock and John Day. Do not enter freeway . 

On left, earth-fill McRay Reservoi r Dam, in danger of collapsing until 

refurbished in 1978. Rounded hills on both si des of road are eroded in 

the McRay beds of Hogenson (1964), a Pliocene fanglomerate. 

Roadcut in the McRay beds. Many more such cuts between here and Pilot 

Rock. 

Good view of Blue Mountains front to left and ahead. The Frenchman Springs 

Member caps highest ridges to left but is absent ahead. 

Across valley to right, saprolite separating the Grande Ronde Basalt and 

the Frenchman Springs Member exposed in railroad cuts. 

Town of Pilot Rock. Bluff at 1:00 is the Pilot Rock, mostly the Grande 

Ronde Basalt but relay station rests on basal flow of the Frenchman 

Springs Member. 

Continue left on U.S. 395. 

Continue right on U.S. 395. 

Good view of unimposing Pilot Rock. Roadcuts for long distance are in N2 

unit of the Grande Ronde Basalt. The Frenchman Springs does not occur 

south of this area. 

Junction with Highway 74. Keep left on U.S. 395. By fall of 1979, mapping 

of the basalt had not been finished beyond this point, so the following 

log is necessarily skimpier than it has been. Some information is taken 

from Walker (1973a) and Brown and Thayer (1966). 

Good views of Blue Mountains front. 

Dip slopes to right of road give clear sense of uplift. 

Roadcut on left exposes fault in the Grande Ronde Basa lt. 

The Grande Ronde Basalt rests on soil developed on quartz diorite of 

Jurassic or Cretaceous age. 

Entrance to Battle Mountain State Park. 

Low-Mg flows of the Grande Ronde Basa lt in roadcut on left (Nathan and 

Fruchter, 1974). 

Coarsely columnar, probably valley-filling flow of the Grande Ronde Basa l t 

over granitoid in roadcut on left. 

Blue Mountains crest, 4270 ft. Structural crest 1-2 km south of here. 

Contact of the Grande Ronde Basalt and the John Day Formation poorly 

exposed. 

Terraces at 10:00 are developed in Pliocene sediments, trapped in structurally controlled basin (Walker, 1973a). 

Junction with Highway 344. Continue straight on U.S. 395. Ridge ahead 

i n the Grande Ronde Basalt. 

26 
(0. 85) 

109.35 

(0. 2) 

109.5 5 

(0. 95) 

no. 5 

(0. 3) 

no. 8 

(0. 35) 

111. 15 

(8. 9) 

120.05 

(1. 6) 

121. 6 5 

(1. 3) 

122. 95 

(1. OS) 

124.0 

(6. 95) 

130. 95 

( 0. 45) 

131.4 

(0. 3) 

131.7 

(0. 8) 

132.5 

. ( 0. 5) 

133.0 

( 0. 5) 

133.5 

(0.1) 

133.6 

(l. OS) 

134.65 

(1.1) 

135.75 

(0. 85) 

136 .6 

(1.1) 

137 .7 

( 3. 05) 

140.75 

(6. 35) 

147.1 

(8. 35) 

155 .45 

(0. 3) 

155 .75 

(0. 55) 

15 6.3 

Start of Camas Creek section through the Grande Ronde and Picture Gorge 

Basalts. 

Pillowed flow of the Grande Ronde Basalt in roadcut. 

Interbed between flows of the Grande Ronde Basalt in roadcut on right. 

Complexly jointed and faulted flows of the Grande Ronde. Some horizontal 

slickensides. 

STOP NO. 12. Roadcut near mile post 52 exposing contact of the Grande 

Ronde and Picture Gorge Basalts. Discovered and described by Nathan and 

Fruchter (1974). Aphyic, normally magnetized flow of the Grande Ronde 

(probably N1 but not known certainly) overlies thin organic-rich interbed resting on highly plagioclase-phyric, magnetically normal flow of the 

Picture Gorge. Chemistry by Nathan and Fruchter (1974) confirms stratigraphic assignments. 

Bridge at mouth of Camas Creek. North Fork of John Day River to right. 

Cross 45th parallel. 

Bridge over North Fork of John Day River. 

Dale. All roadcuts are in the Picture Gorge Basalt. 

Meadow Pass, 4127 ft. 

The Picture Gorge Basalt over the John Day Formation in roadcuts on right. 

Cretaceous diorite on left. 

Slopes to right are foliated diorite. 

Cross Middle Fork of John Day River. 

Hyaloclastite in the Picture Gorge Basalt in roadcut on left. Flow 

probably fills valley eroded into Paleozoic rocks. 

Paleozoic metasedimentary rocks for next 0.9 miles. 

At 3:00, scenic view of the Picture Gorge Basalt forming mesa. 

The Picture Gorge Basalt in roadcut on left. 

View straight ahead of the Picture Gorge Basalt dipping southeast from Blue 

Mountains uplift. 

Summit of Ritter Butte, 3991 ft. 

View ahead of Long Creek Mountain, underlain by the faulted Picture Gorge 

Basalt. 

Town of Long Creek. Turn right off U.S. 395 toward Monument and Kimberly. 

Roadcut on right exposes contact of the Picture Gorge Basalt and John Day 

Formation. 

Geologic marker on left, courtesy of T.P. Thayer, explains history of area. 

Scenic view down valley of the Picture Gorge Basalt over red tuffaceous 

27 
(0. 5) 

156.8 

(1. 7) 

158.5 

(0. 65) 

159 . 15 

(0. 5) 

159.65 

(0.6) 

160 . 25 

(0. 35) 

160.6 

(1. 25) 

161.85 

(1. 8) 

163.65 

( 3. 9) 

167.55 

(0. 75) 

168.3 

( 0. 75) 

169 .05 

(10. 35) 

179 .4 

( 3 .15 ) 

182.55 

rocks of the John Day Formation. 

Approximate contact of the Picture Gorge and the John Day in roadcuts on 

right. 

Hamilton. 

Tuffaceous roc ks of the John Day Formation, here chiefly bioturbated airfall tuff. 

Landslides in the John Day Formation. Altered tuffs slide easily, especially when wet and overlain by basalt. 

Geologic marker describes Sunken Mounta in landslide to left. This and 

other markers in the area were conceived and written by Tom Thayer, who 

worked for many years on the Canyon City 2 degree sheet. 

Thick section of the Picture Gorge Basalt ahead. 

Views ahead and to right of angular unconformity between the Picture Gorge 

Basalt and John Day Formation. 

To right, dike and irregular intrusive body of the Picture Gorge Basalt i rr 

the John Day Formation. The Picture Gorge was erupted wholly within this 

region. Its feeder dikes comprise the Monument dike swarm. Fruchter and 

Baldwin (1975) found good chemical matches between flows and dikes of the 

Picture Gorge, substantiating the notion that the two are indeed related. 

From here to Kimberly itself we will see many of these dikes; only a few 

are pointed out in the log. 

Dead ahead is Monument Mountain, one of the thickest exposed sections of 

the Picture Gorge Basalt. 

Cross the North Fork of John Day Rive r and enter town of Monument. View to 

left of Picture Gorge intrusive bodies in the John Day Formation. 

Roadcut across thick dike. For next 10 mi les , dikes are very common, cutting flows of the Picture Gorge as well as the older John Day Formation. 

Zeolites in pegmatoidal flow of the Picture Gorge Basalt in roadcut on 

right. 

General store in Kimberly. Start of geologic field trip summarized by P.T. 

Robinson (this volume). 

28 
GUIOE TO GEOLOGIC FIELD TRIP BETWF.E 

0 THE JOH 

Kl RERLY A 0 BE 0 , OREGO 

DAY FORNATIO 

WITH F. PHASIS 

Paul T. Robinson, D partment of Ea rth Sciences, University of California , Riverside, California 92521 

and 

Gerald F. Brem, Department of F~ rth Sciences, California State University , Fullerton, California 92634 

I TROD CTIO 

The John Day Formation of north-central Or egon 

is a widespread , largely pyroclastic unit lying heween the Eoce ne Clarno Formation and the Miocene 

Columbia River Basalt (FiRure 1) . The bulk of t he 

unit consists of andesitic to dac itic tuffaceous 

claystone and air-fall tuff derived f rom vents west 

of the pr esent-day outcrops . Th age of the forma -

tion based on vertebrate fossils and K-Ar dating 

ranges f rom approximately 36 to lR m • • B. P. 

Woodbu rne and Robinson, 1977) . Hence , t he formation 

is believed to provide a well exposed depositonal 

record of middle Te rti a r y Cascade volcanism no 

available elsewhere . 

Detailed studies of the formation have been 

c3 rried out by a number o f worke rs in the las 20 

years . St udies in the vicinit o the type area , 

near Picture Gorge, have been carried ou bv Fis he 

(1966a, 1966b, 1967, and 1968 ) , fi her and 

Rensberger (1972) , Fisher and Wilcox, (1960 ) and Ha 

(1962a, 1962b , 1963) . Outcrops in and west of t he 

Blue Mountains have been studied by Peck (1964 ) , 

Robinson (1975) , Robin on and S ensland (1979 ) , 

Swanson and Robinson (1 96 ), and Woodhu~ne and 

Rob i nson (1977) . Reconnaissance geologi c maps have 

been completed for most of the uni ( Swanson, 1969 · 

Robinson , 197 5). De ta iled chemica 1 and mine r a logica l studies of the ash- flow and ai r- al tuffs are 

in pro ress by the autho rs . 

STRATIGRAPH':' 

Genera l Asp cts 

The John Day Formation crop out widely over 

north-central Oregon , occurring in three geog rarhically sepa rat e a r e a s (Figure 1) . The t p localit 

(eastern facies) lies east of the Blue Mountain upli f and inc lud h Co r e dis r ict and th 

. i chell a r a . 

the Ochoco 

or nt in th 

ou h 0 

tuffs and lava fl ow rare 

astern anci sou h rn faci s . 

Co nsid rabl disagr ement xi ts as to th naure and stra !graphic position of th bounciar betwe n th John Day and Clarno Fo r mations . In areas 

29 

east of the Blue Mountains the top of the Cla rno 

fo rmation is gene r ally ma rked by a well developed 

red saprolite . Local incorporation of this mate rial 

into the lowe r part of the John Day Formation 

accoun t s in pa rt for t he red color of the lower 

member (Big Basin member of fishe r anci Rensberger, 

1972) . Waters (1954) recognized a simil a r saprolite 

in the Horse Heaven area in the Blue !ountains 

(Figure 1) and suggested that it also markeci the top 

of the Cla rno Formation at that locality . Howev r, 

Peck (1964) and Swanson and Rob inson (1968) placed 

t he contact between the two formations in the western facies at the base of a widespreaci ash-flow tuff 

(membe r A of the western facies) whi ch unconformabl 

overlies rocks of Cla rno litholog a nd is seve ra l 

hundred feet stratigraphical! above the Horse 

Heaven saprolite . At most localities the John Da 

Formation is uncon fo r mabl overla i n by basalts of the 

Columbia Ri ver Gr ou • Where these a r e missing, the 

fo rmation is unconformabl y overlain b younger un i 

such as the Madras Fo rma tion, the Hascall Fo rmati on, 

or Quaternar basalt. 

East e r n Facies 

Merriam (190 1) defined three membe rs in the 

easte rn facie of the John Day Formation based 

largely on co l o r . Howeve r, mos o f the feat ures used 

by Merr iam to distinguish the members f rom one 

another vary laterally a nd define al t e r ation facies 

rather than stratigraphi units . Ha y (1962a ; 1963) 

pr oposed a 3- fold &ubdivision based on the presence 

o f a widespread ash- flow sheet (Pict ur e Gorge Ignimbr ite) near the middle of the Fo rmation . Fisher and 

Rensber er (1972) divided the uni t into fo ur members, 

nearly compa rable to tho e originally de f i ned by 

Me rriam ( 190 1) . From oldest to younges t, these are; 

a) t he Big Basin Member, a basal sequence of red 

claystone , ) the Turtle Cove ember, cha racterized 

by green, buff, or pale r ed zeolitizeci tuff and 

uffaceous c la st one, c) the Kimberly ~mber 

charac erized by light gr ay to buff , unzeolitized 

tuff and tuf f a c eous sedimen and , d) the Haystack 

Valley Member composed of gra , la rge! fluviatile , 

volcaniclasti r ocks . Se veral of the members, 

parti c ula r ly the Kimber! and Big Basin members , are 

distinguished la r gely on the basis of d i agene tic 

features and , hence, the contacts often transgress 

time - stratigraphic boundaries . 

Bi g Basin Memb r - The Big Basin member is a thin , 

discontinous sequence of red and less abundant 

yellow tuffaceous claystone named for ex posures in 

Big Basin . It is typical! 15 to 40 met e rs thick but 

r a n es f rom 0 t o 75 meters . Locally, thin , discontinuous olivine basalt flows occur near its base . The 

clayst one a re poorly bedded, and c rop out in l ow, 
~-. 

' 

~.I ' / ) 

r ~ 

/ " "' / 

/ 

I.J..J I 

.., I (!) I ' '-.. :::-. ~ ' ) c.; q /' 

Q: r_ 

,'"\~ "- -' r~.J 

\ I.L.I ' I 

,.. _ _ , \ \~ a I 

~ q I 

r-'-J ., u ( \ (/) 

J q ~ ( u (, ( 

' -;:::. ~~ ~ I \ Q: ~~~ ) 

/ I.L.I J ( I- I (/) ....... I I.L.I ,.,._) 

s \..' 

.J,...~ 

122° 

Figur l. 

-<.. 

lnd x map 

Or 

0 

I.L.I 

(!) 

2 

q 

Q: 

I.L.I 

a 

q 

u 

(/) 

q 

u 

.:r 

(!) 

.:r 

I 

I 

I 

I 

h wing 

on . Da 

pr b bl 

rounded hills mant 1 d by a thin "popcornlayer. The red colo r of mos of h unit 

incorporation of r d saprolite from local 

I 

\ 

\ 

' \ 

121° 

0 

s 

of the Cla rno Formation and diagen tic al ra ion of 

air-fall tuff in ~ll-drained upland areas . 

colo ration fo r med in the tuffaceous cla ton 

drainage was poor and th re was l ss opportunit or 

i r on oxidation (Hay , 1962a; Fisher and R nsb rg r, 

(1972) . 

Turtle Cove Member - The Tu rt le Cov m mber i th 

thickest and most widespread unit in th east rn 

facies of the Formation . Throughout mo t of the ar a 

it ranges from abou 12 0 to 50 met rs in hickn ss 

but it thins markedly northwa rd against th Rlue 

Mountain uplift where it is less than 15 met rs thick 

(Fisher, 1967) . It consists chiefly of varicolo r d , 

zeolitized, mostly fine - gr ained tuff and tuffaceous 

claystone with less important air-fall and ash- flow 

tuff . Outc rops in the lower part of the m mb r di -

play a typical ribb d or pinnacled ' badland ' topography ; those higher in he unit ar smoo h and 

r ounded . Colors vary widel , re l c ing varying deg rees of diagenetic alteration . os of the uni is 

light green with thin inte r beds of light gray rna erial; however, br own , ellow , and ligh red colors are 

common . The zeolitized tuffs and tuffaceous claystones were originally composed largel of vitric 

sha rds and pumice f r agments . Pu ice fragmen are 

gene rally less than l mm in length but range up to 4 

mm . Sand- size cr stals and rock fragmen s compri e 

5-1 0 percent of most specimens. r.rystals are chiefly 

andesine feldspar with lesser amoun s of oligoclas , 

lab radorite , and sanidine . Pyroxen , hornblende , 

biotite , magnetite , and ilmenite a r usually pr esent 

30 

50 

120° 11 9° 

XL n of h John Da Formali n 

kn wn out rop , dot d lin 

r pr s n t . 

in 

rh 

form 

Hills 

din 

uni. s 

g r ain d, mod ra 

poor . Cryst als 

consist o plagio 

c li nop yroxen • 

mb r Th K imb 

llow grav , unz oli 

similar to tha of 

Cove member . The boundary h n 

s 

drawn almos ntirel on a color change r 

a f w 

of he 

Pain d 

o e 

a l - ric 

s (Ha , 

ash- flo .. • 

cool in£ 

variations in diag n tic altera ion and chronologicall variable . 

Th Kimb rly m mber has a composit thickne o 

about 300 me rs bu th thi kn s vari s con id rabl becaus of h variabl lower boundarv and 

because erosion in lat ohn Oa ime s r ipp d off 

much of th unit . Outcrops ar usuall smooth, ven 

slop s oft n cover d with ligh g ra , powd ry oil . 

Generally , h rocks form hick , poorl bedd d, 

featu r less sections without dis inct marker uni 
Crystals compr ise about 5 moda l percent of these 

rocks and are chiefly andesine felcispar with minor 

amounts of clinopy rox ene, ma~netit , hornblend e , 

biot ite , and ilmenit e . The absence of quartz and 

sanidine suggests an andesitic to daci tic composition . One distinct i ve rhyolite tuff c haracterized by 

a~ndant black obsidian sharcis occu rs near the base 

of this member (Hay, 1963; Fisher and Rensbe rger, 

1972) . 

~ystack Valley Member The Haystack Va lley member 

comp rises a seq uence of i nterbedded fluviatile and 

lacustrine tuffaceous sediments near the top of the 

fo rmation . It has a max imliDl thickness of about 150 

eters. The membe r consists largely of unzeol itized 

co nglomerate, sandstone, a nd siltstone with some 

lacustrine tuff . Massive air-fall tuffs are commonly 

interbedded with the r ework eci anci cross-bedded 

sedimen ts. Indivi dual beds of co nglomerate and sandstone are often lenticula r, filling steep-walled 

channels cut in to the underlyin~ Kimberly member. The 

composition of the reworked material sug~ests that 

this unit was derived solely from erosion of the 

lwer John Day Fo rmation (Fisher and Rensb r~er, 

1972) . 

Southern Facies 

Li ttle detailed wo rk has been done in the southern facies , which lies south of the Ochoco Mountains . 

~connaissance stud ies indicate that this section is 

mo st like the eas tern facies , consisti ng chiefly of 

fine-grained tuff and tuffaceous claystone but having 

t~ prominent ash-flow sheets in the area south of 

Prineville . 

The southern facies has not been fo rmally divided 

into members but lithologic varia tions are similar t o 

those of the easte rn facies . A sequence of red claystone occur s discontinuously alon~ the base of the 

formation and is similar to the Big Basin member of 

Fisher and Rensberger (1972) . The overlying tuffs 

and tuffaceous claystones can be divideci into ze olitized and unzeolitized sections that correspond 

appro ximately with the Turtle Co ve and Kimbe rly 

Membe rs. 

The lower welded tuff south of Prineville Reservoir is a light g ray to reddish-gray, stony rhyo lite. 

It ranges f r om about 5 to 15 meters thick, generally 

thinning from north to south. The tu ff i s densely 

welded wi t h a good eutaxtic structure, having flat -

te ned pumice f ragments up to 10 em long . Crystals 

co prise abou 1-2% and are chiefly plagioclase with 

traces of magnetit and partly altered biotite . The 

trix is a mic roc rys allin mixture of quartz and 

feldspa r in which the original vitroclastic texture 

has been la rgel oblitera ed . Many of the larger 

pumice fragm nts have been zeolitized . 

The upper a h-flow sheet is gen rally about 10 

me ters thick and consists of light g r ay to reddishgray, den"el weldeci tuff . The tuff is re latively 

fine-grained and aphyric . Flattened pumice f r agments, g nerall le s than 1 em long , define a c rude 

euta xtic structur • These are replaced by a mixture 

of tridymi , alkali f ldspar , and z olite . Small andesitic r oc k f ra gments omprise about 1 modal percent . 

31 

either of these ash-flow sheets can be correlated 

with welded tu ffs in the eas tern or western facies 

and they ar e believed to have originated from separate vents south of the Ochoco Mountains . 

Western Facies 

In the area west of the Blue Mo untains and east 

of the Cascade Range a sequence of largely pyroclastic rocks, 600 to 1200 meters thick , lies between 

the Columbia River Basalt and the Cla rno Formation. 

This seqeuence consists largely of tuff and tuffaceous sediment c ompositionally similar to that of the 

eastern and southern facies of the John Day 

Formation, hut it also inc ludes numerous basaltic to 

rhyolitic lava flows and silicic ash-flow tuffs . 

These rocks have been correlated with the type John 

Day Formation in the eastern facies on the basis of 

stratigraphic position and general lithologic similarity (Hodge, 1932; Waters, 1954; Peck, 1964; 

Swanson, 1969; Robinson, 1975; Woodburne and 

Robinson, 1977). 

In the a r ea west o f the Blue Mountains the John 

Day- Clarno co ntact is placed j ust below the basal 

ash- flow sheet of member A. This ash-flow sheet lies 

uncon fo rmably on rocks of Clarno lithology and is 

locally separated from them by a thin, red saprolite. 

Peck (1964) subdivided this sequence into 9 c onformable members (A-I) based largely on the presence 

of ash-flow sheets and lava flows (Figure 2) . This 

subdivision, with minor additions, is valid over most 

of the western facies (Robinson , 1975) . The only 

major exceptions are in the area between Fossil and 

Lonerock and in the Mutton Mountains. In the Fossil 

area course-grained lapilli tuffs and dacitic to 

basaltic lava flows occur near the base of the formation . The stratigraphic position of these units is 

uncertain but they may be correla t ive with the upper 

pa rt of the western facies. In the Mutton Mountains 

the only ash- flow sheet recognized is that of member 

G. Other ash-fl ow tuffs and lava flows are absent 

and the section cannot be further subdivided. 

Member A - Member A is a widespread sequence of 

tuff, tuffaceous sediment, and welded tuff ranging in 

thickness f rom about 10 to 130 meters. At the base 

is a distinctive ash-flow sheet, up to 35 meters 

thick, that rests unconformably on the Clarno 

Formation . This is overlain by about 30 meters of 

poorly exposed lapi lli tuff, tuff, and tuffaceous 

sediment which in turn is succeeded upward by a 

densely welded, fine- grained ash flow-tuff, usually 

less than 20 meters thick . 

The basal ash- flow tuff is densely welded, 

relatively coarse-grained, and sparsely porphyritic. 

A thin, light ray vitrophyre occurs locally at its 

base but most of the unit is devitrified and sparsely 

lithophysal. Two cooling units, each with a basal 

vitrophyr e , are exposed along the county road west of 

Ashwood; elsewhere only one cooling unit can be 

recognized. The tuff is characterized by a well 

developed eutaxitic structure with flattened pumice 

lapilli up to R em long. Crystals average 5-8% and 

consist chie fly of quartz, sanidine, and plagioclase; 

some specimens also contain traces of green hornblende and opaque minerals. Rock fragments, mostly 

a ndesite and rhyolite, comprise 0.5 to 2%. 
WESTERN EASTERN 

COLUMBIA 

RIVER 

BASALT 

800 

H 

600 z 

0 

I-

<l 

~ 

(/) 0:: 

0 (l) 400 LL -+-

(l) >-

E <l 

0 

z 

I 

0 

200 J 

8 

A 

FACIES 

)-

""' _,;... 

"\ 

...,...... 

~" "" "'"' PICTURE 

GORGE 

IGNIMBRITE 

~ 

~" 

FACIES 

y 

mafic lava flows; 

trachyandesite in Member 8, 

tholeiite in Columbia River Basalt 

1/--1 1 tuff, lapilli tuff, tuffaceous claystone 

and volcaniclastic sediments 

[2] rhyolite ash flow tuff 

intermediate to silicic flows; 

andesite in Clarno Fm. 

rhyolite in Member C 

o~-----r----~~----------~~~~ 

CLARNO 

FORMATION 

ASHWOODWILLOWDALE TWICKENHAM 

Figure 2. Generalized composite stratigraphic columns for the western and east rn facies 

of the John Day Formation showing probable co rrelations. 

This is one of the most distinctive and widespread ash-flow tuffs in the western facies. It can 

be traced in nearly continuous outcrop from Grizzly 

on the south to the vicinity of Cla rno on the north, 

a distance of nearly 80 km. It has been recognized 

in isolated outcrops along the crest of the Blue 

Mountain uplift (Swanson and Robinson , 1968; Swanson, 

1969) and in a small outcrop at the base of the eastern facies along Rowe Creek (Woodburn and Robi nson , 

1977). Correlations between these separate exposures 

are based on grain size, phenocryst assemblage, 

mineral compositions, and stratigraphic position . 

Directly above the basal ash-flow tuff is a 

white, poorly bedded lapilli tuff containing abundant 

pumice . This is followed upward by red to yellowishbrown tuffaceous claystone and interlayered tu ff 

locally containing numerous plant fossils. The upper 

ash-flow sheet is a light brown, fine - grained welded 

tuff with 1-2% plagioclase phenocrysts and 1- 2% small 

rock fragments . A light gra y vitrophyre occurs 

locally at the base but most of the unit is stony 

with very sparse lithophysae . 

32 

Membe r B Member B is a s r ies of dark gray 

trachyandesite flows that crop out ch iefly in the 

Antelope-Ashwood area. This m mber is about 300 

meters thick in the area west of Ashwood but the 

sequence thins gradually to th nor h and south from 

this point . A thinner sequence, pr obably no more 

than 30 meters thick , crops out in the Antelope 

Valley . Locally several flows ca n be recognized, 

s p rated by 5-6 meters o brown tuff and t uffaceous 

sediment . The trachyandesite ypically fractures 

into an ula r, fist- size fragments , forming rounded 

outcrops mantled by talus. 

The trachyandesite is ve ry fine- g rained and 

nearly aphyric . Small phenocrysts of olivine , replaced by smectite, lie in a matrix of tiny plagioclase laths, granula r pigeonite , opaques , and 

abundant interstitial glass. One flow in Eagle 

Valley, south of Ante lope, is holocrysta lline and 

medium grained with an intergranular to subophitic 

texture . Vesicles are sparse except in thin zone s at 

the tops of flows . 
Member C A thick rhyolite flow and domal complex 

in the Ashwood-Antelope area comprises member C. The 

rhyolite flows , which are up to 125 meters thick 

along Wilson Creek, were probably erupted from the 

domal complex along Trout Creek, j ust north of 

Ashwood . The domal complex has crude, nearly vertical columnar jointing , variable flow banding, and a 

~rginal rhyolite breccia interbedded with tuff along 

the southern edge. 

Both the flows and the dome consist of light gray 

to purplish-gray, massive to flow banded, very 

sparsely porphyritic rhyolite. Phenocrysts of plagioclase and quartz comprise less than 2 modal percent 

and lie in a matrix of microcrystalline quartz and 

feldspar . 

Member D Member D is a thin, areally restricted 

sequence of tuff and lapilli tuff that lies above 

rhyolite flows of member C and below ash-flow tuffs 

of member E. The tuffs are light gray to yellowishgray , poorly indurated , and moderately bedded . A 

maximum thickness of about 30 meters is exposed along 

Pony Creek between Ashwood and Willowdale. At most 

localities the unit is less than 5 meters thick . 

Member E A series of densely welded, highly 

lithophysal ash-flow tuffs, up to 120 meters thick, 

comprise member E. These are best exposed along Pony 

Creek between Ashwood and Willowdale where they form 

steep, columnar-jointed outcrops . A light gray basal 

vitrophyre is exposed locally but most of the unit 

consists of stony rhyolite . Outcrops of crystallized 

tuff are characterized by abundant large lithophysae 

arranged in crude layers. The tuffs are very finegrained and generally lack any eutaxitic structure . 

Crystals of oligoclase and quartz comprise 1-2% and 

some specimens have trace amounts of altered pyroxene . Rock fragments are rare or absent . In many 

places the large lithophysal cavities have been filled with opal or chalcedony to form round or oval 

masses known locally as 'thunder eggs' . Peck (1964) 

included the weakly welded ash- flow tuff at the 

Priday agate deposit in the base of member F but 

Robinson (1975) considered it to be the top of member 

E. 

Member F Member F consists of 100 to 300 meters 

of varicolored tuff, lapilli tuff , and tuffaceous 

claystone with interlayered basalt flows near the 

base. The pyroclastic rocks are poorly indurated and 

poorly exposed, often being involved in small landslides or covered with talus from ash- flow sheets in 

overlying members . 

At the base of the member is a coarse- rained, 

well bedded pumice lapilli tuff up to 3 meters thick . 

Overlying this is a 50- to 70-meter-thick ection of 

brick-red, poorly bedded, tuffaceous claystone. The 

claystone is follow d upward by predominately yellow, 

gray, and green altered tuffs and lapilli tuffs. 

In Antelope Valle numerous alkali-olivine basalt 

flows are interbedd d with tuffs and tuffaceous claystones in the lower part of the member (Robinson, 

1969). Individual flows are typically 5-10 meters 

thick and extend along strik for up to 20 km . Outcrops are generally low, rounded hills mantled with 

Small chips of weathered, yellow-hrown basalt . 

The basalts are usually holocrystalline, mediumto coarse-grained , and aphyric . They contain 10-15 

33 

percent olivine , mostly altered to smectite, abundant 

titaniferous augite, plagioclase, ilmenite, and minor 

interstitial alkali feldspar . Local pegmatitic segregations consist chiefly of titanaugite, ilmenite, 

and plagioclase . These highly titaniferous and 

alkalic basalts are chemically distinct from lava 

flows in the Clarno Formation, the Columbia River 

Basalt Group, and younger basalts in north-central 

Oregon (Robinson, 1969) 

Member G This member consist of a basal ash-flow 

sheet, about 15 meters thick, overlain by 30 to 120 

meters of poorly bedded, poorly indurated tuff and 

lapilli tuff . The basal ash- flow sheet is typical! 

reddish-brown, densely welded, and completely cr stalized, forming resistant ridge-like outcrops. It 

is characterized by abundant large phenocrysts of 

soda-rich sanidine mantled by myrmekitic intergrowths 

of quartz and feldspar . It crops out chiefly in the 

Antelope-Ashwood-Willowdale area but a similar tuff 

occurs in the Mutton Mountains about 20 km west of 

Willowdale and an air-fall equivalent occurs in the 

eastern facies. 

The ash-flow sheet shows systematic variations in 

thickness, degree of welding, and crystallization 

from south to north . In the vicinity of Hay Creek 

Ranch, east of Madras , the tuff ranges from about 15 

to 30 meters thick and is densely welded, completely 

devitrified, and moderately lithophysal. Twenty 

kilometers to the north along Oregon Highway 206 

between Willowdale and Antelope, the tuff is 5-6 

meters thick, moderately welded, and completely 

glassy . Farther to the northeast, between Antelope 

and Clarno, an air-fall tuff containing similar 

large crystals of soda-rich sanidine mantled with 

myrmekite occurs in the same stratigraphic position. 

In this area the air-fall tuff is usually about 1 

meter thick but, where extensively reworked it may 

attain a thickness up to 10 meters . This air-fall 

tuff has been correlated with a similar tuff in the 

Painted Hills area east of the Blue Mountains (Hay, 

1963; Woodburne and Robinson, 1977) and is believed 

to provide a tie between the eastern and w~stern 

facies . 

The tuffs and lapilli tuffs above the basal ashflow sheet are light gray to light brownish-gray , 

poorly bedded, and moderately to weakly indurated . 

Outcrops are poorly exposed, often being covered with 

talus from overlying ash-flow sheets. At the base, 

there is usually a 2- to 10- meter-thick sequence .{)f 

lithic lapilli tuff, overlain by 20 to 30 meters 

of light brown, fine-grained tuff with about 5% white 

pumice lapilli, which ive the rock a characteristic 

spotted apppearance . Locally , this is overlain by 

5-6 meters of poorly bedded, light gray, fine-grained 

ash. At the top of the section is a sequence of 

poorly bedded, poorly indurated, light brown to light· 

gray tuff, up to 50 meters thick . 

Most of the tuffs and lapilli tuffs are crystal 

poor, containing 2-5% pla iocase feldspar and traces 

of altered ferromagnesian minerals. Quartz and 

sanidine are rare to absent . Most specimens are 

diagenetically altered, containing abundant zeolite, 

chiefly clinoptilolite . 

Member H Member H consists of a 5- to iS-meterthick basal ash-flow sheet overlain by about 30 to 40 

meters of weakly indurated tuff and lapilli tuff . The 
basal ash-flow sheet is ve r y wiclespr acl ·roppin~ out 

almost continuously f rom th vicinit of Junipe r 

Butt to the a rea northeast of Cla rno (Fi~ur 1) . 

Generally, the tuff b comes thinn r and less d nsel 

we l ded to t he north and northeast . It is typically 

yellowish- to redd ish-brown, v ry fine- gr ainecl , and 

very sparsely phyric . Crystals of sodic pla~io·las 

and quartz are present in trace amounts . At many 

localities t he r e is a light gray , ~lass layer at th 

base but most of the unit is devitrified . Out·rops 

east and south of Madras contain numerous lithophysal 

cavities but such features a r e sparse elsewhere . 

The overlying tuffs and lapilli tuff are poorly 

exposed, forming low, talus-mantled outcrops . They 

consist chiefly of light brown to huff, very poorly 

bedded , fine-grained tuff . Most rocks contain S- 10% 

of small white pumice lapilli producing a distin t 

speckled appearance . Outcrops also contain thin 

layers of light gray, slight ly more resi tant tuff up 

to O. S meters thick . Crystals of pla~ioclase comprise 

1-2% uf most tuffs and small lithic fragments are . 

sometimes present . 

Member I -A basal ash-flow sheet, 20 to SO meters 

thick, and an overlying 200- to 300-meter-thick 

sequence of tuff crops out extensively in the west ern 

facies from the area south of Madras to the vicinit y 

of Fossil . Thicknesses var y considerably clue to 

erosion at the top of the formation . 

The basal ash-flow sheet is thickest and most 

densely welded in the area between Madras and 

Grizzly . Here it is consists of SO to 60 meters of 

densely welded tuff with a thick, black vitrophyre at 

the base . It thins and becomes less densely welded 

to the north, being largely unwelded near Willowdale . 

the tuff is coarse-grained, containing large flattened or irregular pumice fragments and pieces of 

black obsidian . Small oligoclase crystals comprise 

about 1% and rock frgments, chiefly andesite and rare 

shist, make up 1-2 %. 

The overlying tuffs are mostly light ~ray t o 

yellowish-gray, poorly bedded, and massive, but a few 

layers are thinly bedded and cross-bedded . Outcrops 

are steep, even slopes often mantled with ta l us from 

the overlying Columbia River Rasalt . The tuffs contain 1-2% crystals, chiefly andesine, sparse lithic 

fragments, and sometimes small pieces of black, 

vesicular glass. Tuffs in the lower part of the 

member are usually zeolitized but those in the upper 

SO to 100 meters are still glassy but hyclrated. These 

tuffs often contain well preserved vertebrate fossils 

(Woodburne and Robinson, 1977; Dingus, 1979). 

Exposures of the John Day Formation between 

Fossil and Lonerock cannot be correlated directly 

with the facies but most of these rocks consist of 

fine- grained tuff similar to that in the upper pa rts 

of members H and I. At the base of the sequence is a 

c ream-colored to gray, relatively coarse-grained, 

poorly sorted, very poorly bedded, pumice lapilli 

tuff. The tuff has a maximum thickness of about 2SO 

meters directly north of Fossil and t hins to the 

northeast and southwest . No welding is observed in 

t~is unit but the poor sorting ancl lack of bedding 

suggest an ash- flow origin . 

34 

In th vi ·inity of Fossil s v r al la va flows are 

int rb dd d with th basal uff. W t and no rt h of 

Fossil is a r ddish-brown rhyodacit (Robinson, 

1969) , S to 10m te rs thi k, with rud platy j ointing . Farth r ast at 1 ast two flows of alkaliolivin -basalt occur a t the same strati~raph c leve , 

This low r s quenc i lo a lly ove rlain by a 

thin, discontinuous weld cl tuff , possibly o rrela tive 

with th ba al sh-flow sh t of m mb r H. The t uf 

mod rat ly t o d nsely 

w lded with a weakly dev loped utaxitic s tructure , 

A thin lay r of light gra gla s o • urs a t the base 

of the tuff, pa ssing upward into a w kly w lded, 

r ddish-gray, stony rock. Spa rs cry tals a re 

chiefly oligo las • 

Abov th weld d tuff is a 200-to 300-m te r-t hic 

sequence of fine-grain d, moderately indurated, 

poorly b dd cl tuff imilar to that in th upper par 

of members H ancl I . The uppermost tuffs ontain 

small bla ·k f r agments of fr sh v si ular glass; ho~ 

near the base are diagen ti ally alter d and zeolitized . 

Co rrelation Of Easte rn And W 

The w stern faci s of th John Day Formation 

diffe r s from the eastern fa ies in having generally 

coa rser- rained ai r-fall tuffs and tuffaceous claysto nes and more abundant lava flows and a h-flow 

sheets. These differences ar believ d to reflect a 

western source for most of the pyroclasti c mate r ial 

and the pres nc of topographic barr! r along the 

Blue Mountain axis through most of John Day time . 

Two specific corr elations between the easte rn aM 

western facies are proposed (Figure 2) . A small 

outcrop of ash- flow tuff at th base of the eastern 

facies along Rowe Cr ek (Hay , 1963 ; Rob i n on , 1975) 

is correlated with the basa l ash-flow sheet of member 

A in the western facies (Swanson and Robinson , 1968; 

Woodburne and Robinson, 1977). This orrelation is 

based la r gely on pheno ·ryst mineral ogy, in.c luding the 

presence of a distinctive barium-rich sanidine 

(Woodburne and Robinson, 1977) . If this correlation 

is co rrect it implies that uplift along this part of 

the Rlue Mountains took plac after about 36 m. y. 

A distinctive rhyolitic air-fall tuff in the 

ea tern facie (Hay, 1963) i s o r e l a t- wi h the 

basal ash-fl ow tuff of member G and its air-fall 

equivalent in the western facies. The s e tuffs a re 

mineralogically similar, having abundant phenoc r ysts, 

1-4 mm in diameter, of distinctive soda-rich sa nidiM 

mantled with myrmekite . This correlation sugge sts 

that the bulk of t he western facies is equiva l ent to 

about the lower 7S meters of the eastern facies. 

If thesP. co rrelations are correct, the Pict ur e 

Go rge Ignimbrite has approximately the same s ra tigraphic posi t ion as t he basal ash-flow sheet of 

member H of the western facies but these appear to bt 

different units based on texture and mine r alogy 

(Fisher, 1966) . The general absence of ash-fl ow 

sheets in the eastern facies and thickness variations 

within the formation (Fisher, 1967) imply tha the 

Blue Mountains fo rmed a topographic ba rrier t hroughout middle and late John Day time . 
Table 1 

odal Analyses of John nay Rocks 

2 

Groundmass 92 . 7a 97 . 6b 

Rock Fragments 4.0 0 .7 

Sanidine tr 

Quartz tr 0 .3 

Plagioclase 2. 3 l.4 

(An - Content) 35-40 35 

Clinopyroxene 

Hornblende 

Biotite tr 

Opaques 0 . 2 tr 

Zircon tr tr 

Apatite tr tr 

3 

98 . 4 

0.4 

tr 

1. 0 

10- 20 

tr 

tr 

0. 1 

t r 

tr 

4 

94 .9 

l.1 

3. 2 

0. 8 

20-25 

tr 

tr 

tr 

5 

84 . 4 

3.6 

10 .1 

l.6 

0. 2 

tr 

tr 

l. 

2. 

3. 

4. 

5. 

Tuffaceous claystone an~ siltsto f " ne rom western facies (average of 648- 207 , 2 9 Vitric tuff from w stern facies (648-S7-6) -S- ' and -S2-l 1). 

Picture Go rge Ignimbrite , lower coolin · ( 6 g un1t average of 48-4 58a, - 458b , and -4 58c) Basal ash-flow tuff of memher A (Swanson ancl Robinson, 1968) 

Basal ash-flow tuff of member r. (average of 9 specimens) 

a) includes 7. 2 percent pumice fragments larger than 0 . 5 mm 

b) includes 4 . 7 percent pumice fragments la r ge r than 0. 5 mm 

- = not recorc!ed 

CHEMICAL COMPOSITION OF THE JOH DAY FORMATIO 

Rocks of the John Day Formation form t hree distinct mineralogical and chemical groups. Tuffaceous 

claystones and vitric tuffs are chiefly andesitic to 

dacitic, ash-flow tuffs and minor silicic lava flows 

are r hyolitic , and mafic lava flows are alkaliolivine basalt or trachyandesite . 

Tuffaceous claystone and vitric tuff - The original 

composition of the tuffaceous claystone and vitric 

tuff cannot be determined directly by chemical 

analysis . Much of the original glassy mate rial in 

these rocks has been replaced by montmorillonite or 

cl inoptilolite during weathering before burial or by 

later diagenetic alteration. Chemical analyses of 

altered rocks reported by Hay (1963) indicate extreme 

hydration accompanied by leaching of a20 . Chemical 

analsyes of glassy material from the upper pa rt of 

the formation a re equally misleading because of 

extreme etchin , bleaching, and hydration of the 

glass. The ini ial composition of the tuffaceous 

clayston and vitric tuff must therefore be inferred 

from the pyrog nic min ralogy . Crystals in these 

rocks are chi efly andesine feldspar with lesser 

amounts of labradorite, magneti te, py roxene, hornblende , biotite, and ilmenite suggesting that the 

bulk of the material was originally andesitic t o 

dacitic in composition (Table 1; Hay , 1962a; 1963; 

Fishe r and Ren sbe rger, 1972) . Less abundant 

oligoclase-bearing tuff and tuffaceous claystone was 

probably rhyodacitic and a few tuffs in the lower 

pa rt of the forma tion cont aining quartz , sanidine, 

and oligoclase were probably rh olitic in compostion . 

These i nferred compositions are supported by the 

presence of accessory andesit ic, dacitic, a nd 

rhyolitic fragments in many of the samples. 

35 

Silicic ash-flow tuff Ash-flow sheets in both the 

eastern and western facies are rhyolitic, based on 

phenocryst ·mineralogy and whole-rock chemical 

analyses . Phenocrysts are primarily oligoclase, 

sanidine, and quartz with only traces of ferromagnesian minerals (Table 1) . Chemical analyses 

indicate gene ra lly silicic compositions (Tables 2 and 

3), howeve r, the tuffs show very wide compositional 

variations. These perplexing chemical variations 

within individual ash-flow sheets have been noted by 

previous workers (Hay, 1963; Swanson and Robinson, 

1968) . 

umerous chemical analyses of the Picture Gorge 

Ignimbrite and the basal ash-flow sheet of member H 

indicate a consistent chemical alteration pattern 

that is dependent on the porosity and crystallinity 

of the tuff (Table 2) . Very densely welded, 

devitrified zones with little or no observed pri ary 

porosi ty are interpreted to be the least altered 

parts of the ash-flow sheets. Within these zones 

analyses are consistent, alkali elements are subequal 

to each other , water contents are consistently low, 

and petrographi c characteristics do not suggest 

dissolution of glass or deposition of secondary 

minerals. Other zones exhibit increasingly severe 

effects of alteration . Glassy zones are hydrated as 

indicated by water contents or volatile losses of 3 

to 6 weight percent . Typically, hydrated vitrophyres 

contain less Si02, Ti02, and a20 and more 

Fe203, CaO , and K20 than devitrified, densely 

welded tuffs. Alumina and 1g0 have no consistent 

pattern of loss or gain . With minor exceptions such 

as Fe2 03, the chemical differences between 

hydrated vitrophyres and devitrified, densely welded 

tuffs are similar to those reported for evada Test 

Site welded tuffs (Lipman, 1965). 
Devitrified , mod rately to w akly w ld d or 

unwelded tuffs display different ch mical changes 

that become increasingly evere as th porosity 

increa es (Table 2; unpublished data). T pi all , 

Si02 increases and Ti02 , Al2 03 , fgO, CaO, and 

a 20 all decrease with increasing porosity of the 

sample . Both iron and potassium are somewhat high r 

in devitrified, moderately welded tuffs hut d cr a 

in more thorough! silicified unwelded portions of 

the devitrified ash-flow tuffs. 

Chemical changes in devitrified samples an b 

partially correlated with petrographic f atures. Th 

loss of many elements is consistent with observ d 

leaching of pumice lapilli and phenocrysts from 

altered specimens. Iron cont nt i largely controlled by secondary i r on oxides that stain the rocks 

brick red or form liesegang banding. Very high 

potassium samples have minute euhedra of potassiumrich feldspa r in the groundmass or in pumice lapilli, 

and very high silica rocks contain abundant prismatic 

quartz crystals in the groundmass and po r e spaces . 

The ash-flow tuffs are believed to have been 

altered by the same diagenetic event that affected 

the air-fall tuffs and tuffaceous claystones (Hay 

1962a; 1963). The very different alteration of airfall and ash-flow tuffs is due largely to differences 

in texture. Densely welded non-porous vitrophyres 

exhibit chemical alteration typical of leaching by 

groundwater during hydration, such as that reported 

by Lipman (1965). The chemical effects are generally 

small, as might be expected for rocks with no primary 

porosity and little induced porosity. 

Porous glassy material from unwelded basal zones 

of ash- flow sheets and enclosing tuffaceous claystones and vitric tuffs exhibit severe chemical 

changes induced by large volumes of groundwater acting on large surface areas. The low porosity and 

permeability of the vitrophyres precluded significant chemical reaction with the groundwater except 

possibly for hydration and minor leaching. Where cut 

by fractures that increased permeability and 

porosity, the vitrophyres have thin (1-5 em), zeolitic alteration halos along the cracks (Hay, 1963). 

Porous, partially welded to unwelded zones in the 

ash-flow tuffs must have been devitrified prior to 

diagenetic alteration; otherwise, secondary minerals 

similar to those in surrounding claystones and tuffs 

would be expected . Devitrified tuffs contain alkali 

feldspar and quartz in the groundmass Whereas claystone and vitric tuff contain montmorillonite, 

celadonite, or clinoptilolite. The difference in 

secondary mine ra logy of the welded tuffs an~ surrounding claystones is probably not due to differences in initial rock compositons because Ha (1963) 

reported zeolitic alteration of silicic as well as 

intermedia te composition tuff. 

The alteration of upper partially welded to unwelded zones is probably unrelated to vapor phase 

crystallization in the ash flows. This is inferred 

because typical vapor phase minerals such as tridymite are not commonly observed and qua r tz is the 

most abundant silica mineral . Minor chemical changes 

may have resulted from vapor phase activity, but 

these were overwhelmed by subsequent diagenetic 

alteration . 

36 

As uming tha ou r mod 1 of consist nt ch mical 

hang as a fun tion of sampl porosi y and crystaG 

linit is ·orr t, w can infer th original magma 

composition of alt r d unit by compa ring rocks of 

imilar p trographic hara ter . For xampl , a 

rhyolit compo iton is inf rr d for th ash-flow 

h et of m mb r E, whi h has no known fr sh material, 

by ·omparing its omposition to p trographically 

similar alt r d mat rial from th ld d uff of 

m mb r H or from th Pi tur Gorg Ign imbri t (Table 

2) . 

Chemi al analys of al t r d sampl s f r om other 

units sugg st that all of the ash-flow tuffs in the 

John Day Formation wer originally rh oli in 

composition (Tabl 3) . C rtainl minor h mical 

differences betw heets must hav been 

initially pr s nt; how v r, th s ar masked by 

diag nP.tic alt ration eff ts . In gen ral, th ashflow tuffs app ar to hav mod rately al kal i 

rhyolite with sili a contents on th ord r of 75-76%. 

Th se rocks app a r to b ch mically di tin t from 

most of the vitric tuffs and tuffa ous claystones 

which mak up the bulk of th formation . 

Mafic lavas - Although the ro ks are som wha 

alter d r liable chemical analys s hav b n obtained 

for most of th mafic lava flows in th formation 

(Table 4) . Memb r B of th western faci s is a 

trachyand site with mod rat Si02 content . Equivalent rocks are not known in the astern facie outcrops . Basalt flows in the west rn facies are all 

low silica , silica-und rsaturat d alkali-olivine 

basalt with high titania . Minor basalt flow in the 

lower part of th eastern facies hav imilar compositions. 

The trachyandesites and alkali olivin basalts 

probably represent eparate magma puls s, unr lated 

to each other or to the silicic rocks . They form 

distinct chem~cal groups and rocks of intermediate 

composition are not known . In addition, th high 

FeO/MgO ratios and Fe203 contents of th alkaliolivine basalts indicate an evolution along high-Fe 

tholeiitic trends rather than along high silica lines 

toward trachyandesite compositions. 

ORIGI OF THE JOH DAY FORHATIO 

Tuffs, lapilli tuffs, and tuffaceous claystones 

in the western, southern , and eastern facies are 

texturally similar to rocks that represent accumulation of ash falls on the land surface (Hay, 1962a; 

1963; Fisher, 1966b). These rocks become progressively thick r and coarser-grained from east to west 

(Waters, 1954; Peck, 1964; Robinson, 1975), suggesting that the source volcanoes lay to the west of the 

present outcrops . The source volcanoes were largely 

andesitic to dacitic in composition based on pyrogenic mineralogy . o andesitic or dacitic volcanoes 

of John Day age are known in the John Day basin, east 

of the present day Cascades . Previously suggested 

vents in the Horse Heaven Mining District (Waters and 

others, 1951) and at Smith Rock (Williams, 1957) are 

unconformably overlain by John Day rocks (Swanson and 

Robinson, 1968; Robinson and Stensland, 1979) and are 

here considered to be Clarno in age. The absence of 

andesitic or dacitic vents in the John Day outcrop 

area, the evidence for a western source, and the 

similarity of these rocks to younger units sugge t 

that the source volcanoes were located beneath the 

present day Cascade Range. This suggests that 
w 

-.J 

able 2 

Chemical Com1)osition of Fresh and Alte r ed \.,Te lded Tuff 

el'lhe r F. Membe r H 

2 3 4 5 6 7 8 

Si02 76 . 37 77 . 51 83 . 23 75.94 77 . 5 76 .9 80 . 83 75 . 31 

Ti02 0 . 21 0. 21 0. 17 0. 26 0. 17 0 . 16 0. 17 0 . 39 

Al 2o3 12 . 03 11 . 39 8. 74 13 . 07 12 . 2 11.61 11 . 33 13 . 08 

Pe 2o3 2. 13 2. 20 l. 21 1. 20 2. 28 l. 79 0. 92 2. 22 

nO - - - 0. 02 0. 09 0. 03 0. 0 1 tr 

MgO 0 . 11 0 . 06 0. 02 0. 07 0. 35 0 . 03 0. 04 0. 25 

CaO 1. 00 0. 61 0. 31 0. 50 0. 65 o. 18 0. 21 0. 68 

a 2o 2. 29 2. 92 I. 43 4. 41 3. 1 3. 33 2. 96 3.98 

K20 5. 09 4. 47 4. 77 4. 19 3. 7 5. 08 3. 38 3. 95 

r2o5 - - - 0. 02 0. 01 0 . 03 0. 02 0 . 10 

Total 99 . 23 99 . 37 99 . 88 99.66 99 . 98 99 . 14 99 . 85 99 . 96 

LOI 3. 74 1. 02 l. 12 0. 86 6. 0+ 0.4 2 0. 64 1.00 

1. 648- 86 , 648- Sl - 3 , 648- S7 -1 0 , anci DL P 58- 50 (Peck, 1964): Hydrated glass. 

2. 648-41, 648 - 54- 4, PTR-71-4b : Oev i trified, moderately welcied . 

3. 648-S1-4 : De vitr ifieci , weak l y weldeci. 

4. 648 - 129A and 648-1 89 : Oevi trified , very ciensely welcied 

5. DLP 58- 39A f rom Peck (1964): llydrated gl a ss . 

6. 648- 311 : Oevitrified , mocie r a tely weldecl . 

7. 648- S4 - 10 and PTR 71-Sb : Oevitrified , weakly weldecl . 

8. 11 3 Hay (1963) : Oe vitrified , ve r y rlensely weldecl, fresh . 

9. 62 - 137 and 62 - 376 from Fishe r (l966a); 112 Hay (1963); 648- 4558: Hydrated glass. 

10 . 1/ 4 Hay (19 63 ) a nd 648-4 58A : De vitri fie cl, moderately welded . 

11 . 648-4 55C , 648- 4550 ancl 648-4 55F. : Oevitrified , unwelded . 

not cletectecl . 

Picture Go ~e Ignimbrite 

9 10 11 

74 . 07 76.39 82 . l 5 

0 . 25 0 . 35 0 . /.4 

13. 08 12 . 46 9.22 

3. 23 1. 55 1. 03 

0. 04 0. 02 0. 03 

0 . 19 0. 10 0. 04 

l. 28 0 . 59 0. 55 

3. 60 3. 31 2. 41 

4.28 5.14 4. 03 

0 . 10 0. 14 0. 05 

100 . 11 100 . 05 99 . 73 

4. 03 l. 04 0. 90 
Ta hl 

Ch mi al Analvsis o 1. ;t St Alt r d John Oa Formil ion A h-Flow Tuffs 

1 mh r A 1 mhf>r I. 1 mh r F. 1 mh r \. nh r H mh r l Picture 

Gorge 

Low r Tuff Upp r Tuff Ignil'lhri te 

Basal Upper 

Coolin~ Cooling 

Unit Unit 

1 2 3 5 0 7 R 

Si 02 74 . 68 73 . 99 7 ') . 17 

Ti02 0 . 25 0 . 61 0. 20 

Al 203 12. 5 7 12 . 78 13 . 0 

Fe20 3* 2. 9 2. 11 1.77 

HnO 0 . 03 0 . 21 

74 . 51 

0 . 25 

12. RO 

2. 61) 

0 . 06 

7 6. 0 76 . 64 74 . 06 7 5. 94 

0. 2C) n. tR o. 7 o. n 

12 . 5 l l. 31 12 . 4 5 11 . 07 

2. 17 2. 69 • 41 l. ?O 

0 . 12 0. 03 0 . 02 

1g0 0. 23 o. 12 0 . 04 0 . 06 0 . 09 n . s 4 0. 07 n. 19 

CaO 0. 65 l. 01 o. 9 0 . 46 0 . 76 [.()C) 0 . 50 0 . 88 

Na20 3. 6 2. 16 2. 7 

K20 4. 38 6. 49 6. 08 

P2o5 0 . 20 

2. 44 

5. 70 

3. 1 2. 74 3. 64 4 . 41 

5 . I 4. 71 4 . 37 4 . l q 

0 . 02 0 . II o. 02 

To ta l 99. 26 99 . 30 100 . 14 100 . 12 99 . I 0 99 . Q9 99 . o6 99 . 46 

LOI 0 . 62 0. 71 3. 22 l. 10 l. 54 l. 81 0 . 86 3. 46 

*To t al iron as Fe203 

Sample Identification 

1 . 648- la: pa r tially devitrified , rlensel w ld d tuff 

2. 648 - lb: devitrified welded tuff with lea hed out pumic e lapilli. a nd minut qu rt z a nd alkali f lcls par 

crystals in groundmass 

3. 648- 4a : hyd r.ated , clensely welderl tuff 

7 5. 31 

o. 39 

13 .0R 

2. 22 

tr 

0. 25 

0.68 

3. 9 

3. 95 

. to 

99 . 96 

1. 00 

4. 648- 27 and 648 - S1 - 1: crystallized rhyolite flow with 1 ach c1 ph nocrysts and quartz a nd lka li f l ds par 

in gr oundmass 

5 . 648- 41 : devitrified , densely weldecl, lithoph sal tuff wi.th c rystal-lind caviti s; no hin e tion 

6. 648- 34 : glassy , pa r tially hydrated vitrophyre 

7. 648- 129A and 648- 1R9 : devitrified , ver densely weldecl tuff with sph rulit s or v r y fin grain d qua rt z a~ 

alkali felclspar 

8 . 648- 5 , 1)48- 245 , 648- 52- 12 : hydratecl moderat to clensel welded tu f f with p rliti texture 

9. Ana l ysis 3, Hay (19o3) : devitrified, very densel weldecl tu f f 

Table 4 

Average l.hemical Composition of Mafic Lava Flow 

Mel'lber R Memhe rs F. and F Low r Me mber 

I.Je t rn Facies F.a s e rn Fa c i es 

Si02 

Ti02 

Al203 

Fe203 

MnO 

MgO 

CaO 

Na 20 

K20 

P2o5 

55 . 98 

2 . 14 

13 . 79 

13 . 84 

0 . 21 

2. 13 

o.26 

3. 31 

l. 7 5 

0 . 60 

100 . 00 

l. 41 

46 . 78 

. 49 

15 . 21 

16 . 0 1 

0 . 21 

5. 17 

. 79 

3. 0 1 

0 . 78 

0. 55 

TOO. oo 

l. l R 

otes : Member R: average of three analyses from Robinson (1969) 

!embers F. and F: average of 11 analyses in western facies from Robin on (1969) 

Lower Member : ave r age of 2 analyses , in eastern facies (Robinson, 196Q) . Out c rops 

are area below Picture Gorge Ignimbrite; however , corrrela tion t o 

western facies members is uncertain 

All samples recalculated water free, all iron as Fe203 and totaled to 100% 

38 

46 . 0 5 

4.02 

I 5. 72 

Jo.88 

0 . 15 

3. 73 

7. 32 

3. RO 

1. 65 

O. n8 

100 . 00 

l. 98 
ad volc tnism 

the earli s phn 

ila r t o l<t 

m. v . go , nnct hat 

volcani srr1 w r e C'OI'lpnsi ionally 

The ash- flO\-' tu fs of he John Day F'ormation a r 

all rhyoli ic in compos ition , ctistinc ly differ nt 

from mo· of h air- fa ll tuff anrl t u fa ous 

~ost of th ash- flow tuffs o cur in t h 

s and la r a1 va r in on~ in thickn s , 

ah uncta n e of 

and 

a 1 

ost 

erupted f rom v nts along th 

pre sen da Cascade Rang and o have 

no rtheas into h ohn Day Rasin . 

re tric ed to the w t rn faci becaus 

ct r iv d fr oM 

in ag 

a r 

!o~: ntain uplif formed a orog r aphic barri r be we n 

he easte rn nd west rn facies hrough !'lOSt of John 

ay time (Robinson , 197 5) . Th Pi tur e \.orge 

Ignimbrite in th ast e rn faci s can no b cor r la ct 

ith any of he ash- flow sh e t s in h western faci s 

and is believed t o ha ve been erup t d f r om an unknown 

vent in he Ochoco l!oun t ins (F'i he r, 196oa) . Th se 

il icic volcano s pr obably al o acco unt fo r h 

pa rse rhyoli ic ai r - fall tuff and u fac ous claystones in the low r pa rt of the fo rmntion . 

The alkali- ol ivin basal and trachyandesit 

lows in the low r pa r of the o rmation a r mos 

a undant in he w s e rn faci s in th An elop -

Ashwo od- Wi llowd a le a r ea , hut hey also occur as of 

t~ Blue ountain u plift . These ar mostl mall , 

areall restric d flows , clea r l of local origin . 

.catte r ed dikes a nd small accul'lula ions of cinde rs 

probably ma r k v n s f r om which som o th s flows 

·e re er upt d (Swanson , l96q; Robinson , 197 5) . 

1ARY 

Our mod l fo r th volution of vol 

no rth r n Or gon i. s follow : nuri ng 

ande i i c volcAnism of Cla rno li holog 

spread in nor h-e n r al Or on , probabl 

ven c; a l ong h<> nor h ast- t r nding Rlu 

upli ft (Swan s on , 1969) . Tn h g n ral 

0 

h 

39 

pres n da Cascad s , marin sedimen w r hein 

deposi ed along wi h minor nsa l ic r ocks (P ck and 

o hers , 1 64) . Sometime h we n 40 anct 3n m. y . ago , 

Clarno vol ·anism rl c r a d in i n nsity , pr obably 

nding nti r el b 36 m. y . Although the age of h 

uppermo st Clarno rock i not known exa tl , no cal 

alkalin ande i ic ro·ks of Cla rno li holog a r e 

known in th John Day Fo rma ion . 

Initia ion of John Da vol canis about 36 m. y . 

ago s ignified t he eme rgen· e of a new vol ano- t ec oni· 

reg ime in no rth rn Or ego n. \olca noes in the vi i ni t 

of the pr e ent day Cascades began ru pting andesi tic 

t o dacitic pyrocla stic mate r ial tha t was deposited in 

the John Day basin a ash falls . This mate r ial r pr e sents the ea rliest documented volcanic ac tivit 

along the r.as cad r nd . Simul t aneously , vents ast 

of the High Ca scades , between the Cascade Rang an d 

the Blue lountain uplift , we r e e rupting rh eli t e 

ash- f low tuffs , lava flows , and mi nor air-fall tuffs . 

ost of t he ash- flow tu ffs were de rived fr om vents 

we t of the pr esent outcrops , whereas lava flows a nd 

dom s r epre s nt local e ru ptions wit hin th John Oa 

basin . Other vents , mo tl ea st of the major 

r hyoli t ic volcanoes , erupted alkaline ba salt and 

trachyandesite . 

This pat t e rn of volcanism continued until approximately 25 m. y . ago , at whi ch time th r h o litic , 

basal tic , and tr ach andesitic ru ptions ceased . 

And esitic t o dacit i c eruptions con in ued in the 

Cascade Range leading to deposi ion o f th upper pa rt 

of the John Day Fo rmation . 

Deposit i on of the John Day Formation ceased about 

18- 20 m. y . ago (\Joodburne and Robins on , 19 77) coinc ident wit h a probab le hiat us in Ca sc ade volcanism 

( cBirney and o t hers , 1974) . Following a short 

per i od of fol ding and e r osio n , th Columbia River 

Rasalt was e rup ted from vents in the ea tern part of 

the John Day basin beg inn ing about 1o m •• ago 

(Watkins a nd Baksi , 1974 ; M Kee , S1•anson , and Wright , 

1977) . Re newed volcanism in the Ca scade Range is 

reflected in tuffaceou interbed s i n the Columb ia 

River Basalt and in post - Col umb ia River dep sits such 

as t he Hascall a nd 1adras Fo rmations . 

Th r egiona l ignificance of the patte r n of 

Oligocene a nd earl ocene volcanism outlined a bove 

i s not clea r . The different magmas may reflect 

lateral variation in depth of magma ge ne ration , in 

c rus t al thickness or composition , or in tectoni c 

en vironment . 

REF ERE CES CITED 

Col man , R. G. , 1949 The John Day Format1on in the 

Pictur Go r g Quadrangle , Or ego n : I npub . • S. 

th sis , Or e on State niv ., Corvallis, 2 11 p . 

Brown , C. E. and Thaye r, T. P., 1966 , Geologic map of 

the Can on ci ty quadrangle , northeastern Oregon: 

• , . G ol . Survey isc . Geol . Inv . Ma p I - 447 . 

Dingus , L. w., 1979 , The \Ja rm Springs Fa una (mammalia 

Hemingfordian) f rom the western f acies of the Joh n 

Day Formation , Or egon : npub . M. S. thesis , Univ . 

Ca lif. , Riverside . 

Eve rnd n , J . F., Savag , n. E. , Cu rtis, G. H. and 

Jam s , G. T., 1 64 , Pota siuM- a r gon dates and 

th Cenozoi mammalian hr onolog of orth 

Ameri a : Amer . Jour . S i., v . 2n2 , p . 145-1911 . 
Fishe r, R. 196 a , \.eology of a lio n i~nimhri C' 

l a er , John Da Formation , a tern Or on: C:alif. 

Univ . Pub • G ol. Sci. v. 67 , 59 p . 

Fishe r, R. \,, 196nh , Text ural comparison of John Oa 

volcanic i lt ton s w1 h lo s and vol anic ash : 

Jour. Sed . Pe t., v . 36 , p . 706 - 718 . 

Fisher, R •. , 1967 , Earl Te rtia r d formation in 

north-c ntral Oregon : Am . A so . Petrol. Geol . 

Bull., v . 51, p . 111-i 23 . 

Fisher , R. V., 1968 , Pyrogeni minerals abilit , 

lower member of the John Day Formation : Univ 

Calif. Pubs . Geol. Sci., v . 75 , 36 p . 

Fisher, R. V. and Rensbe r ge r, J . H., 19 72 , Physi al 

stratigraphy of the John Oay Formation, c ntral 

Oregon : Calif. Univ . Pubs . Geol. Sci., v . 10 1, p. 

1- 45 . 

Fisher , R. V. and Wilcox , R. E. , 1960 , Th John Day 

Formation in the lonument quadrang le, Oregon : 

U. S. Geol . Survey Prof . Paper 400-R, p. 302-304 . 

Hay , R. L., 1962a, Origin and diageneti a lte ration 

of the lower part of the John Day Formation near 

Hitchell , Oregon : Geol. Soc . America , Ruddington 

Hemorial Hemoir, p . 191-216 . 

Hay , R. L., 1962b , Soda-rich sanidine of p r ocla tic 

origin from the J ohn Oa Formation of Oregon : Am . 

Mineral. , v . 4 7, p . 968- 971. 

Hay . R. L., 1963, Stratigraph and zeolitic diagenesis of the Joh n Oay Formation of Or gon : Univ. 

Calif. Pubs . Geol. ci., v. 42, p. 199-26 1. 

Hodge, E. T., 1932, Geologic map of north- ntral 

Or egon : Oregon lniv . Pu bs ., Geology Ser . v . 1, 

7 p . 

Lindsley , D. H., 1960 , Geolo~y of the Spra 

Quadrangle , Oregon , wi th special emphasis on the 

petrography and magnetic pr opertie of h Pi tur 

Go r ge Basalt: npub . Ph. D. thesis, Johns Hopkin 

Univ . Balti mor e , d ., 236 p. 

Lipman , P. W., 1.965, Chemical composition of glass 

and crysta l line volcanic rocks : . S. Ceo . 

Survey Bull . 1201 - D, p . D1-D24 . 

HcBirney, A. R., Sutter, J . F., aslund , H. R. , 

Sutton, K. G. and White , C. H., 1974 , Episodic 

volcanism in the central Or egon Cascade Range: 

Geology, v . 2, p . 585 - 58 9. 

HcKee , E. H., Swanson, D. A., and Wright , T. L., 

1977, Duration and vol ume of Colum ia River Basalt 

volcanism, Washington, Oregon , and Idaho: ol . 

Soc . America Abstracts wit h Pr ogram , v . 9 , no . 4. 

p. 463 - 464 . 

Herriam , J . C., 190 1, A contribution t o the geology 

of the John Day Basin (Oregon) : Calif. niv . 

Pubs. Dept. Geol. Bull. v . 12 , p . 269 - 314 . 

Peck , D. L., 1964 , Geologic re connaissance of the 

Antelope - Ashwood area, no r h- e n r al Or on, wi 

emphasi s on the John Day Formation of late 

Oligocene and ea r ly iocene a e : . S. Geol . 

Survey Bu l l. 1161 - D, p . 01 - 026 . 

Peck, D. L., Gr ig s, A. B., Schlicker, H. G. , Wells , 

F. G. , and Dole , H. M. , 1964 , Geology of the 

central and northern parts of the Western Cascade 

Range in Oregon: . S. Geol. Survey Prof. Paper , 

449 ' 56p . 

Robinson , P. T., 1969, High titania alkali- olivine 

basalt of north- central Or egon , U. S. A.: Contr . 

Mine r al. Petrol. , v . 22 , p. 349- 360 . 

Robinson , P. T., 1975 , Reconnaissance geologic map o f 

the John Day Fomation in the southweste rn part of 

the Blue Mountains and ad j acent areas, northcentral Oregon: U. S. Geol Survey 1ap I- 872 . 

40 

P. T . .qncl S nsl nd , D., 1979, G ologtc lllap 

SJTti h Ro k a r a , Oregon: t . S. G ol. 

Us· . G ol . Inv . Map T- 11 42 . 

19 , Law ni hlu s his from 

nor h-e n ral Or gon : in C:eologi 'al Surv y 

Re r h 1969 , l . S. Geol. Surv Prof. Pap 

650-B , p . 8-11 . 

Swan n D. A., 1969, R 

t he half o f t h 

1-rh J ff r on , co 

U. S. G ol . Surv 

Swa n on , D. A., and Robin on , P. T., 1968 , Base of the 

John Day Fo rmation in and n a r th Hors Heaven 

mining distri ' , north-c n tral Oregon; ..!.!.: G ological Surv y R a r h , 19 9: lJ . S. G ol. Survey 

Pro f . Pap r 600-D , p . 0154-0 161 . 

Wate rs, A. C., 1954 , John Day Formation wes of its 

type localit ( abst.), G ol. Soc . Am r . Bu ll. , v. 

65 , p . 1320 . 

Wat r , A. C., Brown , R. E., Compton , R. R. , Staples, 

L. w., Walk r, G. w., nd \.Jilliams , H. , 195 1, 

Qui ksilv r d posit of the Hor H aven mining 

district Or gon : . ' , Geological Su rv Bull. . 

969- E, pp . 105- 14 . 

\-/atkins, • D. and Bak i , A. K., 1974 , agn tostratigraphy and oro'linal f lding of th 

Columbia Riv r , St n and Owyh e ha alts in 

Or egon , Washington , and Idaho : Am . Jour . . ci., 

v . 274 , p. 148- 189 . 

Williflms , How 1 , 1957 , A g ologi map of h B nd 

quadrangl , Oregon , and a re onnaissan e geologk 

map of th ntral por ion of he Hi~h Cascade 

ountains : Oregon D p. G ol. Hn . Ind . , in 

cooperation with U. S. G ological Survey , Scales 

1 : 125 , 000 and 1: 250 , 000 . 

Woodburn , 1. 0 . and Robinson , P. T. , 1977 , A new 

lat e H mingfordian Mammal fauna f rom th John 

Da Formation, Or gon , and its stratigraphi c 

implication : Jour . Paleo ., v . 51 , p . 750- 757 . 
. ILES 

(0. 35) 

0. 35 

(0 . 40 ) 

0 . 7 

(0 . 60) 

1. 35 

(0 . 05) 

1. 40 

(0 . 55 ) 

l. 95 

( 0 . 75) 

2. 70 

(0. 45) 

3 . 15 

(0. 40) 

3. 55 

(0. 60) 

4 . 15 

(1.15) 

5. 30 

(0 .4 0 ) 

7 . 10 

( 1. 40) 

5 . 7 

(0 . 55) 

7. 65 

1P K EPLY OR·. 0 

.' IT m OH '1' 0 

Paul T. Robinson, n rartl'l nt of Ea r h S·ience, lniv rsitv of la1ifornia 

R i v r i d , C:a 1 i f o r n i a q 2 2 I 

Kil'lb rly . Turn 1 f on High\ av 1 oward Dr~yvill and John nav • 

Cross orth Fork of ohn nay Riv r . 

High 1 vel r diPl nt at 12 o ' clo·k . Thes are oml'lon along h John na River 

b tween Kimberly and Pi·tur Gorg • 

S op 1 (Op ional) . Dike of 

ohn Day Formation . Dik 

It ha fin - grained margin and 

l.org Rasalt cuts uppe r buff bed of the 

about . 30°\ . and is es ntiallv v rtical . 

pl x joint patt rn . Although dik is about 

10 Plet r wide th how littl baking and onl moderate 

induration . 

Stream gravels in roadcut on 1 ft . 

Dik eros es road at this poin • 

Tuffac ous clay ton and tuffs of upp r John Da Formation in roadcut on 

1 f • From here to Pictur Gorg road uts ar Plostly in John Day rocks . 

High 1 vel pedil'l nts on both sid s of ohn na Riv r ahead . 

Pic ure l.o r g dike on hill ide ast of John nay R· ver a t 2 o ' clo k . 

l.r enish tuffs and tuffaceous la ston s of John nay Formation at o ' lock . 

Stop 2 (Optional) . l.r en bed of John Dr~y Format·on ov rl ing Picture Gorge 

Ignimbrite on 1 ft side of road . Th ignimbrite h r is about 10 et r 

thick and exhibit plat jointing near bas with Pla sive , rounded outcrop 

a ove . Ignimbrite is mod rately w lded with utaxitic structure defined b 

elongate pumic fragments . Quartz bip ramids and feldspa r cr tals omprise 

1- 2% and lithic fragment are sparse . Alter d glass chips and fragm nts are 

also present . Ignimbrite is alt red and zeolitized . 

l.ood 

9 

l.orge 

Good 

at 9 

la 

iddl 

xpo ur s of 

o ' lo k . ot 

Ignimbrit 

gr 

k . 

r e 

n uff 

L dg 

is an 

tuf fa ous 1 stone 

ro ion into pinna le 

of John Day Formation 

Ledge is Picture 

and tuffaceous 

a r light! mor 

lay ton 

la ston s of John Da Formation 

resi tant airfall tuffs interohn na b d at 2 o 3 o 'c lo k with Pi·tur Gorg Ignimbrite forming 

41 
~ 

1'\.) 

121° 

4~0 

WARM SPRINGS 

44 ° ( 

-v<v <o"-' 

0 

PA INTED HILL S 

S TATE PARK 

ocH o co 

o 5 10 15 20 MIL ES 

120° 

0 L ON E ROC K 

M OUNT A IN S 

f 

N 

Figure 3 . Index ;~p of no r th- central Oregon showing field trip route anc 

loc~~ian s af gtopn dcocribod in tu~l . 

0 HAM ILTO N 

0 

LO NG C REEK 

§ 

OREGON 
( l • 5() 

. I') 

(() . 5) 

q, S(l 

(O. R5) 

I 0 . 15 

(0. 5 ) 

IO. CJ(l 

( l. 50) 

12 . 40) 

( 0. 50) 

12. CJ O) 

(0 . 9() ) 

1 . R0 

(0 . 60) 

14 . 40 

(0 . 50) 

14 . 90 

( 2. 15) 

17 . 05 

(1 . 60) 

18 . 6 

(0. 20) 

18 . R5 

( 0 . 90 ) 

19. 75 

(0. 45) 

20 . 20 

(1.2 0) 

21.40 

pro in n l cl g • 

rr~vels in ro<'lclcu 

Th i g ni.mhri 

on 1 f • 

is r>A rtlv slumpe d t owC'lrcl <" riv E> r . S r <1 m 

o For Ar <'l of John nav Fossil Reels tionr~l 

g r ion of John nr~y For c1 ion wi h 1 cl g 

on hoth sid e s o f riv r . ot e 

\ iclcll e 

Tgni Mhrit 

k on w t 

Lm r r cl uffa ou ·lav on s of John n<'l Foma tion <'lt CJ 0 ' lock . 

Larg lancl.licle hlo·ks of Pi.·tur Gorg Rasalt on both sicl s of the road . 

Ca h clral Rock c:~t 1 o ' ·1 ·k hows 1 dg of Pictur Gorge Ignimhrit int rhedd d \vith gr n tuff C'lnd tuffaceous la stone of John Day Formation . 

Ahead , dip in Pictur Gorge flows defines ncline plunging south (upriv r) . 

Sh ar cl s rpentinit , pr su ahl of Tria sic age , Rr own and Tha er (1966) , 

1 ft of r oad . 

F.n r<'ln -

of Turtl 

to Turtl Cover Area , John Day Fossil Red 

Cove r embe r of John Da Formation . 

a tional Park . T pe ar a 

Rriclge aero s r iv r . F.nter Pi ·ture Gorge Rasalt , brought clown b plunge of 

syncline . 

Fault , south sicl up , hrings Cr taceous conglom r ates into contact with 

Pictu r Gorg Ra alt . Conglo e r at exposed on both icies of road at Goose 

Rock 0 . mil s to the outh . 

Stop 3 . p Ro k on ast id of rive r. Complete section of John Da 

Fo rmation eel he r • R cl heds on 1 ft (to the north) a r e the Rig Rasin 

on Cr tac ous Ov rlying green tuffs and 

ston s mak up m r, cha r acterized h ext nsiv 

alt ration and z olitization . Gorge Igni brit occur in th upper part of th Tur tle Cov r ember . The uff to tan h ds in the upp r part 

of th formation comprise the Kimb rly Mh r , chara · te r ized b unze litized 

tuffs and tuffac ous lay tone • This is ov r lain by two flows of Pi tur 

Gorg Ra alt . ote small normal fault offsetting Pictu r e Gorge Ignim r ' te , 

downd r opp d to the no r th . 

Ent r ance to Pi tur e Gorge . Location of t pe section of the basal t. Th 

gorg is not named fo r th graffi t i , orne of it g ologic in the fo r m of 

painted number inco rr tly designating the numbe r of flows . Lowest xpos d 

flow in roaclcut on right . 

Inte r s ction with . S. 26 . Continu straight ahec:ld th r ough Gorg • 

low on a 

ote many 

W 11 xpos d flow on a·t on right . 

L av org , with tuffa ou dim ntary rocks of ioc ne 1a call Formation 

ov rl ing Pi ur Gorg Ra al 

Turn righ sha rpl onto C:r k Roaci . Pro d 0 . 1 mile , tur n right on dirt 

ra k and ontinu for hundr d f to overlook . Stop 4. Dip slop 

of Pi ur Gorg Ra al no r h pr oj ts h n arth valley of John Oa 

Riv r . Pic ur Gorg un'onformably ov rla in b las all fo r mation with 

43 
(? . RO) 

. 2n 

( l. 60 ) 

2 . 80 

( 1. 90) 

27 . 70 

(1. 00 ) 

28 . 70 

(0 . 90) 

29 . 60 

( 0 . 90) 

30. 50 

(0 . 40) 

30 . 90 

(0 . 95) 

3 1.85 

(0 . 30) 

32 . 15 

(1.75) 

33 . 90 

(2 . 05 ) 

35 . 95 

(4 . 90) 

40 . 85 

(0 . 80) 

41.65 

( 1. 00 ) 

42 . 65 

( 1. 90 ) 

44 . 55 

·.s. 8 n n r on 1° . ur 

h o r g • 

urn 1 f 

in Pi ur Co r e 

Hn ci r oc ci 

Rasa l 

ci 

ure 

no r 1 \, (l rei Cll' k 

l Cl nci r r o · ci ur 

r o<td cu 

Fo r tl Oll 

RCl s rll 

Fault cro s s r oaci , do\ ndrorpin , 8S ' <dl h ds on nor h 

ba a l t fo r n x t 0. m i 1 s and h n s s in o v r 1 in 

ici . RoAn r av r es 

. a call Foma ion . 

Fault 

Pi tu r 

r oses r oad , do~ndro pin 

Go r Rasalt on out h 

. a 

ide . 

all b d 

RoCld n 

on 

rs 

0 John n 

r oad l o ' 

anci upl i 

Picture Co r Ba. Cll t and und r l . in sed im n 

exrosur s o o n nay ro' ks on right id o 

through poo r l . xpos d out ·rop of John na Fo r ation for Cl ou 

S all ou · r op of Cr t a eou -on lo 

FlO\S a nd breccia of Cla rno Fo r mCl 

of highway . Roadcut continu in 

next 5 iles . 

rat on r i h id 0 

ion x po d in r o<ld ut 

h ClCl rno Fomation f 

on 

in 

C.ood 

ide 

lv the 

Pinna l s in ullv on lef side of 

breccia . 

hway a r ·ompos do C rno laha r i' 

John nay tuffs ov rli lah ric re- c a of Clarno Fonnation south o f r a 

John Da beds largely ob 'ur d b slump blocks of OV rlyin Pi · ur Gorg 

Basalt . 

Cla rno laharic br ccia on right sid of highwa is v r lain by John Day tuffaceous sedimen which in turn is OV 

Picture Cor e Basalt ove r l in r ed 

cuts for next 5 miles are in Pictur 

tion . 

r la'n b Pi' ur C.o r g Ra al t. 

d'men t s of John ay Forma ion . Road 

Go r g Basa l as road limb th r ough secJune ion of Antone Road and l . . . 2 • Continu traight h ad . Oua rr ou h 

(left) of highwa is in Pi c tur Gorg Basal t. Ridg north of highwa consists of Pi c ture Go r ge Basalt capp db Ra tl snak w ld d tu f f. Fo r ahou 

next 5 . 5 miles i ghwa runs along t op of Pictur Go r ge Basalt wi h 

Rattlesnake welded tuff exposed in hills on right hand side . 

Tr ee cove r ed ridge to south (left) i 0 hoco Ba a t r e ting on Rattlesnak 

Forrrla tion . 

Stop 5 (Op tional ) . Outcrop of Rattlesnake weld d tu ff on 1 ft . Glas 

fin - gr ained , moderat l y to densel w ld d uff with a f w la r g ollaps d 

pumice fragm n t s . Outcrop has well d velop d p aty jointing . 

John nay Formation in road cut with Rattlesnak w 1d d tuff on hill to righ • 

44 
( . ')) 

4 7 . 0 

(I. l ) 

49 . 05 

(I • 90) 

50 . q 

(I . 70) 

5 . 65 

(1 . 90) 

54 . 55 

(0. 20) 

54 . 75 

(0 . 25) 

55 . 00 

(0 . 35) 

55 . 35 

(0 . 95) 

5 • 30 

(0 . 25) 

5 . 55 

(0 . 75) 

57 . 30 

(2 . 00) 

59.30 

( 1. 05) 

60 . 35 

( 4 . 35) 

64.70 

(0 . 70) 

65 . 40 

(2 . 65) 

68 . 05 

(0. 80) 

68 . 8 

(3 . 80) 

72 . 5 

( 1. 70) 

74 . 35 

(0 . 4 ) 

74 . 80 

R i'l snake \ve cl cl u f ·a p s r i cl g e to l f This is w t rnmos oc urr n e of his m mh r in the John Dav draina 

Highlv alt r cl \.la rno ancl si on rig sicl of r oacl . Roacl uts \v.l] b in 

\.larno Formation f r appro ximat ly h n xt 7 miles. 

K yes Summi • K 

Clarno a ncl it 

1ountain , an xhL~ecl Oligo ne vol cano omposed of 

ancl breccias , can he s n at 2 o ' lo k . 

Clarno lahari br '·ia with v r larg ancl ite ·lasts on righ . id of road . 

Plat Clarno anclesit flow in road cut on righ t . 

Turn off for 1it·h 11 busin ss lo~p . Con inue straight ahead on . S. 26 . 

Contact of Clarno Formation and Cretaceous shales and conglom rats on r ight . 

Junction Oregon 206 with l . S. 26 . Continue traight ahead on . S. 26 . 

Turn r"ght on old highwa • Stop 6. Examine Cr ta ·eou sancl t on and cong lomerate . Continu on old highwa for 0 . 40 miles to Or gon 206 . Turn 

sharply l ft on 206 oward Service Creek and Fo il. 

Co nta·t of Cla rno lava flow ov r l ing Cr ac ous rocks in r oad u on right . 

Ou c rop of Cr e a ous sands on , conglom ra e , and shal • \.l rno dik can 

en at 9 o ' clock . 

Roadcut in Cr etaceou 

of Cr ta ou ro ks . 

Sutton ountain. 

shale • 

Straigh 

But e at 2 o ' lo k i Clarno and it on top 

ahead on sk line i Pic ure G rge Ba alt of 

On left ar 

blueschi s . 

P rm· an (?) metasedim nts . 

(Swanson , 196q) . 

Rock ·nclud ph , llit s, rna rbl s , and 

Hills on left a r compo ed of Cretac ous conglome r ate and sand tone . 

Clarno andesites ov rlie C:r ta· ous rock on righ s ide of highway . 

Junction of Oregon 206 wit h Twickenha Road . Turn left toward T\<.•ickenham. At 

right is narrow hand of John Da sediments betw en Clarno F rmation and 

Picture \.orge Basalt . For approximately next 7 miles e road traverses 

Sutton ountain , a shallow ncline of Picture Gorge Basalt . 

Hackl jointing in thick basalt flow on right . 

Ha ·kl ioin ing in hi k ha al low . 

1'wick John Oa River . Thi i on he west side of the 

Su on and Pic ur Co r Rasa t dip as twa rd with John Da 

ro cks s p, eli ff- forming flow at 12 o ' lock and 2 

ar thi k , ha·kl - jointed flows en along road . 

Hogba k on ri ht is Pi tur \.org Ignimhrit interla y r d with tuff and tuffa ous lay tons of John Oa Formation . urn r ous andslid blo · ks of 

Pi tur Gorg Ra alt obs·ur th upper part of the John Oa Formation . 

Pi· ur 

n ·mbri 

\.org Ign "mhrit xpo d on righ • \{hit , r si tant 1 dg 

is rh olit air- fall tuff \Vith la r ge soda - rich anidine 

45 

below igr tals 
(I . OS 

7 5. R 

(O . RS) 

77 . 60 

(0 . 35) 

77 . 95 

(1.15) 

79 . 10 

(0 . 40) 

79 . 50 

(1.25) 

R0 . 75 

(0 . 40) 

R 1. 15 

(2 . 35) 

R3 . 50 

(4 . 40) 

87 . 90 

( 0 . 75) 

RR . 65 

(5 . 60) 

94 . 25 

(2 . 60 ) 

96 . 85 

(O. RO) 

97 . 65 

THIRD OAY 

h li v cl 

John net 

o h <luivetl n 

Fo rr1<1 ion . 

0 etsl-flo u f f o f r1 m h r C: o f \.JC' • rn fA· i . o 

Cletrno 1etv(l flows n ft unci rl i John n ForMa ion . C:oocl vi w on ri gh of 

John OF! outcrops wi h Pj • ur C:or R sal f!hov 

R cl 

i u . 

gr 

. ton s in roAci ·ut on 1 Ar low r John Oetv h ds 1 i g 

w'th ClArno FormAtion . A 12 o ' l o·k a r th huff anci 

S dim n Of th Middl John Da • 

Stop 7. I A k up gull for s v r l hundr t o out ·rop of ha al ash-flow 

tuff of John na Fo rmation . Th tuff is mod rat 1 w lei d with a w 11 dvelop d ut axitic t xtur • It · n ain ahou 5% pheno·r s , chi fl 

quartz and anidin with minor plagioclas • Ras d on lithology , ph no rys 

mineralog , and stratigraphi· po ition thi tuff i ·orr lat d with th ha sal 

a h- flow tuff of th w t rn fa i of th John Oay Formation (Sw- nson and 

Ro hin on , 196R ; l~ood burn and Ro hin on , 1 977) . 

Red tuf fa · eo us clay ton 

lava flows . Road follow 

of ha al John na Formation r sting on plat 

th ontact for a hout th n xt 1. 5 mils. 

Rasal ashflow tuff of John Oa 

ande it flows ·rop out across 

Format · on i 

h r rvoir , 

xpo d at 

on 1 f t. 

nd of d m. 

Clarno 

Clarno 

The Cla rno- John na conta·t ris to thP r'gh and th road 

outc ro ps . Road uts will h in Clarno Forma ion from h r 

nt rs Clarno 

o town of Fo sil . 

High pea k a 9 o ' ·lo ·k i Clarno r1 fie in rusion . 

Red claystones and lahari· hr cias on 1 ft ar int rla r d with Clarno and site flows . 

~igh p ak at 12 o ' clock is Ranch r i Ro k . This is a porph ritic hornhl nde 

andesite plug in Cla rno Formation . 

Junction with Oregon Highwa 19 . Tu rn 1 ft toward Fos il . 

Rutte Cre k Summit . ·cood out·rops of Clarno and sit on hoth id of highwa • 

Kinzua road j un ction . Continu straight ahead toward Fossil . 

Tuffaceous cla stones of Cla r no Formation , in r oad u t on right. 

Junction Oregon 19 and 2 1R . End 2nd da • 

0 . 00 Trip log resumes at junction of Or egon Highway 19 and 21R . Continue straigh 

ahead on Highwa 19 . 

( 0 . 05 ) 

0. 05 

(0 . 50) 

0. 55 

Junction Highway 19 and 21R . Continue straight ahead on Highway 19 . 

Outcrop on r i gh t is Clarno andesite . Hills at 12 o ' clock are Yakima Rasal 

ove r lying John Day Formation . 

46 
(0.35) 

0.90 

(0. 40) 

I. 30 

(0. 30) 

I. 60 

(0.2 0) 

1. RO 

(0. 40 ) 

2. 20 

(0 . 95) 

3. 15 

(0 . 60) 

3.75 

(0.65) 

4. 40 

( 4 . 30) 

8. 70 

(15. 75) 

24 . 45 

(0.95) 

25.4 0 

(0.95) 

26.35 

(0.40) 

26. 7·5 

Rutte Cre k turnoff. Continue straight ahead on Highway 19. Good exposures 

of Colu~hia Riv r Rasalt occur along Rutte Creek . Picture Gorge and Yakima 

Rasalt 'nter.finger near mouth of Rutte Creek where it enters John Da River. 

Red tuffaceous cla stone in roadcut marks Clarno-John Da contact. 

Outcrops in roadcut are rhyodacite flow interlayed with lapilli tuffs of the 

John Day Formation. 

Junction of old highway . Turn right on Hoover Creek road. 

.top 8 . Examine rhyodacite flow and lapilli tuff of John Day Formation . The 

lapilli tuff is probably an unwelded ash-flow deposit. It cannot be correlated with any other ash-flow sheets in the John nay Formation and is 

restricted to the area northwest of the Blue Mountain uplift . 

Road junction--stay on paved road toward Mayville. Note outcrops of Yakima 

Rasalt ahead and to the right, characterzied hy stone rings and stripes. 

ote difference in outcrop character between Yakima and Picture Gorge 

Rasalts . 

Stop 9 (Optional) . Outcrop on right is light colored, poorly bedded tuffaceous claystone of upper John nay Formation . This sequence is t pical 

of the upper John nay in the area west and north of the Rlue Mountains . 

Junction with Oregon Highway 19 . Turn left, back toward Fossil . Road crosses Rasalt-John Day contact and enters exposures of John Day Formation . 

Road junction Oregon Highways 19 and 218 . Turn right on Highway 218 toward 

Antelope and Shaniko . Road cuts for next 19 ~iles will be in Clarno Formation . 

Stop 10 (Optional) . Clarno Unit, John Day Fossil Beds ational Monument. 

Steep cliffs are Clarno mudflow brecia. This sequence is better bedded and 

more stratiform than most of the Clarno Formation . 

Entrance to Camp Hancock, run by Oregon Museum of Science and Industry . 

Locality of the famous Clarno 'nut' beds-- numerous plant and vertebrate fossils in the upper Clarno Formation. 

Cliffs on both sides of highway are basal John Day welded tuff overlying red 

tuffaceous claystones of upper Clarno Formation. 

Stop 11 . Rasal welded tuff of member A of the John Day Formation is exposed 

in road cut . From here the unit can be traced, in almost continuous outcrop , 

to Grizzly , a distance of nearly 50 miles . The unit extends 3-4 miles east 

of this point, where it is overlapped by younger ~embers of the John Day 

Fo rma ti on . This unit is taken as the base of the John Day Formation in the 

Clarno-Ashwood-Antelope area, the type area of the western facies of the 

Formation . In this outcrop the tuff is fairly densely welded, devitrified, 

a nd omewhat richer in lithic fragments than elsewhere . Rock contains about 

5~ cr tals--quartz, sandine, and plagioclase . 

High p ak to north on right hand side of John Day River is Ironside Mountain, underlain by Yakima Basalt with at least one flow of Picture Gorge 

Rasalt at base . The Columbia River Rasalt rests unconformably on the John Day 

Formation . The ledge extending toward the river from the base of Ironside 

ounta i ns is composed of alkali-olivine bpsalt flows interbedded with tuffaceous sed im n ts of the John na Formation, member F. 

The entire area west of the John Da River is a large landslide in which 

hlo ks of Columbia River Basalt ar mixed with tuffaceous sedi~ents of the 

J ohn nay For~ation . On block of Colum ia River Rasalt, about 3 km long, 

47 
(0 . B0) 

7. 5 

(0.40) 

27 . q5 

(A . f\5) 

34 . 60 

(0 . 70 

35 . 30 

(0.50) 

35 . RO 

( 2.R5) 

3R . 65 

(3.1 5) 

41 . R0 

(0 . 55) 

42 . 35 

(0 . 55) 

42 .qo 

(1.4 0) 

44 . 30 

(1.35) 

45 . 65 

(0 . 65) 

46.30 

(3.15) 

49.45 

(0 . 70) 

50 . 15 

(2 .30) 

52.45 

(0.35) 

52.80 

oveci as Ct ·oh<>rent hlock . 

luMp blo ·ks . 

o t p n r 1 htiMfllockv topop r flhv , nrl n11M r 11!': 

C'larno Briel . Ct'ro.s John Dav Riv r . 

BCtsal ashflow uff of John CtV FonnA ion in roAd ·u on ri gh 

overl . ing tuffa · ou s c im nt. of M mh r F. Fr m h r ro ad o te 

ncilici ar for n xt 7 Mil 

Outcrop in ·liff on righ r has lt fl w o John Day Fom 

f m mb r II . 

ion (m mber F) 

ov rlain bv basal ash- low sh 

\"hit tuffa· ous s cii.M nt of M mb r F, John Dav Fo rfTI <l jon ln r aci·u on 

left . 

Crest of hill b tw n J hn Day all y and An 

sediments of m mb r Fin ro d·u Cliffs a 

Basalt overlying John Day Forma ion . Road ·ut 

approximately n xt . 5 mil 

lop Vall y . 

Rand 12 

Whi. u f f <lceous 

Y ki a 

r r i 1 for 

Road junction-- ·ontinu tra ht ah aci on Hi hway 2 1 R. Rim on 

is edge of C:oluMbia Plat au . Irr ular hill in for round a r 

of Yakima Basalt . 

kylin 

lump 

a ead 

1 cks 

Road cut on left i pro ably ra·hy nci 

ForMation . Tra·hyba alt is ciiffi·ul 

ogicall froM YakiMa Ra alt ut h r 

Mi · (" llv or Min ralo h n Da y s ci i n t s . 

Antelope Road ;unc t i n . urn 1 f tow rei I i llowcia l 

Low brown hills on right ar alkali-o1ivin b al 

Day Format ion . 

nci ad ras . 

fl w f m mb r f , J hn 

John Day basalt on right . Lithoph sal w ld ci uff of 1'1 mb r F. , ohn D;w 

Formation , overlies rhyolite flow of 1'1 mber Con 1 ft . 

Welded tuff of meMber Eon left . 

Welded tuff of member E in road cut on lef • Road follows 

E and F for about next 3 miles . Brownish hills on right ar 

capped by YakiMa Rasalt. 

on a ·t of M:mbe r 

John Da y sa 1 t 

Stop 12. Alkali- olivine basalt flow interb cid d with tuf ac ou sedim :1 ts of 

membe r F, John Day ForMation . Rasalt is aphyri with medium-grain d ophi tic 

text ure . Some pegMatitic seg r egations are omplosed of larg ilm ni e a d 

clinopyroxene c r stals . Several sh ar zone cut basalt . 

Stop 13 . Outcrop of gra , weakly welded ashflow tuff of member G, John Day 

Formation . ote abundant large soda sanidine cr stal , sparse lithic 

fr agments , and slightly collapsed pumice lumps . Eastward from his poi~ 

unit is an airfall tuff; to the southwest it pas es into den el weld ed, 

devitrified tuff . ote brown weathering color of John Da ba al at q 

o ' clock . High hills at 10 and 11 o ' clock are aki Ra 

Formation . Basal ash- flow tuff of membe r C is overlain 

sediments. At 12 o'clock , the low ledge just above the 

ash- low sheet of member H. 

uffaC('OU 

he ba sa 

On left , YakiMa Rasalt cuts down into John Dav Formation in old valle y or 

topographic depression . ote columnar jointing in basalt at 12 o ' lock . 

flay 

App rox imate contact of Yakima Basalt and John Da Formation . Roadcu re in 

48 
(0 . 0) 

53 . 10 

(].35) 

54 . 45 

(0. 95) 

55 . 40 

(0 . 40) 

55 . 80 

(0 . 15) 

55 . 95 

(1.25) 

57 . 20 

(0 . 90) 

SR . 10 

(0 . 80) 

5R . 90 

(0 . 70) 

59 .60 

( 1. 90) 

61 . 50 

(0 . 7 ) 

62 . 25 

( 1. 00 ) 

63 . 25 

(1 . 95) 

65 . 20 

( 1. 1 ) 

. 35 

(0 . 95) 

67 . 30 

(1.20) 

6R . 50 

YakimA BAsalt for next 1 mile . 

Fault in Yakima Rasalt at 11 o ' clock; left side down . 

Hogba·ks dipping to west (right) at 1 o'clock are John Day welded tuffs uplif ci h. ma_ior fault. Road cuts on left are landslide blocks of Yakima 

Basalt . 

Low ledges on both sides of road are basal ash- flow sheet of member I , John Day Format ion . 

Jun·tion with l . S. 96--turn right. 

Stop 14 . Basal ash-flow tuff of member I; unwelded glassy tuff characterized 

b high percent of lithic fragments , abundant pumice lumps , and glass chips 

and fragments . Contains 1-2% crystals , chiefly plagioclase . Unit becomes 

thicker and more densely welded to the south . 

Road junction--turn right on old highway . 

Road junction--turn right and return to U. S. 97 . 

Jun tion U. S. 97 - -turn left toward Willowdale . 

\ illowdale 

Road june ion--turn left toward Ashwood . Ledge on right after turn is basal 

welded tuff of member H, involved in a small fold . 

Stop 15 (Optional) . Examine ledge of welded tuff of member H. Rock is red , 

ver fine-grained ash- flow tuff . Unit is not highly welded but is well 

indurated . Contains ver sparse plagioclase phenocrysts and rare rock fragments . Locall , glassy base is preserved . Ahead at 12 o ' clock is landslide 

lobe which moved down from outcrops on skyline . 

Ahead at 12 o'clock is dip slope of lithophysal welded tuff of ~ember E, John 

na Formation, bounded by fault on far side . Hogback at 9 o ' clock is basal 

welded tuff of member H with a glassy base and crystallized top . This is 

overlain at 8 o'clock by basal welded tuff of member I with a thin zone of 

intervening sediments . Above basalt tuff of member I a r e tuffaceous 

sediments, mostly claystones with small pumice lapilli. Hill is capped by 

Yakima Rasalt. 

Road crosses major fault at this point--west (left) side up . At 9 o'clock 

flat-1 ing welded tuffs of member E cap the rhyolite flow of member C, 

separated by small amount of tuffaceous sediment of member D. Just to left 

of his ou ·r p he ntir s quence dips w stward about 40 °, uplifted by 

faul • Fr m h r to A hwood the road will generall go down section thr ough 

h John na Formation and into the Cla rno . Road cuts for next 2. 2 miles 

will b mber F with landslide blo ks of welded tuff from members G and 

H. 

Pon 

ledg 

Rut at 4 o 'clock is apped b basal 

is ba al weld d tuff of memb r G. 

lded tuff of member H. Lowe r 

Contact of uffa eous sediment of mem er F and welded tuff of member E. For 

approxima 1 ne t 5 mil s road u will be in lithoph sal tuff of mem er E. 

S op 16. Examin outcrop of lithoph sal tuff of m mber E. Rock is fine1 weld d r stallized tuff containing sparse plagioclase 

and rar ro·k f ragment ; lo all has glass base • Abundant 

form la er alternating with massiv portions of the tuff but 

49 
(1.20) 

69 . 70 

(0.85) 

70 . 5 

(1.45) 

72 . 00 

(0 . 50) 

72 . 50 

(0 . 30) 

72 . 80 

(0 . 10) 

72 . 90 

(0 . 10) 

73 . 00 

( 1. 50) 

74 . 50 

(0 . 90) 

75 . 40 

(0 . 90) 

76 . 30 

(1.75) 

78 . 05 

(2.10) 

80 . 15 

( l. 20) 

81.35 

( 1. 05) 

82 . 40 

(0 . 70) 

83 . 10 

( 0 . 60) 

83 . 70 

(0 . 70) 

84 . 40 

v o ·cur in v rti ·;1l 

ars to fill a lo~al 

. warms . This tuff v, ri s g r a ly in thickopogrAphi· ci pr ssion on rh olit flow 

1aior out ·rop of tuff and tuffa' ous s dim nts of m mb r n b n a h w lded 

tuff of m mb r F. . F r n xt 0 . mil s \v lei d tuff thin rapidly o ast anci 

north ast . 

Road now runs on top of m mb r F.. Low hill on 1 ft ar rhyoli 

member C. 

fl w of 

Base of m mh r F. r ting on tra·h and sit flow of m mb r R, John Oa 

Formation . 

La er of s dim nt s parating tra ·h and it flows of m mb r B. 

Conta·t b 

of middl 

it of m mb r B and r deli h tuffac ous s diments 

m mb r A. 

Basal w ld el tuff of member A. 

Contact betwe n John Da and Clarno Formation • Road uts will h in Cla rno 

to bottom of valley . ot r d zon at top of Clarno--thi aproli is discontinuou and 1 ss well-dev lop d in this ar than in the ar a st of the 

Blue Mountain • 

Road bends to right. Turn 1 ft on elirt road and pro 

road . 

el 0 , 90 mil o end o 

Stop 17 . At 12 o'clock i steep-sid d rhyolit dom with flow x nding off 

to right. Oom marks the vent from which th rh olit flow of m mb r C, Jo 

Day Formation, was erupted . ot cruel olumnar jointing in dom • Coa rse 

boulder lahar on left is in Clarno Formation . Return to highway . 

Junction with paved road . Continue straight ahead toward Ashwood . 

Ashwood Road junction--t urn right on gravel road towarel Ha Cr k and 

Griz zl • 

Contact of John Day and Clarno Formations in roadcut on right. S op 18 . Bus 

will proceed 0 . 35 miles up road and park . Road traverses a ompl t section 

of member A of John Day Formation . Rasal ash-flow sheet exhibits two cooling 

units, mineralogically and petrographically similar. This is overlain by a 

sequence of poorly exposed lapilli tuffs and tuffa eous layston s which are 

capped by upper fine - g ra ined ash-flow tuff of member A. Entire s tion overlain by trachyandesite of member B. 

Top of hill. Road junction--continue straight ahead and follow left hand 

fork . Tuffaceous sediments of member Dare exposed in roadcut. Here member 

lies between trachyandesite of member R and lithophysal tuff of member E. 

From this point the road runs approximately south and roughly parallel the 

contact between members Band E crossing back and forth for next 5.5 miles. 

Contact of members Band E in roadcut on right . 

Contact of members Band E in roadcut on left . 

Contact of members Rand E in roadcut on left. 

Contact of members R and E in roaduct on righ • L dge on righ is memh r F., 

much thinner here than in Pony Creek Canyon . inor amounts of tuffa ceous 

sediments of member D lie beneath member in roadcuts for the next 0. 5 mil s. 

50 
(1 . ()()) 

R5 . 40 

(() . 20) 

R5 . 60 

(() , 40) 

R6 . 00 

(1.30) 

87.30 

(0 . 45) 

87.75 

(2 . 10) 

89 . 85 

(0 . 50) 

90 . 35 

(1.35) 

91.70 

(2 . 85) 

94 . 55 

(0 . 45) 

95 . 00 

. (0 . 95) 

95 . 95 

(1 . 40) 

97 . 35 

(1.15) 

98 . 50 

(0 . 65) 

99.15 

(1.15) 

100.30 

( 1. 50) 

101.80 

(0.35) 

102.1 

(0 . 70) 

102 . 85 

F.x' 11 nt view of t igh Cas ·ade peaks on right . Oouhle hut te in foreground is 

Tell r P.utt --capped with weld d tuff of l'lel'lh r c. . 

Conta 't of l'lel'lh rs Rand F. in road ·ut on left 

S dil'lents h tw n trachyandesit flows of m mh r R. 

Weld d tuff of member F overlies member R in roadcut on left . Welded tuff of 

memb r F. forms promin nt ledg dipping northwest on hill at 2 o'clock. Just 

ah ad, where road swings left , the hill at 1 o'clock is capped with welded 

tuff of m mher C. . 

Tuffac ous sediments of memh r F are exposed in roadcuts on right . Roadcuts 

are in memh r F for approximate! n xt 4 miles . 

\.Jhite pumic lapilli tuffs in roadcut on right are near the base of member F. 

Good exposures of member F occur on hill at 8 o ' clock . These are mostly red 

to gray tuffaceous cla stones and tuffs . Ledg at top of hill at 3 o ' clock 

is Quaternary basalt . 

Small dam on left . From this point the road swings generall west and rises 

in John Oa Formation . ote landslide blocks of Quaternary basalt on right 

side of road for next 4. 5 miles . 

Fault crosses road , uplifting welded tuff of member Eon west . Fault scarp 

is visible to left, across meadow . 

Road jun·tion to Ha Creek Ranch-- turn right toward adras. Quaternary 

basalt crops out on sk l'ne at 4 o'clock with man landslide blocks below the 

rim . The cliffs at 12 o ' clo k ware welded tuff of member C. , John Da Formation . 

Road junction--turn right toward Willowdale • 

Stop 19 . Examine outcrop of welded tuff of member C. . nit is densel welded , 

cr stallized , and contains sparse lithoph sae . ote abundant large soda- rich 

sanidine cr stals. A major fault runs along the road , tilt ing the welded 

tuff sheet to the west . The low cliff on the west (left) side of the road is 

the basal ash- flow sheet of member H. This is overlain by Quaternary basalt , 

which at this point flowed down old channel of Hay Cr eek . Turn a r ound and 

return to jun tion of Hay Creek Road . 

Junction of Hay Creek Road--turn right . 

Entrance to Hay Creek Ranch--take right hand fo r k of road . 

Lithoph sal w ld d tuff of member E on both sides of road . Small landslide 

a 10 o ' lock . 

Rr w r R s rvoir . Fault runs along front of liff on right, uplifting ledge 

of memb r E. 

Road junction--turn right . After turn, flat-topped hill at 1- 2 o ' clock i s 

basal welded tuff of m mher H, overl ing member G. Hill at 10 o ' clock is 

w ld d tuff of m mber C. . 

R d outcrop at 2 o' lo k is e·tion of tuffa eous sediments of member F . 

Road lil'lbs o top of a h-flow sheet at base of member C. . Here , welded tuff 

dip g n rall south but i folded into a series of broad , open folds . Out-

·rop a ross fi ld at 11 o' lo~k is ba al welded tuff of member I and cliff 

51 
('0 . 7)) 

1 r • ~n 

(0 . 50 

I 04 . 10 

(0 . nS) 

104.75 

(0 . 65) 

105 . 40 

(1.40) 

106 ~0 

(0 . 45) 

107 . 25 

(0 . 55) 

107 . RO 

(0 . 35) 

1 OR . 15 

(0.70) 

lOR . RS 

(1.35) 

110 . 20 

(0 . 40) 

110. 60 

(1 . 55) 

112 . 15 

( 1. so) 

113 . 65 

(0 . 90) 

114 . 55 

(0.25) 

114 . RO 

(1.35) 

116 . 15) 

(0 . 25) 

116 . 40 

(1.20) 

117 . 60 

At I o 'c lock i. "'eldc>rl tu f of I" r1h r l , urlift cl AlonP ;t M;tjor nul t . Far 

r ~k :1 1:' o'l'lo·k is Cr~v Rut in Sr-~i h Ro·k Ar :1 . 

Roe~cl Stop "0 . Tuff in roacl is re~v, rd ssv , Ash-flow tu f Cl ha e 

l' ;:llk e~ho ut 100 vet rrls up roacl to 1 ft flnci Xflmi n hA Sfl l <~sh -fl ow 

r I. r the unit is cl n lv w lrl rl with rt goorl vi.trophvre at 

·rystalliz cl portion ahove . 

l·'eld rl tuff o mer-~h r I · ro. s s roacl . 

L dge on hoth irl of roetd i n uat ernarv ha alt. Road ·JlMhs to top of 

hasalt flow . This flow origin~t cl f r m ~ smetll hi lrl vol ·ano ~hut a nile 

to th east ancl flowecl rlmm old s r ar1 ·hann 1 towarcls ~1acirfl . • 

Po~<i 'unction e~nd ·~ttle guetrd --tt:rn 1 ft . 

Small mound on right i hi ld vol· ano from whi ·h Ouat rnary ha al flow was 

erupted . Grizzlv P ak i at 12 o ' ·]o·k. 

f.dg of Ouat rn~r hasalt. HE>r unit ov rli tra·hyancl it 

R, John Dety Fomation , whi·h i expos d ~n quarry at 1 o ' lo·k . 

ing clips of Grizzl Peetk and Cray Rut in eli tan· at o ' ·lo·k . 

underlain h rhvolit flow prohethl folded into hroad vn·lin • 

of rn fllber 

o t opposRo h 

I elded tuff of meMb r A, John nay Formation, in o~d ·ut. Thi L h 

southernmost outcrop of this unj t. To th outh, olcl Clarno hill ar onlappecl h stratigr~phi ·ally hi h r units of th John nay ForM~tion . 

Roacl junction--turn right towarrl Crizzly . 

Roacl iunction-- turn right towarrl LetMont~ and Prinevill • 

f.dge of Ouaternay hasalt flow exposed on right. 

Roacl has climh cl to top of flat-lying farlra Fomation . This uni , mos ly of 

Pliocene age, consists of fluviatile silt tones , sanclston etncl ·ong lomerates 

with interlayE>red air- fall and ash-flow tuffs anrl hasalt flows . 

SMall outcrop at 2 o ' clock is rh olit iMilar to that of Grizzl P ak, completely surrounded by rocks of Madras Formation . 

Good view of Gray Rutte at 10 o ' clo·k . Low ridg just 1 ft of Cra Rutte is 

capped by saMe rhyolite flow . Low ridge at 9 o'clo·k is YakiMa Ba alt overlying tuffaceous sediments of John Da Formation . Low b nch to right of 

Gray Rutte underlain by Clarno-type flows dipping outh ast . 

Junction with t .s. Highway 26 . Continue straight ahead on rlirt road towa r d 

Culver. Ridge at 3 o ' clock is hasalt ash- flow tuff of member H. ot tha 

strike of John Day is nearl f.- W. Hills on r ight at 1-2 o'clock ar also 

capped by welded tuff of memher H. 

Road runs on surface of adras ForMation . Low hills at 3 o'clo k are ca pped 

hy welded tuff of member H. Flat-topped hutte at 11 o'clock i He~vstack 

Rutte, also capped by welded tuff of memher H. Left of Ha stack Rutte t his 

welded tuff laps onto Clarno- type rocks of Smith Rock . 

Road junc tion--turn left . 

Road junction--continue straight aheacl . 

Roarl .iunction--turn right. Goocl view of Crav Rutte on lef af er turn . 

52 
(0 . 95) 

i!R. 55 

(0. 85) 

!19 . 40 

(0 . 40) 

119 . RO 

(0. 75) 

!20 . 55 

(0 . 95) 

121. 50 

(0. 65) 

12 2. 15 

( 1. 20) 

123 . 35 

(0. 20) 

123 . 55 

(0 . 40 ) 

123 . 95 

(1.30) 

125 . 25 

(0. 20) 

125 . 45 

(0. 50) 

125 . 95 

( 1. 20 ) 

12 7. 15 

(0 . 20) 

127 . 35 

(0 . 50) 

127 . 85 

(0 . 95) 

12 8. 80 

(1.75) 

130 . 55 

(1.95) 

132 .50 

(0 . 40) 

132 . 90 

(0 . 20) 

133 . 10 

(1 • 05 ) 

134 . 15 

( 1.1 5) 

l3 5. 30 

(0 . 60) 

135 . 90 

( 2 . 5) 

138 . 55 

Contact of Madras Formation and John nay Formati on . John Day tuffaceous 

se diments of m m her C form small hill surrounded by Marl ras Format io n. 

Co nt act of John Day sediments with Clarno Format ion . 

Road junction- -turn right . 

Good outcrop of Cla rno Formation on right . Road cuts are in Cla rno Formation 

for next mile wit h John nay contact just ahove r oad on right . 

Road 'unction--turn left. 

Haystack Reservoir at 2 o'clock . Top of ridge just be yond reservoir is 

welded tuff of member H. Just below is a local welded tuff that pinches out 

to the ri ght . 

Welded tuff of member H crosses road . Ridge at 10 o ' clock is lava flow from 

Juniper Butte . 

Road climbs to top of Madras Formation which surrounds the older rocks . 

Rutte at 11 o ' clock is Round Butte , a small shield volcano from which some of 

the basalt flows in the Madras Formation were erupted . 

Road ·unction--turn left. 

Junction U. S. Highway 97 --turn left toward Redmond and Rend . 

Jun ipe r Butte , a rhyolite dome of John Day age at 1 o ' clock . 

La va flow f rom Juniper Rutte at 9 o'clock . 

Welded tuff of member H, John Day Formation in roadcut . 

Th ree Sisters volcanoes in High Cascades visible at 2 o ' clock . 

Appproximate contact of Madras and John nay Formations . Haystack Butte at 9 

o ' clock is capped with welded tuff of member H. 

Gray Rutte at 10 o'clock, capped with rhyolite flow . Laharic breccias o f 

Smith rock at 11 o'clock . 

Road cuts with Madras basalt flow overlain by tuffaceous sediments . 

Madras basalt flow . 

Brid~ a r o Cr ook d R v r r. rg . 

Stop 21 (optional) . Tu rn right to overlook for Crooked River . Gorge was cut 

in Madras Forma ion , then filled in with Quaternary basalt . Present gorge 

was ut through intracanyo n basalt which is visible in canyon walls . Not e 

olumnar join ing in intr acan on ba alt . Return to Highway 97 and turn right . 

Tuff of Smith Rock e xposed at 9 o ' clock . 

Basal flow of Madras Formation in r oadc ut on left . 

T rr bonn • 

T h r ow Rutt • This is a rie s of inder ones marking vents f rom whi c h 

som of the basalt flow of the Madras Formation were erupted . 

53 
(2 . 00) 

140 . 55 

(0 . 90) 

141 . 45 

(0 . 20) 

141.65 

(2 . 05) 

143.70 

(5. 70) 

149 . 40 

( 5. 75) 

155 . 15 

(0. 85) 

156 . 00 

(0 . 25) 

156 . 25 

(1. 55) 

157 . 80 

R dmond . 

Jun·tion Or gon 126--·ontinu traight ah ad on Highwa q7 . 

Junction Or gon 126- -·ontinu traight h ad n Highwa q7 . 

Old hannel cut in Madras Formation nd fill d with Qu t rn ry ba al • 

Low hill on right i 

age . Dome proje·t 

sil'ci· dom of F rk d Horn Rutt , probably John n 

through Ma cl r as F o rm a t i n • 

Junction U. S. Highwa 20-- continu str ight ah ad . 

R nd . 

Road junction- -turn right toward ·it ·ent r . 

End of road log--corner of Franklin and Wall Str t • 

54 
CE TRAL IIIGH CASCADE ROAD IDE GEOLOGY 

BE D, SISTERS, 1CKE ZI E PASS , D S TIN-1 PASS, OREGO 

Edward 1. Taylor 

Department of Geology 

Oregon Stat e University 

I TRODUCTIO 

This geo logic fie ld trip guid is a rev1s1on of 

"Roadside Geo logy , Santiam and 1cKen zie Pass Hi ghways, 

Oregon" (Tay lor, 1968) wi th add i tions pertinent to 

geo l ogy of th e Bend -Sist ers ar a. Checkpoint mileages 

express cumu l a t ive dist ance from Cen ral Oregon Communi y College in Bend, over a rout e whi ch l eads to 

Sisters, around the 1cKenz ie Pass-Clear Lake-Santiam 

Pass highway loop, and back to Bend (see Index Map, 

Fig ure 1) . Pers ons using this guide shou ld anti ipate 

one fu ll day of at l east 140 miles, and should check 

ac essibi li t y to fcKenzie Sununi t during winter, 

spring, and l ate fa ll. 

GE ERAL REMARKS ABOUT CE TRAL CASCADE GEOLOGY 

A calc-a lk alic vo l canic arc has been in ermitentl y ac t ive during the l ast 10-15 m.y . a long he 

eastern part of the cen tral Cascade Range in Or egon. 

The l ate Pl eistocene record of this vo l cani c activity 

is well pr ese rved on the crest of the High Cascades; 

he best exposed r ecord of ea rl y Pl eis ocene, Pliocene, and late 1iocene Cascade vo l canism is fou nd in 

olcanic and volcaniclastic deposits on the east flank 

of the range and i n th e adjacent D schu t es Basin . In 

the followi ng discussion, structural, s ratigraphic, 

and magmatic features of the \ est ern Cascade, Hi gh 

Cascade , and Deschutes Basin subprovinc s are described and their interrelationships are briefly 

summarized. 

Cen tra l High Cascade Province 

The central High Cascade Range i n Oregon is 

chiefly a Pleistocene volcanic platform of overlapping 

basalt and basaltic ande site lav a flows whose aggrega te thickness is gene rally unknown but probably exceeds 4, 000 fee t local ly. This platform is e longa t e 

north-sou th and is 20 -30 miles wide . typica l volcano of the platform is a broad shield of light -

co lored, vesicular basaltic andesite with a inder 

ha has e n in aded b plugs and radial 

is o en exam le e pos d in cross-se ion 

erosion include phinx Bu e sou h of 

posi c s rue ur r 

shie ld base 10 mile 

band ."or h Sis 

Finger d Jack . 

pla form w re a i 

duccd small c i nd r 

Deer Bu e nor h of Los 

k 

of 

omon a 

!Iushe 

es 

s, and 

ne 

55 

cinder cones are no less abundant but they are not as 

well preserved. Examples of glaciat ed remnants of 

Pl eistocene cones include Bluegrass Butte , Condon 

Butte, and Sco tt fuunta in near McKenzie Pass and ~xwe ll Butte, lloodoo Butte , and Cache Mounta i n near 

Santi am Pass. 

systematic temporal inhomogeneity exists wi th -

in the High Cascade pl atform . Earl y Pleistocene 

lavas were predominantly high-alumina olivine thol eiites i n vesicu l ar, thin, widespread units, commonly 

with pronounced diktytaxitic textures. Later Pl eistocene l avas were predominantly high-alumi na basaltic 

andesites i n th ick, platy un its, genera lly with pi lotaxitic t extures. The arly ba salts crop out i n 

greater abundance and arie t y along the weste rn and 

eastern margi ns of he platform and i n the wa lls of 

deeply glaciated canyons· however, identical basalts 

do occur at highe r levels. Later basaltic andesites 

cover most of th e pl atform but th ey are also found at 

lower stratigraphic l evels . Examples of ea rly basal ts a re well exposed along the west margin of the 

pla tform at Cupola Rock in Lost Creek canyon, in cuts 

of Highway 126 north of Trai lbri dge Re servoir, and in 

th e va ll ey of Hackl eman Creek west of Fish Lake. 

Early basalts along he east ma r gin of he p l a t orm 

are widespread in the upper 1e tolius Riv e r valley and 

in th e vicinity of Sisters. 

A systema tic spatial inhomogeneity also exists 

within the High Cascade platform. West of Bend, in 

th e vicinity of South Sister, silicic volcanic rocks 

are interbedded with and r est upon the mafic pl a t form 

ro cks . A silicic highland of some 15 mil es br eadth 

was produced by th e development of rhyolite , rhyodaci e , and dacite domes surrounded by andesit e , dacite, 

and rhyodacite la vas and ash-flow tuffs. ~st of 

this highland is mantled by mafic cinder co nes and 

la as of the Triangle Hill group and by composit volcanoes such as Broken Top. Examples of inte r bedded 

andesites, dac i t es, and rhyodacit e lavas are common 

i n upper Squaw Creek and Tumalo Creek canyons . Two 

units of bla k andesitic ash-flow tuff (Century Drive 

Tu f and hevlin Park Tuff) and one unit of pink, dei rified, da itic ash-flow tuff (Dese rt Spr i ng Tuff) 

were erupted f rom the highland and are we ll exposed 

wes of Bend. Two rhyoda it~ as h-flow tuffs (Tuma lo 

Tuff and Lava Isl and Tuff) and one ex ensive lapill ifall pumice deposit (Bend Pumice) of High Cascade 

origin are a lso exposed near Bend but were probab l y 

erupted from a vent south of he silicic highl an . 

Sou h Sist er volcano is chiefly andesite with 

minor dacite and rhyodacite. Broken Top (east of 

South Sister) is basaltic andesi t e with minor i nteredded dacite and rhyodacite lavas and small-volume 

ash-flow uffs . 1idd l e Sist er (north of South Sister) 

is basal wi h minor basa lti andesite, andesite, 

da i e and rhyodacite. South Sist e r and nearby volanoes are relatively l a t e products of long-continued , 

ompositionally diverse, and localized silici magma-
c \' 

• 

0 

0 

30------4-----------r-----r-----------------+-

X ' THREE 

~ FINGERED 

<) JACK I 

( 

t;)) 

W\ err 

u l 

XM{}(JNT 

D 

OREGON 

MILES 

KILOMETERS 

( WASHINGTON 

( I ~-----

...._ ~t 

SISTERS 

0__.!..-126 

lz6 

\ 't: I \--~----------------, 

~\ 0 ( (j) \ 

4.} 

l)l 

:::-NORTH 

MIOOLE X SISTER 

SISTER \ 

l SOUTHX 

SISTER ( 

,/ "><iJ ROK EN 

( TOP 

r-' 

( 

\ 

15 

25 

44·~---4----------------~----------------~----------------~ 

FIGURE l, INDEX MAP , ROADSIDE GEOLOGY, BEND, SISTERS, McKENZIE PASS, AND SANTIAM PASS. 

56 
tism. 

In s umma r y , the 

not th simpl e P I i · 

Rang is 

f a nd iL 

t xtbo k 

lavas 

sub rdinal 

d thi<· 

in .isoDeschut Basin Pr vin 

Th ast rn mar in 

platform is m rk d by a v r 

th late lioc n and Plio 

Early platform intra anyo n lavas xt nd 

miles ast of the Cascad foo th ills a nd 

ont a t with 

Formati n. 

as much as 5 

isolat d 

Pl istoc n vol ano s of basalt and basaltic and sit 

rest on Plioc n ro ks of th D s hutes Basin . Examples of High Cas ad intra a ny on lavas oc ur in low r 

M tol ius Riv r canyo n, D p Canyon, and n a r Squaw 

Creek, Bull Flat, and Tumalo Cr k b tw n Sist rs and 

B nd. Isolat d vol ano s of Hi gh Cas affinity 

includ Squaw Ba k Rid g , Long Butt , and Pilot Butt 

and Awbr y Butte n ar B nd . 

Th D schu tes Formation ontains str am-d posit d 

silt , sand, and grav l, and sitic-to-rhyodacitic ashflow and a h-fall tuffs, and interb dded basalt flows . 

Th basalti 1 vas w r rupt d from ind r on s a nd 

fissure v nts within the D hut s Basin but th picla tic a nd vol anicla tic r o ks w r hiefly of 

Cascade prov nan Close to the Cas ad s, th formation b om s thi k r, basaltic a nd sit lavas pr -

dominat , and th . vol a ni lastic ro ks b com disconint rb d . Indicators of transport dir tion 

volcani lastic s dim nts point astwa rd. 

Deschutes Formation sourc vol a no s w r coincid nt 

with, or not fa r r mov d from the High Cascad axis . 

One d epl diss ct d r mnant of a n and sitic D -

schutes Formation sourc vole no is located at th 

bend of M tolius River, 12 mil s east of Mount Jeff rson . 

Strata of th twe n Warm 

h 

of 

57 

by lavas 

f a ul -

rpr tation can be summariz d 

1 . D schu Formati n ba lti 

lava , and ash-flow 

f r m vol a no s n ar th 

axis and flow d astward into 

n xly inlava s , 

d 

Basin m r than 4 . 5 m.y . ago . Rock units d posit d by this pro ess have be n trac d from 

Gr n Ridg to th D schut s Riv r. Th y r v al 

a continuou , gentle astward dip of onl l-2 

d gr s . Therefore, Green Ridg cannot represen t a tilt d-up fault block. It is, instead, 

a r mnant of tabl ast rn Cas ad foothills 

and its rocks still r st on an initial pal aslope. 

2. Th maximum ag y t obtain d by radiom tri 

dating of ce ntral High Cascad platform ro ks is 

3 . 9 m.y. Th r fo r , Deschutes Formation ro ks 

and so urc vol ca noe must li b n ath the Cascade platform and displac ment of the astmarg.in fault syst m probably xceeds (and might 

gr atly x d) 3,000 feet. 

3. IE Gr en Rid ge has not be n displac d upward 

r lativ to the D schutes Basin, th Cascade 

axis has be n displa ed downward. This pr suma -

bly occu rr d about 4.5 m.y . ago, terminating d -

position of D schut s Formation ro ks . 

Is i possibl that a who le rang of Plio n comp -

sit vol ano s fou nd e r e d and was buried ben ath the 

Pleisto n Hi gh Cas ad platform? Su h an int rpr -

tation is strongly suggest d by availabl vid nc . \Hth approp riat informality, this hypoth tial s emblag of volcanic rocks might be all d th 

"Plio-Cascades ." 

Ea t Margin of th Central West rn Cascade Provin 

Stratigraphi and structural r lationships at 

th w st rn margin of the central High Cascade platform ar obs ured by mo r extensiv rosion, thi c k r 

all uv ial ov r, and mor l uxuri ant v g tation than 

along th east rn margin. How ver, striking similar~tle are vid nt . Isolat d Pleisto n volca no s 

of basalt a nd basaltic and site o cu r in th \ st rn 

Cascade at 1 ast 20 miles west of the platform. 

Examples ·nclud Harter Mountain, a cinde r con and 

flow near Quartzv ill , and Battle Ax Mountain. Early 

High Cascad platform basalts on 

lavas in the adja nt \ s inlud diktytaxitic ba cany n of M K nzi e Riv r imilar r ocks in the canyon 

of o rth S ntiam River. 

Ro k of the W stern Cascad s adjac nt to th 

High C s ade platform ar pr dominantly late 

and Plio ene mafic lavas (Armstrong a nd 

o her , 1975; Sutter, 1978) with subordinate ash-flow 

and sili ic vol anic domes. These rocks hav~ 

n included in the S rdin Formation by Peck and 

oth r (1964) but should be assign d to the Outerson 

F rmation of Thay r (1937). Thy ar equivalent in 

litholo y and ag to th D schutes Formation . They 

g n r all flat-lying x ept wher they are offs 

northw st-trending n nnal faults of small displac~ 

m nt. Although man units of th Outerson Formation 

w r v ~t d along th ast rn dg of the West rn 

Cas ad s , v r 1 and sitic ash-flow tuffs and one 

ba alti a nd sh-flow tuff app ar to have moved 

w stward from cad vol anoes. Exampl s of 

Out rson Form tion ro ks n b s en along th southto-north re tlin of mountains whi c h includes Fris-
s ll Poin , Bunchgr.1ss un .tin, 

1 un , in , E hl 1 untain , Crc ~:nl 

Pyramids, and Coffin Butte. 

Out r 

he same age as 

lower (Arm trong and 

Bn \"d r Rid gL' , lro n 

I until in Thr 'l' 

1 ngth , this aul s n 

ati n of th e es arpm n and by la vas of th 

Cascade pla tfo rm deposit d a ain t th ba 

esca rpm nt. 

Sard in e . 

Littl e Butt e . etc . 

Western 

Cascades 

Sj)< I C L' . 

Hiq/7 

Cascades 

Silicic 

Composite 

Volcano 

Green 

Ridqe 

Deschutes 

Basin 

FIGURE 2. DIAGRAMMATIC CROSS SECTION OF WESTERN CASCADE J HIGH CASCADE J AND 

DESCHUTES BAS INJ OREG ON . 

If it can be d mons rat d tha the High Cas cad 

axis was displaced downwa rd along margi nal boundary 

f ults relative to th D schutes Basin a nd W stern 

Cascad s du rin g the int rval 3 to 4 . 5 m. y . ago, th 

Hi gh Cascad platfo rm should b vi ew d as a Pleistoc n f ill within a Plioc ne grab n (Figur 2) . On 

th basis of my Ovm limited xp r i nc , I b liev 

tha t this co ndit io n p r vail d north a 1 ast as far 

a s Moun Adams nd ou th a 1 a t as far as Cr ater 

Lak , bu t was no n ssa r ily con tinuous in tim or 

58 
liLES 

( 1. 2) 

1.2 

(0 . 3) 

1.5 

(l . 2) 

2. 7 

(0 . 3) 

.o 

ROADLOG FOR CE TRAL HIGH C SCADE GEOLOGY 

BE D, SISTERS , !CKE ZIE PASS , A D SA TIAM PASS OR EGO 

Edwa rd M. Ta lor 

D partm nt of G ology 

Or gon Stat Univ rsity 

Start in B \.Ja y a nd wport Ave. in valle b t\.J n 

Awbr a nd d W. (out of town) on wport Av .-Shevlin 

Park Road. Ro d uts ahead ar fl in lavas asso iat d with inder co n s 

S.W . of A\vbr y Butt . Awbr a shi ld volcano of diktytaxitic basalt that 

was probably last activ during arly Pleistoc ne. It r sts upon tuf ac ous 

dim nts a nd lavas of th upp r part of h D s hutes Formation . Four High 

Cas ad ash- flow tuffs mov d W-to-E b twe n Awbr y and Ov rturf Butt s and ar 

now s n in scatter d outcrops and in xcavations, eastward to th Deschute 

Riv r . The ash-flow units ar : (1) D sert ~Tuff (black to gray, da itic, 

poorly w ld d, larg black pumice fr gm nts) , ov rlain b (2) Tumal Tuff 

(oran , rhyoda itic, poorly \.J lded), overlain by (3) Lava I la nd TuffTcr am 

to buff, rhyodacitic, complet ly d vitrifi d, ov rlain~(~v~Park Tuff 

(black, a nd sitic , mod ratel \.J lde d and rather well sorted along ash-flow ----

m rgins) . 

Right-hand urv . ot pumic quarri s on left . They r veal welded Tumalo 

Tuff with a basal, thin whit i ntralay r r sting on Bend Pumice which has been 

xt nsiv ly quarri d for aggragate . ote a red Plei~ ~e of altered 

cind rs on right-hand road side. 

L ft-hand c urv . Th las 0 . 5-mile of ro dway has follow d the Tumalo F~ult 

which b hind this poi nt xt nds 2 mils S.l2E . and ah ad of this point ext -nds 

i n a . 43\..J. dire tion for 1 mile, th n ontinu s . 38\..J . for 11 miles mor . 

Rocks on th S.W. sid of th fault hav be n displac d downward along each 

s gm nt but not uniforml in amount . Roadwa ah ad 1 ads ov r a narrow High 

Ca d mafi lava flow that mov d S.W. to . E. Th lava flow and und r lying 

Spring Tuff, Bend Pumi , and Tumalo Tuff have all ben displaced b th 

Tumalo Fault . Th ov r l ing Sh vlin Park Tuff has not b n displac d her but 

has b n ffs t alan th S. E. segm nt of th fault . orthw t of th 1 va, 

th urfac i mantl d b th nonw ld d faci s of the Sh vlin Park Tuff . 

gr d · 

J I 8 mil 

d h 

trip . 

h vlin Pa rk 

59 

Follow park road to bridge 

id of r k where 

b dd d lapilli and ash of th 

k bl ck of d ns ly 

d pumi fragments . Roadof Tumalo Cre k anyon, ut 
High Cas d L va Fl w eN 

Formation Lavas and Tuffs 

Tumalo 

High Cascad Lava Flow 

Cind r Con 

2 . 7 

Four As h-Flow Tuffs 

FIGURE 3. GEOLOGIC SKETCH MAP FROM MILE 0.0 TO MILE 16. 6 

60 

n Basalt 

Butt 
( 2. 2) 

5.2 

(0 . 7) 

5. 9 

(1. 3) 

7.2 

(0 . 4) 

7.6 

(0.6) 

8.2 

(1. 7) 

9.9 

(1. 8) 

11.7 

(0.4) 

12.1 

Roadway ah ad pass s W. of Tumalo Butte ci nder one whi h is older than th 

ash-flow tuff uni Road uts on .W. flank of co ne expos fine-grain d marginal fa is of Sh vl·n Park Tuff. Lev 1 terrain . of Tumalo Butte is und rlain 

by Pl isto en grav 1 deposits which occur stratigraphically between Tumalo and 

Shevlin Park Tuffs and hav b en ext nsiv ly quarried for construction aggr -

gat . 

Sharp turn right. Roadway for next mile is on top of a High Cascade mafic lava 

flow whi h mov d W. to E. Lava is overlain by Pleistoc ne gravels. 

L ft turn leading to N.E . downgrade, margin of lava flow. Roadcuts on righ 

e xpos basal co nta c t of lava resting on red, oxidized soil that develop d upon 

Tumalo Tuff . Nonweld d, pink upper zone of Tumalo Tuff has been remov d by 

erosion from lowlands to the . , revealing bedrock of orange, welded Tumalo 

Tuff between patches of Pleistocene gravel . 

Junction of Johnson Road and Tumalo Market Road. Desert Spring Tuff is exposed 

in vicinity of the junction and rests upon Deschutes Formation lavas . Turn 

left onto Tumalo Market Road . 

STOP NO . 1 : Broad shoulder on left side of road opposite orange-colored roadut. Examine vertical s ction through Desert Spring Tuff . From ere k bed to 

road 1 v 1: Partly collapsed black pumice fragments in pink, devitrified, 

weld d matrix are concentrated at base, but nearly absent above. From road 

1 vel to top of unit : Hydrated, orange and yellow, poorly welded matrix with 

abundant fresh, noncol lapsed pumice fragments . Alluvial gravels on tuff are 

overlain by Bend Pumi ce . Exposed in quarry a few 100-feet W: Bend Pumice 

overlain by very poorly welded and nonwelded Tumalo Tuff which contains a finegrained white intralayer at its base. Exposed in roadcut and quarries a f w 

100-fe t E: A NW-SE normal fault which dropped the section about 12 feet on 

the .E . side. Bedded pumice deposits are exposed at roadway curve; they belong to the basal member of Bend Pumice and consist of ash-fall and reworked 

pumice ash and lapilli. 

Turn left onto Tumalo Reservoir Road; proceed west. Pleistocene aliuvial 

gravels mantl the nearby t rrain. Quarries intermittently visib1e to the 

north have b en xcavated wherever the overburden of alluvium and Tumal0 Tuff 

is sufficiently thin to permit extraction of Bend Pumice . 

Tyl r Road . R commended side trip to examine welded facies of Tumalo Tuff . 

Follow Tyler Road 0 . 3- mile S. , stop at base of grade; outcrops are N. E. of the 

road. Thi wa n r t th . marg·n of th Tumalo ash flow . Pumi e fragments 

s 1 rg 1 oot ontain ores of the only fresh, white, nonhydrated glass to 

b ound in this unit. Along roadway ahead, alluvial gravels give way to 

at r d out rops of D schut s Formation mafi c lavas as the Tumalo Fault is 

approa h d . 

End of Tum lo R s rvoir Road : turn right and cross brid e over outlet str am 

from Upp r Tum lo R s rvoir. D sert Spring Tuff forms S. bridge abutm nt; 

D hut Form tion lavas ar xpos d along th N. ba nk of the stream. This 

r ad foll w h Tumalo Fault for the next 7 miles . 

STOP NO. 2 : Examin 

Tuff exp s d along 

fin grain d mar inal faci s f and sitic Shevlin Park 

.E. sid of road. Bull Flat (W. of the road) is underlain 

6 1 
(1. 8) 

13 . 9 

(l. 3) 

15 . 2 

(l. 4) 

16 . 6 

(l. 2) 

17 . 8 

(1. 1) 

18.9 

(l . l) 

20 . 0 

(0 . 5) 

20 . 5 

(4 . 4) 

24 . 9 

(0 . 7) 

25 . 6 

b 0 

vium 

h 

vi 1 

n 

Boundar b tw .n g 

Hill on th \ . id 

and it lava low . 

scarpm nt on th E. 

Old Pla invi w School 

Highwa · 20 . In this 

an earl Hi gh Ca a d 

broadl to th . E . 

umi · 

wn-d rc pp 

r Tuma l R s 

a d is th 

F rma ti 

0 th r ad . 

Fi 

E. 

n 

ur 

l 

3 nd 4. 

rop 

. \ . ' 

0 s hut 

rk Tuf , and a lluu1 R a ' iva i on of 

h tuff uni Ls a nd I I uin r 

to th 

whi c h 

b s 1 

i d 

dd d 

na 

ught 

i 

cti n with 

S. E. and 

pr d 

Int rs c tion of Gist Ro d with Hi ghw 20 . Turn l ft on 20 t ow rd Si t r 

Highwa cu rv l ft 

Sist rs, Pleistoce n 

are common . 

Junction Hi ghwa s 20 

r ound r oadcut 

alluvium a nd in 

and 126 . Pro 

Plei t oce ne alluvium, mo t of whi ch 

alluvium is und rlain in this a r a 

basalt flows . 

b 

in 0 hu 

nspi uou 

Form tion lav s . 

oute r ps o Pli n 

d \ . through Sist rs. Si ter 

has b n d posit d by Squ w Cr 

\vid pr ad Hi gh Cas ad dikt 

June t ion Highwa 20 and 24 2. Follmv 24 2 \v. t oward K nzi P 

vi w of Thr Sist rs volcanoes on S.W. skylin . 

62 

From h r t 

m fi e la va 

is built on 

k . Th 

a xi i c 

Ex llen t 
26 . 8 

Formation Lavas and Con s 

s Fo rrna ion La v 

High Cas ad Basaltic Lavas 

Tr ac 

High Ca ad B And i 

FIGURE 4. GEOLOGIC SKETCH MAP FROM MILE 16 .6 TO MILE 27 .1 

63 

d 

salt 
(l. 4) 

26 . 8 

(0 . 3) 

27 . 1 

( 2 . 7) 

29. 8 

(2 . 3) 

32. 1 

(0.9) 

33. 0 

(0.6) 

33.6 

(3 . 4) 

37.0 

(3. 0) 

40 . 0 

(0 . 3) 

40 . 3 

L \ 

i 

diktyt X 1 b· S Jt 

h highway as nd 

h' fly outw h 

Boundary b tw n g olo i sk h m~p , Figur s 4 and 5 . 

L ft-hand c urv at sit of Cold Spring . 

basaltic and it lava flow whos ur 

Lake, 9 mil s. \ . of h r Th highway 

the nex t l.l miles, th n a nd t~ 

o the r 1 . 1 mil es . 

Fourmil Butt e n abo ut 

Pl isto ne mafic 

lava flows issu d from on 

25 sq uar mil s . Th w stern par 

c ne moraine and outwa h d posits . 

Sha rp right-hand turn . Th highwa 

al moraine d po it d along th 

s urf 

urv 

id of 

i from th marg·n oE a 

n r So u h M thi u 

of t h ' f 1 tv or 

flatv for anFo urmi i on of 10 1 te 

nd Bl ck But te . So many 

ld now cov rs ppr xima ely 

buri d und r lat Pl i oaround th E. 

lat Pl ist 

nd of a 3-mil latern gla i r . Li le 

Butte , j ust . of the highw , is a t rminal mora'ne . 

Seco nda ry road This point mark 

basaltic lava from Belknap Crat r . 

flow for th n xt 3 . 2 mil s . 

th t r nmo t xt n ion of Holoc n 

Highway will follow th S. margin of th's 

Windy Point . Glaciated promontory of ba altic nd it lavas and cind rs a 

the .W. base of Black Crater . Excellent vi w of th bro d shield o Mount 

Washin gton s urmount d by glacia t ed s ummit on nd plug. In for ground i 

Holocene basaltic a nd esite lava from Yapoah C n . Farther .W. is th'· lava 

f ield of Little Belknap . Highwa ahead will follot the margin of Yapoah lava 

for 3 miles. Outcrops of glaciat d basaltic and from Bla k Crater and 

basalt from smaller Pleistoc ne i nd r on s n S. E. of h highway. 

Highway cuts through E. lev e of Ya poah flow. S v ral flow units ca n b s n 

in whi ch thin lenses of dense la va a r e sepa r at d by thick r lay rs of rubbl 

STOP NO. 3 : Dee Wright Observatory . From th obse rva tory roof th followi ng 

landmarks are seen pro ceeding clockwise f r om true north in azimuthal d gr es. 

0 Mount Jefferson: Andesitic composite volcano . 

7 Cache Mountain: Glaciat ed basaltic andesite vol cano, 0 . 9 m. y . old, wit h 

late Pleistocene basalt cinder co nes and lavas on summit. 

11 Bald Peter (far horizon): Basaltic andesite volcano, 2 . 1 m.y . old, de eply 

glaciated , constructed against the N. end of the Green Ridge fault escarpmen t . 

20 Dugout Butte (forested foreground): Glaciated diktytaxitic basalt flows 

of the High Cascade platform. 

64 
~;:== == ==;--~ 

33 . 6 33 . 0 

32 . 1 

High Cascade 

Basaltic Andesite 

High Cascade Basalt 

High Cascade Basalt 

Lat ral Moraine 

Terminus of Lava from Belknap Crater 

South 

Belknap 

Lava 

Lava From 

Belknap 

Crater 

La va 

La va 

FIGURE 5. GEOLOGIC SKETCH MAP FROM MILE 27.1 TO MILE 34.5 

AND FROM MILE 34.5 TO MILE 44.6 

65 

I 
30 Green R"dge (far horizon): N-S fault block moun ain , 20 miles long . 

Escarpment faces W. Lavas on cr st are ?-6 m. y . ol . 

40 Black Butte (background) : B s ltic cinder cone located at S. end of Gr en 

Ridge . Older han any of he o her visible High Case e volcano s ; well 

preserved form is due to lack of glaciation east of the a cade Rang . 

Bluegrass Butte (foreground): Gl ciated b saltic andesi e cinder cone . 

Ridge E. of cone is a lateral moraine. 

82 Black Crater (fills most of eastern sec or): Lat Pleistocene basaltic 

andesite volcano. "Cra er " is actua ly a glaci l cirqu open o the N.E. 

105 Unnamed Cascade summit ridge composed of glaciated basaltic andesi e 

to cinders, bombs, and lavas which issued from a -mile-long ch in of cones. 

155 Probably was the site of spec acular lava fountains nd eruptions of 

unusually large and abundant volcanic bombs during the late Pleis ocen . 

168 North Sis er (elev . 10 , 085 fee) . Basaltic and si e composi e vo cano on a 

broad shield . Central plug and dike systems exposed by glacial erosion . 

At base of North Sister stand Yapoah Cone (lef ) an Colier Cone (righ ) . 

174 Middle Sister (elev . 10,045 feet): Composite volcano supporting Co ier 

Glacier . Predominantly olivine basalt porphyry but also contains flows of 

basaltic andesite, andesite, dacite , and rhyodacite. Younger han Nor h 

Sister. 

178 Summit of Little Brother and ridge W: asaltic composite volcano with 

exposed plug and dikes. Older than North ister. 

188 Four-in-One cinder cone (below skyline): A ridge-cone breached in four 

places by andesite lava flows, 2600 years ago . Part of a N-S alignment of 

19 vents. 

197 The Husband (on skyline, partly obscured): Although of late Pleis ocene 

age, it i s one of the oldest and most extensively dissected volcanoes in 

this region . The exposed plug of basal ic andesite is 1/4-mile in diameter. 

218 Condon Butte : Late Pleist ocene cinder cone surrounded by glaciated lava 

field of basaltic andesite . Nested summit era ers. Knob visible at left 

base is an unnamed glaciated dome of rhyodacitic obsidian . 

235 Horsepasture Mountain (far horizon): Western Cascade peak. 

256 Scott Mountain: Small summit cone on a broad glaciated basa tic shield . 

282 South belknap Cone: Cone was formed and breached 1800 years ago, then 

surrounded by basaltic andesite lava from a nearby vent about 1500 years 

ago . 

285 Unnamed twin steptoes (in foreground lava field): Glaciated basaltic anand desite volcanoes surrounded by lava from Little Belknap . 

306 

309 Belknap Crater (summi t cone on skyline): Focal point of a l ong-continued 

and complex episode of Holocene basalt and basaltic andesite volcanism. 

The broad shield which fills the N.W. view is 5 miles in diameter ; it is 

estimated to be 1700 feet in maximum thickness and 1 . 3 cubic miles in 

volume . The volcano probably contains a core of cinders which inter66 
(0 . 3) 

0 . 6 

(0 . 5) 

41.1 

(1 . 3) 

2 . 4 

(0 . 6) 

3. 0 

(1 . 0) 

4 . 0 

h ummit 

s 

\ . 

main 

ir v nts . 

321 Littl B lknap : ano, built 2900 ar ago on h 

E. flank E th 

340 Mount ~ shingt f t) : Gla iat d r mn nt of a larg basalti 

C n ral plug ~s lank d b -S swarm of dik 

an 

ri 

\.Jh 

h 

ral 1 

h 

Highw 

dm.;rn 

and it omposit 

in ummit on 

h 

d 

Th 

n 

The visitor aero 

tub collaps d and 

lava by th rmal contraction and los 

onsolidation . On th E. sid o the main hann 1 is 

visitor mi ght w 11 imagin the hann 1 in full flood , 

high a th 1 v cr st nd spilling ov r th ast rn flank . 

lava diminish d molt n int rior drain d and th 

subsid d . Lacking support, large s gm nts of th tipp d 

ca usin g d p, irr gular t nsion c racks to op n in 

f rd a vi w of on of th s c racks and o vet bas of th 1 v 

Yapoah flow W. margin . Tr s grow r adil upon 

par l upon th old r Lit le Belknap flow . This 

r d with blocks , int rstiti 1 scori , nd 

, noti 

fly from 

oh r n t . In g n ral , 

clj.._ma te and 

highway 1 ves th 

dark soil . It is 3 

cone . 

f a Littl B lkn p lava tongu which pour d S. E. 

r 0 . 

in 

uth 

in i m r r nt 

and in its nbundan 

67 

he light- olor d b drock m rgimov d . \ . from h r tow rd a 

\vh r 

1 v 

vall y has be n buried 

it ross s an alluvial 

from South B lknap 

B lkn p lav i distinct 

urf on 1 r g 

in - gr in d glom r ph no-
st~ L'f p l.1 g i h ' l ,lst' , u l i v inL' , .tnd t·l in(l p. r llx n · . Th i s l nv<l \vas v ru p l ' d I ROO 

v ,;lt"S .l)!,l. Tn Lll' . i s ;1n l':-;t' •IIPnl v i. '\v o S. s i {'P' of lk l k nnp v o l t'd no . '1\vin 

Cr <llL' r .· c ind ' r ' t ) l1 ' i s visibl ' t(l Lll • I . : i.l \v ;t s ;H· tivt' L )O() '<H s ;tgo . 'X L 

1 . 1 mill's '~ lds v ' r g i ;J · i ·1L " d l ;1v;1 r o m Co ntl n n Bull t' . 

0 . ) 

. 6 B undar b t\v 11 g o l tg i · sk ' t t h m'-lp s , l· i g ur s "' <1nd 

(0 . 7) 

5. 3 

(1. 2 ) 

46 . 5 

(0 .8) 

47.3 

(0.4) 

47 . 7 

(0 . 4) 

48 . 1 

(3 . 3) 

51.4 

(2.1) 

53.5 

Thr 

a n 

hi 

L f t-hand urve; highway 

basalti a nd sit lava f r 

E. of this point, spill d 

canyon to a point 9 mil s 

Parkin g a r on 1 ft 

Colli r Con 

way cuts a nd 

thin, slaggy 

walls of th Lost Cr 

The Husband r nco unt 

b 

Hairpin c urve t o the right with parking r a . 

trail about 0 . 8 mil s t o Linton Lak . Tr il 

levees and th ntral lava gutt r, ea h 

composition , but all from Colli r Con . 

tween 500 a nd 2500 year ago . With r e p 

ma r g in 

Th 

min g l 

d fr 

lier Cone is similar to th much-studi d M xi a n vo] 

lier lava dammed Linton Cr k t o fo r m Linton Lak 

upon 

f o red 

l ava 

nt 

t rn 1 b of 

3000 f t 

r of th 

Follow 

d lava 

h mi cal 

b -

ColCo l -

Parking area and trail to Proxy Falls. Re omm nd d sid trip. On th s hort 

trail to the falls is a view of th Deer Butt plug a nd associat d ind r con 

enclosed within la vas of a shi ld vo lca no . A a r s ult of i nt n gla iation 

on the . canyo n wall, the entire butte is xpos d in c r oss ti n. Th trail 

68 
eN 

Earl High 

Cascade 

Lava From Sims Butt 

Lava From South Belknap 

Lava From Twin 

Husband 

From Colli r Cone 

FIGURE 6. GEOLOGIC SKETCH MAP FROM MILE 44.6 TO MILE 57.9 

69 
(2 . 5) 

56 . 0 

(1. 9) 

57 . 9 

(0 . 9) 

58 . 8 

(3 . 6) 

62 . 4 

(l. 0) 

63 . 6 

(1. 3) 

64 . 9 

J ad 

fall 

b d 

r 

Boundar 

gl 

Cl ar a r 

it 

For th 

gla 

ad 

vol ani 1 

b 

Road to th 

dar 

tuff is a ri s o 

intracanyon basalt flow whos 

x 11 ntl y expos d. Road 

highway r v al m fi 

nd 

70 

rm i nu · . M n 

d b prings 

Pr xy FalL i · h 

7. During h 

hat a m Jl 

miL d \vn n 

w 

rom 

ri. n t r -

a 

rod y 

d of 
71.4 

oY 

70 . 9 

67 . 6 

Late Pleistocene Termi nal 

E rly High Cas d Ba lt 

Out rson F rm tion 

Mo unt i n 

ad 

nd Basaltic 

From 

Butt 

FIGURE 7. GEOLOGIC SKETCH MAP FROM MILE 57.9 TO MILE 72.3 

71 
( 2. 7) 

67.6 D r Cr 

High Ca 

(2 . 0) 

69 . 6 Oll 

(l . 3) 

70 . 9 STOP 

(0 . 5) 

71.4 

tur 

llO\ving 

oli vin 

(most 

i 

or of any oth r parti ulat 

the basal 6 fe t of the uni 

thick with sharp int rfac 

ge neous interior . Additi 

(1) 

Right- hand cu rve; h droelectric powerhouse on opposite 

Water is co nducted to the facilit through a tunnel rom 

2 miles W. Highway cuts on the right how ev r 1 sprin 

aquifer of High Cascade pillow basalt . 

72 

d, 
(0 . 9) 

72 . 3 

(0 . 5) 

72 .8 

(2 . 4) 

75 . 2 

(0 . 7) 

75 . 9 

(l.l) 

77 . 0 

(l. 5) 

78 . 5 

(l. 7) 

80.2 

(0.5) 

80. 7 

(l . ) 

8 . 1 

B und r b tw n g olo 

Hi gh\vay fill 

r st f a 

th upp r M 

n xt mil . 

South m r gin 

8 mil . E. , nt r 

p ur d in a do ub l 

s 

Cr k v 11 y . B gi nning of a lo ng g rade up ont o th 

thin Hi gh Cascad basalt flows which ar intraca nyon t o 

Riv r . Th flows a r w 11 xpos d in hi ghway cuts for the 

basalt flow. Lava f rom th . bas of Belknap Cra t e r, 

Mc K nzi Ri v r ca nyon he r , 1500 years ago . The flow 

liffs t o th \ . . a nd spr a d out upon the ca nyon 

s downstr eam . During th e Ple istocene, prior to 

ld volcano a nd r la ted lava fields, the entire 

f r om ic b twe n Mount Washingto n and Black 

r a t this po int . Lava flowed through this valca nyo n man y tim s ; t h intraca nyon basalt flows 

last 2 miles we r e so derived . 

S r i of highway c uts a nd cliffs t o th ri gh t xpos many thin lava flows 

dat d at 1 . 2 m . . Th a r e probably a part of a larg early High Cascade 

basaltic a nd sit shi l d volcano wh ose central plug and flanking lavas can be 

s n i n th opposit wall of McK nzi Riv r ca nyo n, 1 mile W. 

Sahali Falls parking lot . Re comm nd d scenic · Two thi c k flows of basalti a nd sit lava from th Sand Mountain chai n of cinder co nes (3 . 3 miles E. of 

h r ) mo v d into Mc K nz ie Riv r ca nyo n in this a r a, 3000 years ago . The terminus of th fi r s t flow is marked b Koosah Falls, 0 . 4 mi le s downstream . The 

s ond flow , wh os t rminus is mar~ed by Sahali Falls, is seen in the highway 

c ut ah a d . 

Road on r ight to Cl ar Lak . Cl ar Lak is 1 . 5 miles long, fed b large 

sp ring , and is mor than 120 f et d ep in pla es . It wa s fo rmed behind a dam 

of lava whi h issu d f r om a cind r con S. of Sand Mountain and poured across 

th M K nzi Riv r . Rising wat rs inundat d a standing fo rest; several dozen 

s na gs a r s t ill r oo d on th lake floor. \ood f rom t he snags and charcoal 

from b n ath h ast-sho r lavas a r both 3000 C- 14 years o ld . 

South margin o Holoc n lava and out le t c hannel f r om Fish Lake . The lava came 

from a r l v n of th Sand Mountain cha in of cinde r cones and dammed Hackleman Cr k t o fo rm Fish Lak 3800 years a o . The lava is basaltic and relative-

! thin with smooth rop surfa · tr e molds are ammon . 

Road t o 

73 

ba -

fl w from as h 

appa r nt l was not th 

om tim s sensitiv i nd icators 

The jun -

n s pa r a t occasions: 

Crat r and Sand Mountain 

. ) ( 2) ubs qu ntly 

j un tion), a nd (3) 

f a h rat r (bl k 
8 . 1 

Hi gh Cas a d Ba a lt 

78. 5 

·ili gh Cascad 

Ba s alti c And es it s 

77 .o 

L A V 

80 . 7 

Lava Fr m 

S,nd luntain 

h in Con s 

75 . 9 

Lav From 

B lkn p r t r 

75 . 2 

L t Pl is o n 

In trac nyon B lt 

Hi gh Cascade Basal t i And s i s 

FIGURE 8. GEOLOGIC SKETCH MAP FROM MILE 72.3 TO MILE 83. 5 

74 
( 1 . 4) 

83 . 5 

(0.6) 

84 . J 

(0.6) 

84. 7 

(0.6) 

85.3 

(1. 0) 

86.3 

(1. 5) 

87.8 

(1 . 5) 

8 . 

lava front ab ut 200 f t S. f th jun tion). 

Boundar b tw n g sk t h maps, Figur s 8 and 9 . 

nt to th highway on right, is a short lava tub 

r py rust on its st rn flo r. 

Sawy r's Cave , adjawith a well-pres rved 

Gradual right-hand urv aero s a lava fi ld compos d of basaltic andesite 

fl ws (S.E. of th highway) from ash Crat rand ounger flows ( . W. of the 

highw y) fr m Littl ash Crat r. Fin ash from a~h Crater fell on both of 

th flows, but th resulting deposit ·s thin and to be se n it must be recov r d from int rstic s in th lavas . 

R ad to quarri s in Little ash Crat r, left . R commended side trip, but not 

uitabl for a arg bus. Follow grav 1 road to top of cone . \ atch for trucks. 

Littl ash Crat r has be n breached on th \v. by lavas which extend from the 

bas of th breach d area, westward for 1. 7 miles . The quarried areas are 

litt red with discard d volcanic bombs and bomb fragme nts up to 4 feet in diam t r . The top of the con affo rd s a god view of the maturely dissected 

W st rn Ca cad escarpment and nea by High Cas ade Holocene cones and lava 

fi lds . Follow a grav 1 road W. and . from th co ne to Highway 22 and examine 

th lar road cut . of 22 . It r v als a later Pleistocene terminal moraine 

m ntl d with a thin la r of Mazama ash, overlain by a bed of fine mafic ash 

from ash Crat r . This is ov rlain b coarse rock fragments blasted from bedrock and morain ov r th Little ash Crater vent . The top layer in the c ut 

consists of mafic lapilli from Littl ash Crater . Tur n E. on Highway 22. 

San iam Jun tion; Highways 20 and 22 . Continu E. on 20 . Late Pleistocene 

lat ral morain s form ridges to th and S. of th junction area, outlining 

th last advanc of gla ial ic . Holoc n volcanic activity in the junction 

rain lud s : (1) arly basalt flows that moved . W. toward Lava Lake from 

v n s b ash Crat rand Sand ountain, (2) basalt flows from ash Crater, 

(3) d v o th Lost Lak chain of con s and associated basalt flows, 

and (4) and sit flows from th S. and . \ . bases of ash Crater. 

During phas of activity at ash Crater, Little ash Crater ~as 

form d and br a h d by flows of basalti and site . All of these vol ca nic epis d s d posit d 1 va or ej eta or both in th vicinit of Santiam Junction. 

Broad 1 ft-hand curv 

thi part of th chain 

and th resulting ridg damm 

Th on f the highway 

on of Lost Lake chain of cinder cones. In 

w r constructed across a glacial canyon 

Cr k to form Lost Lake (one mile ahead). 

rater 1000 feet wide and 300 feet deep. 

In highwa uts and on hillsid . of th highwa are several outcrops of columnar and plat - jointed basaltic and sit lava from a glaciated cone, 1 . 5 miles 

Lak . Old r rocks in the S. wall of this ca nyon have been 

0 . Th 1 v o v rl in b a m ntl of reworked 

L k nd Li tle ash Crater. 

advancing 

75 
L va Fr m l.i lt l 

c n , La L ' r a l Mo r a i n 

Los L Lak ' ' n •s 

!lo g 1 Rock And •s i · Dome 

High Cascade Basalts Blue Lake Crat r 

FIGURE 9. GEOLOGIC SKETCH MAPS FROM MILE 83.5 TO MILE 91.6 

AND FROM MILE 91.6 TO MILE 99.8 

76 
(1 . 9) 

91. 2 

( l. ) 

2. 6 

(2 . 5) 

15 . L 

(0 . ) 

95 . 5 

(1 . 0) 

9 . 5 

(l. 3) 

97 . 

(2 . 0) 

99 . 8 

(1 . 5 ) 

101. 3 

( 2 . ) 

I 0 . 

Summit of Santiam Pass . orth rn margin of a larg glaciated field of basalti c 

h spr ad E. and W. from cones a nd lava v nts along the High Cascade 

from Highway 20, S. to Mount Washington. It should be noted that 

ind r , so pr val nt on road cuts between Little ash Crater and Suttle 

b n d p sited by snowplows and are not part of the natural str atiTrailh ad to Square La ke , left. From this poi nt to the E. base of the High 

C s ad platform, basaltic a nd esite lavas of r everse paleomagnetic polarity 

d in highway c uts . 

v rlook, right. Blu Lake occupies a Holocene c rater sur r ound e d by 

rim o c ind rs , spatter, and bombs of basalti c a ndesite, ejec ted 3500 yea rs 

Although no lava appeared, the eruption must have occurred with co nsid e rviol n , for most of the cra t er was blasted out of solid bedroc k and 

1 rg fragm nts were scatter ed i n all Qjrections. The landscape is blanketed 

with c ind r s fo r 3 mil s E. and S. E. 

ha v 

th y 

curv . Highway uts in thick, platy-jointed basalti c andesite flows 

n dat d by K-Ar at 0.54 m.y., but must be at least 0 . 7 m. y . be ca use 

ss rev rs paleomagn tic polarity . 

Midpoint of long downgrade . of Suttle Lake. Glacial drift of a late Pleistoen lat ral moraine which forms an E- \ ridge 2 miles long and 600 fee t high is 

xpo d in sid highway cuts . Suttle Lake r ests in an elongate basin enc los d by lat ral a nd terminal moraines . Cache Mountain and Mount Washington 

ar o the S.W. 

Lak , right . ext 0.5 miles of highway will lead through late 

t rminal morain s . This r ep r esents the lowest elevation rea c hed by 

an ext nsion of th ic sheet which accumula t ed between Mount -Washington and Thr e Fing red Jack . Th moraines are covered with 0.5 t o 1.0 feet of 

fin ash from th Sand 1ountain chain of cind e r co nes and by approximately 1 

foot of younger cinders from Blue Lake Crater. 

Boundary b t w en g ologic sketch maps, Figures 9 an d 10. Hi ghway leads over irr gular su r fac und e r lain by a widespread fiel d of Pleistocene diktytaxiti c 

basalt flows . 

J un 

_____ Highway lQ, turn left on r oad to Metolius River a nd Camp 

ahead will lead cloc kwis e around the base of Bla ck Butte. The 

t high and 4 miles in diame t r. It is a composit e co ne of 

nd 1 t d at th S. of the Green Ridge fault 

n . Turn ri h n . . F. . . RD 1 

77 

, Bl Butte i older than many 

Thr e Fi nge red Ja k a nd Mo unt 

dow f the C s ades. It 

flanks , r ocks on its urlav s posse s reverse 

but th standard err ors 

d lava f i ld of 

"H ad f M toli us . " 
107 . 0 

High Cascad Basalts 

High Cas ad 

Cr n Rid Faul 

of D s hu 

F nnat ion 

109.4 

V l no 

L v s o D s hu s 

,. 

).' 

6:' . ·~ 

~ 

" 

Format ion 

,, ::a. ::-==::: .. -=--';:. ~ .... 

I 

FIGURE 10. GEOLOGIC SKETCH MAP FROM MILE 99.8 TO MILE 119.0 

78 
(0 . 2) 

104 . 1 

( 1. 9) 

106 . 0 

(l. 0) 

107 . 0 

(0 . 3) 

107 .3 

(2 . 1) 

109 . 4 

(4 . 3) 

113 . 7 

( 5. 3) 

119 . 0 

(0 . 7) 

11 . 7 

( . ) 

12 . 

(3 . 0) 

127.6 

S c d, U. S . F. S. . RD 1430, turn~ a nd follow thi road uphill 

f r Th Riv r m rg s from la r g springs at th b of 

Bla k Butt , 1 mil his point . Bla k Butt and yo un g r Cascad 1 va 

hann 1 of a n rth- Elowin g an tral M tolius Riv r . Groundwat r 

still m v s through thi cha nn J, m th . margin of Black Butt 

1 va . Larg b uld r s f Bl k Butt abundant along this road, but no 

ut · rop will b s n. 

L ft-hand urn f r om 

mitt nt o ut rops of 

n or th n xt 1 

f Black Butt t o W. lop of Green Rid g . Int rs parat d by flow breccias will be 

find vi wpoin n ar top of grad . Parts of one of the most signitural f atur s on th E. flank of the c ntral Cascad , n m ly the 

fault blo k, an b s n from this point. Th west- fac ing s arp -

. fo r 15 mil s and rises 2000 f t above th Metolius Riv er 

Basalt, basal ic a nd sit , a nd and sit lavas with interbedded ashlow a nd air- f 11 tuffs ranging in ag from 5 . 0 to 9 . 2 m. y ., o c ur in the face 

of th scarpm nt; th y hav b n trac d E. into th co rr lative Deschutes Formation . M gmas of Bla k Butt probabl asc nd d through the Green Ridge fault 

z n and a w 11 pr rv d ma fi cind r cone and lava flow of Pleistoc ne age 

also r sts on th fault trace, 3. 0 mil s of h r . Several High Cascade 

p aks can b s n t o th W. Most promin nt ar Mount \. ashington (S . l . ) , Three 

Fing r d Ja k (W . ) , and Mount J fferson ( .W. ) . 

J un ction with Allingham Road, . S . F . S. o . RD 1120 . Tu rn right and follow 

RD 1120 south . Th hillsid visibl about 0.5 mil s E. is the S. end of the 

main Gr n Ridg s a rpm nt · the roadwa y leads across part of a ramp structure 

b w n two parall 1 faults . Large altered boulders of Deschutes Formation 

lava wil l b s n a ong the roadside for the next 5 miles . 

Jun tion with road 1 ading E. (RD 1139) . Continu south on . S. F . S. RD 11 

to rejoin Hi ghway 20 . 

Jun tion with Highwa 20 . Turn 1 ft toward Sisters. From her to Sisters the 

highwa c ross s a n arly flat alluvial plain hi fly co nsisting of late Pleisto e n outwash ands and g ravels from Hi gh Cas ad glacial s stems . The alluvium r sts up on ea rl Pl istoc n lava flows and is overlain by late Pleistoc n cind r cones and a so iated lavas. 

Boundary b twe n G ologic Sk tch Map , Figur s lOand 11 . Junction of Highwa s 

20 nd 242 ; Town of Sist rs. Pro eed E. through Sist rs . 

J un - i n 

J un i n 

n x 2 

whi h pr 

h 

Hi hw y 20 nd 126 . Pro 

79 

d . E. n 20 toward B nd. 

oward B nd on 20 . For the 

High Cas ad basalt porph r 

in th D schut s Basin . 

hw ' 20 . Low hills on 

lti and sit 1 vas n ar 

1 v 1 p rt of highwa for 2 . 2 
Tr of Tumal 

11 9 . 7 

E· r lv High as ad' 

/ ln Lr can 

/ Porphvrv 

D . hu s Form ion L·1V 

on B· s, 1 L 

FIGURE 11. GEOLOGIC SKETCH MAP FROM MILE 119 .0 TO ILE 129.1 

80 
(1 . 5) 

129 .1 

(0 . 2) 

129 . 3 

(2 . 6) 

131 . 9 

(1.1) 

133 . 0 

(0 . 5) 

133 . 5 

(1. 2 ) 

134 . 7 

(1. 3) 

136 . 0 

( 3. 3) 

139 . 3 

mil s ah ad is und rl ai n by D s rt Sprin Tuff , obs ur d by alluvial cov r toward ll . W. 

B unda ry f G oJ gi Sk t h Maps , Figur s 11 a nd 12 . 

ull y 

a h fl 

xpo d pink D s rt Spring Tuff in bo th banks . The 

this a r a , t owa rd Dry Canyon . Hi ll 0 . 7 miles E. is an 

ind r on . 

Jun ti n of Cou h M rk t Roa d and H'ghwa y 20 . Th low hill E. of the highway 

a m 11 blo k o upp r D schut s Formation mafic lava bounded by northwestr ndin g no rmal faults . Doz ns of s uch fa ults o cur . E. of the Tumalo Fault 

n Gr n Rid g a nd B nd~ th y are part of th e Brothers-Sisters Fault Zone . 

Laidl \.J Butt 

c tiv aft 

thi ar a . 

th ri ght. Laidlaw is a Pl istocen c inder cone which was 

rt Spring Tuff, Bend Pumi c , and Tumalo Tuff were deposit ed in 

Right-ha ~d urv and downgrad to floor of Des chutes River valley. Highway cuts 

along h grad xpos pink nonw ld d Tumalo Tuff in a position close to the 

. E. ma r gi n of th Tumalo ash flow . The ash flow moved 3 miles farther . E. and 

n unknown, but probably much great r distanc eastward, where it is now buried 

b n ath Pl isto n lava fields . 

Highwa y 20 ross s Des chutes River . Alluvium on the valley floor has been quarri d fo r g r a v 1 . Highway grade up the E. valley wall exposes Bend Pumice and 

Tumalo Tuff (which are underlain b D sert Spring Tuff at water level), overlain 

by grav 1 b ds . At th top of the grad , basaltic lava flows rest upon the 

grav ls. 

On of sev ral Highway cuts through basaltic lava which exposes pressure ridges 

in ross s ction . The lava was moving .W. in this area from late Pleistocene 

v nts on th . fla nk of ewherry Volcano . The Deschutes River was forced westward to its present co urse along the west margin of the lava fiel d. The city of 

Bend, E. of the river, has been built upon this lava field. 

Junction of Highway 97 and route to city center . Turn right and proceed to 

downtown Bend. Turn right again at Newport Avenue. Proceed to College Way and 

completion of field trip circuit at 142.0 miles. 

81 
Laidlaw Butt 

Cind r Co n 

QAL Unde r lain by 

High Cascade Ash- Flow Tuffs 

eN 

Ea r ly Pleistocene 

of Awbr ey Bu t te 

D~~» rt , p ri ng Tuf 

Pl i s O<. _n ' Bas 1ts 

Fr om or t h Fla nk 

of wb >rr 

Vol a no 

FIGURE 12. GEOLOGIC SKETCH MAP FROM MILE 129.1 TO MILE 142. 0 

82 
REFERE CES 

Armstr ng , R. L., Tayl r , E. 1., Hal s , P. 0. , nd Park r, D. J. , 1975, K-Ar dat s for 

v 1 ni r ks, ntral Ca ad Rang of Or gon : Iso hron/\~ t, o . 13, p . S-10 . 

Pe k' D. L. ' 

G olog 

. S . G ol. 

r, H. G. , W lls, F . G. , and Dol , H. M. , 1964, 

ntral a nd north rn parts of th W t rn Cascade Rang in Oregon : 

Pr f. Pap r 449, 56 p . 

Pet rson, Cr h, E. A. , Taylo r, E. . , and St n land, D. E. , 1976 , C ology and 

min r 1 r so ur of D hut C unty, Or gon : Or . D pt . Geol . Min . Ind . , Bull . 89, 

66 P· 

Scott, \ . E. , 1977, Qu t rnary gla iation a nd vol anism, etolius River area, Oregon : 

G 1 . So . Am . Bull . , v . 88, p. 133-134 . 

Sutt r, J. F. , 1978, K-Ar ag s of C noz i v lcani ro ks from the Oregon Cascades west 

of 121 °30 ': Iso hr n/W st, o . 21, p . 15-21. 

Ta lor, E. M., 1968, Ro d id g ology, Santiam and McK nzie Pass Highways, Oregon : Ore . 

D pt . G ol . Min . Ind . Bull . , v . 62 , p . 3-33 . 

--------~~~ · 1978, Fi ld g ology of \~ . B:ok n Top Quadrangl , Oregon : Ore . Dept . 

G ol. Min . Ind. , Sp cial Paper 2, SO p . 

Thayer , T. P., 1937, P trology of lat r T rt ia r y and Quaternary rocks of the north- central 

Cas ad Mountains in Or gon : G ol . So . Am . , Bull . 48, p . 1611- 1652 . 

83 
84 
NEWBERRY VOLCA 0 , ORECO 

orman s. MacLeod , David R. Sh rrod, U. S . G olog ical Survey, M nlo Park , California 94025 

La wr nee A. Chitwood , U. S . Forest Se rv ice , Bend, Or egon 97701 

and 

Edwin H. McK e , U. S . Geological Su rvey , Menl o Park , Califo rn ia 94025 

GEOLOGIC SUMMARY 

ewbe rry Volcano , centered a out 20 miles 

southeast of Bend , Ore on , is among th lar~ s 

Quaterna r y volcanoes in the conterlllinous ~nited 

States . It covers an area in xcess of 50() mi , and 

lavas from it xtend nor hward many ns of miles 

beyond the volcano . The highest poin on the 

volcano , Paulina P ak wi th an elevation of 7 , 984 ft , 

is about 4 , 000 ft higher than the errain 

surrounding the volcano . The gently sloping flanks, 

embellished by more than 400 cinde r cones , cons is 

of basalt and basaltic andesite flows , anclesitic to 

rhyolitic ash- flow and air-fall tuffs and other 

types of pyroclastic deposits , dacite to r hyolite 

domes and flows , and alluvial sediments produced 

during periods of rosion of the volcano . A 

Sewberry ' s summit is a 4 - to 5 - mile-wide calde ra 

that contains sc nic Paul ina and Fast Lakes . The 

caldera has be n the site of numerous Holocene 

eruptions , mostly of rhyoli ic composition , tha 

occurred as r c ntly as , 400 years ago . 

Many geologists have tudied various a pects of 

Newberry Volcano startin~ wi h I. C. Russell (1905) 

11ho visited i durin a horseback r econnaissance of 

central and east rn Oregon in 1903 . Howell Williams 

(1 935 , 1957) mapped the flanks of the volcano in 

re connaissan c and studi d the caldera in mor 

detail. His outstandin work forms the asis for 

subsequent investi ations , most of whic h have 

focused on caldera rocks o r oung flank asal 

fl ows . o comprehensive study has be n made of the 

geology of ewberry ' s forest- covered and azama ashcovered flanks even though they form more than 95 

per cent of th a rea of the volcano. As part of a 

geothermal resource investieation of central and 

eastern Oregon , the first three autho r s have mapped 

the sixteen 7- l/2 ' quadrangles that cover the flanks 

of ewb erry at a scale of 1 : 62 , 500 , and 

reinterpre ed and par ly remapped the calde ra . 

Highly g neralized geologic sketch maps are shown in 

igures 1 and 2 . The new mapping and K- Ar dating by 

he las r qui r ant ial r in erpr a i o n 

of h of f o rma ion of 

i . 

ewb rr l i 40 mi l 

Ca cade Ra ng in a Re 

Volcano in Ca li fo rni a 

vo . ) . Bo h 

b 

con s , s imil Ar 

he si s o ru p i o ns 

obsi dia n f lows during th las 

c ivi within 

h 

Lak 

his 

y ars . 

e n 

and 

85 

ewbe rr y lies at the west end of the High Lava 

Plains , a terrain formed of Miocene to Quate r nary 

basalt flows and vents punctuated by rhyolitic domes 

and vent complexes (\~alker and othe r s , 1967 ; Greene 

and others , 1972; Walker and olf , this vol.) . The 

rhyolitic rocks show a well- defined monotonic age 

progression starting at about 10 m. y . east of Harney 

basin and decreasing to less than 1 m. y . at 

ewbe rr y ' s eastern bo rd er (Walker , 1974 ; MacLeod and 

o thers, 19 75; McKee and others , 19 76) . ewberry 

rhyolites appear to be a continuation of these ageprogressive rhyolitic r ocks . 

A complexly faulted te rrain surrounds 

e wberry . The northeast-trending \~alker Rim fault 

zone impinges on e wberry ' s southern flank but 

offsets only its older flows . A zone of faults that 

offsets olde r flows on the lowe r northern flank 

extends northwestward into the Cascade Range at 

Gre n Ridge . Although e wberry lavas obscure the 

r ela tions of the Walker Rim and Gr een Rid e fault 

zones , they likel join ben ath e wbe rr y and 

represent but one cu r vin fault system. The 

Brothe r s fault zone, a major west- northwest- trending 

zone of faults, extends ac r oss the extreme 

northeastern flank but does not apparentl offset 

surficial ewberry flows . It probably extends a t 

depth to join or abut against the Green Rid e - Walke r 

Rim fault zones . 

The north and south flanks of ewberry Volcano, 

which extend the g r ea test distances f rom the summit 

calde r a , are almost exclusively venee r ed by basalt 

and basaltic andesite flows and associated vents . 

The basalt flows form much of t he surface in a broad 

region extending far nort h of the vo lcan o (Peterson 

and Gr oh , 1976) as we ll as southward to the Fort 

Rock basin . Individual flows are a few feet to more 

than 100 ft thick and cover areas of less than 1 

squa re mile t o many tens of square miles . Flow 

ma r gins are commonly well preserved even on older 

flows, but the flows are complexely interwoven and 

it is difficult and time consuming to trac& 

individual flow boundaries . Most flows are of block 

or aa type; pahoehoe surfaces occu r locally on a few 

lower flank flows . Lava tubes are common , and some 

ext nd uncollapsed for distances of 1 mile (Greeley , 

1971); som lower flank flows may have been fed by 

tube systems . Casts of trees occur in many flows, 

particularly the younger ones . 

The basalt and basaltic andesite flows can be 

readil divided into t wo groups on the basis of 

their a e relative to Mazama ash ( 14c age 6 , 600-

, 700 years) derived from the volcano at Crater Lake 

70 miles dis ant . The t/,oungest lava flows over lie 

azama ash, and their 1 C ages range f r om 5, 800 to 

6 , 380 ars c14c ages of this magnitude are 

en rally about 800 years younger than actual 
10 MILES 

C:::===~~;;;;;;;;;;;;;;;--

8 

8 

E] 

'o a 

lr 

( Pl ANA l ION 

Und•lle•tnt•JteO DJ\Jit and bd\JII•c anoe''' llow$ 

(l avJ Bulle llow I IJb I d) Jill Pie• locene 10 Holotene 

Rn otne 10 dJ ne dome~ llows pum•ce ••no~ and tnt 

compte ~s earl Pter~tocene 10 nolocen 

Undrlle•enuated pum•c IJII~ ~h llows and Jllu•ral 

deoosns ol 1h~ uppe• IIJnk PI 1\IOcen ano tiulocen 

Ash !low tulls and aqgtut•nat 0 p •ocla~t•c roc \ 01 upp , 

we 1 llan . Pte•\toc n 

l .tpllh lull ol west llan Pters1o ene 

Und•llerenuJied Jllu •al oeposn~ wnn •nte•Dedded IJP•Ih lull 

(0111 Hh !low tull and pum•c tall deP0\11\ Plt•stocene 

A h !low lull ol Teepee OrJv. Ple•stocene 

flu •al and lacustflne ~ed•menls P1e•slocene and Phocen 1'1 

Ba Jlt ba IIIC andesne and ande~•le llows a h llow lull~ 

and pum•ce tall deposns of 1he Ca caoe Rano Ple•stocene 

Ba sall llows and mleroedded c•nd rs and to11a deoosns 

1a1 M•ocene 

Rhyotne 10 noesne flows dome ano py•octasi •C roc s 

of lhe Pme Mountam area earl M•ocen 

C1nde1 cones and lr su•e en1 

/ Con1ac1 

Fault 

f•eld lfiP 10u1e 

3 f 1eld HIP SlOP 

1 o •sopacn of oung pum•ce ta ll 

Figure 1 . Geo logic ske tch map of ewberry Volcano . Geolo y of he cal ra is shown in fiqur 2 . 

ages). Carbon for isotopic dating was obtained from 

carbonized root systems at the bases of lava tree 

casts (Peterson and Groh, 1969) and from beneath 

cinder deposits that extend as plumes leeward of 

cinder vents related to the flows (Chitwood and 

others, 1977). The young flows may have erupted 

during a much shorter period of time than the age 

spread indicates, perhaps as little as a few weeks 

or several years . All other flows are covered by 

Mazama ash and are older than 6,700 14c years; 

surface features on some flows suggest a relatively 

young age, perhaps 7,000 to 10,000 years, o hers ar 

likely several tens or hundred of thousands of years 

old . 

Seventy- three basaltic to andesitic flow rocks 

or bombs from associated vents analyzed by Higgins 

(1973) and Beyer (1973) contain 48 to 60 percent 

Sio 2 • The mean silica content of these rocks is 

about 54 percent, similar to that of analyzed rocks 

from the adjacent Cascade Range . Of these analyzed 

Newberry rocks, only about 20 percent contain less 

than 52 percent Si02 , about 60 percent lie between 

52 and 56 percent, and the remaining 20 percent have 

56 to 60 percent. Thus the dominant analyzed rock 

type is basaltic andesite similar to many rocks in 

the Cascade Range. The analyzed ro cks are generally 

biased toward younger rocks, however, and many of 

86 

the older voluminous flows on the lower flanks are 

basalt, mostly of high-alumina type (table 1 , co · 

1). The 6 , 000-year-old flows con tain 52 5 

percent Si02 and are basaltic andesites 

col. 2) . 

More than 400 cinder cones and 

have been identified on the f lanka of 

other volcanoes in the world contain so many . 

are concentrated in three broad zones that join o 

the upper part of the volcano. The eastern zone is 

a continuation of the High Lava Plains zone ~ 

basaltic vents and parallels the Brothers aul 

zone; except for cones high o n the east flank , 

cones in this zone appear re la tively old . 

northwestern zone of vents is colline a r with the zone 

of faults on the lowermost flank that ext ends to 

Green Ridge in the Cascade Range, and ~ 

southwestern zone is collinear with the Walke r R! 

fault zone. Fissures and alined cinder 

generally parallel the belts in whi ch they occur. 

The distribution of the vents, and particularly of 

alined vents and fissures, suggests that the 

northwest and southwest zones, and perhaps t he 

faults that they parallel, are part of one broad 

arcuate zone that c urves in the vicinity of 

Newberry's summit . Some ali ned cinder co nes and 

fissure vents near the s ummit occur in arcuate zones 
cald r a rim arallel to th 

P fractures ; som occ ur ring 

caldera side is downd copped · 

a nd likely lie along 

along faults wh os 

Most of the ci nd r cones a r e well pr se rv ed 

oving to thei r high porosity and cons quent 

abso rption r ather than run off of wat e r. Larger 

cones a r e as much as 500 f t high , typical co nes a r e 

.OO to 300 ft . Most a r marked by summit c r a ters 

and flows em r f r om their bases . Ci nd e rs 

dis persed by prevailing winds during e ruptions form 

a rons extend ing le ward from some co nes s uch as 

L~ va But t e (Chitwood and others, 1977) (stop 1, road 

log) · Fissure vents co nsist of long ridges or 

trenchli ke depressions formed by cinders, spatter 

and agglutinate flows · Small pit craters are 

developed along some fiss ure vents . Cind e r cones 

and fissure vents on the lower flanks are generally 

devoid of fragments of rhyolitic r oc k, wh ereas many 

of those highe r on the volcano contain rhyolitic 

inclusions (stop 3, road log) · 

Shieldlike vents occur at Spring and Green 

Butte 0 the southwest fla nk and Green Mountain t o 

the northwest of e wb erry . They are 1 to 3 miles 

ac ross , have gent le slopes, and are more faulted and 

olde r than most surficial ewberry flows . 

any of the hi lls on Newberry's flanks are 

rh olitic domes • In add it ion , pumice rings, 

obsidian flows , and s:nall rh yolite or obsidian 

protrusions occur in many places . Most of the domes 

form rounded hills, such as McKay Butte on the west 

flank , that are 100 to 500 ft high and up to 4,000 

ft ac ross . The largest dome, which fo rms Paulina 

Peak (s t op 6, road log) , extends southwe stward from 

the cal dera wa ll for 3 miles . Its very elongate 

ou tcrop suggests that it was emplaced along a 

no rtheast-trending fissure or fault; an obsidian 

flow c r ops out farther down the slope on a direct 

extension of the axis of the Paulina Peak dome and 

cay have been e rupted from the same bu ri ed fissure 

or fault . 

Several small rhyolitic outcrops may be the 

tops of rhyolite domes that are more extensive at 

dep th . An exampl e is along Paulina Creek on the 

11est flank of ewberry wher e obsidian irregularly 

invades basaltic andesite flows over a small a r ea . 

n addi tion, the common occurrence of rhyolite as 

ragmen ts in cinde r cones on the upper flanks 

attests to the relative abundance of rhyolite at 

depth on the upper part of the volcano . 

K- Ar ages were determined on six rhyolite domes 

and flows. The ages range f rom 400,000 to 700,000 

yea rs, although many undated rhyolites are probably 

yo unge r. Some small spinal protrusions, domes, and 

pumice rio s on the upp r sou h as flank may b 

less than 10 , 000 yea rs o ld . In con cas t o he 

relativ ant iqui of many rhyoli es on h flanks , 

hose in h cald r a a r e commonly youn er han 

Mazama ash and as young as 1 , 400 years . 

Ash flows , pumice falls , mudflows , and oth r 

pyroclas i c d posits ac common on th w st and eas t 

flanks of ewb cr y and ar lik ly pr ese nt at d pth 

on he nor h and sou h flanks b low he v neer of 

basalt and ba sal ic andesit flows . The olde s t 

known unit occurs on th lower north ast and east 

fla nk. and consists of a wide sp r ead rhyolitic ash 

flow identifi d by G. W. Walk c (p rs . commun., 

1973) during r conna issanc mappin of adjac nt 

areas to h ast . It has b en r ferr d to 

informally as h Teep Draw ash flow fo r ou crops 

87 

along Te pee Dr aw (stop 14 , r oad log) . The Teepee 

Draw ash flow crops out along r avine walls for at 

least 6 miles toward the caldera bef o r e being buried 

by younger r ocks . Along some ravines it exceeds 70 

ft in thickness, but it may be conside r ably thicke r 

becaus e its base is exposed only wher e kipukas of 

older basalt project t h rough it; also the upper part 

is e rod ed . The ash flow c r ops out roughl y over a 

50° quadrant of the volcano, and , as it is 

apparently older than the su rficial rocks on the 

other flanks , may be present at depth completely 

around the volcano . Or iginal volume of the unit is 

dif f icult to estimate without inf o rmation on 

distribution at depth on the other flanks, but it 

likely is much more than 10 cubic miles . The Teepee 

Draw ash flow probably relates to an early, perhaps 

the earliest , period of caldera collapse . 

Farther up the northeast flank the Teepee Draw 

ash flow is buried to progressively greater depth by 

alluvial sediments derived by erosion of rocks 

higher on the slopes. Interbedded in or under lying 

this alluvium are basalt and andesite flows, and 

several other ash- flow tuff units (stops 12 and 13, 

r oad log) . Some of these ash fl ows are widespread, 

others occur in only a few scattered localities 

(commonly plastered on the caldera side of c inde r 

cones) . Most of these post- Teepee Draw ash flows 

are characte rized by dark-colored, probably dacitic, 

pumice . 

The west flank of the volcano contains two 

major tephra units. The older unit forms most of 

the lower two-thirds of the slope and overlies 

basalt flows and vent deposits that in many places 

are deeply eroded. This unit also occurs on the 

northeast flank where it occurs h igher in the 

section than most of the ash flows. Although 

locally over 200 ft thick, it is rarely exposed. 

Most of the scree- covered road cuts along t he paved 

road on the west flank that leads to the caldera ar e 

in this unit (stop 2, r oad log) . The unit consists 

of gray to black ash, lapilli , and small bombs and 

abundant accidental lithic fragments . The lapilli 

and bombs have characteristic cauliflowerli ke 

surfaces and virtually all contain angular to 

r ounded inclusions of rh yolite , dacite , and 

andesite; some inclusions are fused and f r othed . 

Trenches dug in the deposit show that it is 

massive . In no place have we seen any indication of 

collapse of pumiceous lapilli or welding. All of 

the lapilli and bombs, as wel l as the ashy matrix , 

have the same nonnal natural remnant magnetizat i on, 

as measured by a field fluxgate magnetometer, which 

suggests emplacement temperatures above the Curie 

point for the entire unit. Most of the unit was 

likely deposited as pyroclastic flows . I n one area 

the unit is palagonitized and much more indurated 

and h s c har act ris i s more like that of a lahar • 

Lapilli and bombs id ntical to those in this unit 

a r e an ubiquitous and voluminous component of most 

alluvial deposits on the vol cano . Furthermore, 

numerous g ravel pits b eyo nd the flanks of the 

volcano contain similar lapil li as a major c omponent 

of the g ravel. The original volume of the unit was 

probably several cubic miles . Its eruption cou ld 

have been accompanied by caldera collapse . 

The second majo r tephra unit on the west fla nk 

forms the smooth and gentl dipp ing upper part of 

the flank ex tendin for about 2 miles from the 

caldera rim. At localities farthest from the rim 

(stop 4 , road log) t he un it consists of many thin 

ash- flow units, common! 3 to 20 ft thick . They are 

reddish to brownish i n colo r and consist of 
and esit ic sc ria and pumjc and ac id nt.ll 1i h ie 

ra m n s in a p o r 1 o rt d 1i hi - and 

cryst 1-r i h ash matrix. Bas s of i nd i v id ual un its 

are commonly w l d d . Towa rd the cal d r a rim, th 

ash fl ws prog r ess i ve l chan chara t r a nd i n ma ny 

place n ar th rim, a s at Pa u l ina Falls ( t p 5 

road lo ) , thi k units hav the app aranc of 

agglu tinate flows . Th se d posits probably 

re p resent hot co-ignimbrit la dep sits. 

Alluvial d posits occur ov r broad areas of th 

northeast, lower southeas , and upper south flanks 

where they form round d slopes with virtually 

nonexistent exposure. Most of th deposits ar 

gravel and sand, but the occur rene of boulders as 

float at some horizons along sides of ravines 

indicates that boulder beds ar present also; pumic 

falls and ash flows are interbedded in the alluvial 

deposits in some areas. B cause most of th 

uppermost slopes of the volcano in areas where th 

alluvium is present are veneered by young pumice 

falls, we were not able to determin th origin of 

the deposits near the caldera . They may be fluvial 

but equally well may be of glacio-fluvial or glacial 

origin . Farther down the slope they are probably 

entirely fluvial, representing broad alluvial fans. 

Much of the uppermost northeast, east, and 

southern flanks extending for about 1 to 2 miles 

outward from the caldera rim are formed of pumic 

and ash deposits derived from vents within the 

calde r a . Most of the deposits are younger than 

Mazama ash , but scattered holes dug through th m 

show that similar deposits underlie Mazama a h in a 

few places . Much of the east flank of the volcano 

is covered by an extensive pumice fall (stop 11, 

road log) derived from the vent for the Big Obsidian 

flow in the caldera (Sherrod and MacLeod , 1979) • It 

extends as a plume oriented • 80° E., is well over 

10 ft thick near the caldera rim , and thins to about 

10 in . at a distance of about 40 miles from the 

caldera . 

Williams (1935, 1957) first recognized that th 

4- to S- mile-wide depression at the summit of the 

v olcano is a caldera; Russell (1905) had originally 

s uggested that it was a large glacial cirque . Owing 

to the absence of known ash flows on the flanks, 

Williams (1957) interpreted the caldera as resulting 

f r om "* * * dr ainage of the underlying reservoir 

either by subterranean migration of magma, or, more 

likely , by copious eruptions of basalt from flank 

fiss ures * * *, " with summit collapse occurring 

along ring fractures . Higgins (1973), on the other 

hand , inte r preted caldera formation as due t o 

tecto nic- volcanic collapse along fault zones that 

supposedly inte r setted at the summit . As noted by 

Peterson and Groh (1976), the main axis of the 

Brothers fault zone lies far north of the caldera, 

r ather than crossing the summit, and faults within 

the zone do not appear to offset Newberry lavas . 

Also, most of the faults shown by Higgins on the 

upper part of Newberry that he .uses as part of his 

structural interpretation could not be corroborat ed . 

Ash flows and other. tephra units are now known 

to be common and voluminous on the flanks . Thus the 

cal dera seems much more likely to be the result of 

v o luminous tephra eruptions from magma chambers 

b elow the forme r summit with concomitant collapse of 

the summit in a manner similar to that of most other 

calderas the size of ewberry ' s or larger. As there 

were several major tephra eruptions, it seems like l y 

that coll apse oc c urr ed severa l times, each collapse 

nvolvin~ areas smaller than that o f the presen t 

88 

a l d r a . 

l n r p r ted 

a 

ly . h 

1 n s 

p r s n 

Cfl l d rn 

Th g o f c a ld ra s an d of 

consid r r La d t o th 

p orly known. Higgins (1973) 

a Holoc n 

of wid spr ad 

many rocks on th 

flank , ar 

y ars old , so abs n e 

is not m aningful. 

volcano s on th ast 

whi h ar about th 

obvious glacial f 

n arly a million or 

The ring fractur s along which 

occurr d ar not expos d , yet their g n ral locatio 

is indicated by arcuat zones (fi.g . 2), visible 0 

aerial photographs . I is not possibl to re la e 

individual ash-flow units to coil ps along s pecific 

ring fractur s . Also it is possibl that collapse 

occurr d along parall 1 ring fractur s at the sa e 

time . Two par all 1 walls on th southea s side 0• 

the cald ra present an int resting problem io 

interpreting th origin of th caldera '-'al 

sequences (des c rib d lat r) . If th inn r wall is 

the young r, th n the area b tw en the inner and 

outer wall may b f o rm d of old caldera-fil 

deposits . Th inn r wall is thi ckly mantled ~ 

young tephra d posits, and th only e xposur s are a 

relatively small ar a of rhyolit in the lower par 

of the slop • The remaind r of th slop on this 

wall dips 30° t o 40°, n ar he angle of r pose, and 

ha s a very uniform smooth shape . If the youn 

tephra deposits were locally und rlain by indurated 

roc ks , as are present in th other walls, it see.cs 

likely that th slope would b mor irr gular . Thus 

this caldera wall likely is dominantly or entirely 

form d of relatively unconsolidated fragments 

rocks, probably cal dera-fi ll de posits. Th rhyolite 

exposed loc ally at the base of the wa ll may be a 

faulted dome that was originally within the 

caldera . The upper part of the northeast ~o~ all , 

above exposures of c ald ra wall rocks , ma y a lso be 

formed of caldera-f ill depo i ts, the older c aldera 

wall bein farther north . 

The walls of the c aldera are mostly cove r ed by 

younger deposits (talus, pumi c e falls, etc . ) and the 

wall rocks are only loc ally expo s ed. They are 

described by Williams (1935) and in mor e detail by 

Higgins (Higgins and Waters, 1968; Higgins , 1973) . 

The most continuous e xposure s ar a l ong he north 

wall extending from the Red Slid , nort h of Pau lina 

Lake, to the northeast obsidian f low, whi ch dr apes 

the wall north a s t o f F.as I.a Th bas of he 

wal l sequence is f o rmed o f platy rhyolit e . Higher 

in the sec tion are basaltic ande site flo , 

palagonite tuff, c inder and agg l ut i nat ed s pa tter 

deposits, and at the t o p of the ex pos ur es are 

palagonite tuffs. 

Exposur e s in the e ast wall ar e f o und on ly fr 

a bo ut midway on East Lake northward t o the Sheeps 

Rump, a cind e r cone along the wal l at the no rtheas 

co rner o f the cal de r a . The i nt e rnitten t exposure 

c onsist of basaltic and e sit e or and esit e fl o~. 

palagonite tuff, a nd pumice deposi t s. The last 

range from unwe lded a nd un collaps ed pumice to 

densely weld ed deposits . 
0 

t:=-' 2 MILES 

~ l'umottl 

IEJ Rh, It .1 ~so~ an .lome 1n oootr asJOn~ 

@"u Una 1!er n'J.d~t' 4 p m ,. dOd d h ar o : 

PaiJ onott •ull 

~ Condt• cone' an sure n depo ,,, 

~ Flu oat ana latus 11nt aepo ' s 

B sal a allot anaesote and andes• e loO ¥> S llota:l buroed b 

pumoce and ash Ou ) 

Caldera wall roc s we st wall roc sho n separately Ool) 

Ash llow ulls and agglutonaled pyroclas oc roc s ol upper 

wes1 llan 

4 Foeld !lop sop 

--- foeld :11p rou1e 

Rom ol caldera 

Probable caldera ron lrac1ure 

Faull 

= Axos ol oun9 pumoce Ia I 

a 

<t 

z 

Pt re 2 . Geolo,ic ske ch map o wberry cal ra . 

Other than xposures of rhyolite south of East 

Lake , the inner sou t h caldera wal l is unexposed and 

is covered y hick pumice fall deposits . The ou t e r 

sou th wall has e xposu re s o nly near the Big Obsidian 

flow and fa rthe r we s t below Paulina Peak. The best 

exposure s , midway between Paulina Peak and the Big 

Obsidian flow, consist of a lower platy rhyolite, 

overlain by basaltic andesite flows, dikes, and 

cinder deposits with local inte rbedded pumice fall 

deposits, and with a cliff-forming obsidian flow at 

the top . At the west end of this exposure are 

outc rop s of bedded pumiceous tuffs that have been 

fused near their contact with the overlying obsidian 

flow ; part of the appa rent base of the obsidian flow 

is fused tuff even on the ea st e nd of the o ut c r op . 

South of the Big Obsidian flow vent are exposures of 

andesitic or dacitic ash flows a nd basaltic andesite 

flows , and below Paulina Peak a ledge of basalt 

c rops ou t in one small area . 

Most of the caldera floor is formed of 

rhyoli ic rocks including domes, flows, and 

pumiceous phra d posits (a h lows, pumice falls, 

explosion breccias) , bu basaltic andesit and 

andesite flows and pala onite uff rings occur in 

sev ral pl c s . Williams (1935) and Higgin (1973) 

have prov id d mu ch us f ul data on the floo r ro cks , 

although w int rpr t th r lative a s of many of 

the units diff r ntly . Th floor rocks r pr sent 

but th ip on he ic bur with r spec to the total 

fill pr sent in th cald ra . A cor hole , curr ntly 

bei ng drilled fo r th U. S. Geological Surv y has so 

far p n trated 1, 700 ft of cald ra-fill deposits, 

mos tly pumic ous phra, wi h a few rhyoliti c and 

dacitic flows, and lak sediments 1 , 000 f bel w he 

su rfac • 

89 

Mazama ash is a useful datum wi t h wh ich to 

roughly subdivide the calde ra r ocks as olde r o r 

younge r t han about 6 , 700 14c yea rs . All the mafic 

rocks a re older than Mazama except fo r those along 

the Eas t La ke fissu r e . The East Lake fissu re has 

no t yield ed c arbon fo r da ting, but the summit 

basaltic a ndesite flows along the same fissure less 

than a mile north are about 6 , 090 14c years old (S . 

w. Robinso n, written commun . , 1978) . Included in 

the pre- Mazama basaltic rocks are the interla ke 

basaltic andesite flow (east shore of Paulina Lake), 

Red Slide cinder vent and flows (no rth side of 

Paulina Lake) , Sheeps Rump cinder cone and flow 

( northeast side of East La ke ) , and east- rim fissure 

a nd the associated flow f r om it that extends to East 

La ke . The palagonite tuff rings that occur 

southeast of Pau l ina Lake (Little Crat e r) and near 

the south sho re of East Lak e (stop 10, road log) are 

also pre-Mazama in age . Rhyolitic rocks of preMazama age include two domes along the south shore 

of Paulina Lake , a large obsidian flow in the 

no rthe ast corner of the caldera, an obsidian dome 

that c r ops out near the caldera wall south of East 

Lake with an obsidian flow that extends northward 

from the dome to East Lake, and a poorly exposed 

dome(?) south of the Central Pumice cone . In 

addition , rhyolitic pumice falls, lacustrine 

d posit s (i . e ., east shore of Pau lina Lake), and 

landslide deposits (no rt h of Paulina Peak) are known 

to locally underlie Mazama ash. 

The post -Mazama rhyo litic deposits occur in the 

east rn half of the caldera (fig . 2) . They include 

obsidian flows, pumice r ings and cones, ash flows, 

pumice falls , and oth r pumiceous tephra deposits. 

Hydration-rind dati n by Friedman (1977) indicates 

that th Central Pumice cone, b e twe en East and 
Table 1. Repres ntativ ch mical analys s of N wb rry rocks . 

[n . r . not reported) 

Lava Butt 

Flank basalticbasalt andesite 

Paulina Falls Paulina 

andesitic- P ak 

Si02 ------49.1 56 . 0 60 . 43 71.07 

Al 2o3 - - ---17.6 16 . 1 16 . 06 14 . 92 

FeO+Fe 2o3 - 9. 0 7. 7 6. 51 2.81 

MgO ------- 8. 9 4. 49 l. 71 . 22 

CaO -------10.0 8. 2 4. 58 1. 08 

Na 2o ------ 2.6 3. 72 5. 71 6. 04 

K20 ------- .43 l. 25 1. 54 3.03 

H20+ . 71 n.r. .53 • 17 

H20- • 10 n.r. • 07 .01 

Ti0 2 1.0 1. 13 1.30 . 29 

P205 • 31 n . r. .59 . 05 

MnO . 16 . 14 . 15 . 09 

co2 <. 05 n . r. .61 <. 05 

Column 1. Higgins (1973, table 6, col. 62) . 

Column 2. Beyer (1973, table lc, LB-4) . 

Column 3. Higgins ( 1973, table 4, col. 19) . 

Column 4. Higgins (1973, table 4, col. 27) . 

Big Obsidian 

flow 

72 . 02 

14.61 

2. 43 

. 164 

• 85 

5. 16 

3. 89 

n . r • 

n .r. 

. 24 

n.r • 

. 064 

n . r . 

Column 5. Laidley and McKay (1971), average of 66 analyses. 

90 
Paulina Lakes , and th Interlak and Game Hu 

bsidian flow that c r op ou n r h and south of i , 

are only sligh 1 youn r than Mazama ash ( 14c ag , 

6, 00- 6 , 700 years) · Th East Lake obsidian flow 

are appar nt l ab u 3 , 00 y ars old , and the Bi 

Obsidian f low is about 1 ,400 y ars o ld . 

Widespr ad pumic ous t phra deposits cov r much 

of the east rn par of th calde ra. They underlie 

the East Lake obsidian flows and probably are 

slightly olde r than the Cen ral Pumice cone and th 

obsidian flows on its no r h and south sides . Th se 

tephra deposits may hav b n d rived from several 

different vents, but hol s dug through th m indicate 

that they cons is dominantly of 5 to 10 f t of 

oassive to poorly bedded pumic with accidental 

fragments (palagonit tuff , basalt , rhyolit , etc . ) 

overlain by s veral fe t of w 11- bedded mud - armored 

puoice, accr tionary lapilli, ash and pumice (stop 

9, road lo ) . 

The oungest period of volcanism within the 

caldera wa s associa t d with the vent for he Big 

Obsidian flow. It began with rupt ions that 

produced a wid spr ad pur:lice fall that cove rs the 

southern part of the calde ra and he astern fla nk 

of the volcano (Sher rod and Mac Leod , 1979) . 14c 

ages of 1, 72()_±200 (Higgins , 1969) and 1 ,550+120 (S . 

\.'. Ro inson, written commun ., 1978) years were 

ob tained on ca r bon directly benea th the fall. On 

the basis of thickness measurements from 150 holes 

dug through the pumice fall, the axis of the fall 

extends 80° E. , a way from the vent for the Big 

Obsidian flow. At about 5-1/2 miles from the vent 

the fall is ~bout 12 ft thick, and at 40 miles it 

dec reases to abou t 10 in . 

The pumice fall wa s followed by eruptions t hat 

produced an ash flow that extends ove r a broad area 

between the Big Obsidian flow and Paulina Lake . It 

is we ll exposed in r oadcuts n ar the Big Obsidian 

flow (stop 7, road log) . 14c a es of the ash flow 

are 1 , 270+60 and 1,390_±200 years (Pierson and 

others, 1966 ; Meyer Reubin, in Fr iedman, 1 977 ) , with 

an olde r age (2,054_±230 years) obtained many years 

ago by Libby (1952) . Th final event wa s th 

eruption of the Big Obsidian flow and th domal 

pro trusion that marks its vent . Sligh t collapse 

occu rr ed over a one- half- mile-wide area around the 

ven t before the flow was erupted . The flow ex tends 

no rt hward f rom near the outer caldera wall to near 

he paved r oad in the caldera and , in its northern 

part , partly filled an older pumice ring (stop 8 , 

road log) . 

Considering the long time ov er which e r uptions 

took place on ewber r y , the volcano should be 

considered dormant but capable of future e rupti ons 

even though about 1, 300-1,400 years ha ve transpired 

since th last erup ions . e wb erry is ideally 

suit d for thos who wish o div rse volcanic 

fea ur s . I s rocks rang wid ly in composition , 

and exampl f r om it could be us d to illustrate a 

nearly compl a las of h yp s of produc s of 

volcanism . 

LIST OF REFERE CES 

Beyer, R. L. , 1973 , a rna di f rentiation a 

wb rry c r a t r in c ntral Or on: Eu en , 

niversi y of Oregon, Ph . D. th sis, 84 p . 

Chitwood , L. A., Jenson, R. A., and Groh , E. 

1977 , Th of ava Bu tte: Th Or Bin, v . A.' 

39 ' no . 10 , p . 157-164. 

Fri dman , Irving, 19 77' Hydration datin of 

vol canism at wberr Crat r, Or ~on : . s. 

91 

Geological Surv y Journal of Resea r ch , v . 5 , 3, p. 337 -34 2. no . 

Gr le , Ronald, 1971 , G olo y of selected lava 

tubes in the Bend area , Or egon : Or egon 

Depa rtme nt of Geolo y and Mineral Indust r ies 

Bulletin 71 , 47 P• 

Gr e ne, R. C., Walker , G. \.1 . , a nd Co r co ran, R. E., 

1972 , Geologic map of the Burns quadrangle, 

Oregon : U. S. Geological Su rv ey Miscellaneous 

G olo ic Investigations Map I - 680, scale 

1 : 250 ,000 . 

Higgins , W., 1969, Airfall ash and pumice lapilli 

deposits from Cent ra l Pumice cone, ewberry 

calde ra, Or gon , ~ Geological Su rvey Research 

1969 : U. S. Geological Survey Professional Paper 

650-D, p . D26 -D32 . 

____ 1973 , Petrology of ewberry volcano, ce ntr al 

Or egon : Geological Society of America Bulletin, 

v. 84 , p . 455 - 488 . 

Higgins, M. w., and Waters , A. c. , 1968, ewber r y 

caldera field trip , ~ Andesite Confe r ence 

Guidebook ; Oregon Depa rtment of Geology and 

Mine r al I ndus tries Bulletin, p . 59- 77. 

Laidley , R. A., and McKay, D. s., 1971, Geochemical 

examina ion of obsidians f r om ewberry calde ra, 

Ore on : Contributions to Mineralogy and 

Petrology , v . 30, p . 336 - 342 . 

Libby, w. F., 1952, Chicago radiocarbon dates III: 

Scienc , v . 161 , p . 6 73- 681. 

acLeod, S. , Walker, G. w., and McKee, E. H., 

1975 , Geothe rma l significance of eastward 

inc r ase in age of upper Cenozoic rhyolitic domes 

in southeast Oregon : Second United ations 

Symposium on the Development and Use of 

Geo thermal Resources Proceedings, v. 1 , p . 465-

4 74 . 

McKee , E. H. , acLeod, S., and Walker , G. W., 

1976, Potassium-argon ages of Late Cenozoic 

silicic volcanic rocks , southeast Oregon : 

Isochron/West , no . 15, p . 37 - 41 . 

Peterson, • V., and Groh , E. A., 1969, The ages of 

some Holocene vo lcanic erup tions in the ewberry 

volcano area, Ore on : The Ore Bin, v . 31, 

P • 73- 87 . 

-------~1976 , Geology and mineral resources of 

Deschutes County , Oregon : Or egon Department of 

Geology and Mineral Industries Bulletin 89, 66 p . 

Pierson, F. J ., Jr ., Davis , E. M. , and Tamers , M. 

A., 1966, University of Tex as radiocarbon dates 

I : Radiocarbon , v. 8, p . 453- 466 . 

Russe ll, I . c., 1905 , Preliminary report on the 

geology and water r esour ces of central Oregon : 

.s. Geological Survey Bulletin 252 , 138 p. 

Sherrod, D. R., and MacLeod, N. S., 197 9, The last 

eruptions at Newberry volcano, central Oregon 

[abs . ] : Geological Society of Ame rica, Abstracts 

with Programs, v. 11, no . 3, p . 127 . 

Wa lk r, G. w., 19 74 , Some implications of La te 

Cenozoic volcanism t o geo therma l potentia l in the 

High Lava Plains of south- central Oregon : The 

Ore Bin , v . 36, no . 7, p . 109-1 19 . 

Walker, G. w., Peterson, • V., and Greene, R. C., 

1967, Reconnaissance geologic map of the east 

half of the Crescent quadrangle , La ke, Deschutes, 

and Crook Counties, Orego n: u.s. Geological 

Survey Miscellaneous Geologic Investigations Map 

I-493 , scale 1 : 250 , 000 . 

Williams , Howell , 1935 , ewberry volcano of central 

Oregon: Geological Society of America Bulletin , 

v . 46 , p . 253-304 . 

1957, A geologic map of the Bend quadrangle , 

~on, and a r econnaissance geologic map of the 

c ntral portion of the High Cascade Mountains: 

Ore on Depar ment of Geology and Mineral 

Industries ~p, scales 1:125,000 and 1:250,000. 
92 
... 

RO DLOG FOR 

NEWBERRY VOLCANO, OREGON 

Norman S. MacLeod, David R. Sherrod 

U.S. Geological Survey, Menlo Park, California 94025 

Lawrence A. Chitwood, U. S. Forest Service, Bend, Oregon 97701 

and 

Edwin H. McKee, U.S. Geological Survey , Menlo Pa rk, California 94025 

The route of this field trip (figs. 1 , 2) leads southward from Bend across the 

basalt and basaltic-andesite flows of Newbe rry's northwest flank and eastward up the 

wesL flank where pyroclastic units are visited. Several stops in the caldera, including 

an overview stop provided by Paulina Peak, show the diversity of caldera rock units. 

The trip continues down the east and northeast flanks with stops to see several ash- flow 

tuffs and returns to Bend across the apron of basalt flows that extend far north of 

ewberry . Total mileage of the trip is about 118 miles, all on paved or improved gravel 

roads that are readily traversed by passenger cars or buses. Mileages are approximate, 

all turns and stops are underlined, and old and new (in parenthesis) U.S. Forest Service 

road numbers are shown where appropriate. The 14c ages of young basalt flows are most ly 

from Peterson and Groh (1969). 

Miles 

o.o 

(0 . 7) 

0. 7 

(3.2) 

3.9 

(2 . 8) 

6. 7 

(3 . 3) 

10 . 0 

(0 . 7) 

10 . 7 

Junction u.s. Highways 97 and 20 in Bend, Oregon. 

south on U. S. Highway 97. 

Elevation 3, 630. Head 

Diktytaxi tic olivine-phy ric high-alumina basalts exposed in rail road underpass and intermittently for the next 11 miles to south are typical of 

Newberry's lower flank flows. Northwest-trending faults that extend in a 

broad zone through the Bend area offset the flows . A typical analysis of a 

lower flank flow is shown in table 1, column 1 . 

Road 1821 ( 18) leads eastward to · interesting lava tubes (Boyd Cave, 9 miles; 

Skeleton Cave , 12 miles; Wind Cave, 13 miles; and Arnold Ice Cave, 13 miles) 

Ash-flow tuff exposed on east side of U.S. 97 near draw. Ash flow is 

widespread around the Bend area and was derived from the Cascade Range west of 

Bend. Ou tcrops and float near rail road west of road show a wide r ange i n 

p mice composition . 

Roadcuts expose cind r plume that ext e nds northeast f rom La a Butte . Charcoal 

from ba se of cinders he r e yielded 14c age of 6,160_±65 years (Chitwo od and 

others, 1977). 

orthwest- trending faul exp sed on east side of high ay offsets diktytaxitic 

ol~vine basal • ote you n "Gas Line" basaltic-andesite flow s outh of fault 

(1 Cage ,80~1 50 to 6,160_±65 yea rs, (S. w. Robinson, written commun . , 1978) . 

93 
( 0 . 5) 

11. 2 

( 1. 8) 

13.0 

(1.2) 

16.0 

(2.4) 

18.4 

(2.4) 

20.8 

(3.2) 

24.0 

(0.6) 

24.6 

( 1. 2) 

25.8 

(1.1) 

26.9 

Turn we t t o Lava La nd s a nd dri v by and a ro s th 

Lava Butt • ot that the l ava f l ow m r g s from th 

Lav a Butt 

sou h sid 

low t o to p of 

o f th but te . 

STOP 1 at top of Lava Butt (elevation 4, 970 ft) . The ba s ltic a ndesit flow 

derived from Lava Butt xtends northward more than 5 mil s and we s ward 3 

miles to th Deschut s River. A chemical analysis of th flow i s s hown in 

table 1, column 2. It is on of many basalti andesi flows on Newb rry that 

have 14c ages of about 6,100 years . S v ral, including th "Gas Line " and 

Mokst Butte flows, can be seen from this stop . No e also h many c inder 

cones on the north and northwest flank, an area formed mostly of basalt and 

basaltic-andesite flows and associated vents, most of which are youn r than 

the pyroclastic units that will be visited on the east and west flanks. A 

panorama of the Cascade Range, particularly of the Bachelor Butte, Brok n Top, 

and Three Sisters area , can be viewed from here . South of Lava Butte , 

Pleistoc ne (and Pliocene?) lacustrine and fluvial sediments of the La Pine 

basin lie between Newberry Volcano and the Casc ade Range . Return t o Highway 

97. 

Turn south on U. S. Highway 97 . 

Road on east leads to Lava River Cave, 1/4 mile from highway . The mile-long 

lava tube is now a state park and is open to the public (lanterns can be 

rented at the entrance) . 

Sun River Junction. Road on east goes by Camp Abbot Cinder Pit (3/4 mile) t o 

Lava Cast Forest (9-1/2 miles) . The cinder pit corzains brilliant blue , 

green , gold, and red cinders . Lava Cast Forest flow ( C ages, 6, 380_±130 and 

6,150_±210 years) contains numerous well-d eveloped tree casts. A mile-long 

paved trail leads past many casts. Other young flows accessible by this road 

include Forest Road (5, 960±100 years) , Mokst Butte, and Lava Cascade 

(5,800±100 years). 

Fall River Junction. 

5,870±100 years). 

Road on east leads to Sugarpine Butte flow ( 14c age, 

Cross old logging railroad grade that went to the now abandoned town of 

Shevlin. Many hundreds of miles of roads on Newberry are on or along old 

railroads developed about 50 years ago when logging in the area was at a peak . 

Approximate boundary of Newberry lavas and sediments of the La Pine basin . 

Gravel in this area is mostly composed of black lapilli with rhyolitic 

inclusions like those that form the lapilli tuff at stop 2. The s e can be seen 

in this area in several pits, including the garbage dump (Southwest Landfill), 

east of the highway . 

La Pine State Recreation Area junction . To west 5 miles are ex c e l lent 

exposures of fluvial and lacustrine sediments, including diatomite, along the 

scenic Deschutes River. 

Turn east 

2129 (21). 

(elevation 4,205 ft) on Paulina Lake Road , U.S . Forest Service Road 

Road c rosses Mazama ash-covered alluvial sediments . 

94 

....... 
(2 . 8) 

29.7 

(0 . 2) 

29 .9 

(0. 5) 

30 . 4 

(3. 2) 

33.6 

(2. 9) 

36.5 

(0. 4) 

36.9 

(0. 6) 

37.5 

(0 . 7) 

38 . 2 

(0 .1) 

38. 3 

(0 . 1) 

38 . 4 

Cross Paulina Creek . Fed by Paulina Lake , it is the only stream on Newberry. 

Outcrops of Newberry lava flows near road. Farther along the road these flows 

are locally buried by lapilli tuffs like those at stop 2 as well as by gravel. 

At this site on September 19, 1979, The Cascade Rattlesnake Award was 

presented to J. P. Lockwood before a cheering audience of 80 geologists. 

Road 2045 provides access to three rhyolite domes at and near McKay Butte (KAr age, 0.6±.1 m.y., may be reversely polarized). 

STOP 2 (elevation 5, 420) at large scree-covered slope on north side of road. 

The lapilli tuff at this stop is one of the most widespread tephra units on 

Newberry's flanks . The unit is rarely exposed but the lapilli in it are 

distinctive so that float from the unit can be recognized. On the west flank 

the lap ill i tuff covers an area of about 30 square miles, with basalt flows 

cropping out locally beneath it; higher on the west flank it is overlain by 

ash-flow tuffs like those at stop 4. The lapilli tuff on the northeast flank 

occurs higher in the section than the ash-flow tuffs to be seen at stops 12, 

13 , and 14. It is also a major constituent of gravel deposits on all sides of 

the volcano. The lapilli tuff is deeply eroded but locally exceeds 200 ft 

thick, and original volume was probably 5 to 10 cubic miles . It consists of 

dark gray to black lapilli, and less common blocks and bombs, in an ashy 

lithic-rich matrix. Sorting is very poor and in observed outcrops the unit is 

massive . Most lapilli are finely vesicular to frothy, have cauliflowerlike 

surfaces, and contain small, generally angular, rhyolite, and more mafic 

inclusions. The normal polarization of the lapilli and matrix suggests that 

they were above the Curie point temperature when deposited. The poor sorting 

and massiveness of the unit suggest that it was a hot pyroclastic flow or 

flows . The large volume of the unit permits speculation that its eruption may 

have been accompanied by caldera collapse. Continue upslope on road. 

Upper snowmobile parking lot. 

of here. 

Rhyolite domes or flows occur north and south 

Contact of lapilli tuff and cinder cones. These vents probably fed flows 

exposed farther down the road that under lie the lapilli tuff. Cinder cones 

are buried on their east (caldera) sides by ash-flow tuffs like at stop 4 . 

Float of andesitic ash-flow tuff along road. 

Turn south to U. S. Forest Service pit F-17, also known as "Mixture Butte." 

STOP 3 (elevation 5 , 960 ft) at "Mixture Butte," a cinder cone with rhyolite 

and pumice inclusions . The pit is on the north side of a horseshoe-shaped 

cone that is buried by ash-flow tuffs on its east (caldera) side. Rhyolite 

and pumice, as well as basal and andesite, occur as inclusions in the cinders 

and bombs and as accidental f ragments in the deposits . Some inclusions have 

been fused and have flowed . The pumice, in blocks to more than 2 ft wide, 

commonly shows bands of different colo r and probably composition. A mile- long 

rhyolite dome crops out about 1 mile to the southwest, contains phenocrysts 

similar to those in the pumice and rhyolite inclusions and may extend at depth 

95 
(0.1) 

38 . 5 

(0 . 6) 

39.1 

(0 . 9) 

40 . 0 

(0.2) 

40.2 

(1.1) 

41.3 

(1. 0) 

42 . 3 

(0.1) 

42 . 4 

(0.1) 

42.5 

(0 .3 ) 

42.8 

b neath th cind r con • P ssi ly th pumic is d riv d f rom a buri d pumice 

rin associated with th dome. Many ci nd r co n s a nd flo ws on th upp r flank 

of the vol ano cont in rhyolit inclusions . Rhyolite domes a nd flows crop out 

in many plac s and may b much more xtensiv at depth. R turn to Paulina 

Lake Road . 

Turn east on Paulina Lake Road . Area b twe n here and stop 4 is almost 

entirely underlain by andesitic ash-flow tuffs; found as large slabs of float 

and less commonly as outcrop . 

Turn south on Road 2131 (2121) (elevation 6 , 020 ft) . 

West of road is eroded cinder cone with associated flows that are partly 

buried by ash-flow tuff. 

STOP 4 (elevation 5950). Park at road intersection and walk 500 ft eastward 

along small side road to exposures of ash-flow tuff along side of ravine . 

Andesitic ash-flow tuffs crop out over a 5-square- mile area of the upper west 

flank of the volcano and grade near the rim to pyrocla s tic deposits that have 

characteristics more nea r ly like agglutinate deposits , as at stop 5 . The unit 

consists of ash flows as little as 1 ft thick to as much as 30 ft . The ash 

flows are pumiceous, rich in lithic fragments and crystals, and reddish 

brown . Even thin units commonly show welding . At many localities collapsed 

- pumices are brilliant blue and green . The ash flows are very near vent 

deposits, perhaps from a ring fracture near the west side of the caldera, and 

have relatively small volume . Ash- flow tuffs similar to these occur at 

scattered localities on the upper east flank . Return to Paulina Lake Road. 

Turn east on Paul ina Lake Road~ 

Turn north into Paulina Falls parking lot. 

STOP 5 Paulina Falls (elevation 6,240 ft). Paulina Creek drains Paulina Lake, 

less than 1/2 mile to the east. The indurated rocks that form the cliffs of 

the falls are agglutinated andesitic pyroclastic deposits formed of many thin 

to thick units. Similar rocks occur along the caldera wall for about 1-1/2 

miles north of the creek . They grade down the creek into ash flows like those 

at stop 4 and represent very near vent deposits, perhaps from a ring fracture 

bordering the west side of the caldera . Most of the deposit is probably 

co-ignimbrite lag, many units being entirely fall, others stubby agglutinated 

pyroclastic flows. A chemical analysis fo r this locality is shown in table 1 , 

column 3. The less indurated rocks below the cliffs are formed of poorly 

sorted and rudely bedded ash, l ap illi , and blocks with abundant accidental 

ithic fragments. Some beds low in the section contain accretionary and mudarmored lapilli. The contact betwe e n the lower and upper unit is 

gradational. Probably the eruptions first deposited relatively cold material, 

perhaps from phreatic eruptions, and temperatures increased to the point where 

all f ragments were agglutinated at t ime of deposit ion. Lap illi tuffs like 

those at stop 2 occur at creek level only a few hundred feet downstream from 

the falls. Return to Paulina Lake Road. 

Continue east on Paulina Lake Road . 

Turn south on road 2160 (500) to Paulina Peak. 

96 

...... 
(1. 3) 

44 . 1 

(0 . 6) 

44.7 

( 1.1) 

45 .8 

( 1. 3) 

47.1 

Buried con tac t between upper wes t flank pyroclastic flows and Paulina Peak 

rhyolite d ome. 

View northward of gently sloping upper west flank fo rmed mostly of ash-flow 

tuffs like at stop 4. Note rhyo lite domes at and near McKay Butte ( • 60° w., 5-1/ 2 miles). 

View of numerous cinder c ones on south flank and long narrow Surveyor flow 

c14c age 6080±100 years). 

STOP 6 Paulina Peak (elevation 7,984 ft). Before looking at the caldera, walk 

to peak for view southward. Note fault-bounded Walker Mountain (S. 42° w., 35 

miles). Faults of the Walker Rim zone extend to Newberry's lower south 

flank. Some of the older rocks, such as the reversely polarized dacite flows 

on Indian-Amota Butte (S. 30° E., 11 miles), are offset several hundred feet 

by northeast-trending faults; faults that cut the nearby basalt flows have 

vertical offsets of less than 100 ft and the faults disappear upslope below 

younger flows. Other distant features include: 

~1) Bald Mountain (S. 12° w., 28 miles), a rhyolite dome on the south side of 

a poorly preserved 4- or 5-million-year-old caldera with widespread ash-flow 

tuffs that crop out from Fort Rock Valley east of it, to Walker Mountain. 

(2) Cascade Range. From south to north on a clear day you can see Mount 

Shasta, Mount Scott, Crater Lake, Mount Thielson, Diamond Peak, The Three 

Sisters, and nearby Bachelor Butte and Broken Top, Mount Washington, Three 

Fingered Jack, Mount Jefferson, Mount Hood, and Mount Adams. 

(3) Fort Rock Valley southeast of Newberry with numerous pa l agonite tuff 

rings, cones, and maars. Included in these are Fort Rock in the middle of the 

valley, and beyond it about 39 miles from here is Table Rock, the site of a 

field trip by Heiken and Fisher (this vol.). 

( 4) To the east are China Hat (N. 85° E., 11 miles) and East Butte next to 

it. They are 0.8- and 0.9-m.y.-old rhyolite domes on the west end of th~ belt 

of age transgressive rhyolite vents. Note the 5-m.y.-old rhyolites of the 

Glass Buttes dome complex on the skyline (S. 80° E., 60 miles). The High Lava 

Plains, a broad zone of Miocene to Quaternary basalts in addition to rhyolite 

domes and flows, extends from Newberry beyond Glass Butte. 

Now walk to the north along and beyond the fence bordering the cliffs of 

Paulina Peak to a viewpoint about 50 ft below the elevation of the parking 

lot. Please use caution in this are a--t he cliffs are 500 ft high. The 

generallized geologic map of the caldera (fig. 2) should be referred to for 

identifying features. From this location you can see the inner and outer 

(near pumice flats) walls of the south side of the caldera. The caldera wall 

rocks have been described by Higgins and Wa ters (1968) and Higgins (1973). 

The south wall near the Big Obsidian flow consists of platy rhyolite, basaltic 

andesite f lows, scoria and c inder deposits, and an overlying obsidian flow. 

At the west e nd of the wall exposures is a thick sequence of near-vent pumice 

and ash deposits that have been fused near their contact with the obsidian 

flow . The north and east caldera walls contain rhyolite and basaltic andesite 

flows and pyroclastic rocks, as well as palagonite tuff; the east wall also 

contains n e ar vent welded rhyolitic pumice deposits. Basaltic vents and flows 

occur on the north wa ll near Paulina Lake (Red Slide) and East Lake (East Lake 

fissure , Sheeps Rump), and a long fissure vent occurs near the top of the east 

wall. All of these vents and associated flows are pre-Mazama ash in age, 

97 
(4 . 3) 

51. 

(0 .1) 

51.5 

(0.3) 

51.8 

(0 .5) 

52 .3 

(0.3) 

52 .6 

(0 . 1) 

52 .7 

(0 . 2) 

52 .9 

exc pt th E st L k Fi sur • It is th uth rnm 

rift zone s quen of b s lt flow and v nts, ab 

contains abundant inclu ions of obsidian, rhyol it , 

1970) . 

t v n t of t h n r h we 8 t 

6,100 y ars old, and 

• (Higgin and Wat rs, 

Youn rhyolitic flows, pumi on s, rin and other t phra d posits are 

widespread around East Lake. llydr tion-rind dating by Fri dman 0977) 

indicates that the East Lak obsidian flows ar bout 3 ,500 y ars old and that 

many of the other obsidian flo ws and pumi con s (Interlak flow, Gam hut 

flow, Central Pumi e cone) ar 4 ,500 to 6, 700 years old. 

The youngest period of volcani m was associat d with the v nt for th Big 

Obsidian flow . Eruptions b gan with a widespread pumice fall that now covers 

much of ewb erry's east flank and which will be seen lat r a\ 4stop 11. 

Higgins (1969) obtained carbon from ben ath th fall that has a C age of 

1, 720+250 years, and we found carbon at another locality with an age of 

1,550j)20 years (S. w. Robinson, writt n commun., 1978). Isopachs of the 

pumice fall clearly show that it was erupt d from at or very n ar the v nt for 

the Big Obsidian flow (Sherrod and MacL od, 1979). Later eruptions produced 

the Paulina Lake ash flow (stop 7) 1, 300 to 1, 400 y ars ago. Th final ev nt 

was the eruption of the Big Obsidian flow and the domal protrusion that marks 

its vent. Slight collapse occurred over a 1/2-mile-wide area around the vent 

before the flow was erupted . 

Paulina Peak dome extends about 3 miles southwest down the flank and is about 

1 mile wide . It is marked by la rge rills parallel to its axis that formed 

during expansion of the surface of the dome much as era ks form on fr nch 

bread. The age of the dome is not yet known, but an obsidian flow that occurs 

on axis with the dome farther down the flank is 0 . 4 m. y . (E . H. McK e , unpub. 

qata, 1979) , and Paulina Peak dome may be similar in age . An analysis of 

rhyolite from the Paulina Peak dome is shown in table 1 . 

Return t o Paulina Lake Road . 

Turn east on Paulina Lake Road . 

Paulina Lake campground provides a convenient lunch stop . 

Roadcuts expose Mazama ash covered by a few inches of fine ash from Newberry 

eruptions. Holes dug through the Mazama ash show that it overlies sand and 

gravel deposits. 

Road traverses south side of two rhyol ite domes, over lain by Mazama ash and 

bordered on south by landslide deposits. 

Roadcut exposes rudely bedded ash and pumice deposits, perhaps the remnant of 

a pumice ring associa ted with the rhyolite domes . 

Westernlimit of Paulina Lake ash flow . 

STOP 7 Paulina Lake ash flow . Concrete boxes on south side of road preserve 

collecting site for carbon in ash flow . Originally dated by Libby ( 1952) as 

2,054±230 14c years, subsequent dates indicate a younger age (1 270~0 years, 

Pierson and others, 1966; and 1, 390±200, Meyer Reub in , l:.!!_ Fr iedman, 1977) • 

The ash flow extends from the Big Obsidian flow to the shore of Paulina 

Lake . Ridges and furrows on the ash flow are apparently primary features , and 

98 
(0 . 4) 

53 . 3 

(0 . 4) 

53 . 7 

(0 . 1) 

53 . 8 

(0 .1) 

53 . 9 

(0 . 4) 

54 . 3 

(0 . 5) 

54 . 8 

(0 . 5) 

55.3 

(0 . 3) 

55 . 6 

their orientation suggests that the vent is located beneath the southern part 

of the Big Obsidian flow (Higgins , 1 97 3) . Although pumice is generally 

abundant , the ash flow in many places is composed almost entirely of ash . The 

absence of collapse and welding indicates the deposit had only a moderate 

temperature wh en emplaced , but sufficient for oxidat ion to give the deposit a 

slightly pinkish color . Continue eastwa r d on r oad ove r ash flow. 

Road on north leads 

along sho r eline of 

lake deposi ts. 

to Little Crater Campgro und and inte r esting ex posures 

palagonite tuff and silicified pumiceous , fossiliferous, 

Tu rn Right to parking lot next to Big Obsidian flow. 

STOP 8 (elevation 6,370 ft) Big Obsidian flow. Before wa lking to the flow, 

note that the pinkish Paulina Lake ash flow ove rlies weather ed p um ice deposits 

of a pumice ring on the east side of the parking lot . Lo st La ke pumice ring 

is partly cove r ed by the Big Obsidian flow, but its northern part can be seen 

from the trail ascending the obsidian flow. The trail provides interesting 

exposures of flow-banded obsidian , pumiceous obsidian, brown streaky obsidian 

that was forme rly pumiceous before it collapsed , and various feature s 

indicating the flow behaved in both plast ic and brittle manner duri ng its 

emplacement . Laidley and McKay (1971) did extensive analytical work on the 

Big Obsidian flow a nd showed that is is essen tially uniform in compos ition. 

Their average of 66 chemical analyses is shown in table 1 , column 5 . Return 

to parki ng lot a nd to main road; note exposures of pal agoni te t uff of Little 

Crater north of road. 

Continue eastward on paved road. 

Road on south leads t o drill site of U. S. Geological Su rvey core hole . Note 

Central Pumice cone to north. 

Game Hut obsid ian flow is e xposed on north side of road. 

STOP 9 (elevation 6, 460 ft) . Pumiceous tephra deposits of the East Lake 

area. Roadcuts expose massive t o rudely bedded pumice and ash with l arge 

accidental blocks overlain by mud-armored pumice, accretionary lapilli, 

pumice, and ash beds. Bomb sags occur in the upper bedded part and indicate 

that the beds wer e probably wet and cohesive . A hole dug vertically through 

the deposits indicates that they are about 14 ft thick and overlie Mazama ash , 

which rests on obsidian. Tephra deposi ts , similar to the exposures here, 

occur over most of the eastern part of the caldera except where buried by 

youn r units (East La k obsidian flows , etc . ) . The vent or vents for the 

deposit ar not known, and co nce ivably may lie in East Lake. The obsidian 

that unde r lies the deposit is part of an ex tensive obsidian flow tha~ most ly 

on th basis of topographic e xpression , probably extends to the south (inner) 

wall of the calde ra, its vent appa rently being an obsidian dome near the 

wall . Con inue eastward along r oad . 

Turn left to East Lake Campground and walk eastward along shore . 

99 
(0.1) 

55 . 7 

(0 .1) 

55.8 

(0.6) 

56 .4 

(0.4) 

56.8 

(0.3) 

57.1 

(0.4) 

57 .5 

(0.2) 

57 .7 

( 1.1) 

58 . 8 

(0 . 3) 

59 . 1 

STOP 10 (elevation 6, 382 ft) . P 1 onite tuff f East L k • Th p lagonite 

tuff is overlain by Mazama ash and by pumic ous t phra d posits Jik those at 

stop 9. Many of the beds in th palagonite tuff ar formed of mud-armored 

lapilli and accretionary lapilli, presumably indi ating that th xpos d part 

of the unit was deposited above water. ote lso several bomb sags. About 

halfway along the shoreline exposures is a contact b tw n two palagonite tuff 

units; the western unit is youn er and locally is plastered verti~ally on the 

eastern unit. The eastern palagonite tuff contains very abundant and large 

accidental blocks of palagonite tuff that are mor thoroughly palagonitized 

than is the tuff they occur in. From the shore you can look westward to see 

the Central Pumice cone, northward at the East Lake fissure, and northeastward 

at the Sheeps Rump and the obsidian flow that extends to the northeastern 

shoreline. Return to the main road. 

Turn east on paved road . 

Hot springs occur along shore near headland formed of palagonite tuff. 

Temperatures as high as 80°C have been measured, but the hot spring water is 

diluted by lake water. 

Turn east on gravel road, U.S. Forest Service Road 2129 (21). Not exposures 

of andesite near road intersection. This flow has been traced southeastward 

to the fissure that occurs at the top of the east wall of the caldera. This 

fissure contains rocks of highly variable composition and phenocryst content, 

as does the flow . Cinders, bombs, and spatter associated with the fiss ure can 

be seen from several places along the road. 

Pumiceous tephra deposits exposed in roadcuts are similar to those seen at 

stop 9. 

Road here crosses over andesite flow, but it is buried by Mazama ash and 

pumiceous tephra like at stop 9. 

Pumice fall derived from vent for Big Obsidian flow forms a thin surficial 

unit that thickens as we progress up the road toward the south. 

Top of east rim of caldera (elevation 7,008 ft) covered by pumice fall about 8 

ft thick. 

STOP 11 (elevation 7, 010). Pumice fall derived from vent for Big Obsidian 

flow. The fall is exposed in a large hole dug in roadcut on north side of 

road . The fall is about 9-1/2 f t thick here, but the axis of the fall is 

about 1/2 mile to the south. 14c ages of the fall are 1, 720_±250 (Higgins, 

1969) and 1,550_±120 (S. W. Robinson written commun., 1978) years. One hundred 

and fifty holes dug through the pumice fall (and most not in roadcuts as at 

this site) show that it forms a v~ry narrow plume (at the 10-in. isopach it is 

6 miles wide by 40 miles long) oriented N. 80° E., from the Big Obsidian flow 

vent . The orientation of the plume is the same as the current prevailing wind 

as indica ted by deformed trees on higher ridges and buttes (for instance, on 

The Dome, a cinder cone immediately south of here). Pumice lapilli and blocks 

constitute most of the unit, but accidental fragments of basalt, rhyolite, 

obsidian, etc ., are common. Ash forms a conspicuously small part of the 

unit . Sizes of the pumice and lithic fragments decrease regularly eastward. 

100 
(0.9) 

60.0 

(2. 0) 

62.0 

(4. 7) 

66.7 

(0. 9) 

67.6 

(1.5) 

69.1 

(0.8) 

69.9 

(0.5) 

70.4 

(0.1) 

70.5 

(3.4) 

73.9 

(0.4) 

74 . 3 

(4 . 4) 

78 . 7 

(0 . ) 

79 . 6 

Within about 6 mil es of the vent the upper part of 

thin ash beds, generally 1 in. or less thick. They 

groundsurges, ash washed from the eruption cloud, 

pulses. Continue east. 

the unit contains a few 

may be products of small 

or decline of er upt ion 

View back to west of The Dome, a cinder cone partly covered by pumice fall. 

Road down east flank traverses the pumice fall. At the intersection of the 

road with Road 1821 (18) the pumice fall is about 3.3 ft thick. 

Note Red Hill, a young cinder cone to north; pumice fall once covered it but 

now is eroded. 

Turn south on Road 1821 (18) (elevation about 5,250 ft) which passes on west 

side of China Hat, an 0.8-m.y.-old rhyolite dome. 

Road intersection (Road 2228), keep to left on 1821 (18}. 

Note young basalt flow on west. 

pumice fall. 

Ash-flow tuff crops out beside road. 

Section line 23- 26 marker on tree . 

It is over lain by Mazama ash and Newberry 

STOP 12. Ash-flow tuff (elevation about 5, 070 f t). Walk eastward on small 

dirt road to outcrops of ash-flow tuff. Note the exposures in roadcuts above 

the small cliffs. This ash flow crops out over a north-south distance of 5 

miles but is overlain in places by alluvial deposits and basalt flows. It is 

buried on the west by similar rocks. The base of the ash flow is not exposed 

and the top is eroded, but it occurs in outcrops as much as 25 ft high . The 

ash flow is dark colored and contains abundant gray to black pumice in an ashy 

lithic-rich matrix. This ash flow may correlate with an ash flow that crops 

out locally for a distance of about 10 miles northward from this locality, and 

higher in the section than the ash flows to be visited at stops 1 3 and 14. 

Tu rn around and go back (northward) on Road 1821 (18). 

Cut on east side of road exposes pumice fall over Mazama ash . Old ra ilroad 

grade to east leads to road on west side of China Hat where pumice falls and 

ash flows are plastered on the side of the rhyolite dome. 

Road 2129 (21) on west, continue northward on 1821 (18). 

Road on left leads to site of Brooks-Scanlon Camp No . 3, used in the heyday of 

log ing in the 1930's. Exposure of alluvium on left . The alluvium extends up 

the slope to the caldera rim. 

Ravine . South of the ravine are exposures of basalt with local windows of 

rhyoli t i ash- low tuff. orth of draw rhyolitic ash- flow tuff and a thin 

veneer of gravel form he surficial rocks to and beyond the next turn at 82 . 2 

miles. 

101 
(2 . 5) 

82 . 1 

(0 . 1) 

82.2 

(2.1) 

84.3 

(2.1) 

86 . 4 

(1. 5) 

87.9 

1. 2) 

89 . 1 

(0 . 7) 

89 . 8 

(0 . 6) 

90 . 4 

Brook -S canl on Camp No . 2 . Form r log in it wa ast of road. 

Turn west on Road 2113 (1835) (elevation 4 , 885 f ) Road is along broad rid e 

covered by grav 1 deposits that overli ash-flow tuffs. 

STOP 13 . Ash-flow tuff (elevation 5 ,170 ft). Park i n wid a r a on so uth side 

of road and walk southward to outcrops along side of ravine. This darkcolored ash-flow tuff, characterized by porphyritic obsidian as a common 

accidental lithic constituent, is formed of black to dark-gray porphyritic 

pumice in an ashy lithic- and c rystal-rich matrix. It crops out along ravine 

walls at this general elevation for a distanc e of about 4 miles, b eyo nd wh ich 

it is buried by basal t flows . Farther upslope i t is buried be neath alluvial 

deposits and basalt flows. The unit is similar to the ash flow visit ed at 

stop 12 but has a different lithic component and is much mor e c rystal rich. 

Ash flows, like those at stop 12, occur further north; some oc c ur plaster ed on 

the sides of cinder cones (e.g . , Orphan Butte) . A thick sequence of lapilli 

tuff, identical to that at stop 2 on the west fla nk, occurs about 2 miles 

northeast o f this locality and h ighe r in the section. Turn around and r eturn 

eastward on road 2113 (1835) . 

Continue on road 2113 across road 1821 (18). Ro~d traverses area of rhyo litic 

ash-flow tuff like at stop 14 , overlain by thin gravel veneer. Exposures of 

ash- flow tuff can be seen along many small ravines north and south of road. 

Kipuka of basalt projects through rhyolitic ash-flow tuff south of road . 

Turn north on Road 2013 (13) (elevation 4,665). 

Basalt flow on east was derived from a ven t east of China Hat . 

STOP 14 (elevation 4 , 540 f t) . Teepee Draw ash flow. Walk west from the 

border of the National Forest to outcrops of ash- flow tuff on sides of 

ravines . Note that basalt flows crop out on east side of nearest ravine and 

on west side of the far ravine immediately north of the ash-flow tuff . This 

rhyolitic ash-flow tuff covers a large area on the lower eastern and 

no rthea stern flank and is buried to the northwest and south by basalt flows. 

Toward t he caldera gravel on the surface of the ash flow thickens to the point 

whe r e float and outcrops of the ash flow can no longer be found . This is the 

oldest known tephra unit on the volcano. Plagioclase separated from pumic e in 

it yielded an age of 0 . 7 m. y . (McKee and others , 1976), but unfortunately a 

high atmospheric a r gon content resulted in an uncertainty as large as the 

age . The a s h flow and overlying units are normally polarized . At this 

locality t he lower pa r t of the ash flow is uncollapsed and consists of large 

pumices in an a s hy lithic- and crystal-rich matrix . The pumices show 

progressive collapse upward and the top of the unit is welded . Note the 

scattered fossil · fumaroles in the cliff face . Toward the caldera the unit 

becomes progressively more densely welded . The base of the unit is exposed 

only where kipukas of basalt project through it and the top is eroded . Along 

some ravines, however , the unit is more than 70 ft thick with the base not 

exposed . This ash- flow tuff probably was associated with an early, if no t the 

earliest , period of caldera collapse . I t and the ash-flow tuff at s top 13 are 

older than the tephra deposits of the west flank and may occur at depth 

entirely around the volcano . Look to the east to see Pine Mountain, a complex 

102 
(0 .9 ) 

91 .3 

(2 .0) 

93 . 3 

(0 . 4) 

93 . 7 

(1 . 1) 

94.8 

(2 .5 ) 

97.3 

( 1. 7) 

99.0 

(2 .0) 

101. 0 

(16 . 4) 

117 . 4 

(0. 7) 

118 . 1 

of flows a nd intrusions mostly of dacitic to rhyolitic composition that are 

about 21 m. y . old and that probably correlate in part with the John Day 

Formation . Continue north on road. 

Turn no rthwestward on r oad 2043 ( 2 3). 

Ou tc r op of ash-flow tuff west of road. 

Outcrop of ash-flow tuff on subdued hill east of road is the most 

northeasterly known outcrop. Eroded alluvial fan deposits derived from 

Newberry and Pine Mountain cover the distal part of the ash flow and it is 

also buried on the west by basalt flows from Newberry . 

To the west is Horse Ridge, a highly faulted sequence of older basalt flows 

and near-vent deposits. The faults are part of the Brothers fault zone. 

Junction with U.S. Highway 20. Turn west. 

Viewpoint . Dry river canyon is 800 f t wide and 300 f t deep at this point. 

For next 0.8 mile roadcuts and canyon walls expose lavas of Horse Ridge. The 

basalt flows, with interbedded tuff and breccia, have yielded a 7 -m.y. K-Ar 

age. 

u.s. Highway 70 from here to Bend crosses diktytaxitic high-alumina basalts 

derived from Newberry that cover much of the low-lying r egion to the north. 

Road on north leads to Pilot Butte. Although young in appearance, this cinder 

cone is probably many hundreds of thousands years old, as it is locally 

o verlain by an early Pleistocene pumice fall. 

End of trip at Junction of U.S. Highways 20 and 97 in Bend, Oreg. 

103 
104 
HI H L V PL I BR THER F L T ~ 1 E T HAl-( EY BA I , I{E · 1 

zeorg Iker U. . eologic I urv y M nlo Park, liforni 94025 

nd 

Bruce olf Central r gon ommunity allege Bend, regan 

RY 

The fi ld trip through the High L v PI ins province of 

central nd south-c ntr I regan (fig. I) provid s 

reconnatss nc ov rvi w of the Cenozoic volcanic geology 

long the Brothers fault zone from B nd to Harney Basin th 

site of vents for sever I sh-flow tuffs that cover thousands 

of squar ktlom t r of southe t Oregon nd h ve volum s of 

hundreds of cubtc kilometer ost of this are has been 

mapped only in r conn iss nee (W lker and others 1967; 

Greene nd others 1972), and many inter sting det ils of the 

geolog have yet to be studi d. From e st . to :vest t.h 

geologic record general! progr sses forw rd tn ttm wtth 

some of the youngest volcanic ro ks at the we t (or Bend) end 

of the trip and the oldest at th ast end in and n ar Harney 

Bastn. This d scription is an oversimplific tion with respect 

to the basaltic rocks but the silicic rocks sho a ell-defined 

age progression in silicic volcanism described by lker (1974) 

and Macleod and others (1976). 

., . 

<.."' 

; · I .,<- "' 

; \ \)" ....... \ 

" I •ae.c 

~ I 

0 R E G 0 N 

119' 

Figure 1.- Ind x m p of r gon hawing the High Lava Plains 

and appro imate outlin of Harn y B sin (dott d line). 

105 

Idaho border ( alker, 1977), obscur s much of the older 

Cenozoic geology along the Brothers fault zone. 

Th High Lava Plains are cont!guous ith nd 

gr da tiona! into the Basin and Range provtnce to the south, 

and many I te Cenozoic volcanic rocks nd fault structures 

are common to both provinces. For instance some oi the 

larger north- nd northeast-trending faults, characteristi c of 

the Basin and R nge, ppear to change to a northwest trend 

and blend into th Brothers fault zone (Walker, 1977). A 

comparatively sharp boundary sep rates the High Lava Plains 

from the Blue ountain province to the north, here older 

Cenozoic and pre-Cenozoic rocks have been brought to the 

surface in the Blue Mountain-Ochoco ountains uplift. 

Older Cenozoic volcanic and tuffaceous sediment ry 

rocks are exposed along the northern margin of the High Lava 

Plains and in kipukas (inliers), such as Pine ountain, and 

incl ude parts of the Columbia River Basalt Group nd the 

John Day and Clarno Formations (Walker and others, 1967). In 

most parts of the province, however the oldest rocks are 

aphyric and phyric basalt with plagioclase phenocrysts and 

minor andesite flows of middle(?) iocene ge. These flows, 

whi ch have generally been referred to the teens Basalt, are 

exposed principally along the southern and southeastern 

margin of the province; sections hundreds of meters thick are 

beau tifully exposed in the glaciated canyons high on Steens 

Mountain. An average chemic I composition of these 

mtddle(?) Miocene flows (table I, column l) indicates 

moder te to low silica content and a somewhat higher th n 

normal ~ alumina con tent; this ver ge is not a f ir 

representation, however, of the chemical div rsi ty of th se 

rocks, hich v ry considerably in their proportions of 

pi gioclase, mostly labradorite, and olivine. Isotopic ges of 

the b salt flows indicate most ere er upted bout 15 m.y. go 

(Baksi, and others, 1967; Laursen and Hammond, 1974; W lker, 

and others, 1974). 

The basalts and andesites were over I pped by 

tuffaceous sedimentary rocks nd subsequently by widespread 

sheets of ash-flow tuff. The ash-flow tuffs, which re of 

rhyolitic com position (table I, columns 3, 4, nd 5), spre d 

laterally over tens of thousands of squ re kilometers of 

ancestral Harney Basin and form three principal units that 

r about 9, , nd 6 .5 m • . old. Th tot I volume of erupted 

rhy~litic tephra is unknown but is in the range of 1,000- 2,000 

km . Eruption of this tephr was accomp nied by collapse 

into evacuated m gma chambers, which w s partly 

responsible for the development of the large structural 

d pression of Harney Basin. The depression was subsequently 

filled wi th younger ash-flow tuffs, tuffaceous sedimentary 

rock , and basalt flows and palagonitic sediments-all of 

middle(?) and late Cenozoic age (Piper, and other, 1939; 

aldwin, 1976; Greene and others, 1972). In places, the 

tuffaceous sedimentary rocks have been diagenetically 

altered to ben toni tic clay minerals, zeolites, and potassium 

feldspar (Walker and wanson, 1968), particularly where the 

sediments were deposited in ancient lakes that filled the 

lo er parts of the structural depression. 
120° 

10 

I 10 

L -, 10 20 

10 20 

30 40 

30 MIL S 

50 KILOM TEAS 

~' I c R 0 ',·L ___ __ _ 0 

HORSE RID~~~' GE Mi ca\1 l~ ----- , , 

' ' 2 

~~\ Brothe;s oX~ ! _, / · PI '\\ I '~~ -- ... EWBERRY VOLGA 0 DUNTAI \ ,_-.... ../ 3 . I 

,<,.....,...">.. (2 1) ' \~ ._ , 

HAMPTO BUTT 

/J CJ ) '\ "~ ...._ .Y CHI A HAT • . EAST BUTTE \..,'-.'f-.. ' 

(0.8) .) "{_! "'\ 

• QUARTZ ' '- f -...\FREDERIC 

- - - -~ -- -- --- ~~)TAl -- \~~ ~ ?~1- (~T;)E_ 

\ \(.. ..Y / ' 

L A 

" \ 

umbe rs in pa r entheses are selec ed K/A r ages on rhyoli e dom s 

Fi gure 2A, B and C .--Map showing route of field trip long U .. High 

to Harney Basin, m jor tructural elem nts, nd select d K/, r dom es. 

106 

y 20 from nd 

ges on rhyolt t 

1.··· 

... ·, 

I 
c 

A 

10 

J. 

20 

R 

120° 

30 MILES 

K 

G R A 

-- ~- .... -·-· .. -.. ~ r .. 

~ 

/ I .. -· ' : 

., 2 

107 
I . 

1.·· 

I 

._J 

I 

L_. -. :-:-;--.-

""" • SOUA BUTTE 

TIRE TAl 

7) 

gon tire 

(5.7) 

G R A 

---.-

/ 

T 

Buchanan 

10 __) 

• .. . · 

108 

"!. ·. ~ ·. 3 ·. (\) . 

~ .. 

I 

I 

--- ~ 

2 

• DUC BU 

(10) 

43' 
TJb C .--:\,erJ 'l' ( r 

. t lr ·tc oJs .. dtt c ...111 rh 

---1 --- - 2--

I 2 . 7 

-\ 2 .7 

Fez .3 . 26 

F 7. . 5 

\lg .O<J 

9.0 .90 

'.J 3.0 3. 

1.0 3.7 

HzO I. .43 

Tt 2 2.2 .1 0 

Pz 5 .4 

, \n .2 .06 

I. 

2. 

3. 

5. 

typt( .J l) h 'llll t' ._tl C IIIJ.> ~I !11.)11~ vl 

Itt 1 • ro · · . 

_3 _____ ---------T --

7 ) . 

, 

II. 

2 . 

.2 

.4 

3. 

4.9 

. 21 

.05 

.o 

7 .7 77 .0 

12. 3 12 . 1 

2.'1 .9 

.3 .2 

.5 .6 

. 15 

3.5 

. ') 

. 16 

. 15 

.1 7 

n ke 

' alcu dt d 

In th e t rn part of the pro inc , the m i ddle and 

late Miocene basalt flows, sh-flo tuff , and sediment r 

rock are mostly buried beneath young r !at Miocene, 

Pliocen , and u ternary bas It flows (Williams, 1957; 

Peterson and Groh, 1965; Walker nd thers, 1967) th t 

erupted from wid ly sea ttered cones, shield volcanoes, and 

fissures. Char coal from trees inund ted by several you7q 

flo s on the northwest flank of ewb rry Volcano h v C 

ages of about 6,000 years (Peterson and Groh, 1965, p. 9; 

Chitwood, and others 1977). Throughout much of th 

western part of the province most of these younger rocks ar 

mantled with a widespread but discontinuous sheet of 

unconsolida ted coarse to fine pumice fragments, some of 

which was erupted about 7,000 years ago from Mount 

Mazama, a large prehistoric Cascade Range vole no that 

co llaps d to form the ca ldera now occupied by rater Lake, 

nd om from v n t in th c ld ra of b rry Vol no. 

109 

gc 

b . ~!ti c (p 

Th 

tuffaceous 

and 

r 

t dome 

1)41 111 nd th 

thi ckn 

qu w, Palomino, 

of 

crt p r llt' l ltneM 

cJ t nd utt1<> dg • 

t th ~c t n .1r · wbcrr} 

prob bly resurg nt dome related to v ry ob cur 

calderas tn Harney Basin or to ring-frac ture zone 

Frederick Butte. 

Mo t of the older b salt flows in teens 

ount in re associat d ith major dike s~ drms vi ibl on 

glaciat d c nyon walls on th east f c f the high t ens 

block; younger flows are associ a ted with broad shi ld 

volcanoes of lo relief, with cinder zones, or i th poorly 

exposed fi sure zones. In pi ce the rising b s It magm 

encountered ground or surf a e water, which au ed 

fragmentation of the ba alt and violent ste m 

Where the explosions ere of madera te intensity b 

cones, tuff rings, and tuff ridges formed; th be t e 

of the e features are found bout 40 to 60 km south of th 

field trip route at Fort Rock, T bl Mountain, t. Patrick 

Mountain, and Seven Mile Ridge, all ei t her within or margt n 1 

to Fort Rock nd Christmas L ke Valley, and in the we t nd 

of Harn Basin, near Iron \ountain. More viol nt steam 

explosions bl sted out prominent craters, including uch 

fe tures as Hole-in-the-Ground. 

large t and most p c t cuJ r ruptive 

c nter wberry Volcano ne r the western end of the 

province. It is a compara tively young, Pleistocene and 

Holoc ne vole no, about 65 km long and 40 km wide 

surmount d by calder th t contains East nd Paulin Lakes 

nd v riety of volcani c features and products; t he walls of 

th c lder xpos s ver I varieties of both basaltic nd 

rhyoli ti c rocks, including sh-flo tuff, th t indic te it has 

h d a mor com plex volcanic history th n most of the other 

vole in th r gion (.VIac l od, and oth r , this volume). 

of th High L v PI ins 

th northwe t-trending Brothers 

to be on of the fund men tal 

gon. Th zone is continuou ly 
"~ I ~~ 

/'.. ~" \ \ ( ..... , \ '-' --....., 

S · 

~'\. 

118° 

120° 

25 0 25 50 75 MILES 

I I I I I I I I I 

25 0 50 1 00 KILOMET RS 

\ \-

1 -----..... ------ -..... " / _......... , ) I ......__........- ~ ....._._,~ 

I 0 . Burns '\. Newberry Vole. 

" I "--- ' .......... --- -------

D1amonu \ 

Craters / 

I 

Jordan 

Crater 

0 R E G 0 

-............ 

N 

.......... __ ~....,...-- " 

Cow Lake 

Craters 

1170 

42° ----r---------~~---------.----------~---------.---------.----------~--------~r 

Figure 3.-- We t-northwest-tr nding belt of Middle(?) nd l te u t rn ry ba l t fi ld . 

110 
B ks1, . K ., ork , 

lltr,.ll r'•on. :\t 

· d . Cod 

bsidi n 

vole nic 

R. E. J 972 

u .. 

1-680, 

r y of 

rocks, 

on: 

r t rs 

111 

P t r on , , . .. ( d .), 19 

g IO~Il I gu1d boo 

0 p..1rt men 

57 51 p. 

\1n ral lndu trte 

portion High 

Dep rtrnent eology and 

cooper ti on 1 th U. . ologi 

nd l :250,000 . 

of 

1 r 

.11 t 

olcan1sm to 

Ind 

ks o 

F1eld 

c ntr· 

reg on 
112 
ROADLOG FOR HIGH LAVA PLAINS, BROTHERS FAULT ZONE TO HARNEY BASIN, 

OREGON 

G org W. W lker, U .. G ologic 1 urvey, Menlo Park, aliforni 94025 

and 

Bruce Nolf, Central Oregon Community College, Bend, Oregon 

This road log gives point by point information on the geology from Bend to Harne Basin and 

into the Diarr:ond Craters ~rea at the southea:t mar gin of Harney Basin. The route of the field trip 

and the locatiOns of some 1mportant geograph1c landmarks are shown on figures 2A, 2B, and 2C; the 

road log is keyed to these figures by mileages between principal points of interest and by field trip 

stops. 

ILES 

(0.8) 

0.8 

(3.5) 

4.3 

(16.3) 

20 .6 

Junction U .. Hwys. 97 and 20 in Bend, Oregon, heading east on U. Hwy. 20. 

Jet. with Pilot Butte Road. Pilot Butte, a late Pliocene or early Pleistocene cinder cone, 

is lapped by a pumiceous air-fall dated at between 1 and 2 m.y. 

For next few miles we travel on Pleistocene basalt cut by northwest-trending faults with 

displacements of up to 40 m; physiographic relief reduced or eliminated by sedimentation 

in downthrown blocks. 

Jet. of road to Alfalfa. For several miles traverse young basalt partly covered by 

Mazama sh. Highway begins climb through older (7 m.y.) near vent basalt flows and 

flow breccias that characterize Horse Ridge. Northwest-trending faults, with up to 150 

m vertical offset, cu t these basalts. 

STOP NO. 1. At road jet. Geologic features that can be seen from this point include 

(from oldest to youngest): 

a. Pine Mountain to southeast, a kipuka of silicic to intermediate volcanic rocks dated at 

about 21 m.y. 

b. To west and north, faulted late Miocene 7 m.y.) basalt of Horse Ridge. Note fault 

scarp north of and parallel to the highway. North of this fault are several additional and 

parallel normal faults which have localized basal tic vents in the form of small lava cones 

and cinder cones. 

c . To southwest the large volcanic edifice is Newberry Volcano. (Refer to MacLeod and 

others, this volume.) 

d. To south, China Hat and East Butte represent silicic domes near west end of the age 

progr ssion of silicic domal rocks along the Brothers fault zone (MacLeod and others, 

1976). Age of China Hat is 0.78 .!_0 .20 m.y. and East Butte is 0.85.!_ 0.05 m.y. 

e . Low terr in to south and southeast is underlain by young basalt flows and lacustrine 

nd fluvi tile s dim nts of prob ble Pleistocene age. 

ot th t norm l f ulting along this p rt of the Brothers fault zone is bracketed by the 7 

m.y. b 1t of Horse Ridge nd the basin fill. Faulting of Newberry Volcano and of flows 

in nd ba in indicate some displacements in middle Pleistocene to Holocene time. 

113 
(3.8) 

24 .4 

(5.9) 

30 .3 

(2.4) 

s e 1 

highway 

f ul t th 

thin-b dd d 1 u trin dim nt in ro d ut on north id of 

nd 1 o on north id of highw y, h Ion norm 1 

Millican. ote c rp of noth r ch Ion f ult north of highw y. 

econd flo from top of b it xpo d on s rp north of high y h b n 

d t d at 6.5 m.y . 

32.7 Younger, mu h I s f ul t d flows int rb dded with gr v lly s dim nts dj . nt to 

highway . 

( 1.2) 

33 .9 Jet. with tate Hwy. 27 . 

(7 .2) 

41.1 0 . 2. at Broth rs rest r . Flat topp d ridg to northwe tis c pp d by basalt of 

(7 .0) 

late Miocene age, prob bly compar ble in g to b salt of Hor Ridg . Irregul r tr ecovered hills to the north and ridg s forming north rn skyline r compos d of Eoc n to 

middle Miocene volcanic nd vole nicla tic rocks, including th Pictur org B s It nd 

the John ay and Clarno Form tions. Foreground is underlain by u tern ry b salt nd 

sediments. Source for some younger flows is low conical hill to thee st-north t with 

tank on top. Rounded hill to south of highw y is rhyoli t of unknown ge, prob bly older 

than the age progression. 

48 . 1 Jet. with road south to Frederick (Fredrick) Butte. Fault scarp north of highway is in 

late Pliocene or Pleistocene basalt. 

(2.7) 

50 .8 Jet. Camp Creek road . Turn northeast off highway on side trip. Go 3.6 miles to c ttle 

guard. STOP NO . 3. Rim to the northeast of cattle guard is 3.9 m.y. old ash-flow tuff 

which apparently came f rom vent area south of highway . Frederick Butte, to southsouthwest, is 3.9 m.y. old and composes one of several rhyolite domes on the east m rgin 

of a circular collapse area, probably the source of this ash-flow tuff. 

(7 .4) 

Note that the ash-flow tuff and overlying basalt are cut by faults and here are downdropped south of the fault. South of highway both basal tic and rhyolite vents can be seen 

as low hill . Hampton Butte, to east, is a volcanic pile that includes older rocks and 

intrusive rhyolite . The Clarno Formation crops out on west side and intrusiv rhyolite 

forms conspicuous hill on south side of Hampton Butte. Return to U.S. Hwy 20 and 

continue east. 

58 .2 South of highway both north and south facing fault scarps are visible that cut Pliocene 

and Pleistocene basalts (younger than ash-flow tuff from Frederick Butte vent complex). 

Note ash-flow tuff rims north of highway. 

(3.4) 

61.6 Hampton Station. As we approach Glass Buttes, the faulted basalts on both sides of 

highway are probably Pliocene in age. Some of these basalts appear to lap against the 

rhyolite domes of Glass Buttes. 

11.0 

72.6 Jet. Buck Creek Road, to G. I. Ranch . 

(2 .4) 

7 5.0 Jet. with BLM road to south. Side trip to outcrops of dated obsidian associated with 

Glass Buttes volcanic complex. Drive 2.7 miles to small reservoir. 

STOP NO. 4. Outcrops of obsidian, dated at 4.9 m.y., on both sides of wash. Glass 

Buttes complex is elongate in northwest-southeast direction, approximately parallel to 

the Brothers fault zone. At the east end of this group of rhyolite domes are opal! te-type 

mercury deposits and a thermal well. Return to highway. 

Note large mafic vent north of highway that fed Plio-Pleistocene basalt flows. Proc eed 

east on U.S. Hwy. 20. 

114 
(6.4) 

81.4 

(3.4) 

84.8 

( 11.4) 

96.2 

(6.2) 

102.4 

( 1.3) 

103 .7 

(5 .3) 

109.0 

(0.7) 

109.7 

( 1.9) 

111.6 

(0.5) 

112 . 1 

(4.0) 

116. 1 

(3.1) 

119.2 

( 1.3) 

120.5 

(2 .5) 

123.0 

(3.1) 

126.1 

(2 .8) 

128.9 

(2.7) 

131. 

(3 .5) 

135. 1 

Juniper- covered hills north of highwa y are another rhyolitic compl e x e longa te northwestsoutheast parallel to the fault zone. 

Squaw Butte visible ahead . quaw Butte sits in the middle of another rhyolitic complex, 

dated at 5. 1 m.y ., surrounded by basalt whi ch in places laps onto the rhyolite . 

Dry Mountain north of the highway is a large rounded volcanic pile of faulted middle 

Miocene hypersthene andesite lapped by 6-m.y .-old ash-flow tuffs. A few miles north of 

Dry Mountain, pre-Cenozoi c rocks are exposed, some as old as Devonian. Va lley north of 

highway and south of Dry Mountain is filled with Pleistocene gravel and a few thin 

Pliocene and Pleistocene basalt flows. 

Jet. U.S. Hwy 395 (Riley). Fault scarps cutting Pliocene or possibly late Miocene basalt 

flows to the north and northe st of highway. 

Sharp peak on southern skyline is Iron Mountain, a rhyolite dome dated t 2-3 m.y. This 

appears to be the only exception in the well-defined age progression in sili c ic volcanism. 

Enter area under la in by palagoni ti c (basal ti c ) bedded tuffs of Pliocene and le istocene 

age. 

Ditch on north side of highway provides best exposures of these tuffs. 

STOP NO. 5: Highway rest area. 

Palamino Buttes, south of highway, consists of 6.5-m.y.-old rhyolite; adjacent to Buttes 

on east is eroded basaltic cinder cone. Buttes lapped by basalt flows dated elsewhere at 

2-3-m.y . Both the basalt and under lying rhyolite are cut by northwest-trending normal 

faults with decreased offset of the basalt. 

South of highway for next several miles, rim of 2-3-m.y. basalt underlain by tuffaceous 

sedimentary rocks generally referred to as part of the Harney Formation. These 

sedimentary rocks rest on the Rattlesnake Ash-flow Tuff. 

Exposure of Rattlesnake Ash-flow Tuff in roadcut. 

Outc rop of rhyolite south of highway and addi tiona! outc rops to northeast and north of 

highway. These rhyolites are part of the Burns Butte volcanic complex which has been 

dated at 7.8 m.y . Within the complex are some mafic c inder cones. 

Borrow pit on south side of highway exposes one of these mafic cinder cones. Clastic 

dikes cutting these cinder cones have recently been described by Peterson (1978). 

City limits of Hines, Oregon. 

J e t. of U.S. Hwy 20 (also U.S. Hwy 395) and State Hwy. 78 in the center of Burns, 

Oregon. Burns lies at the northwest margin of a nearly circular are that represents 

central Harney Basin, an area characterized mostly by flat-lying younger fill and some 

dune deposits. Central Harney Basin li es within a much larger structurally depressed 

r a tha t coi ncid s a ppro xi mat e ly with the physiographi c basin outlined on fig. 2. The 

str uc t ur I basin e volved in part c oncu rrently with the eruption of late Miocene ash-flow 

tuffs, hich re tr a v rsed by t he ne xt st ge of the field trip. 

Turn no rt h on U .. Hwys. 20 an d 395. 

Jet. U .. H y. 20 nd 395, turn nort h on 395. 

. 6: Expos ur , a bout 20 m t hick, of Ra ttlesnake Ash-flow Tuff, which has been ~--=--=~-=--:-=- d t d t bou t 6.4 m. y. on t h b sis of numerous analy es. A single flow and cooling unit 

of rhyolitic as h-fl ow t uff that r sts on bedded sedimentary rocks. onsists of a well 

115 
( 1.0) 

136.1 

(0.7) 

136.8 

( 1.3) 

138.1 

( 4. 1) 

142.2 

(0.0) 

0.0 

( 1.7) 

1.7 

(10 .5) 

12.2 

d -v lop db a ! vitrophyr , n 

Vents for thi nd underlying 1 t Miocen h- flow tuff r buri db n th young r fill 

in th 1 wer central part of H rn y B in. nly indir ct nd m g r vid nee is 

available s to their location and chara t r. 

T P NO. 7: Prater Cr ek Ash-flow Tuff exposed in ro d cut. ingl flow nd ooling 

unit that is 8.4 m.y. old, a nd is slight ly mor m fie than th Rattle nd D vine 

anyon sh-flow Tuffs. This unit is ch r terized by low cry t I cont nt · nd bundant 

1 

lithophysa . Maximum thickness is bout 30 m; its xtent is poorly known b use it is . 

largely buried by young r materials including th R ttlesn k Ash-flow Tuff. Unit is 

underlain by bedd d tuffaceous edim nt ry ro ks; int r- sh- flow tuff units thin way 

from central H rney asin. 

Cross railroad tr cks. 

STOP NO. 8: Upper non-welded part of vine anyon Ash-Flow Tuff. olumned shflow tuff across creek to the north-northwest is welded nd compact d low r p rt of the 

same unit, samples of which have been d ted . from a number of widely separated 

localities at about 9.2 m.y . 

STOP N . 9: Outc rops on both sides of road of Devine a nyon Ash-f1~w Tuff. A single 

cooling, probably multiple-flow unit that co vers more than 18,000 km entered on the 

east half of Harney Basin and extending north, south, and e t of the basin. M ximum 

thickness is about 35 to 40 m, although most of the unit is less than 10 m thick. 

Isopaching suggests a source caldera in the vi cinity of Burns, perh ps with its west 

margin in the vicinity of Burns Butte. Contains s much s 30 percent crystals and 

crystal fragments, mostly sodic sanidine, ome quartz, and minor, ubi qui to us green ironrich clinopyroxene. Thinner distal ends tend to b glassy, where s thicker tions 

exhibit more de vi tr ifica tion and locally extensive vapor-phase a! ter tion. 

At this point turn around and return to Burns. 

LOG FROM BURNS TO DIAMOND CRATER 

Jet. of U.S. Hwys. 20 and 395 with State Hwy. 78 . He d east on 78. 

Jet. State Hwys. 78 and 205. Turn south on 205 . 

STOP NO. 10 : Viewpoint on spur gravel road, south rim of Wrights Point. Wrights Point 

consists of a 2.4-m.y. valley-fill basalt flow which h s been erod d into positive relief. 

The point extends 6.5 km from here eastward; the vent is several kilometers to the 

west. Visible beneath the basalt flow are some of the best exposures of bedded 

tuffaceous sedimentary rocks of the Pliocene Harney Form tion . 

From this point looking to the south-southwest, the juniper- covered hill is Dog Mountain, 

original site of the type section of the Harne y Formation. Inasmuch s Dog Mountain is a 

palagonite tuff ring, atypical of the formation, road cuts on Wrights Point have been 

recently de signa ted a reference section (Walker, in press). The tuff ring is late 

Pliocene. West of Dog Mountain are a number of additional young basaltic vents (tuff 

rings, cinde r cones, lava cones, and dikes). East and southeast of Dog Mountain are man 

more young basaltic vents, including those at Malheur National Wildlife Refuge 

Headquarters, at Coyote Buttes, and Diamond Craters. 

Steens Mountain (elev. 9,733 feet; 2,967 m), a west-tilted fault block capped by the 

middle Miocene Steens Basalt dominates the southeast skyline. On the east-southeast 

skyline is Duck Butte, a large rhyolitic dome complex about 10 m.y. old, considered by 

116 
( !0.7) 

22.9 

(0.9) 

23.8 

( !.6) 

25 .4 

(2.4) 

27 .8 

(3.1) 

30 .9 

(3.0) 

33.9 

(6 .6) 

40.5 

(0 .8) 

Mac Leod and others (1976) as probably the oldest rhyolitic uni t within the 

progression. 

Excellent view to the west of the rhyolitic dome at Iron Mountain. 

ge 

Bridge t Narrows. Some geophysical data suggest that one or more source calderas for 

the late Miocene ash-flow tuffs are centered about here (H. R. Blank, or 1 

communication, 1974). Harney Lake, to west, is the current sump of the Harney Basin 

drainage system. Low hills to the east are young basaltic cinders, palagonite tuffs, and 

related flows. 

J e t. with road to Malh ur National Wildlife Refuge Headquarters and to Princetor. , 

Oregon. Along this road are numerous young basaltic vents, mostly characterized by low 

rounded hills consisting of red cinders, common cored bombs, and agglutinated basalt. 

STOP NO . 11: Roadc ut in a c rystal-poor pumi c i te that is thoroughly a! tered by 

zeoli tization (erioni te, clinoptiloli te, etc) and may represent either an ash-flow tuff or a 

pumice slurry deposit that flowed into shallow water. Pumice fragments are neither 

c olla psed nor welded . 

Excavated basaltic ci nder cone to east of highway . Rim on west side of road is welded 

pumiceous ash-flow tuff, probably Rattlesnake Ash-flow Tuff. Cap on Saddle Butte is 

basalt flow interstra tified with the Harney Basin deposi tiona! sequence. 

Exposures in low roadcut of non-compacted, non-welded, partly altered crystal-rich 

Devine Canyon Ash-flow Tuff. Alteration suggests that this also may represent 

deposition of ash-flow in wet en vironment. 

STOP NO. 12 : On moderately well welded crystal-rich Devine Canyon Ash-flow Tuff (9.2 

m.y .). This is overlain by pumiceous ash-flow tuff to the west and is underlain locally by 

as much as 150 m of bedded tuffaceous sediments which lap south and southeastward 

onto middle Miocene ( %15 m.y.) Steens Basalt. The overlying pumiceous t uff also thins to 

the south. 

Note the several northwest-trending fault scarps in which units are stepped up to the 

south. This is a prominent northwest-trending zone which can be traced almost 100 km. 

41.3 Exposures of densely welded Devine Canyon Ash-flow Tuff. 

(0 .8) 

42 . 1 Turn left on paved road to Diamond Oregon. 

( 1.5) 

43 .6 Rimroc k on both sides of road is Devine Canyon Ash-flow Tuff. View to south-sou thwest 

of large northeast-trending, east-facing scarp along the fault zone that sep rates t he 

west-til ted High Steens block from the Jackass Mountain block. This is one locality near 

the southern margin of the Brothers fault zone where a northeast-trending fault sy tem 

c urves into a northwest-trending system. 

( 1.2) 

44 .8 Excellent profile to the north of the Diamond Craters volcanic complex. This compl x 

has been studied by Peterson and Groh (1964). From here can be seen a few vents and 

loc 1 gentle doming of basalt flows ithin the complex. The valley to the north and east 

i und rlain by surfici 1 deposits resting on Devine Canyon Ash- flo Tuff which is faulted 

( 4 .I) 

and underlies th i mond Crater comple x. 

48.9 Turn north on gravel road to iamond Crat rs. 

( 1.0) 

49.9 L ft at "Y" on ro d to iamond Cr ters. Shortly beyond this point we see basalt flows of 

(2.2) 

th iamond Cr ter complex th t 1 p onto evin Cany n Ash-flow Tuff. 

52.! o d jet. at borro pit in inder cone. The princip 1 p rt of th complex is west and 

north of this point. 

117 
v il bl nd 

a. n hill top, approximately 1 km north of borrow pit j t., w th outh m rgin 

of a 2.3-km-long northwest-trending gr b n long th n longat dom 1 

structure in the basalt. This gr ben w s pp rently the i te of li tt1e or no eruptiv 

act1v1ty. ummit co llapse prob bly result d from withdr w I of lav from highstanding magm body beneath this up- r ch d tru tur . The complex in ludes fi v of 

thes elongate domal tructure all of whi ch oriented in northw st- outh 

direction . 

Four of the five are ch racter iz d by various d grees of ummi t fr ture nd coil ps . 

In the near foreground to the outh is a prominent pit rat r nd relat 

Many of these young t vent r localized t the margin of dom 1 stru tur s ( t point of 

maximum flexure) and represent points of lav egr s t times of ummit co li p e. \ 

The tendency to use degree of vegetative co ver to d t rmine r 1 tiv 

be misleading. For example, in the foreground to the south, the 

sagebrush cover laps onto the I ss veget ted flow. 

Based on evaluation of weathering features in this dry environment, we b Jieve th t thi 

basal tic complex is late Pleistocene in · age, rather th n Holocene. Thus f r no m ter ials 

suitable for isotopic dating have been found, but n age of 17 ,000 +2 ,000 ye rs w s 

obtained by Friedman and Peterson (1971) on hydration rinds of rhyolitic m terial 

enclosed in basalt bombs. 

Jackass Mountain, to the west-southwest, is the high point on the J ck ss Mount in fault 

block which is bounded by prominent scarps. Most of the rims to the south are fault d 

and eroded Devine Canyon Ash-flow Tuff. Some patches of R ttlesn ke Ash-flow Tuff 

are present on the back (west) slope of teens Mountain. The giant west-dipping teen 

Mountain block is bounded on the east by precipitous northeast-trending, 1 ,800-m-high 

fault scarp, and is lapped on the west by sedimentary rock containing a B rstovi n 

vertebrate fauna and by late Miocene ash-flow tuffs and interbedded sediment ry rocks. 

NOTE: For those interested, a loop road between Frenchglen and northern atlow V lley 

traverses the dip slope and crest of the High Steens. 

The lower country to the north of the Diamond Craters com plex is loca lly under! in by 

accumulations of young basalt, mostly valley-filling flows. Some of these flows 

effectively closed off the pluvial drainage from Harney Basin. 

On the skyline to the north, older rocks crop out north of the Brothers fault zone. 

b. Approximately 2 km west of the graben is a large vent area referred to by Peterson 

and Groh (1964) as the central crater complex. This complex is localized by a partly 

filled northwest-trending graben at the crest of another elongate domal structure. 

Probably graben development and eruptive activity were at least in part 

contemporaneous. More than 30 separate vents have been identified within this 

depression . Vent activity ranged from quiet emission of lava to highly explosive volcanophreatic events. Blocks of Devine Canyon Ash-flow Tuff and associ a ted units are found 

in the tephra; exotic blocks up to 50 m in maximum dimension are present. Other 

features within this central crater complex include exhumed conduits, driblet spires and 

spatter cones, and small-scale collapse structures. 

118 
A FIELD TRIP TO THE MAAR VOLCANOES OF THE FORT ROCK - CHRISTMAS LAKE VALLEY BASIN , OREGON 

G. H. Heiken , Geosciences Division , Los Alamos Scientific Laboratory , Los Alamos, NM 87545 

R. v. Fisher , Dept . of Geolog y, University of California , Santa Barbara , CA 93106 

N. V. Peterson, State of Or egon , Dept . of Geology and Mineral Industries , 

Grants Pass, OR 97526 

The Fort Rock - Chr istmas Lake Valle y basin is a former lake basin that e xisted from 

late Pliocene through late Pleistocene time . The basin is about 64 km long and 40 km 

wide ( Fig . 1) . Eruptions of basaltic magma occurr ed along faults that trend diagonally 

across the basin and adjacent highland , forming maar vol canoes within and o n lake margins 

and forming cinde r cones with flo ws beyond the lake margins (Peter son and Groh, 196 3 ; 

Heiken , 1 97 1 ) . 

The purpose of this field trip is to visit several of the maars and a maar complex in 

and near the basin . 

Road Log: (c umulative distance , in miles; stop- to- stop mileages are in pa-rentheses) 

0 . 0 

( Fig . 2) . 

La Pine Junction ; intersection of Oregon State Highways 97 and 31 . 

down Highway 31 , to the southeast. 

Figur ~ 1: Location map. 

119 

Proceed 
_. 

I'\) 

0 

0 

0 

0 

(] Cb 

Fl<lA. be,. and Doll on 

the dOw1"1hroY<o"n ·t: c 

cs c p<» •' S at tn 

0 

(P 

lui ' con e) one " ' I"Ql Pltoc c nc 

0 C .noc r cone ) 

lat e ooso

~ . t flows 

0 

•ll og.-

10 

oO 

0 .: 0 0 

{J 

1Q m ll c ) 

20 ~m 

( u 0 -

~ ~ c~ o ~ 

~ I 0 ~ 0 0 ~ o o Dpo o 

a L•T 1 TOP 0 

0 

\) 

0 GREEN 0 

•• ~ So 

0 

c. ··-·?"V4; HAYE S BUTT["-- -----

',\ ~r:,' {- ; 

··, -$-/( 

• ', (,so 

Figure 2 : Map of Fort Rock- Christmas Lake Valley Basin , sho wing route of fie J:j .r·1.p . 

43. 30 ' 

43. 15' 
( 1. 0) 

1.0 

( 1. 9) 

2 . 9 

(7 . 1) 

10 . 0 

(0 . 3) 

10 . 3 

Large meadow ; good view of Newberry Volcano to left (north) . 

Railroad crossing ; as is typical of the area between La Pine Junction and Fort 

Rock Valley, the surface (except for steepest slopes) is mantled with Mazama 

pumice from Crater Lake. 

Bend in road; Moffitt Butte tuff ring is straight ahead. 

Moffitt Butte is on left ; its base is about 50 m east of road in the woods . 

Moffitt Butte 

Moffitt Butte is a dissected tuff ring, 1400 min diameter and 120m high. It is a 

praninent topographic feature , but is obscured by the forest from the road. Although not 

associated with a lake basin, as is the case for Big Hole and Hole-in-the-ground, Moffitt 

Butte is a tuff ring composed of hyaloclastic t uffs. Rising magma may have encountered 

permeable aquifers beneath the cone . A line of tuff rings between here and the Fort Rock 

Basin are along a topographic low between Fort Rock and the La Pine basins. 

The crater floor of Moffitt Butte is about 80 m above the surrounding plain. A parasitic vent, and small tuff ring, 510 m in diameter, is located on its southwestern flank . 

The deposits consist of sideromelane lapilli-tuff in graded and ungraded beds, 3 to 30 em 

thick. Near the main ring crest is an· unconformity dipping 20 ° into the crater that 

truncates beds di pping outward at 35° (Fig . 3) . Rocks above the unconformity consist of 

a 1m thick bed of angular basalt blocks and 18m of very wel l -bedded lapilli-tuff. 

The c rater of the parasite vent is fil led wit h lava that issued from a dike on its 

northwest edge . 

(2 . 8) 

13 . 1 

( 1. 5) 

14 . 6 

Road cut through pressure ridge or large tumuli in a basalt flow . This is 

typical of many road cuts between here and Fort Rock Valley ; basalt flows are 

from vents on the southern flank of the Newberry Volcano; overlain by Mazama 

pumice . 

Junction of State Highway 31 with Rock Creek Road (gravel ) . Exposed in road 

cut south of highway are remnants of an unnamed tuff ring (Ridge 28 of 

Peterson and Groh , 1963) ; gray to orangish-brown (partly palagonitized ) , 

well-bedded hyaloclastites. Most units contain accretionary lapilli. 

NW 

4soo·F 

44oo· E 

NE 

4soo· E. 

44oo· E? 

SE 

sw 

Figure 3 : Cross-sections through Moffitt Butte , Klamath County Oregon . The upper cross 

section is through the main body of the tuff ring . The lower c ross-section is through a 

parasitic vent on the southwest flank of the ring; the lava-filled crater is close to the 

highway . 

121 
(5.0) 

19 . 6 

Big Hole 

J nction , t · t Highw y 31 , Big Hol Ro 

( so u t h) to en t r Big H o l e ( no i n cl u d 

lurn of'f 

in th 

Y 31 to right 

A g r avel road from this junction go cir cul r , 1820 m 

t th cr st and 

o n h northeast 

iameter maar cra t er . Deposits o f the 

exten 1800 to 2500 m be yond th c rater rim . 

si e , along High wa y 31 (Big Hole Butte) . 

The tuff ring is composed o f moder te- to .J 11-b sid romelan e lapilli-tuff n 

tuff breccia , in beds 5 em to a met r thi k. Th ccia b ds i nclu por phyrit ic 

basalt bl oc ks up to 2 . 5 m in iamet r ; the r a r bund nt be ding pl n gs c us d by 

impact of these bl oc ks into once w ter-satur t as h b ds dur ing th ruption . Best 

exposures o f these de posits are along gullies on the east rn r i m o f th maar . Con volute 

bedding within the rim deposits is w ll xposed alo ng th east r i m (Fig . 4) . 

Within the crater is a 152 m-wide ledge that ap pe a rs to beth top o f la r g bloc 

that slumped into the crat r, possibl y during t~ ruption. Collaps i nt o th c r ate r of 

such large blocks would explain the la r ge volum e o f the c rat r an sm ll p rcentage of 

xenoliths within the ejecta . 

(0 . 6) 

20 . 2 

( 0 . 4) 

20 . 6 

( 4 . 5) 

25 . 1 

On the right are. well-bedde hyaloclastic tuff deposits o Big Hole tuff ring. 

Deposits are thicker on the northeast rim , probably ca us ed by prevailing winds 

from the southwest during the· eruption . Th e th ic ker posit is called Big 

Hole Butte , although it is not a separate struct ure o r ve nt . 

Good exposure of Big Hole tuffs on ri ght side o f Highwa y 31 . 

Junction of State Highwa y 31 with th r oa o Hole-in-the- gr oun (HIG) (Pr oceed along Boundary Road , 11245) . The side trip t o HIG will not be included in 

the overall mileage . Forest Road 245 is gravel ; f ollow sig ns or 2 . 8 miles 

(one l eft turn and one right turn t o the west rim and to HIG overlook) . 

Hole- in- the- ground 

Hole- in- the - ground is described by Pete rson and Groh ( 1961 , 196 

(1971) . Lorenz's (1 971) abstract is as f ollo ws: 

and by Lorenz 

"Hole-in- the- Groun is a volcanic explosion crate r o r maar 

located in Central Oregon on the edge of For Rock basin . At the 

time the c rater wa s formed betwee n 13 , 500 and 18 , 000 ear~ ago a 

lake occupied most o f the basin and the site o f the eruption was 

close to the water level near the sho re . The crater is now 112 to 

156m below the original ground l eve l and is surroun e by a r im 

that r i ses another 35 to 65 m higher . 

The volume of the c rater below the or iginal surface is onl y 60 

pe rcent of the volume of the ejecta . The latter contains onl y 10 

percent juvenile basaltic material , mainly s id erom elane produced by 

rapid quenching of the lava. Most o f the ejected mat e r ial is fin e 

grained , but some of the b locks o f ol e r r ocks r ach im nsio so 8 

m. The argest blocks are concentrate in fo ur ho r izons and reached 

distances of 3. 7 km from the center of the e ra ter . Ace ret ionar y 

apilli, impact sags, and vesiculated tuffs are well develope 

The crater was formed in a few days or we eks by a series o f e xplosions that were triggered when basa l ti c magma r ose along a northwest- trending fissur e and came into contact wit h abundant ground 

water a t a depth of 300 to 500 m below the surface. Aft er the initial ex plosion , repeated slumping and subsidence along a ring-fault 

l ed to intermittent closures of the vent , changes in the suppl y o f 

ground water , and repeated acc umulations of pressur e in the pipe . 

Four majo r ex plosive events resulted from pressures o f over 500 bars 

in the orifice of the vent . Ejection ve loci ties during these 

period s reached 200 meters per second . The c orresponding pr essures 

122 
123 

his r g·on. Con olute b ddd oll m nt-l.k mov m nt of 
. 

• . .. 

! 

• I 

· ... 

CENTER FOR VOLGA, OLOGY 

'1'. ' , _, 

.. ....... "":' .. 

:. 

.. -· ·,._ .- ..... 

+: ... .... ~\ '· .. I 

.. .... 

Figure 5 : Topographic and geologic map of Hole-in-the-Ground . 

feet . Field control by J . W. Hawthorne . 

124 

. -

~ ••• u .. a-.Jt ""' ·' 

Pyroclaatlo o j oct o 

n.,-.toulo 

Co,.l_.roloe 

- 1'11wu- l&a< llo.l 

Opp.r 1p1Jil>T1t e 

o f "-79rl T\off 

Contour interval is te 
.,..,. ..... Lake beds 

,_.,,. · .. ( Pyrocla~llc rocks of H.G. 

"/JJ..JO'. Porphyrllrc basal!, lype 4 

llllllr:tu:! !"'orphyrrllc basal! , 1 ype 3 

·.;;;. ~ FrnOIJrarnod basal!, type 5 

I,ll Drillhole t\o. I , No. L: 

+++ In trusive basalt 

L: H 

400 600 BOO IOOOm 

Figure 6: Geologic section through Hole-in-the-Ground based on surface exposures, drill 

holes and geophysical data. Interpretation by Lorenz (1971). 

and velocities during intervening, less violent stages were in the 

range of 200 to 250 bars and about 130 meters per second. 

The kinetic energy released during the most violent eruptions was 

approximately 9 x 10 20 ergs and the seismic events that must have 

accompanied these explosions had a magnitude of about 5. Ejecta 10 

centimeters in size were thrown to heights of 2 to 3 kilometers and 

the eruption cloud may have reached 5 kilometers or more·. The axis 

of eruption was slightly inclined toward the southeast; the form of 

the vent seems to have had a more important influence than wind. 

Base surges that accompanied some of the explosions left deposits of 

vesiculated tuff. 

The total energy derived from the basaltic magma was of the order 

of 5. 7 x 102 3 ergs. Most of this energy went into heating of ground 

water and the enclosing country rocks; only a small part, possibly a 

tenth was released by expansion and vaporization of the water and 

mechanical processes, such as crushing, acceleration and ejection of 

debris. 

Geophysical measurements indicate a domical intrusion below the 

crater floor and extending upward as a ring dike around the margins 

of the c rater." 

Return to State High way 31 . 

( 1. 7) 

26 .8 

26 . 8 to 

28 . 2 

Picnic grounds on right. 

Section (to p to bot tom) through ( 1 ) basalt flow ( from flanks of Newberry Volca no?) and (2) Peyerl Tuff ; a section consisting of 4.0 m.y. old pyroclastic 

flows and tuffaceous sediments . The section is e xposed along Highway 31 in 

r oad c uts. I f you stop, please leave your auto at the top of the hill in the 

pi c ni c grounds and stay on the edge o f the road; there are nl.ITlerous fastmoving logging truc k s on this highway~ 

125 
( 0. 9) 

29 . 1 

( 6. 4) 

35.5 

( 0 . 1) 

35 . 6 

(0 . 5) 

36. 1 

fort Rock 

Junction, Oregon State Highway 31 and Fort Rock Road. Turn left (east) on 

Fort Rock road. Before reaching the lake basin floor, the road cross s several north-south trending horsts and grabens th t cut through a section consisting of the Peyerl Tuff and Pliocene(?) age basalt flows . On the left 

(north) of Fort Rock road along an east-west line to Fort Rock re the r mnants of four deeply eroded basaltic vents. Onl y small remnants of lava lakes 

remain, but most are surrounded by aprons of rounded cobbles and pebbles consisting of palagonitized hyaloclastic tuffs. The vent closest to Fort Rock 

(Beggars Heel) has a cave where some of the oldest man-made artifacts in 

Oregon were found . 

Downtown Fort Rock and intersection with Fort Rock State Park Road (follow 

signs) . 

Turn left to Fort Rock State Park (if you stay o n this road and follow th 

signs, you can reach Hole-in-the-Ground from the east side). 

Fort Rock State Park 

Fort Rock , with its spectacular wave- cut cliffs , is an isolated maar volcano within a 

monotonous, flat lake basin (Peterson and Groh, 1963). The wav e-cut remnant is 1360 m in 

diameter and 60 m high , and the present crater floor is 6 to 12 m above the floor of the 

lake basin. The south rim has been breached by waves of the former lake , providing easy 

access to the crater . A wave- cut terrace occurs 20m above the floor of Fort Rock Valley 

( fig. 7). 

The maar is composed of orange-brown lapilli-tuff in beds of 1 em to 1 m thick that 

can be traced from within the era ter to the outer flanks . Graded beds with accretionary 

lapilli are common . 

Inward- dipping beds are parallel to the crater walls (fig . 7) and suggest that the 

crater is funnel shaped; the innermost beds dip inward at angles of 20 to 70 degrees . On 

the west side of Fort Rock is a distinct angular unconformity where the deposits, truncated by slunping into the crater, are plastered with younger beds. These younger beds 

are part of a continuous pyroclastic sequence on the outer flanks. An incipient slump is 

visible on the east flank where the sequence is in-place with quaquaversal dips a way from 

the rim crest; fracture planes associated wit h the slump , dip in ward at 40 degrees , the 

same angle as the surface of the unconformity on the opposite side of the crater . 

Fort Rock is typical of most of the smaller, isolated maars of the Fort 

Rock-Christmas Lake Valley basin , such as Table Mountain , Flat Top, Lost Forest, Green 

Mountain SW and Green MountainS ( fig. 2) . 

(7 . 00) 

43 . 1 Return to junction of fort Rock Road and Highway 31 and turn left (south), 

down Highway 31 toward Silver Lake . 

An alternate route, from the center of downtown fort Rock , is south along 

Lake County Road 513. This is a gravel road that is a short-cut to Silver 

Lake, but the bridges will not support heavy trucks or buses . If you take 

this alternate route of 16 . 5 miles, you wil l go between a north- south 

trending normal fault ( west ) and the Connley Hills (east) . As you leave the 

lake basin, near junction with County Road 510 - C you will see the northern 

end of the Connley Hills , a line of domes of intermediate composition . Remnants of a small maar are plastered onto the northernmost dome, near the 

former la ke shoreline . South of the Connley Hills is Hayes Butte , a basaltic 

shield . These volcanoes stood as an island in the lake that was present here 

during late Pliocene-Pleistocene time . 

To the west , several broad , older ( Pliocene ) maar volcanoes are exposed in 

a low ridge, partly buried by basalt flows . a gravel road to the we st ( turn 

right, 5 . 3 miles south of the town of Fort Rock) crosses one o f these maars . 

It is on pr ivate land , ho wev er . Please ask for permission to enter before 

doing so . 

126 
0 

/ 

/ 

/ ' 

1 

as 

300 m. 

I 

, 

I 

, / 

" 

/ 

/ 

/ 

~--

Qt 

' '-,',,, ' '<. 

.... _ ... _ -----,, 

.. . ·~ · "· 15. 

I 

I 

I 

I 

\ 

\ 

\ 

\ 

-- - - --; I / 

\10 \ 

\ 

I 

...,..... 

30 

30 ( 

2?.-l -" 

-RO-\ 

36 

-"' 

I 

I 

I 

I 

I 

I 

I 

,~·/ 

/ Qs 

I 

I 

' 

I 

I 

I 

I 

I 

I 

I 

... 

. ?" 

C. 

Figure 7: Fort Rock tuff ring . A. "MAP. Qs - Pleistocene 1 ake sediments , mostly d iatomites . Qt- tuffs, lapilli- tuffs and tuff breccias. Scalloped line is an unconformity . 

Dot ted line is the cliff edge . B. Ob lique aerial photograph , from the southwest . C. Cross-section . 

A 
(0 .4) 

43.5 

( 3. 1) 

46.6 

(9.4) 

56.0 

( 2. 8) 

58.8 

( 1 • 9) 

60.7 

(6.4) 

67. 1 

(2.0) 

69. 1 

(2.0) 

71 . 1 

( 1. 1) 

72.2 

After c rossing a small divide, you will enter Paulina Marsh; no out c rops 

until you reac h the intersection with Highway 31 n ar Silver Lake . 

Road cut (Highway 31); park at base or top of road cut. Be careful; stay o n 

the side of the road. There are good exposures of part of the Peyerl Tuff, a 

sequence of pyroclastic flows and interbedded sediments , as much as 150 m 

thick, that crops out along the west side of Fort Rock Valley. ·Radiometri c 

ages of 4.47 + 0 .84 and 3 . 35 + 0.44 m.y. were determined for sev eral of the 

pyroclastic flows (MacLeod et - al., 1976). The source for these pyroclastic 

units appears to be a 7 to 10 km wide caldera near Wart Peak, that is about 15 

km WSW of this stop. 

Junction, Highway 31 and Wi ckiup Springs - Starns Well Road. The ridge on the 

horizon to the west is near the east rim of the Wart Peak Caldera, identified 

by N. MacLeod. Due south is a cinder cone of late Pliocene(?) age. 

On the east side of the road is a poorly exposed Pliocene(?) maar, the 

Wastina Maar of Peterson and Groh (1963). 

Straight ahead (south) is Hager Mountain, a 5 . 9 m. y. old silicic dome on the 

southwestern rim of the Fort Rock - Christmas Lake Valley Basin (MacLeod et 

al., 197 6). 

Exposure of lake sediments in road cut (Highway 31) . 

Town limit ; Silver Lake. As you drive along Highway 31 between the town of 

Silver Lake and Christmas Valley Road, note the wave-cut terrac es and notche s 

along the basin rim, especially northward around the "island" formed by Hayes 

Butte, Connley Hills and the Table Rock Maar Complex. The maar s are straight 

ahead and north of the highway. 

The Silver Lake graben (south) is the keystone of a broad arch broken by 

normal faults; Pliocene age basalt flows exposed in this arch slope northward 

into and under the lake sediments of the Fort Rock - Christmas Lake Valley 

Basin. 

Junction, Oregon State Highway 31 and Christmas Valley (Arrow Gap ) Ro ad. Turn 

left (north). Dunes south of the junction rim Silver Lake, the marshy rP.mnant 

of a much larger Pleistocene Lake. 

On the right (east) is the Table Rock maar volcano complex. The l owest c liff 

is a basalt flow from Hayes Butte that underlies the west edge o f the c omplex . 

Dirt road into the center of the Table Rock maar complex l eaves the pav ed 

county .road (to the right). Continue east and return to this junction after 

visiting stop A. 

Stop A. J ust b e yo nd the pass , a t t h e b ottom o f g ull y c ut nto the maar 

complex , stop. Proceed on foot up a jeep trail to the right ( east) . 

TABLE ROCK MAAR COMPLEX (STOP 2) 

General Description 

Table Rock is a tuff cone located 14.5 km east of the village of Silver Lake, o n t he 

shore of Silver Lake, one of the few remnants of a much larger Pleistoc ene l ake that once 

filled the Fort Rock-Christmas Lake Valley Basin (Fig . 2) . The Table Rock tuff c one is 

part of a maar comple x consisting of the cone, two large tuff rings and six smal l er tuff 

rings or eroded vents. 

This complex forms an elongate , NNW-trending oval 5 . 6 by 8 . 8 km. The highest point, 

about 395m above the basin floor, is the crest of Table Rock. 

Silver Lake graben, located immediately south of the tuff-ring c ompl ex, is the ke ystone of a broad 25 -km-wide structural arch that forms the southwest boundary of the l ak e 

basin . The normal fault defining the east wall of the graben (through which Highway 31 

128 
. ········· 

__.. .......... 

.· 

TABLE ROCK 

LAKE COUNTY,OREGON 

J 

·. \\ .. I . ~ EXPLANAT ION '-.......;..__ I • • • • • CE NTER of MAAR 

';A CRATER 

---RI M o f MAAR 

jt; A STOPS 

VENT NUMBER 

• HORIZON TAL BEDS I 

I 

~''o 

SYNCLINES 8 1:+ AN TICLI NES OUE 

TO SLUMPI NG 

CH ANNELS 

I 

i· 1N ~ ~:~ [J GRAVEL BAR 

- ~BASALT DI KES 

;:; ~BA LT AVA LAK E z 

w 

u 

g_ 

v• D MAAR DEPOS I TS 

j D I TE RBEDOEO 

c. SEDI MENTS 

0 

I TER SE ODED BASALT F"LO 

FROM HAYES BUTTE 

OOm 

I ' u ~ ,..-- -, . / " / .· ..... ~ \\--.. 

\ 

·. ..... · 

\ RANCH 

' ' '---

\ 

J 

I /+ 

/H / - / 

Figure 3r~ : Table Ro c k tuff ring comple>-.. 

129 
continues o e r Picture Rock Pass) is p rallel to the long x is of th maar compl x and 

possibly lies beneath it . The fault , howev r, doe s not c ut any of the tuff d posits . 

The tuff ring complex ov e r lies a 220 m thick ection of lake sedim nts and i nterbedded tuffs , and sands and gravels derived from the Connley Hills . Th se pr edominantl y 

lacustrine sediments overlie the Pi c ture Rock Basal t ( H mpton , 1964) , f el dspathi c di ktytaxitic basalt unit of Pl1occne age . The basalt is well-exposed along th south rn 

edge of the lake basin, es peciall y along Highway 31, south of the T· bl e Ro c k Complex . 

The Picture Rock basalt, consisting of individu 1 flows up to 9 m thi c k , with an aggregate thickness of over 230m , dips into the basin wh r it is th main aquif r tapped for 

irrigation (Hampton , 1964). South and east of the basin , shallow lacustrin e and flood 

plain deposits and maar eposits re int rbedded with the Pi cture Ro c k bas lt ( Walker et 

al ., 1967) . 

Stop A, table Rock Comple x 

Park on the county road , walk east along th gull y south of vent 6 ( bluff on north) 

for about 350 m. Sediments underlying the Table Roc k Complex are well-exposed here (Fig . 

8). In contrast with well-bedded diatomites found on the east side of the complex, 

these consist of interbedded volcanic litharenites , lithic arkoses , diatomaceous siltstones and lapilli-tuff that form an outwash apron around the Connle y Hills . The Connley 

Hills , located northwest of Table Rock co nsist of a basaltic shield and i ntermediate t o 

THICKJ\ESS 

(1-IETE RS ) 

DESCR IPTION 

Massive tuff- breccia, wi th blocks of the 

und erlyi ng units . 

\.:....;1'- - -----::------------ ------- - - Gray pumice - be11r in g s ;, nd or. e ; \Ol cani c 

4.5+ 

lith arenite . We ll -bed e d . 

1\'h i te d ia t on ace ou5r. dSto-ne ; 

Gray p~ mic e lapilli tuf! -------- ----

Well-bedded , subnature coarse san d tone; 

tuffaceous volcanic litha;cnitc. The 

sandstone contains abundant reworked 

pumice fragmen ts . 

h'hite , well - bedded sandy cl zystone; 

possibly diatomaceo us . 

Poo rly bedded su ma ture medium to coa:se pumice-bearing lithic subarkose. Pum1c e content in c re as es ne ar he base to a ou t 

60 \ of the rock . 

Well-bedded gray or white r dium sandstone· sub-lithite-arenite. Interbe dde d 

with ihe sandstone are a few thin layers 

of conglomerate , made up of well - rou nde d 

pumic e fragw.ents . There i s some crossbedding. 

Talus covers the base of the section . 

;;-!. .J ure GJ: · ..... cct. on o sediments exposed under the northwest co rner of the Table Rock 

L f ring canplex . 

130 
· 10m -----;4 

Fi.sur:_e 9 : Diagram -or -:Jor"rrna i.on pattern dL the tuff-breccia (stippled) sediment 

(pl "'.t.ll) ~ . .- <Htl-cict , belo w tuff ring 112, table Rock tuff r ing com plex. Uppermost layers of 

sandstone ahd mudstone are intruded into the basalt tuff- breccia as dikes . Deformed 

bedding planes are preserved with in the dikes, although nearly pinched off where the dike 

enters the breccia . Bedd ing below the contact is also deformed by small scale faults . 

silicic domes and flows . They we r e an island , 6 . 4 km wide and 19 km long , throughout the 

late history of the lake basin . The sediments are higher than those of the basin and may 

have been deposited in a small depression between the large tuff ring (2) and the hilli 

to the north . 

The sediments are deformed beneath thick units of hyaloclastic tuff- breccia and penetrate the tu ffs as mudstone dikes ; the dikes vary from <2 em to 3 m thick . They have 

sharp , irregular boundaries and bulbous , ovoid and sheet- like shapes (Fig . 9) . The entire sedimentary section is deformed here. Away from the contact wi th the tuff breccia 

these units are flat , but here they dip beneath the tuff ring at angles of 30 to 40 

degrees. Similar features are visible in finely - lam·nated diatomite beds under the 

7 -mile Ridge Comple x ( see map , Fig. 2) . It isn't known i f the tilting is due to subsidence around the crater or to loading by the massive tu ff-brec cia . 

The plastically deformed sediments , many of which retain original bedding features 

within dikes , suggest that the sediments were wate r - saturated when buried by the overlying massive hyaloclastic breccias. 

The unstabl foundation upon whi c h he uff-ring compl ex rests may also account for 

s e large-scale deformation of h yaloclastic deposits on its eastern flank . Broad anticlin sand synclines within the tuff ring d posits may , in some i nstances , be c aused by 

slumping . This is especially tr1..1e f or vent 2 , a broad ring with deposits up to 120 m 

thick . 

The bluff immediate y north of Stop A · s part of vent 6 , a smal l tuff ring about 240 

m in d · ameter with deposits 25 - 30 m thic k . The ou ter slopes o f the ring have been 

eroded , leaving cliffs on all sides e xcept the SW edge , wh ere i ts deposits lap onto tuff 

ring 2 . Typ icall y, the eposi s consist of well-bedded , yellow- bro wn sid eromel ane 

lapilli-tuff in 1- 60 em thick beds. Ab out five pe r cent o f the deposit consists of 

angular basal bloc ks . 

Stop 8 , Table Rock Comp l x 

A small vent and tuff ring ( ent 8) e xposed in cross-sectio n along the c liff cuts 

h yaloclastic tuff and tuf -brecci a o f the tuff ring 2 . The vent is about 33 m in diamet r ; the tu ff ring remnant has a radius o f 120m . The ve nt has v e rtical walls and is 

13 1 
filled with near-v rtical , concentri c b s o f yellow-brown 

em to 2 m thick . Near the cent r of th v nt some of th 

Some beds can be traced from within th vent into Lh 

ring 2 . 

10) . 

o f tuff 

Ve rtical beds a r in rprete s follows : uring Lh w ning ph s s o f · cti v it y, 

vertical vent walls wer e pla ster ed with bed after b d of coh siv sh until th v nt w- s 

f illed. Unlike several o ther sm<:lll vents simil8r to this one , th r e is no c ntr 1 cor 

of massive tuff-breccia . 

Between Sto ps Band C there are w ve- c ut cliffs; t Stop C, th wa v - c ut f e ture 

breaches the north end of tuff ring 2 . The 120 m wide br ach through wh ich th road 

passes, con tains a gravel bar consisting of w 11-round d cobbles a nd p bbl es o f palagonite tuff and tuff-breccia eroded from tuff ring 2 . 

Stop C, Table Rock Complex 

Visible here is a steep-walled , U-shaped channel filled with w ll-bedd d tuff . The 

channel trends and plunges north a way from the approximate center of v ent 2 . Thi s is one 

of 23 U- shaped channels cut into the rim deposit~ o f vent 2 . They rang from 1 to 2 1 m 

deep and 2 to 30m wide . Several cha nn els are traceable for about 150m , but origin al 

lengths are not known . 

The steep sides and U-shape of the c hannels re similar to those in deposits of maar 

volcanoes near Rome , Ital y (Losacco and Parea , 1969 ; Mattson and Alvarez , 1973) and Ko ko 

Craters , Hawaii ( f isher , 1977) . Channels are interpreted to have bee n c ut by base 

surges . Infilling beds , which thin and curve up against channel sides with some 

e xtendi ng over the sides onto the rim beds of the tuff cone , we re deposited by base-surge 

flows and by air-fall. The absence of consolidated palagonit tuff cobbles within the 

cha nne ls indicates that the y were cut prior to palagonitization and induration of the 

te phr a . 

East of the channel , along the inside rim of vent 2 , the e ffects of palagon iti zation 

on hyaloclastic tuffs are well shown . Well-b edded sideromelane tuffs have b een hydrothe rmall y altered to massive , featureless orange-brown to dark brown , brittl e palagonite 

tuff . The contact between slightly palagonitized bedded rocks and hydrothermal l y 

palagonitized tuffs is sha rp; it is irregular and c rosses bedding planes ( fig . 11) . Only 

T 

I 

18m 

I 

J_ 

Figure 10 : Sketch o f the cliff face in which vent 8 , Table Rock tuff ring com le .~~ 

e x posed . Heavy dots outline the ground surfac e prior to the e rupt ion o f vent 8 . A remnant of a small tuff ring around the vent is visible . 

132 
Figure 11 : Bedding within the Table Rock maar comple x is often d e str o yed by palagon i -

ti zat ion of the we ll-bedded tuffs by late-stage hydrothermal activity. The dotted line 

t r aces the co ntact between partly palagonitized and completel y pa l agon itized tuff. 

the most distinctl y g rad ed beds, with coarse la pilli at the base are preserved in the 

hydrothermally altered zones. Beds abov e the contact are slightl y altered sideromelane 

tuffs ; individual sideromelane pyroclasts consist of a core o f brown glass, rimmed with 

or a nge or yellow-or ange pal agon i te . Sane pyroclasts, especially smaller ones , are completely altered , but their relict forms are pr eserved . There are also traces o f zeolite 

and calcite c ement between pyroclasts . 

Below the hydrothermal contact , sideromelane pyroclasts are completel y alte red to 

palagonite , and the rock is c rossed by dessicatio n(?) cracks that break it do wn into 10 

to 40 ~m particles . With the "homogenization" of the rock, grain size differences 

responsibl e for visibility of bedding and sedimentary structures are destroyed. The end 

product is a massive , brittle rock composed of cl ays, zeqlites , i ron oxide and calcite 

cut by NW-trending to W-tr e nding joints (vertical) . 

Massive , altered areas within craters suggests that the alteration is due to reac tion 

of sidercxnel ane pyroclasts ( basalti c glass) with steam seeping through ejecta . This 

process is being observed at Surtsey , Iceland (Jakobsson , 1978). 

Stop D, Table Rock Complex 

Continuing east and then sout h to the rim of vent 2 , then back down to the central 

c rater area, many physical characteristics of maar volcanoes may be s een , including 

planar graded and reversely graded l apilli-tuffs, cross-bedding ( with current directions 

uphill, out of crater), bedding plane sags ( block sags), and convolute b eds . 

StopE , Table Rock Comple x 

Park vehicles near the base of Table Rock and walk southeast about 500 m to the edge 

o f a depression about 45 m deep and 360 m in diameter and open to the lake basin on the 

east side . The area , designated as vent 4 , formed a crater in tuffs of vents 1 and 2 , 

with crater wa lls sloping inward at angles of 30 to 85 degrees. Parallel to and covering 

the crater walls are well- bedded , steeply dipping layers of lapilli- tuff , with an aggregate hie kness o f 60 m ( Fig . 12a) . Yellow- bro wn la pill i - tuff and tuff- brecc ia are present in beds ranging in hickness from 1 mm to 1 m. These beds di p steepl y wi thin the 

ven and may be traced out o f the v en onto the r emnants of a sm all maar deposit overlying tuffs o f vents 1 and 2 ( Fig . 12b) . Wet , stic ky hyaloclastite tephra were plastered 

onto crater wa lls during the waning phases o f the eru pt ive activity ; many hav e remai ned 

i n place , but some sections have slum ped and over tur ned . 

Lake sediments co nsisting of white diatcxnaceous mudstones occur within the crater o f 

ven 4 . These de posits lie nearl y 30 m abov e the la ke basin floor and were de posited in 

a c rater la ke apart fr the larger basin . 

Within the southern part of the c rater of vent 4 is a prominent oval col umn o f tuff 

( 18 m high , 90 m long) that may have been the co nduit for a small vent (5) ( Fi g . 13a). 

The col umn co nsists o f co ncentric , steepl y di pping t uff beds that c ut ac ross a nd therefore are younger than the c rater la ke sediments o f ven t 4 . Within 2 em o f the t uff-lake 

sedime nt contact , the flat-lying la ke sedime nts a re brittle , (a ffected the rm all y by the 

h y a 1 o c 1 a s tic t e ph r a o f the co n d u i t?) Be s o f s i d e rom e 1 an e t u f f a nd 1 a pill i-t u f f d i p 

133 
- -----,,--- --------------------·------. Ind ex Hap (/ I t-<--- '00'- - ~ 

u 

20 

J.-

N 

~ 

.. u' 

Figure 12a: Sketch map of ven areas 4 and 5 Table Roc tu ff ring complex . The map is 

represented on t he i ndex map (inset) as a black square . An outline of the Tabl Rock 

uff ring complex is shown in he ind x llldp . Th pres nt crater edge of ven 4 is sho\'tn 

wit h a double line . Plain areas are bedded tuffs sti ppled areas are crater la e 

sediments and alluvium diagonal lines represent l ake sedimen s of t he For RockChri stmas L ke Valley Basin . Vent 5 is located at .he south edge of the crater of en 4. 

Figure 12b: Cross-sections U-U' and V-V' . The alternating stipple-line pa tern r epr ese nts bedded uffs of tuff r ing 2, he cant i nuous l i ne pa tern represents bedded tuffs of 

vent 4, alternating line-dash pa erns are era er lake sediments, and doted patterns 

represent lake sedi men s underl ying the tuff ring complex . 

134 
.. 

T ~ e 

e N 

~45 m. 

1 .h~ 

S EDS . 

s 

Figure 13a : Sketch map of vent area 5 , Table Rock t uff ring complex . Scalloped lines 

represent the most pronounced unconformities , wit h the scallops on the upper beds . The 

oval , concentric arrangement of the bedding is shown by the arrangement of attitudes. 

~--------------------------~------------~ 

S'lf 

1n'ell- ~ 

tuffs 

Figure 13b : Field sketch of the southeastern end of vent area 5 . 

inward from the outer edges of the conduit of vent 5 at angles of 30 to 80 degrees (Fig . 

13) . The center of the conduit consists of massive tuff breccia containing blocks of 

lake sediment . As at vent 8 (Stop B) and vent 4, concentric beds with steep dips suggests that cohesive tephra was plastered or1to crater walls during ( waning phases?) of 

explosive activity . The vents may have been progressively clogged with tephra , somewhat 

analogous to clogging of pipes with grout . 

Stop F, Table Rock Comple x 

Proceed up the dirt and gravel road to the top of Table Rock . Table Rock is an erosional remnant of a tuff cone constructed above lake level on the southern rim of vent 2 . 

At present it is a symmetrical cone about 1530 m in diameter at the base , tapering to a 

diameter o about 360 m at a height of 360 m above the surrounding plain . The cone is 

c apped with flat-lying basalt which once filled the crater , but erosion has modified the 

original cone , exposing he once-ponded basalt lava lake (Fig . 14) . Dikes extend north 

and SSE of the crater lake, parallel to the long axis of the tuff ring complex. 

On the lower flanks of the cone , the rocks are mostly hyaloclastic tuffs ; yellowbrown or orange sideromelane and palogonite lapilli-tuff occurs in 1 mm to 2 m thick 

beds. ear the surrrnit , the uppermost hyaloclastites are overlain by 1.5 to 6 m of massive black or red cinders and bombs from fire-fountaining (Strombolian eruption) that 

preced d the filling of the crater with lava (Fig . 15) . 

The lava lake is vertically jointed high-alumina basalt (Table 1) . Blocks of the 

lava hav slumped toward the east , leaving 2-3 m high scarps along NNW-trending fra ctures. 

Why is the shape of Table Rock tuff cone different from the broad , low rnaars of most 

of the complex? A possible answer is the depth of explosive steam generation where 

135 
T 

6Jl 

1-

r-------------------------------

G?-AY , APHANITIC BAS.4LT 

TACHYLITE CINDERS AN D SPATTER ·----------- --- ----- SIDEROMELANE TIJFF ; EAK ED )J £AR 

CONTACT 

GRAY SIDER O~ ELA E T FF 

PARTLY PALAGONITIZED SIDER0-

1-!ELANE TUFF 

Figure 14: Section of tuff exposed below the lava lake on the west and south side of the 

Table Rock tuff cone . A zone of tuffs ( sideromelane) immediately belo\o.' IJhe former lava 

lake is not altered to palagonite . Tuffs elsewhere in the tuff co ne are partl y alterE:d 

to palagonite, implying that dehydration of the tuffs immediately below the lava lake 

decreases the instability and resistance to weathering. 

ascending magma interacted with water. The broad, low tuff rings may have resulted from 

shallow phreatanagmatic explosions where magma nearly reached lake level . Shallow steam 

e x plosions would likely produce broad e xplosion c rate rs, low fragm ent trajectories and 

base su r ges resulting in broad, low rim deposits of the tu ff rings . The base of the 

Table Rock tuff cone is , however , located on the r im o f vent 2 above former lake level . 

About 210 ·m below the cone base is a highl y permeabl e aquifer with in basalt flows whi ch 

lies beneath essentially impermeable diatomite beds. In contrast with shallow e xplosions, phr eatomagmatic explosions within the aquifer may have been guided by the co ndu it , 

r esulting in higher angle trajectories and a higher, steep-sided co ne . 

There is an excellent view of the en tire complex and the Fort Roc k-Christmas Lake 

Valley basin from the top of Table Rock: 

South- The edge of the basin, consisting of normally fau lted Picture Rock Basalt of 

Pliocene age (Hampton , 1964). Silver Lake is located within a graben that is the ke ystone of a broad arch. The Table Rock Complex is situated along one of these normal 

faults , but not cut by it . 

Also visible are wave- cut terraces and beach deposits that border the basin at an 

elevation of 1336 m. The lake basin is about 64 km long and 40 km wid e , with nl.ITierous 

island s such as Hayes Butte and the Connley Hills , NW of Table Rock . The la ke probab l y 

ex isted f r om mid(?) Pliocene to late Pleistocene time. During that period, eruptions of 

basal tic magma along normal faults that cut across the basin produced maars within and 

immediately adjacent to the lake and ci nder cones and flows beyond the lake margins . 

East - 13 km east of .Tab l e Rock and overlapping the basin margin is Seven-M ile Ridge, 

a NW- trending group of five overlapping maars. The com plex is 12 km long and 3 . 2 to 4 . 8 

km wide . The best preserved maars are at the basin edge above the former lake level; 

they drape over a 61 m high fault scarp . The two northernmost maars have been eroded by 

wave action to flat- topped mesas, 9 to 18 m high . 

Stop G, table Rock Complex 

This stop , at the southern end of the table Rock Complex, is best reached by dirt 

roads fran near the junction of State Highway 31 and the southern route to the village of 

Christmas Valley . Vent area 9, a small , tuff- filled conduit similar in size to vent 5 , 

crops out as a 6 to 9 m high knoll at the edge of a wave-cut terrace . It is about 30 m 

136 

.
Table 1. Chern ical analyses of basalts from the Fort Rock- Christmas Lake Valley Basin . X- ra y fluorescence anal yses by G. Heiken . 

Sample No. 2 3 4 

Si02 50 . 54 50 . 34 50 . 67 52 . 39 

Al 2 o

3 16 . 18 15 . 67 17 . 28 16 . 02 

FeO (total) 10 . 03 10 . 82 10.87 8 . 49 

Mg O 6.63 8 . 08 7 . 05 6.48 

CaO 9 . 90 10. 33 10 . 12 9 . 73 

Na 2o 2 . 70 3 . 09 2 . 42 3 . 33 

K2 0 0 . 59 0 . 39 0 . 39 0.86 

H20+ 0 .2 0 . 0 0 . 0 0.8 

H

2 0- 0.2 0.3 0.6 0.6 

Ti02 1. 37 1. 68 1. 37 1. 23 

MnO 0.19 0.19 0.19 0.16 

Total 98.53 100 . 9 100 . 95 100 . 09 

1. Basalt flow in the lava lake of tuff cone 3. Table Rock tuff ring complex . 

2 . Lava Lake in the crater of the north tuff ring, Table Mountain. 

3. Basalt fran a dike which crosses the largest tuff ring in the Seven-Mi l e Ridge tuff 

ring complex . 

4. Paho ho e f low f rom Lava Mountain . 

w 

ooo· 

••00' 

TUFFS OF 

tlll11tluntu 

RING~~ u --~~------ --~------ ~~~ on 

Figure 15: Cr oss-section , Table Ro c k. 

137 

E 

_ I:: 
P..assive 

/ / I 

/ / I J -

Figure 16a : Sketch of 'Jent 9 

arrangeme nt of tu ff bed s arou nd 

Table Rock tuff ring complex sho\Jing t h cone n ric 

he outer edges and massive tuff in lhe c n r . Stop G. 

Fi gure 16b: Ske tc h map of vent 9. Do ted lines r present 

are actuall y le ss t han a centimeter to several meters hick . 30 mete rs long . 

138 

he geoemetry of beds \vh ich 

The scale on he righ t is 
in diameter . Tuff beds , 1 em to 75 em thick , are concentric and are plastered onto vent 

walls (walls now unexposed). The core of the vent is massive , palagonitize tuff ( fig . 

16) . 

stop H, Table Rock Comple x 

About 4 50 m west o f stop G , on the so u the a s t e r n fl an k o f v en t 1 , on 1 y two r i d g e s 

remain as erosional remnants of vent 1. These ridges rise about 95 m above the lake 

basin floor . Originall y thjs maar may have been nearly 3 km in diameter . 

The ring is composed of palagonitized sideromelane tuff , lapilli-t uff and 

tuff- breccia . The rocks form brownish- gray deposits on the outer flanks and ridge tops 

and orange-bro wn deposited within the crater . On the flanks, the tuff beds are in uniform , 1 to 2m thick layers . Within the crater , ho wever , hydrothermal activity has completely palagonitized and "homogenized'' the tuff as is the case in vent 2 . 

Deposits in some parts of ring 1 are deformed by slumping and are characterized by 

con~Lolute bedding . The 61 m thick section of tuffs at Stop Hare folded into a steep, 

overturned anticline in sharp contact with underlying , undeformed beds ( f ig . 7) . The 

glide plane for this slump is with in the tuff sequence that dips outward (SE) from the 

vent at an angle o f 6 degrees . 

Figure 7a : Slumps ructure in bedded tuff on he east edge of Tuff Ring 

S op H. 

able Rock . 

Figure 17b: (Below) . Diagrams illus ra ti ng he his ory of even s leading to th 

ti on of he slump illustra ed in Fig . 1Gb . 

E::p:anat:.o n: 

So lid lines 

repre sen t tuf f. 

DasJ-,ed line s r present l ake 

sedimen t. 

b. Forma ion of anti l ine- like stru lure t ypica l of tuff 

rings by deposition of tuffs dur~ng cruption (s). 

c. Mas es of bcd~~d tuff slump 1nto the crat r. . d. Un s ~b le tuff re m ining on the oute r fl ~~ k o f t he r1ng 

slides to~, :1 rd the southeas t a l ong a orc :-1 etent det ch -

r.l~n t hori::on . 

139 

forma-
END OF FIELD TRIP 

Return to Bend , via Oregon Highway 31 . For more information on th e r gion, see the 

enclosed references . 

REFERENCES 

Fisher, R. V., 1977 , Erosion by volcani c base-surge density currents : U-shaped cha nnels . 

Geol. Soc . Amer . Bull . 88 , 1287-1297 . 

Hampton, E. R., 1964 , Geologic factors that control the occurr e nce and availability o f 

ground water in the Fort Roc k basin , Lake County, Or egon . U.S . G ol . Surv. Prof . 

Paper 383B , B1 - B29 . 

Heiken , G. H., 1971 , Tuff rings: e xample s from the Fort Rock - Christmas Lake Vall e y 

basin, south- central Oregon . Jour . Geophys . Res ., 76 , 5615 -562 6 . 

Jakobsson, S. P., 1978 , Envir onmental factors con trolling the palagonitiza tion of the 

Surtsey tephra, Iceland . Bull. Geol. Soc. Denmark, 27 , 91-105 . 

Lorenz , V., 1970 , Some aspects of the eruption mechanism of the Big Ho le maa r , central 

Oregon. Geol. Soc. Am er . Bull. 81B, 1823- 1830 . 

Lorenz, V., 1971 , An inv estigation of volcani c depressions . Part I V. Origin o f 

Hole- in-the-ground , a maar in central Oregon . NASA Progr ess Report , NGR-38-003-012 , 

113 p . 

Lusacco, U. and Parea, G. C., 1969 , Saggio di un atlante di strutture sedimentarie e 

post-sedimen tar ie oss e rv ate nelle piroclastiti del Lazio . Atti Soc. Nat. e Mat ern . di 

Modena, 94, 30 p. 

MacLeod , N. S., Wa lke r, G. W., and Mc Kee , E. H. , 1976 , Geothermal significance o f 

eastward i ncrease in age of upper Cenozoic rhyolite dcxnes in southeastern Oregon . in 

Proc. , Second United Nations S ymp osium on the Development and Use of Geothermal 

Resour ces, 465-474 . 

Mattson, P. A. and Al varez, W., 1973 , Base surg e deposits in Pleistoc ene vol c anic ash 

near Rome. Bull. Vol c anol., 37 , 553-572 . 

Peterson , N. V. and Groh , E. A., 1961 , Ho le-in-the-ground , central Oreg on . Meteorite 

Crater or volcanic expl osion? The Ore Bin, 25, 73 - 88 . 

Peterson, N. V. and Groh, E. A., 1963, Maars of south- central Or egon . The Ore Bin , 25 , 

73 -89. 

140 
... 

ROADLOG FOR FI ·LD TRIP TO MEDICI E LAKE HIGHLAND 

Ju lie M. Donne lly-No la n, U.S . Geological Survey, MS 18, 345 Mi dd lefield Rd., 

Menlo Park, CA 94025 

Eugene V. c·ancanell., Cascadia Expl orati on Corporati on, 3358 Apostol Rd. 

Escondido, CA 92025 

John C. Ei chelberger, Geol ogi ca l Res earch G-6, Los Alamos Sc ientific Laboratory, 

Los Al amos, NM 87545 

Jon H. Fjnk, GeoJogy Department, Stanford Un i versi ty, Stanford, CA 94305 

Grant He i ken, Geological Research G-6, Los Alamos Scientific Laborator~ Los Alamos, 

NM 87545* 

MILES (i ntervaJ mi leage in parentheses) 

0 Leave Cimarron Mote l , 3060 So . 6th St . , Klamath Falls, and head east 

3. 1 At "Y" i n road, keep r ight to avoid going toward Winnemucca . Conti nue south on 

Oregon Hi ghway 39 . (14 . 0 ) 

17.1 

(2 .0 ) 

19. 1 

( 1. 9 ) 

21.0 

(12 .8 ) 

33 . 8 

(3 .3 ) 

37.1 

Hi ghway turns east at Merrill. Pass through town and continue east to 

Malone Road . Turn r i ght . Proceed south to State Line Rd . 

Jog left to State Li ne Liquors, then r i ght, and continue south toward Fi sh and 

Wildlife Headquarters. Continue south a l ong fau lt scarp toward 

Lava Beds National Monument . Turn l eft at "T." 

STOP 1. Captain Jack's Stronghold. 

Geologically , this stop illustrates fl ow-end features in a high-alumina basalt 

flow. The stronghold is one of the major features at Lava Beds National 

Monument, and Aaron Waters has contributed hi s gee-historical study of the 

Stronghol d to the pub lished gui debook. One of the Park Rangers, Patti Easterla 

(whose grandfather was born in the Stronghold during the Modoc War ) has agreed 

to gu i de us through th is natural fortress and expla i n how so few Indi ans were 

ab e to hold off so many U.S . soldi ers for so long . 

The unusua topography here , which was so critical to the Modoc 's defense, is 

explained by Wa ers (this volume ). 

Drive west from Captain Jack 's Stronghold, fac ing di rect y toward the 

north-south faul scarp known as Gillem's Bluff. Pre-Highland lavas are 

exposed jn he scarp , wh ich is one of several steppi ng up to the west. As the 

road turns south , note he Big Horn Sheep Enc losure where some 35- 40 sheep 

currently 1 · ve. The Park Service is attempting to reintroduce the sheep wh ich 

were once nat i ve but were killed off around 900. 

* Au thors are listed alphabet·cally except f i rst au hor. See Acknowl edgements for 

further crE-dits. 

141 
Dorris 

0 

Miles 

N 

® 

5 

0 5 10 

Kil ometers 

Klamath Falls 

.@ 

@). 

142 

Locat i on map for 

f ield tri p to 

Med ic i ne Lake Highland 

Q stop numbers 

OREGON 

CALIFORNIA 
(12 . 9 ) 

50 . 0 

(1 . 7 ) 

51.7 

(0. 2) 

51.9 

(0. 3) 

52 . 2 

(2 . 8 ) 

55 . 0 

(4 . 6 . 

57 . 6 

( 6 . 3) 

63.9 

( 0 . 1 

As you leave the f aul t scarp and hea d southeast, cross the De vil 's Homest ead 

f l ow wh i ch is very young i n a ppearanc e . It a ppar entl y vented a t Fl eene 

Chi mn eys , spatt e r con es si tt·ng astri de the southwa rd conti nuation of the 

Gillem's Bl uff fau t s carp . 

Vi si tor Center 

Turn le f t on t o Va ent i ne Cave Road 

STOP 2 . Va lent i ne Cave . 

This is one of near l y 300 l ava tube caves i n Lava Beds Nati onal Monument. Many 

of t he more i mp orta nt a nd a ccessi bl e caves ha ve been mapped i n de ta il by Aaron 

Wa t ers and hi s f i el d assi stants, at the request of the Nati ona l Park Service . 

Waters has a l so wri tten des c r i pti ons to accompany the maps, one of wh i ch 

descri bes Valent i ne Cave (Waters, 1976 ) . 

"Val enti ne is an i nteresti ng, c ean and vari ed cave . Here one can see a 

Java - tube system which is almost undamaged by post-lava collapse . The 

cave shows most of the features to be found i n l ava tubes: among them 

pahoehoe floors, l ava pools, and l ava cascades; well-deve l oped ava~ es 

and dri pstone on ce ilings and wa l ls; extraordinarily i nteresti ng l ava 

benches, one of wh i ch marks a hi gh - l eve l stand of viscous lava that 

attempted to crust outward from the wa lls; and still another ki nd of 

bench made by the penetrati on and "bul l doz i ng" away of collapse blocks 

and rubb l e as mo l ten l ava under hydrau lic pressure forced i ts way through 

the tube . Large pillars ar·ound wh i ch the lava stream div i ded and 

reun i ted are present i n the upper part of the cave . The centra l part of 

the cave shows "nteresti ng l ava fa lls and cascades through wh i ch the l ava 

stream was transferred from a hi gher l evel to a l ower l eve l . Downstream 

from thi s area of breakdowns, the Va l ent i ne tube subdivi des i nto 

distri butaries wh i ch are g radua lly phased ou t downstream by f illing wi th 

lava ." 

Return to ma i n road and turn r i ght . 

Dr i ve past Vi si tor Center aga i n and turn l eft onto di rt road to Med i cine Lake . 

Mammoth Crater (opti ona l stop ) . Pi t crater at s ummit of broad, l ow, basal t i c 

s hi e l d . Sour ce of many of the l avas i n the Monument, probab l y i nclud i ng the 

f l ow that mad e the Strongho l d (Waters, thi s vo l ume . ) 

Conti nue south, fi rst on paved and t hen on di rt road . 

Turn sharp r i ght and dri ve one- tenth of a mi l e along west si de of spatter 

cones. 

64 . 0 Park and wa l k north al ong spatter cones, then up onto flow for 

(0 . 1) 

64 . 1 

( . 8) 

65 . 9 

There are numerous ree molds in the area i mmedi ate l y north of t he l ast of 

the severa spa ter cones that a r e aligned a pproxi mately north- south . The 

lava was apparen ly ve ry f lui d when i t fi rst emerged, flowing around the 

rees i n i s pa h . Many of he molds are l arge enough to c l imb down i nto . 

Mos t ar e 10-1 5 fee t de e p· a few had snow at t he bottom all summer . About 200 

feet north of he last s patt er cone , the fl ow has a r ougher surface and there 

a ppear to be no mor e mol ds . 

Return o mai n r oad and turn r i ght; conti nue south . 

Tu rn right on good di rt road . 

143 
( l. 0) 

66 .9 

(0 . 8) 

67.7 

(0.2) 

67 . 9 

(0 . 3) 

68 . 2 

(2 . 1) 

70 . 3 

(2 . 4) 

72 . 7 

( 1. 4) 

74 . 1 

(0 . 3) 

74 . 4 

(0 . 3) 

74 . 7 

(1 . 5) 

76 . 2 

(7 . 2) 

83 . 4 

STOP 4. Cracks. 

Park and hike northwest about 200 feet to narrow d]rt track. Turn 1 ft and 

wa k about half a mile jnto an area of recent cracks . The larg stone ·s 30 

to 40 feet deep and ha snow at the bottom. The exposed s quenc is pumice 

over scori a. bo h ove lying platy andesite that is we ll-displayed in the 

ragged edges of the cracks. Mos cracks trend north to northeast her , with 

the west side down where offset can be determj ned. Th is is the same. offset 

d"splayed at the next part of Stop 4. 

Fau ed c i.nde cone . ------

Return to d"rt road track and wa k northwest to pumice-covered cinder cone . 

Climb up on cone and stand on east side of fault scarp. The scarp is red and 

bare of pumice , thus fault movement fo llowed deposjtion of the pumice . The 

fau lt trends north-northeast and at each end is covered by a very young 

rhyo i t ic f ow . 

There is an exce llent view to the northwest from this spot . Eat lunch and 

then proceed north along the fault to Crater Glass Fl~~ · Climb up on the 

flow and examine the sp·nes on the surface . Note the scarcity of pumice on 

top of the flow desp ite the abuncance of pumice surrounding it. 

Return to cars and proceed northeast to junction with main road . 

Park for STOP 5. Glacial Striations. 

Andesite is polished and grooved in a northwesterly direction. See Anderson 

(1941) for a description of the glaciation of the H"ghland . 

Return to cars and proceed northeast to junction with main road. 

Turn right and drive south over rim of caldera as described by And erson 

(1941) . 

Vi ew of Med·c i ne Lake basin and the young Med ici ne Lake flow (dacite) . 

Fo low the road down and south past the dacite flow . 

At the Ranger Station, turn r ight . Proceed along ma i n road, keeping to the 

r i ght at next intersection wi th a good road . 

Turnoff to Little Mt . Hoffman . Keep right . 

At Ljttle Glass Mounta i n, turn left and follow east edge of flow . 

STOP 6. Park for f i rst Little Glass Mounta in stop . See accompan i ng a rticle ~H. Fi nk for description . 

This stop offers an oppor ni y o erv he di ffer nt units wh ich mak e up 

the f low strati graphy. First we note the layer of tephra wh ich covers the 

ground surrounding the f low. Isopach maps indicate that the source of this 

tephra is under Li tt l e Glass Mountain (Heiken, 1978 . 

Return to junction and turn left, driving around north side of Little Glass 

Mountain . 

STOP 7. Park for second Little Gl ass Mountain stop. See accompan i ng article 

by J . H. Fink for descr·ption . 

Turn around and return to Medicine Lake . 

Medici ne Campground . 

144 
AY 

6 . 

88 . 9 

( . 2 ) 

93 . 

(9 . 7 

02 . 8 

( 0.6 ) 

03 . 4 

(0.8 ) 

04 . 2 

(2. ) 

106 . 6 

(0 . 4 

07 . 0 

( 0 . 2 ) 

07 . 2 

r::p_ " .. . ' ,... e 

a .; ·:: c :o. . "" . .. : ef" . 

t.e 

, oa nc r _ef _ass Mo . _a::., 

e an 

mos as a and andes.:. e. 

Roac u c 0 T r ef 

Sharp ef r on 0 d'~ roac (si gn po·n s o La va Beds . M. ' 0 mi. 

Road u c : 0 Keep .J. ef 

rn ef on 0 pum.ice - covered road . 

Road ·u ct'on . Keep r·gh 

Tur sharp ef 0 0 arrow roa runn ng para e 0 f ow fron 

STOP 8. 

Park and wa k up bu ozed road on o f ow . This ocation was chosen 

spec i f.:. cal y o show he mix·ng of magmas i nvo_ved ·n he genes·s of he 

G ass Moun a·n dac· e- rhyo · e f4ow (An erson, 933 · Eichelberger, 974, 

975 ) . 

The north quarry s·te affords an opportu i y o see he ex remes n ava 

ypes produced duri ng a s· g_e erup ·on of e G ass Mounta i n vo cano . 

·r ee . vents a o g a northwes - southeas trend· g issure were active 

during h's eruption wh ich occu rred within he _ast hou and years . Nine 

he 

nor hwes s and the sou heastern- mos vent produced domes, wh ile lavas of he ~ e ven i g h ee ve s coa esced o form he G ass Moun ai f ow, 

km3 me . T e f ow s af th ee daci ic eas ern lobes wh ich ra de d over a· n by rhyo · e obes. 

We are now a h where the of the rhyo ite obes stopped as it 

ov he a dac · e obe . The dacite o e f owed a 

6 km ven · our present loca ion · s 3 1/ 2 km 

ven hern f l ow ven probably a so fed his lobe and the 

f ow ven may have con ributed ava as we 1 . The rhyo i e lobe came 

from he nor hern fl ow ven 3 km is ant. Howeve , here was no 

he erupti on between the wo he l obes merge i n a 

con ·nuous surface west of h vents. or is any disconti nu i ty in 

i ho ogy si nce the ac i e obe g ades ups ream o rhyoli e at the middle 

and sou h v n s . Var·a io i n i ho l ogy wi th i n o es occurs over a horter 

145 
( l. 4) 

108.6 

( 3. 9) 

112 . 5 

(9 . 8 ) 

122 . 3 

( 0. 9 ) 

123 . 2 

dis ance norma o s ream f ow, efl ec i ng the s rong velo i y gradien i n 

hat direct·on dur · ng emp acem nt. H nee "young r," nd h for mor 

rhyol · ic , ava occurs n the ·nner porti on of eac h lobe . 

Contrast be ween he ol er a nd youn app r nt i n flow morpho logy 

as we as itho _ogy . The rhyoli e i m thicker than th dac i e 

obe on the same s ope . The dac·te var·e f om dense , massi ve 1 va wi th some 

f ow banding to ves · cu a r, scori ceous, n brecc i ated rna r ia , 

par icu ar y abundan on he f low surface . The dacite a l ways contains 

abundant basa tic xeno ·ths nd a larg porion of th phenocry s within 

he dac·te were de~ived from di aggrega ·on of h se xenoli hs . The rhyo lite 

vari es from dens e obs·d·an to very ight pumice wh i ch is confin d to th 

surface. Obsid·an has been f orced upward hrough the surf ce pumice i n 

pressure ridges. I nteriors of bo h he daci e and rhyolit lobes ar massi ve 

lava . 

Samp les of the basa l t ic xenol.ths ha ve a densi y of 2 . 15±0 . 04 g/cm3 , 

·ndependent of the vesicu arity or comp osi i on of the host lava . The mass 

densi ty of the xenoliths (i . e . density wi hout ve s·cles) is 2 . 79±0 . 01 

g/cm3 . Densi t i es of non- vesicular host avas ar e as f ollows : obsidian 

near midd l e vent--2. 38±0 . 0 g/cm3 dacite at this stop--2 . 49 g/cm3, 

dac i te at distal end of this l obe-- 2 . 55 g/cm3. 

Development of his compositiona ly zoned f ow can be viewed as a consequence 

of "bubbli ng" of basa lt· c magma hrough a rhyol · ic magma cham er . Bu l k 

composition and phase assemblage of he dacite are attri butable to addi i on 

of basal tic materi al o rhy ol . tic magma . The rounded shape of the xenol i ths 

and disti nctive "q ue nch " textu e indi cate that basa t was i ntroduced into the 

chamber as a liqu id , was chi led aga i ns rhyol.tic magma, and consequently 

underwent rap j d crysta . · zation. Vesicula ion accompanying crys allizati on 

produced a layer of l ow- density foam a he ase of the rhyolitic magma 

body . Th is foam was removed by f o a j on as i formed and became 

concentrated at the roof of the chamber. The vo l ume i ncrease due to 

j nf ati on of he foam triggered he erup ·on wh ich 

first tapped the foam- rich hybrj d dacite at the top of the cham er. 

Eruptj ons of similar avas prior to he G ass Mountain even suggest hat the 

chamber i s a l arge- volume, long- i ved feature beneath he High and . Known 

densi t i es of the materials ev·dence that flo ati on occurred , and reasonab l e 

esti mates of water content of basaltic magma cons ra·n the depth of his 

chamber to wjthi n about 10 km of he surface . 

The road onto the fl ow provides easy access to the surface of the rhyoli e 

l obe . The c limb takes 15- 20 minu tes and provides views of he summ·t dome 

which was extruded from the north f l ow vent , the d ·stal end of he dac i te 

lobe, and many other i nteresti ng features of Glass Mountain and its environs . 

Drive east on narrow road back to ma i n road . Turn right . 

Junction wj th paved road . Turn sharp right and return toward Medicine Lake . 

Turno ff to Glass Mounta i n. Keep le ft . 

STOP 9 (opti ona l) . 

At second turnoff to Glass Mounta in , park at i ntersection and hike north i n 

to explosion crater. This is one i n a northeast- trend·ng series of expl osion 

craters wh i ch extend for about three mi l es. These craters expose the local 

strati graphy wh i ch here is andesite over porphyriti c Lake Basalt . 

Cont i nue on ma in grave ll ed road kee ping f i rst r igh t, then le ft, at 

i ntersecti ons wi th other good roads, to four-way i nt ersection . 

146 
(3 . 4) 

126 . 6 

( 6 . 1) 

132 . 7 

(0 . 7) 

133 . 4 

(0 . 6) 

134 . 0 

( l. 4) 

35 . 4 

( 2 . l) 

137 . 5 

( 1. ) 

138.6 

(0. 8) 

139 . 4 

(0 .9 ) 

140 . 3 

( 2 . 1) 

142 . 4 

(0 . 8 

153 . 2 

(0 . 9) 

54 . 1 

Turn right toward Med ici ne Lake. Continue past Med icine Lake campgrounds and 

Ranger Stat1on w st toward Little Gl ass Mounta in . 

Little Mt . Hoffman turnoff. Turn left. 

STOP 10. L"tt1e Mt . Hoffman 

Lunch and panoramic view (weather permitting) of Mt . Shasta, Lassen Peak, 

and Mt. McLaugh lin , with Little Glass Mountain directly below us . 

Bottom of Little Mt . Hoffman . Turn left at junction . 

At Littl e Glass Mountain, keep r ight . 

Turn l eft at ma i n (Harris Spring) road . 

Road to Lost Spring and Tennant . Turn right . 

At junction wi th road to Tennant, continue stra j ght ahead toward Lost Spring . 

STOP 11 . Andesite Tuff 

Th·s is nearly the southernmost mapped exposure of this ash- flow tuff. Here 

i t is nonwelded to partjally we lded and you can see both the unoxidized 

buff- colored matrix having dark gray pumices, and the reddish to 

reddish-brown matri x col or wh ich is more typical of this wides pread and 

distinctive unit . It extends at least 200 feet uphill to the west and is 

nonwe l ded to partially we lded throughout . According to mapping by Hughes 

(1 974), the tuff is exposed a mile and a half to the north at at elevation of 

6400 feet, its highest known occurrence . It is also mapped by Weaver (1975) 

four miles to the southwest . Hart (1975) mapped large areas of it in the 

northeastern quarter of the Bray 15 ' quadrangle, just west of the Medicine 

Lake quadrangle . Hughes, Weaver, and Hart were students of S. A. Mertzman at 

Frankli n and Marsha ll College, and some of their work is summarized in 

Mertzman (1977) . 

The thickest apparent exposure of the tuff is right here, but its 

poorly-wel ded character suggests that the tuff may simply be coating the 

surface of the h1llside to the west . To the north, the tuff is exposed on 

he south side of Mt . Dome, a mos 20 mil es from here . To the northeast, we 

have mapped it east of the Ca llahan flow at about 5800 feet in elevation . In 

the areas to the north and east, i is a very th·n unit, always less than 20 

feet thick and typi cally 2- 5 feet thick . The source of the tuff is unknown 

but may be burjed under younger avas to he nor hwest of Medicine Lake . If 

the tuff's average hickness is taken to be 20 feet and i ts areal extent to 

be 250 square miles, jts total erupted volume i s one cubic mile . Although 

its volume js m)nor i is very important as a stratigraphic marker . 

or·ve a ten h of a mile fur her down the road turn around, 

and return to Harris Spring road . Turn l eft. Proceed north on good road 

pas ca tle guard . 

Turn right on nex good dirt road. 

Follow oad east then south just pas junction . Park . 

147 
(0 . 9) 

155 . 0 

STOP 12 . Andesi te tuff and 

Porphyr j ic oJjv·ne ba aJt overl· s ndesj te a nd und rlie and si te 

tuff. Wa k up on top of tuff to g t vi w sou th toward th Highland . 

Dj rectJy to he sou h near Dock We ll, 5 mils from h re the tuff 

overlie these same two un i ts as we J two other andesi s nd two silicic 

domes. Anderson (1941) thought th t the domes had i ntruded i nto 

two- hundred -foot thjckn ss of and s · e tu ff, but deta iled mapping show that 

the tuff flowed around the domes , la pping up onto the ea tern one. Th tuff 

is actual y very thi n, and it appears unstra i f"ed ecause i ha flow d down 

a steep slope and because mo t blocks are vi ewed from the top . Samples 

col ected near Dock We ll yi eld a norma magnet i c polarity . 

Further to the east, but west of the Ca ll ahan flow, the tuff is expo ed in 

patches as high as 5600 feet . It overl·es an olivine andesite, two ba alts, 

and an ol der rhyolite flow . East of the Callahan flow i t overli es a glassy 

porphyritic dac i te and an andes·te, as we 1 as an older sequenc of a phyric 

rhyol i te and dacite on top of porphyrit· c pheruliti c rhyolite . The pre-tuff 

sequence i n each case ·s complex, wi th al rock types from basa t through 

rhyoljte represented. The post-tuff sequence consi sts most y of silicic 

andesites pil ed up around Medic i ne Lake and young basalts on the f lanks of 

the volcano. The youngest eruptjons are bi moda , wi th basalts lower and 

rhyoli tes hi gher on the vo l cano. 

Turn around and return to paved road . Turn ri ght . Abou 4 l/2 mi les further 

north, there ·s a good paved road to he r ight . Keep left and proceed 

northwest i nto Red Rock Va l ley, then we st and northwe st to Ma cdoel and 

Highway 97. Turn r ight and go north through Dorris toward Klamath Fa lls. 

For those returning to the Klamath Falls a irport, turn r i ght at Joe Wri ght 

Road (sign points to Ki ngs ey Fi eld wh ich is the Kl amath Falls airport) . 

Acknowledgements 

Donnelly-Nol an was pri nc i pall y responsible for the road l og, but Ciancane li , Fi nk, and 

Heiken provided va l uable assi stance in choosing fie l d trip stops. Eichelberger wrote 

the description of Glass Mounta i n (Stop 8), c·ancane lli measured the temperature at the 

Hot Spot. L. Hose provided valuab le assi stance wi th f"eld tri p log ·stics and a l so 

discovered and mapped the exposures of andesite tuff east of th Callahan fl ow. 

We wish to thank P. Easterla of the staff at Lava Beds Nati onal Monument f or gui ding us 

through Captain Jack's Stronghold and A. C. Waters for contri buting the unpu lished 

description of Va l enti ne Cave . We also thank the U. S. Forest Service and the U. S. Park 

Service for their cooperation and assistance. 

References 

Anderson, C. A., 1933, Vo can·c h istory of Gla s Moun ain, nor hern CaJ" f ornia : 

American Journal of Science, vol . 26 , p. 485 - 506 . 

1941, Volcanoes of the Med ici ne Lake High and, Ca liforn i a: Universi y of 

California Pub lications, Bulletin of the Department of Geo l ogica Sciences , vol . 25, 

no . 7, p. 347- 422 . 

Eichelberger, J. C., 1974, Magma contamination within the vo can ic pile : origin of 

andesite and dacite : Geol ogy, vo l . 2, p. 29 - 33 . 

______ 1975, Ori gin of andesite and dacite : evi dence of mi xing at GJass Mounta·n i n 

Ca liforn i a and at other circum- Pacifi c volcanoes: Geol ogi cal Soc i ety of America 

Bul leti n, vol. 86, p. 1381 - 1391. 

Hart, W. K. , 1975, Geology of the northern one - half of the Bray Quadra ng le : map to 

148 

-
accompany unpub lished B.A. thesis, Frank lj n and Marsha l l College, scale 1:62,500. 

He i ken, G. , 1978, PJi ni an- type eruptions i n the Medic i ne Lake Highl and, California, and 

the nature of the under .y·ng magma: Journal of Vo lcano logy and Geothermal Research, 

vol . 4, p. 37 5-40 2 . 

Hughes, J., 1974, GeoJogy of the western Medici ne Lake High lands and Garner Mt. Ar ea , 

map to accompany unpub lished B.A. thes·s, Franklin and Marsha ll College, scale 

1:62,500. 

Mertzman, S. A., Jr. , 1977, The petrology and geochemistry of the Med icine Lake 

Vo Jcano, California : Contributions to Mineralogy and Petrol ogy, v. 62, p. 221 - 247 . 

Waters, A. C., i n press, Captain Jack ' s Stronghold : this volume . 

____ 1976, Valentine Cave : Nati onal Park Service unpublished document. 

Weaver, S. , 1975, Geology of the southeast Bray quadrang le, California: map to 

accompany unpubljshed B. A. thesis, Frank l"n and Marshall College, scale 1:62,500 . 

149 
150 
CAPTAI JACK'S STRONGHOLD 

(The Geolog i c Events that Cre ated a a t ura l Fo r tress) 

Aar on C. Waters 

308 Moore Street, Santa Cruz , Ca liforn i a 95060 

Map by David Kimbrough and Aaron C. Wate rs 

ntroduc t ion 

" . . ... reached by a si de road, the v isitor comes 

w capta i n Jack's s tronghold, an area of natural rock 

trenches and rock she l ters i n the l ava flows, where a 

tiny band of Ind i ans he l d hundreds of troops at bay 

for a mo st 5 months .. . .. The record tells the ta le 

of a motley army of bad l y tra i ned sold i ers, l ed by 

inept of fi cers on a battlefiel d of the Ind i an's 

choosi ng about wh i ch he army i nte l ligence had 

absolu te y no i nfo rmation . Gi ven such cond i t ions, 

tragedy wa s i nev i table for set tl e rs on t he nor th 

shore of Tu l eLak e , and for Jack's and Hooker J i m' s 

bands of Modocs . " (Keith A. Murray, i n preface to 

Erw in R. Thompson, 1 97 1 , !iodoc War: Argus Books, 

sacramento, Ca l if., 188 p . plus appendices) . 

Last o f the Ca liforn i a Ind i an upr isings, the 

Modoc wa r (November 29, 1872 t o June 4, 1873 ) has 

been chronicl ed by many news pape r wr i ters, 

istori ans, soc i a l scient ists , and o the rs . The 

wri t ten record, however, is blur red and contrad i cto ry 

as to the causes , mot i ve s, heroism, and savagery of 

princi pa l pa t i c i pants on bo th si des. Parts of the 

record have been g r eatl y fic t i ona li zed and 

romant i c i zed, and s_ome even fa l s i f i ed as Jeff C. 

Riddle sco rnf u ly notes : 

"I have read Cap . William T. Dr annan' s 

~k , 'Thir ty Yea rs on the Pl a i ns,' wh e re he wrote 

aoou t t he Modoc warr i o rs . Accord i ng to what he s ays, 

he captu r ed and k i lled more Modoc warriors than Capt . 

ack rea lly had when he corranenced fighting. Jack had 

only f i fty-two warriors in a l l. I knew every one of 

them. ("Jeff C. Ri ddle, 19 1 4 (repr i nted 1974 ) , The 

Indian History of the Modoc War, Urion Press, Eugene, 

Oregon, 295 p . ) 

Our r eport and accompanying map makes no attempt 

"to set the record straight" wi th regard to what has 

been reported about the histor i c and soc i olog i cal 

roots of the Modoc War. Inste ad we investigate the 

ques ion repea dly asked by almos eve r y wr i er : 

H~ was i possibl ha 53 Modoc men , aided (or 

encumbered ?) by wic as many women and chil d ren , 

wi hs ood a sieg through h dead of win er , rou d 

3!JO u.s. roops e ngaged i n he f irst major ass aul , 

wi hdrew und ec ed after re pulsing second assa ul t 

of 650 t oops suppor ed by mor t ars and howi zers , and 

hen only a few days later s aged a succ essf ul ambush 

and i nflic ed 25 fa ali ies upon a patrol composed o f 

59 en li s ed m n a nd 6 of f i c r s? 

One par t o f he a ns wer o th is qu es ion is ha 

~ Indi ns chos a sup rb, bu by no me ans un i que , 

natur I fo r r ess i n wh ich to mak e he i r s tand. Th is 

for ress , how ve , is on l y one part of he t o tal 

equa ion . The v i al po i n is ha th Modocs knew 

horough l y nd i n de ail h na ur o f he t erra i n 

15 1 

south of the shore l ine of Tule Lake; the army was 

tota lly i gno rant of this l andscape's military 

advantages . Wr i ters about the Modoc War clearly have 

not understood the real nature of the terrain in 

which the Indians holed up any more than did the U.S. 

troops and their officers. One historian writes 

vaguely about the Modocs "d isappear i ng i nto the 

Schonchin flow" as if this barren patch of recent aa 

l ava had some myth i ca l power t o swa l low up the 

Ind i ans and hi de them f r om pursuing Army patrols. In 

fact th i s a l most tree l ess expanse of sma l l and l oose 

blocks o f l ava would be the worst possible place for 

the Ind i ans to try to hi de; and the Modocs avoided 

the Schonchin f l ow compl etely. Some wr i ters have 

assumed that the Stronghold is "within the Schonchin 

f l ow," but the distal end of the Schonchin flow is 

3.8 k ilometers ( 2 .4 mi l es) south of the Stronghold, 

and the source of the flow is at the east base of 

Schonchin Butte, another 6.4 kilometers (4 miles) 

further south. The Schonch i n f l ow played no part in 

the Modoc war, except i t seems possible that a few 

members of Capta i n Jack's band may have staged the 

Thomas-Wright ambush out of fear of being trapped by 

this army patrol aga i nst the inhospitable west edge 

o f the Schonchin fl ow. 

We wi ll f i rst describe the terrain in and near 

the Stronghold, as seen through the eyes of a 

geol og ist . Then we return to the Modoc War and 

discuss, i n terms of terrain, the consequences of the 

first and second assau l ts by the Army upon the 

Stronghold, followed by an analysis of how the Modocs 

were able to withdraw from the Stronghold undetected , 

and a few days l ater stage the disastrous 

Thomas-Wright ambush. 

The Terrain 

Today the country in and adjacent to Captain 

Jack ' s Stronghold consists of four kinds of 

topographic surfaces (see map ) . 

l . The Tu le lake pl ain, which in 1872-1873 lay 

benea t h the waters of Tule Lake. Since 1906 over 3/ 4 

o f the a r e a o f th is l arge but sha llow lake has been 

rec l a imed f or farmland . 

2. Low lands, of l ow relief, underlain by lobes 

a nd tongues of sol i dified lava, border the shoreline 

both to the east and west of the Stronghold. The 

l owl ands are rough surfaced in places, but the 

variat i on i n height is 2 meters or less. The lava 

tongues g r ade at the shoreline i nto pillow lavas 

i nterspersed wi th sandy beaches of hyaloclastic 

debris (black glass sand and pillow-like ellipsoids 

o f ba salti c lava, wh i ch formed wh ere the molten lava 

tongues were quenched and granulated by entry into 

the waters of former Tule Lake). 
DAVID L. KIMBROUGH AN D AARON C. WATE RS , 1976 

SURFACE FEATU RES 

~~~ 

D 

D 

PLATEAU REMNANTS 

(Flat- topped remo ons of o once contonuous 10\ o- llow surface. whoch 

on other ports of the mop has been IONered. broken and tilted onto baSins 

and schollendomes. l 

AREA OF LARGE SCHOLLENDOMES 

(Tilted and broken solodofled crust of a former lava llow Molten lava . 

droonong out from beneath ots already solldofoed crust, caused collapse of 

the crust onto a very rough landscape of schollerdomes. small bosons. 

collapse po t s, deep c rocks, and plies of talus) 

LOWLA NDS (South and East of former shoreline ) 

(A lower lava - flow surface between the schollendomed 

area and the former shorelo ne of Tu le lake Choroc terozed 

~ h:'={ sha llow bosons, low schollendomes. few crocks. 

FORMER LAKE BED ( North and West of forme<" shoreline) 

( Mostly lake - beds slits. now under cult ovotoon except near 

former shoreline lake bed recloomed tor farmland 1906-1918.) 

GEOLOGIC FEATtJ RES 

$1 \C ENTRAL CRACK 

>€TERS 

~ SCHOLLE NDOME 

8 ' 0 

8 ' 

COLLAPSE BASIN 

COLLAPSE PIT 

bet wee 

voews and nile 

cealed lndoon snoper Many 

outposts were rebuolt bv t 

Army after occupotoon. 

others hove tumbled 

nto ruons 

U S ARMY FORTIFICATIONS 

G l oose blocks of roc 

D 

! 

> 

were pol ed onto I thiC 

walls of voroous shapes Most 

ore 0 5 to 1 m eter hogh The 

abundant Arrrry fortofocot oons shOWn 

w thon the central area of thos mup were 

never used on bottle, they were buo lt olte 

the Modocs hod abandoned the Strongold To guard ogoonst a OOSSible 

return attock the Army con 

structed rock walls. and man 

small emplacements that orovoded shelter for 2 to 5 men 

Some fortoflcot oons outsod e 

the ce nt ral area, however. were has 

ol y thrown up by the soldoers before and 

aurong the Second Assault on the Stronghold 

Apr ol 15-17, 1873 In the d osostrous First 

Assau lt . J anuary 17 t he troops learned 

rom the Modocs to va lue shelter behond 

rocks 

SYMBOLS 

....<-.__ 

~ 

~ 

TRAILS 

( Woth brodge ....... and stoors 

ROAD 

STRIKE AND DIP 

(Wot hon schollendome areas. shOws 

slope of land , short line ond ocotes 

he d orect oon of t olt ) 

TRAIL NUMBERS 

(Stot oons on the sell- guod ong trool 

tnrough Coptoon Jock's Stronghold) 

~CT~iC C~KS( II 

FORM ER 

-·- SHORELINE 

OU POS~ 

~ MOOOC 

_d 

152 

~os 

*\..-~ \..-0 

G+-..J... 

~~0 00 

ROUGH HUMMOCKY SUR FAC E 

WITH MANY SCHO LLE NDOMES 

LAKE 

oG+-..J... 

0~~ 0 

PLATEAU 

BEO 

..--- . :1'? A <oY<:J 11'1/~~ 

~os 

*\..-~ \..-0 

MC 

"-

MOOC 
-·--- I ~ J · .. ~ (j 

I f\__.? 

I 

/ /o 

0 

0 

... 

~ 

(<11\1. Ds 

=-

0 iJJ) ~ ( )L /J) PLATEAU 

PLATEAU 

0 

~ 

@ 

153 

\. 

c 

( 

~ " &. 

D -ar----. 

0 ~· v' 

C) 

0 LOWLANDS 

" 

0 c 

0 

~'\_ . • 

0 D J ... 

"" 

\.. ~ -

~ 

~ 

I - TECTONIC CRACKS 

~0· 

CONTOURS (meters) 

(near former Lake Shore) 

--------10 -------

--------5 --------

--· -- • -- O(Lake Shore on 1873) 

-------·s--------

SC"LE 

0 20 40 100 200 "ETERS 

0 100 200 XlO 400 ~ ~EEl 
3 . .:..T:.:.h=--..::....:...::..::..:~ 

where he 

is h 

map ) 

out from he f ow fron , and a so drained out from 

benea h i s solidified crus in a plexus of 

underground ava tubes . Therefore the plateau 

surface in the Stronghold is dimpled wi h smal sag 

bas·ns and by vertica -wa led col apse pits (see 

map) . Moreover , the plateau edges which were close 

to the original flow front have " turn-down margins" 

produced as the plateau surface sagged outward and 

was riven with deep cracks and fissures as it wa s 

rafted forward and let down o a lower elevation 

wh ile molten materia wa s escaping from benea h i 

4. Schollendomes. A marginal area of large 

schollendomes is present on three sides of the 

plateau remnant in the Stronghold area, and it 

continues to border other pl ateau remnants for many 

miles to the east and southwe st . These schollendomes 

are detached parts of the former plateau crust wh ich 

have broken into e ongate domes characterized by a 

deep central clef along their top . Many deep 

f issu res and minor cracks branch from he central 

cleft . (For description and ske ch of a schollendome 

see the Explanation accompanying the mapl . 

We now describe in more detai the geologic 

nature and origin of each of these four topographic 

units as they occur within the area of the 

accompanying map . This will give the background and 

pe rspective necessary to understand how the 

topography was effectively used by the Modoc 

defenders of the Stronghold . 

Tulelake Plain , Contrasted With the Tule Lake of 1872 

In 187 2 the area north of Captain Jack's 

Stronghold l ooked very different than it does today. 

Instead of the grain fields laced with irrigation and 

drainage ditches , the wate rs of Tule Lake lapped 

against a steep slope of crazily tilted ava blocks 

that formed a schollendome field between the 

shoreline and a flat-topped plateau 12 to 20 meters 

above the l ake. The change from lake to farms was 

brought about by diversion of Lost River to irriga e 

ar id lands farther west . Over a period of about 12 

years (1 906- 1918) the lake shrank to less than l/4 of 

its former area, and the hyaloclastic sil s and sands 

on the ake bed , fortified with organic matter from 

the tule swamps that flourish in shallow water , 

became rich farm land . The position of the original 

shoreline i n 187 3, and the shape of the present land 

surface immediately adjacent o it , are shown by 

5-meter contours on the accompanying map. 

Only over a stretch of about 500 meters (l/3 

mile) did he shoreline of Tu e Lake come against the 

be lt of large schollendomes hat r im the plateau 

remnant of he Stronghold . Both east and west the 

shoreline is against lava lowlands that rise only 0 

to 3 meters above the level of the former lake . The 

site of a non-permanenl Indian village that occupied 

a rocky ledge just above lake level on the eastern 

lowland one kilometer (0 . 6 mile ) northeast of the 

center of he Stronghold (se map) can be identified 

154 

y 

h 

bi r d 

0-m 

vill 

~----~ ----~ ~~~~~~~~~~ . -In rea above th level 

s art wi h pla eau nd th n 

proceed o areas of lower el vati on . This is the 

order in which th three opographic uni s (pla eau 

surface, schollendomed margin, and lowlands) , wer e 

developed geologically. Each repres n s a pha se 

ur ing h clima·x and dying ou of a major episode of 

volcanic ac ivity . Tracing them in ord r of 

development makes i easi r to und rs and how h 

na ural fortress used by th Modocs was formed . 

The of h plat au (zippi em area on he 

op of an unusually hick l va flow which 

his area from he nor h . umerous 

he margin of the pla eau penetra e 10 

below its top; this dep h is p rhaps l/ 3 

thickness of the lava flow at the ime 

of its greates infla tion during the clima x of he 

erup ion . The flow is one of a gr a number of lava 

flows which spread north and eas from vents in and 

near Mammoth Crater , located 15 kilom ers (9 .4 

miles) airline to the south. The molten lava , 

however , did not travel all this dis ance on he 

surface of the ground. Ins ead i was transmitted 

most of the way through underground lava tubes . (For 

an ana ysis of various parts of the lava-tube sys ems 

see he maps and reports on display a the Visi or 

Cen er in Lava B ds ational Monumen ) . Small 

distributary tubes fanning out from one major branch 

of this intricate system of large lava tubes became 

active within he flow that forms the plateau at 

Captain Jack ' s Strongho ld during the last stages of 

eruption, as we show la ter. 

The distribution of remnan s of he plateau 

surface indicates hat this hick flow reached its 

farthes north extent within the area of Captain 

Jack ' s S ronqhold. From h r h~ irr g 1ar1y 1 

flow front trended about South 65° Eas to and 

beyond Fern Cave; and in the opposite direction it 

runs approxima ely South 30° Wes for abou 3 

kilometers . A most points, however, he exact 

posi ion of he flow fron can b inferred only 

within a width of about 500 meters, because in almos 

all places the flow's margin was broken up and partly 

inunda ed by late movements of mol en lava wi hin the 

interior of the flow after i s fron and crus had 

partly solidified . 

After the flow front stopped moving and a 1- to 

15-meter thick crus had congealed on op of the 

flow, a resurgence of volcanism sent large volume s of 

molten lava through the long syst m of tubes . Lava 

from d istal branches of these tubes entered the s ill 

molten interior of the flow, lifting i and putting 

so much hydraulic stress agains he flow front hat 
> 

en Ia a brok hrough in num rous places. Dur i ng 

~hi s max imum pha s o f ac · vi ty he s i 11 

en ava benea h h au crust began to creep 

oward hese new breaches, se i ng up skeins of flow 

toward he points of low st stress. During he 

waning s ages large par s of the flow fron and par s 

of he crust of he flow began to break up nder 

hese stresses and were rafted slowly forward on he 

bac of the moving currents below . At he base of 

he flow front small lobes and tongues of mo ten lava 

escaped to h surface and flowed on to the nortf'l. 

After eruptive ac ivity a he dis ant vents had 

ceased the mol ten interior of the flow and the 1 ava 

fi l ing he underground tubes continued to drain out , 

eaving many parts of he plateau crus unsupported . 

sch parts sagg d downward , rep acing the space lef 

by he departing lava from the interio . Large 

asses of he solidifi d flow fron , and finger-like 

ongues f the crus e xtending hundreds of me ers 

os ream from the flow fron , sagged down and were 

d~agged forward as the lava beneath them 1 eaked ou 

1 ote on the map the many small oval collapse basins , 

and the long finger-like basins with schollendomes 

... hich indent the plateau). 

wherever he edge of the plateau "turned down" to 

rep ace the escaping lava beneath , deep tensional 

cracks and fissures formed in the turned down fJap . 

n places parts of these fissured margins were rafted 

away, forming schol1endomes . Nearly all edges of the 

p ateau remnants became an intricate maze of deep 

L ssures which greatly impede travel across t is 

errain . Where shallow dis ributary lava tubes 

drained ou , post-lava col apse of the plateau 

surface i nto the ubes has left numerous 

vertica l -wa led collapse pits and collapse trenches. 

Inspect he area of the map at the end of the 

plateau remnant where the Modocs took up residence . 

Each of the fain lines represents a crack or fissure 

too wide to jump over without extreme caution , and 

for each line on the map there are many more cracks 

in the ground (omitted from the map) . These 

fissures, especially the big ones at or near the top 

of the turndown flaps , are the "natural trenches " 

used by the Modocs . Note that hey are nearly 

con i nuous along he edge of he pla eau on three 

si des of the Modoc encampment . These plateau-edge 

fissures, however , ~e re only he last line of 

defense; the Modocs a so used rock clefts on the tops 

of schollendomes beyond the plateau margin as sentry 

outposts . 

The Schollendomed Marg in of the Lava Plateau . 

The map shows much better than words how this frontal 

par of h hick flow ha form d h 

bro en up and ransform d i n o a 

of schqllendom s . Schol l ndomes 

crus h 

and 

Jac ' s 

edge o 

end of 

Only h larg s 

indi ca d on our map. 

also h " urndown basins" 

1n o unbrok n par s of he u, ar 

nd 

rs 

h 

155 

wi h small e r s chollendome s whi c h range down i n si ze 

t o schollen no 1 rg han a sma room and only a 

me er or wo in heigh . Time did not permit s to 

map the sma l ler schollen. Moreover, the cl t er of 

ines ha would result from plot ing all hese 

cracks and fissures would have made the map 

unreadable . The unbroken expanses of whi e paper i n 

this par of he map , however , do give a false 

impression that some of the wider basins in the 

schollendomed area might be easy to cross . From a 

distance they a so look easily traversible, but 

actually they are deceptively cruel to a person who 

ries to cross them in a hurry . We have used the 

word hummocky in places on the map to indicate that 

such surfaces are quite irregu ar, rough with various 

sized schollendomes, and riven with hundreds of 

cracks and fissures . 

Several high schollendomes outside the plateau 

remnants provide excel lent viewpoints over the 

shoreline and lowland areas . The Modocs developed 

them into we ll-camouflaged sniper positions by piling 

loose fretworks of rock in or around parts of the 

central crack . 

Lowlands Bu ilt of Pillow Lavas and Hyaloclastic 

Debr is. Numerous outcrops where the lowlands meet 

the former shoreline of Tule Lake show that lava 

advancing from the south flowed into the lake . 

Molten lava changes on contact with a water body into 

pillow lavas and hyaloclastic deposits . These 

materials filled in the edges of the original Tule 

Lake , pushing the shoreline northward much like a 

delta grows at he mouth of a large river . For 

details of the mechanics of formation of pillow lavas 

and various kinds of hyaloclastic deposits consult 

Fuller (1931) and Waters and Fisher (1 971 } . 

In a quarry deve loped for road building materials 

near the site of the former Indian village (see map}, 

we can examine typical e xamples of pillow lavas with 

chilled glass rinds, pillow breccias, and granulated 

slag-like bits of hyaloclastic material (basalt 

glass) - the three kinds of deposits 

characteristically formed where molten basalt 

advances into a water body. In nearby areas molten 

lava also escaped directly into the lake through lava 

tubes . Good e xamp les are on the Hovey Point 

peninsula wes of the Stronghold , and there are 

several additiona l vent areas and low maar craters to 

the east between the St:onghold and Hospital Rock . 

In places molten lava also erupted directly in to the 

lake, building larger underwater volcanoes at The 

Peninsula , Juniper Butte, and Pr isoners Rock -

localities several miles to the east and south of the 

Stronghold . 

Most of th lowland area probably was built into 

the lake by he quenching of lavas that are slightly 

o lder han he flow that formed the plateau . The 

a e ongues of lava that broke through the 

s chol lendomed front of the plateau flow, however , 

have also assis ed in pushing the lake front farther 

to the north . 

The lowland ar as are easily traversed . Al hough 

h lava surface is minu ely rough , and is in places 

diversifi d with small schollendomes about a meter 

high, h area shows non of the deep fissuring, 

arg schollendom s , or broken areas of talus that 

make h adjacent scholl ndomed area so difficult to 

rav rse. Mor over , waves and wind have scattered 

hyaloclastic sands and silts over the surface of many 

Jowl nd ares, assuring exc ll n mobility for 

ravel . Soldi rs in raining on the lowland areas 
near Gillem's Camp , or on the hyalocl stic flats near 

Hospital Rock , probably had no concept of the ind of 

terrain they wou ld meet in their assaults on the 

Stro ngho ld . 

The Modoc War 

In l ate November of 1872, two groups of Modoc 

Indians were encamped at their winter villages, 

located about 1/2 mile apart on either side of Lost 

River a few miles upstream from whe re it entered Tule 

Lake . On November 29, a surprise confrontation just 

at dawn with a patrol of soldiers sent to put the 

Modocs on the Klamath Indian Reservat ion ended in a 

shootout, followed by withdrawal of the Indians from 

each village. One group, led by Captain Jack, and 

the other by Hooker Jim, each headed for their 

Stronghold on the opposite (south) shore of Tule 

Lake . Jack's group, which also included the women 

and children from both villages, paddled the 13 miles 

(21 kilometers) across the lake from the mouth of 

Lost River during the night of November 29-30 . The 

following day they were joined by Hooker Jim's small 

group of warriors, who had ridden horses along the 35 

mile route around the east end of the lake, and had 

taken revenge for the fatalities incurred in the 

early dawn confrontation by killing all men at 

isolated ranches and settlements along the north and 

east shore of Tule Lake. In the wake of the 

revenging Modocs, the widowed women and ch ildren of 

the settlers began the long walk across Stukel 

Mountain to the security of other white settlements. 

Men of two entirely different cultures, neither of 

whom fully comprehended the rights or the motives of 

the other, were now at war. 

The Stronghold. Examine the map and note the 

nature of the natural fortress which was to be the 

home of the Modocs for the next 5 months . Here the 

northern tip of the plateau surface overlooks a bay 

in the south shore of Tule Lake. The part of the 

plateau closest to the lake is a rounded table 

approximately 150 meters in diameter - about the size 

of a modern football stadium. It is bordered on 

three sides by a field of large schollendomes; to the 

south a neck about 50 meters wide connects it with a 

larger remnant of the plateau. 

Within the area chosen for the Modoc's living 

quarters the plateau surface is dimpled by 8 collapse 

pits, each a vertical-walled hole 2 to 15 meters in 

diameter and 3 to 8 meters deep. They formed where 

parts of the plateau's crust caved into small 

underground lava tubes. Floors of these pits are 

covered with large angular boulders tumbled from the 

roof ana wa lls. In places one can burrow around 

between these boulders, especially beneath the 

overhanging walls of parts of the pits, and find 

small chambers each of which would give protection 

for 1 to 5 individuals against the rain or snow , and 

also from the strong cold winds that sweep the 

plateau in winter and spring. Three of these 

collapse pits are easily visited from the inner trail 

constructed by the Park Service: Captain Jack's 

Cave , Schonchin John's Cave, and Family Cave (see 

map) . Nearby a small mound of pahoehoe lava served 

as a rostrum from which the Modoc leaders could 

address their people. 

Loose rocks picked up from the surface of the 

plateau were piled into low dry-walls, forming a 

partial breastworks around parts of the camp. (These 

fort i fications were later rebuilt into much thicker 

and higher walls by Army sold'ers after the Modocs 

had wi thdrawn from the Stronghold) . The main defense 

156 

positions used by th Modoc s , however, are he dee 

natural cracks nd creva sses along th top of the 

tur ndown edges of the plat au, a nd similar f~ssures 

along th tops of high schell ndome s which r1ng tr 

sid s of he Stronghold. The more stra tegic and 

important of these Modoc defense trenches are labe , 

on the map. Note that they form a si nuou s lineal ·: 

the entire northwest margin of th plateau, and tha.' 

they curve i nto a natural u-shaped ambush line wh ic: 

bars access to the Stronghold from the nearest po1

, , 

of the lakeshore . The floors of these natural ' 

defense trenches wer e cleared of loose debris so ,. 

the defenders could pass rapidly along them. Shor · 

radial routes by wh ich one can walk to various pans 

of the trench system from the central Stronghold 

without encountering difficult crevasses were we l 

known to the Modoc defenders. 

Beyond this natural trench system lay scatt~~ 

Modoc outposts - high isolated overlooks with 

unimpaiced views of the surrounding country. Most 

such outposts are located in the central cracks of 

the highest schollendomes, and were further 

camouflaged by piling loose fretworks of rock thr 

which a sniper's rifle could be extended unobserved: 

No doubt additional Modoc outposts with in the area " 

the map have gone unrecognized during our mapping ~ 

(see the description of Modoc Fortifications on ~ 

map). 

Contrary to the statements of several writers, 

these natural defense features of the stronghold an 

not unique. Many other table-like remnants of the 

lava plateau have even more formidable and 

deeply-crevassed turndown edges. The unique value 

the stronghold chosen by the Modocs was its prox ia: 

to the shoreline of Tule Lake. A constant supply 0:· 

water, and of some food from wocus root, waterfow , 

fish, and fresh-water clams was thus assured. hl ~ 

its location denied communication for an enemy usi; 

the easily traversible route along the lake shore. 

Moreover, the Modocs were well aware of an easy 

escape route to the south over the flat surfaces of 

scattered remnants of the lava plateau, whereas one 

unfamiliar with that terrain would flounder painfu: 

and slowly across the heavily fissured and 

schollendomed country that surrounds these plateau 

remnants. 

Still another unusual topographic feature, a 

natural cattle corral, wa s vital in helping the 

Modocs withstand the winter siege. Just west of ,e 

Stronghold encampment is a small and deep collapse 

basin, bounded on three sides by the steep and 

heavily crevassed sides of three large schollend~! 

na on the fourth {east) side by the s eep and deep: 

fissured turndown flap of the plateau. Miracu lous . 

these bordering fissures along the edge of the 

plateau die out about 200 meters to the south. Hen 

a smooth and easily traversed slope leads down off 

the surface of the plateau and northward through a 

narrow "gate" across the end of the southern 

schollendome into the natural cattle corral. Stray 

cattle on the southern plains, and others captur~ : 

Modoc raids, wete driven north across the plateau 

remnants and into this natural corral, where they 

could be securely penned in by piling a wall of rOC1 

and brush across the narrow gate (see map). Thus a· · 

adequate supply of beef was available throughout ~ 

winter. 

Assault of January 17 , 1893 . Meanwh ile the Ar ; 

stung by their lack of success in "rounding up" t~ 

Indians and shocked by the murder of 14 settlers ~ 

the retreating Modocs after the Lost River 
confrontation , wee preparing for a second attempt. 

Addi tiona troops wer ca led i n from other Army 

5 t 5 , and groups of ~olunteers ~rom Oregon and 

~lifornia (whose enlistment per1od wa s only for 30 

d

a 5 we e hastily or ganized and haphazardly 

ay · d · d l trained . Also recru 1 te were unorgan1ze vo unteers 

and abor support f r~m near by towns and r a~ches. By 

ear y January 400 " fJghting men " we re "r aring to 

go . " Lieutenant Colonel Frank Wheaton set January 17 

for the at tack on the Modoc' s encampment. At that 

time the ir natura l fortress wa s unnamed; it soon 

became known as Captain Jack's stronghold. 

colonel Wheaton' s strategy was a p i ncers movement 

from both west and east. Three hundred men were to 

~committed to battle; 100 held in reserve. Captain 

Green's cavalry (but dismounted as foot troops) a long 

~o~ith contingents of Oregon and California vo l unteers 

~o~ere to bear the brunt of the fighting and attack 

from the west. Captain Bernard's smaller command 

~o~a 5 o advance from the east, primarily a s a holding 

force to keep the Modocs from escaping a long the 

a eshore lowl ands. On January 16 the t roops moved 

into position from the i r training areas . Green's 

for ces marched 13 miles across rough country to a 

it ion on the bluff over looki ng the southwe st 

corner of Tule Lake (near Gillem's camp on modern 

aps) . On the same day Bernard's force advanced west 

over the low ands adjacent to the south shore of Tule 

Lake, but because of a heavy fog they probed too far, 

drew the Modoc's fire, and three men were wounded. 

Bernard's group then withdrew to an area near 

Hospita l Rock (about 3 kilometers east of the 

Stronghold ) and camped for the night. 

The morning of January 17 was cold, and a heavy 

f~ encompassed the area around the lakeshore. 

Wheaton and Green's troops, a lthough on the march at 

da~, were slow i n making their way down Gi llem 's 

bluff and getti ng organized into a li ne of 

5 irmishers as they advanced toward the Stronghold. 

o doubt Modoc scouts were fully aware of the troop 

oovements since early dawn, but it was after ll a . m. 

fore Modoc snipers opened fire . The debacle that 

followed has been chronicled by many; the most 

detai l ed account of number of men, their positions, 

and their movements during the "battle" is in 

Thompson (1 971, Chapter 4, p . 33-45 ) . After wounding 

and killing several men the Modoc's re linquished a 

few of their outposts, thus leading the advanc i ng 

troops eastward until they were enmeshed in the chaos 

of deep cracks and crevasses within the schollendomed 

area . Here the Modoc's fire from their natural 

defense trenches above was accurate and deadly; 

casualties mounted, and yet not an Indian had been 

seen by h befuddl d roops . By midaf er noon a 

hought of charging up on o the pl ateau was 

abandoned; some pa r t s of he li ne we re already in 

retreat leaving heir dead on he f ield . Captain 

Green personally led an attempt to round he 

Stronghold along he lakeshore a nd mak e contac with 

Bernard's command on the e ast. They suffered many 

casualties; mos retr ated or were killed, but a few 

en remain d cone aled behind boulders until 

darkness , and then made heir way over to Bernard's 

position . Cap ain Gr e n w s among them. 

Bernard ' s group had also ben in roubl during 

he day . Casual i s w re i nf lic ed by un se n Modoc 

snip rs . The hough no as difficult as 

hat on x nsi vely schollendomed , 

crevass s . 

assaul on 

re rea was 

reach rous groups of 

arning by si gnals that the 

sid had f iled , a mor orderly 

10 p .m. on h as si d a nd i 

157 

con inued through the night. 

The day after the rout the Modocs searched the 

battlefield and recovered much valuable booty 

(Riddle , 197 4, p. 56 ) . They found the ground covered 

with ammun i ion , rifles and other kinds of guns where 

the Oregon voluntee rs had stampeded in bad order . In 

the area where mos t of the casua l t ies of the soldiers 

had occurred were 9 carbines and 6 belts fil l ed with 

carbine cartridges. Also recovered was considerable 

field equipment, boots and clothing. 

The outcome of the January 17 assau l t was thus a 

spectacular victory for the Modocs. Casualties of 

the Army and the Oregon and Ca lifornia volunteers 

totaled 37, and 6 of the dead were left on the 

field. The Modocs had no casualt i es. In Thompson's 

words (1971, p. 43 ) "Three hundred men had been 

unabl e to make the slightest dent on the magnificient 

union of lava and Indian ski lls." 

In many written accounts the heavy fog is blamed 

for the Army's debacle, but a person thoroughly 

f am iliar with the terrain can argue effectively that 

the fog worked to the advantage of the Army, not the 

Modocs . The Modocs early defense was accompl is hed 

entirely by snipers in Modoc outposts. From their 

secondary natura l defense trenches at the top of the 

plateau, other Modoc defenders could not see through 

the fog and determine which parts of the Army's line 

were hung up on fissured ground, ~hich parts were 

advanc i ng, and which were routed and in retreat . 

Therefore they could not concentrate their limited 

manpower to the points where it would be most 

effective - as they did so successfully later in the 

disastrous Thomas-Wright ambush. 

The Winter Program of "Gradual Compression". The 

events between January 18 and April 11, 1873 are well 

summarized in Murray (1968) and additional detail can 

be found in Thompson (1 971, p. 45-65) . The Modoc 

Shaman (Curley Headed Docter) had assured the 

warriors that they were invincible, and would not 

suffer a single fatality . After the easy repulse of 

the January 17 attack they had every reason to 

believe him . The Army was humiliated. Changes in 

command took place: Brigadier General E. R. S. Canby 

and Colonel Alvan C. Gillem were placed in charge, 

and steps were taken to greatly increase the number 

of soldiers. The Oregon and California volunteers, 

who had been so eager to do battle prior to the 

January 17 assault now had their fi ll of fighting the 

Modocs; most of them disappeared the moment their 

30-day enlistment was up . Irresolute and conflicting 

orders also came from the War Department in 

Wash i ng on, D.C. A strong feel i ng arose that it 

might be better to try to negotiate with the Modocs 

than to engage them in battle. A Peace Commission of 

5 members was set up, but rapid changes occurred in 

its membership, and the Commission did not truly have 

the power to give answers with regard to the Indians 

two chief concerns: l . Establishment of a small 

reservation for the Modocs in the Lava Beds; 2. 

Escape from the hangman's noose for those who had 

murdered the Oregon settlers after the Lost River 

confrontation . Moreover the Indians were growing 

i nc rea si ng ly concerned with the steady bu ild up of 

roops nearer and nearer to their Stronghold . Canby 

was engaged in a program of "gradual compression." 

By early Ap ril , Gi llem's Camp, at the southwest 

cor ner of Tule Lake, and only 3 miles from the 

Stronghold, contained 350 soldiers. On April 6 

Capta i n Ma son moved 5 companies of foot soldiers to 

Hosp ital Rock, only 2 mi les to the east. 
Dissens ion bout fu ure ac a so arose in th 

Modoc's camp . F om heir poin he mee ings 

wi h he Peace Commission had b n compl e y 

frui less , and immediately af m eting Canby 

had mov d addi ional troops n o he 

S ronghold . Th is program of "gradual compression " 

after each meeting cou ld only lead o frustration , 

and a final e xplosion of he Modocs . 

One of he Mo ocs proposed ha on more P ace 

m eting be held a wh ich General Canby, A. B. 

Meacham , he head of he Peace Commission, and the 

other members in attendance wou ld be murdered . This 

audacious idea cap ured the fancy of many of the 

Modoc wa rriors . They reasoned that if the General in 

charge of operations , and the head of the Peace 

Commission were simu ltaneously put out of the way the 

soldiers might wi thdraw. Cap ain Jack knew better , 

and vigorously opposed the plan , but some of his 

col eagues taunted him as ? coward , pushed him to the 

ground, and placed women's clothing upon him . His 

manhood thus called into quest ion, Jack agreed with 

the plan, and said that he would be the one to murder 

Canby. 

This treacherous p an was learned by Toby Riddle 

(Winema ) , a Modoc woman married to a white man, Frank 

Ri dd le . The Ri ddl es had served as in er pr eters 

dur ing the Peace negotiations . She warned Canby and 

Meacham of the plan and begged t hem not to hold 

another me eti ng. They d id refuse a meeting for the 

fo llowi ng day , but after Captain Jack formally 

requested a meet i ng for Apr il ll , to be held i n the 

"peace tent" l ocated between Gil em's Camp and the 

Stronghold , Canby agreed. He "assured the 

Commissioners that the Modocs wou ld 'dare not molest 

us because (our) troops commanded the situation '" 

(Thompson, 197 1 , p. 59 ) . 

The meeting wa~ he ld . Canby and Jack i mmed iately 

argued over peace terms . Jack gave the sign, pul ed 

h is gun, and killed Canby . Simultaneously Thomas was 

killed . Meacham wa s shot several times, but sur v ived 

because Toby Ri dd le deflected Schonch i n John' s aim. 

She a lso stopped an attempt by Boston Charley to 

sea p Meacham. L. S. Dyar and Frank Riddle ran the 

moment Jack pu lled h is gun, and each s uccessfully 

evaded bu lle ts fired by pu rsui ng Modocs . 

Assault of Apri l 15-1 7, 1973. The tragedy of 

Apr il 11 spurred both the Wa r Depa r tment and the 

loca l Army command into action. Gi llem 's and Mason' s 

sol diers were a lready pois ed on either si de of the 

stronghold . Mason had 300 men, and in add ition 70 

Warm Spri ngs Indians were riding from their 

rese r vat ion to the north to join him as scouts in 

time or the battle . The western command under 

Colone l Gillem and Capta i n Gr een already had 375 

soldiers at Gillem 's Camp. Sur ely 675 troops plus 70 

Ind i an scouts wa s sufficient to capture or 

e xterminate 53 Modoc warr i ors! Wheaton had realized 

that Coehorn mortars would be more effective than the 

l ow-tra jectory howitzers i n displ ac ing the Modocs 

from caves and rock trenches. Many mortars had 

arrived and wer e ready to be moved into action . 

Wheaton , from his January 17 e xperience , had also 

pa ssed on to his successors that the east side of the 

Stronghold was much more vulnerable to attack than 

the west side , but this e xcellent advice wa s totally 

d isregarded . 

Gillem se Ap il 15 for the assaul . I s 

planning , execution , and final resul s wer e almost a 

carbon copy of the assault of January 7th, e xcept 

hat his time he scenario was played in slow 

158 

mo ion . On he n i ght o April 14-15 

edged ging rly [orward i n 

occupi d roughly h s m 

of January 17 had re ch d 

fire . Bu i n h iss cond were 

prepared to s y nd hold on . roo s on he 

w s side wer moving into posi ion und r cov r of 

arkn ss soldi r los his footing mong he jagged 

rocks nd his rifl ccid n ally fired . Thus he 

Modoc ' s were alerted , and th ir crie-s o warning were 

passed along hroughout th p rim ter of th 

Stronghold . The soldiers immediately bivouacked in 

plac , throwing up rock forts as shelters. 

The soldiers on th east sid had alr ady 

cons ructed numerous low forts of loose rock , and a 

roc k- wall lin of defense before he date for the 

assault was set . The lesson of th va lu of rock 

s helters had been well-le rn d in the January 17 

assault . Wh e n the Modocs c ied oul their aler , he 

troops on the east paused for the night i n these 

s helters. 

The ne xt day (Ap ril 15) mortar and howit zer 

shells were pour ed i n o th Modoc ' s e ncampm n , and 

the troops began a cautious and slow ad vance . They 

immediately drew the Modoc ' s fire . Some success was 

attai ned i n the early afternoon when Gre n's troops 

displaced a few Modoc snipers, who escaped safely to 

their defense trenches on the margin of the plateau, 

but not until t hey had taken a toll of casualties 

from the advancing troops . As the night of Ap ril 

15-16 approached, Gr een ordered his troops to 

straighten their line and build forts of loose rock, 

or else find shelter in rock crev ices for the night . 

The advance on the east side had been e ven mor e slow 

and cautious. Throughout this ni ght the artillery 

sent bursts from their mortars and howitzers toward 

the Modoc e ncampment. 

On the second day of fight ing (April 16) the 

artillery continued to pour mortar and howitzer 

shells into the Modoc' s lair, and the foot soldiers 

advanced slowly . An attempt to push forward strongly 

on the south flank of the we st side fai led 

comple tely . On the eastern front Mason ' s troops ca~ 

und er sniper fire from their rear as well as in the 

direction of advance, and were pinned down for most 

of the day. It became obvious that it would be too 

costly to try to take the Strongho ld by direct 

cha r ge . The str ategy was changed to attempting a 

simu ltaneous advance along the lakeshore which would 

cut off the Modoc ' s water supply . Reports disagree 

on whether a real junct ion of the t wo forces was 

made , but at least the soldiers approached close 

enough to bring any Modoc see ki n wat r und r f ir 

Throughout the night mortar shells were lobbed 

continuously at the Modoc's positions , and this 

activity was doubtless far more effective than the 

desultory and inaccu r ate rifle fire of the foot 

troops aga inst an enemy that they could not see . At 

any rate , it was during this night that the women, 

children , and most of the Modoc warriors withdrew to 

the south , undoubtedly fo llowing the wel l-known route 

across the flat-topped remnants of the la va plateau 

along wh ich the Modocs had driven cattle into the 

natural corral . A few Modoc war riors remained 

through most of the night to harass and taunt the 

troops "in very plain , if not classical English" 

(Thompson, 197 1 , p . 74 ) . 

On the morning of Apr il 17 the artillery stopped 

pouring shells into the Stronghold , a nd troops on 

both east and west sides began a cau t ious advance . 

No sounds came from the Modoc's posi ion , no shots 
~ere fired , and so soldiers i n par s of th Army ine 

advanced more apidly and soon we on op of the 

p ateau with in the Stronghold area . They found it 

completely dese te . Many soldiers, however , delayed 

and wer still hiding behind their rock shelters. 

co onel Gillem , angry at their unwillingness to fight 

"got upon the highest rock availab e and ordered 

repeatedly ' Forward ,' ' For ward ," unt il finally 

Mason' s Troop G under Captain Bernard came up to join 

th Green in sweeping the area ." (Thompson, 1971 , 

p. 75 1 . 

It wa s an empty " s weep . " Over 650 Army regulars 

had spent 3 days and 3 nights i n "battle" and had 

suffered 23 casualties (6 killed , 17 wounded) . Their 

attempt to " round up" or else "e x termi nate" the 

Modocs was a complete failure , even though they now 

occupied the Modoc ' s Stronghold . 

The Modocs , however , had suffered a few 

casualties as well , a nd so the Shaman's p ronouncement 

of i nv i ncibility was discredited . It is reliably 

reported that one Modoc wa rrior was k illed by a 

mortar shell , and very li kely t wo additiona l Modocs 

di ed from the shelling . Sergeant Fit zgera ld reported 

that the severed head of a Modoc was be i ng kicked 

about by the troops after their entry into the 

stronghold, and Wi liam Simpson , an Engl ish newspap r 

reporter , drew a pictu re of a soldier hold i ng the 

sea p of a Modoc man, erroneously reported o be 

scar-Faced Charley (who , however , was still alive and 

out of the Stronghold) . On e aged Indian squaw, 

crawling through the rocks in an attempt to get to 

water wa s captured, and then summarily shot through 

the head . In this war sa vagery was not restricted to 

people of one culture or one color . 

The Modoc's Withdrawa l Route . Much nonsense has 

been written in Army Reports, as well as by 

historians and other writers, about the route by 

which 150-17 0 people , most ly women and children, were 

able to escape from the Stronghold undetected . It 

was inconceiva ble to the Army command that they could 

slip away so silently in the night without the 

soldiers ' knowledge . The Warm Spr i ngs Indians, hired 

as mercenar ies , were suspected of being traitors to 

their contract and of a llowing the Modocs to "escape 

up a gu lley , " near the li ne that the Warm Springs 

held at the southeast e nd of the easter n front . 

Historians , as well , have appealed to t he finger-like 

collapse draws south of the Stronghold , plus 

connivance with the Warm Springs scouts. Yet an 

observer who walks through these draws on the ground 

finds them so cluttered with schollendomes an d ri ve n 

with deep cracks and crevasses that it would be quite 

impossible to get such a large group through , a o ng 

with their dogs and horses in one night's tim . Ev n 

more fanciful are h wri n sta m n s i n sam 

serious reports hat he Modocs " slipped pas the 

Warm Springs scouts in a large lava rench " (no such 

"trench " or "gu lley" is th re) , or the less ambiguous 

(bu qu ite impossibl ) s a ement ha they 

"disapp ared into he Schonchin Flow." 

To a opographic engineer 

equi pp d with mod rn airpla n 

errain , the escape rou of 

They simply wa l ked south and 

brisk pac on th remnants of 

plat au (see map), avoiding 

dimpl par s of h pl 

and Scholl ndom s whi ch 

pl au r mnan s . Th 

h on long which h 

s ray and s t ol n ca l 

or o a g ologis 

photographs of he 

he Modocs is obvious . 

basins which 

turndown flaps 

bard rs of th 

he sam s 

driv n ov r 100 

n tur l corral from 

15 

h p ains and hi hl nd s o the southeast . 

In the area of our map the northern end of this 

trail is shown by a broken line symbo . ate that 

its closest point to the Wa rm Springs posi ion is 

about 1/ 2 mile (750 met rs) but hat i is no f r 

from the Modoc natura defense trench s at the top of 

the turndown fla p on the west edge of the plateau . 

Here the Modocs had tota ly brok n an effort of th 

soldiers to "push forward strongly " during the 

previous day ' s figh i ng . 

Rock Fortifications Construe ed by the Army . 

Where had the Modocs gone? Would they return? Was 

thei r disappearance only a ruse preliminary to a 

surprise attack? Colone l Gillem and his troops did 

not know. It was obvious that a few Modocs were 

still around , for the occasional crac k of a snip r ' s 

rifle was heard, and now a nd then an Indian could be 

seen out of rifle range walking on the southern 

plateau remnants , and others bathed in t he lake . So 

the ne xt few days after the occupation were s pent i n 

hastily throwing up rock forts a nd rock walls for 

defense in case the Modocs decided to return . Over 

200 of these fortifications have been located and are 

l arge e nough to be shown by appropriate s ymbo ls on 

our map . Another 30 or 40, includi ng a long but 

discontinuous rock wall , w re put up by Mason ' s 

troops before the Ap ril assault - they constitute the 

easter nmost li ne of forts and wa lls shown on the map. 

Dur ing this activity the troops also discovered 

that the easiest travel in the area is on the surfac 

of the lava plateau , and they selected and fortified 

with 1 0 larger and stronger "hollow-square" forts an 

east-west li ne of easy travel across the plateau 

remnants south of the Modoc' s former livi ng 

quarters. The outer trail, constructed by the Park 

Service, follows this li ne of forts in its east-wes 

course (see map). 

The Thomas-Wright Ambush . Colonel Gil lem decid d 

to send out patrols i n an attempt to find out wh ere 

the Modocs had gone . Bernard headed a cavalry patrol 

to the east and northwest , and a second patrol under 

Perry rode to the southeast on the same day . Near 

Dry Lake some Wa rm Springs Indians, ridi ng with 

Perry , caught , killed and scalped two Modocs . Wh en 

t he Wa rm Springs returned on Ap ril 21 they also 

brought i nformation that some Modoc war riors were in 

the Lava Beds about 4 mi les south of the Stronghold 

near a light colored butte (now known as Hardin 

Butte) . Gillem first sent a small patrol to 

i nvestigate , and then dec ided to se nd a large patrol 

to see if it was feasible to set mortars on Ha rdin 

Butte to shell the Modocs in their new hideout. It 

se ms clear from other evidence that only a few Modoc 

warriors w re near Ha rdin Butte . Th e route of 

withdrawal of the Modocs was southeast toward Big 

Sand Butte and Dry Lake . 

The large patrol started for Har din Butte from 

Gi llem's Camp on Ap ril 26 . Captain Evan Thomas, son 

the Ad jutant General (retired) Lorenzo Thomas, was in 

charge . With Evans were the sons of two other Ar my 

Generals : Lieute nant Thomas F. Wright and Li e utenant 

Albion Howe . None of the t hree had any e xper ience 

fighti ng Indians . The patrol moved along slowly fr om 

Gillem' s Camp , and just before noon they entered an 

amph itheat r bounded by low ridges on the e ast, 

sou h , a nd w st , a nd locat d near the west base of 

Hardin Bu It is reported that "old ha nds " among 

he foot soldi rs had ben worried on the march by 

th lack of x rience of th offic rs , a nd by the 

car 1 ss way in which the patrol trudged along in a 
compac 

sides . 

becam 

and ac 

umn without deploying skirmish rs to th 

Ind d it is repor d tha Serg ant Rom r 

isgusted, climbed up the ridg on th w s 

ed as a Elan i ng gu rd for pa r t of he urn y . 

When h pa rol reached he floor of h 

amphitheater Captain Thomas alled a h lt fo r lunch . 

The troops sa in small groups on the sandy ground , 

and many took th ir boots off. While the me n w r 

resti ng and eating, Captain Thomas it~ thr e o h r 

soldiers began o climb he ow ridge on he sou h t 

the head of the amphitheater . Befor e he y had gone 

far rifle fire came from the ve r y pile o f rocks that 

they had set out to reach. Lieute nant Wr ight react d 

at once by o r dering a " set of f ou rs" up the ridge to 

the southeast; these four men immediately were fi red 

on; they turned and ran back to Wright's position in 

the middle of the amphitheater. Thomas then ordered 

Wright to advance with all of Company E onto the 

opposite ridge (west of the amphitheater}. Again th 

Modocs fired from rock p iles i n the top of th is 

r i dge, but Wr ight and some of his men continued, only 

to di e in the attempt. Others turned and r an. 

Meanwhile Lieutenant Cranston had taken 5 men to th 

northeast , hoping to c limb h i gh e no ugh on to Hardi n 

Butte to signal Gillem's Camp. All six were killed 

when they reached the Butte. 

The soldiers i n the amph itheater were i n a state 

of confusi on- many broke and ran toward Gillem' s 

Camp. Those who remained were pinned down on the 

sandy floor with little or no protection by eit her 

rocks or bush. Most were killed or wounded . 

Although the fi rst survivor of those who ra n r e ached 

Gillem's Camp before 2 p.m., his story was not 

believed . Rescue par ties we re not organized or under 

way unt il nearly dark. It wa s the following day 

before they picked up the wounded , a nd bur ied the 

dead at the points were they found them. No Modocs 

we r e at hand to contest the operati ons. The return 

to camp was not attempt ed until after dar k. It was 

shortly after dawn ·on Ap ril 28 whe n hey straggled 

into Gillem ' s Camp. 

This stunning vic tory of the Modocs was 

accomplished by Scar-Faced Charley wi th only a few 

men. Most of the Modoc warriors we re on the other 

side of the Schonch in Flow, far to the southeast . 

Cha rley's smal l band evidently remained behind to 

check on what wa s going on in the Stronghold and at 

Gillem' s Camp, and a lso to p rotect the rea r of the 

depa rting Modoc's. It is li kely that he had less 

than a ha lf dozen men , and they may ha ve s aged th 

ambush on the spur of the mom nt because of the E ar 

of being trapped against the west wa ll of the 

Schonch i n Flow. The only crossing of th is flow that 

is negotiable withou t hours of slow plodd ing over 

tre cherous loose 1 va blocks is j ust nor h of h 

base of Hard i n Butte . 

From the standpoint of the Army the results of 

this battle are succinctly summed up in a report from 

Lieutenant Jocelyn, stationed at Camp Wa rner : "We 

have sickeni ng news aga i n from the Lava Beds . " 

Twenty-five men were killed , and 16 wounded -

mos t of them seriously . In h istor i an Thompson' s 

words (1971, p . 91) : "Two-thi rds of the patrol had 

fallen vic im to he Modocs accurate rifle fire . The 

rest had run ." Aga in a ring of rock clefts and loose 

boulders in a schollendomed area had been used w·th 

expert pr ecision by Modoc s nipers . 

Dissension , ~h~ _!!~ ra ~1 , ~_9 2u~. Why 

had he Modocs lef their nat ural fortress? We 

160 

surmis 

rol s . 

c au s 

th 

Obvi 

including worn 

f act that th 

WO 

wa rriors . Th Shaman's claim rs wou ld n ver die in c . 

No m tter what th reasons, the t as k o the Ar y 

in rounding up the Modoc s now becam one of movemen 

a nd flui di t y, instead of a sieg . In this new 

si tuation th Modocs, deprived of n arby sour ces of 

wate r and food, and encumbe r ed by worn n, c hildren a 

the aged, could not for ve r with sta nd t he probing 0 

c avalr y patrols from a well- quipp d a nd w 11- fed 

ar my of over 1,000 men. Ne verth less th retreatin 

Modocs were still a dangerous fore to cope with . 

May 7 t he y raided a train of 4 wagons , drov off ~ 

escort of 15 to 20m n - 3 of whom suffer d wou ndsand captur d 11 mul s and 3 hors s . Th wagons were 

empty - the train was on its way to pick up 

qua r terma s t r s uppli es - so t h Modocs burn d them. 

Three day s later , at dawn on May 10, th Modocs 

s urrounded an Army patrol c amp d for th nig ht nea r 

Dry Lake. Th is t i m , however, t he soldi rs stood 

t heir ground and drove th Mod cs off. Eight troo~ 

were wou nded, 3 of whom lat r di d, bu h Modocs 

also lost two m n kill ed, one of th m th popul ar 

leader Ell en' s-Man George . 

Dissension and b tr ayal now b cam important 

factors in bringing the fig hting to an nd . The 

Modocs quarr l d bit e ly aft r th Dry L k ba tle, 

and split into t wo camps . One group who form rly 

li ved mai nly a long Hot Cr ek , n r Dor ris, urned 

west toward th ir origi nal hom la nd. Th o her and 

s ma ller group under Captain Jack remain d for he 

present in the ne ighbor hood of Dry Lak e a nd Big Sand 

Butte. Army patrols continu d o reconnoi t r throuq .. 

this area for days, occasional l y s eing on or mor e 

Modocs. Rock fortifications hat the soldiers pu t . 

near Dr y Lake and south of Big Sand But re 

num rous , and well preserv d o this day . Moreover, 

the v isitor who will search out the highes points . 

this nearly flat plain generally will b r ward d ~ 

finding the op n fretwor k of s on s wi h which a 

Modoc s n i p r had camouflaged his observa ion 

position. 

Th Ho Springs group ran into roubl whil 

tr ve ling west . Colon l J fferson C. Davis , who 

succeeded to Canby ' s command , arrived in the lava 

beds on May 2. He soon broke up the large 

concentrations of soldiers at Gillem' s Camp, the 

Stronghold , a nd Hospilal Rock into many roving 

patrols o f lar ge size who would li ve in the Lava s 

"wher e they could fight a the first oppor unity , or 

could rest and take things easy , like the Indians. " 

Two such patrols pi c ked up the rail of the Hot Cree 

band . One caught up with and gave chase to the 

I nd ians - who scattered in all directions hrough t 

rock s , mountain mahogany , nd junipers. Bu 5 Modocs 

were ki lled - 3 women and 2m n - and se v ral women , 

children and horses were captured . Dav is ordered hts 

Indians , bu 

chase som Modocs 

report d that th 

h n continue to harass hese 

y could again tak up he 

o the Fairchild Ranch and 

d to surrend r 
(Thompson, 1971, p. OS ; Riddle , 1974, p . 28 . 

oavis s n t wo I nd i an wome n hat he had emp loyed on 

other missions o see k ou the Modocs a nd g i ve hem 

the t e ms of surre nde r. 

John Fai r c h ild a nd his wi fe also we nt up he 

mounta i n wi h them. Aft r a s hor t d iscussion 

sca r- Faced Charley t ook he lead i n persuad i ng his 

peop e to s u r nde r wi thou fu r her b oorls he d . 

Eventually 63 Modoc m n, wome n a nd child re n cam o f f 

the mount ai n a nd s urr nd e r ed. Th ey were a 

sorry-look ing g roup : c lo thes i n t att ers; ma ny lame , 

sick or i nj ured; a nd "hal f-n a ked ch ildr n a nd a ged 

squaws who could scarcely hobhl ". Hooker Jim, " he 

Lost Ri ve r murdere r," wa s no t amo ng them, bu h c ame 

i n and s urrender e d a li ttle l a er . 

Dav is' a vowed purpose , howeve r , wa s to catch 

captain Jack and to hang hi m forthwith for the murder 

of canby . He l os t no time in acce l e r at i ng the 

search . Dav i s had earn d the va l ue of hi ri ng 

Ind i an s to catch Indians, so he immediately hired 

steamboat Frank, Hooker Jim , Bogus Charley, and 

Shacknasty Jim to f i nd Captain Jack (Thompson, 1971, 

p. 11 0; Riddle, 1974, p 164) . These were the very 

same Modoc s who had taunted Captain Jack into kil l ing 

Canby . Dav is also sent along the t wo Indian women 

who had helped arrange the surrender of the Hot Creek 

group . 

Wi th i n a week the se Modocs , now turned informer , 

and ri d i ng we ll-fe d Army hors e s , had located Captain 

Jack' s band c amped a l ong Willow Creek in a wi l d and 

mounta i nous area east of Cl ar Lake. A large 

contingent of troops moved in , and wh i le the traitor 

Bogus Charley was ta l king wi th Captain Jack about 

surrender , the troops came upon the Indian camp a few 

hundred meters away and fired a volley of shots into 

it (Ri ddle , 1974, p . 147) . The hunted Modocs 

scattered , leaving most of their guns , ammunition, 

and equipment. Eight Modoc women and some children 

were taken prisoner, among them Jack ' s two wives a nd 

ch i ld. Jack and a few others dashed into a thicket 

of willows, waited unt i l darkness, and then escaped. 

The chase cont i nued. Before it was over Jack and 

the few members stil l left in his ba nd had raced 

approximate l y 200 mi les, f i rst in the rough country 

a l ong Wi llow Creek, then to Lange l l Valley and Bryant 

Mountain in Oregon , and back to C ear Lake and the 

l ower part of Wil l ow Creek again. But the odds 

against survival were too overwhe l ming; the mounted 

patrols, now aided by skill ed Indian trackers, had 

picked up his trail at many points, and on June l, 

1873 they closed in . Aga i n Scar-Faced Charley helped 

in pursuading both Modocs and soldiers to refrain 

from fu r he r bloods hed. Capta in Jack, along with two 

men , th ree worn n, and s e ve r al ch il d ren s urrendered. 

Another Modoc warr i or, Be n Lawve r wi th his wife, boy, 

and aged fa her and mo he r came i n a few days l ater. 

The r ema i nde r of he Modoc' s s t o ry i s a record 

tha contri but s li tt l of honor to ei her v ic tor or 

vanquis hed . Davis , i nte n on r e v ng i ng Canby, 

plann ed to hang Ca p ain J ack pub licly wi hi n a f ew 

days a ft e r cap ur e , but he wa s frus rate d by a ruling 

from Was hi ng on ha t p nal i t s mu s b decided by th 

courts . Th res ults of th trial w re predictable . 

Cap ain J ack , Sc honch i n J o hn, Bos on Char l y , a nd 

Black J im wer hang d . Slolock a nd Barncho, who a lso 

were i n th vici n i ty o f he p ac t n but d i d no t 

a k pa r t i n he a ctua l ki llings , w r e s n e nc d t o 

ti f a Alcatr a z. Fi ve wound d Modoc prison rs wer 

killed by wo unknown a ssa si ns wh ile b ing 

t ra ns por d t o j i l a Fo r Klama th. Ano her Modoc 

16 1 

comm itted suic i d a ft er cap ure . Th remai n ing 

Modocs , 55 m n, women , a nd children , w r ba n ished 

o a barren tr act o f la nd, 5-3/ 4 s q uare mil s in re 

(2 . 5 miles on a si d ) n ar the Qua paw I nd ian Agency 

i n Ok ahoma . Confi nement , dise se , a nd poverty t h n 

a comp ished more than had th effor t s o f over 1 , 000 

troops . Some ew sur v ivors were gi ve n permission to 

retu rn t o t h Kl ma h Res er va t i on i n Or gon a ft r 

1909 . 

The Modoc' s e t ernal manum nt is t he nat ur al 

fortre ss o f t he St r onghold , which remai ns j ust the 

s ame today as whe n i t was for med by th congeali ng 

nd ere p i ng fo rwar d of a large flow of l ava mo r e 

t ha n on mill ion ye ars ago . 

Ref e r e nce s 

Fu ller, R. E., 193 1, The aqueous chi ll ing of basa l t 

lava on the Columb i a River Plate au; Amer i c a n Journa l 

o f Science, v. 21, p. 281-300. 

Murray, Keith A., The Modocs and The ir War: 

University of Ok l ahoma Pr e ss, Norman, 1958 , 346 p. 

(An objective, interesti ng, and well-written account 

by a historian and sociologist who has s erved as 

Park Ranger at Lava Beds National Monument). 

Ridd l e , Jeff C., The Indian History of the Modoc 

War: First printed in 1914; republished in 197 4 by 

Urian Press, Eugene , Oregon, 1974 , 295 p . 

(A fasc i nat i ng book written i n s impl e, 

s raightforward l anguage by the son of the Modoc 

woman Toby (Winema ) Ri dd l e , who served as interpreter 

during the Peace meetings. Jeff Riddle knew all of 

the Modoc warriors and his reports of conversations 

among them in their own language give a vivid picture 

and a new dimension of many of the deliberations 

among the Indians , and also between Modocs and 

whites). 

Thompson, Erwin N., The Modoc War: Argus Books, 

Sacramento, Calif., 1971, 188 p. plus illustraions 

and maps. 

(A comprehensive account with particular emphasis on 

numbers , distribution, personnel, and objectives of 

he various military units engaged in a struggle. 

Outstand i ng documentation by notes and references to 

many little known army, newspaper , and private 

records that are not easi ly available). 

Waters, A. C. , and Fisher, R. V., 1971, Base 

s urges and their deposits: Cape l inhos and Taal 

volcanoes; Journal of Geophysical Research, v . 76 , p. 

5596-561 4. 
162 
PRE-HOLOCENE SILICIC VOLCAN ISM ON THE NORTHERN AND WESTERN MARGINS OF THE MEDICINE LAKE HIGHLAND, CALIFORNIA 

Stan ey A. Mer zman 

Department of Geo ogy, Frank li n and Marsha ll Co lege, Lancaster PA 17604 

A s rae ------

~ ed ici ne Lake Hi gh and is an area of diverse 

vo-canism whose extrusi ves span he l ast 1 . 5 to 2 

: li on years of earth hi story . The enigmatic 

Andes· e Tuff of C. A. Anderson (1 94 1) has been 

prov 'si ona y pl aced i nto he genera strati graph ic 

sequence as one un i w'thin the collage co ll ec i ve l y 

known as he "pre-Med i cine Lake andesite " vo lcan i cs. 

his proposed pos · tioning results from f i el d, 

etrogra ph ic , and chemical ev i dence wh i ch suggests 

the ex is ence of a series of andesi t ic 1 ava flows 

~ are coeva l i n age or slight y younger han the 

Andes· e Tuff. One of these equ i va l ent l ava flows 

has g'ven an age of 0 . 21 ± 0~ m.y. ol d. If th is 

correlaUon ·s accu ate, then the conclusion that the 

A des· e Tuff represents an event wh i ch occurred 

r:or to he i ni i al e upti ons of he Med ici ne Lake 

and, therefore, he uff cannot be 

he deve opment of he Med i ci e Lake 

caldera . 

Additional s'l'cic volcan ism, i nc uding rhyo li ic 

es and flows, is consi derably o der ( 0 . 43 to 0 . 95 

.y. ·a fac which i ndicates no direc genetic li nk 

eed ex ·st be ween the rhyolites and he tuff. 

ast y, he rhyo li tes can be di vided i nto wo 

separate groups on the basis of Si 02, ne i he of 

~ich ove l aps t hat of the Hol ocene glass f l ows, a 

ea ure f or wh ich the Med ·ci ne Lake High and i s we 

known. 

Severa ecent summaries by Macdona l d (1972 and 

IIi li ams and McBi rney ( 979 concern i ng the o i g in of 

co l apse calderas suggest a number of these features 

formed concurrently wi h he eruption of ash-flow 

ffs . oble 0 969) suggested tha such a 

re~ationsh p may in fact exp a i n the ori gi n of the 

Med ic ' ne Lake ca dera i n nor hern Ca li fornia. In 

arti cular he p opos ed that a un i 

A PSi e ff may correl a e wi h he 

Med ;c· ne Lake caldera and her he 

ex rusion ·of h "old ,.. shie d-forming andesi te " uni 

o C. A. Anderson (1 94 1) . Th i i n erpre ation wa s 

a'so rei erat d by A. T. And rson 0 976 ) . On the 

oher hand, r zman ( 977) i n rpre d th s ol' vi ne 

andes· e lavas a he i n' ial e uptions of h 

: edici n Lake volcano and heref o neces a 

P"e-ca dera i n ag a conclusion s i mil ar o of 

r. A. Anderson (1941 ) . Thes con radic ory 

'nterpreta ions ar abe ted by a ack of outcrop n 

cri ica areas a d he · nab i i y o obtain a reliab e 

K/Ar age da e f h~ And Tuff, a di ffi cul ty 

a isi ng: from i t la rg seal a tmospher ic argon 

contamina i on . v r, f'e _d mapp i ng by Walter 1 975) and er zman ( unpub lished da a) i nd ica t he 

ex is nee of and si ic lavas wh ich appea r to be 

163 

coeva wi h the andesitic ash-flow tuff un i t . 

Subsequent ana l yti ca l studies concerni ng these 

t uff-equi va l ent avas has apparently resolved the 

contradictory i nterpretati ons prev i ousl y outli ned and 

prov i des a rud i mentary strati graphy on wh ich to bu ild 

future research i n the Medic i ne Lake Hi ghland . 

Geologic Setti ng 

The Med'c ine Lake Hi ghland, located approxi mate l y 

50 km eas -northeast of Mount Shasta in northern 

Cali forn ia , is the southern extensi on of a 

discont'nuous belt of shield- li ke composit e volcanoes 

wh i ch li e 30-50 km east of the High Cascade 

stratovol canoes . The northern extension of this belt 

i nc ludes Newberry vol cano in centra l Oregon and 

Si mcoe volcano i n south-central Wash i ngton . Previous 

research i n the Med icine Lake High l and has been 

recent y summari zed by Mertzman (1977) and He i ken 

(1978 ) and so the wri ter will present only the 

highlights of the geolog·c history . Over the past 

several years three dozen K-Ar analyses have been 

performed as part of an ongoi ng attempt to ascerta i n 

the geochronolog·c development of the region 

surrounding the Med icine Lak e H'ghl and and extend i ng 

toward Mount Shasta . To date, all he samples from 

wi hin the region covered by Fi gure l are less than 

wo mi lion years old. The oldest l avas are the 

basa l tic rocks of Timber Mounta i n volcano, wh i ch is 

l ocated jus off Fi gure l approx i ately 9 miles 

sou heast of Lava Beds Nat i ona Monument . These 

l avas range from 1. 7 to 1. 9 million years old . Thin 

f lows of low po assium high- alumina olivi ne thole iite 

a r e we l exposed along the porti on of Gi llem's Bl uff 

fa u t scarp wi hin Lava Beds Nat ional Monument and 

vary from 1. 0 to 1. 4 m.y . old . In add i tion, numerous 

olivi ne tholeii t e fl ows of nearl y i dentical 

pe rography and chemistry outcrop both around the 

peri phery and i ntercal ated with lavas of the Med icine 

Lake High land and range i n age from approximately 1.4 

m.y. old to Late Pl eistocene to Recent . Mov ing from 

the Hi ghl and westward there are approximately a dozen 

mos y andesi ic composite volcanoes within the Bray, 

Dorris a nd Moun Dome quadrangl es whose ages fall 

etw n 0 . 3 and 1. 3 m. y . old . It is within the 

contex of th 's volcanic hi story that the silicic 

gneou s a ti v'ty on the northern and western margins 

of he Highland must be viewed and i ts ori gin 

eventua l y expl a i ned . 

DATA 

K-A results 

The K/ Ar da a are shown i n Tab l e l wi h the exact 

ample locations depicted i n Figure l . Potassi um was 

dete rmi ned together wi th all he other major elements 

utilizing a lithium etraborate fusi on wh ich results 

i n a one i nch gl ass d 'sc . The actua l ana ysis was 
MT 

DOME 6 

/ ....... ·. ) 

' I 

LAVA BEDS 

NATIONAL 

MONUMENT 

/ ...... .. 

I 

i 

,... i 

CONTACT 

INFERRED CONTACT 

6 MOUNTAIN PEAK 

CALDERA RIM 

5 km 

[] RECENT SILICIC . . VOLCANICS 

ANDESITE TUFF 

. i D 

i ·- ··- ··- ··- ··- ··- ··- ·· - ··-··: 0 OLDER SILICIC t1@ VOLCANICS 

FLOWS EQUIVALENT 

OF ANDESITE TUFF 

41 °40 ' 

LITTLE 

6 HORSE 

PEAK 

6 BLACK 

MTN 

N 

1 

Figure 1. Geologic map of selected rock units from the Med icine Lake Highland area. The stratigraph ic column 

is based in part on K/Ar dating. 

performed usi ng an automated vacuum X-ray 

fluorescence spectrometer. Argon was determined by 

pipette-spi ki ng with 38Ar; the isotopic 

measurements performed with a MS- 10 mass 

spectrometer. This i nstrument di spl ays virtuall y no 

memory and no mass 36 background. The argon fusion 

was carri ed out on 3- 6 grams of 8-42 mesh fragments 

of who le rock wh i ch had been ultrasonically c l eaned 

i n dist ill ed water and alcohol , dried i n an oven and 

pre-baked in a vacuum at 250°C. The uncertainty i n 

the age i s estimated utilizing a 0.3% uncertainty i n 

the argon spike, a 2% uncertainty in sample 

homogene i ty, and an absolute uncertai nty i n the 

potassium of 0.01% K20. 

The silicic rocks dated in this i nvestigati on are 

from rhyo litic flows and domes. The contact 

relati onships between the domes and the surrounding 

vol canics gene rally provide rather inconclusive 

informati on. These rhyolites range from 0 . 43 to 0. 95 

m.y. old with relatively small uncerta i nti es . This 

spread of ages falls within the range previously 

quoted for the composite volcanoes l ocated generall y 

west of the Med icine Lake Highland . Any theory which 

attempts to explain the petrogenesis of th is region 

over the past t wo million years must come to gri ps 

with this s pace- time-compositi on re lationshi p whi ch 

ex i sts between these two groups of volcanic rocks. 

164 

The Andesi te Tuff i s the one ma j or i gnimbri t i c 

un i t i n the regi on and i ts area l di stri buti on i s 

out li ned i n fi gure l . In general , it i s a poorl y 

welded ash-flow tuff that compl etely lacks any 

strati fi cati on . However, wh ere the base is exposed 

( e .g. i n the southern porti on of the area now known 

as the "Bighorn Sheep Enc l osure" on Gillem Bl uff) 

substanti al fl atteni ng of pumi ce l ap illi has occurred 

together wi th si gn i fi cant l y more we l di ng . The wri ter 

has made three attempts t o radi ometri ca l ly date this 

unit usi ng a moderate l y we l ded sampl e, a poorl y 

wel ded sampl e, and a p agi oc l ase separate ( no 

potassium fe l dspar present ) . None of these attempts 

were successful due to l arge scal e atmospheri c argon 

contaminati on . Subsequent deta il ed fi eld work in t he 

area south of Dock We ll by Wa l ter (1 975) and myse l f 

(See porti on of fi gure l de li neated as "Flows 

equi va lent of Andesi te Tuff" ) l ed to the i sol ati on of 

several andesi t i c fl ows whi ch appeared to be the same 

age or sl ightl y younger than the Andesi te Tuff. 

Petrographi c and chemical studi es supported the 

mappi ng i nterpretati on wh i ch has been fu rther 

substanti ated by reconna i ssance mi croprobe mineral 

analysi s . These data i nd i cate that smooth 

compositi ona l gradati ons as well as a l arge measure 

of overl appi ng occur between the proposed equi valent 

l ava flows and the tuff wi th respect to the 

orthopyroxene, c li nopyroxene and pl agioclase 

felds par. In fa ct, the plagioclase phenocrysts wh i c 
TABLE 1. Analytica l results of K-Ar dating.a 

Sample 

Number 

Map Location 

Number 

Rhyolite domes 

ML49 18 

S~l 17 

5681 15 

0. 0269 

0. 0412 

0. 0443 

Lava flow equivalent of Andesite Tuff 

263Cl 12 0. 0065 

Earliest Medicine Lake volcano lavas 

366W 

SM25c 

A 

8 

0. 00115 

11 . 62 

68.04 

5. 55 

3.18 

2.12 

0.74 

4. 42 

4. 63 

3.18 

2. 14 

0.85 

0.85 

K-Ar age 

(m.y.) 

0.43±0 . 04 

0.61±0 . 03 

0.95±0.14 

0 . 21±0.05 

0.09±0 . 05 

0.08±0.04 

aConstants used are: 4°K/total K 1. 167 x 10-4

; 

x

8 = 5.81 x lo-11

yr1 

bRad i ogen i c argon, 10-lO moles . 

cThe age is slightly different from that reported in Mertzman (1977) as a result of 

recalculation with the new K-Ar age constants of Dalrymple and Mank inen (1979). 

occur in both the flows and the tuff have vi rtually 

the same zoni ng pattern in addition to highly similar 

chemistri es. One of the proposed tuff-equival ent 

ava flows gi ves an age of 0.21 ± 0.05 (Table 1) . 

I submit that this age closely reflects the t i me of 

eruption of the Andesi te Tuff and, at the very worst , 

reflects its mi ni mum age. 

The Andesite Tuff controls the topography on the 

northwest side of the Med ici ne Lake Highland onto 

whi ch the earliest of the Med icine Lake basalti c 

andesite flows were extruded . Two strati graphically 

equi va l ent lava flows, from the northwest and the 

southwest flanks of t he Medicine Lake volcano (B and 

A respecti ve ly ·n figur e 1) have given ages of 

0.08±0 . 04 and 0.09±0 . 05 m. y. old . Since these 

flows represent bo h the initial eruptions of the 

~dicine Lake volcano as wel l as the bulk of the 

extrusi ves wh ich compose the Highland (equiva lent to 

he shield-forming platy olivine andesite of C. A. 

Anderson, 1941) the conclusion can be d awn tha the 

entire 900- 000 m thickness of lavas related to the 

edicine Lake volcano (See Mertzman, 1977) has 

accumulated over the past 100,000 years. A corollary 

of this conclusion is that the caldera-forming event 

must be <100,000 years old; unfortunately no 

minimum age can yet be assigned since no 

rampart-forming andesite , wh ich represen s syn-to 

post-caldera volcan ism, has her tofore been 

radiometrically dated - a void which will be filled 

over the next several months. Two additional points 

are worthy of no e : since the stratigr aphic 

thickness of the tuff noticeably increases in a 

165 

southeasterly direction and yet it does not outcrop 

on the eastern and southern margins of the High l and, 

i t seems safe in assuming that the vent area for the 

tuff lies buried beneath the younger andesitic lavas 

of the Med icine Lake volcano. Secondly, the gross 

age distinctions wh ich exist between the rhyolitic 

domes and flows and the tuff-equivalent andesi tic 

l avas on the northwest margin of the Highland, which 

differ by a factor of 3 to 4, indicate separate 

pul ses of vol canic activity rather than the rhyolite 

and the tuff being time correlative . The latter 

i nterpretation was suggested as a possibility by C. 

A. Anderson (1941) . 

Chemistry 

Major and trace element chemist ry was performed 

by X-ray fluorescence spectrometry and the resulting 

data together with CIPW norms are reported in Tables 

2 and 3; the exact sample locations are depicted i n 

Fi gure 1 . For trace element analysis one gram of 

dried whole rock powder was mixed with 0 . 5 g of 

microcrystalline cellulose and pressed i nto pellets 

using a course grade of ce llulose as a back i ng. 

Trace element data reduction was achieved by the mass 

absor ption correction method outl i ned by Hower (1959 ) . 

Ana l yses of the Andesite Tuff are f ound in Table 

2; typically these samples have relatively large loss 

on i gnition va lues, ranging from 1 .87 to 4. 52% by 

weight, a point wh ich encouraged the writer to 

calculate the CI PW norms on t he basis of recalculated 

ana lyses deri ved by normalizing to 100 percent 
Major and trace elemen ch mis ry ogether 

with C.I.P . W. norms of he Andesite Tuff. 

The exact sample locations are dep ic ted i n figur e l. 

TABLE 2. 

Map location number 1 2 ~ 5 6 7 8 

Sample number SW236 SM107 SW253 SM97 SM72 27601 WH67 SM88 

Si02 59 .26 60 5 60 .95 61 . 02 61 . 06 61.29 61. ~6 

Al

2

0

3 2~ 15 .73 17 .00 15 9~ 16 .66 15 .77 15 . 27 

FeO 3. 10 0. 91 l. 69 l. 2 1.92 l. ~1 l. ~6 

Fe2o

3 

3. 28 88 36 53 3. 56 26 3. 88 

Ti 02 1.03 0. 90 0.95 0. 92 0. 90 0 . 90 0. 86 

MnO 0. 12 0. 11 ~ 0. 11 0. 10 0.11 0. 10 0. 

MgO l. 98 1.86 1.90 l. 89 l. 7~ 2. 28 l. 65 

CaO 3. 83 4. 08 3. 71 19 9~ ~. 08 3. 76 

Na2o 55 27 67 ~ .60 . . 72 79 . 71 

K20 1.53 2. 28 2. 17 2. 28 2. 36 2 .82 2.46 

P205 0.22 0. 20 0. 19 0. 20 0. 21 0. 42 0. 20 

LOI * 3. 26 3. 32 2. 68 3. 19 ~ l. 87 .?~ 

Tota l 00 ~0 98 .99 100 . 41 100 .29 100 . 07 100 . 00 98.79 

- - ----·- -----

Q 16 . 17 17 .08 15 .58 15 ~8 ~. 73 12 8~ 16 . 26 

c 2. 74 0. 69 

Or 9. 31 1~ 08 13 . 12 13 .88 1~ 35 16 .98 15 . 17 

Ab 39 . 63 37 . 77 ~0 ~3 ~0 08 ~ 1.10 . 30 ~1. 0 

An 18 . 09 17 .79 17 .57 16 .59 17 .81 13 ~5 13 8~ 

Di l. 57 2. 71 0 .71 .~ 3 3 . 33 

Hy 6.63 11 8~ 3. 59 ~.13 20 2. 7~ 2.88 

MT 4.90 0. 71 3. 22 2 . 3~ 02 2 . 34 2.65 2.20 

Ilm 2. 01 . 79 1.85 1.80 l. 76 l. 7~ l. 70 l. 03 

Hm 61 2~ 3. 05 0. 89 2. 73 2 . 22 1.25 

Ap 0.53 ~9 0.~5 ~8 0.50 0. 99 0.49 0. 32 

----

TR ACE ELEMENTS (p. p.m. ) 

Rb 30 5~ ~6 5~ 51 72 64 68 

Sr 373 389 373 387 39~ 377 360 293 

Ni 6 2 6 4 2 6 5 

Zr 275 262 260 255 263 2~2 273 213 

Ba 570 552 640 588 622 523 571 656 

y 33 26 23 17 24 31 21 25 

v 115 101 99 121 106 72 94 30 

Rb/Sr 0.080 0. 139 0. 123 0.1~0 0. 129 0. 191 0. 178 0. 232 

K/ Rb ~23 351 392 351 38~ 325 319 333 

*Loss on ignition 

166 
TABLE 3 · 

MaP locat i on number 

Sampl e number 

Si02 

Al

2

o

3 

FeO 

Fe2o

3 

Ti 0

2 

MnO 

MgO 

CaO 

a

2

o 

K

2

0 

P205 

LOI 1 

Total 

-Q -

c 

Or 

Ab 

An 

Di 

Hy 

Mt 

1m 

Hm 

Ap 

Rb 

Sr 

Ni 

Zr 

Ba 

Rb/Sr 

K/Rb 

' Loss on i gn i t ion 

ND:not detect ed 

Ma j or and t rac e el ement chemi s t ry together wi th 

C.I . P. W. norms of the lava fl ow equ i va l ent of the Andesi te Tuff (9-12) . 

Also i nc l uded a re si x ana l yses ( 13-18 ) of ol der si l i cic vol can ic rock s . 

9 

5481 

59 .55 

16.50 

4 . 36 

l. 43 

o. 74 

0. 10 

3. 16 

5. 56 

3.58 

2. 04 

0. 19 

1.49 

98 .70 

13. 30 

12.40 

31.16 

23. 59 

2.89 

12 .63 

2. 13 

l. 45 

0. 45 

56 

438 

30 

241 

575 

33 

99 

0. 128 

302 

The exact sampl e l ocati ons are depic ted i n figure l . 

10 

ML57 

60 . 50 

16.7 1 

1.88 

4.72 

0.9 1 

0. 12 

2.66 

4.88 

4. 18 

2. 03 

0.17 

1.48 

100 . 24 

15.15 

12 . 15 

35 . 81 

21. 10 

l. 79 

5.88 

3. 86 

l. 75 

2. 12 

0 . 40 

48 

412 

14 

229 

528 

30 

92 

0 . 11 7 

351 

11 

259Cl 

60 . 59 

16 . 45 

3.74 

1.68 

0. 69 

0.10 

2.89 

5.25 

3.69 

2 . 36 

0.19 

1.21 

98 .84 

14 . 09 

14 . 28 

31 . 98 

21.87 

2.89 

10 .60 

2 . 49 

l. 34 

0.4 5 

59 

415 

19 

212 

523 

24 

96 

0. 142 

332 

12 

263Cl 

60 .84 

16 .98 

4. 56 

l. 41 

0.79 

0. 11 

3. 45 

5. 74 

3. 85 

2. 14 

0. 24 

0. 72 

100 . 83 

11.59 

12 . 63 

32.54 

22 .71 

3 . 43 

13 . 01 

2.04 

1.50 

0. 56 

i3 

ML55 

71.08 

14.25 

0. 70 

l. 78 

0. 32 

0. 07 

0.53 

l. 74 

4 . 73 

3. 19 

0.08 

0.98 

99 . 45 

27 . 48 

0. 04 

19 . 14 

40.65 

8. 24 

l. 34 

l. 58 

0.62 

0. 72 

0.19 

14 

47Bl 

71.30 

14 . 32 

1.07 

l. 78 

0.44 

0. 08 

0. 42 

l. 92 

5. 10 

3. 02 

0. 11 

0. 95 

100 . 51 

26 . 09 

17 . 93 

43.34 

7. 30 

1.21 

0. 49 

2 . 44 

0. 84 

0. 10 

0. 26 

TRACE ELEMENTS (p . p.m. ) 

54 77 67 

433 176 234 

28 ND 3 

245 

542 

19 

106 

0. 125 

329 

167 

217 

737 

28 

10 

0. 438 

344 

233 

728 

20 

16 

0.286 

374 

15 

5681 

71.47 

14 . 48 

0 . 91 

1.63 

0. 31 

0.06 

0. 51 

l. 66 

4.78 

3.18 

0 . 09 

0.87 

99 . 95 

27 .68 

0 . 37 

18.97 

40 . 82 

7. 72 

1.28 

2 . 25 

0. 59 

0. 09 

0. 21 

63 

131 

3 

194 

729 

18 

10 

0 . 481 

419 

16 

SM38 

71.99 

14. 11 

0. 60 

2.23 

0. 45 

0. 05 

0. 34 

1.24 

4.87 

4. 28 

0.08 

0.36 

100 . 60 

24 .63 

25 . 23 

41.11 

3. 99 

1.27 

0. 26 

0. 79 

0. 85 

l. 68 

0. 19 

123 

91 

ND 

367 

790 

48 

22 

1. 352 

289 

17 

SM5 l 

71.96 

12.93 

0.82 

0.42 

0. 21 

0. 04 

0. 08 

0. 86 

4.06 

4. 63 

0.04 

0. 29 

99 . 34 

31.81 

27 . 62 

34 . 68 

3. 42 

0. 55 

0. 81 

0. 61 

0. 40 

0. 09 

128 

70 

ND 

189 

664 

54 

6 

1.829 

300 

18 

ML49 

76.38 

12.13 

0 .70 

0.38 

0. 05 

0. 04 

<.02 

0 . 46 

4. 46 

4.42 

0.01 

0.44 

99 . 47 

32 . 92 

26 . 38 

38 . 11 

0. 03 

1.83 

0.07 

0. ?6 

0 . 10 

0.02 

148 

2 

ND 

130 

85 

99 

ND 

74 . 0 

248 
anhydrous t otals . Ca s in t his i gh i~t all y all 

he tuff samp es become more da citic than andesi ic 

i n bulk composi i on. The on ly add i t i ona l face of 

their chemistry worthy of hi gh li ghti ng at th i s t i m 

i s that three of e ight samples are pera l umi nous. 

Comparison of the chemistry of the tuffs with that of 

the four samples for the proposed equ i va lent 

andesi ti c lava flows yi elds a pattern of l ower SiO 

and Na20 and higher MgO and CaO for the lavas. 

Also, the percentage of phenocrysts is higher in the 

lavas than in the tuff samples (See Table 4) . These 

trends are analogous to those reported from numerous 

other ignimbritic areas (e .g., Lipman and Others, 

1966; Ratte and Steven, 1967; Christiansen, 1979), 

further substantiating this writer's claim of a 

co-genetic relationship linking the lavas and the 

andesitic tuff. 

The rhyolitic volcanics form two distinct groups 

on the basis of their Si02 contents: a low Si02 

TABLE 4 

Modal analyses ( volume percent ) . The data reported 

are average values (8 and 4 samples, respecti vely ) 

which cover the enti re range of petrographic 

variation wi thin both the Andesite Tuff and the 

lava flows proposed as being equivalent to the tuff. 

Mineral Andesite Tuff Tuff equivalent lavas 

Olivine Trace Trace 

Orthopyroxene 3. 2 3.9 

Clinopyroxene 1.1 1.8 

Amphibole Trace 

Opaques 2 . 3 1.2 

Plagioclase 5. 2 21.8 

Groundmass 89.3 71.3 

group (71-72%) and a high Si02 group ( >74 . 5%) . 

Exami ni ng he age and chemi ca l da t a concurrentl y 

produces a noteworthy correl ati on be ween decreasi ng 

age and i ncreasi ng Si 02 content . Further 

documentati on of th is emergi ng pattern i s presently 

underway. Whether or not th is age-composi t i on trend 

can be genet i cally l inked to a vi abl e crysta 

fracti onation mode l awaits mi nera l compositi on data 

concern i ng phenocryst-forming phases, thus enabl i ng 

some quanti tative mode l l i ng to be performed. It is 

a l so i ntri gu i ng to note that the chemistry of these 

ol der rhyol i tic extrusi ves does not mimic that of the 

Recent silic i c g l ass f l ow acti vi ty i n the Med i ci ne 

Lake High l and (Ei chelberger, 1975; Heiken, 1978; 

Mertzman, i n press) . Table 5 presents average 

chemical ana l yses for the three Recent g l ass f l ows . 

It is ev i dent that these data form a third group, 

i ntermed'ate between the two groups defi ned by the 

ol der si l icic vol can i sm . Suffi ci ent chemistry is 

168 

cur en l y a vailabl all p rform d wi h s m 

analy ical proc du res , o pro ably 

in ufficien s mp li ng a nd a naly ic l unc rtai nt y as 

po en ti a l f a tors produci ng h cl ump- Ji k ater~ 

r ather h n a conti nuum o f l var ia t ion. 

Wh the r or no t th is ol der (or episod es) of 

silicic volcan ism is monog ne ic or pol y n t ic , s: 

esearch pr oblem currentl y und r study u i lizi ng 

micr oprobe and iso epic t echn i qu 

TABLE 5. 

Major and trace element analyses of three Holocene 

glass flows from the Med i c i ne Lake Highland. (n : 

number of samples analyzed) . 

Little 

Glass Glass Northwest 

Mountain Mountain Glass fl aw 

n 5 4 3 

Si02 73.4 1 73.57 73. 23 

Al 2o

3 13 . 62 13.68 13 . 56 

FeOT 2 . 13 2.1 2 2. 05 

Ti02 0. 27 0. 27 0.28 

MnO 0. 03 0.04 0.04 

MgO 0.35 0 . 22 0. 30 

CaO 1.29 1.20 1.25 

Na2o 4. 10 4.11 4. 21 

K

2

0 4.28 4. 29 4. 28 

P205 0.03 0. 05 0. 05 

LOI * 0 . 40 0.34 0.49 

Total 99.91 99 . 89 99.74 

Rb 159 162 154 

r 107 113 108 

Ni ND ND ND 

Zr 229 215 223 

Ba 74 1 728 722 

v 6 8 3 

Rb/Sr l. 486 l. 434 l. 426 

K/Rb 223 220 231 

*Loss on i gnition. 

ND = not detected. 
~ 

Anal y i ca l da ta concern i ng the o der s ilic i c 

agma ic ac · vi y on he nor he rn flanks of Medi c i ne 

ake Hi gh and is presented . Av a il able K/ Ar r esu ts 

ive a 0 . 43 t o 0 . 95 m. y . range for the rhyol iti c 

0 can ism which c an be di vi ded chemi ca lly i nt o l ow 

si ica (71- 72%) and hi gh siU ca ( >74. 5%) groups . 

Both of hese g oups are clearl y di sti ngui sha bl e from 

he Hol ocene gl ass flows wi th respect t o chemistry. 

he area ex tent a nd chemical varia bi lity of t he 

And esite Tu f f is outli ned. The past i na bi lit y to 

rad iometrica lly da t e the time of erupt i on of t he tu ff 

has tentati ve l y been ci r cumvented by a na l yz i ng one of 

several lava fl ows wh i ch we r e erupted concurren tly 

wi h or ve ry shortly after the tuff. The age of t he 

equi va lent flow i s 0 . 21 ±0.05 m. y. ol d, a date that 

cl earl y es tab lishes the erupti on of the t uff as an 

even t preced i ng the deve l opment of the Med i c i ne Lake 

andesi t ic volcano and, therefore, not correl ati ve 

with t he format i on of its summit ca l dera . 

ACKNOWLEDGEMENTS 

I thank t he Research Corporati on for a Cottre ll 

co l ege Sci ence Grant, the Frank lin and Marsha ll 

committee on Grants, and the Ameri can Ph il osoph i cal 

Soc i ety for f i nanc i al support of the f i e l d work. I 

a so thank Dr . James Aronson of Case Western Reserve 

Un i versity for access to hi s K/ Ar l aboratory as we ll 

as for his t i me l y advi ce . The automated vacuum x-ray 

fl uo rescence spectrometer was pu r chased with grants 

froo the Pennsy l vani a Sc i ence and Engi neering 

found ation, the Nat i ona l Sc i ence Foundati on, the Gu l f 

Oil Company, and the Fl e ischmann Foundati on . 

REFERENCES 

Anderson , A. T., 1976, Magma mi xi ng, petrol ogi cal 

proc ess and vo l canol og i cal tool ; Journa l o f 

Vol canology a nd Geotherma l Research, v. 1. p. 3- 33. 

Ande rson, C. A. , 1941, Vo l canoes of the Med i cine Lake 

gh :~ , Ca lifornia: Un i versity of Ca l iforn i a 

Pub li cations, Bull eti n of the Department of 

Geolog· ca Sc i ences, v. 25, no. 7, p. 347 - 422 . 

Christi ansen, R. L., 1979, Coo ling uni ts and 

composi t e s hee t s i n re l ation to ca l dera structure: 

Geol ogi cal Soc i ety of Ameri ca Spec i a l Paper 180. 

Da rympl e , G. B. , and Mank i nen , E.. A., 1979, Rev i sed 

geomagneti c pol ari ty t i me scal e for the i nterva l 0- 5 

m.y. B. P.: Journa l of Geophysica l Research, v. 84, 

p. 615- 626 . 

Ei chelbe rge r, J . C., 1975 , Ori gi n of andesi te a nd 

dacite : evi dence of mi xi ng at Gl ass Mounta i n in 

Cali forn i a and at other ci rcum- Pac i fi c volcanoes: 

Geologi ca l Soci ety of Ameri ca Bu ll et i n , v. 86 , p. 

381-1391 . 

He i ken , G. , 1978 , Pli ni an- ty pe erupt i ons i n the 

Medicine Lake Hi gh l and , Ca lif or ni a , a nd the na tur e of 

the und er l yi ng magma : Journa l o f Volcanol ogy a nd 

Geotherma l Research , v. 4, p. 37 5- 402 . 

Hower , J . , 1959 , Matri x correcti ons i n t he x-ray 

spectr ograph ic trace e l ement ana l ysi s of roc ks a nd 

mine rals : Am eri can Mi nera l ogist , v. 44 , p. 19- 32 . 

169 

Li pman, P. W. Chris i ansen , R. L. , and O' Connor, J. 

T. , 1966 , A composi iona l ly zoned ash- f l ow shee t i n 

southe r n eva da : U. S. Geologica Su vey Professional 

Paper 524- F, 47 p. 

Macdona l d, G. A. , 1972 , Vo lcanoes Prenti ce- Ha ll , New Jersey, 510 p. 

Mertzman , S. A. , 1977 , Th e petrol ogy and geochemi stry 

of th e Medi ci ne Lake volcano , Ca l i f ornia : 

Con t r i buti ons to Mi neralogy and Petrology , v. 62 , p. 

221- 247 . 

Me rt zman, S. A. , i n pr ess, The petrogenes· s of Recent 

silicic magma tism i n the Medici ne Lake Hi gh land: 

evi denc e from cognate i nc l usi ons found at Li ttl e 

Glass Mounta i n, Ca li forn i a : Geochimi ca et 

Cosmoch i mi ca Acta. 

Nobl e, D. C. , 1979, Speculations on the ori gi n of the 

Med i c i ne Lake cal dera : Oregon Department of Geology 

and Minera l Industries Bu l leti n 65, p. 193 . 

Ratte, J. C., and Steven T. A., 1967, Ash f l ows and 

rel ated vol can i c rocks associ ated with the Creede 

ca l dera, San Juan Mounta i ns, Colorado: U. S. 

Geol ogi cal Survey Professi ona l Paper, 524 -H , 58 p. 

Wa l ter, R. C. , 1975, Geol ogy and petrol ogy of the 

northwest portion of the Medic i ne Lake Highl and, 

California: Unpub. B. A. thesi s, Franklin and 

Marshall Coll ege. 

Wi ll iams , H. and McBirney, A. R., 1979, Volcanology: 

Freeman , Cooper and Co . , San Franci sco, 397 p. 
170 
S RFACE STR CT RE OF LITTLE GL SS ,tO . TAL 

Jon H. Fink 

Geology Department, Stanford niversity, Stanford, CA 94305 

(Pr sent address : Depa rtment of Geology , Arizona Stat niversity , Tempe Arizona 85281) 

The purpose of these two stops is to observe and 

interpret the structural re lations exposed at the 

surface of a very young rhyolitic obsidian flow . 

Before surface structures can be discussed howev er , 

he flow stratigraphy and the origi na l attitudes of 

foliations must be determined . Unfortunately these 

Holocene obsid'an flows do not have any well exp osed 

cross sections so we are limited to observations of 

now fronts and to comparisons with older, more 

d)ssected flows . At these stops we will first define 

he stratigraphy and d iscuss the attitudes of 

foliations as seen in flow fronts; then we will go 

on o the upper flow surface and see the variety of 

structural re la tions exposed there . 

l m SURFACE BRECCIA 

5 m FINELY VESICULAR PUMICE 

15m 

9m 

2m 

3m 

OBSIDIAN 

COARSELY VESICULAR 

PUMICE 

BASAL BRECCIA 

TEPHRA 

Figure 1 . Schematic cross section through a 35 m 

hick rhyo i ic obs ' dian flow a ed on 

observation of Lit le Glass Moun ain 

and dissect d flows in ew Mexico and 

Lipari, Italy . 

Stop Dl - Nor heast lobe , Lit le Glass Mountain 

Before w can interpret he deformed flow 

structure w must be able to recogniz he undeformed 

structure . This stop offers an oppor uni y to 

observe he different units wh ich make up the flow 

171 

Figure 2 . Flow fron , Li tle Glass Mounta in . 

Photo· foliations; interpre ation . 

Complex y fo ded coarsely vesicu ar 

pumice (c) overlain by obs'dian (o) and 

finely vesicular pumice (f) . Base 

obscured by talus (T . 
stra ig raphy . Firs we no e he layer of ephra 

which covers he ground urrounding he flow . 

Isopach maps indica e tha the source of this tephra 

is under Little G ass Mounta in (Heiken , 1978) . 

If we l'ook a blocks wi hin he talus pile w can 

distinguish princ i pal types based on r.olor nd 

vesicularity: black glassy obsid~an, brown to 

green'sh grey coarsely vesicul ar pumice, and whi ish 

grey f inely vesicu ar pumice . These hree rock types 

have uniform chemical composition (73% Si 02) and 

all exh 'b 't f ow ayering, but they differ in 

density, with the obsidian havi ng he highest . In 

the flow front above the talus pile these three rock 

types appear as coherent units, with foliations 

generally conforming to the contacts between units. 

[I] Talus 

Bo h i n he flow fron s and on the upper flow 

surfac he coarsely and fi ne l y vesicu ar pumice 

uni are nearly ah.rays separa d by obsidian; they 

do not a ppear in conformable contact . Furthermore 

he coars pumice nearly al ways underlies h ' 

obs'd'an, so hat the apparent stratigraphic seque, 

is ( upward ) : tephra coarsely vesicular pumice, · 

obsidian, fi nel y vesicular pumic . Above the teph~ 

and on op of he flow ar brecc ias comprised of 

blocks o f he other three flow units (Figure 1) . 

In older more dissected rhyolite and rhyolitic 

obsidian flows, f oliations near the base are 

generally horizontal whereas those near the top are 

more nearly vertical . The depth of the transition 

from vertical to horizontal flow-layering varies f 

0 25 50m D Coarsely ves1cular pum1 ce 

D Obsidian 

0 Finely vesicular pum ice 

t 

N 

172 

Figu re 3 . 

Map of part of 

northeast lob , 

Little Class Mountain. Compressional fold axes marked 

by 1 ines with 

arrows; axes of 

fractures mark d by 

lin s with on or 

two cross bars 

(two bars indicates 

sepa rations across 

frac tur e of more 

than 5 m) . Dotted 

lines indicate 

margins of f racture 

surfaces . 

-
fl ow to fl ow and within a gi ven flow, however it 

genera llY l ies with i n the top 25 % of the flow . Thus 

with i n the strati graphy seen in Li ttl e Gl ass Mounta i n 

the contact between the coarse pumi ce and obsi di an 

layers may be consi dered to be ori gi na lly 

horizonta l. Mapping the ori entati ons of this contact 

on th e fl ow surface i nd i cates the deformation . 

Figure 2 shows the three units exposed i n a fold 

wi th i n t he f l ow front . Th is fo l d and others like it 

Figure 4 • Compressi ona l folds i n fl ow front, 

northwest lobe, Li ttle Gl ass Mountain: 

a) overturned fo l d, plunging into 

flow. 

b) closer view of sam fold . 

tre ched vesicles show 

x ension di rection associa ed 

wi h foJd i ng . 

173 

are also exposed on the upper f l ow surface . 

About 20 m south of th i s fold i s an antic l i ne of 

coarse pumice wh i ch appears to be dissec t ed by a 

valley 3-4 m deep . Later we will see that these 

va l leys wh i ch trend perpend i cu l ar to the f l ow front 

are fractures which commonly bisect anti cli nes c ored 

by coarse pumice . If we climb onto the flow through 

this fracture and cli mb the northern wa ll , we will be 

able to see much of the area covered by the map i n 

Figure 3. 

Figure 5 . o·ap·r rise accompa nied by fracture of 

surface . otice that f ractur e a xis 

corr es ponds to earlier anticlinal axis 

and hat f · ne pumice be ween he 

diapirs forms tight s ynclines . 

~: 
.. 

1 Ta lus 

L Coarsely vesicular pum 1ce 

N f Obs1d1an 

D Fmely ves icular pum1ce ~( 

Figure 6. Map of part of northwest l obe, Li ttl e Gl ass Mounta in . Symbols same as Fi g. 3. 

The most prominent structure i n th is part of the 

map area is a fracture about 2 m deep wh i ch cuts 

through coarse pumice. One can easi ly see that the 

foliation patterns in the two opposi ng wa lls 

correspond exactly . In addition, the ent i re coarse 

pumice area is enclosed by an area of obsi dian, wh i ch 

i n turn is surrounded by fine pumice . The fo l iati ons 

within the coarse pumice and i ts contact wi th the 

obsi di an defi ne an ant i c li na l structure wh i ch pl unges 

to the northwest (into the flow ) . 

20 m to the north is a seri es of severa l more of 

these pl ung i ng, fractured, coarsely vesi cu l ar pumi ce 

antic lines . The obsi di an adjacent to each anticli ne 

gets pushed l atera lly away from the fracture pl ane, 

and where two fractures para ll el each other the 

obsidi an and fine pumice may get sandwi ched into a 

tight synclinal confi guration. In Fi gure 3, these 

syncli na l axes para ll el to the fractures are marked 

by heavy dots. Not i ce that the separati on or 

s pread i ng across each fracture i ncreases toward the 

f l ow front, suggesting that these fractures form, at 

l east i n part, i n response to circumferenti al 

spread i ng near the front of an advanc i ng flow lobe. 

174 

The coars e l y vesi cu l ar pumi ce borderi ng each of 

the fractures i s fo l ded, with fo l d axes perpend i cular 

to the fracture axes. This creates a pattern of 

coarse pumi ce domes. In fi gure 3, the fo l d axes are 

i nd i cated by heavy li nes wi th arrows, whereas the 

fracture axes are marked by cross bars. These fo lds 

generally l ie perpend i cu l ar to the f l ow di recti on a 

are i nterpret ed t o be s urface f olds caused by 

compressi on, si milar to the ropes on pahoehoe basa l 

f l ows ( Fi nk and Fl etcher, 1978 ) . 

The basal posi t i on and re l ati ve l y l ow densi ty of 

the coarse l y vesi cu l ar pumi ce form the basi s f or 

grav i ty instability wi th i n the f l ow structure. The 

i nstab ility can gi ve ris e to regul arl y spaced di ap in 

of coarse pumi ce . On aeri a l photographs of both Bi 

and Little Glass Mounta i ns, such regu l arl y spaced 

areas can be clearly seen. Depend i ng on thei r 

positi on relati ve to the f l ow front these areas of 

coarse pumi ce may be subjected to subsequent fold ing 

or fracturi ng or both, resulti ng i n the compl ex 

structural pattern observed on th is portion of Li t t e 

Glass Mounta i n. 
I 

I 

N 

', \~ - ~ ~<'~ 

,sf ( .~ ~ I 

" 

t \ 

-+- 30 \ 

~ 

10 

Fi gure 7 . Map of large ridge adjacen 

r~ac ured ar a on nor hw s 

\ 

\ 

\ 

\ 

) 

/ 

" \. 

to 

lobe . 

Sop 12 - Northwest lob , Li l e Glass Moun ain 

I 

I 

A his locality we will see s i milar structures 

as before , but he re fractures are more developed. 

Al hou h he entire stratigraphic sequence occurs 

here , we will concentrate on structures found within 

a large area of coa rse ly vesi cular pumice . Th i s area 

has a broad doma l struc ure and is bordered by 

obsidi an . 

175 

The firs s ructure is loca ed abou 2/3 up he 

f ow front, 20 m south of the small park i ng area . 

Here coa se pumice and obsidian wi h very l arge 

vesicles (over 10 em i n ·arne er) form an overturned 

f old, whose axia p ane plunges in o the f l ow ( Fi gure 

4) . S re ching of vesic les i ndicates he extension 

direc ·on associated wi th fo ldi ng. Mi nera liza i on of 

he ve sicles occurred prior to th is fo l ding as the 

coa i ngs are a l so stre ched . Wi thin th i s fo l d the 

obsid"an dra pes over the coarse pumice , and hi gher up 

i n the front the obsidian forms a syncline cored by 

fi nel y vesi cular pumice . These f olds wi th axes 

parallel o the f l ow front conti nue on the upper flow 

surface . 

Climbing to any high poi nt on the upper surface 

one can see several 3-5 m deep fractures trendi ng 

normal to the flow front. Here again, examination of 

the fo liation patterns in the opposing walls shows 

the large amount of lateral separation associated 

with these fractures, up to 40 m i n some cases. The 

obsidian and f i ne pumice wh ich stratigraphically 

overlie he coarse pumice are genera lly upturned 

around the margi ns of th is area, but within the area 

they have been sandwiched into t i ght synclines and 

even "subducted" between adjacent fractures. 

Al though he cumu l ati ve extension associ a t ed with 

these fractures is over 100 m, this "subduction" 

could conserve the volume of the flow. Fi gure 5 i s 

an interpretation of the i nterac tion between upward 

rising coarse pumice a nd downward propagati ng 

fractures . 

Folds wi th axes normal to the flow di rection can 

also be s een i n th is area, and these extend laterally 

into the higher ridges to the south. Detailed 

exami nation of the structure of he highest of these 

ridges ( F"gure 7 ) shows that i t also is interrupted 

by a seri es of subparallel fractures, with separation 

i ncreasing toward the flow front, again due to 

extension associ ated with radial expansi on of the 

flow. 

Aeri a l photographs of th i s lobe (Figure 8 ) show 

several other areas of coarse pumice and obsidian 

with irregular spac i ngs along the flow direction. 

These have been interpreted as diapirs and the 

spacing has been related to the stratigraphic 

thicknesses and viscosities of the di fferent textural 

units . 

In summary, Little Glass Mounta i n, being nearly 

erosion-free, offers a rare opportunity to observe 

t he surface structure of a rhyolitic obsidian flow 

just as i t appeared upon coo i ng . The structural 

relations seen here could only be detected on older 

flows wi th great difficulty because erosi on and 

vegetation prevent continuous examination of 

fo liation patterns, and subsequent alteration 

obscures the textural differences originally present . 

References 

Fi nk, J. H. , 1978, Surface structures on obsidian 

flows: PhD thesis, Stanford University, 175 p. 

Fi nk, J. H. , Gravity i nstability in the Little Glass 

Mounta i n rhyolitic obsidian flow, northern 

California: Tectonophysics, in press. 
Figure 8. Photo, part of northwest lobe, Li ttle Gl ass Mounta i n. Lava moved from l eft to r i gh t . Da rk 

fractured areas are pri mari ly coarse l y vesi cular pumice . The l ocati ons of Fi gures 4 and 7 are 

indi cated . 

Fink, J. H., Surface fo lding on rhyoli te flows: 

Geo logy , in press . 

Fink , J . H. , and Fletcher, 1978, Ropy Pahoehoe: 

Surface folding of a viscous fluid : Journal of 

Volcanol ogy and Geothermal Research, v. 4, p. 

151- 170 . 

He iken, G. 1978, Plinian type eruptions in the 

Med ici ne Lake Highland, California and the nature 

of the underlying magma : Journal of Volcanology 

and Geothermal Research, v. 4, p. 375- 402. 

176 
HOLOCE E PL I IA TEPHRA DEPOSITS OF THE MEDICI E LA KE HI'HLA D, CALIFOR I A 

Gran t Heiken 

Los Alamos Scienti f ic La bo r ato ry , Geosc ie nces Di v ision , Los Al amos , Ne w Me x ic o 87545 

Co nden s ed from : PLI NI AN - TYPE ER UPTIONS I N THE MEDICI NE LAKE HIGHLAN D, CALIFOR I A, AN D THE NA TU RE OF THE 

UNDERLY I NG MAGMA , Jo ur . Vole . and Geo t he r m. Re s ., 4 , 375 - 402 (1978) . 0 Els evi e r Sc i. Pub . Co . , Amsterd am 

INTRODUCTION 

The Med icine Lake Highland i s a young volcano 

located 55 km east o f Mount Shasta , on the west e rn 

ma rgin o f t he Modoc Plateau . The vol cano is east of 

the north- south-trending chain of High Casc ade volcanoes and within a 100- km diameter depression filled 

wi th vol c anic roc ks of Plioc ene to Holocene age . It 

ov erlies a plateau consisting of late Tertiary tuffs , 

l ava flo ws and sediments, c ut by north- to northwest- trending normal faults (Po wers , 1932 ; Anderson , 

194 1) . The volcanic center is loc ated at the intersection of s e veral of these fault systems. 

The highland i s a 1 km thi c k, 25 km-diameter 

sh ield , composed of andesi tic and basa l tic flows 

( Ande rson , 1941) . There is an 8 km by 6 km, 100- 200 

m deep c a l dera at the shie l d summit . Noble ( 1969) 

believes that an andesitic pyroc lastic flow , interbedded with andesi t ic flo ws of the shield , may have 

been associated with caldera collapse . Andesitic, 

daciti c and rhyolitic flo ws were erupted along the 

caldera rim and partl y bury the wal l s. Construct io n 

of the shie l d and c a l dera format i on are believed to 

have occ urred ma i nly during Pliocene to Pleistocene 

time ( And e rso n , 1941 ) . During the time e lapsed 

be tween the earlier eruptions of andesite , dac ite and 

r hyolite and more recent activity , there was g l aciation of the highland and substantial soil devel oped 

( An der son , 1941 ) . 

Holoc ene a c tivity was bimodal, consisting of 

basal tic vol c anism , mainly on the shield flan ks and 

eruption of tephra and l avas of intermediate t o rhyolitic c anposition within and c lose to the rim o f the 

caldera . 

The purpose o f th i s paper is to present data on 

the Ho loc ene pumi c e deposits of the Medi c ine Lake 

Hi ghl and and t o speculate on their origin . Th ese 

de posi t s wer e erupted from Glass Mountain. a fl ow of 

rhyolite- dac ite on the northe ast rim o f the cald era 

and fran Little Glass Mountain , l oc ated a few ki l ometers west o f the c a l dera ( Fig . 1) . 

Age o f the Holocene de posits 

Underl ying Holoce ne rh yolitic and ba saltic t e phr a 

exposed in t he highland is a red or ye ll ow soil , 

developed on both flo ws and older te phra . Th is soil 

is estimated t o be mo r e than 15 , 000 yea rs old ( D. 

i ller , perso na l commun ica t ion . 1977 ) . The old est 

tephra ove r l yi ng the soil ar e f ine-gra i ned bas al t ic 

ash la yers near Little Glass Mo un tain and no rthwest 

of Glass Mountain. The y r a nge fr om 0 . 5 to 14 em 

th ic k and ar e r easo na bl y fr esh ( sider omel ane gl ass is 

well pr ese r ved) . I t is possi ble tha t these t e phr a 

are fr om Pai n t Pot Cr a t e r or some o f the yo ung er 

*Th e 1 ' C measurements we r e made by Iso t opes . Inc . a nd 

the sam ple was collected by C. D. Mi ll er from the 

northwest slo pe o f Little Mt . Ho ffman ( Fig. 1) . The 

sam pl e consi sts o f wood , leaves , bark a nd cone s . 

177 

cones in Lava Beds Nationa l Monum ent (Table 1) . 

Immediately north of Glass Mountain, the soil has 

devel oped on an older pumice bed ( Ch e ste r man , 1955) . 

Chesterman dated trees that were growing i n this soil 

and buried by younger pumice beds to indic ate the age 

of the Holocene Glass Mountain tephra. Overlying the 

lowest Holocene basal tic tephra unit are the pumice 

deposits of Little Glass Mountain , with a radiocarbon 

date of 1065 + 90 years*. As will be discussed 

later , the tephra are similar to those of Glass Mounta i n . Luckily , one of the more extensive tephra beds 

from Little Glass Mountain has a distinctive yellow 

hue . By following this bed it was determined that 

Little Glass Mountain tephra underlie tephra units 

erupted from Glass Mountain. The age of t he Little 

Glass Mountain flow is not known. but is believed to 

be only slightly younger than its tephra deposits ; 

they ma y have been part of the same eruptive 

sequence . 

Within the zone of overlapping Glass Mountain and 

Little Glass Mountain tephra , there is a thin (5 em) 

bed of basa l tic ash , believed to be from one of the 

young cinder cones on the northwest flank of the 

shield. 

The uppermost tephra beds in the highland were 

erupted from Glass Mount ain and have a radiocarbon 

age of 1360 + 240 years ( Chesterman , 1955) . The 

ana l yses were-of carbon from tree trunks buried by 

the Glass Mountain pumice falls . The Glass Mountain 

flo ws have been dated at 130 to 390 + 240 years by 

the 1 'C method (Friedman, 1968) ; these dates were 

based on wood from standing trees damaged by the 

flow. There is an inconsistency here : field evidenc e indicates that little time elapsed bet ween the 

pum i ce eruptions and extr usion of the flows at Glass 

Mounta i n , whereas radiocarbon dates i ndicate a hiatus 

of 1000 years. The older date of 1360 + 2 40 years is 

of t e phra overlying tephra dated at 1065 + 90 years . 

Tephra from Glass Mountain also overlie the very 

well-preserved High Hole Crater cind e r cone and Burnt 

Lava Flow . The youth of these basa l ti c vents is also 

i ndicated by the presence of tree trunks burned by 

t he flo ws, re st i ng o n the f low ma rg in s . 

I n summary , most Holocene silicic r ocks nea r the 

h i g hl and summit and the most youthful basal tic cones 

and fl o ws on the highland flanks we r e contemporary 

and may have erupted during t he l ast 1100 years. 

RHYOLIT IC TEPHRA FROM THE HOLOCENE ACTI VITY 

Litt le Gla ss Mountain 

Te phra from Littl e Glass Mountain form a highl y 

el onga t e e l l i ps e wit h its long a x is orie nted 

so uth we st-nor t he ast ( Fig. 2) . Measurable tephra 

la ye r s are presen t up to a distanc e of 25 km and 

i nd iv i dua l rhyoliti c lapilli possibly from Little 

Gl a ss Moun ta in have been found 50 km west-southwest 

o f t he ven t . ( The most distant lapilli were found by 

D. Mi ll er on the sl opes o f Mt . Shasta ) . The area 
Ba S • LI rLO• S W( .!11 1' 10 N£ 0 , ,.. 1' (l1 

c::=J I•S•lfiC CI .,. O(IIt CO,..[S 

M0L.0C(h( 0( P0$ 11 S 0 V(Iitl.Y I ... G A 

• tu - 0( ( L.OP(O SOil. . 8(l.I (Y(0 

TO I( • 11 00 T ( A .. S Ol.O 

-<...,.... / CALOflltA lt l lll I AJill n (lt$0H , 

r ''" '' 

PUWICE STONE 

WTN . 0 

&@PA INT POT 

~ '""'" 

l AVA 8( 0 $ N lt,f 

-----

0 

I 

I 

I 

I 

I 

__ I _) 

·~)o -~~----------------------~L-----------------~~~~~~~~----------~1 

121•4&' ,,..,,:;• 

Fig . 1. Distribution of Holocene deposits believed to be <1100 yea rs old in the Medicine Lake Hig land. 

The fissures near Crater Glass Flow are open and well pr eserved. Based on U.S . G. S. 15 ' topog aph' 

quadrangle ( Medicine Lake and Timber Mountain , Cal i fornia) , the geologic map o f Anderson (194) a fie l d work by the author . 

TABLE 1 

Age ~el a 

RhyollllC 

Tephra of Glass un a1n , •·c date (by . F. L• bby •n : Ches ennan, 1955) of 1360 : 240 and 1107 : 

380 yea~s. 

flows of age? 

ephra o Li le Glass un a1n , 

" Cage (by Isotopes. Inc . ) of 065 : 90 years . 

Basalt ic 

Bu~n Lava Flow and H1gh Hole C~ate~ . a we 11- p~ese Ned c 1 nde~ cone . 

II 11-p~eser ed basal 1c ephra of Pa1n 

Po c~a ~. o e~la1n by L• le Glass 

unta 1 n teph . 

So 1l developed on andes• e f lows and c• nde~s (L a e 1scons•n o~ early 

Tioga; 0 . Miller, pe~sonal C(lfTTilun ic auon , 1977) ,. 10 ,000 - 15 ,000 years B.P. 

178 
._ • ML - 25 (PU MICE PIT SECTION) 3. 7 km NE 

@ = SfM -II (PUMICE STONE Mtn. Sec.) 1.8 km SW 

• • OTHER MEASURED SECTIONS 

....- / 

/ 

/ -, // //// - -... / / / / \ • / ' , / 'l I ,, / / / //, I / • ' 

/ / / ~/ (o •• 

/ / / OE> ~) I I / 1 / >- 'f ' I 

I / /,1 ( (l) L'G ) / / • _,.. I / I 1 I 1 - . I / 1 • 

I / I \ . ,..-1/ I 

I I fo '----;.." I I I ~ / I 

/ I o"? • '~ ,~ • / • I 1 1 1 o· / I I I /' .-..--" 

I I / I / ---- ~ I I I I /I ,o ...... -- o J l /// l I I I / / // 0 3km 0 

"--!..._ 0 

I I • / / 1---'--L--i I \ -.t __ ., ' // { 

I \ _ ,' / ....... -' // 

/ 

// 

fig . 2 . Isopach map o f Little Glass Mountain (d ashed ) and Glass Mountain (solid) tephr a . Stippled 

areas are flows at the two vent areas. Dots are the locations of measured stratigraphic sec tions . The 

cross near Glass Mountain is the location of the thickest section measured. 

covered by measurable tephra layers is about 200 km 2 ; 

the tephra have a volume of 0 . 046 km 1 • 

ithin 3 km southwest of Little Glass Mountain, 

ther e are five tephra units , consisting of mainly 

reversely graded , pale gray pumice and lapill i- si ze 

b oc ks in co arse gray ash. 

These units range in thickness from 20 to 50 em 

and each contai ns between 5 and 20% lithic fragments . 

The onl y evidence for a hiatus within these units is 

a thi n (1-5 em) coating of orange-brown clay at the 

surface of the third tephra unit . Contacts be.tween 

all other un its exhibit no evidence of weathering or 

erosion . Between 3 and 8 km southwest and 6 km 

northeast of Little Glass Mountain , there are t wo 

tephra units; the lowest, overlying a well-developed 

soil , co nsists of a 0 - 24 em thick , mediLITl to coarse 

gray ash sometimes with a slight yellow hue ; the 

up per unit consists of a 13-64 em thick , generally 

reverse l y graded , lapilli-bearing coarse ash . It is 

possi ble that the break bet ween units 3 and 4 , near 

the vent c orresponds to the break between the t wo 

ephra units farther fran the vent. Beyond 9 km , 

onl y one tephra la yer is present . This tephra bed i s 

most readily correlated to the upper of the two 

e phra units, located closer to Little Glass t-1ountatn , on the basis of its greater thickness ( Fig . 3) . 

All of the tephra units erupted (rom Little Glass 

Mountain appear to have been deposited as air-fall . 

There appear to have been two periods o f explosive activit y at Little Glass Moun tain ; the only 

evidence for this , ho wever , is the cla y coating 

developed on the top of a tephra la yer ( f ig . 3) . The 

teph ra are overlain by the flo ws of Little Glass 

Mountai n , a rhyolite flo w wi Lh a volume of 0 . 3 km' . 

The flow has bur ied th vent area . 

Glass Mountain 

Pumi ce deposits from Glass Mountain form an 

trr gular ellipse , with the long axis tre ndi ng 

east- north ast ( Fig . 2) ; the deposits with measur able 

179 

thi c knesses cover an area of 320 km 2 and have a volume of 0 . 09 km 1 • Within an approx imate range of 3 km 

east and northeast of the summit of Glass Mountain, 

there are multi ple tephra bed s . Within a pumice pit 

loc ated 2 . 8 km east of the summit, e even tephra 

beds , with a c ombined thickness of 3. 7 m, overlie a 

red-brown soil devel o ped on cinders . The tephra 

units c onsist of reverse y graded or massive pumice 

block and lapilli-bearing coarse gray ash, 5-102 em 

thick . Several of the uppermost beds co nta in 10-30% 

lapilli and coarse ash-si ze lithic fragments (mainly 

angul ar obsidian fragments and cinders) . Sharp depositional contacts e x i st between these beds; there was 

no erosion or weathering at the bedding plane surfaces . Several tephra units exhibit a pale orangepink discoloration near the top of e ach bed; the 

alteration may be due to mild vapor-phase activity 

that left a thin film of hematite stain on pyroclast 

surfaces . Between a distance from the vent of 3-11 

km east, 3-11 km north and 1 ( ?) - 3 km south and 

west, there are two or three tephra beds with combined thicknesses of 22- 131 em . Both consist of 

rev ersel y graded or massive beds of gray medium-ash 

to lap ill i-si ze tephra. Due to the remarkable unifo rmity of the Glass Mountain tephra , it was not 

possi ble to corela e a ny o f these beds with units 

described near the ven t . Units 1, 7 and 11 of the 

Pumic e Pit section are only tentatively correlated 

with the more distant units, because they are the 

thic kest units. Beyond the range of mul tiple beds 

there is only one bed of massive coarse ash and 

la pilli ; this rapidly thins t o zero . 

All tephra beds from Glass Mountain drape the 

underlying terrain and exhibit no cur rent structures; 

there i s no ev ide nee for deposition by flow. After 

depo sition of rhyolite tephra , the Glass Mountain 

dacite-rhyolite flows were erupted along a northnorthwest-trending fissure. In addition to the main 

body of Glass Mountain there are , along the fissure, 

nine small domes . Associated with the domes are 
ASH, E:=J MEDIUM to COARSE 

LAPILLI 

~ PU MICE BLOCKS 

20 km SW 

lm 

0 

8km SW 

, VT7y-,·"T. · ,..,.. .-ORANGE CLAY 

2km SW 

COATING on 

PUMICE 

Fig . 3. Tentative correlation of tephra units along a line southwest from Little Glass 

may have been two episodes of e xplosive activity at Little Glass Mountain . 

untain . There 

small , c rescent-shaped pumice cones , consisting of 

angular , 2-45 em long pumice lapilli and blocks . 

Ot her silicic vents 

The Crater glass and Medicine Lake glass flows 

appear to be approximately the same age as Glass 

Mountai n and Little Glass Mount ain , but neither 

ex hibited much e xplosive activity. There were only a 

fe w lapill i or blocks erupted from these vents; there 

is no evidence for extensive tephra deposits associated wi th them . 

DISCUSSION 

Vents for the Holocene eruptions of silicic 

tephra and lavas are located along the oval caldera 

r im and along fissures parallel to the rim. Glass 

Mountain and associated small domes were erupted 

along a fissure trending northwest-southeast . Th e r 

is no clear ev idence that Little Glass Mountain was 

erupted from a fissure . There is, ho wever , an 

arcuate trend , concentric to the caldera rim that 

includes the Crater Glass Flow ( Fig. 1) and several 

e n-echelon open fissures. The f issures, with a total 

length o f several kilometers , do not appear to be 

associated with an y of the normal faults of the 

region and are believed to be an expression of rhyolite dikes that a r e near the surface , but d id not 

erupt . Crater Glass Flow was erupted f r om a segment 

of one of these fissures . 

It was suggested by And erson (1941) that the 

caldera at Medicine Lake collapsed along cone fractures that later acted as conduits for Pleistocene 

and Holocene silicic magma . This may indeed be the 

case ; e ruptions along the caldera rim and along fi ssures concentric to that rim may indicate the presence of several co ncentric , inward dipp ' ng cone 

sheets located near the surrrnit o f the Medi cine Lake 

shie d . Cone fractur systems are developed over 

plutons during intrusion and have been observed in 

many intrusive-vo lcanic com ple x es throughout the 

world (Anderson , 1937; Koide and Bhattacha r ji , 1976) . 

Cone fractures dip in ward , from the ground surface , 

at angles of 45 ° to nea r y vertical . If the models 

for cone sheet formation develo ped by Anderson and 

180 

Koid e and Bhattachar j i are corr ct , then cone fractures may e xtend downward from the sur f ace xpr ession 

of calderas and cone ntric fissures to a magma body 

that is considerably narrower than the calder a . 

The remarkable similarity and contemporaneity of 

the eruption at Little Glass Mountain and Glass Mountain , located 15 km apart , suggest that they wer e 

erupted from the same body of magma , possibly along 

cone fractures . Eruptions at the two vents have the 

following in common : ( 1) the same eruption sequence, 

consisting of -8~ tephra and 92-93% flo w; (2) identical major-element c ompositions for the tephra ; (3) 

nearly identical tephra character istics , as outlined 

earlier ; (4) similar ages ; (5) the lavas have identical trace element compositions (Co ndi e and Hayslip, 

1975) . The uniqueness of trace element data for 

individual magma bodies has led to the use of these 

data to 'fingerprint' tephra deposits for th purpose 

of correlation (e .g . , Borchardt et al. , 1971 , Howorth 

and Rankin , 1975) . 

The total volLJTle of Holocene silicic tephra and 

lavas is about 1.2 km 3 • On the basis of the smal 

volume of eruptive rocks, it is possible Lo make the 

inference , albeit weak , that the magma body is small . 

The caldera has a voll.llle of about 8 km 3 ( An derson, 

1941) . If the caldera volLITle is a reflection of the 

approximate size of the magma chamber , this is still 

a relatively sma ll chamber . Also , the caldera volune 

rn y reflect the earli r phase of ndesitic vol canism 

and not that of the Holocene silic1c eruptions . 

The ratio of 1: 9 for tephra to flows , observed at 

Glass Mountain and Little Glass Mountain also fits 

general model for small sili cic magma chamb ers 

developed by Smith (1976) . Interpretation of the 

structural setting and the petrology of the silicic 

tephra and lavas erupted in the Medicine Lake Highland dur ing Holocen e time supports the hypothesis 

that there is a small silicic magma body below the 

highland . Concentric cone fractures may have 

developed over the apex of a body located at a depth 

of 4- 8 km ; this depth is consistent with the model 

presented by Loide and Bhattacharji (1976) and with 

observed intrusive-volcanic complexes and elsewhere 

in the Cascade Range (e .g . Erikson , 1969) and Per u 

(Cobbing and Pitcher , 1972) . If it is true that only 
small cooling body of magma is present below the 

~ighland shield, a reassessment of the area as . a 

en ial geothermal area may be necessar y. Th1 s 

~ pothesis should b e kept in min during he 

d~ elopment. of the highland which has been classified 

as • known geothermal resource area ' (God win et al . , 

1971) . 

The tephra blankets fit a general description of 

deposlts having b een f ormed uring Pl inian or 

sub-Plinian er upti o ns . A Plin1an eruption is exceptionallY violent and ejects co pious pL.Ill ice (Escher , 

1

933 ; Wa l ker and Croasdale , 1970) . Tephra wer e 

deposited as air fa ll with no ev.idence fo: deposi ~00 

as pyroclasti c flows . An erupt1on of th1 s klnd f1ts 

the model outlined above of vesiculated rhyolite 

magma erupted from the vola tile- ri c h upper portion o f 

a small magma chamber . 

Deposition of tephra in beds may be due to 

i ndi id ual eruptions relate to time of refilling of 

a fissure , vent blockag e , or variat io n in wi nd 

in tensit y over the volcano. I n several tephra units 

from Glass Mountain and in all un1ts from Little 

Glass Mountain , co ncentrations of lithic fragments at 

the base of each bed indicates clea ring of vents o r 

erosion of vent walls during e x plosive activity . 

After the volatile-rich top of the magma cham ber 

was erupted as tephra , the bulk of the lavas with low 

(<20- 0$) vesi c ularities were erupted . 

CO NCLUSIONS 

(1) Ho locene activit y in the Medicine Lake Highland consisted o f basaltic eruptions on the flanks 

and eruptions o f silicic tephra and lavas near the 

s nmit . The rhyoliti c eruptions at Class Moun ain 

and Little Glass Mountain occurred during the last 

1100 years . 

(2) Air - fall tephra consists of ver y poor y 

sorted lapilli that are mostly pL.Illi ce pyroc lasts . 

The tephra falls formed e ongate ovals , e x tending 

northeast of Glass Mountain and south wes of Little 

Glass Mountain . The te phra deposits are similar to 

those deposited during Plinian eruptions . 

(3) Pumice pyroclasts are remarkably homogeneous 

and consist of blocky , angular forms . Most have 

elongate simple and compound vesicles and vesicularities of 45 - 60$ . All of the pyroclasts c~nsist of 

homogeneous rh yolite g l ass . 

(4) The Medici ne Lake Highland is located at the 

intersection of several normal faults ; a ' wea k spot ' 

that allowed pooling o f basal tic magmas needed for 

crustal mel't:.i ng or a ' conduit ' f or the rise of diapirs of silicic magma . 

(5) The contemporaneity and phys·cal and chemical 

similarit y of te phra a nd lavas e rupted at Glass Mountain and Little Glass Mo untain , located 15 km a pa r t , 

suggest t hat t he y were e r u pted from the same ma gm a 

bod y . The y ma y ha v e e r upted along cone fra ctures 

developed above the magma c hamber. The r elatively 

small vol ume of Holocene silicic eruptions , small 

caldera volL.Ille , and a ratio of 1:9 for tephra and 

flows suggest that the magma body had an appro x imate 

volL.Ille of between 2 and 8 km 3 • If this hypothesis is 

correct , the highland might have less potential as a 

geo thermal resource than was previousl y believed. 

(6) The blocky, angular pumice pyrocla sts may 

have developed by : (a) vesiculation and elongation 

of vesi cles by flo w within 1-2 km o f the surface 

vent ; ( b) cotm~inution and isruption of the brittle , 

vesic ul ated magma by an expansion , or ' re l ief' wave 

passing down into the vent from he magma-atmosphere 

inte rface . 

18 1 

ACK 0 LED ME TS 

Samuel Gallegos , Jr . was an able f1e d and laboratory assistant thro ghout the summer of 1976 . 

wish to thank D. Mann anJ T. Gregor y for t.h ir help 

as well . I benefited from discussions of the 

Medicine Lake tephra with J . Eiche berg r , B. Cro we , 

and C. D. Miller . Howe Williams and an anonymou 

reviewer provide usef revie ws of the manuscript. 

This research wa s s ppor ed by the Departmen of 

Energy, Division of Basic Energ y Sciences under contract W-7 405 -ENG- 36 . 

REFER£ CES 

And e rson , E. M., 1937 . Cone-sheets and ring-d ykes : 

The dynamical e xpl anation . Bull. Vo canol. , 1 : 

35- 40 . 

Bor c hardt , G. A., Harward , M. E. and Schmitt, R. A., 

1971 . Co rrelati o n of vo lc ani c ash eposits by 

a ctivation analysis of g ass separates. Quat . 

Res. , 1: 247 - 260 . 

Che sterman , C. W. , 1955 . Age of an o bsi ian flow at 

Glass Mountain , Siskiyou County , Californ ia . Am. 

J . Sci ., 253 : 418- 424 . 

Cob bing , E. J . and Pitche r , W. S . , 1972 . The Coastal 

Batholith of central Peru . J . Geol. Soc. London , 

128 : 421 - 460 . 

Condie , K. C. and Hayslip , D. L. , 1975 . Young b ' mod al vol c anism at Medicine Lake volc ani c cent r , 

northern California . Geochim . Cosmochim. A ta, 

39 : 1165 - 1178 . 

Erikson , E. H. Jr ., 1969 . Petrol og y o f he composite 

Snoqualmie batholith , central Cascade Mountains, 

Washington . Geol. Soc . Am . Bull . , 80 : 2213-2239 . 

Friedman , L. , 1968 . Hydrat10n rind dates rhyolite 

flows . Science , 159 : 878- 880 . 

God win, L. H. , Haigler , L. B., Rioux , R. L., White , 

D. E. , Muffler , L. J . P. and Wayland , R. G. , 

197 1. Classification of publi c lands valuable 

for geo hermal steam and associated geothermal 

resources. U. S . Geol . Surv . Circ . , 64 7. 

Howorth , R. and Rankin , P. C., 1975 . M ti-element 

characterization of glass shards from stratigraphicall y correlated rh yolitic tephra units . 

Chern . Geol ., 15 : 239 - 250 . 

Koide , H. and Bhattacharji , 1976 . Formation of fractures around magmatic intrusions and their role 

in ore localization . Econ . G ol. , 70 : 781-799 . 

Smith , R. L. , 1976 . Ash- flo w magmatism ( abstr . ). 

Geol. Soc . Am . Abst r . Progr . , 8 : 63 3- 634. 
182 
MECHANISM OF MAGMA MIXING AT GLASS MOUNTAIN , MEDICINE LAKE HIGHLAND VOLCANO, CALIFORNIA 

John C. Eichelberger 

Geosciences Division , University of California, Los Alamos Scientific Laboratory, Los Alamos, NM 87545 

ABSTRACT 

Mixing of basal tic and rhyolitic magmas at Glass 

Ho ntain appears to have been driven by vesiculation 

0

/ basaltic magma as it intruded ~ rhyolitic magma 

chamber. Rapid cool1ng of basal t1c magma formed a 

mafic foam whi ch floated and becam~ concentrated at 

the roof of the chambe.r . Foam-r 1ch lava emerged 

first during the erupt10n, and became the hybrid 

dacite of the distal end of the flow . The c .hamber is 

robably a relatively large volLme, long-llved fea-

~ure , lying within 10 km of the surface beneath the 

caldera. Eruption of similar lava at Crater Lake 

shortlY before caldera collapse supports this interpretation. This mechanism of mixing between silicic magma 

stored in a crustal chamber and basal tic magma 

feeding the chamber is controlled by initial water 

content of basaltic magma, and implies that dry 

basaltic magma would remain at the base of the 

chamber. The eastward change from andesi tic to b imodal volcanism in this portion of the Cascade Range 

may be due to an eastward decrease in water content 

of parenta l basaltic magmas . 

INTRODUCTION 

The largest and most imposing product of Holocene 

activity at the Medicine Lake Highland Volcano is the 

Glass Mountain lava flow (Fig ure 1). The most perplexing feature of this flow is its lithologic 

variety . The 1 kms of lava which comprises the flow 

ranges from dull , stony, porphyritic dacite charged 

with fine-grained mafic xenoliths to shiny, black, 

phenocryst-free and xenolith-free obsidian. The 

range in silica content is from 66 wt ~ to 73 wt ~. 

Anderson (1933) described the geologic setting 

and general petrologic features of the Glass Mountain 

Glass Mounta in Flow and Domes 

D RHYOLITE 

~ RHYOLITE WITH INCLUSIONS 

D BANDED LAVA (RHYODACITE) 

• DACITE 

D BRECCIA ZONE 

• SAMPLE LOC ATION 

+ VENT 

KILOMETER 

F1gure 1 . Map of Glass Mountain sC¥nple locations and lithologic units. Three digit numbers refer to 

sampl es discussed by Eichelberger ( 1975) . Single digit numbers designate sample sites for density measurements discussed in this paper . Modified from Eichelberger (1975). 

183 
I FERREO 

CALDERA 

C:J R( C£ S I IC IC A A 

- CROSS SECTION 

\ GLASS MOUNTAIN 

4:1 

I 

_..../ 

H IGH HOLE 

CRATER 

oil " ANOERSO (1 941) 

Figure 2. Map showing distribution of Ho ocene avas 

of the Medi cine Lake Highland . 

flo w. Additiona data on its age and history were 

provided by Finch (1928) , Anderson ( 94 ) , Chesterman 

( 1955) , and Friedman (1968) . More recently , Condie 

and Hayslip (1974) an Mertzman ( 977) presented geechemica and petro ogic data for Highland avas , and 

I ( Eiche lbe rger, 1975) stud·ed the compositiona 

zonation of the Glass Mountain flow as a means for 

understanding relationships among magmas o f the 

basalt-andesite-dacite-rhyolite family : Mapping and 

sampling of lava streams within the flow revealed 

that the eruption was continuous and , in genera , 

proceeded from homogeneous da ci te through highly 

hete r ogeneous banded rhyodacite to rhyo ite. This 

suggested a canpositional zonation of the parent 

magma body in whi ch dacitic magma graded down ward to 

rhyolitic magma . Electron microana ysis of phases 

within the dacite indi c ated that most of the phenoc rysts, An 75 plagioclase , Fo 80 olivine , and o 1 En 

Fs 1 & augite, were derived from disaggregation of the 

cr ystal-ri ch basalti c xenoliths . The dark acitic 

bands of the heterogeneous rhyodacite are streams of 

basaltic debris from disintegrating xeno iths . This 

process was more advanced in the dacite into which 

the rhyodacite grades, yie d ' ng an intermed ' ate magma 

that was megasco pi c a ly homogeneous except f o r residual xeno liths. Since this ybr · magma was concentrated at the top of t e c amber , and since the Highland shield through whi ch the magma passed contains a 

hi g h pro po r t · on o f m a f i c 1 a v a f o ws and c i n d e r cone s , 

it was concluded that mafic material had s o ghed off 

the roof of the magma chamber and contam · nated the 

top of the magma body. However, in subsequent 

studies of avas of similar c omposition from othe r 

Casc ade volcanoes , found s·milar xenoliths and 

recognized evidence that the xenoliths were iq i 

when they came in contact w· t si ic · c magma 

(Eiche berger , 978) . This e ' de nce · ncl des : 

184 

1 

1. Roun e shape o f xenoliths . 

2 . Texture indicating rapi cr ysta liz· tion to 

jus above soli us . 

3. Incorporation o f ph e no c rysts from the host 

silicic magma in rinds on the o uter portions 

of some large xenoliths. 

4. Out ward dec rease in grain size in some arge 

xenoliths . 

5 . Reaction or resorption of phenocrysts in the 

host lava indicating heating o f the silic ' c 

magma during mi xing . 

Although the parental rhyol itic magma of Glass Moun -

tain lacked any phenoc rysts to record a thermal ev en 

associated with mixing , significant c r ystalliza ion 

should have occurred if cool rock had been stirre 

into the chamber . Lac k of such cr ystallization suggested that mi x ing at Glass Mountain might have involved wo magmas rather than magma and roc k , and 

that a reevaluation of the data was in order . 

The arrangement of vents on the Highland , sil cic 

vents associated with a caldera structure and flanked 

by numerous mafi c vents ( Figure 2) , raises the possibility that mafic magma is supplying a sha lo 

silicic chamber ( Figure 3) . However , such a mec hanism would introduce mafi c magma to the base of the 

chamber and woul seem to produce a composi tiona 

zonation opposite to that observed in the Glass untain flo w. Evi ence that mixing occurs by floatation 

o f basal tic foam in silici c c hambers (Eichel berger, 

1979) affords a so ution to this problem and prov ' es 

a basis for un erstan ing the evol tion of the Medici ne Lake magma chamber . 

VAPOR EXSOLUTION DURI G MIXING 

Mix ing of arge batches of magma requires flow on 

a arge scale within a magma cham ber . A lik l y mechanism of large sea e f ow and stirring is convection . 

Introduction of mafi c magma into the base of a 

chamber would strong y heat silicic magma i n the 

ower portion o f the chamber and should induce re atively rapid co nvection ( Ri ce and Eichelberger , 1976 ; 

Sparks and others , 1977) . But beca use the densit y of 

basa t is high , any mi x ing of heated si icic magma 

with basalti c materia l would inhibit co nvection . For 

example , the ensity decrease due to heating silicic 

magma by 100°C could be o ffset by addition of only 2 

volume percent basalti c material. The s ystem old 

probab y remain stratified , with the silicic and 

basaltic la yers co nvecting separatel y, and mix · ng 

imited to diffusion at the inte rface . Of co urse , if 

the basal tic material were less dense than the si icic magma , heating of silicic magma and mixing w' th 

basal tic material would both decrease density of 

magma in the lower portion o f the sil1c ic ayer , 

resu t ing in strong pward flo w. Despite the h ·gh 

ens· ty o f phases cr ysta · zed by basa tic magma and 

present ·n abundance · n the basalti c xenoliths contained n andesitic and acitic avas , it appears 

that the b k ensity of these xenolith s · s ac tua 

less than tha of the avas in w ich they occur . The 

reason fo r this apparen contradi ctio n is that the 

xenoliths invariably contain ab n ant , fine, rou 

uniform y · stributed vesi cles . A though vesicu ation can occur at very sha ow depth d ring eruption, 

these vesic es may be o f deep , primary origin . As 

state before , the te x ture of the xeno i ths suggests 

rapid cr ystallization o f basaltic melt in contac t 

with cooler silicic magma . Our ing the si x ho urs 

required for a chil wave to propagate 0 em into 

basa tic magma , co rresponding to roughl y the 1 argest 

common iameter of he xenoli hs , a h ydrat ion fr ont 

could advance o n y 0 - 2 em into the s · icic magma, 

based on a a of Sha w ( 97 ) . Thus, heat s 
0 

N 

0 

I 

RECENT MAFIC L AVAS MAF IC L AVA S 

s 

5 km 

I 

Figure 3 . A north- south cross section through the center of the Medici ne Lake Highland volcano (line o f 

cross section shown in Figure 2) , with major f eatures projected onto the profile . Vertical exaggeration 

of surface topography is 2X. Si l icic magma cham ber is shown at depth discussed in text and intermediate 

in size between the minimlJTl case , a small body at intersection of cone sheet caldera fractures (Hei ken, 

1978 ) and the maximum possible ex tent marked by position of flanking mafic vents . 

extracted from the basa lt orders of magnitude faster 

t han water, so that the basalt behaves nearl y as a 

closed system with respect to water. Under l ow to 

moderate crustal pressures a vapor phase exsolves 

bec ause water is co nta ined entirel y within the melt 

phase of the system and the melt fraction decreases 

wi th cr ystalli zation from almost 1.0 to about 0 .1. 

The result is a basaltic foam la yer at the base of 

the silicic magma body that can float and mix. 

Floatation of the foam exposes fres h basal ti c magma 

to coo l silicic magma , s o that the process of foam 

formation continues . Dispersal of foam through the 

silicic magma is due t o co nvec t i ve flow , dr i ven bo th 

by therma l expans i on and the positive !!.V phase 

change . Photomicrographs showing the Glass Mountain 

xenoliths to be a c rystal-r ich foam are 'pr esented in 

Fi gure 4 . 

DENSITY MEASUREMENTS AT GLASS MOUN TAIN 

To test the h ypothesis that mixing is driven by 

vapor ex solution, samples of Glass Mountain lava and 

xenoliths were collected for densit y measurements. 

Several pairs of lava and xenolith samples wer e coll ected fran sites ( Figur e 1) near the vent , middle , 

and distal end of the longest lava stream, representing "rhyolite with i nclusions," dacite near the 

transition to rhyodacite , and the most ma fi c dacite 

lava of the flow, respectively (Eichelberger , 1975) . 

Density was determined by measuring dry a nd submerged 

weight of samples which ranged in size from 300 g to 

600 g. Results are given in Table 1. The obsidian 

near the vent has no porosity and a very narro w range 

of density (smaller than anal ytical error) , 2 . 38 + 

0. 01 g!cml . Dacite from mid way down the stream is 

2.49 g/cml . Dacite from the distal end is 2 . 55 

g/cml , reflecting its more mafic and cr ystal- rich 

character. Bulk density o f the xenoli ths fall in a 

surprisingly narrow range , 2 . 15 + 0 . 04 g/ cml. This 

densit y appears little affected by the vesic ul arity 

o f the host lava , since a xenolith in a pliTliceous 

(1.49 g/cm 3 ) sample has a densit y of 2 . 10 g/ cm l . In 

order to determine th m ss density of xe nolith 

material , xenolith samples w re ground to a fin sand 

185 

to destroy the vesicles . Grain size of the sand 

averaged 100 \lm and ranged from \lm to 400 \lm. 

Exaninat ion of polished grain mounts revealed that 

grinding opened at least 99 ~ of the pores. The sa nd 

was then we ighed dry and submerged . The submerged 

measurement was made after the sample was subjected 

to a vacu um for 20 minutes, and water added under 

vaculJTl . The resul ting densi ty of 2 . 79 + 0 . 01 g/ cml 

was expec ted for th i s predominantly plagioclase and 

pyroxene mater i al . Using the ave rage values for bulk 

and mass den s ity gives a porosity of 23 ~. Thus the 

xenolithic material is much denser than the lava , but 

becaus e of high porosity, t he xenoliths wou ld have 

floated in the magma c hamber . This explains why the 

most xenolith-r ich lava emerged first . 

CONDITIONS OF MIXING 

In view of the evidence that the xenoliths 

floated , and because densities as a fun ction of pressure and temperature of the materials involved are 

known or approximately known , constraints can be 

placed on the depth o f the magma chambe r ( Figure 5) . 

Following previous definitions (Eic he l berger, 1979 ) , 

Ps and Px are the mass and bulk densities of the 

basaltic xenoliths , so that Px = p 8 ( 1-Xv) + Pvxv, 

wher e Py and XV are the densi'!y ancr vollJTle fract i on 

o f the vapor phase , respectivel y . Bu l k density of 

the foam (p ) versus pressure was calculated at T = · 

900°C f o r bu1k water c ontent i n basa l tic magma of a . 3~ . and 5~ . by we ight , usi ng t he following assl.ITlp-:-

tions: 

l . The vapor phase is water . 

2 . The xenolithic mat eri al (exc l usive of vapor) 

i s 10% melt and 90% c rystals. This estimate 

is based on i nspection o f thin secti ons of 

x enolith s (g ra in s i ze i s too sma ll for 

acc ur ate modal an a l ysis) and is consisten t 

with the obser ved ma ss density . In the 

general ca se , degree of c rystall i zation is 

depende nt largel y on tempe rature contrast 

bet ween the basaltic and s ilicic magmas . 
A 

Figure 4 . Photomosaics of basaltic xenolith in banded rh yodacite lava from Glass Mountain . Views are 

reflected light (A) , t r ansmitted light ( 8), and transmitted light with crossed polars (C) . Bar length 

is 1. 0 mm . Note abundant , 0 . 1 rom-diameter vesicles in xenoli th ( A) and absence of vesicles in most of 

the host lav a , except for a large vesicle at the lower left edge of the xeno lith . Lava at the bottom is 

contami nated with basaltic material and contains some vesicles. In 8 , the xenolith is a dark round 

object with prominent white plagioclase crystals. Pyro x ene is also a major phase, with olivine , oxides , 

and glass present in lesser amounts . The large vesicle associated with the xenolith is an irregular 

white area . Lava co ntaminated with basaltic material sho ws up as dark streaks . InC, the cr ysta l-rich 

cha racter of the xenolith is apparent . Contaminated lava contains more cr ystals than uncontaminated 

lava . 

3. The xenolithic material is incompr essible . 

In fact , the melt become s less dense with 

increasing pressure because mor e water is 

retained and plagioclase and pyr oxene compress, but the effects are small , opposite 

and nearly equal . 

The measured value of Pg was used , correcting for 

thermal expansion of plagioclase and pyroxene 

(Skinner, 1966) . Data of Burnham and Jahns (1962) 

wer e used for solubility of water in melt, and data 

of Burnham and others ( 1969) were used for density of 

the vapor phase ( pV) . The resulting curves (Figure 

5) sho w the density of the foam formed during mi x ing 

at a specified pressure and water content of basal tic 

magma . Portions of Px versus pressure curves for 

constant porosity of foam are also shown. 

Density of magma i n the chamber before mixing can 

be estimated for elevated temperature and pressure by 

assuming that rhyolitic glass and melt behave in a 

manner similar to glass and melt in the s ystem albite 

+ water (Burn ham and Davis , 1971) . The petrologic 

186 

data indicate that magma in the chamber was entirely 

rh yolitic melt. Hence the density measurements for 

rhyolite obsidian were used . Wat er content strongly 

affects both thermal e x pansion and compressibilit y of 

rhyolite glass and melt . Ava ilable analyses give 

water contents of Glass Mountain obsidian of a few 

tenths of a percent (A~derson , 1941) . Therefore, PR 

versus pressure at 900 C was calculated for 0 and l 

wt . $ water . Knowledge that the xenoliths floated 

constrains xenolith density to those portions of the 

Px curves left (lower density) of the dry p curve . 

Estimates of water content for high alumina ~asalti c magma of the type involved in mixing range up ward to 

5 wt . $ (Anderson , 1974 ; Rose and others , 1978) . This 

constrains pressure to a region above (lower pressure) the 5 wt.$ water pX curve . Since the xenoliths 

have most likely experienced continuously decreasing 

pressure since formation , it is unlikely that they 

ever possessed higher than their present (sur fa ce) 

porosity (XV) , whi ch constrains xenolith density to 

the right (highe r density) of the XV= 0 . 23 curve . 

• 
TABLE 1. Densities of Glass Mountain Samples 

Sample 

(Fig . 1) 

1A 

1B 

1B 

1C 

10 

1E 

2A 

2B 

3B 

3B 

3B 

Averages 

1B 

1D 

2A 

Average 

w 

a::: 

::::> 

(/) 

2 

4 

(/) 6 

w 

a::: 

a.. 

8 

Bulk Density of 

Xenolith(g/cm3 l 

2 . 14 

2 . 13 

2 . 14 

2. 20 

2 . 18 

2 . 10 

2. 10 

2 . 23 

2. 15 

2 . 13 

2 . 11 

Den s i t y o f ho s t 

lava(g/cm3 l 

2. 40 

2 . 37 

2. 37 

2 . 38 

2. 39 

.1 . 49 

2 . 49 

2 . 49 

2. 25 

2 . 42 

2. 55 

2 . 38~0 . 01 

2 . 49 

2. 55 

Sample 

Description 

Rhyolite obsidian 

Pumice 

Nonvesicular dacite 

Vesicular dacite 

Nonvesicular dacite 

Rhyolite obsidian 

Nonvesicular dacite , site 02 

Nonvesicular dacite , site 1/3 

Mass Density , Xenolith (g/cml) 

-- px AT CONSTANT XH

2

o 

·············· px AT CONSTANT Xy 

---- PR AT CONSTANT XH20 

+ MEASURED p, T • 20•c 

2 . 79 

2 . 79 

2. 78 

2. 79 ~ 0. 01 0. 23 

10 

I 

201-

a.. 

w 

0 

e CALCULATED p, T = 90o•c JO L-----~-------L~----~------L-----~~L-~J---~ __ L__L __ ~ 

2.0 2.2 2.4 2.6 2.8 

DENSITY (9 /cm 3 ) 

Figure 5 . Density versus pressure (depth) for materials from Glass Mountain . p is bulk density o f 

basaltic foam ( xenoliths) , pR is density of the reservoir magma , and pB is mass densit y o f xenolithic 

material . The possible region for the Glass Mountain magma chamber is shaded . Arr o ws show path for 

ascending basaltic magma with 3 \oft.~ water . See text for further discussion . 

187 
These constraints bound the shaded region in Figure 

5. If we knew that the basalt initially contained 3 

wt.$ water, which is probably more reasonable since 

Medicine Lake basalt eruptions are much less gaseous 

than the Fuego eruptions for which the 5 wt .$ water 

estimate wa ·s made, then the possible region would be 

a small triangle bounded below by the p X curve for 3 

wt.$ H2 0 , to the right by the appropriate pR curve , 

and above by a horizontal line extending rrom the 

intersection of the pX curves for 3 wt . $ H2 0 and 23$ 

porosity (foam with XH . O = 0.03 above this line would 

have XV >0. 23) . Regrettably, water content of basaltic magma is a major unknown, but it seems likely 

frcxn these considerations that the Glass Mountain 

magma chamber lies within 15 km and probably within 

10 km of the surface. 

A possible path of ascending basal tic magma is 

shown as arrows in Figure 5 , ass~.ning XH 0 = 0 . 03 for 

basalt and XH = 0 to 0.01 for rhyolite ~ The basalt 

ascends throu~g the crust , slowly beccxning less dense 

as indicated by the steep path from 10 kb to 2 kb. 

Densities along this path were calculated using 

average Medicine Lake basal t ( Anderson, 1941) and the 

model of Bottinga and Weill (1970) . Note that at 2 

kb the magma is still vapor undersaturated. However, 

at this depth it enters the Glass Mountain magma 

chamber, undergoes rapid cr ysta llization wh ile 

® 

exsolving a vapor phase, and forms a foam with P 

2 . 74 g/cm 3 , XV ~0 .1 , and'\ ~2 34 g/cm 1 • If t 

fo m remained rigid but leaked after formation it 

would follow a constant porosity curve to the surface. If it maintained an equilibrium por pressure 

and did not leak, it would follow a constant water 

curve and blow up during ascent. The pat.h shown is 

intermediate: some leakage and some expansion to the 

observed XV = 0.23. The Glass 1-buntain xenoliths 

probably did leak, since most xenoliths have a large 

vesicle at the lava/xenolith interface , as shown in 

Figure 4 . In flat ion of the foam does work , energy 

for which is supplied largely by cr ystallization. At 

2 kb, energy available from crystallization is near y 

two orders of magnitude greater than energy required 

for inflation. 

THE GLASS MOUNTAIN ERUPTION 

Important implications of this discussion for 

Glass Mountain are that water played an important 

role in magma evolution and that the chamber lies at 

shallow depth. A probable reconstruction of the 

eruption sequence is shown in Figure 6 . Basaltic 

magma intruded the chamber and formed a foam whi ch 

floated and mixed (in a mechanical sense) with magma 

in the chamber . The subsequent eruption first tap~d 

Figure 6. Probable eruption sequence at Glass Mountain . (1)Mafic magma intrudes shallow rhyolitic 

magma chamber and erupts nearby at High Hole Crater. (2) Vesiculation of chilled mafic magma forms foam 

which floats and mixes in the convecting chamber. (3) Vesiculation in chamber triggers eruption, which 

first taps hybrid dacite concentrated near roof of chamber . (4) Eruption continues with extrusion of 

rhyolite magma . Rhyolitic tephra which underlies Glass 1-buntain may have erupted early in this 

sequence, as an illll1ediate response to intrusion of mafic magma into the chamber , before mixing was far 

advanced . Alternatively , low density of water-rich rhyolitic magma at the top of the chamber ma y have 

prevented further rise of foam-rich currents , so that a small rhyolitic cap remained near the chamber 

roof a:1d erupted as tephra just before the hybrid dacite . 

188 
.. 

t.he hybri whi c h was concentrated at the to p o f the 

chamber . Evi e nce that vapor exsolution occurred 

ithin the c hamber suggests that mi x ing tri gge r ed the 

eruption . Assumi ng that 0 . 2 km 3 of the flo w is a 1: 3 

mixture of basalt and rh yolite , then the vol um e of 

vapor exsolved within the chamber was 0 . 01 km 1 Some 

of this almost c ertainl y lea ked f r om the f oam and 

dissolved in the strongly water undersaturated rh yolitic melt . Ne vertheless , a ra pid volLme i ncrease 

within the chambe r could most easily be accommodated 

by extrusion of material from the cham ber . Thus , the 

mixing and eruption events were proba bl y closely 

spaced in time , and ma y corr elate with the e x trusion 

of mafic magma 10 km to the south on the fla n ks of 

the Highland at High Hole Crater and its associated 

Burnt Lava Flow . Similar rh yolite and h ybr id dacite 

have erupted be fore near Glass Mountain and el s e where 

near the summit of the Highland . These ma y be proucts of repeated intrusion of basal tic magma into a 

rel atively large , shallow , long- lived magma chamber 

beneath the caldera (Figure 3) . This would account 

for the liquidus or superheated condition of some of 

these lavas , as indicated by their lack of phenocrysts . 

Although water content of basal tic magma is a 

major unkno wn, the high initial densit y contrast 

between basal ti c and reservoir magmas (Eichelberger . 

979) constrains the Glass Mountain chamb er to 

shallow depth . Both silicic composition and scarcity 

of phenocrysts in the reservoir magma contrib ute to 

this high densit y contrast. A remarkabl y similar 

high density contrast mixture eru pted from Mt . Mazama 

~ form the Llao Rock dacite flo w, just prior to the 

Crater Lake eve nt . The subsequent climactic tephra 

eruption shows that Mt . Mazama over la y a large shallow reservoir containing cr ystal-poor silicic magma , 

with mafi c magma at the base of the reservoir . The 

Gl ass Mountain data suggest that this condition now 

exists beneath the Medicine Lake Highland . 

Another consequence of this hypothesis is that 

mi xing between magma stored in a c rustal reservoir 

and mafi c magma feeding the reservoir is controlled 

by the water content of the mafic magma . Mi x ing 

uring rep l enishment of c rustal magma reservoirs 

offsets compositional effects of fractionation or 

crusta l melting , so that eruption products of reservoirs fed by wet mafic magma will tend to be intermediate in c anposition . Volcanism resulting from dry 

mafic magma will be bimodal because mafic magma 

remains at the base of the reservoir without mi x ing . 

Anderson (1 97 4) suggested that Medicine Lake basalts 

are drier than those to the west of Mt . Shasta . This 

may ac c ount for the bimodal character of volcanism 

east o f the High Cascades, farther from the plate 

boundary and the source of water . 

ACKN OWLEDGMENTS 

This work was supported by the Office of Basic 

Energy Scie nces , U. S. De partmen t o f Energy. David 

Mann pre pared the thi n s ec t ion s ho wn i n Figure 4 . 

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