MISCELLANEOUS FIELD STUDIES DEPARTMENT OF THE INTERIOR MAP MF-1379-A UNITED STATES GEOLOOICAL SURVEY

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e=occi'c"a~,,BUO~===='======~'====='c====~''====='5 Kl _u I' El E~s \.~---~ CONfOUR INTERVAL 40 FE ET AREA OF MAP NATIONAL GEODE r1c VERTICAL DATUM OF 1Y29

APPROX IMATE MEAN DECLINATION, 1982

CORRELATION OF MAP UNITS

SEDIMENT ARY DEPOSITS VOLCANIC ROCKS

Holocene

121" 45' 121°30' a, ,wrlh. The lower andesite flows southeast and east of Mount llood Studic!s Related to Wilderness 122° 15 122" 00' QUATERNARY --1•------~------~--~ are chemicall y different from those on the 1Hood River Counties, Oregon. '"" quartz-p)'rite-limonite veins. The lower aodesi te flo"8 are aasignnd a L~te Miocene age on the h,qsis of three K-Ar age '" dete.rminatlons from J.li~e (1%9), and on the stratigraphic relatlon with the underlying Rhododendron Formation. On the e,,st ,, TER f lARY aide of Mount Hood, Wise (1969) dcternin,•d <, K-Ar ,ge of Miocene 7.0±0.8 m.y. on an an,lesite flow ,tt Polatlte Greek. From t he west side of Mount Hood, a K-Ar age of 5.8±0.8 m.y. was 45'30 "' determined from an andeaite fl<>w in the upper Clear Fork area, SUMMARY OF !HE GEOLOGY OJ,' THE fr and ,lll age of 5.S±Q.7 m.y. from an andesite flo1< on Zig~ag MQ[mT HOOD WILDERNESS, ORECON Mountain (Wise, 1969) . Volcanism 1Mount Hood Wilderness lles along the crest of the Cascade Ranee in Clackamab and !Iood River Counties, Oreg. Mount Hood is a Qu,it,,rnary INTRLJSIVR ROCKS (MIOGFNE)--~ledium-gray a.1desite plugs containing stratovol cano, still potentially active (Crand ell, 1980), which rises " phenocrysts of plagioclase, olivine, augite, and hypersthene in a cn. The plugs intrude Wilderness covers 47,100 acres (19,061 ha) an.n 10,000 and 29,0110 years B. F. magnetite, and hematite. 1'.hese flows ,,re pe trographicallv Mountain have pene t rated the quartz dlorite; ln ane rlr111 hole, Rhodode"dron Formatio". The qusrtz dlorite of Laurel Hill probably was (Crandell, 1980). Patches of pr~Fraser and Fraser moraines, too similar to andesite flows from vents at Cloud Cap, The Pinnacle, the quartz diorite is overlain by a lower andesite flow (Covert cmplaeed during late Miocene tlme, and drill-hole data Indicate that It ls a small to be included at this map scale , hnvte b"en usecl by and Vista Ridge. They appear to he llt"r than the Pliocene and Meyer, 1979). Wise (1969) i ndicated that the 11.6±1.2 "'·Y • la~ger body than surface outcrops might indicate. The intrusion tnay be Crandell (1980) to date lava flows on the north flank of Mount flows; however, they are not irt cont,,ct with lavas from Mount K-Ar age that he obtained on the intrusion, is older than relat"d tu othter quartz diorlte intrusions to thc sot1theast .,nd southwest. Hood Hood geologic relations indicate. Bickerman (1970) obtaiecd two The intrusion partly underlies the south edge of ~igzar; Mountain and may have additional K-Ar ages of 8,4±0.6 and 8.0±().6 m.y. on the pushed t he mountain upward, causing f racturiSANDY GLACIER VOLCANO (PLEISTOCENE)--Black to dark-gray olivine int1'."uaion. A dike cutting the intcusion was dated by llfrkerman upper .~ndeslte flovs were emplaced dur i ng Pliocene time. Upper .~ndesite fl<>w~ basalt flows, pyroxene artdesite aed hornblende andesite flows , (1970) as 5.0±1.0 m.y. Wf're extruded from several centers nnd flawed over early Miocene andesite lava Sarnes, F. l'., and flutler, W., 1930, The structure and stratigraphy of the LAVAS OF N:OUNT HOOD (HOLOCENE AND PLEISTOCENE)--IJndifferentlated lava and interlayered tuffa and br eccias. Weathered rocks are orange flows of late Miocene age. Pe~rogrsphically, the andeaite flows are all very Columbia River gorge and Cascade Mountains in the vicinity of Mount flows and pyroclaatic rock.a of Mount Hood. The cone is built up brown. Phenocrysts are commonly labradorite, olivine, and T< Rl!ODODRNDRON FO&'IATION (MtoC!l~E)--Varicolored volcaniclastic rocks similar. Hood: Eugene, University <>f Oregon, M.A. thesis, 73 p. of approximately 70 percent olivine and pyroxene andcstte flows hypersthene and locally augite and hornblende (with opaque iron aed aedesite lava flaws. The lower part of the unit is composed The Sandy Glacier volcano of Pleistocene age is exposed on the west stde Beeson, M. H., Keith, T. E. C., and Bargar, K. E., 19BO, Secondar y and 30 percent interlayered pyroclastic flows (Wise, 1968, 1969; oxide rims). Grou~dm,,ss nf much of th" fresh rock ls glassy; mostly of mudflmos and pyroclaatic debcia. Hornblende andesit,· of Mount Hoorl "he r e it has been pa r tly eroded bv g l actat (o-,,. Th e well­ mineralization i.n the Mt. Hood area, Or,,gon [ah11.]: Geolog ical Society White, 1979). Hornblende daclte tephrn ~nd m"dflnw~ mantle the crystalline groundmass consists of plagloclase laths, olivine, and hypursthene a'1desitc flows, about 3 m thick, arc abundant at developed coec is composed of basgrams, v. 12, no. 3, P · 96. older lava fto.,s. A plug of hornblende dacite ts expnsed iu the clinopyroxene, and magnetite. Phenocrysts of plag;loclase and the top of the unit. The volcaniclasrfr rocks are usually southeast under Mount Hood, Geophysical evidence indicates that sever,.l Beeson, M, H., and Moran, M. R., 1979, Stratigraphy and structure of the c r ater area. Olivine andesite flova are medium-gray porphyritic h<>rnblendu from hornblende andestte outcrops along the upper part gray, greenish, and g,:eenish gray; purple rocks are conspicuous Pleistocene volcanoes may have been buried by Mount Hood (G. Flanagan nnd n.L. Columbla Riv"r llas

Sandy River are as long as 5 mm. Chemical compositions of locslly. Lava flows are medium gray, greenish grav, dark brown, Williams, written commun., 1981). and Riccio, J. A., eds., Ceothermal resource asse~s,oent o"r Mount Hood: allgite, hypersthene, and olivine (commonly rlouoed with flows from the Sandy Glacier volcano range from basalt erupted and reddiah brown. Volcaniclastic roclcs consist of epiclastic Construction of the present cone of Mount Hood began spproximately U.S. Department of Energy Contract No . AC06-77-ET-28369, Report hypersthene) in a holocrystalline groundnasa composed mostly of during the earlier stages of activity to andeaite erupted during siltstone, sandstone, and conglomerate af reworked volcanic 730,000 years B.P. when andesitic lava flows poured out over the Plioeenf! and RL0-1040-TI, p . 5-77. plagioclase laths, anhedral augite, and minor hypersthene and the later stages (Wise, 1969), Outcrops of the Sandy Glacier material; pyroclastlc tuff, rnudflows, and volc,inf< hre,·cl,i; and Pleistocene volcanic rocks. The huge cone contlnuc,cl to grow to nt l ~ast its Bickecraan, Michael, 1970, K-Ar ages of Laurel Hill pluton and dike, Orego,i: magnetite. Pyroxene andeaite flows are medium-gray porphyritic volcano occur on the west side of Mount Hood mainly at the head volcanic debris of indeterminste, probably mixed, origin. present height, and probably higher (Crandell, 1980), until about 29,000 yeats The Ore Bin, v. 32, P• 211-215. rocks with subhedral to anhedi::al phenocrysta of hypersthene, of Muddy Fork, and in the upper part of Sandy River. Partly Hypersthene andesite and hornblende andes! t e ar" the mos t collltnon B.P., which was the beginntng of the Fraser Glaciation. Andest te flows then Callaghan, Eugene, and Budd(ngton, A. F., 1938, Me talliferous mineral deposits al!gite, and andesine in a holocryatalline groundmass co,.posed palagoniti~ed buff- to orange-weathered olivine basalt tuffs and clast constituents where volcanic rocks type can be determi,1ed. ceased, and after the glaciers retceated ahout 15,000 years B.P., volca:i.ic <>f the Cascade, Range in Oregon: U. 8. Geologlc,ll Sarvey Bulletict 893, mostly of plagioclase laths and anhedral augite with minor breccias exposed in Ladd Creek may be part of the Sandy Glacier Pumice fragments are abundant in tuffs, espe~ially those that activity was charac~erized by pyroclastic eruptions of hornblende dacite, dome 141 p. hyperstheae, magnetite, hematite, and, locally, ..::: -cristobali te. volcsno. The main vent of Sandy Glacfor volcnno vas probably crop out along the noi::th side of Zigzag Mountai,1. Abundant saiall extrusion, and mudflows. The three major eruptive per iods during this time Covert, w. F., and Meyer, R. J., 1979, Geothermal observation wells, Mt. Hood, Light-gray to pink porphyritic hoi::nbleade dacite pyroelastic located near the head of Muddy Fork. Andesite dikes striking plagi.ocl ase (andeaine) phenocryata are characteristically have been designated by Crandell (1980) as Poi.~llle, 12,000 to 15,000 years Oregon: TJ.S. Department of Ellergy Report for Contract No. material has pheeocrysts of dark-reddish-brown to black about N. 40° W. and dipping steeply southwest crop out at the prominent an outcrop sl!rfacea. Phenocrysts are most commonly B.P.; Timberline, 1,500 to 1,800 years R.P.; and Old M"lrl, 200 to 300 ye.~rs Dl!-AC08- 78ET-P28417 for peciod 4 October 1977 to g July 1979, Northwest hornblende, hypersthene, ,,ndes1ne, and locally minor auglte. The head of Muddy Fork. Flows making up the cone dip about 20° to olivine, hypersthene, ,iuglte, aTid andeslte. Grounrlmnss consists B.P. Crater Rock, just south of the S1.L"ltmit of Mount Hood, is a hornblende Geothermal Corporation, 28 p. ground~ss is holocrystalltne and composed of plagioclase, .C - 30° to the east and southeast under Mount Hood. Wise (1969) mostly of plagioclase, augite, and magnetite. Virtually all of dacite dome thnt w,is empl,1ced durfn,: the Old Maid event. Ftrnarolic activity Crandell, D. R., 1980, Recent eruptive history of Mount Hood, Oregon, and crtstobalite, hypersthene, augite, magnetite, hematite, and determined a whole-rock K-Ar age for Sandy Glacier volcano of the valcantclastic rocks of the Rhododendro.n Formatlon have been hag been continuous around the dome since emplacement. Minor erl!ptive f)<)tential hazards from future eruptions: U.S. Geologic.~1 Survey Bulletin locally minor sanidineO The relations of the lavas forming Mount 3.2±().3 m.y. (:miilion years) (Pliocene). A ,aicroporphyritic affected by low-temperature hydr<>therm,, 1 .~ lter.,tlnn . La umontJte, activity was reported in 1859 and 1865 (Crandell, 1980) and may hav" deposited 11,92, 81 p. Hood are revealed by e:s:posures in deeply dissecte d stream and holocryst.,lllne andesite sample collected during thls study gave mo rdenite, stilbite, heulandite, montmorillonite, celadonlte, and local thin ash layers on the slopes of Mount Hood. Green, S. M., Weaver, C. S., and Iyer, fl. M., 1979, Seismlc studies at the Mt. glacial valleys on the flanks of the volcano. Many flows are a K-Ar "Ile of l.29±0,05 m.y, (Pleistocene.) (J. Markman and J. expanding chlorite are common alteration r.iinerals. Tuffs of The general atructural pattern in the Mount Hood area is typical o f the Hood volcano, northern Casca~e Range, Oregon: U.S. Ge<>lng1cal Survey only about 3 m thick oc less, and have somewhat irrCascade Range (Venkatakrishnan and others, 1'180). The fracture pattern is Open-File Report 79-1691, 27 P • d1strlbut1on since they fl owed as some,,1h4t v t scous mat,.rial down n_.,arly agt"ees with obsecved geologic field relations which affected by local propylitic alteratinn resnlrlng 1n depos l Lton generally east-northeast and north-northwest. Miocene lava floc,s are more Hull, D, A., and Riccio, J. F., eds., 1979, Geothermal resource asaesment of chi'!' slopes of the volcano. Intracanyon flo•., sequences, such ea indicate that the Sandy Glacier volcano is younger than the of pyrite, qusrtz, and epld<>te. Propyl!tlc ~lter.,tion <>f the fractured than the Pliocene and younger rocks . The rocks on Zigzag [fountain Mount Hood, U.S. Department of Energy Contra.ct No, AC06-77-ET- 28369, Barrett Spur on the north side of M,rnnt Hood and Mississippi Head Pliocene lava flows Rhododendron Formation near the contact with the west "dge o.f the appear particularly to reflect the reglonal structural pattern, although that Repott RU>-1040- 1'!, 273 P• on the southwest side, may be as thick as 200 m aud consist of quartz diorite of Laurel Hill has pradu,:ed secondary quartz, area may have been affected by local uplift during emplacement of the quartz Mankinen, E. A., and Dalrymple, G. B., 1979, Revised geomagnetic polarity time several ZO- to 50-m-thick flows. The s,unmit area of the volcano T"" ' UPPER ANDESUE Fl.OWS (PLIOCENE)--Medium-gray miccoporphyritic olivine epidote, c hlorite , pyrite, and z"olltes. Ther e ls little diorite of Laurel Hiil and p08slble associated intrusions that have rrot yet scale for the interval 0-5 m.y. B.P.: Journal o f Geophysical Reseai::ch, consists of fumarolically altered coarse andesitic vent andesite flows .('rolll seversl vent areas, Phenocry,;ts are olivine alteration of the holocrys talUne andesite flows in the upper been exposed by erosion. v. 84, P• 615- 6Z6. brt,cc/.a. At the west edg" o f the s,umnit, andes:lte flows from an (commonly vith hypersthene rims), plagioclase (An45), part of the Rhododendron Formatlr>n. The Rhc>dr>dendron Form,,tl<>n On the sol!th side of Mout1 t Hood, a N. 4S0 w.-trending fault that dips Nehring, N, L., Wollenberg, H. A., and Johnston, D. A., 1981, Gas analyses of older vent are exposed. The central part of the summit area ia hypersthene, and augtte. "Phenocryst size is usually 0.5 to l mm crops out along the south, west, ,incl north sides c,f ?.ip:zai steeply to the northenst has been active since the Old Maid eruptive event, fumaroles from Mt. Hood, Ot:cgon: u.s. Ceologlcal Survey Open-File Report unstable because of alter,Hion nnd the loose bre,clated nature of long, but may be as long ss 2 m,a . Gro"ndmnss ls holocryst;illinP M<>un tain. Barnes snd Butler (1930) described the Rhododendron 200 to 300 years B.P., as shown by displacement of the block and ash f low on 81-236, 9 p. the rock. The main vent of ct-he volcano must have been slightly and composed of plagioclase (andeslne), nuglte, magne tlte, and, Fonnntlon from bluff e,o: ~osurea at the type locality (N 1;2 Sec. 2, the east side of Zigzag Canyon just above timberline. A parallel faulf along Rite, Alan, and Iyer, H. M,, 1981, July 1980 Mt. Hood earthquake swarm: U.S. to the north of the present swumit atJ sor.ie time. Minerals locally, hematite. Minor glass is locally present as microblebs T. 3 S., R, 7 E. a!ld SE /4 sec. 35, T. 2 s., R. 7 E.) on the south Lost Creek is buried beneath block and ash material f r om Mount Hood and is Geological Survey Open-File Report 81 - 118, 24 p. typical of fumarollc alteration ,rnd depd'.dtlon, ~,,.,,h. as native in the groundmass. Chemical analyses of n,"nerous upper mdeslte aide of Zig zag M011nt,~in ne,,r the town of Rhododendron. l'!aximurn possibly responsible for, or associated with, quartz veins and pcopylitic Robison, J. H., l'orcella, L. S., snd Gannet, M. w., 1981a, Data from sulfur, oµat,..c- and @-cristoballt e, and Fe oxides-hydroxides, flows suggest that the vents are not g"netlcally ,.,tated to eacli thickness here is 1,400 ft (415 m); however, Old Maid Flat drill alteration in the lower andesite flows. Evidence from outslde the map area gentherm.~l test wells near Mount Hood, Oregon: u.s. Ceolor,kal Survey coat fractures and fil l pore spaces in the summit andeaites other (\iiae, 1969). Vent areas for upper andeslte flows on the hole 1-2 on the north aide of Zigzag Mountain penetrated more shows that the "olcano forming Mount Hood appears to occupy a north-gouth- Open-File Report 81-1002, 24p. (Beeson and others, 1980). Fumarolic activity in the area of the north and west sides of Mount Hood are partly covered by younger than 2,000 ft (600 m) of Rhododendro" Fo rm ,,t!an (Covert and trending graben (.Beeson and Moran, 1979). , Robison, J. H., Keith, T. I':. c., Beeson, M. H., and llarg.~r, K. E., 1981b, Map hor nblende dacite dome (Crater Rock) that was emplaced during volcanic racks of Mount Ilood and coexti::uded andesites fro:n Meyer, 1 979). Wise (1969) suegeats that poggihle vent centers Selsmic activity in the vicinity of Mount Hood is presently:I being showing geothermal investigations in the vicinity of the Mount Hood volcanic activj:ty about 200 years B.P. is altering the dacite satellitic vents at Vista Ridge and The Pinnacle. Vent area for for the toffs and lava flows sre coO"ered hy la"as o f Mount continuously monitored. Studies by Westhusiag (1973) showed mo~tly sh.,llow Wilderness, Clackamas and Hood Rive,: Coueties, Oregon: U.S. Geol ogical ,composing thu dome and dei,oaiting red, yellow, and white the flows on Zigzag Mountain is probably at the east end of Honrl. The thick ruff s ec tion in the Burnt Lake ares on Zigzag 5-10 km depth) shocks that had epicenters near the cente,: of the volcano. In Survey Miscellsneous Field Studies Map MF-1379-B, scale 1:62,500. incrustations. Temperatures of presently active vents sre as Zigzag Mountain. Vent area for fto1r, a 16-day study, he recorded 53 near events, all of which he attributed to Trimble, D. E., 1963, Ceology of Portland, Oregon, and adjacent areas, U.S. high as 91°c, which is above boiling for the .ctl titude (10,150 ft east side of Mount Hood is either Badger "flutte (not shown on Rhododendron volcanism. The forrn.Hion thins raptdl y t o the tectonic, rather than volcanic, origin. Green, Weaver, and Iyer (1979) Geological Surve;' Bulletin 1119, 119 p. (3,171 m)) (Nehring aed others, 1981). The absence of revecsed map), ,ln eruptive center appcoximately 1.5 mi (2 km) southeast of north, east, and sol!th (Wise, 1%9). fossil plants and pollens reported on a saismic network of 16 stations, operatf.nnal bet1rmation (Wolfr, 1962) and stratigraphic 1977 and December 1978, during which time 10 local events were recordJ d, the lit1eara of the northern part of the Cascade Range, Oregon: Oregon indicat es that the volcano began erupting not more than 730,000 of Monnt Hood. The upper andcstte flovs cap ridees and overlie relations in the Portland Hills area (not on map) west of Mount most intense of which had a magnitude of 3.4. Three of these evenrl's were Dep,,rtment of Ce<>logy and Mineral Industries Special Paper 12, 25 p. years l!.P. (l1arikinen and Dalrymple, 1979). 1'fo i n cone-building lower andesite flows but do not appe>1r to have be<>n so faulted Hood (Trlmble, 1963) indicate a late Miocene age for the located within 3 km south of Mount Hood, and seven 1rted to have been produced severa :tJ miles A., and Riccio, J. F., eds. , Geothermal resource assessment of Mount Qac ANDES1TE OF CLOUD CAP {PLEISTOCENE)--Meditr.n- to darlc-gray olivine respectively. Both ages arc within the range of 2 to 5 rn.y. for andesite flows that overlie the Rhododendron Formation on Zigz,~g southwest of the Wilderness ln the Still Creek-Cheeney Creek area (Callagan Hood: U.S •. Department of Energy Contract No. AC06-77-ET--28369, Report andesite flows thnt eruptted f rom a ;,ent at Cloud Cap. A ,;cquonco the Pliocene (Ceologtc Names Comm ittee, ll, S, Geological Survey, Mountain has been dated by the K-Ar method as 5.5±0.7 m.y. (Wiae, and !:ludd ington, 1938). With the e::tception of a few uopt"oductive claims ,1n t he RL0-1040-TI, p, 78-126, of flve flows having an accwnulated thickness of approximately written co!llmUn., 1980). Vo lcanism was probably sporadically 1969). In and 1mme,d iately adjacent to the Wilderness area, the vicinity of Lost Creek on the north side of Zigzag Mountain and Laurel Hill on Wise, W, S., 1958, Geology of the Mount Hood volcano, ln Dole, H. M., ed. , 500 ft (150 m) extends nonheaatward from the Cloud Cap vent continnOllS thro ughout much of the Pli ocene Rhododendron rests upon middle Miocene basalt flovs of the the south side, no mining activity haa been noted ln the Wlld e rness (Douglas And esite conference guidebook: Oregon Department ofGeology and Mineral (Wise, 1969). Most are f)<)rphyritic; some are aphanitlc. The Columbia River Basalt Group (Beeson artd Mor.m, 1979; H . .J. Mey<'r, Causey, written commun., 1980). Considerable iaterest has been generated Industries Bulletin 62, J>• 81- 98. andesite flows are chemically similar to those f rom satellitic LOWE:R ANDF.SITE FLOWS (MIOCENE)--Light- to ;:iedium-gray olivine and "rltten commun,, 1980). Although the Colwobla Il.lver Basalt Croup regarding the geothermal energy potential of 'lount Hood since the acea '-'"-S 1969, Geology and petrology of the Mt. Hood area--A study o f High Cascade vents to the west at The Pinnacle (Qap) end Vista Ridge (Qbav), two-pyroxene andesite flows with interlayered tuffs and doe8 not crop out within the Wildei::ness, drill hol es 1n Old Maid designa ted a KGRA (Known Geothermal Res<>urces Area) in 1970, and several ---volcanism: Ccological Society of America Bulletin, v. 80, P• 969-1006. but are ,1ot genetlcally related to the lavas of Mount Ilood (Wise, breccias. Locally, andesites are red hrown ovfng to o~idstion of Flat on the north side of Zigzag Mountain have penetrated the exploratory geothermal and heat-flow holes have been drilled in areas adjacent Wolfe, J. A., 1962, A Miocene pollen sequence from the Cascade Ra"3e of 1969). The activity at Cloud Cap followed the mair, cone­ iron in mafic phenocrysts. Phenocryats are andesine, olivine, Rhododendroll Formatiot1 resting nn the Columbia Ri ve,: Basalt Group to the Wilderness (Covert and Meyers, 1979; Hull and Riccio, 1979; Robison and northern Oregon: U.S. Geological Survey Professional Paper 450-C, building stage of Mount Hood but preceded the Fraser Claclation hypersthene, and augite; gronndmass i s usually holocrystalline (Beeson and Moran, 1979; Covert and Meyer, 1979) others, 1981a, 198ib). p. C81-C84. (29,000 years B.P.) (Crandell, 1980) and composed of plagioclase, augite, hematite, and magnetite. Flows at the base of the unit, overlying the Rhododendron ANDESITll OF ·rtiE PINNACLE (PLE!STOCENE)--Medtum- to dark-gray oll."foe Formation in the vicinity of Zigzag and I.1st Chance Mountains, andesite with phenacrysts nf plagioclase, olivine (wlth O[l<.~que are distinctively ~oarsu porphyritic andesite with phenoccysts of iron oxide rims) and orthopyroxene in a groundmass of plagioclase plagioclaae (andesiue) and hypersthene as long ,,s 10 mm. Unless a:id magnetite. Three f lows that extend north1 those f rom be easily distinguished from andeslte flows in the upper part of the underlying Rhododendron Formation. On the basis of rock Cloud Gap v"nt to the e,1st "nd Vlsta Ridge vent to the .,est, but FAULT, SHOWINC DIP--Dashed whtere ,ipproxlmately located; dotted where distribution and rock chemlstry, Wlse (1969) determined that the are not genetically related to the lavas of Mount Hood (Wise, uncertain, Ball snd bar on downthrnwn side 1969). Activity at The Pinnacle followed the main cone-huilding lover andesite flows were derived fro,n several different vents. stage of Mount Hood, but part of The Pinnacle vent may be cov,.red The distinctive coarse porphyritic hypersthe,,ne andesite flows on STRIKE AND DIP OF LAVA FLOWS AND BEDDING IN VOLCAN1CLAST1C LAYERS by some of the youngest of the lavas of Mount Hood. Activity at Zigzag Mountain and .similar flows north and northwest of nount The Pinnacle had ceased by the begfaning of the Fraser Glaciation Hood are probably associated vith the Tertiary (upper niocene) intrusive rocks (Ti), or vents, on Zigzag Mountain and to the (Crandell, 1980) BO!JJl;DARY OF MOUNT HOOD WILDEKN!:::SS

GEOLOGIC MAP OF THE MOUNT HOOD WILDERNESS, CLACKAMAS AND HOOD RIVER COUNTIES, OREGON

By

T.E.C. Keith, M.H. Beeson, and K.E. Bargar

lntenor- Geological Suney, Res ton Va - 1982

l'or sale by Branch of Distribution, U.S. Geolug,cal Survey. 1982 Box 25286, Federal Center, Denver, CO 80225