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USFS Rd 1018

USFS Rd 15

Scott Pass

McKenzie Pass

v o l c a n i c f i e l d

USFS Rd 1514

Hwy 242

"Glisan Pinnacle"

"Prouty Pinnacle"

 "Racetrack

 Meadow"

Hodge Crest

Park

Meadow

Cascade Lakes Hwy

Broken Top

volcano

Mt. Bachelor

Ski Area

Mount Bachelor

lava flows

MIDDLE SISTER

NORTH SISTER

SOUTH SISTER

BROKEN TOP

NEWBERRY FLOW

CONE

6302

"CRAG 6400"

CONE

6315

KNOB

6435

DOME

5803

COLLIER

CONE

"KNOB 6453"

"BLACK

 HUMP"

DOME 7930

KNOB

6760

KNOB

6482

HILL

6485

"EGAN CONE"

"D E V I L S C H A I N"

"D E V I L S C H A I N"

SIMS

BUTTE

OBSIDIAN

CLIFFS

LANE PLATEAU

THE HUSBAND

SPHINX

BUTTE

THE WIFE

THE HOUSE

ROCK

KOOSAH

MTN

TALAPUS

BUTTE

KATSUK 

BUTTE

KALEETAN BUTTE

CAYUSE

CRATER

TROUT CREEK

BUTTE

MILLICAN

CRATER

ROCK MESA

LE CONTE

CRATER

LITTLE

BROTHER

Brush Draw

YAPOAH

CRATER

Camp Lake

Linton Spring

East 

 Fork 

 Park 

 Creek

Fro g

Camp Creek

North Fork Why chus Creek

Sink Cre

ek

Whychus

Creek

"Falls Fork"

Whychus Creek

"Todd Creek"

Snow Creek

South Fork

Obsidian

Creek

Mesa Creek

Creek

Hinton Creek

Separation

Goose

Creek

Moraine

Lake

Soda Creek

Devils

Lake

 Upper Chush

Falls

Trout Creek

Swamp

Alder Creek

Park 

Fork

West

Creek

James

Creek

C h a m b e r s

L a k e s

121°37'30"

44°15'

44°07'30"

44°

121°52'30" 44°

44°07'30"

44°15'

121°52'30"

121°45'

121°45'

H. Prouty Glacier headwall on northeast face of South Sister. Radially dipping stacks of andesitic lava flows and 

intercalated oxidized scoria were excavated by Prouty Glacier, exposing cross-sectional view of last 33 ka of summit 

activity in 350 m of vertical relief. Dipping stacks are truncated at about 9,800 ft elevation, forming ragged rim of a broad 

paleocrater. Gray, stratified, dacitic phreatomagmatic deposit 150 m thick makes up upper slopes of edifice, capped only 

by thin ledge of mafic agglutinate on skyline. Beneath Hodge Crest (left shoulder of edifice) is another thick, gray 

phreatomagmatic deposit, this one cut by many dikes that altered host fallout, which erodes into colorful spires. Little 

Ice Age moraines in foreground are barren and unstable. Photograph by John Scurlock, 2007.

H

I. View northwestward of South Sister and “Devils chain” rhyolites to its left. Sparks 

Lake basin is in foreground. Unforested “Devils chain” is string of nearly contiguous, 

virtually uneroded rhyolite lava domes and flows aligned north-south along 5 km 

stretch on southeast apron of South Sister. Largest of these is Newberry flow (right 

center, below South Sister), only 2.5 km from summit of South Sister. All were extruded 

about 2,000 years ago, during the most recent eruptive episode at South Sister, which 

also ejected pumice fall that still mantles much of the area. Photograph by George 

Cagwin, 2009.

I

J. South Sister edifice, viewed northward from east shore of Sparks Lake. Visible are five rhyolitic units of different 

ages: Unforested 2-ka rhyolite of “Devils chain” rests on forested 35-ka flow lobe from adjacent Devils Hill dome. Three 

pale-gray spurs form sharp salients that extend to elevations only 550 m below darker andesitic summit of South Sister. 

Each spur is a petrographically distinctive rhyolite lava, and they yield ages of (from left to right) 51 ka, 30–24 ka, and 35 

ka. On skyline, South Sister summit and Hodge Crest are separated by cirque of Lewis Glacier. Photograph by George 

Cagwin, 2009.

J

E. Northwest flank of South Sister. 

Skyline epaulet high on right shoulder is 

thin agglutinate sheet that fell during 

most recent summit eruption. Headwalls 

of cirques expose thick red cinders that 

are intercalated with stack of radially 

dipping lava flows that built current cone. 

Photograph by Judy Fierstein, 2006.

E

F. Aerial view of northwest summit of South Sister. Lost Creek Glacier and its cirque are in center, and Eugene Glacier 

and its cirque are toward bottom left. Summit mafic agglutinate mantles its right shoulder, covering gray fragmental 

dacite deposit from explosive eruption about 24 ka. Fingerlike lobes of dacite lava dribble down northwest ridge atop 

fragmental deposit and beneath capping agglutinate. Entire edifice visible here was built between 30 and 20 ka. 

Photograph by John Scurlock, 2007.

F

G. East face of Middle Sister, 350 m high. Shingled stacks of thin mafic lava flows and intercalated blocky rubble constitute 

most of upper half of Middle Sister cone. Ice-sculpted pile of lavas at right, variously called “Black Hump,” “Prouty Point,” 

or “Step Sister,” has 190 m of relief and consists of at least five flows. Hayden Glacier is below both summits; 25-m-thick 

cliff in front of right side of glacier is terminal lobe of fissure-fed andesitic scoria, spatter, and fountain-fed lava flow that 

drapes south flank of North Sister; dated at 21±6 ka, this flow (unit anh) is much younger than North Sister but about same 

age as Middle Sister and “Black Hump.” Well-formed moraine in foreground was left by Hayden Glacier in Little Ice Age 

time. Photograph by Andrew Calvert, 2002.

G

D. View southeastward across saddle between North and Middle 

Sisters. Small ice-mantled peak in saddle is dacitic “Black Hump.” In 

left foreground, ice-ravaged mafic edifice Little Brother is separated 

from North Sister by Little Ice Age trough of Collier Glacier. Both 

North Sister and Little Brother expose numerous oxidized scoria falls, 

whereas smooth black Middle Sister cone is cloaked by mafic lava 

flows. Brick-red ridge extending north (left) from North Sister is part 

of younger north-south vent alignment. Beyond South Sister cone (at 

upper right), which has several pocket glaciers, is smooth cone of 

Mount Bachelor behind it and rugged, barren Broken Top volcano to its 

left. Photograph by John Scurlock, 2007.

D

A. East faces of Middle Sister and North 

Sister. Dacitic “Black Hump” is in saddle 

between them. Altered zones are conspicuous on summit of Middle Sister and east ridge 

of North Sister. Diller and Hayden Glaciers 

have left complex of Neoglacial moraines, 

debris principally supplied by erosion of 

eastern one-third of Middle Sister cone. Thick 

mafic lava flows form “Prouty Pinnacle” and 

“Glisan Pinnacle” atop North Sister, helping 

slow destruction of fragmental edifice. 

Photograph by John Scurlock, 2007.

A

B . East face of North Sister, 750 m high. Consists of about 100 thin mafic lava flows 

and intercalated layers of red scoria, capped by thick summit lava flows. Remnant of 

Thayer Glacier feeds cirque lake. Pervasively altered yellow-orange buttress on right, 

250 m thick, consists of east-dipping, palagonitized, ash-rich fragmental flow deposits. 

Photograph by John Scurlock, 2007.

B

C. Steep northeast face of North Sister. Remnant of Villard Glacier is in cirque below. 

Thick summit lava flow makes up “Glisan Pinnacle,” which overlies hundreds of thin 

rubbly lava flows that are intercalated with red-oxidized agglutinate and scoria falls 

exposed on 800-m-high face below. Flows that form ridge at upper right are thicker 

than those on most of edifice. Ash-rich phreatomagmatic fallout layers in lowest 150 m 

are pervasively altered to yellow-orange palagonite and riddled with dikes, many of 

which also cut through overlying stack of thin lava flows. Photograph by Judy Fierstein, 

2009.

C

Any use of trade, product, or firm names in this publication is for descriptive purposes only 

and does not imply endorsement by the U.S. Government

For sale by U.S. Geological Survey, Information Services, Box 25286, Federal Center, 

Denver, CO 80225, 1–888–ASK–USGS

Digital files available at http://pubs.usgs.gov/sim/3186/

Suggested citation: Hildreth, Wes, Fierstein, Judy, and Calvert, Andrew T., 2012, Geologic 

map of Three Sisters volcanic cluster, Cascade Range, Oregon: U.S. Geological Survey 

Scientific Investigations Map 3186, pamphlet 107 p., 2 sheets, scale 1:24,000. (Available at 

http://pubs.usgs.gov/sim/3186/.) Printed on recycled paper

Geologic Map of Three Sisters Volcanic Cluster, Cascade Range, Oregon

By

Wes Hildreth, Judy Fierstein, and Andrew T. Calvert

2012

1000 0 1000 2000 3000 4000 5000 6000 7000 FEET

1 .5 0 1 KILOMETER

SCALE 1:24 000

CONTOUR INTERVAL 40 FEET

NATIONAL GEODETIC VERTICAL DATUM OF 1929

1 1/2 0 1 MILE

APPROXIMATE MEAN

DECLINATION, 2012

15 / ° 1 2

MAGNETIC NORTH

TRUE NORTH

Geology mapped by Wes Hildreth and Judy Fierstein, 2000–2009; K-Ar and Ar/Ar 

dating by Andrew T. Calvert

GIS database and digital cartography by J.E. Robinson and Judy Fierstein

Edited by Taryn A. Lindquist; digital cartographic production by Kathryn Nimz

Manuscript approved for publiation October 27, 2011

Base from U.S. Geological Survey, 1:24,000, provisional 1988: 

Broken Top, Linton Lake, North Sister, South Sister, Trout Creek 

Butte

Projection and 1,000-meter Universal Transverse Mercator 

grid, zone 10 

1927 North American Datum

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Map 3186

Sheet 1 of 2

Pamphlet and CD-ROM accompany map