2200

1800

2400

800

1600

1400

600

1200

1000

2600

2800

3000

400

3200

3400

3600

2600

1000

3400

2000

2000

2600

3200

3200

1000

1800

1800

2600

2400

2000

1400

3200

3200

2200

2200

2400

600

1600

2600

3200

1200

3000

1600

2200

800

1800

2200

1800

2800

1600

2000

2800

1200

2000

600

1400

3000

1800

2000

1600

3200

2600

3400

2600

800

2000

2200

1600

2400

1600

2800

1400

1400

1200

1600

1800

2000

2200

2600

2400

2800

2400

1800

1800

1200

1800

1600

2600

2000

2200

2200

2600

1800

2400

1200

1800

1400

400

200

2000

1200

31 32 33 34 35

19 20 21 22 23

28 27 26 30 29

16 15 14 18 17

4 3 2 6 5

7 8 9 10 11

16 15 14 18 17

19 20 21 22 23

12

13

24

27 26 25 30 29 28

7

18

19

30

1

36

24

25

31

19

30

6

13 18

G I F F O R D P I N C H O T

N A T I O N A L F O R E S T

C O L U M B I A R I V E R G O R G E

N A T I O N A L S C E N I C A R E A

B E A C O N R O C K S T A T E P A R K

Hamilton

 Mtn

Little Beacon

 Rock

Beacon

Rock

Three Corner

Rock

PIERCE

NATIONAL WILDLIFE REFUGE

COLUMBIA RIVER

Hardy

Slough

Hardy

Falls

Rodney

Falls

Deer

Creek

Creek

Washougal

River

Prospector

Creek

Steep Creek

Hamilton

Creek

Stebbins

River

Washougal

Hard

Scramble

Creek

Hardy

Creek

Woodward

Duncan

Creek

McCloskey

Creek

Creek

Little

Creek

Timber

Creek

Lily

Lake

HARDY

RIDGE

PACIFIC

CREST NATIONAL

SCENIC

TRAIL

NATIONAL

SCENIC

AREA

BOUNDARY (INDEFINITE)

NATIONAL

SCENIC

AREA

BOUNDARY

(INDEFINITE)

PIPELINE

PIPELINE

DUNCAN

CREEK

ROAD

WOODWARD

CREEK

ROAD

SCOTT

ROAD

KUEFFLER

ROAD

Croswell Butte

14

CG-1400

CG-1400

CG-1400

CG-1500

CG-1500A

CG-1408

CG-1440

W-2000

W-2070

W-1430

CG-1430

W-1430

W-2010

CG-1420

CG-1420

W-2000

W-2000

CG-2090

Qa

Qa

Qa

Qa

Qa

Qa

Qa

Qa

Qa

Qa

Qa

Qa

Qobg

Thba

Thba

Thba

Thba

Twfsh

Twfsh

Qamc

Qamc

Twfss

Twfss

Qlbr

Thtb

af

af

af

Qls

Qls

Qls

Qls

Qls

Qls

Qmbc

Qmbr

Tecu

Tecu

Tecu

Tecu

Tecu

Qmdc

Qmdc

Qmdc

Teha

Qmum

Tggc

Tipd

Tipd

Qmwc

Tgo

Tgo

Tgo

Tgo

Tgo

Tgo

Tgo

Qt

Qt

Qt

Qt

Qt

Qt

Qt

Qt

Qt

Qt

Qt

Tgsb

Tgsb

Tgsb Tgsb

Tgwr

Tgwr Tgwr

Tgwr

Tgwr

Tgww

Tgww

Tgww

Tha

Tha

Tha

Teha

Qls

Qa

Qa

Qa

Qa

Qa

Qa

Qa

Qa

Qa

Qa

Qa

Qa

Qa

Qa

Qa

Tb

Tha?

Tvs

Tvs

Tvs Tvs

Tvs

Tvs

Tvs

Tvs

Tvs

Tvs

Tvs

Tvs

Tvs

Tvs

Tvs

Tvs

Tvs

Tba

Tba

Tba

Tba

Tba

Tba

Tba

Tba

Tba

Tba

Tba

Tba

Thba

Thba

Thba

Thba Thba

Thba

Thba

Td

Td

Td

Td

Td

Td

Td

Thd

Tdi

Thtb

Tvs

Thtb

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Tecl

Tecl

Ta

Tia

Tia

Tia

Tia

Tia

Tiah

Tecu

Tecu

Tecu

Tecu

Tecu

Tecu

Tecu

Tial

Teha

Teha

Teha

Teha

Tiba Teha

Tiba

Qmhs

Tehd

Tid

Qmlc

Tepa

Tepa

Tepa

Qmus

Qmwc

Qmwc

Qt

Qt

Qt Qt

Qt

Qt

Qt

Qt

Qt

Qt

Qt

Qt

Qt

Qt

Qt

Tgsb

Tgsb

Tgsb

Tgsb

Tt

Tt

Tt

Tt

Tt

Tt

Tt

Tt

Tt

Tt

Tt

Tt

Tt

Tt

Tt

Tt

Tt Qtdwu

Qtdwu

Qtdwu

Tgww

Tgww

Ta

Ta Ta

Ta

Ta

Ta

Ta

Ta

Ta

Ta

Ta

Ta

Tha

Tha

Tha

Tha

Tha

Tha

Tha

Tha

Tha

Ttw

Ttw

Ttw

Ttw

Qls

Qa

Qa

Qa

Qa

Qa

Qa

Qa

Qa

Qa

Qa

Tvs

Tvs

Tvs

Tvs

Tvs

Tvs

Tvs

Tvs

Tvs

Tvs

Tvs

Tvs

Tba

Tba

Tba

Tba Tba

Tba

Tba

Tba

Tba

Tba

Tba

Tba

Tba

Thba

Thba

Thba

Thba

Thba

Thba

Thba

Twfsh

Td

Td

Td

Thd

Thtb

Thtb

Thtb

Thtb

Thtb

Thtb

Thtb

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Tecl

Tecu

Tecu

Tecu

Tecu

Qt

Tiba

Tepa

Tidcr

Tgo

Tgo

Tgo

Qt

Qt

Qt

Qt

Qt

Qt

Qt

Qt

Qt

Qt

Qt

Qt

Qt

Qt

Qt

Qt

Qt

Tgsb

Tqd

Qtcr

Tt

Tt

Tt

Tt

Tt

Tt

Tt

Tt

Tt

Tt

Tt

Tt

Tt

Tt

Tts

Tts

Tts

Tts

Ta

Ta

Ta

Ta

Ta

Ta

Tha

Tha

Tha

Tt

Thba

Tba

Qa

Qa

Qa

Qa

Tvs

Tvs

Tvs

Tvs

Tvs

Tba

Tba

Tba

Tba

Tdi

Qls

Qls

Qls

Qls

Qls

Qls

Tia

Tia

Tia

Tia

Tiba

Tid

Tid

Tgb

Tob

Qt Qt

Qtdwu

Qtdwu

Qtdwu

Ta

Ta

Ta

Ta

Ta

Ttw

Ttw

Qa

Qa

Qa

Qa

Tvs

Tba

Tdi

Qls

Tia

Tiba

Tiba

Tid

Tidcd

Tidcd

Tidcr

Tob

Qt Ta

Qa Tia

Tt

Tba

Ttw

Tt

Thtb

Tvs

Tba

Tecu

Tecu

Qls

Tha

Qls

Tecu

Tecu

Tecu

Thba

Thba

Tha

Tba

Tba

Tvs

Tvs

Tvs

Tecu

Tecu

Qt Qt

Tba

Tt

Tt

Thba

Qa

Qls

Qt

Tt

Ta

Tvs

Tvs

Tvs

Qdf

Qdf Tvs

Tvs

Tvs

Qac

Tt

Qls

Qls

Qls

Tvs

498±7 ka

19.50±0.05 Ma

1,055±22 ka

1,056±9 ka

20.1±0.1 Ma

19.48±0.04 Ma

21.59±0.31 Ma

21.94±0.06 Ma

22.79±0.13 Ma

1,065±5 ka

22.60±0.06 Ma

24.30±0.11 Ma

25.79±0.55 Ma

24.04±0.06 Ma

<23.14±0.18 Ma

821±24 ka

879±7 ka

84.2±5.1 ka

55.8±6.1 ka

58.4±6.4 ka

4

7

4

2

6

5

9

1

7 8

2

2

7

5

5

5

5 7

6

6

19

15

12

37 13

51

37

11

12

30

12

27 34

25

22

42

26 28

30

35

34

31

21

33

22

26

25

40

16

20

12

31

17

25 19

37 37

38

33

36

13

17

29

36

37

15

19

17

19

22

23

15

17

25

34 17

20

10

15

19

32

24

17

25

22

24

22

23

28

15

12

33

13

30

15

22 18

10

18

5

47

39

32

23

28

?

?

?

Hamilton

Mtn

Beacon

Rock

Three Corner Rock

PIERCE NWR

Deer

Creek

Creek

Prospector Creek

Steep

Creek

Hamilton

Creek

Stebbins

WASHOUGAL

Hard

Scramble

Ck

Hardy

Creek

Woodward

Creek

Duncan

McCloskey

Creek

Creek

Little

Creek

Timber

Creek

Lily

DUNCAN

CREEK

ROAD

WOODWARD

CREEK

ROAD

KUEFFLER

ROAD

Rock

Creek

RIVER

14

GIFFORD

PINCHOT

NATIONAL

FOREST

Lake

COLUMBIA

OREGON RIVER

WASHINGTON

LOOKOUT MOUNTAIN QUADRANGLE

45°45'00"

122°07'30" 122°00'00"

45°37'30" MULTNOMAH FALLS

QUADRANGLE

BONNEVILLE DAM QUADRANGLE

BOBS MOUNTAIN QUADRANGLE

BRIDAL

VEIL

QUADRANGLE

Archer

Mtn

Birkenfeld Mtn

STATE PARK

BEACON ROCK

C O L U M B I A R I V E R G O R G E

N A T I O N A L S C E N I C A R E A PACIFIC

CREST

NATIONAL

SCENIC

TRAIL

CG-1400

CG-1400

CG-1500

CG-1440

W-2000

W-1430

W-1430

CG-1420

CG-2090

W-2000

W-2000

INDEX MAP SHOWING GEOGRAPHIC AND CULTURAL FEATURES OF BEACON ROCK

QUADRANGLE (ORANGE RECTANGLE) AND VICINITY ON HILLSHADE IMAGE DERIVED

FROM BARE-EARTH LIDAR SURVEYS.

CG-1408

Tgwr

2,000

1,000

FEET

3,000

C

SEA LEVEL

4,000

2,000

1,000

SEA LEVEL

FEET

3,000

4,000

C'

Thickness of surficial units exaggerated

1,000 feet = 305 meters

Qmdc

Qls Tgo Tgo

Tgsb

Tgwr

Twfsh

Tecu

Tha

Twfsh

Qls

Tha

Qls Tecu

Qt Tgo

Tha

Qt

Teha

Tgo

Qa

Qt

Tecl

Tecu Qt Tggc SECTION B–B'

Qls

Hardy Creek

Little Creek

Woodward Creek

Duncan Creek

Qmwc

2,000

1,000

SEA LEVEL

FEET

3,000

4,000

2,000

1,000

FEET

3,000

4,000

B'

1,000 feet = 305 meters

B

SEA LEVEL

Thickness of surficial units exaggerated

Prospector Creek

Stebbins Creek

Hardy Creek

Hamilton Mountain

Timber Creek

Tgo

Tgsb

Tgww

Qt Tecu Qls

Qt

Qt

Tecl

Teha

Teha

Qt

Qls

Tecu

Qmwc Tecu

Qt

Qls

Thba Thba

Thba

Thba

Tha

Tha

Qls

Tba

Tts

Tts

Tt

Tts

Thba

Tha

Qa

Tvs

Tvs Tvs

Tdi Tia

Tba

Tba

Ttw

Ttw

Qa

Ta

Tecu

Tgo

Tiba

Qls

Ta

Ta

Tha

SECTION

C-C'

2,000

1,000

SEA LEVEL

FEET

3,000

4,000

2,000

1,000

FEET

3,000

A

SEA LEVEL

4,000

A'

Thickness of surficial units exaggerated

1,000 feet = 305 meters

Three Corner Rock

Qa

Qls

Qls Qls

Qls

Qt Tidcr

Tvs

Tidcd

Tvs

Tvs

Tts

Tt Tt

Tt

Ta

Tba

Tba

Tba

Thba Tha

Tecu

Tecu

Tgo

Tgsb

Twfsh

Thd

Qtcr

Tba

Tha

Tba Tt Tvs

Tba Tvs Tvs

Tvs

Ta

C'

C

B

A

A'

B'

Gabbro (Oligocene?)—Light-gray, medium-grained gabbro exposed on ridgecrest 

north of Timber Creek; configuration unknown; inferred to be a sill. 

Composed of plagioclase, olivine, clinopyroxene, and Fe-Ti oxide with 

intergranular texture. Pervasively altered to albite, stilbite, smectitie, hematite

VOLCANIC AND SEDIMENTARY ROCKS

Eagle Creek Formation

Eagle Creek Formation (Miocene)—Sequence of weakly lithified, dark-brown to 

light-gray, predominantly coarse-grained volcaniclastic rocks and local 

interbedded lava flows (Wise, 1961, 1970; Waters, 1973; Hammond, 1980). 

Overlies lava flows of Hamilton Creek and underlies Grande Ronde Basalt; 

both contacts are erosional unconformities, hence unit thickness is highly 

variable, with maximum about 425 m on east flank of Hamilton Mountain. 

Fossil flora from locality near type area in Oregon, southeast of quadrangle, 

indicates an early Miocene age (Chaney, 1959). Divided into two informal 

members on basis of compositional and textural characteristics: 

Upper volcanic member—Composed dominantly of light-gray to white, 

moderately indurated to friable diamictite (chiefly lithic-pyroclastic-flow and 

lahar deposits) and interbedded fluvial conglomerate, sandstone, siltstone, and 

airfall tuff; pumiceous beds rare. In map area, best exposed in landslide 

headscarp on southeast side of Hamilton Mountain; elsewhere, areas underlain 

by this unit typified by boulder-strewn surfaces that superficially resemble 

those underlain by till. Clasts, as large as 3 m across, chiefly porphyritic 

pyroxene- and hornblende-phyric andesites and dacites broadly similar to 

interbedded lava flows. Plagioclase from a dacitic lithic pyroclastic-flow 

deposit yielded an 40Ar/39Ar age of 19.48±0.04 Ma. Locally includes andesite 

and dacite flows:

Pyroxene andesite—Dark-gray, porphyritic to seriate, pyroxene±olivine± 

hornblende-phyric andesite flows within the upper member; platy to columnar; 

to 15 m thick. Contains phenocrysts of plagioclase (15–25 percent; as long as 

10 mm), clinopyroxene (2–5 percent; 1–2 to 6 mm across), orthopyroxene 

(2–4 percent; 1–2 mm across), Fe-Ti oxide (0.4 percent; ≤0.5 mm across), and 

olivine (≤1 percent; 0.5–1 mm across); partly to completely replaced by 

smectite; with sparse chromian-spinel inclusions and reaction rinds of 

orthopyroxene and magnetite) or green-brown hornblende (≤1 percent; 0.5–1 

mm across) in a hyalopilitic to pilotaxitic groundmass. Plagioclase separated 

from flow on ridgecrest southeast of Three Corner Rock yielded an 40Ar/39Ar

age of 20.07±0.3 Ma

Hornblende andesite—Distinctive, coarsely porphyritic, dark-gray flows and 

flow breccia of hornblende-pyroxene andesite and basaltic andesite; exposed 

nearly continuously in Hardy Creek north of Rodney Falls; also forms Hardy 

Falls; to 90 m thick; configuration suggests flow occupies paleovalley cut into 

slightly older volcaniclastic strata. Typically columnar, with columns as wide 

as 3 m across transected by closely spaced cross joints. Phenocrysts of blocky, 

complexly zoned plagioclase, (10–20 percent; 1 to 5 mm across; locally 

glomerocrystic), orthopyroxene (1–3 percent; 1 to 3 mm across; variably 

replaced by pale yellow smectite), clinopyroxene (about 1 percent; 1 to 3 mm 

across), prismatic orange-brown hornblende (about 1 percent; 0.5 to 3 mm 

long; largely to completely dehydrated to fine-grained assemblage of 

clinopyroxene, Fe-Ti oxide, and biotite), equant to amoeboid quartz (<1 

percent; 0.5 to 1 mm across; typically with thin rind of granular pyroxene), and 

Fe-Ti oxide (<1 percent; 0.5 mm across) in a plagioclase-microphyric 

pilotaxitic groundmass. Analyses show 56–58 weight percent SiO2

 and higher 

K2O (>1.3 weight percent), Ba (>450 ppm), Sr (>1000 ppm), Ba/Nb (>100), 

Sr/Y (>59) than older andesites (Ta, Tha) in map area. 40Ar/39Ar age of 

19.50±0.5 Ma was obtained from plagioclase of sample collected at top of 

Hardy Falls

Hornblende dacite—Light-gray, porphyritic, hornblende dacite; forms 

rubbly-surfaced dome in headwaters of Woodward Creek. Contains 

phenocrysts of blocky white plagioclase (12–15 percent; subhedral to 

anhedral; 4 to 10 mm across; much altered to kaolinite), prismatic green-brown 

hornblende (8–10 percent; 0.5 to 2 mm long; Fe-Ti-oxide-rich dehydration 

rinds), biotite (<0.5 percent; 0.5 to 1 mm across; partly replaced by coarse 

hornblende), and Fe-Ti oxide (<0.5 percent; 0.5 mm across) in felsitic 

groundmass

Lower fluvial sedimentary member—Dark-green to brown, fluvially deposited 

conglomerate, sandstone, siltstone, and rare debris-flow beds; extensively 

altered to low-temperature clay-bearing assemblages but less indurated than 

underlying rocks. Thickness highly variable; unit thin or absent in most of 

quadrangle but as much as 270 m thick in area east of Hamilton Mountain. 

Conglomerate generally clast supported, ranges from well sorted to poorly 

sorted; clasts include various volcanic rocks but porphyritic rocks like those of 

underlying lava flows (Thba, Tha) dominate

Lava Flows of Hamilton Creek

Lava flows of Hamilton Creek (Miocene)—Sequence of porphyritic to seriate 

andesite, basaltic andesite, and rare dacite flows and locally interbedded tuff 

breccia exposed in upper Hamilton Creek valley and on steep slopes to east 

and southwest of Three Corner Rock. Conformably(?) overlies volcaniclastic 

rocks (Tt and Tvs); contact with superjacent Eagle Creek Formation is 

profound erosional unconformity marked by thick saprolitic paleosol. 

Maximum thickness about 425 m. Mapped as lava flows of Three Corner Rock 

by Hammond (1980) and as andesite flows of Three Corner Rock by Phillips 

(1987). 40Ar/39Ar plagioclase ages of 21.94±0.06 Ma and 21.59±0.31 Ma were 

obtained from two flows in this unit

Andesite—Dark-gray to brown, chiefly porphyritic to seriate, predominantly 

low-SiO2, pyroxene-andesite flows and flow breccia. Individual flows 

typically 10 to 20 m thick; commonly columnar jointed. Petrographically 

similar to associated basaltic andesites (Thba), with phenocrysts of 

plagioclase (15–30 percent; to 7 mm long; locally glomerocrystic; slightly 

altered to clay), clinopyroxene (≤6 percent; to 2 mm across), orthopyroxene (≤

7 percent; to 2 mm across; slightly altered to pale yellow smectite), and Fe-Ti 

oxide (≤1.5 percent; ≤ 0.5 mm across) with or without olivine (0–2 percent; to 

2 mm across; some with chromian spinel inclusions; typically with thin rinds 

of fine-grained granular pyroxene±Fe-Ti oxide; totally replaced by 

smectite±hematite) and rare orange-brown hornblende (0.5–1 mm long; 

largely to completely replaced by fine-grained pyroxene+Fe-Ti oxide) in an 

intergranular to intersertal to pilotaxitic groundmass of plagioclase, pyroxene, 

Fe-Ti oxide, apatite, and interstitial smectite±quartz

Basaltic andesite—Dark-gray to brown, porphyritic to seriate, basaltic andesite 

flows and flow breccia. Individual flows typically 10–20 m thick; commonly 

columnar jointed. Typically contains phenocrysts of plagioclase (20–35 

percent; to 5 mm long; locally as glomerocrysts; slightly altered to clay), 

olivine (trace–6 percent; to 3 mm across; some with chromian spinel 

inclusions; typically with thin rinds of fine-grained granular pyroxene±Fe-Ti 

oxide; partly to completely replaced by yellow-orange smectite), 

clinopyroxene (0.5–5 percent; to 2 mm across) with or without orthopyroxene 

(0–3 percent; 1 mm across), Fe-Ti oxide (<1 percent; 0.5 mm across), and rare 

orange-brown hornblende (<0.5 percent; 0.5 to 1 mm long; largely dehydrated 

to fine-grained pyroxene+Fe-Ti oxide) in an intergranular to intersertal 

groundmass of plagioclase, pyroxene, Fe-Ti oxide, and interstitial glass 

(variably altered to smectite)

Tuff breccia—Dark-brown, indurated, monolithologic diamictite composed of 

porphyritic clasts compositionally similar to associated lava flows (Thba, 

Tha); forms lenticular bodies as thick as 50 m. Interpreted as syn-eruptive 

debris-flow deposits

 Hypersthene dacite—Scattered flows of light-gray, platy, sparsely to moderately 

porphyritic, pyroxene hypersthene dacite near eroded top of Hamilton Creek 

unit directly south of Three Corner Rock and in headwaters of Woodward 

Creek; maximum thickness about 35 m. Phenocrysts of plagioclase (4–11 

percent; 1 mm across; locally altered to montmorillonite) and orthopyroxene 

(0.5–1.5 percent; 0.5 mm across; largely replaced by smectite), and Fe-Ti 

oxide (≤0.5 percent; ≤0.5 mm across) in a felsic, micropoikilitic groundmass

Strata Older Than Lava Flows of Hamilton Creek

Basaltic andesite (Miocene and Oligocene)—Medium- to dark-gray, aphyric to 

sparsely phyric, rarely porphyritic, basaltic andesite flows and flow breccia 

stratigraphically below lava flows of Hamilton Creek; locally includes 

andesite flows and clastic interbeds too small or poorly exposed to map. 

Phenocrysts of plagioclase (0–30 percent; 1 to 4 mm long; locally 

glomerocrystic; slightly to highly altered to clay minerals), olivine (0–5 

percent; 1 to 2 mm across; rare chromian spinel inclusions; altered to 

smectite±hematite), clinopyroxene (0–3 percent; 1 to 3 mm across), with or 

without orthopyroxene (<1 percent; 1 to 2 mm across; partly to completely 

replaced by smectite), and Fe-Ti oxide (<0.5 percent; 0.5 mm across) in an 

intergranular to intersertal to trachytic groundmass of plagioclase, pyroxene, 

Fe-Ti oxide, and interstitial glass variably replaced by smectite. A flow that 

crops out on the Stebbins Creek-Washougal River divide yielded an 40Ar/39Ar 

plagioclase age of 25.78±0.55 Ma 

Andesite (Miocene and Oligocene)—Medium- to dark-gray, aphyric to sparsely 

phyric, pyroxene andesite flows stratigraphically below lava flows of 

Hamilton Creek (Thba, Tha); locally includes basaltic andesite flows and 

clastic interbeds too small or poorly exposed to map. Phenocrysts of 

plagioclase (<10 percent; 1 to 4 mm long; locally glomerocrystic; slightly to 

highly altered to clay minerals), clinopyroxene (≤2 percent; 1 to 3 mm across), 

and (or) orthopyroxene (≤2 percent; 1 to 2 mm across; partly to completely 

replaced by smectite), and microphenocrysts of Fe-Ti oxide in a pilotaxitic, 

cryptocrystalline, or micropoikilitic groundmass

DESCRIPTION OF MAP UNITS

SURFICIAL DEPOSITS

Artificial fill (Holocene)—Unconsolidated sand, gravel, and crushed rock 

forming raised railroad bed south and east of Beacon Rock

ALLUVIAL AND MASS WASTAGE DEPOSITS

Alluvium (Holocene and Pleistocene)—Unconsolidated silt, sand, and gravel. 

Most mapped deposits consist of stratified sand and cobble to pebble gravel, 

but alluvium in Hamilton Creek valley consists largely of andesite boulders 

as large as 4 m across, largely recycled from the Eagle Creek Formation 

(Tecl and Tecu)

Debris-flow deposits (Holocene)—Diamicts forming small fans where short 

steep tributaries enter Hamilton Creek. Consist of angular to subrounded 

blocks of andesite (Tha) and Eagle Creek Formation (Tecu)

Talus deposits (Holocene and Pleistocene)—Unsorted accumulations of angular 

blocks beneath cliffs. Extensive talus aprons in southwest part of map area 

where Grande Ronde Basalt overlies Eagle Creek Formation and in northeast 

part of map area where lava flows (Thba, Tha) overlie volcaniclastic beds 

Tvs, Tt)

Landslide deposits (Holocene and Pleistocene)—Diamictons of unsorted, 

angular bedrock and surficial material transported downslope en masse. 

Chiefly deep seated slumps and rockslide, earthflow, and debris-flow 

deposits. Most mapped slides head at arcuate scarps and exhibit bulbous toes 

and hummocky, poorly drained surfaces. Typically formed where lava flows 

overlie weakly consolidated sediments. Large Skamania Landslide Complex 

(Palmer, 1977) and landslides below Hamilton Mountain are composite 

features formed during multiple slide events (Pierson and others, 2016). 

They involve failure of weakly consolidated Eagle Creek Formation (Tecu) 

where overlain by Columbia River Basalt Group or Quaternary volcanic 

rocks; failure plane for landslide complex in Hamilton Creek valley is within 

deeply weathered volcaniclastic rocks (Tvs, Tt) beneath lava flows of 

Hamilton Creek (Tha, Thba)

Bridge of the Gods outburst-flood deposits (Holocene)—Unconsolidated 

gravel underlying bench bisected by Hardy Slough. Poorly exposed in map 

area but exposures to east show poorly sorted, subangular to well-rounded, 

clast-supported cobble gravel. Clasts mostly derived from Tertiary volcanic 

and sedimentary rocks of the Cascade Range and the Columbia River Basalt 

Group. From texture and position, inferred to have been deposited by 

outburst flooding from breaching of the Bonneville landslide, which blocked 

the Columbia River between A.D. 1425 and 1450 (O’Connor and Burns, 

2009)

Terrace deposits of upper Washougal River (Holocene and (or) 

Pleistocene)—Unconsolidated to weakly consolidated cobble gravel capping 

strath terraces along Washougal River. Tread elevations range from 780 to 

980 ft (240 and 300 m). Deposits typically <6 m thick, poorly sorted to 

moderately well sorted, and imbricated; matrix of coarse sand. Clasts as 

large as 30 cm across, well rounded to subrounded, composed of 

Cascade-derived volcanic and granitic rocks; lack weathering rinds

ROCKS OF THE BORING VOLCANIC FIELD

Basaltic andesite of Beacon Rock (Pleistocene)—Medium-gray basaltic 

andesite (55–56 weight percent SiO2

) that forms 250-m-high, steep-sided, 

massive to columnar-jointed monolith of Beacon Rock; interpreted as 

exhumed core of cinder cone (Williams, 1916; Hodge, 1938; Hammond, 

1980; Hammond and Korosec, 1983; Waters, 1973). Texturally variable: 

rock near base of monolith is dense to microvesicular and contains sparse 

phenocrysts of olivine (<2 percent; 1 to 2 mm across; chromian spinel 

inclusions), plagioclase (<0.5 percent; 1 mm long), and clinopyroxene (<0.5 

percent; 0.5 to 1 mm across), in a coarse-grained, intergranular to trachytic 

groundmass of plagioclase, clinopyroxene, orthopyroxene, and Fe-Ti oxide; 

rock at summit is highly vesicular, oxidized, and contains olivine, 

plagioclase, and clinopyroxene phenocrysts in an intersertal groundmass with 

substantial devitrified glass. Chemically somewhat evolved (Mg# (atomic 

ratio 100Mg/(Mg+Fe2+) with Fe2+ set to 0.85 × Fetotal) =57–60) calc-alkaline 

composition with low Ni (60–80 ppm) and Cr (80–90 ppm) and moderate 

K2O (0.8–0.9 weight percent), Ba (230–260 ppm), and Sr (660–720 ppm) 

contents; Ba/Nb relatively low (25–28). Normal magnetic polarity. 40Ar/39Ar 

age is 58±5 ka (Fleck and others, 2014); young age indicates host cinder cone 

was removed by the latest Pleistocene cataclysmic floods (20–15 ka; Waitt, 

1980, 1985, 1994, 1996; Atwater, 1986; Benito and O’Connor, 2003; Clague 

and others, 2003; O'Connor and Benito, 2009), the largest of which were 

deep enough to completely submerge monolith (Benito and O’Connor, 2003)

Basaltic andesite of Boyles Creek (Pleistocene)—Scoria cone and lava flow 

northwest of McCloskey Creek; apparent source of several flows in adjacent 

Bobs Mountain quadrangle (R.C. Evarts, unpub. mapping). Block-lava flow 

on northwest flank composed of porphyritic microvesicular basaltic andesite 

(approximately 54 weight percent SiO2) with phenocrysts of olivine (4 

percent; 0.5–1 mm across, some as large as 2 mm across; chromian spinel 

inclusions) and yellow-green clinopyroxene (4 percent; 0.5 mm across, rarely 

as large as 1 mm across; typically as glomerocrysts of subhedral to anhedral 

crystals) in a trachytic to intersertal groundmass of plagioclase, 

clinopyroxene, Fe-Ti oxide, and variable amounts of interstitial pale red 

glass; rare sieved plagioclase xenocrysts to 1 mm across. Calc-alkaline with 

Mg#=62 and moderately high K2O (1.2–1.5 weight percent), Ba (400–440 

ppm), and Sr (960–1,030 ppm) contents; Ba/Nb=40–60. Normal magnetic 

polarity (Fleck and others, 2014). 40Ar/39Ar ages of flows in Bobs Mountain 

quadrangle range from 220 ka to 286 ka (Fleck and others, 2014)

Andesite of McCloskey Creek (Pleistocene)—Dark-gray, platy, microvesicular, 

calc-alkaline andesite (57 wt percent SiO2); overlies Columbia River Basalt 

Group near southwest corner of map area; erupted from vent marked by 

dome about 1 km to west of map area (R.C. Evarts, unpub. mapping). Rubbly 

flow surface composed of meter-sized angular blocks of modestly vesicular 

andesite. Contains sparse microphenocrysts of olivine (<1 percent) and 

hornblende (<1 percent; largely dehydrated to fine-grained granular pyroxene 

and oxide) in a trachytic to pilotaxitic groundmass of feldspar, pyroxene, 

Fe-Ti oxide, and minor interstitial pale brown glass. Moderately rich in K2O 

(1.4 weight percent). Normal magnetic polarity (Fleck and others, 2014). 

40Ar/39Ar age of 270±11 ka was obtained from locality in adjacent Bobs 

Mountain quadrangle (Fleck and others, 2014)

Basaltic andesite of Hard Scramble Creek (Pleistocene)—Eroded cone of 

agglutinate and associated blocky-carapaced lava flow of microvesicular 

basaltic andesite (54–56 weight percent SiO2) directly west of Hard Scramble 

Creek; flow approximately 10 m thick. Contains microphenocrysts of olivine 

(variably oxidized; with granular pyroxene rinds and chromian spinel 

inclusions), plagioclase, yellowish-brown clinopyroxene, and 

microcrystalline pyroxene+Fe-Ti-oxide aggregates (probable products of 

hornblende dehydration) in a seriate trachytic groundmass of plagioclase, 

clinopyroxene, orthopyroxene, and Fe-Ti oxide; hornblende and large, sieved 

feldspars are probably xenocrysts. Chemically, lava is calc-alkaline with 

Mg#=58–60, and potassic (1.4–1.6 weight percent K2O) with Ba=480–570 

ppm, Sr=840–945 ppm, and Ba/Nb=54–63. Normal magnetic polarity (Fleck 

and others, 2014). Sample collected just west of quadrangle boundary 

yielded 40Ar/39Ar age of 280±5 ka (Fleck and others, 2014)

Basaltic andesite of upper McCloskey Creek (Pleistocene)—Small cinder cone 

and lava flow(s) of microvesicular, olivine-microphyric basaltic andesite 

(about 53 weight percent SiO2); flows exhibit thick (1–3 m diameter) 

columns with pronounced subhorizontal platy parting and rubbly oxidized 

flow tops; thickness probably <10 m. Contain small olivine phenocrysts (3–5 

percent; 0.5 to 1 mm across; chromian spinel inclusions) and rare plagioclase 

laths (as long as 1 mm) in a fine-grained trachytic groundmass of plagioclase, 

clinopyroxene, Fe-Ti oxide, and minor interstitial glass. Calc-alkaline and 

relatively primitive (Mg#=66) with comparatively low K2O (0.6–0.7 weight 

percent) and about 330 ppm Ba, 830–910 ppm Sr, and Ba/Nb about 40. Age 

unknown but probably older than basaltic andesite of Boyles Creek (Qmbc). 

Normal magnetic polarity determined from oriented hand sample (J.T. 

Hagstrum, written commun., 2012)

Basaltic andesite of upper Hard Scramble Creek (Pleistocene)—Single flow of 

light-gray, olivine-microphyric, calc-alkaline basaltic andesite (54 weight 

percent SiO2) exposed in upper reach of Hard Scramble Creek; about 5 to 8 m 

thick; vent location unknown, presumably buried by younger volcanic 

deposits. Contains microphenocrysts of olivine (about 5 percent; 0.5 mm 

across) in a fine-grained trachytic groundmass of plagioclase, clinopyroxene, 

and Fe-Ti oxide. Distinguished from nearby lavas by lower TiO2 (0.9 weight 

percent), K2O (0.6 weight percent), Ba (170 ppm), and Sr (587 ppm). Age not 

known but probably older than basaltic andesite of Hard Scramble Creek 

(Qmhs). Normal magnetic polarity determined from oriented hand sample 

(J.T. Hagstrum, written commun., 2012)

Basalt of Little Beacon Rock (Pleistocene)—Medium- to dark-gray, 

sparsely vesicular, porphyritic to seriate basalt (51–52 weight percent 

SiO2) that forms eroded vent complex at Little Beacon Rock and flow 

remnant banked against Eagle Creek Formation in landslide headwall 

to northwest. Little Beacon Rock consists of coarse agglutinated 

scoria cut by irregular to tabular basalt intrusions, including a 

chemically similar but fine-grained, microvesicular, 

olivine-microphyric variant not found elsewhere. Typically contains 

phenocrysts of olivine (5–9 percent; 0.5 to 2 mm; variably oxidized, 

with abundant chromian spinel inclusions) and rare plagioclase (to 2 

mm long) in a holocrystalline, trachytic groundmass of plagioclase, 

clinopyroxene, olivine, Fe-Ti oxide, and local traces of brown 

hornblende and phlogopite. Calc-alkaline, medium-potassium (about 

0.9 weight percent K2O), relatively primitive (Mg#=65–67) 

composition with 350–450 ppm Ba, 700–900 ppm Sr, and 

Ba/Nb=40–55. Normal magnetic polarity (Fleck and others, 2014). 

40Ar/39Ar age of 498±7 ka obtained for sample collected from the lava 

flow (Fleck and others, 2014)

Basaltic andesite of Duncan Creek (Pleistocene)—One or more flows of gray, 

porphyritic, basaltic andesite (52–54 weight percent SiO2) in scattered 

outcrops along western headwall scarp of Skamania Landslide Complex. 

Source vent probably was to east in area now occupied by landslide deposits. 

Phenocrysts of olivine (4–5 percent; 1 to 2 mm, rarely to 4 mm across; 

chromian spinel inclusions; variable replaced by iddingsite) with or without 

plagioclase (<0.5 percent; 0.5 to 1 mm long) and (or) brown clinopyroxene 

(≤0.1 percent; 0.5 mm across) in a holocrystalline trachytic groundmass of 

plagioclase, clinopyroxene, Fe-Ti oxide, and minor interstitial glass. 

Calc-alkaline, medium-potassium (0.9–1.1 weight percent K2O) composition; 

distinguished by exceptionally high Nb content (16–19 ppm) and low Ba/Nb 

(about 15). Reversed magnetic polarity (Fleck and others, 2014). 40Ar/39Ar 

ages of 821±24 ka and 879±7 ka obtained for samples collected in Duncan 

Creek and north of Archer Mountain (Fleck and others, 2014); analytically 

more precise older age is preferred

Basaltic andesite of Little Creek (Pleistocene)—Aphyric basaltic andesite 

(53–54 weight percent SiO2) flow forming bench on east side of Hardy 

Ridge. Source vent probably to west in area now occupied by Skamania 

Landslide Complex. Trachytic, microseriate texture with plagioclase laths to 

1 mm long in matrix of fine-grained granular pyroxene and Fe-Ti oxide. 

Mildly tholeiitic and evolved (Mg# about 54) with relatively low Ba content 

(about 150 ppm) and low Ba/Nb (19–23). Magnetic polarity undetermined. 

40Ar/39Ar age of 1,055±22 ka (Fleck and others, 2014)

Basaltic andesite of Three Corner Rock (Pleistocene)—Massive plug and 

associated lava flow of seriate, dense to microvesicular, light- to 

medium-gray basaltic andesite (52–53 weight percent SiO2) (Hammond, 

1980). Plug forming bare prominence of Three Corner Rock is coarsely 

jointed, breaks into blocks several meters across; scattered remnants of red 

cinders and ash directly east of Three Corner Rock indicate plug was 

emplaced into since-eroded cinder cone. Blocky-surfaced lava flow about 15 

m thick extends along ridgecrest to east of plug for about 1.5 km. Consists of 

variably oxidized olivine (0.5–1 mm across; abundant chromian spinel 

inclusions), plagioclase (typically 0.5 to 1 mm long), greenish-brown 

clinopyroxene (0.5 to 1 mm across; commonly in multi-crystal clots), and 

Fe-Ti oxide (<0.5 mm across) with a holocrystalline, intergranular to 

trachytic texture. Calc-alkaline, medium-potassium (0.7–0.8 weight percent 

K2O), and relatively primitive (Mg#=66–68) composition; has low Ba/Nb 

(20–25). Reversed magnetic polarity (Fleck and others, 2014). 40Ar/39Ar age 

of plug is 1,065±5 ka (table ; Fleck and others, 2014)

Basaltic andesite of Woodward Creek (Pleistocene)—Flows of gray, aphyric to 

olivine+plagioclase-microphyric, basaltic andesite (52–54 weight percent 

SiO2) exposed in headwaters of Woodward Creek; plug surmounted by 

eroded cinder cone exposed in high eastern headwall scarp of Skamania 

Landslide Complex. Distinguished by 1 to 2 percent embayed to spheroidal 

pyroxene xenocrysts as large as 3 mm across, probably derived from 

underlying Miocene units (Tecu, Tha); orthopyroxene xenocrysts invariably 

rimmed by intergrown olivine and Fe-Ti oxide; holocrystalline, trachytic to 

intergranular groundmass composed of plagioclase and olivine 

microphenocrysts in matrix of granular pyroxene and Fe-Ti oxide; numerous 

microcrystalline gabbroic and pyroxenitic xenoliths 1–2 mm across. 

Chemically variable (Mg#=68–60), K2O=0.7–0.9 weight percent, with 

relatively low Ba contents (130–174 ppm) and low Ba/Nb (16–24). Normal 

magnetic polarity (Fleck and others, 2014). 40Ar/39Ar age of sample from plug 

is 1,056±9 ka (table 4; Fleck and others, 2014)

COLUMBIA RIVER BASALT GROUP

Wanapum Basalt (Miocene)—Dark-gray to black, variably vesicular to 

microvesicular, aphyric to sparsely phyric lava flows of high-TiO2 tholeiitic 

basalt and basaltic andesite; exhibits blocky to columnar or hackly jointing 

patterns and vesicular flow tops; upper few meters locally thoroughly 

weathered to red saprolite or laterite. Textures resemble those described by 

Long and Wood (1986) for basalt flows in the Columbia Basin. 

Nomenclature follows Swanson and others (1979b), Beeson and others 

(1985), and Martin and others, (2013)

Frenchman Springs Member—Most voluminous and widespread member of 

Wanapum Basalt (Tolan and others, 1989; Reidel and others, 2013). Overlies 

Sentinel Bluffs Member of Grande Ronde Basalt near southwest corner of 

quadrangle and on Birkenfeld Mountain (informal name) east of Hamilton 

Creek, just east of the map area (shown on index map)

Basalt of Sentinel Gap of Beeson and others (1985)—Poorly exposed flow 

of blocky jointed, microphyric basalt to basaltic andesite; overlies basalt of 

Sand Hollow (Twfsh) in upper McCloskey Creek area. Contains sparse (< 1%) 

phenocrysts of plagioclase (to 10 mm long), brown augite (to 1 mm across), 

and olivine (to 1 mm across) in abundant opaque glass. Lithologic 

characteristics, stratigraphic position, and chemistry indicate this flow is a 

high-P2O5 variant of the informal basalt of Sentinel Gap of Beeson and others 

(1985), which is stratigraphically near the top of the Frenchman Springs 

Member

Basalt of Sand Hollow of Beeson and others (1985)—Blocky to columnar 

jointed, sparsely but coarsely plagioclase-phyric basalt to basaltic andesite; 

overlies Sentinel Bluffs Member of Grande Ronde Basalt; approximately 25 m 

thick. Distinguished by scattered large (to 35 mm across) phenocrysts and 

glomerocrysts of amber plagioclase (<0.5%); groundmass typically 

microphyric, intersertal to intergranular, composed of plagioclase, brown 

augite, Fe-Ti oxide, olivine (partly altered to clay), and red-brown interstitial 

glass; microvesicles typically filled with yellow-brown isotropic clay or 

calcite. Stratigraphic position, chemistry, petrography, and normal magnetic 

polarity (J.T. Hagstrum, written commun., 2006) indicate correlation with 

informal basalt of Sand Hollow of Beeson and others (1985), erupted about 15 

Ma (Beeson and others, 1985; Barry and others, 2010, 2013)

Grande Ronde Basalt (Miocene)—Light-gray to black flows of tholeiitic basaltic 

andesite; vesicular to microvesicular, aphyric to microphyric to sparsely 

plagioclase-phyric; flows exhibit relatively high SiO2 and low TiO2 contents 

characteristic of the Grande Ronde Basalt of the Columbia River Basalt 

Group (Swanson and others, 1979b; Mangan and others, 1986; Beeson and 

others, 1989; Reidel and others, 1989a; Hooper, 1997; Reidel and Tolan, 

2013). Unconformably overlies Eagle Creek Formation. Flows typically 

consist of thick, hackly jointed zone (entablature) above thin basal 

columnar-jointed zone (colonnade); some flows are blocky to 

columnar-jointed throughout; vesicular uppermost parts of flows may be 

crudely columnar; pillow lava and pillow breccia are found locally at base of 

some flows. Typical samples are intergranular to intersertal, with lathlike 

plagioclase, granular clinopyroxene, and Fe-Ti oxide crystals in abundant 

dark glass; textures resemble those described by Long and Wood (1986) for 

correlative flows in Columbia Basin. Basalt erupted from vents in eastern part 

of basin and entered northwestern Oregon and southwestern Washington 

through a wide gap in the Cascade Range (Tolan and others, 1989; Beeson 

and Tolan, 1990). Age determinations in Columbia Basin indicate 

emplacement of Grande Ronde flows between 16 Ma and 15 Ma (Barry and 

others, 2010, 2013). Nomenclature is that of Reidel and Tolan (2013); lower 

case text designates informal units

Sentinel Bluffs Member—Uppermost member of the Grande Ronde Basalt. 

Flows distinguished by relatively high MgO (4.5–5.2 wt percent MgO) and Cr 

(≥20 ppm) contents and normal magnetic polarity (Reidel and others, 1989; 

Reidel, 2005). Jointing patterns vary from blocky/columnar to 

columnar/entablature. Four chemically distinguishable flows occur at top of 

member in map area although only two or three are present in most locations. 

Flows range from 20 to 35 m thick; maximum thickness of member is about 75 

m. Stratigraphic position and chemical and paleomagnetic characteristics 

indicate equivalence to Sentinel Bluffs Member of Reidel (2005) and Reidel 

and Tolan (2013), which is within the N2

 magnetostratigraphic unit of 

Swanson and others (1979b). Chemical compositions and stratigraphic 

positions of flows indicate they belong to three of the compositional types 

identified in the Columbia Basin by Reidel (2005): the basalt of Museum, the 

basalt of Stember Creek, and the basalt of McCoy Canyon

Winter Water Member—Single flow of basalt with moderately low MgO 

content (3.8–3.9 weight percent) and normal magnetic polarity; contains 

scattered plagioclase phenocrysts and glomerocrysts 1 to 3 mm across in 

aphyric to sparsely microphyric groundmass. Underlies Sentinel Bluffs 

Member (Tgsb) and overlies Ortley member (Tgo); about 40 m thick on ridge 

west of Hardy Creek; as thick as 60 m near southwest corner of quadrangle. 

Stratigraphic position and chemical and paleomagnetic characteristics indicate 

equivalence to Winter Water Member of Reidel and Tolan (2013), which is 

within the N2 magnetostratigraphic unit of Swanson and others (1979b). 

Distinguished paleomagnetically by a low inclination (J.T. Hagstrum, written 

commun., 2006, 2007)

Ortley member—Flows of black, aphyric, intersertal to intergranular basaltic 

andesite with relatively low MgO (3.4–3.7 weight percent) and TiO2 (1.9–2.03 

weight percent) contents and normal magnetic polarity (J.T. Hagstrum, written 

commun., 2006, 2007, 2008). As many as five Ortley flows present in map 

area; underlies Winter Water Member (Tgww) and overlies Grouse Creek 

Member (Tggc). Individual flow units range from 20 to 90 m thick; total 

member thickness generally 120 to 200 m except at Hamilton Mountain, where 

a ~200-m-thick flow and associated foreset-bedded pillow breccia occupy a 

paleocanyon and total member thickness exceeds 320 m (Wells and others, 

2009). Stratigraphic position and chemical and paleomagnetic characteristics 

indicate equivalence to informal Ortley member of Reidel and Tolan (2013), 

within the N2

 magnetostratigraphic unit of Swanson and others (1979b)

Grouse Creek member—Flow of black, aphyric, intersertal to intergranular 

basaltic andesite with relatively low MgO (3.65–3.70 wt percent) and TiO2

(<2.0 weight percent) content. Underlies Ortley member (Tgo) and overlies 

Wapshilla Ridge Member (Tgwr) in box canyon near southwest corner of map 

area. Chemically similar to overlying Ortley flows but has reversed magnetic 

polarity (J.T. Hagstrum, written commun., 2010); approximately 30 m thick in 

map area, thickens to south. Stratigraphic position and chemical and 

paleomagnetic characteristics indicate equivalence to informal Grouse Creek 

member of Reidel and Tolan (2013), within the R2 magnetostratigraphic unit of 

Swanson and others (1979b)

Wapshilla Ridge Member—Thick flow of black to dark-gray, aphyric to rarely 

phyric, typically microphyric basaltic andesite with relatively low MgO 

(3.4–3.5 weight percent) and high TiO2 (2.3–2.4 weight percent) contents; 

reversed magnetic polarity (Tolan, 1982; J.T. Hagstrum, written commun., 

2007); to 70 m thick. Stratigraphically lowest Grande Ronde Basalt unit in map 

area; underlies Grouse Creek member (Tggc) and unconformably overlies and 

pinches out against Eagle Creek Formation (Tecu). Stratigraphic position and 

chemical and paleomagnetic characteristics indicate equivalence to Wapshilla 

Ridge Member of Reidel and Tolan (2013), within the R2 magnetostratigraphic 

unit of Swanson and others (1979b)

ROCKS OF THE ANCESTRAL CASCADE VOLCANIC ARC

INTRUSIVE ROCKS

Intrusive pyroxene dacite (Miocene)—Light-gray, densely porphyritic, 

quartz-pyroxene dacite intrusion that intrudes Eagle Creek Formation (Tecu, 

Tehd) in lower Hardy Creek. Phenocrysts and glomerocrysts of blocky 

plagioclase (about 40 percent; 2 to 6 mm across), orthopyroxene (about 5 

percent; 1 to 2 mm across; partly altered to pale yellow smectite), 

clinopyroxene (about 3 percent; 1 to 3 mm across), equant to amoeboid 

quartz (about 5 percent; 0.5 to 1 mm across), Fe-Ti oxide (≤0.5 percent; 0.5 

mm across), and hornblende (<0.5 percent; 0.5 mm across; completely 

dehydrated to fine-grained pyroxene+Fe-Ti oxide) in a felsic 

cryptocrystalline groundmass. Petrography and chemical composition, 

particularly high Sr and Sr/Y, indicate affinity with Eagle Creek Formation 

so may be not much younger than 20 Ma

Intrusive andesite of Lily Lake (Miocene)—Light-gray, platy, aphyric andesite 

that forms conical hill southwest of Lily Lake (informal name). Trachytic to 

pilotaxitic texture; composed of blocky plagioclase, granular clinopyroxene, 

orthopyroxene, and Fe-Ti oxide, and dark-brown interstitial glass + 

pale-yellow smectite. Chemical characteristics, such as high Sr and Sr/Y, 

suggest affinity with early Miocene Eagle Creek Formation, although aphyric 

texture contrasts with typically porphyritic Eagle Creek rocks. Only large 

intrusion in map area known to postdate lava flows of Hamilton Creek 

(Thba, Tha); may also intrude Eagle Creek Formation but contacts buried by 

landslide debris

Intrusive basaltic andesite (Miocene or Oligocene)—Dikes, sills, and plugs of 

aphyric to porphyritic basaltic andesite; texturally diverse, broadly similar to 

compositionally equivalent flows (Tba, Ta); generally much less altered than 

host rocks

Intrusive andesite (Miocene or Oligocene)—Dikes, sills, and irregular intrusions 

of aphyric to densely porphyritic pyroxene andesite and minor dacite; 

texturally diverse, broadly similar to compositionally equivalent flows (Ta, 

Tha). Intrusions in lower (Oligocene) part of stratigraphic section, along 

Washougal River and west of Timber Creek, tend to be highly altered like 

their host rocks whereas those at higher stratigraphic levels, south of Stebbins 

Creek, are generally fresher than their host rocks; some of these may be 

subvolcanic intrusions related to lava flows of Hamilton Creek (Thba, Tha)

Intrusive dacite (Miocene and (or) Oligocene)—Black, vitric, sparsely 

porphyritic, pyroxene-dacite intrusions; broadly similar to compositionally 

equivalent flows (Td); generally much less altered than host rocks

Diorite (Miocene and (or) Oligocene)—Scattered small intrusions of medium- to 

dark-gray to greenish-gray, seriate to porphyritic, pyroxene diorite in 

northwest part of map area. Composed of plagioclase (to 3 mm long), 

clinopyroxene (0.5 to 1 mm across), olivine and (or) orthopyroxene (0.5 to 1 

mm across) in intergranular to hypidiomorphic groundmass of plagioclase, 

pyroxene, Fe-Ti oxide, and minor interstitial quartz. Moderate to pervasive 

alteration to assemblages of albite, smectite, chlorite, titanite, calcite, 

hematite, prehnite, zeolites

Quartz Diorite (Miocene and (or) Oligocene)—Light-greenish-gray, 

medium-grained, seriate, pyroxene quartz diorite exposed on north valley 

slope of tributary of Deer Creek. Composed of phenocrysts and 

glomerocrysts of plagioclase (about 25 percent; 1 to 4 mm long; variably 

altered to zeolite minerals), clinopyroxene and orthopyroxene (1–2 percent 

each; 1 mm across), olivine (<1 percent; 1–2 mm across; totally replaced by 

brown smectite; granular orthopyroxene rinds), and Fe-Ti oxide (<0.5 

percent; 0.5 mm across) in a fine-grained hypidiomorphic groundmass of the 

same minerals plus interstitial quartz, smectite, and traces of orange-brown 

hornblende

Intrusive complex of Deer Creek (Miocene)—Intrusions of light-gray to buff, 

porphyritic, pyroxene dacite and rhyolite underlying slopes in Deer Creek 

valley and nearby areas. Outcrops typically composed of frost-heaved, 

unvegetated, angular blocks to 0.5 m across; resemble and grade into talus 

(Qt)

Rhyolite—Similar to dacite but contains equant to embayed quartz phenocrysts 

(about 2 percent; 0.5 to 1 mm across) and lacks hornblende. An 40Ar/39Ar age 

of 22.60±0.06 Ma was obtained for plagioclase from large intrusion east of 

Deer Creek

Dacite—Phenocrysts of chalky white plagioclase (about 15 percent; 1 to 4 mm 

long; typically altered to albite, montmorillonite, calcite), clinopyroxene (<1 

percent; 0.5 to 2 mm across; commonly rimmed by late-magmatic, pale-brown 

hornblende; variably altered to smectite, calcite), orthopyroxene (<1 percent; 

0.5 to 1 mm across; totally altered to smectite), and microphenocrysts of Fe-Ti 

oxide in a felsitic groundmass of altered feldspar, quartz, Fe-Ti oxide, and 

smectite, with or without hornblende

Intrusive andesite of Hard Scramble Creek (Miocene?)—Sill-like body of 

coarsely porphyritic pyroxene andesite exposed in Hard Scramble Creek and 

on hill to north. Contains phenocrysts of plagioclase (30–35 percent; 2 to 5 

mm long; typically forming glomerocrystic clumps), clinopyroxene (<1 

percent; 1 to 3 mm across), and orthopyroxene (<0.5 percent; 1 to 3 mm 

across) in groundmass of granular to skeletal to spherulitic crystals of 

plagioclase, pyroxene, and Fe-Ti oxide in dark-brown, devitrified glass; 

smectite amygdules. May be subvolcanic intrusion related to andesitic lava 

flows of Hamilton Creek (Tha). Maximum 40Ar/39Ar age of 23.14±0.18 Ma 

obtained from plagioclase separate

Volcaniclastic sedimentary rocks (Miocene and Oligocene)—Diverse 

assemblage of continental volcaniclastic rocks of inferred epiclastic origin. 

Consist of thin- to thick-bedded, well-sorted to poorly sorted siltstone, 

sandstone, conglomerate, breccia, and massive, poorly-sorted diamictites 

containing clasts as large as 1 m across; includes beds of pumiceous 

pyroclastic rock and lava flows too small or poorly exposed to map. Beds most 

commonly light green to olive green or greenish gray but also white, tan, or 

brown. Texturally and compositionally immature; most abundant clasts are 

volcanic rocks petrographically similar to interbedded lava flows and tuffs; 

other components include plagioclase, Fe-Ti oxide, and pyroxene crystals, 

pumice, vitric ash, fine-grained plutonic rocks, and plant remains. Moderately 

to pervasively altered to assemblages of clay minerals, zeolites, calcite, and 

quartz; interbedded lava flows are generally less altered. Interpreted as debris 

eroded from older and penecontemporaneous volcanoes and deposited by 

normal fluvial and lacustrine processes in low-lying medial to distal 

intervolcano environments. Dominate the lower part of the stratigraphic 

section in the map area. Tend to be poorly exposed, but nearly continuous 

outcrops exist in Prospector and Deer Creeks

Tuff (Miocene and Oligocene)—Heterogeneous unit composed dominantly of 

massive, indurated, light-green to brown andesitic to rhyolitic tuff, pumiceand lithic-lapilli tuff, and tuff breccia; inferred to be mostly of pyroclastic-flow 

and lahar origin but locally includes significant reworked debris. As mapped, 

includes tuffaceous rocks interbedded with and gradational to subordinate 

pumice-poor epiclastic sedimentary rocks. Nonwelded to densely welded 

pumiceous laiplli tuff (ignimbrite) commonly contains phenocrysts of 

plagioclase (to 25 percent; as long as 3 mm; commonly broken) and pyroxenes 

(<5 percent; 0.5 to 1 mm across) and Fe-Ti-oxide microphenocrysts in matrix 

of pumice and shards; original glass typically devitrified to cryptocrystalline 

feldspar and quartz or replaced by smectitic clay, but some densely welded 

vitrophyre preserved locally; commonly contains sparse volcanic lithic 

fragments, and charred wood. Alteration assemblages similar in character to 

those described for volcaniclastic sedimentary rocks (Tvs). Plagioclase from a 

vitrophyric ignimbrite northeast of Three Corner Rock yielded an 40Ar/39Ar age 

of 22.79±0.13 Ma. Some extensive densely welded tuff beds mapped 

separately:

Welded tuff (Oligocene)—Beds of densely welded tuff in valley of Stebbins 

Creek. Beds typically 20–30 m thick, traceable for as much as 4.5 km. 

Generally devitrified, rarely glassy; some superficially resemble flow-banded 

silicic lava flows, but vague fiamme and broken crystals are apparent in thin 

section; rheomorphic folding observed locally. Plagioclase separated from 

welded tuff in lower Stebbins Creek valley gave an 40Ar/39Ar plagioclase 

age of 24.04±0.06 Ma

Dacite (Oligocene)—Small, sparsely phyric to seriate, pyroxene dacite flows and 

flow breccia scattered throughout northeast-trending belt extending from Hard 

Scramble Creek to headwaters of Stebbins Creek. Black and obsidian-like 

where fresh, light-brown to gray where devitrified or altered. Composed of 

plagioclase (1–10 percent; 1 to 2 mm long), clinopyroxene (<2 percent; 0.5 to 

1 mm across), orthopyroxene (<2 percent; 0.5 to 1 mm across; variably 

replaced by brown smectite), and Fe-Ti oxide (<1 percent; ≤ 0.5 mm across) 

phenocrysts in a pilotaxitic to micropoikilitic (devitrified) groundmass

Tuff of Stebbins Creek (Oligocene)—Densely welded, sparsely porphyritic 

ignimbrite exposed in upper Stebbins Creek and traceable for 4.5 km in map 

area and at least 2.5 km to north; averages about 35 m thick. Light-brown to 

gray stony tuff distinguished by extremely flattened pumice clasts, some 

longer than 10 cm. Contains euhedral to broken phenocrysts of plagioclase 

(1–3 percent), orthopyroxene and clinopyroxene (≤0.5 percent; 0.5 mm across; 

variably replaced by smectite, calcite), and microphenocrysts of Fe-Ti oxide in 

a devitrified cryptocrystalline groundmass of feldspar, quartz, Fe-Ti oxide, and 

green-brown hornblende; shard texture obscure except in rare vitrophyre. 

40Ar/39Ar age of 24.30±0.11 Ma obtained for this unit

Basalt (Oligocene)—Flow of olivine+plagioclase-phyric basalt exposed near 

mouth of Stebbins Creek. Contains phenocrysts and microphenocrysts of 

plagioclase (9–12 percent; to 4 mm across; variably replaced by stilbite), 

olivine (5–7 percent; 1 to 3 mm across; chromian spinel inclusions; partly to 

completely altered to chlorite, calcite, serpentine, hematite), and clinopyroxene 

(1–2 percent; ≤1 mm across) in an intergranular, extensively altered 

groundmass

Olivine basalt (Oligocene)—Flow of greenish-gray, olivine-phyric basalt within 

volcaniclastic section (Tvs) in upper Timber Creek. Phenocrysts of olivine 

(about 7 percent; 1 to 2 mm across; abundant chromian spinel inclusions; 

largely replaced by yellow-green smectite) and rare clinopyroxene (0.5 to 1 

mm across) in a coarse-grained trachytic groundmass of plagioclase, 

clinopyroxene, Fe-Ti oxide, and minor interstitial smectite. Calc-alkaline but 

low-potassium composition with exceptionally low Zr (46 ppm), Nb (3.6 

ppm), and Y (15 ppm) contents and low Zr/Y (3.1)

EXPLANATION OF MAP SYMBOLS

Contact—Solid where location is accurate; long-dashed where location is 

approximate; short-dashed where location is inferred

Fault—Long-dashed where location is approximate; short-dashed where location 

is inferred; dotted where location is concealed; queried where existence or 

extent is uncertain. Ball and bar on downthrown side. Arrows show relative 

horizontal movement

Syncline—Trace of axial surface; long-dashed where location is approximate; 

dotted where location is concealed; showing direction of plunge

Strike and dip of beds

Strike and dip of platy parting in lava flows

Strike and dip of eutaxitic structure in pumiceous tuff

Basaltic andesite or andesite dike

Cinder cone

Hyaloclastite breccia

Volcanic plug

Approximate limit of propylitic alteration—Hachures face altered rocks

Sample locality showing 40Ar/39Ar age determination—Showing age±1-σ

error

af

Qa

Qt

Qdf

Qls

Qobg

Qmbr

Qamc

Qmhs

Qmdc

Qmlc

Qmwc

Qtdwu

Qtcr

Thtb

Thd

Teha

Tecl

Tha

Thba

Td

Tb

Tob

Tepa

Qmbc

Qlbr

Tehd

Twfss

Twfsh

Tipd

Tial

Tiba

Tia

Tid

Tdi

Tqd

Tidcd

Tgww

Tgo

Tggc

Tidcr

Tiah

Tecu

Tts

Tgwr

Qmum

Ttw

Tgsb

Qmus

Tgb

Tt

Tvs

Ta

Tba

879±7 ka

10

25

5

Qmum

CORRELATION OF MAP UNITS

SURFICIAL DEPOSITS

INTRUSIVE ROCKS VOLCANIC AND SEDIMENTARY ROCKS

0

20.0

22.0

24.0 

26.0 Oligocene

Pleistocene

Holocene

QUATERNARY

TERTIARY

Tia Tiba

Unconformity

0.05

0.25

0.50

1.0

Qmus

Qmdc

Qtcr

Qlbr

Qmwc

Qmlc

Qmhs

Qmbc

Miocene

Tha Thba Thtb Thd

Tba Ta

Qmbr

Qamc

56±5 ka

1,065±5 ka

1,056±9 ka

1,055±22 ka

879±7 ka

498±7 ka

Twfsh

Tgsb

Tgww

Tgo

Tggc

COLUMBIA RIVER BASALT GROUP

Tgwr

Unconformity

Local unconformity

Erosional unconformity

Unconformity

Tecu Tepa Tehd

Eagle Creek Formation

20.07±0.03 Ma

19.50±0.05 Ma 19.48±0.04 Ma

Twfss

25.8±0.5 Ma

? ?

? ?

ROCKS OF THE

BORING VOLCANIC FIELD

15.3

16.0

Teha

Tipd

Tial

?

? ?

21.59±0.31 Ma

Approximate

age, in Ma

[Age of epoch boundaries from GTS2012 (Gradstein and others, 2012). 40Ar/39Ar ages are from table 4]

Tecl

ROCKS OF THE ANCESTRAL CASCADE VOLCANIC ARC

Lava Flows of Hamilton Creek

Strata Older Than Lava Flows of Hamilton Creek

Tdi

Tid

? ?

?

Tqd

?

?

Tiah <23.1±0.2 Ma

?

Tidc Tidcr d

22.6±0.1 Ma

Tgb

?

?

Intrusive Complex of Deer Creek

21.94±0.06 Ma

af

Qa Qt

Qls

Qobg

Qtdwu

ALLUVIAL AND MASS WASTAGE DEPOSITS

Qdf

Tvs

Td

Tt Tts

Tob

Tb

24.3.±0.1 Ma

24.0±0.1 Ma

22.9±0.1 Ma

Ttw

WASHINGTON

MAP LOCATION

MAGNETIC NORTH

TRUE NORTH 15.6°

APPROXIMATE MEAN

DECLINATION, 2017

1000 0 1000 2000 3000 4000 5000 6000 7000 FEET

1 .5 0 1 KILOMETER

SCALE 1:24 000

CONTOUR INTERVAL 40 FEET

1 1/2 0 1 MILE

Base-map topography, stream network, and roads compiled from 

bare-earth digital elevation models derived from lidar surveys acquired 

in 2005 and 2009 by U.S. Army Corps of Engineers and in 2005 by 

Washington Department of Natural Resources

Outlines of water bodies derived from 2009 National Aerial 

Photography Program orthoimagery

All data reprojected to Universal Transverse Mercator projection, 

Zone 10, North American Datum of 1983

Cultural data and political boundaries taken from U.S. Geological 

Survey, 2011, Beacon Rock digital topographic quadrangle

Geology mapped by R.C. Evarts, 1997–2012; 

assisted by P.A. Dinterman, 2002

Digital cartography by R.S. Nicholson 

Edited by J.L. Zigler; digital cartographic 

production by Kathryn Nimz and D.L. Knifong

Manuscript approved for publication 

September 24, 2016

(STABLER)

) M

AD

ELLI V

ENNO

B( 5066

5065

5064

5063

5062

5061

5059

5058

5057

5056

5055

5054

5053000mN

T 3 N

T 2 N

(GUMBOOT MTN)

(LOOKOUT MTN) 569000mE 570 571 572 573 574 575 576 577

E6 R

E7 R

) NI ATNU

OMSBO

B( 5065

5064

5063

5061

5060

5059

5058

5057

5056

5055

5054

5066000mN

T 3 N

T 2 N

(MULTNOMAH FALLS) (BRIDAL VEIL)

(TANNER BUTTE)

570 571 572 574 5 575 577000mE 69 576

5053

45°37'30"

122°

40'

42'30"

45°45'

122°07'30" 5' 2'30" 122°

45°45'

42'30"

40'

45°37'30"

122°07'30" 5' 2'30"

Geologic Map of the Beacon Rock Quadrangle, Skamania County, Washington

By

Russell C. Evarts and Robert J. Fleck

2017

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

This map was printed on an electronic plotter directly from digital files. Dimensional calibration may vary 

between electronic plotters and between X and Y directions on the same plotter, and paper may change 

size due to atmospheric conditions; therefore, scale and proportions may not be true on plots of this map.

For sale by U.S. Geological Survey, Information Services, Box 25286, Federal Center, Denver, CO 80225, 

1–888–ASK–USGS

Digital files available at https://doi.org/10.3133/sim3367

Suggested citation: Evarts, R.C, and Fleck, R.J., 2017, Geologic map of the Beacon Rock quadrangle, 

Skamania County, Washington: U.S. Geological Survey Scientific Investigations Map 3367, scale 1:24,000, 

pamphlet 61 p., https://doi.org/10.3133/sim3367.

ISSN 2329-132X (online)

https://doi.org/10.3133/sim3367

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Map 3367

Sheet 1 of 2

Pamphlet accompanies map