Geologic Map of the Beacon Rock Quadrangle, Skamania 

County, Washington

By Russell C. Evarts and Robert J. Fleck

Pamphlet to accompany

Scientific Investigations Map 3367

2017

U.S. Department of the Interior

U.S. Geological Survey

Beacon Rock along north shore of Columbia River, Hamilton Mountain in background.

i

Contents

Introduction.....................................................................................................................................................1

Geographic and Geologic Setting......................................................................................................1

Previous Geologic Investigations in the Western Columbia River Gorge...................................1

Acknowledgments................................................................................................................................3

Synopsis of Geology......................................................................................................................................3

Cascade Volcanic Arc...................................................................................................................................3

Stratigraphic Relations and Nomenclature......................................................................................3

Oligocene and Early Miocene Strata Older than the Lava Flows of Hamilton Creek................5

Lava Flows of Hamilton Creek ............................................................................................................5

Eagle Creek Formation.........................................................................................................................7

Intrusive Rocks......................................................................................................................................8

Metamorphism and Hydrothermal Alteration ..................................................................................9

Columbia River Basalt Group.......................................................................................................................9

Grande Ronde Basalt .........................................................................................................................11

Analytical Issues........................................................................................................................11

Wapshilla Ridge Member.........................................................................................................14

Grouse Creek Member..............................................................................................................14

Ortley Member............................................................................................................................14

Winter Water Member..............................................................................................................15

Sentinel Bluffs Member............................................................................................................15

Wanapum Basalt.................................................................................................................................16

Frenchman Springs Member...................................................................................................16

Rocks of the Boring Volcanic Field ...........................................................................................................16

Alluvial and Mass-Wastage Deposits ......................................................................................................18

Landslide and Talus Deposits ...........................................................................................................18

Holocene and Pleistocene Alluvium of Tributary Streams ..........................................................19

Holocene Outburst-Flood Deposits..................................................................................................19

Structural Features......................................................................................................................................19

Faults.................................................................................................................................................... 20

Geologic Resources ................................................................................................................................... 20

Geologic Evolution...................................................................................................................................... 20

References Cited......................................................................................................................................... 21

Appendix 1.................................................................................................................................................... 26

Figures

1. Regional setting of Beacon Rock 7.5’ quadrangle.....................................................................2

2. Simplified geology of Beacon Rock quadrangle showing distribution of rocks of

ancestral Cascade volcanic arc, Columbia River Basalt Group, and Quaternary

volcanic rocks ...............................................................................................................................4

3. Chemical characteristics of volcanic rocks from Beacon Rock 7.5’ quadrangle

(analyses recalculated volatile-free) ........................................................................................6

4. Cliff composed of upper Eagle Creek Formation on southeast flank of Hamilton

Mountain ........................................................................................................................................7

5. Plot of Sr/Y versus Y for igneous rocks in Beacon Rock 7.5’ quadrangle .............................8

6. Typical jointing patterns exhibited by lava flows of the Columbia River Basalt Group.....10
ii

7. Stratigraphic nomenclature, ages, and magnetic polarities of the Columbia River

Basalt Group................................................................................................................................12

8. Chemical characteristics of Grande Ronde Basalt flows in the Beacon Rock 7.5’

quadrangle...................................................................................................................................13

9. Paleomagnetic directions for Grande Ronde Basalt flows in the Beacon Rock

7.5’ quadrangle............................................................................................................................15

10. South face of Hamilton Mountain showing ~200-m-thick intracanyon Grande

Ronde Basalt flow overlying basal foreset-bedded pillow breccia of same flow

on the left and Eagle Creek Formation on the right...............................................................16

11. Selected chemical characteristics of Pleistocene volcanic and subvolcanic

rocks in the Beacon Rock 7.5’ quadrangle.............................................................................17

12. Paleomagnetic directions for Quaternary volcanic rocks in the Beacon Rock 7.5’

quadrangle...................................................................................................................................18

13. Three Corner Rock, a mass of basaltic andesite that occupied the throat of a

now-eroded, approximately 1-Ma scoria cone .....................................................................19

Tables

1. Chemical and modal analyses of igneous rocks of the ancestral Cascade

volcanic arc, Beacon Rock 7.5’ quadrangle, Skamania County, Washington..................34

2. Chemical analyses of the Columbia River Basalt Group, Beacon Rock 7.5’

quadrangle, Skamania County, Washington ..........................................................................45

3. Chemical and modal analyses of Quaternary volcanic rocks, Beacon Rock 7.5’

quadrangle, Skamania County, Washington ..........................................................................55

4. Summary of 40Ar/39Ar incremental-heating age determinations, Beacon Rock

7.5’ quadrangle, Skamania County, Washington ...................................................................60

5. Paleomagnetic data for Columbia River Basalt flows and Quaternary volcanic

rocks, Beacon Rock 7.5’ quadrangle, Skamania County, Washington ..............................61
1

Introduction

Geographic and Geologic Setting

The Beacon Rock 7.5′ quadrangle is located approximately 

50 km east of Portland, Oregon (fig. 1), on the north side of 

the Columbia River Gorge, a scenic canyon carved through the 

axis of the Cascade Range by the Columbia River. Although 

approximately 75,000 people live within the gorge, much of 

the region remains little developed and is encompassed by the 

292,500-acre Columbia River Gorge National Scenic Area, 

managed by a consortium of government agencies “to protect and provide for the enhancement of the scenic, cultural, 

recreational and natural resources of the Gorge and to protect 

and support the economy of the Columbia River Gorge area.” 

As the only low-elevation corridor through the Cascade Range, 

the gorge is a critical regional transportation and utilities 

corridor (Wang and Chaker, 2004). Major state and national 

highways and rail lines run along both shores of the Columbia 

River, which also provides important water access to ports in 

the agricultural interior of the Pacific Northwest. Transmission 

lines carry power from hydroelectric facilities in the gorge and 

farther east to the growing urban areas of western Oregon and 

Washington, and natural-gas pipelines transect the corridor 

(Wang and Chaker, 2004). These lifelines are highly vulnerable 

to disruption by earthquakes, landslides, and floods. A major 

purpose of the work described here is to identify and map 

geologic hazards, such as faults and landslide-prone areas, to 

provide more accurate assessments of the risks associated with 

these features.

The steep canyon walls of the map area reveal extensive outcrops of Miocene flood-basalt flows of the Columbia 

River Basalt Group capped by fluvial deposits of the ancestral Columbia River, Pliocene lavas erupted from the axis of 

the Cascade arc to the east, and volcanic rocks erupted from 

numerous local vents. The Columbia River Basalt Group 

unconformably rests on a sequence of late Oligocene and early 

Miocene rocks of the ancestral Cascade volcanic arc, which 

underlies most of the map area. The resistant flood-basalt flows 

form some of the famous landforms in the map area, such as 

Hamilton Mountain. Extensive landslide complexes have developed where the basalt flows were emplaced on weak volcaniclastic rocks.

Previous Geologic Investigations in the Western 

Columbia River Gorge

The spectacular exposures in the Columbia River Gorge 

have attracted the attention of geologists for well over a century. 

Although observations and interpretations regarding specific 

aspects of gorge geology go back as far as the Lewis and Clark 

expedition of 1804–1807, the first comprehensive description 

of geologic relations in the Columbia River Gorge was by 

Ira S. Williams (1916). His popular account was published as a 

beautifully illustrated geologic road guide shortly after completion of the Columbia River Scenic Highway in Oregon. The 

report and accompanying map were the first to describe the 

major stratigraphic units visible in the walls of the gorge. He 

distinguished four major rock units: tuff and conglomerate that 

he named the Eagle Creek Formation, Miocene Columbia River 

basalt, cobbly gravels assigned to the Satsop Formation, and 

young andesitic and basaltic lavas and tuffs, which he called 

the Cascades Formation. All except the post-basalt gravels are 

present in the Beacon Rock area. Despite marked changes in 

geologic concepts and terminology in the past century, later 

workers have largely retained the basic stratigraphic framework 

outlined by Williams.

Hodge (1938) published a wide-ranging synthesis of more 

than a decade of work by him and his students in the Columbia 

River Gorge. Lacking adequate age constraints or chemical 

data to distinguish lavas of varying age, many of his concepts 

regarding the stratigraphy, structure, and geologic history have 

proved to be incorrect. Most subsequent studies (Chaney, 1959; 

Lowry and Baldwin, 1952; Trimble, 1963; Wise, 1970; Waters, 

1973; Swanson and others, 1979a; Hammond, 1980; Tolan, 

1982; Hammond and Korosec, 1983; Tolan and Beeson, 1984; 

Fleck and others, 2014) have been restricted to limited areas or 

have emphasized specific aspects of gorge geology. Wise (1970) 

mapped the area directly east of the Beacon Rock quadrangle 

and showed that the Eagle Creek Formation of Williams (1916) 

consists of two lithologically distinct volcanogenic units of 

the western Columbia River Gorge and discussed causes of 

the extensive landslides found north of the Columbia River. 

Swanson and others (1979b) established a formal stratigraphic 

nomenclature for the Columbia River Basalt Group and applied 

it to exposures in Washington. These revisions were incorporated into the reconnaissance geologic map of the southern 

Washington Cascade Range by Hammond (1980), who also 

broke out lithologic packages within the sub-basalt rocks of the 

ancestral Cascade volcanic arc, mapped the Plio-Pleistocene 

volcanic rocks, and inferred several major structures. Hammond 

and Korosec (1983) provided chemical analyses and a few K-Ar 

ages for some of the young volcanics.

The most recent geologic map that includes the Beacon 

Rock quadrangle is Phillips’ (1987) compilation of the 

Vancouver 30′ x 60′ quadrangle at 1:100,000 scale, which was 

prepared as part of the state geologic mapping program of the 

Washington Division of Geology and Earth Resources (Walsh 

and others, 1987). The geology of the Columbia River Gorge 

area portrayed on this map is taken without modification from 

the small-scale regional reconnaissance maps of Swanson and 

others (1979a) and Hammond (1980).

Allen (1975) briefly discussed Quaternary volcanoes 

(Boring lavas) of the greater Portland region. Palmer (1977) 

summarized information on the huge landslides along the north 

bank of the Columbia River, and Pierson and others (2016) 

mapped landslides in the western Columbia River Gorge based 

largely on interpretation of digital elevation models derived 

from lidar data.

Our new work expands upon these previous efforts in 

several ways. It shows lithologic variations and structures 

within strata of the ancestral Cascade volcanic arc and provides 

extensive chemical analyses and 40Ar/39Ar incremental-heating 
2

Figure 1. Regional setting of Beacon Rock 7.5’ quadrangle. 

A, Map showing major tectonic and physiographic features of the Pacific Northwest. 

B, Simplified geologic map of Vancouver 30’ × 60’ quadrangle, modified from Phillips (1987).

Willamette River

Lake Merwin

Yale Lake

CASCADE RANGE

COAST RANGE

Yacolt

Vancouver

WASHINGTON

Portland

Ridgefield

Battle Ground

Columbia River Gorge

Columbia

Washougal

Tualatin Mountains

River

0 10 20 KILOMETERS

0 5 10 MILES

45°30'

122° 123°

46°

B

Scappoose

Saint Helens

Woodland

Troutdale Tualatin

Basin

OREGON

PORTLAND BASIN

River

AREA OF MAP

?

Lewis

PORTLAND HILLS FAULT ZONE

*

120° 125° 49°

45°

00

100 200 KILOMETERS

MILES 50 100

OREGON

CALIFORNIA

BRITISH COLUMBIA

WASHINGTON

NEVADA

COAST RANGE

CASCADIA SUBDUCTION ZONE

CASCADE RANGE

PACIFIC OCEAN

A

PUGETWILLAMETTE

LOWLAND

Mount Hood

Columbia River Gorge

Area of figure 1

B

EXPLANATION

Holocene floodplain of Columbia RiverQuaternary sediments and Pliocene to Miocene sedimentary rocks, undividedQuaternary to Pliocene volcanic and volcaniclastic rocks and depositsMiocene Columbia River Basalt Group

Tertiary granitic rocksPaleogene volcanic and volcaniclastic rocks, undifferentiatedPaleogene marine sedimentary rocks
3

ages for these rocks. A more detailed stratigraphy for Grande 

Ronde Basalt, founded on abundant new chemical and paleomagnetic data, is presented. Similarly, the young volcanic rocks 

previously mapped as the Boring Lavas are differentiated based 

on their chemical and paleomagnetic properties, radiometric age 

determinations, and inferred source vents. The new chemical 

analyses are provided in tables 1–3, 40Ar/39Ar determinations in 

table 4, and paleomagnetic data in table 5.

Acknowledgments

Landowners and land-management agencies granted 

access that was essential for detailed mapping in the Beacon 

Rock quadrangle. We particularly thank Brian Poehlein of the 

Washington Department of Natural Resources; Erik Plunkett, 

supervisor of Beacon Rock State Park; and Denny Mohan 

of Longview Fibre Company. Philip A. Dinterman assisted 

with fieldwork in 2002. A lidar survey covering the map area, 

kindly provided by Stephanie Zurenko and Terry Curtis of the 

Washington Department of Natural Resources, was used by 

Karen Wheeler and Zenon Valin at the U.S. Geological Survey 

(USGS) to generate field sheets and the base map. Diane M. 

Johnson-Cornelius and Richard M. Conrey of Washington 

State University produced the high-quality chemical analyses 

required to differentiate Columbia River Basalt Group flows, 

and our USGS colleague Jonathan T. Hagstrum provided 

high-quality paleomagnetic data. Kenneth Bishop and Judith 

Fierstein of the USGS made available essential laboratory facilities. We obtained water-well drillers’ logs from the Washington 

Department of Ecology website at https://fortress.wa.gov/ecy/

waterresources/map/WCLSWebMap/default.aspx. Lee Walkling 

aided in obtaining information from the Washington Division of 

Geology and Earth Resources Library in Olympia, Washington. 

We have benefited immensely from discussions on various 

aspects of the regional stratigraphy, structure, and geologic history with Roger Ashley, Richard Conrey, Trevor Contreras, Paul 

Hammond, Jim O’Connor, William Phillips, Patrick Pringle, 

Stephen Reidel, Michael Sawlan, David Sherrod, James Smith, 

Terry Tolan, Richard Waitt, and Ray Wells. O’Connor provided 

descriptions of some Quaternary deposits along the Columbia 

River. We appreciate the careful reviews by Ralph Haugerud 

and Paul Stone that resulted in significant improvements in the 

content and presentation of this map.

Synopsis of Geology

For the past 40 m.y., the Cascade Range has been the locus 

of an episodically active volcanic arc associated with underthrusting of oceanic lithosphere beneath the North American 

continent along the Cascadia Subduction Zone. Late Oligocene 

to early Miocene rocks of the ancestral Cascade volcanic arc 

underlie most of the Beacon Rock quadrangle and comprise 

three unconformity-bounded stratigraphic packages (fig. 2). The 

lower part of the stratigraphic section in the map area consists 

of diverse, moderately altered, volcaniclastic rocks and lesser 

lava flows of late Oligocene to early Miocene age. An angular 

unconformity separates these rocks from an overlying and 

less-altered sequence of early Miocene porphyritic andesite and 

basaltic andesite flows, herein informally named the lava flows 

of Hamilton Creek. The top of this unit, locally marked by a 

thick paleosol, is an unconformable contact with weakly lithified volcaniclastic deposits of the early Miocene Eagle Creek 

Formation. Arc volcanism in southern Washington waned after 

the early Miocene.

In the middle Miocene, huge volumes of basaltic lava, the 

Columbia River Basalt Group, erupted from fissure vents in 

Idaho and eastern Washington and Oregon. Between 16 Ma and 

12 Ma, several of these flood-basalt flows crossed the Cascade 

Range through a broad lowland and entered western Oregon 

and Washington. In the Beacon Rock quadrangle, these flows, 

mostly Grande Ronde Basalt, are banked against the eroded top 

of the Eagle Creek Formation and mark the north margin of the 

Miocene trans-arc valley.

Following cessation of flood-basalt eruptions, the ancestral 

Columbia River incised a canyon located south of the map area. 

During the late Pliocene, voluminous volcanic activity related 

to a northward-propagating intra-arc rift filled this paleocanyon, 

forcing the river northward to its present position. Young volcanism in southern Washington has been more dispersed and less 

vigorous than that to the south. Several small extinct volcanoes 

have been mapped in the Beacon Rock quadrangle; they are the 

northeasternmost centers of the Pliocene to Quaternary Boring 

Volcanic Field (Treasher, 1942; Trimble, 1963; Allen, 1975; 

Evarts and others, 2009).

In the latest Pleistocene, cataclysmic outburst floods 

poured through the Columbia River Gorge. Their effects in 

the gorge were primarily erosive and they left only sparse thin 

deposits in the map area, although they undercut canyon walls 

and may have triggered some of the large landslides along the 

Columbia River.

Deformation in the area is characterized by mild folding, 

with attitudes in the older beds generally striking northeast to 

north-northeast and dipping moderately (15–30°) southeast. 

Decreasing dips in progressively younger strata indicate that 

folding occurred gradually throughout the Miocene, but deformation of the 16-Ma Grande Ronde Basalt Group is limited 

to gentle tilting (2°) to the southwest. Only a few small faults 

have been mapped; none are known to have moved during the 

Quaternary.

Cascade Volcanic Arc

Stratigraphic Relations and Nomenclature

Williams (1916) applied the name Eagle Creek 

Conglomerate to volcaniclastic rocks that underlie Columbia 

River Basalt flows in the Columbia River Gorge. The unit was 

renamed the Eagle Creek Formation by Chaney (1918), who, 

in subsequent publications, described its fossil flora in detail 

and eventually assigned it an early Miocene age (Chaney, 1944, 

1959). Wise (1961, 1970) and Waters (1973) recognized that the 

volcanic-arc strata in the area directly east of the Beacon Rock 
4

quadrangle consisted of two dominantly volcaniclastic but lithologically distinct units separated by an erosional unconformity. 

Wise (1970) therefore redefined the Eagle Creek Formation as 

“the sequence of volcanic conglomerates, sandstones, and tuffs 

that are typically exposed south of the Columbia River along 

Eagle Creek.” This definition restricted the name Eagle Creek 

to those rocks above the unconformity, which included all of 

the fossiliferous beds studied by Chaney (1944). Wise (1970) 

assigned the more altered and deformed strata below the unconformity to the Ohanapecosh Formation, which has its type area 

in Mount Rainier National Park, more than 100 km north of the 

map area.

Hammond (1980) traced the unconformity at the base of 

the Eagle Creek Formation westward into the Beacon Rock 

quadrangle, where the stratigraphic section contains numerous lava flows and is more heterogeneous than to the east. He 

divided the folded and altered strata beneath the unconformity 

into three units, from oldest to youngest: the Ohanapecosh 

Formation, the Stevens Ridge Formation, and the informal 

lava flows of Three Corner Rock. Although the stratigraphic 

relations shown by Hammond (1980) are broadly correct, the 

nomenclature he employed is not adopted here. Correlation 

of the Oligocene and earliest Miocene rocks in the Beacon 

Rock quadrangle with the Ohanapecosh (chiefly volcaniclastic 

River

Washougal

45°45'00"

122°07'30" 122°00'00"

45°37'30"

EXPLANATION

Quaternary unconsolidated 

deposits

Quaternary volcanic rocks

Tertiary intrrusive rocks

Columbia River Basalt Group

Eagle Creek Formation

Lava flows of Hamilton Creek

Tertiary volcanic and 

sedimentary rocks

Fault—Dashed where 

approximately located; 

dotted where concealed; ball 

and bar on downthrown side 

Columbia River

Figure 2. Simplified geology of Beacon Rock quadrangle showing distribution of rocks of ancestral Cascade volcanic arc, Columbia River 

Basalt Group, and Quaternary volcanic rocks.
5

sedimentary rocks) and Stevens Ridge (chiefly pyroclastic 

rocks) Formations in their type areas at Mount Rainier National 

Park is questionable. This correlation was based largely on gross 

lithologic similarity, but subsequent regional mapping (Evarts 

and Swanson, 1994) and age determinations (Vance and others, 

1987) demonstrate that the strata in the two widely separated 

areas are neither coeval nor laterally contiguous. Furthermore, 

broadly equivalent strata elsewhere in southern Washington 

have been given other names (Wilkinson and others, 1946; 

Roberts, 1958; Hammond, 1980; Phillips, 1987), and no reliable 

criteria have been found for mapping contacts between these 

various units. For these reasons, we employ strictly lithologic 

names or, where appropriate, informal lithostratigraphic names 

in reference to the strata that Hammond (1980) mapped as 

Ohanapecosh and Stevens Ridge Formations.

The upper part of the pre-Eagle Creek section in the map 

area consists almost entirely of porphyritic lava flows, which 

Hammond (1980) informally named lava flows of Three 

Corner Rock. The detailed mapping presented here confirms 

the existence of these flows and shows that the basal contact of 

this flow-dominated unit is an unconformity. However, because 

Three Corner Rock is a Pleistocene volcanic plug (see Rocks 

of the Boring Volcanic Field section), we rename these Tertiary 

rocks the (informal) lava flows of Hamilton Creek.

Oligocene and Early Miocene Strata Older than 

the Lava Flows of Hamilton Creek

Lithologically diverse, northeast-striking and southeastdipping strata that predate the lava flows of Hamilton Creek 

underlie much of the Washougal River drainage basin in the 

northwestern part of the map area. As much as 2,500 m of these 

rocks are exposed in the map area and comprise a lower volcaniclastic section, a middle section dominated by lava flows and 

ignimbrites, and an upper volcaniclastic section.

Volcaniclastic sedimentary rocks (Tvs) with rare small lava 

flows form the stratigraphically lowest part of this sequence in 

the map area. Thin- to medium-bedded, fine- to coarse-grained 

fluvial deposits predominate and are well exposed in the beds 

of the Washougal River and its tributaries, Prospector, Deer, 

and Timber Creeks. Thick, massive, pumiceous lapilli tuffs, 

probably deposited by pyroclastic flows, are subordinate but 

abundant locally, as along the Washougal River near the mouth 

of Stebbins Creek.

The lower volcaniclastic-dominated section is conformably overlain by a section composed of basaltic andesite (Tba), 

lesser andesite and dacite (Ta, Td), rare basalt (Tb, Tob), and 

lapilli tuff and tuff breccia (Tt, Tts). This flow-dominated 

section is best exposed in Stebbins Creek. Most of the flows 

are aphyric to sparsely phyric (<10–12 percent phenocrysts, 

although a coarsely porphyritic basaltic andesite at the base of 

this section holds up the ridgecrest between Timber Creek and 

Stebbins Creek; plagioclase from this flow gave an 40Ar/39Ar 

plateau age of 25.78±0.55 Ma, table 4). Thin (<2 m) sedimentary sections, commonly capped by orange-brown paleosols, 

separate many of the flows. Owing to differential erosion, 

the flows tend to form pronounced ribs separated by swales 

underlain by the sediments. As a result, the dipping flows form 

a flatiron topography that is particularly pronounced on the 

northwest bank of Stebbins Creek. Most of these lavas have 

medium-potassium, tholeiitic compositions (fig. 3) and are typical of late Eocene to early Miocene volcanics of the southern 

Washington Cascade Range (du Bray and others, 2006; R.C. 

Evarts, unpub. data).

The interbedded lapilli tuff and tuff breccias (Tt) are 

generally pumiceous, and two of them are densely welded 

ignimbrites that, like the lava flows, tend to form tabular 

ridges. One of these is a sparsely phyric, lithic-poor, totally 

devitrified, hypersthene-dacite tuff that rests on the coarsely 

porphyritic basaltic andesite flow noted above and was traced 

for more than 3 km in lower Stebbins Creek. The other, stratigraphically higher, ignimbrite is the tuff of Stebbins Creek 

(Tts), a stony to vitric, sparsely plagioclase- and pyroxenephyric, rhyolitic tuff that was mapped from lower Stebbins 

Creek northward for 7 km. It exhibits extremely flattened 

fiamme, contains very few lithic fragments, and typically looks 

more like a flow; in a few places, rheomorphic flow folds 

were observed, indicating deposition on a sloping surface. 

Plagioclase from the tuff of Stebbins Creek yielded an 40Ar/39Ar 

plateau age of 24.30±0.11 Ma.

The middle lava-flow and ignimbrite section interfingers 

upward with the upper volcaniclastic section that, in contrast to 

the lower volcaniclastic section, consists largely of pyroclasticflow deposits. Massive, green, nonwelded lapilli tuffs predominate, but densely welded beds, some vitric, are present locally. 

They are dacitic and more densely porphyritic than the tuffs 

lower in the stratigraphic section. Two widely separated welded 

tuffs, each within 150 m stratigraphically of the unconformable contact with the overlying lava flows of Hamilton Creek, 

yielded plagioclase 40Ar/39Ar plateau ages of 24.04±0.06 Ma and 

22.79±0.13 Ma (table 4).

Lava Flows of Hamilton Creek

The lava flows of Hamilton Creek consist of 

two-pyroxene±olivine bearing andesite (Tha) and basaltic 

andesite (Thba) flows, flow breccia, and local lensoidal 

diamicts (Thtb) of probable laharic origin. The unit is thickest 

near Three Corner Rock, where it occupies a small paleovalley 

incised into the top of the subjacent volcaniclastic section. It 

thins to the south and northeast, in part due to erosion prior to 

deposition of the overlying Eagle Creek Formation. A coarsely 

porphyritic subvolcanic andesite body in Hard Scramble 

Creek (Tiah) may mark the vent for these flows; only a single, 

sparsely porphyritic dike has been found to cut the lava flows 

of Hamilton Creek. Chemically, the lava flows of Hamilton 

Creek possess transitional tholeiitic to calc-alkaline compositions (table 1; fig. 3) and differ from older lavas in the map 

area in their generally lower contents of FeO*, TiO2, and P2O5, 

higher contents of Al2O3 and Sr, and higher Sr/Y and Ba/Nb. 40Ar/39Ar ages obtained for plagioclase separates from two 

Hamilton Creek flows indicate they were emplaced at 22.0 to 

21.6 Ma (table 4).
6

Older volcanic rocks

Columbia River Basalt Group Tertiary intrusive rocks

Pleistocene volcanic rocks

Eagle Creek Formation: Lava flows of Hamilton Creek

lava flows

blocks in debris flows

EXPLANATION

0

2

4

6

8

45 50 55 60 65 70 75 80

THOLEIITIC

32.0 17.3

15.5

CALC-ALKALINE

C

10

FeO*/MgO, IN WEIGHT PERCENT

 SiO2, IN WEIGHT PERCENT

0

1

2

3

4

45 50 55 60 65 70 75

Low-potassium

Medium-potassium

High-potassium

B

 SiO2, IN WEIGHT PERCENT

K O, IN WEIGHT PERCENT 2

A

Dacite

Rhyolite

Trachyte

Andesite Basaltic

andesite Basalt

Trachyandesite

Basaltic

trachyandesite Trachybasalt

0

2

4

6

8

10

12

45 50 55 60 65 70 75 80

 SiO2, IN WEIGHT PERCENT

K O2 +Na O2 , IN WEIGHT PERCENT

Figure 3. Chemical characteristics of volcanic rocks from Beacon Rock 7.5’ quadrangle (analyses 

recalculated volatile-free). A, K2O+Na2O versus SiO2 contents, showing IUGS classification 

(Le Maitre, 2002). B, K2O versus SiO2 contents, showing low-, medium-, and high-potassium fields 

extrapolated from Gill (1981, p. 6). C, FeO*/MgO versus SiO2 content, showing separation into 

tholeiitic and calc-alkaline rocks according to Miyashiro (1974). FeO*, total Fe as FeO.
7

Eagle Creek Formation

The Eagle Creek Formation underlies the crest of the 

divide between the Washougal and Columbia Rivers and is 

widely distributed in the valley of Hamilton Creek. In the map 

area, it rests unconformably on the lava flows of Hamilton 

Creek; whereas, to the east it overlies older volcaniclastic rocks. 

In many locations, a saprolitic paleosol as thick as 15 m developed on the subjacent rocks (Wise, 1961, 1970; Waters, 1973). 

Both upper and lower contacts of the Eagle Creek Formation are 

erosional surfaces of considerable relief, so its thickness varies 

substantially. The maximum known thickness of about 400 m is 

in Hamilton Creek (Wise, 1970).

The Eagle Creek Formation in the map area is lithologically similar to that described by Wise (1961, 1970) from the 

region to the east. It consists largely of fluvially deposited 

conglomerate, breccia, sandstone, and diamicts (paraconglomerates of Wise, 1970) deposited by debris flows. In comparison to older volcaniclastic rocks (Tvs, Tt), Eagle Creek beds 

are less consolidated and metamorphosed. East of Hamilton 

Mountain, the formation is divided into two informal members. 

The lower member (Tecl) consists largely of fluvially deposited conglomerate and sandstone with a few relatively fine 

grained debris-flow beds. Clasts include a variety of volcanic 

rocks but are dominated by porphyritic rocks similar to those of 

the underlying lava flows of Hamilton Creek, presumably the 

source. Conglomerate matrix and finer grained sediments are 

considerably altered to smectitic clay, giving the rocks darkgreen to brown hues. Most of the area mapped as Eagle Creek 

Formation in the Beacon Rock quadrangle consists of the upper 

member (Tecu), which in Hamilton Creek valley rests sharply 

but conformably on the lower member. The upper member is 

dominated by poorly sorted breccias deposited by lahars (fig. 4) 

but also includes fluvial pebble- and cobble- conglomerate and 

tuffaceous sandstone, some densely packed monolithologic 

breccias probably emplaced as lithic pyroclastic (block-and-ash) 

flows, and a few lava flows. On the east valley wall of Hamilton 

Creek, a breccia that carries andesite megablocks many meters 

across may be a debris-avalanche deposit. Clasts consist chiefly 

of dense, porphyritic andesite and dacite; pumice clasts are 

Figure 4. Cliff composed of upper Eagle Creek Formation on southeast flank of Hamilton Mountain. Consists of poorly sorted 

conglomerate of presumed laharic origin with minor, discontinuous sandstone beds. Large boulder near edge of cliff (arrow) is about 

2.5 m across.
8

sparse and largely restricted to sandy beds and conglomerate 

matrix. Upper-member rocks are much less altered than those of 

the lower member. They are generally light colored and weather 

to light-gray to white soils that contrast with the darker soils 

derived from older rocks and reddish-brown soils developed on 

the Columbia River Basalt Group. Because the resistant volcanic clasts in the conglomerates readily separate from the weak 

matrix, areas underlain by the Eagle Creek Formation form 

boulder-strewn landscapes resembling those of moraines. Clasts 

in Eagle Creek conglomerates and diamicts are predominantly 

porphyritic, pyroxene- and hornblende-phyric, intermediate to 

silicic volcanic rocks (table 1).

Lava flows are rare in the Eagle Creek Formation to the 

east (Wise, 1970) but several (Tepa, Teha, Tehd) are present 

in the map area (table 1; fig. 2). They consist of pyroxene- and 

(or) hornblende-phyric basaltic andesite and andesite that are 

compositionally similar to clasts in the associated lahar and 

block-and-ash deposits. A distinctive, densely phyric, pyroxene 

dacite (Tipd) crops out in the lower Hardy Creek area, where 

contact relations indicate it intrudes Eagle Creek Formation lava 

flows and clastic rocks. Northwest-trending shears are common 

in this area and suggest emplacement of the dacite along a 

fault zone. Petrographic and chemical affinities with the host 

rocks suggest it is a shallow intrusion of probable Eagle Creek 

age. The Eagle Creek lavas and clasts in debris-flow deposits 

exhibit calc-alkaline compositions and are clearly distinguished 

from older andesitic and dacitic flows by their less altered 

nature, nearly ubiquitous hornblende phenocrysts, and generally higher Sr/Y (>20, fig. 5) and Ba/Nb. 40Ar/39Ar plateau ages 

on plagioclase separates from a lithic pyroclastic flow and two 

lava flows indicate deposition of the Eagle Creek Formation 

between 20.1 Ma and 19.4 Ma (table 4).

Although both members of the Eagle Creek Formation 

were deposited in fluvial settings, their lithologic differences 

record a change in source material. The lower member consists of debris eroded from a pre-existing volcanic highland, 

most likely the stratovolcano that produced the lava flows 

of Hamilton Creek. The upper member, in contrast, reflects 

contemporaneous volcanism. Wise (1970) and Waters (1973) 

interpreted the Eagle Creek Formation as a large debris fan 

originating at one or more active volcanoes. This interpretation 

is consistent with all major characteristics of the upper 

member, including relatively limited compositional range 

and distinctive chemistry of its clasts, abundance of lahar and 

lithic pyroclastic-flow deposits, ash-rich matrix of many beds, 

high-energy depositional environment, and presence of interbedded lava flows. Northwestward thickening of the Eagle 

Creek Formation suggested to Wise (1961, 1970) that the 

eruptive source area was located north of the map area. Eagle 

Creek beds are absent in this area, however, implying that 

erosion has completely destroyed the original volcanic edifice, its nature and location therefore unknown. Because Eagle 

Creek eruptions produced abundant lithic pyroclastic flows, 

which are typically generated by collapse of growing andesitic 

and silicic domes (Cas and Wright, 1987; Freundt and others, 

2000), the source area was more likely a dome field rather 

than a single large stratovolcano. If so, the only remains of the 

Eagle Creek volcanic center would likely be small subvolcanic 

intrusions scattered throughout the area north of the Eagle 

Creek Formation outcrop belt. Three possible Eagle Creek vents 

are mapped in the Beacon Rock quadrangle. One is the hill in 

the center of the map area informally named “Croswell Butte” 

that consists of a blocky-surfaced deposit of hornblende dacite 

(Tehd); because highly viscous dacite cannot flow far from 

its source vent, this outcrop is likely a dome or shallow plug. 

A second possible vent is an intrusion of fresh andesite located 

about 4 km to the east, near Lily Lake. The Lily Lake body 

(Tial) is aphyric, unlike most Eagle Creek clasts, but possesses 

certain distinctive chemical characteristics, such as a high Sr 

content and high Sr/Y, that indicate an affinity to Eagle Creek 

magmas. Finally, a densely porphyritic dacite body cutting 

the Eagle Creek Formation in Hardy Creek (Tipd) also has 

relatively high Sr and Sr/Y and may be a late Eagle Creek-age 

subvolcanic intrusion.

Intrusive Rocks

A wide variety of intrusive rocks were emplaced into the 

older part of the Oligocene to early Miocene section of the 

Beacon Rock quadrangle. They range from phaneritic, variably 

altered, gabbro, pyroxene diorites, and quartz diorites (Tgb, 

Tdi, Tqd), presumably emplaced at considerable depth, to finegrained, locally vitric, mafic to silicic rocks (Tiba, Tia, Tid) 

forming (possibly subvolcanic) dikes, sills, and plugs.

The largest intrusions in the map area, in the Deer Creek 

drainage, consist of moderately porphyritic dacite (Tidcd) to 

rhyolite (Tidcr) distinguished by chalky white plagioclase phenocrysts in a buff to light-gray, felsitic groundmass. These rocks 

are highly jointed, extensively frost-heaved, and commonly 

disintegrate to barren talus. Plagioclase from a rhyolite sample 

on the ridge east of Deer Creek yielded an 40Ar/39Ar plateau age 

of 22.60±0.06 Ma.

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60

Sr/Y

Y (ppm)

Older volcanic rocks

Tertiary intrusive rocks

Lava flows of Hamilton Creek

Eagle Creek Formation lavas

EXPLANATION

Figure 5. Plot of Sr/Y versus Y for igneous 

rocks in Beacon Rock 7.5’ quadrangle; ppm, 

parts per million.
9

The ages of other intrusions in the map area are poorly 

constrained. Those that cut the lowest stratigraphic horizons 

near the northwest corner of the quadrangle are generally altered 

as much as or more than their host rocks and may predate the 

pervasive low-grade metamorphism affecting those rocks (see 

next section). Elsewhere, many intrusive bodies are less altered 

than their host rocks and could be much younger. An 40Ar/39Ar 

experiment on one intrusion, exposed in Hard Scramble Creek 

(Tiah), did not yield a specific age but provides a maximum 

age of 23.14±0.18 Ma, permissive of a Hamilton Creek source 

vent (table 4). The only intrusions known to cut Hamilton Creek 

flows are the plug of platy aphyric andesite forming a conical 

hill south of Lily Lake (Tial) and an isolated sparsely phyric 

andesite dike about 4.2 km to the west. Neither of them petrographically resembles either the lava flows of Hamilton Creek 

or rocks of the Eagle Creek Formation, although, as noted 

above, the Lily Lake body is chemically similar to flows in the 

Eagle Creek Formation.

A distinctive, densely phyric, pyroxene dacite (Tipd) 

crops out in the lower Hardy Creek area, where contact relations indicate it intrudes Eagle Creek Formation lava flows and 

clastic rocks. Northwest-trending shears are common in this 

area and suggest emplacement of the dacite along a fault zone. 

Petrographic and chemical affinities with the host rocks suggest 

it is a shallow intrusion of probable Eagle Creek age.

Metamorphism and Hydrothermal Alteration

The ashy matrix of Eagle Creek Formation volcaniclastic 

rocks has typically been converted to iron-poor montmorillonite 

but otherwise the unit is unaltered (Wise, 1961). Underlying 

rocks, however, have been subjected to zeolite-facies regional 

metamorphism similar to that described from other areas in the 

southern Washington Cascade Range (Fiske and others, 1963; 

Wise, 1970; Evarts and others, 1987; Evarts and Swanson, 

1994). This region-wide metamorphism reflects burial within 

the relatively high heat-flow environment of an active volcanic 

arc.

The degree of alteration in the sub-Eagle Creek rocks of 

the map area increases downsection. Alteration of the lava flows 

of Hamilton Creek is generally limited to partial replacement of 

olivine, orthopyroxene, and interstitial glass by iron-rich smectite. Alteration of associated tuff breccia is similarly limited. 

The weak alteration is consistent with shallow burial depths, 

less than 0.5 km, beneath the Eagle Creek Formation.

Metamorphic effects are more apparent in the volcaniclastic strata below the lava flows of Hamilton Creek, especially 

in originally vitric tuffs, most of which have been pervasively 

converted to iron-bearing smectites with or without celadonite, 

clinoptilolite, or analcime. In the lowest part of the sequence 

northwest of Stebbins Creek, minerals indicative of somewhat 

higher temperatures (albite, stilbite, laumontite, prehnite, pumpellyite, chlorite) are widely developed.

The highest temperature metamorphic assemblages in the 

map area, characterized by the presence of epidote, occur in 

an area of abundant dikes near the mouth of Stebbins Creek. 

Epidote-bearing propylitic alteration is closely associated with 

concentrations of intrusive rocks elsewhere in the Cascade 

Range and reflects hydrothermal activity rather than simple 

burial (Grant, 1969; Evarts and others, 1987; Evarts and 

Swanson, 1994). The exposed dikes, despite their abundance, 

seem insufficient to account for the intensity of propylitization in the Washougal River area, indicating that they may 

be offshoots from a substantial intrusive body in the shallow 

subsurface.

The timing of metamorphism is difficult to ascertain. Peak 

metamorphism presumably corresponds to the time of maximum burial and (or) maximum intrusive activity. Maximum 

burial in the map area occurred during deposition of the Eagle 

Creek Formation, about 20 Ma. This is consistent with regional 

relations that indicate a substantial decline in arc volcanism in 

southern Washington prior to eruption of the Columbia River 

Basalt Group (Evarts and Swanson, 1994). The few intrusive 

rocks in the area that have been dated yield early Miocene ages. 

Metamorphism in the northeastern part of the map area may be 

related to emplacement of the large granitic Silver Star stock, 

5 km to the west, which has a K-Ar age of 19.6±0.7 Ma (Power 

and others, 1981). These observations suggest that metamorphism culminated in the early Miocene.

Columbia River Basalt Group

In the Miocene, between 16.5 and 6 Ma, huge volumes 

of tholeiitic flood basalt, erupted from fissures in southeastern Washington and adjacent Oregon and Idaho, formed the 

Columbia River Basalt Group. Some of the largest flows 

crossed the Cascade Range through a broad lowland and ultimately reached the Pacific Ocean (Beeson and others, 1989; 

Pfaff and Beeson, 1989; Beeson and others, 1989; Wells and 

others, 1989, 2009). The Columbia River Gorge is located near 

the northern margin of the Miocene lowland, where the flows 

are banked against the dissected erosional surface of the Eagle 

Creek Formation. Most of the flood-basalt flows erupted during 

a brief period between 16 and 15 Ma and constitute the voluminous Grande Ronde Basalt and Wanapum Basalt (Beeson 

and others, 1989; Tolan and others, 1989; Reidel and others, 

1989a, 2013; Reidel and Tolan, 2013; Barry and others, 2010, 

2013; Baksi, 2013). After emplacement of the Wanapum Basalt, 

flood-basalt eruptions became less frequent, and the ancestral Columbia River incised a deep canyon that confined the 

youngest flows (Waters, 1973; Tolan and Beeson, 1984; Beeson 

and others, 1989). The locations of the erosional remnants of 

Columbia River Basalt Group flows in the map area are shown 

on figure 2. Figure 6 portrays the stratigraphic nomenclature of 

the Columbia River Basalt Group employed in this report and 

shows the stratigraphic position of units found in the map area.

Columbia River Basalt Group flows possess the general 

character of pahoehoe lavas (Self and others, 1997; Thordarson 

and Self, 1998; Vye-Brown and others, 2013) and typically 

exhibit one of two distinctive cooling-jointing patterns referred 

to as blocky/columnar and entablature/colonnade (fig. 6; 

Waters, 1973; Long and Wood, 1986; Beeson and others, 1989; 

Tolan and others, 2009). Flows with entablature/colonnade 
10

SHEET FLOWS

INTRACANYON FLOW

AXIS OF CANYON OVERFLOW AREA

flow-top breccia

fluvial sediments

10 m

30 m

BASAL PILLOW LAVA

BASAL COLONNADE

ENTABLATURE

UPPER COLONNADE

pahoehoe surfaceblocky jointshackly slender columnsfanning columnsplaty jointsvesicular, chilled and locally rubbly base

COLUMNAR/BLOCKY FLOW

pillow lava/hyaloclastite breccia columnar/blocky jointing UPPER COLONNADE

ENTABLATUREBASAL COLONNADE

vesicular flow top

vesicular flow top

Figure 6. Typical jointing patterns exhibited by lava flows of the Columbia River Basalt Group (modified from Tolan and others, 2009).
11

jointing (the Type I flows of Long and Wood, 1986) have a 

basal zone of well-formed columns, 0.5 to 2 m across, overlain 

by a hackly-jointed entablature composed of thinner, poorly 

developed columns. The entablature commonly constitutes more 

than 50 percent of the flow thickness. Although much more 

densely jointed, entablatures, due to the interlocking nature of 

their curviplanar joints, are more resistant to erosion and tend 

to form the largest and most continuous cliffs. Blocky/columnar 

jointing (equivalent to the Type III flows of Long and Wood, 

1986) is typically associated with thinner flows and consists of 

poorly developed vertical columns and horizontal cross joints. 

Internal jointing in some flows, however, may be complex, with 

multiple colonnades or entablatures (Type II flows of Long and 

Wood, 1986; McMillan and others, 1989; Tolan and others, 

2009). Flow tops are vesicular and commonly weathered.

As noted by others (Reidel, 1998; Self and others, 1997; 

Thordarson and Self, 1998; Vye-Brown and others, 2013), 

defining precisely what constitutes a lava flow in flood-basalt 

provinces is not entirely straightforward. For mapping purposes, 

a flow, as used here, consists of a body of basalt with a jointing pattern that indicates it was emplaced and cooled during a 

single continuous event (sometimes referred to as cooling units 

(Reidel, 1998)). We have observed apparent contacts between 

cooling units that are chemically identical, suggesting that 

these contacts separate sheet-like flow lobes emplaced during a 

single eruptive episode. Contacts between lobes record breaks 

in deposition that range from a few hours to several years (Self 

and others, 1997; Thordarson and Self, 1998; Vye-Brown and 

others, 2013). The collection of flows and lobes produced by 

a single sustained eruption constitute a flow field composed of 

lavas with generally uniform compositions. Although flow tops 

are almost universally vesicular, internal vesicular zones are not 

uncommon (Long and Wood, 1986; Tolan and others, 2009). 

As noted above, individual flows may also show internally 

complex jointing. Furthermore, jointing patterns in individual 

flows may change laterally over distances of less than 100 m. 

These complications can render distinguishing flow contacts 

from internal boundaries difficult. The most reliable field 

indicators of interflow contacts, none that are common, include 

sedimentary interbeds, tree molds, oxidation, basal pillow 

lava-hyaloclastite zones, pipe vesicles, and flow breccia; cooling directions inferred from features on joint surfaces (DeGraff 

and others, 1989) are also useful. Despite the impression of 

sheetlike layers one gets from the seemingly continuous cliffs 

and stair-step topography in the Columbia River Gorge, detailed 

mapping and geochemistry suggest that Columbia River Basalt 

Group flows in the map area can vary substantially in thickness 

over relatively short distances.

Grande Ronde Basalt

Grande Ronde Basalt flows are generally aphyric to 

sparsely plagioclase-phyric, medium-potassium, tholeiitic basaltic andesites (fig. 3) that are readily distinguished from those 

in the other formations of the group by their generally aphyric 

character and comparatively high SiO2 and low TiO2 contents 

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

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

Reidel and Tolan, 2013). We estimate that about 12 individual 

eruptions are represented by flows exposed in the map area.

Grande Ronde Basalt flows in the map area range from 

aphyric to sparsely phyric with widely scattered plagioclase 

phenocrysts or glomerocrysts less than 10 mm across. Some 

flows are distinctly microphyric, with abundant plagioclase 

microphenocrysts ≤1 mm long. They consist largely of plagioclase, augite, pigeonite, Fe-Ti oxide, apatite, and dark glass; 

some flows contain minor olivine or orthopyroxene (Reidel 

and others, 1989a; Reidel and Tolan, 2013). Textures range 

from intergranular to intersertal; samples from colonnades are 

more coarsely crystalline than those from entablatures, which 

typically contain 35 to 65 percent glass (Long and Wood, 1986). 

In general, Grande Ronde flows are petrographically indistinguishable, although variations in the abundance of phenocrysts 

or microphenocrysts are locally useful.

The widespread and originally horizontal Grande Ronde 

Basalt flows potentially constitute excellent stratigraphic 

datums for analysis of post-middle-Miocene deformation 

(Beeson and others, 1989; Beeson and Tolan, 1990), but the 

remarkable physical and chemical similarity of these flows 

stymied early attempts to construct an internal stratigraphic 

framework for this thick formation. Swanson and others 

(1979b) showed that emplacement of the flood-basalt field 

spanned three paleomagnetic polarity reversals, allowing subdivision of the formation into four magnetostratigraphic units 

(fig. 7). Chemical analyses demonstrated that Grande Ronde 

flows could be divided into relatively high-MgO and low-MgO 

groups (fig. 8), with a gap at about 4.0 weight percent MgO 

(Reidel, 1983; Mangan and others, 1986). Employing a combination of petrographic characteristics, stratigraphic position, 

chemistry, and paleomagnetic properties, Reidel and others 

(1989a) developed an informal stratigraphic nomenclature for 

the Grande Ronde Basalt. We find this nomenclature useful and 

have adopted it, as revised and formalized by Reidel and Tolan 

(2013), for our mapping in the western Columbia River Gorge 

(fig. 7). Reidel and Tolan (2013) divided the Grande Ronde 

Basalt in the Columbia Basin into 25 formal and informal 

members based primarily on chemical composition; six of these 

members, mostly within the N2 and R2 magnetostratigraphic 

units of Swanson and others (1979b), are recognized in the map 

area. The members defined by Reidel and Tolan (2013) typically 

contain several chemically distinguishable flows or packages of 

flows, each probably equivalent to a flow field emplaced during 

a single, sustained eruption.

Analytical Issues

Assignment of Grande Ronde flows to Reidel and Tolan’s 

(2013) members requires precise chemical analyses to detect the 

subtle compositional differences between units. Interlaboratory 

bias and weathering effects complicate assessments. To eliminate potential analytical bias, all analyses reported here 

(table 2) are from the Peter Hooper GeoAnalytical Laboratory 

at Washington State University using common instrumentation 

and standards. Although internally consistent, our data cannot 
12

Asotin Member

basalt of Huntzinger

Wilbur Creek Member

basalt of Lapwai

basalt of Wahluke

Umatilla Member

basalt of Sillusibasalt of Umatilla

Priest Rapids Member

basalt of Lolo

basalt of Rosalia

Roza Member

Frenchman Springs Member

basalt of Lyons Ferrybasalt of Sentinel Gapbasalt of Sand Hollowbasalt of Silver Falls

basalt of Ginkgobasalt of Palouse Fallse Falls

Eckler Mountain Member

basalt of Dodgebasalt of Robinette Mountain

13

NN

NN

N

RR

14.5

NN

15.0

Shumaker Creek member

13.5

T,R

NN

N,E

N

EE

Lower Monumental MemberIce Harbor Member

Buford Member

Elephant Mountain Member

basalt of Goose Islandbasalt of Martindalbasalt of Basin City

Pomona Member

Esquatzel Member

Weissenfels Ridge Member

basalt of Slippery Creek

basalt of Tenmile Creek

basalt of Lewiston Orchards

basalt of Cloverland

6 N

8.5

N

R

N

10.5 R,T

R

N

NNNN

12

FORM

ATION MEMBER AGE (Ma) MAGNETIC POLARITY

R

SUBGROUP

erosional unconformityerosional unconformity

erosional unconformity

erosional unconformityerosional unconformity

local erosional unconformity

local erosional unconformity

local erosional unconformity

local erosional unconformity

GROUP

SADDLE MOUNTAINS BASALT WANAPUM BASALT

COLUMBIA RIVER BASALT GROUP

Slack Canyon MemberCold Springs Ridge MemberHoskins Gulch MemberFields Spring MemberIndian Ridge MemberWinter Water MemberButtermilk Canyon memberArmstrong Canyon member

Ortley member

Meyer Ridge Member

Sentinel Bluffs Member

Grouse Creek member

Mount Horrible member

China Creek Member

Frye Point memberDowney Gulch memberBrady Gulch memberKendrick Grade member

Center Creek member

Skeleton Creek member

Rogersburg memberTeepee Butte MemberBirch Creek member

Buckhorn Springs Member

N2

15.5

IMNAHA

BASALT

N1

N0

R1

16.0

16.0

basalt of Museumbasalt of Spokane Fallsbasalt of Stember Creekbasalt of Airway Heightsbasalt of California Creek

basalt of McCoy Canyonbasalt of Pruitt Drawbasalt of Joseph Creekbasalt of Limekiln Rapids

FORM

ATION MEMBER AGE (Ma) MAGNETIC POLARITY SUB- GROUP

Wapshilla Ridge Member

GROUP

GRANDE RONDE BASALT

COLUMBIA RIVER BASALT GROUP

YAKIMA BASALT SUBGROUP

YAKIMA BASALT SUBGROUP

?

T,R

R2

Figure 7. Stratigraphic nomenclature, ages, and magnetic polarities of the Columbia River Basalt Group, modified from Reidel and others (2013), Reidel and Tolan (2013), and Reidel (2005). For the Grande Ronde Basalt, formal members of Reidel and Tolan (2013) designated by M (Member), informal members designated by m (member). Magnetic polarity abbreviations are N, normal; R, reversed; T, transitional; E, excursional; subscripts refer to magnetostratigraphic units of Swanson and others (1979b). Units mapped in Beacon Rock 7.5’ quadrangle are shown in blue italics.
13

be compared directly to analyses performed in other labs or 

even with older data from the Washington State University laboratory (for example, Reidel and others, 1989a) that were produced using different equipment and data-reduction procedures.

A major difficulty in making correlations among Grande 

Ronde flows is accounting for the effects of chemical weathering, which can modify rock chemistry sufficiently to produce 

erroneous member assignments (Wells and others, 2009). 

Consequently, we rely mainly on analyses that meet two criteria 

related to extent of weathering. First is degree of hydration 

as recorded by loss-on-ignition or by anhydrous totals. We 

judge samples that have loss-on-ignition exceeding 3.0 weight 

percent, or anhydrous totals less than 97 weight percent, as 

possibly affected by weathering, although simple hydration 

will not affect normalized compositions. The second and more 

important criterion is total Fe calculated as FeO (FeO*) for 

Sentinel Bluffs Member

Winter Water member

Ortley Member Wapshilla Ridge Member

Grouse Creek member N2 R2

EXPLANATION

0.24 0.26 0.28 0.30 0.32 0.34 0.36 0.38 0.40 0.42 0.44

1.70

1.80

1.90

2.00

2.10

2.20

2.30

2.40

P2O5, IN WEIGHT PERCENT

TiO2, IN WEIGHT PERCENT

B

A

MgO, IN WEIGHT PERCENT

3.00 3.50 4.00 4.50 5.00

1.70

1.80

1.90

2.00

2.10

2.20

2.30

2.40

TiO2, IN WEIGHT PERCENT

Figure 8. Chemical characteristics of Grande Ronde Basalt flows in the Beacon Rock 7.5’ 

quadrangle. N2 and R2 are magnetostratigraphic units of Swanson and others (1979b). A, TiO2 versus 

MgO contents. B, TiO2 versus P2O5 contents.
14

normalized analyses (Wells and others, 2009). Most fresh 

Grande Ronde flows contain at least 11 weight percent FeO* 

(see Hooper, 2000; Reidel and Tolan, 2013), and we therefore 

consider samples with less than 11 weight percent FeO* as 

potentially altered. Importantly, this is true even for samples 

with anhydrous totals above 98 weight percent and for samples 

that appear fresh in hand sample and thin section. We interpret 

the reduction in FeO* to reflect long-duration chemical reaction 

with groundwater. This process can affect elements such as 

MgO, TiO2, and P2O5 that are used to help distinguish Grande 

Ronde Basalt members. Therefore, we do not rely on data from 

rocks that have less than 11 weight percent FeO* for member 

assignments. Most analyses in table 2 are unaltered according to 

this criterion.

We also employ laboratory paleomagnetic analyses to 

distinguish Grande Ronde Basalt flows in the map area (table 

5). This approach has two major advantages over field determinations with a portable fluxgate magnetometer. Firstly, chemical 

magnetic overprints resulting from post-emplacement weathering can be detected and eliminated, producing less ambiguous 

results. Secondly, the data provide paleomagnetic inclinations 

and declinations as well as polarities, potentially allowing 

chemically similar flows within a magnetostratigraphic unit to 

be differentiated (Wells and others, 1989, 2009).

Grande Ronde Basalt flows in the map area exhibit a pronounced onlap relation with the south-sloping paleovalley wall 

underlain by the Eagle Creek Formation. As a result, the number 

of flows and overall thickness of the formation decrease northward and the unit is absent north of the Columbia River valley. 

Our detailed chemical stratigraphy reveals major variations in 

the thickness of individual flows across the map area. These 

variations reflect, in part, the lobate character of Columbia 

River Basalt flows (Self and others, 1997; Thordarson and 

Self, 1998; Vye-Brown and others, 2013), which is probably 

best developed at locations near the province margins (Reidel, 

1998). However, some of the more dramatic thickness changes, 

exemplified by the thick Ortley member flow at Hamilton 

Mountain, clearly record significant erosion during the relatively brief periods separating emplacement of Grande Ronde 

Basalt flows. The resulting intracanyon relations between 

Grande Ronde flows have not been described from elsewhere 

in the flood-basalt province. Such enhanced interflow erosion 

is likely restricted to areas at the margin of the province, where 

major tributaries of the ancestral Columbia River encountered 

the flow field and were shunted along its edge, carving canyons 

that were filled by succeeding lava flows (Wells and others, 

2009). Another manifestation of interaction between basalt 

flows and tributary streams is the abundance of pillow lavas and 

hyaloclastite in the Grande Ronde section of the map area.

Wapshilla Ridge Member

The lowest Columbia River Basalt Group flows exposed 

in the map area belong to the Wapshilla Ridge Member (Tgwr) 

of Reidel and Tolan (2013). They are reversely magnetized, 

have relatively low MgO contents (3.3–3.6 weight percent) and 

possess higher TiO2 (2.3–2.4 weight percent) and P2O5 contents 

(about 0.4 weight percent) than most other low-MgO Grande 

Ronde flows in western Oregon and Washington (table 2; fig. 8; 

Reidel and others, 1989a; Beeson and others, 1989). Wapshilla 

Ridge flows are nearly aphyric but contain abundant plagioclase 

microphenocrysts; this microphyric character is indicative but 

not diagnostic. Only the uppermost Wapshilla Ridge flow in the 

Columbia River Gorge crops out in the Beacon Rock quadrangle; it is found on the slopes of Hamilton Mountain, on the ridge 

west of Hardy Creek, and in a box canyon near the southwest 

corner of the map area. In the first two locations, it overlies 

and pinches out northward against the Eagle Creek Formation; 

in the third locality it probably overlies older Wapshilla Ridge 

Member flows that are buried by talus. This flow is distinguished from older Wapshilla Ridge flows by a slightly but 

consistently higher P2O5 content (≥0.40 weight percent; fig. 8) 

(Wells and others, 2009; R.C. Evarts, unpub. data).

Grouse Creek Member

The Grouse Creek member (Tggc) directly overlies the 

Wapshilla Ridge Member in the western Columbia River Gorge. 

Grouse Creek flows, like Wapshilla Ridge flows, are reversely 

magnetized and belong to the R2 magnetostratigraphic unit of 

Swanson and others (1979b). Grouse Creek flows are generally 

aphyric and generally not microphyric (Reidel and Tolan, 2013). 

Only one Grouse Creek member flow crops out in the map area, 

at the head of the box canyon near the southwest corner of the 

map. It is characterized by moderately low MgO (about 3.6–3.7 

weight percent) and relatively high P2O5 (about 0.36 weight percent) contents (table 2; fig. 8) and an unusual, low-inclination, 

magnetic direction (table 5; J.T. Hagstrum, written commun., 

2010).

Ortley Member

Ortley member (Tgo) flows in the map area are lithologically and chemically similar to underlying Grouse Creek 

member flows (table 2) but exhibit normal magnetic polarities 

(Reidel and others, 1989a; Beeson and others, 1989; Reidel and 

Tolan, 2013). Most Ortley flows in the Beacon Rock quadrangle 

exhibit entablature/colonnade jointing patterns and are aphyric 

but not microphyric. The Ortley member is the thickest Grande 

Ronde Basalt member in the map area, and Ortley flows are 

some of the thickest individual Grande Ronde flows. Thick 

Ortley member sections underpin the ridges east and west 

of Hardy Creek. All paleomagnetically sampled flows in the 

Beacon Rock quadrangle belong to this member (fig. 9).

The landslide scarp that forms the high cliff of the south 

face of Hamilton Mountain provides a spectacular exposure 

of an intracanyon Grande Ronde Basalt flow (fig. 10; Waters, 

1973; Wells and others, 2009). There, a 200-m-thick Ortley 

member flow occupies a paleovalley incised through the 

Wapshilla Ridge Member into the underlying Eagle Creek 

Formation, probably by a tributary of the ancestral Columbia 

River that entered the trans-arc lowland from the north. The 

basalt flow consists of a thick entablature over a thin, curving 

colonnade, banked against Eagle Creek Formation sedimentary 
15

rocks on the east and overlying a thick deposit of pillow breccia 

and palagonitic hyaloclastite on the west. The hyaloclastic 

section, characterized by steeply north dipping foreset beds, is 

as thick as 100 m but pinches out abruptly to the north. Pillow 

fragments in the hyaloclastite are compositionally similar to 

the overlying flow, suggesting that they are products of the 

same eruption. The clastic wedge is interpreted as a delta that 

formed in a temporary lake created when the lava flow blocked 

the south-flowing tributary of the ancestral Columbia River. 

Later in the eruption, the lake water apparently drained away, 

allowing the basalt to flow down the delta front and fill the 

paleovalley. 

Winter Water Member

The Winter Water member (Tgww) is the uppermost 

low-MgO N2 member in the Grande Ronde Basalt (Reidel and 

others, 1989a; Beeson and others, 1989; Reidel and Tolan, 

2013). Winter Water flows can be differentiated from most other 

Grande Ronde flows by the presence of scattered (<1 percent) 

plagioclase phenocrysts and radial, spokelike glomerocrysts 

(Tolan, 1982; Reidel and others, 1989a; Reidel and Tolan, 

2013). In addition, some Winter Water flows exhibit an unusually shallow paleomagnetic inclination (Reidel and others, 

1989a; Wells and others, 1989, 2009; J.T. Hagstrum, written 

commun., 2010). Two chemically and paleomagnetically distinguishable Winter Water flows crop out in the western Columbia 

River Gorge (Wells and others, 2009; R.C. Evarts, unpub. data) 

but only the upper one is present in the Beacon Rock quadrangle. Its chemical composition (table 2) is similar to that of 

some Grouse Creek and Ortley flows but it can be distinguished 

by a consistently lower Ba (<600 ppm) content, normal magnetic polarity, shallow paleomagnetic inclination (table 5; J.T. 

Hagstrum, written commun., 2010), and relatively abundant 

plagioclase phenocrysts. This flow exhibits a colonnade/entablature jointing pattern in which the entablature constitutes more 

than 90 percent of the flow thickness. It forms an imposing 

60-m-high cliff on the west side of the box canyon near the 

southeast corner of the quadrangle but thins northward and is 

very thin or absent about 1.5 km to the north, where the Sentinel 

Bluffs Member rests directly on the Ortley member. The same 

Winter Water member flow crops out on the crest of the ridge 

west of Hardy Creek, where it is about 40 m thick, but is absent 

from the Grande Ronde Basalt section on (informally named) 

Birkenfeld Mountain, 5 km to the north. Its absence probably 

reflects erosion at the edge of the Miocene paleovalley prior to 

emplacement of the younger Sentinel Bluffs Member.

Sentinel Bluffs Member

The Sentinel Bluffs Member (Tgsb) is the uppermost 

member of the Grande Ronde Basalt (Reidel and others, 1989a; 

Reidel, 2005; Reidel and Tolan, 2013). Sentinel Bluffs flows 

are characterized by relatively high MgO contents (4.4 to 5.0 

weight percent; table 2; fig. 8) and normal magnetic polarity. 

Some flows contain widely scattered plagioclase phenocrysts 5 

to 15 mm long. Tolan (1982) referred to these as high-MgO N2

flows. Sentinel Bluffs Member flows in the western Columbia 

River Gorge typically exhibit a blocky to columnar jointing 

style except near flow margins, where an entablature/colonnade jointing pattern may be found (Tolan, 1982; Beeson and 

others, 1989; Reidel, 2005). In the Beacon Rock quadrangle, 

the Sentinel Bluffs Member crops out in scattered localities on 

and southeast of the Washougal-Columbia divide. Its maximum thickness is about 75 m on Birkenfeld Mountain east of 

Hamilton Creek.

Reidel (2005) divided the Sentinel Bluffs Member into six 

compositional types, each associated with a stratigraphically 

confined lava flow or flow package; three of these compositional types, comprising four flows, are found in the map 

area (but were not mapped separately). The uppermost flow is 

assigned to the basalt of Museum of Riedel (2005), which is the 

youngest Sentinel Bluffs unit in the Columbia Basin. This flow 

is vaguely to distinctly microphyric and exhibits a colonnade/

entablature jointing pattern. It is distinguished from the other 

Sentinel Bluffs flows by a combination of high MgO and low 

TiO2 contents (table 2; fig. 8). The basalt of McCoy Canyon, 

the oldest and most voluminous Sentinel Bluffs compositional 

type (Riedel, 2005), is represented in the map area by two 

flows with distinct compositions, including one that has a lower 

Cr content than other Sentinel Bluffs flows in the quadrangle 

(table 2). Although McCoy Canyon flows crop out in several 

localities, the only place where both flows occur is on Hardy 

Ridge, where the lower-Cr flow overlies the higher-Cr flow, a 

relation observed at other locations in the western Columbia 

River Gorge (R.C. Evarts, unpub. data). A flow with a composition higher in MgO than the basalt of Museum, lower in TiO2

270 90

180

N

Figure 9. Paleomagnetic directions for Grande Ronde Basalt flows 

in the Beacon Rock 7.5’ quadrangle. Dots show lower-hemisphere 

(normal) directions. Thin lines outline areas of 95 percent 

confidence intervals. All sampled flows exhibit normal magnetic 

polarity and are assigned to the informal Ortley member of Reidel 

and Tolan (2013).
16

than the basalt of McCoy Canyon, and lower in P2O5 and higher 

Cr than either (table 2) is found at scattered locations in the map 

area. It is relatively thin (<20 m thick) and generally exhibits 

blocky/columnar jointing. Where in contact with other Sentinel 

Bluffs Member flows, as on Birkenfeld Mountain, it always 

lies above the basalt of McCoy Canyon and below the basalt of 

Museum. Flows of this composition and stratigraphic position 

in the Columbia Basin belong to the basalt of Stember Creek of 

Riedel (2005).

Wanapum Basalt

Wanapum Basalt flows are generally more mafic, darker, 

and more coarsely crystalline than Grande Ronde Basalt flows 

and characteristically contain scattered plagioclase phenocrysts 

and glomerocrysts as large as 5 cm across (Mackin, 1961; 

Swanson and others, 1979b). Compositionally, Wanapum flows 

have much higher TiO2 contents (Swanson and others, 1979b; 

Hooper, 1997). The Wanapum Basalt has been divided into 

formal members (Swanson and others, 1979b; Tolan and others, 

1989; Reidel and others, 2013); the Frenchman Springs Member 

(Beeson and others, 1985; Martin and others, 2013) is present in 

the map area.

Frenchman Springs Member

Flows assigned to the Frenchman Springs Member crop 

out between McCloskey and Duncan Creeks in the southwestern part of the quadrangle and at the summit of Birkenfeld 

Mountain. In both locations, they directly overlie the Sentinel 

Bluffs Member of the Grande Ronde Basalt. In the Columbia 

Basin, these units are separated by a sedimentary interval, 

the Vantage Member of the Ellensburg Formation (Swanson 

and others (1979b). In the map area, as elsewhere throughout 

western Washington and Oregon, the Vantage Member is very 

thin (<1 m) or absent (Beeson and others, 1989), but the Grande 

Ronde-Wanapum contact is readily mapped based on the 

distinctly different physical and chemical properties of the two 

formations.

Martin and others (2013) divide the Frenchman Springs 

Member into five units on the basis of composition, paleomagnetic properties, stratigraphic position, and lithologic features. 

Most Frenchman Springs Member outcrops in the map area are 

assigned to the basalt of Sand Hollow (Twfsh) of Beeson and 

others (1985) and Martin and others (2013). The basalt of Sand 

Hollow is the most voluminous unit of the Frenchman Springs 

Member and one Sand Hollow flow reached the Pacific Ocean 

(Beeson and others, 1985; Wells and others, 1989, 2009; Martin 

and others, 2013). Sand Hollow flows are distinguished by the 

persistent presence of amber plagioclase phenocrysts as much 

as 35 mm long and by relatively high Cr content (37–45 ppm).

Two small knobs on the flat ridgecrest south of McCloskey 

Creek consist of similar basalt but contain only scarce 

plagioclase phenocrysts and display distinctly lower MgO 

and Cr and higher P2O5 contents (table 2); these outcrops are 

assigned to the basalt of Sentinel Gap (Twfss) of Beeson and 

others (1985) and Martin and others (2013).

Rocks of the Boring Volcanic Field

Volcanic eruptions in and near the map area ended after 

the Eagle Creek Formation was deposited and did not resume 

until the Pleistocene, when several, small, short-lived volcanoes 

developed. These small centers consist of cinder cones that 

mark vents for associated lava flows of olivine-phyric basalt, 

Qt

Qls

Tecu

Tgo

hyaloclastite

Figure 10. South face of Hamilton Mountain showing ~200-m-thick intracanyon Grande Ronde Basalt flow overlying basal 

foreset-bedded pillow breccia of same flow on the left and Eagle Creek Formation (Tecu) on the right. All visible flows belong 

to the informal Ortley member (Tgo) of Riedel and Tolan (2013). Arrow points to cliff of Eagle Creek Formation shown in figure 

3. Qt, talus; Qls, landslide deposits.
17

basaltic andesite, and low-SiO2 andesite (fig. 3). They are part 

of the Boring Volcanic Field (Evarts and others, 2009), a collection of several dozen monogenetic vents scattered throughout the region from Portland to Beacon Rock (Hodge, 1938; 

Treasher, 1942; Trimble, 1963; Allen, 1975; Tolan and Beeson, 

1984). Products of eleven such centers have been identified in 

the Beacon Rock quadrangle based on field relations, petrography, geochemistry (table 3; fig. 11), paleomagnetic properties 

(table 5; fig. 12), and 40Ar/39Ar ages (table 4). They are distributed in two roughly north-south-oriented belts: one from 

Three Corner Rock to Beacon Rock and the other south of Hard 

Scramble Creek. Vents for eight units have been located (seven 

within the map area); vents for the other three have apparently 

been eroded, buried by younger volcanic rocks, or displaced by 

landslides.

Three Quaternary volcanic units in the map area erupted at 

about 1 Ma (table 4): the medium-potassium basaltic andesites 

of Three Corner Rock (Qtcr), Woodward Creek (Qmwc), and 

Little Creek (Qmlc). The chemically similar basaltic andesite 

of Duncan Creek (Qmdc) is about 200 k.y. younger. All four 

units are notable for low Ba/Nb (15–25), atypical of volcanic-arc magmas (Gill, 1981). Three Corner Rock (fig. 13) is 

a massive lava plug that fills the vent of a completely eroded 

scoria cone. The source for the Woodward Creek lavas, bisected 

by and well exposed in the high eastern headwall scarp of the 

Skamania Landslide Complex, consists of a plug partly encased 

in agglutinated scoria. Woodward Creek flows are notable for 

the presence of dark spheroidal pyroxene xenocrysts, some 

with reaction rinds of fine-grained olivine as large as 3 mm in 

diameter. The basaltic andesite of Little Creek is a flow remnant 

Zr/Nb

10

20

30

40

50

60

70

8 10 12 14 16 18 20 22

Ba/Nb

0

Qmdc

Qlbr

Qmbr Qtcr

Qmwc

Qmhs

Qmus Qmbc

Qmum

Qmlc

B

0.0

50 52 54 56 58 60 62 64 66 68 70

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Ba/Sr

Mg#

Qmhs Qlbr

Qmdc

Qmbr

Qmlc

Qmwc

Qtcr

Qmbc Qmum

Qmus

A

Figure 11. Selected chemical characteristics of Pleistocene volcanic and subvolcanic rocks 

in the Beacon Rock 7.5’ quadrangle. A, Ba/Sr versus Mg# (atomic ratio 100Mg/(Mg+Fe2+) 

with Fe2+ set to 0.85 × Fetotal). B, Ba/Nb versus Zr/Nb. See Description of Map Units for unit 

identities.
18

banked against the east side of Hardy Ridge. It may have issued 

from the same vent as Woodward Creek. The Duncan Creek 

unit consists of several small flow remnants scattered along the 

top of the western scarp of the Skamania Landslide Complex. 

The location of its source is unknown but probably to the east in 

the area now underlain by landslide debris. 

Most of the other Quaternary volcanic rocks in the map 

area are normally magnetized and were emplaced during the 

Bruhnes polarity chron. Little Beacon Rock, composed of 

coarse agglutinated scoria cut by irregular to tabular basalt 

intrusions, is the dissected core of a small volcano that 

generated a basalt flow (Qlbr) that overlies the Eagle Creek 

Formation to the northwest. It was active at 498±7 ka (table 4). 

The elevation of the vent complex at Little Beacon Rock is 

about 250 m lower than the flow remnant, suggesting that the 

vent complex is a large slump block.

The Quaternary volcanic rocks west of McCloskey Creek 

are on the east edge of a short north-south cluster of volcanoes 

in the adjacent Bobs Mountain quadrangle (Evarts and others, 

2009; R.C. Evarts, unpub. mapping). Two scoria cones mark 

the vents for a series of basaltic andesite flows that moved 

westward into the ancestral Washougal River. These flows, 

the basaltic andesite of Boyles Creek (Qmbc) and the basaltic 

andesite of Hard Scramble Creek (Qmhs), are distinguished by 

their relatively high contents of large-ion lithophile elements 

such as potassium, barium, and strontium (table 3; figs. 2, 10). 

40Ar/39Ar age determinations for this unit in the adjacent 

Bobs Mountain quadrangle show that most of these eruptions 

occurred between 260 ka and 290 ka (Fleck and others, 2014). 

During this time span, the andesite of McCloskey Creek 

(Qamc) issued from a dome at the south end of the McCloskey 

Creek cluster, about 1 km west of map area (R.C. Evarts, 

unpub. mapping). Two small mapped volcanic units, the basaltic 

andesite of upper McCloskey Creek (Qmum) and the basaltic 

andesite of upper Hard Scramble Creek (Qmus) have not been 

dated and their magnetic polarity is unknown. They underlie 

the other flow units in the McCloskey Creek cluster and differ 

chemically, with much lower K2O contents; they presumably 

record an older eruptive episode.

The youngest volcano in the map area, as well as in the 

entire Boring Volcanic Field, is the basaltic andesite of Beacon 

Rock (Qmbr). This 250-m-high monolith (see cover photograph) has long been recognized as the exhumed neck of an 

ancient volcano, presumably a cinder cone (Williams, 1916; 

Hodge, 1938; Hammond, 1980; Hammond and Korosec, 1983; 

Waters, 1973). Its 58-ka 40Ar/39Ar age is surprisingly young considering the normal decay rates of cinder cones (Wood, 1980). 

The explanation must be that the enclosing scoria deposits were 

completely washed away by the late Pleistocene cataclysmic 

floods (16,000–12,000 14C yrs. B.P.; Waitt, 1985, 1994; Atwater, 

1986; Benito and O’Connor, 2003; Clague and others, 2003); 

the largest flood would have totally engulfed Beacon Rock 

(Benito and O’Connor, 2003).

Alluvial and Mass-Wastage Deposits

Landslide and Talus Deposits

One of the most prominent geologic features in the Beacon 

Rock quadrangle is the huge Skamania Landslide Complex 

(Qls), one of the largest of a series of landslides that extends 

nearly continuously along the north side of the Columbia River 

Gorge from Cape Horn to Dog Mountain (Palmer, 1977). The 

complex underlies about 35 km2 of the map area and consists of 

several geomorphic domains that apparently represent landslides of different ages. Like most of the large gorge slides, the 

Skamania complex is closely associated with exposures of the 

mechanically weak Eagle Creek Formation, particularly where 

overlain by thick sections of resistant Grande Ronde Basalt 

or Pleistocene lava flows (Hodge, 1938; Wise, 1970; Waters, 

1973; Palmer, 1977; Hammond, 1980). At least some large slide 

events in the map area, however, evidently result from failure of 

water-saturated Eagle Creek strata without benefit of a superincumbent load. Waters (1973) emphasized the role of the thick 

saprolitic paleosol at the base of the Eagle Creek Formation in 

lubricating the Bonneville and other gorge slides, but, in the 

map area, failure planes appear to occur within as well as at the 

base of that unit. Periodic repairs to Washington State Route 14 

and secondary roads, as well as the existence of incipient scarps 

behind the present landslide headwalls, testify to the active 

nature of parts of the slide complex.

Although the Skamania Landslide Complex is by far 

the largest, landslides are common in all areas underlain by 

the Eagle Creek Formation, such as those on the flanks of 

Hamilton Mountain, in upper Hamilton Creek, and in the 

headwaters of Deep Creek. Landslides are dispersed throughout 

270 90

180

N

Qmdc

Qlbr

Qmlc

Qmbr

Qtcr

Qmwc

Figure 12. Paleomagnetic directions for Quaternary volcanic 

rocks in the Beacon Rock 7.5’ quadrangle. Dots show lowerhemisphere (normal) directions. Open circles show upperhemisphere (reversed) directions. Thin lines outline areas of 95 

percent confidence intervals. See Description of Map Units for unit 

identities.
19

areas underlain by older rocks as well, where they are generally associated with weathered or zeolitic volcaniclastic beds 

(Tt, Tvs), and the large landslide complex in Hamilton Creek 

valley formed where lavas flows of the Hamilton Creek rest on 

volcaniclastic rocks.

In addition to landslides formed by discrete failure events, 

talus deposits (Qt) created by slower incremental disintegration 

are common beneath cliffs composed chiefly of lava flows. In 

some places, the basal contacts of these units are buried for 

many kilometers by talus aprons; particularly extensive deposits 

are present at the base of the lava flows of Hamilton Creek in 

the northeast part of the quadrangle and beneath the Grande 

Ronde Basalt on the ridge west of Hardy Creek, the west slope 

of Hamilton Mountain, and Birkenfeld Mountain. Barren, 

post-landslide talus accumulations are also common at the base 

of steep landslide scarps.

Holocene and Pleistocene Alluvium of Tributary 

Streams

Most of the streams in the map area, including the 

Washougal River, occupy bedrock channels. The local accumulations of alluvium (Qa) along them consist predominantly 

of coarse sand and gravel. The alluvium in Hamilton Creek is 

exceptionally coarse, composed of boulders as large as 4 m in 

diameter that have weathered out of the Eagle Creek Formation.

The Washougal River in the map area is commonly flanked 

by strath terraces, with surfaces 2 to 5 m above the riverbed, 

that are mantled by as much as 6 m of cobble gravel (Qtdwu). 

These are little weathered and probably late Pleistocene or 

Holocene.

Holocene Outburst-Flood Deposits

The bench directly east of Beacon Rock is underlain by 

gravel inferred to have been deposited by outburst flooding 

from breaching of the Bonneville landslide, which blocked the 

Columbia River about 7 km upstream between A.D. 1425 and 

1450 (O’Connor and Burns, 2009). Breaching was likely soon 

after damming, seemingly before A.D. 1482, judging from correlative outburst flood deposits downstream that underlie Mount 

St. Helens Kalama-age (A.D. 1479 or 1482) tephra.

Structural Features

Oligocene and early Miocene strata of the ancestral 

Cascade volcanic arc generally strike northeast and dip 

southeast in the map area. Dips within the three major 

unconformity-bounded packages exhibit a clear decrease with 

time (see cross sections A–A′ and B–B′). Abundant bedding 

Figure 13. Three Corner Rock, a mass of basaltic andesite that occupied the throat of a now-eroded, 

approximately 1-Ma scoria cone. Barren rock outcrop is approximately 35 m tall.
20

measurements in the older, volcaniclastic-dominated strata give 

an average dip of about 25°. Although most deviations from this 

trend are attributable to localized deformation around intrusions, 

the flattening of dips in the northwestern part of the map reflects 

regional structure. Attitudes in the lava flows of Hamilton 

Creek are not as obvious because depositional contacts are 

rarely exposed and commonly irregular. Most measurements in 

this unit are from platy parting in lava flows, although variable attitudes of platy parting approximate original horizontal. 

These measurements are consistent with trends inferred from 

tracing individual flows within the unit; both trends indicate 

a regional dip of 5° to 15°. Volcaniclastic beds of the Eagle 

Creek Formation are insufficiently exposed in the map area to 

obtain reliable attitudes, but pronounced platy parting in some 

lava flows dips south to southeast at less than 10°. These dips 

probably are primary and represent deposition on the surface of 

a south-sloping fan. Tracing of individual flows shows that the 

Grande Ronde Basalt strikes northwest and dips southwest at 

about 2°.

The differential dips displayed by the unconformitybounded rock packages indicate that the Oligocene strata were 

deformed prior to deposition of the lava flows of Hamilton 

Creek and that further mild southeasterly tilting took place 

prior to deposition of the Eagle Creek Formation. Late 

Oligocene strata in the Washougal area, approximately 6 km 

to the southwest, are oriented similarly to those in the Beacon 

Rock quadrangle (Evarts and others, 2013), but correlative 

beds in the poorly exposed intervening area instead strike west 

or southwest and exhibit shallower dips (R.C. Evarts, unpub. 

mapping), suggesting that these beds have been gently warped. 

Corresponding strata in the adjacent Bonneville Dam quadrangle strike west-northwest and dip south at low angles (Wise, 

1961, 1970; R.C. Evarts, unpub. mapping), defining a shallow, 

southeast-plunging syncline that transects the northeastern part 

the map area. Broad open folds in pre-middle Miocene strata 

are typical of the Cascade Range of southwestern Washington 

(Wise, 1970; Hammond, 1980; Evarts and Swanson, 1994). 

These structures are large, and defining the trend of the fold 

axes requires mapping over areas much larger than a 7.5-

minute quadrangle. While most folding apparently occurred 

prior to emplacement of the lava flows of Hamilton Creek, 

tilting of those flows indicates that deformation continued into 

the early Miocene. Whether deformation continued during 

deposition of the Eagle Creek Formation and the Grande Ronde 

Basalt is uncertain, because dips in the Eagle Creek Formation 

are likely depositional. The onlap relation between the 

Columbia River Basalt Group and the Eagle Creek Formation 

may simply record emplacement of the massive basalt flows 

against an eroded, south-sloping, volcano flank that locally 

defined the north margin of the middle Miocene trans-arc lowland (Tolan and Beeson, 1984). The slight regional southwest 

tilting that affects the approximately 16-Ma Grande Ronde 

flows may relate to late uplift of the Cascade Range and (or) to 

mild, long-term, north-south compression as manifested in the 

Yakima fold belt (Beeson and others, 1989; Beeson and Tolan, 

1990).

Faults

Only a few faults, with east-west, northeast, or northwest 

strikes, were mapped in the Beacon Rock quadrangle; all are 

steep and exhibit only minor vertical offsets. Those cutting only 

Oligocene rocks are marked by outcrops of sheared, brecciated, 

or altered rock; the displacements and lateral extents of these 

structures are highly uncertain. North of Hamilton Mountain, 

an east-west fault offsets Grande Ronde Basalt flows by about 

25 m. Faults in the upper McCloskey Creek area that juxtapose 

Columbia River Basalt Group flows against the Eagle Creek 

Formation appear to display larger displacements, but relief on 

the erosional unconformity between these two units makes it 

impossible to determine offsets with confidence. Some or all of 

these faults may be primarily strike-slip structures that accommodated the region-wide, clockwise, vertical-axis rotations 

determined from paleomagnetic studies (Beck and Burr, 1979; 

Wells and Heller, 1988; Hagstrum and others, 1999). No faults 

definitely cut Pleistocene volcanic rocks, thus none are known 

to have moved during the Quaternary.

Geologic Resources

Known geologic resources available in the Beacon Rock 

quadrangle are limited to nonmetallic industrial materials, 

chiefly aggregate for road construction and similar purposes. 

Small rock pits that provide material for logging roads in the 

northern part of the map area have been developed in various 

lava-flow and intrusive units. Road metal has also been obtained 

from large blocks of Grande Ronde Basalt in the Skamania 

Landslide complex.

Geologic Evolution

The ancestral Cascade volcanic arc initiated by 40 Ma and 

was continuously active for 20 m.y. (Priest, 1990; Smith, 1993; 

Evarts and Swanson, 1994; Sherrod and Smith, 2000). Volcanic 

activity diminished significantly by the middle Miocene, coincident with eruption of flood basalt flows of the Columbia River 

Basalt Group. The oldest rocks in the Beacon Rock quadrangle 

record deposition within a regional intra-arc fluvial system that 

received episodic influxes of debris flows, pyroclastic flows, 

and lava flows from contemporaneous volcanic centers. These 

late Oligocene to early Miocene rocks exhibit minor to pervasive zeolite-facies metamorphism owing to shallow burial. 

Localized propylitic alteration in the Washougal River area 

probably reflects the existence of a shallow, unexposed intrusion of substantial size. Shortly after deposition, these strata 

were folded and partly eroded. At about 22 Ma, porphyritic 

mafic to intermediate lava flows (lava flows of Hamilton Creek) 

buried the eroded surface of the mildly tilted older strata. The 

preserved flows were probably deposited on the lower flanks of 

a volcanic edifice that was centered to the west, perhaps in the 

area of Hard Scramble Creek. These eruptions were followed by 

another period of nondeposition, lasting less than 2 m.y., when 
21

tilting of the strata continued and the Hamilton Creek volcano 

was deeply eroded. This erosional episode produced a surface 

with more than 100 m of topographic relief. A thick saprolitic 

soil profile developed on uplands between drainages. Between 

20 and 19.5 Ma, this surface was buried by a south-sloping 

fan of coarse volcaniclastic debris (Eagle Creek Formation) 

probably derived from a field of andesitic and dacitic domes 

(Wise, 1970; Waters, 1973), including some located in the map 

area. Cessation of Eagle Creek volcanism marked the beginning of a long hiatus in arc volcanism in southern Washington 

and northern Oregon, when a 60-km-wide valley, the Cascade 

trans-arc lowland of Beeson and others (1989) and Beeson and 

Tolan (1990), developed through the ancestral Cascade Range. 

This lowland, which apparently existed during deposition of the 

Eagle Creek Formation (Beeson and others, 1989), was presumably occupied by an ancestral Columbia River.

Between 16.5 and 12 m.y. ago, massive flood-basalt flows 

entered western Oregon and Washington from the east through 

the trans-arc lowland. The route of the modern Columbia River 

in the map area closely coincides with the northern margin of 

the lowland, which is delineated by the unconformable contact 

between the Grande Ronde Basalt and Eagle Creek Formation. 

During a roughly 1.2-m.y. period, voluminous Grande Ronde 

Basalt and early Wanapum Basalt flows inundated much of the 

back-arc region, forming vast horizontal sheets that filled the 

subsiding trans-arc lowland (Beeson and others, 1989; Beeson 

and Tolan, 1990). The flows issued so rapidly that the ancestral 

Columbia River was rarely able to re-establish integrated drainage between eruptions. The discontinuous distribution and variable thickness of many Grande Ronde Basalt flows in the map 

area, however, indicate that they possess a more lobate form in 

this area at the margin of the flood-basalt field than they do in 

the Columbia Basin. Although the lobate character may reflect 

emplacement dynamics (Self and others, 1997; Thordarson and 

Self, 1998), we believe that inter-eruption erosion by streams 

along the northern margin of the trans-arc lowland was also a 

factor. Streams flowing out of highlands to the north were able 

to locally incise the flows and underlying Eagle Creek beds 

between emplacement events, as shown by the paleocanyon 

fill relations of an Ortley member flow that is well exposed on 

Hamilton Mountain (fig. 10). The existence of such streams 

implies that a major trunk river was localized by the edge of 

the flow field during intervals between basalt eruptions. Such a 

stream would have captured and transported away any sediment 

carried in from the north by tributary streams, thus accounting 

for the virtual absence of sedimentary interbeds within the 

nearby Columbia River Basalt Group. Observed banking of 

Grande Ronde Basalt flows against the Eagle Creek Formation 

demonstrates that flood-basalt flows flowed around a highland 

composed of andesitic to dacitic volcanic centers and did not 

completely blanket a low-elevation, low-relief Cascade Range 

in southernmost Washington at this time (Mackin and Cary, 

1965; Mitchell and Montgomery, 2006).

The youngest Columbia River Basalt Group flows in the 

Beacon Rock quadrangle belong to the 15-Ma Frenchman 

Springs Member of the Wanapum Basalt. At least two later 

flood-basalt flows passed through the trans-arc lowland, routed 

by ancestral Columbia River canyons located south of the map 

area (Waters, 1973; Tolan, 1982; Tolan and Beeson, 1984).

Arc volcanism was renewed in the late Pliocene, about 

4 to 3 Ma, as voluminous low-potassium tholeiite flows erupted 

in the Cascade Range south of the map area (Tolan and Beeson, 

1984; Conrey and others, 1996a, 2004). This influx of basaltic 

volcanic material from the northern Oregon Cascade Range 

eventually forced the ancestral Columbia River northward to its 

present location (Tolan and Beeson, 1984).

Beginning about 2.6 Ma, mafic volcanic activity began 

to spread westward from the Cascade Range into the Portland 

Basin to form the Boring Volcanic Field (Treasher, 1942; 

Trimble, 1963; Allen, 1975; Conrey and others, 1996b; Evarts 

and others, 2009). Somewhat later, volcanism extended northward across the Columbia River (Hammond and Korosec, 

1983), and several small, monogenetic eruptive centers formed 

in and adjacent to the map area within the past million years.

The modern Columbia River Gorge was formed as the 

Cascade Range axis arched upward during the past 2 m.y (Tolan 

and Beeson, 1984; Mustoe and Leopold, 2014). Tilting of 

Grande Ronde flows in the map area probably reflects a combination of this broad uplift superimposed on mild north-south 

compression that produced the Yakima fold belt (Reidel and 

others, 1989b, 2003).

The quadrangle lies well beyond the limits of Pleistocene 

glaciers emanating from the Cascade Range. No evidence exists 

for glacial ice in the Washougal drainage and no sediments of 

likely outwash origin are found in the map area. Cataclysmic 

outburst floods from glacial Lake Missoula coursed through the 

Columbia River Gorge in the latest Pleistocene (Bretz, 1925; 

Trimble, 1963; Allison, 1978; O’Connor and Baker, 1992; 

Waitt, 1994, 1996; Benito and O’Connor, 2003) and probably 

during earlier Pleistocene glaciations. Erosion of bedrock by the 

huge floods was probably limited, but they doubtless excavated 

significant quantities of talus and other unconsolidated debris 

from the base of gorge cliffs (Waitt, 1994), as well as removed 

the scoria deposits housing the Beacon Rock plug. In this reach 

of the Columbia River, the largest floods were about 240 m 

deep (Benito and O’Connor, 2003). Most of the large slope failures along the gorge, such as the Skamania Landslide Complex, 

appear to postdate the cataclysmic floods, but the floods may 

have played a role in promoting slides by preferentially eroding 

the weakly consolidated Eagle Creek Formation and thus undercutting and oversteepening slopes capped by lava flows.

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Appendix 1. 40Ar/39Ar incremental-heating data for samples from the Beacon Rock 7.5' quadrangle, Skamania County, Washington.

[See figure 18 (sheet 2) for sample locations and table 4 for unit assignments. Preferred age shown in bold; for plateau ages, steps used to calculate age shown in 

bold. Analytical techniques described in Fleck and others (2014). --, not determined; Wtd, weighted]

Step (°C) mol 39Ar % 39ArK Rel % 40Ar* 40Ar/39Ar 37Ar/ 39Ar 36Ar/ 39Ar K/Ca Cl/K Age (Ma)

06BV-G633 Plagioclase IRR253-87 J = 0.00211592

550 1.66216E-16 8.1438 7.8075 58.13962 393.14141 0.28946 0.00099 0.065460 23.273±2.613

625 6.0442E-16 29.6138 26.7119 18.95227 403.77861 0.15803 0.00095 0.026646 26.185±0.935

700 5.309E-16 26.0117 44.8319 10.73890 395.60875 0.12882 0.00098 0.020453 24.729±0.941

775 2.92748E-16 14.3433 38.7901 14.10144 386.48832 0.13545 0.00101 0.023488 27.844±1.295

850 1.17425E-16 5.7533 18.1085 30.74764 373.55515 0.18795 0.00106 0.085942 28.019±2.796

925 1.01343E-16 4.9654 19.2457 27.60780 153.86344 0.11773 0.00306 0.111912 22.441±2.727

1000 3.09423E-17 1.5160 3.1920 88.37206 299.83922 0.37206 0.00140 10.785405 13.386±10.173

1100 4.86903E-17 2.3856 13.1353 46.27887 342.75000 0.23029 0.00118 0.219630 29.770±5.399

1250 1.4832E-16 7.2670 29.7490 37.72643 321.79051 0.17817 0.00128 0.051954 53.634±2.217

Wtd Mean Plateau age (Ma) MSWD = 1.3 25.780±0.548

Intercept = 293.0±6.1 Isochron age (Ma) MSWD = 1.11 26.166±1.014

Integrated age (Ma) 27.629±0.689

06BV-G700B Plagioclase IRR253-82 J = 0.00221534

550 1.55961E-16 0.6988 22.5447 16.71654 11.67381 0.04696 0.04460 0.00235 15.115±2.390

625 6.78876E-16 3.0419 38.7861 15.34791 12.93587 0.03532 0.04021 0.00024 23.839±0.678

700 1.53969E-15 6.8991 63.3062 9.77171 14.08902 0.01599 0.03689 0.00047 24.785±0.327

775 2.68925E-15 12.0502 85.2486 7.08893 14.59076 0.00752 0.03561 0.00017 24.225±0.177

850 3.88034E-15 17.3872 92.8930 6.54898 14.71305 0.00559 0.03531 0.00025 24.387±0.125

925 4.27408E-15 19.1516 93.7448 6.47058 14.57904 0.00535 0.03564 0.00007 24.315±0.128

1000 3.25692E-15 14.5938 86.4504 6.98395 14.46110 0.00715 0.03594 0.00015 24.201±0.151

1075 2.59891E-15 11.6454 92.5231 6.53369 14.27976 0.00555 0.03640 0.00009 24.228±0.177

1125 1.31179E-15 5.8779 81.7519 7.42387 14.03922 0.00842 0.03703 -0.00004 24.319±0.358

1200 8.69857E-16 3.8977 68.7224 8.50482 12.55449 0.01242 0.04145 0.00056 23.402±0.501

1400 1.06149E-15 4.7564 56.8427 10.78381 14.52248 0.01972 0.03578 0.00081 24.569±0.415

Wtd Mean Plateau age (Ma) MSWD = 0.84 24.300±0.112

Intercept = 295.5±7.6 Isochron age (Ma) MSWD = 1.05 24.290±0.091

Total gas age (Ma) 24.220±0.068

06BV-G685 Plagioclase IRR253-81 J = 0.00223501 

550 2.6877E-16 0.4102 6.0401 84.056142 8.84204 0.26970 0.05899 0.03162 20.476±3.427

625 8.59066E-16 1.3113 69.2354 8.685938 8.95685 0.01147 0.05823 0.01378 24.230±0.447

700 1.98469E-15 3.0294 86.1146 6.879232 9.19131 0.00573 0.05674 0.00402 23.874±0.204

775 3.69796E-15 5.6445 92.9978 6.417668 9.17245 0.00401 0.05686 0.00128 24.050±0.127

850 6.60559E-15 10.0826 95.4603 6.240220 9.28752 0.00348 0.05615 0.00187 24.007±0.085

925 9.70208E-15 14.8090 97.6099 6.123124 9.32759 0.00303 0.05591 0.00176 24.087±0.075

1000 8.88974E-15 13.5690 94.1271 6.304977 9.29239 0.00378 0.05612 0.04284 23.919±0.079

1075 1.05007E-14 16.0280 97.7726 6.109434 9.18887 0.00296 0.05675 0.00100 24.072±0.075

1150 8.80687E-15 13.4425 92.6957 6.450329 8.95849 0.00403 0.05822 0.00262 24.091±0.083

1250 1.23125E-14 18.7934 84.1123 7.101271 8.80606 0.00621 0.05924 0.00183 24.064±0.080

1400 1.88692E-15 2.8801 82.1836 7.292973 9.00098 0.00684 0.05795 0.00144 24.150±0.210

Wtd Mean Plateau age (Ma) MSWD = 0.58 24.041±0.060

Intercept = 293.2±4.4 Isochron age (Ma) MSWD = 0.89 24.046±0.061

Total gas age (Ma) 24.030±0.028
27

Appendix 1. 40Ar/39Ar incremental-heating data for samples from the Beacon Rock 7.5' quadrangle, Skamania County, Washington.—

Continued

Step (°C) mol 39Ar % 39ArK Rel % 40Ar* 40Ar/39Ar 37Ar/ 39Ar 36Ar/ 39Ar K/Ca Cl/K Age (Ma)

03BV-G122 Plagioclase IRR253-80 J = 0.00229562 

550 1.9251E-16 0.6072 42.6916 23.63018 17.92146 0.05076 0.02893 0.00220 41.214±2.145

625 6.84855E-16 2.1600 38.0070 16.54504 18.38332 0.03974 0.02819 0.00085 25.807±0.779

700 1.66028E-15 5.2365 66.1625 9.22046 18.86875 0.01571 0.02746 0.00036 25.049±0.333

775 2.82126E-15 8.8982 84.6697 6.96925 19.23067 0.00888 0.02694 0.00006 24.241±0.180

850 3.86859E-15 12.2015 89.4242 6.52316 19.34642 0.00763 0.02677 0.00021 23.967±0.133

925 4.32794E-15 13.6503 90.7341 6.35989 19.64489 0.00737 0.02636 0.00026 23.716±0.157

1000 3.71851E-15 11.7281 81.2835 6.92487 20.30514 0.00994 0.02549 0.00023 23.147±0.172

1075 2.43643E-15 7.6845 89.4591 6.54402 19.69367 0.00772 0.02630 0.00044 24.059±0.203

1150 1.79815E-15 5.6713 75.4627 8.64367 18.90978 0.01235 0.02740 0.00162 26.771±0.288

1250 6.48583E-15 20.4562 94.8805 6.97525 19.38640 0.00651 0.02672 0.00132 27.169±0.122

1400 3.71156E-15 11.7062 93.5362 6.80669 20.41203 0.00707 0.02536 0.00086 26.162±0.164

Wtd Mean Plateau age (Ma) none

Intercept = 351±99 Isochron age (Ma) MSWD = 14 23.124±1.623

Integrated age (Ma) 25.142±0.133

Age Minimum (Ma) (Max Age) 23.144±0.183

02BV-G51A Plagioclase IRR201-1 J = 0.00478381 

Step (°C) mol 39Ar % 39ArK Rel % 40Ar* 40Ar/39Ar 37Ar/ 39Ar 36Ar/ 39Ar K/Ca Cl/K Age (Ma)

550 8.9932E-16 0.9139 0.0208 61.47521 15.85845 0.20791 0.03060 -- 11.098±4.182

625 2.62332E-15 2.6659 0.4228 6.17870 18.50077 0.01700 0.02620 -- 22.687±0.605

700 4.65142E-15 4.7270 0.6935 3.82415 19.25469 0.00906 0.02510 -- 23.043±0.369

775 8.06215E-15 8.1931 0.8224 3.27421 19.47233 0.00712 0.02480 -- 23.397±0.259

825 9.25085E-15 9.4011 0.8675 3.08623 19.47400 0.00653 0.02480 -- 23.264±0.198

875 1.23668E-14 12.5677 0.8989 2.92920 19.42311 0.00614 0.02490 -- 22.882±0.189

925 1.4873E-14 15.1146 0.8684 2.97248 19.23560 0.00641 0.02514 -- 22.432±0.186

975 1.74318E-14 17.7150 0.8976 2.92074 19.09835 0.00606 0.02530 -- 22.778±0.183

1025 7.17961E-15 7.2962 0.6217 4.17984 18.91016 0.01040 0.02560 -- 22.576±0.219

1075 6.25732E-15 6.3590 0.8571 3.01930 18.49766 0.00635 0.02620 -- 22.478±0.207

1150 4.49298E-15 4.5660 0.8377 3.20131 18.22926 0.00658 0.02650 -- 23.283±0.232

1250 5.30634E-15 5.3925 0.7897 3.34670 18.13803 0.00718 0.02670 -- 22.947±0.219

1350 5.00651E-15 5.0878 0.7925 3.35999 18.31225 0.00720 0.02640 -- 23.121±0.225

Wtd Mean Plateau age (Ma) MSWD = 2.1 22.789±0.132

Intercept = 295±16 Isochron age (Ma) MSWD = 2.5 22.748±0.264

Integrated age (Ma) 22.755±0.136

06BV-G650 Plagioclase IRR253-86 J = 0.00213787

550 5.83788E-16 1.3070 71.6447 8.58382 5.22688 0.00964 0.10004 0.01351 23.646±0.473

625 1.88675E-15 4.2241 87.4890 6.85629 5.18719 0.00430 0.10081 0.00641 23.067±0.153

700 3.71793E-15 8.3239 94.3855 6.23519 5.47046 0.00266 0.09557 0.00204 22.638±0.102

775 5.66797E-15 12.6897 96.7699 6.08305 5.82202 0.00224 0.08978 0.00100 22.649±0.082

850 7.24149E-15 16.2126 96.7406 6.07487 6.25688 0.00236 0.08351 0.00167 22.618±0.079

900 5.64182E-15 12.6311 97.1921 6.04661 6.53984 0.00234 0.07988 0.00180 22.623±0.084

960 5.3727E-15 12.0286 97.3495 6.04005 6.65635 0.00234 0.07848 0.00193 22.636±0.084

1025 4.31441E-15 9.6593 91.1534 6.35215 6.69505 0.00371 0.07802 0.06314 22.293±0.092

1100 5.84032E-15 13.0756 92.4171 6.38748 6.34817 0.00335 0.08231 0.00764 22.720±0.092

1200 4.39874E-15 9.8481 91.6589 6.40671 6.17099 0.00348 0.08468 0.00335 22.599±0.099

Wtd Mean Plateau age (Ma) MSWD = 1.5 22.599±0.061

Intercept = 269±28 Isochron age (Ma) MSWD = 1.9 22.698±0.081

Integrated age (Ma) 22.639±0.113
28

Appendix 1. 40Ar/39Ar incremental-heating data for samples from the Beacon Rock 7.5' quadrangle, Skamania County, Washington—

Continued.

Step (°C) mol 39Ar % 39ArK Rel % 40Ar* 40Ar/39Ar 37Ar/ 39Ar 36Ar/ 39Ar K/Ca Cl/K Age (Ma)

05BV-G437 Plagioclase IRR253-83 J = 0.00219861

550 2.49645E-16 0.9304 0.3404 804.46108 21.28735 2.71891 0.02430 0.04428 10.982±27.580

575 3.45343E-16 1.2879 1.8304 285.48357 20.24176 0.95394 0.02557 0.02154 20.887±7.179

600 2.86951E-16 1.0698 68.3150 8.32630 20.68857 0.01457 0.02501 0.00965 22.731±1.048

650 7.17922E-16 2.6766 61.3218 9.23221 20.62020 0.01771 0.02510 0.00720 22.625±0.467

700 1.08275E-15 4.0369 74.9164 7.41524 20.60276 0.01193 0.02512 0.00440 22.202±0.298

775 2.5131E-15 9.3693 86.6183 6.33784 20.68135 0.00853 0.02502 0.00226 21.943±0.157

850 3.70591E-15 13.8166 75.2990 7.30469 20.65020 0.01175 0.02506 0.00364 21.985±0.143

925 5.13166E-15 19.1318 61.6434 8.93493 20.68155 0.01725 0.02502 0.00300 22.015±0.159

1000 3.52997E-15 13.1615 84.3676 6.46038 20.55590 0.00904 0.02518 0.10450 21.785±0.143

1075 1.84136E-15 6.8686 95.0882 5.76518 19.89296 0.00640 0.02603 0.00666 21.901±0.191

1150 1.41665E-15 5.2886 92.0766 5.84356 18.69794 0.00667 0.02771 0.01480 21.481±0.191

1250 5.99243E-15 22.3621 97.4560 5.66545 19.27165 0.00576 0.02688 0.00120 22.048±0.096

Wtd Mean Plateau age (Ma) MSWD = 1.05 21.943±0.056

Intercept = 295.0±1.3 Isochron age (Ma) MSWD = 1.5 21.907±0.142

Integrated age (Ma) 21.860±0.161

02BV-G52A Plagioclase IRR248-78 J = 0.00276831 

550 1.31094E-16 0.0157 -6.2312 17.63126 218.87917 0.12356 0.00205 0.25599 -6.337±4.510

625 5.67747E-16 0.0678 17.6943 13.31649 244.21438 0.10421 0.00180 0.26046 13.983±1.545

700 6.76344E-16 0.0808 29.5569 9.71182 227.34679 0.08565 0.00196 0.15955 16.797±1.137

775 5.82629E-16 0.0696 52.6237 6.87545 138.24987 0.04902 0.00345 0.04408 19.781±1.162

850 7.83399E-16 0.0936 57.0138 7.30261 88.69470 0.03500 0.00557 0.06671 21.956±0.844

925 1.44616E-15 0.1727 71.5191 5.86323 113.89289 0.03694 0.00426 0.03409 22.509±0.592

1000 2.46518E-15 0.2944 52.1640 7.49636 123.19778 0.04599 0.00391 0.46484 21.139±0.475

1100 5.28557E-16 0.0631 33.1587 12.76443 65.50597 0.04684 0.00766 0.05160 21.962±1.257

1250 1.19242E-15 0.1424 40.8427 14.77957 95.49602 0.05581 0.00515 0.03929 31.898±0.779

Wtd Mean Plateau age (Ma) MSWD = 1.5 21.592 ± 0.314

Intercept = 291±54 Isochron age (Ma) MSWD = 1.8 21.704±3.550

Total gas age 

(Ma)

21.683±0.314

04BV-G281A Plagioclase IRR248-79 J = 0.00269623 

550 7.78611E-16 0.8048 63.8260 5.98304 6.06039 0.00897 0.08623 0.04601 18.294±0.614

625 3.70929E-15 3.8341 74.5527 5.46825 6.72614 0.00653 0.07766 0.01794 19.533±0.181

700 8.5138E-15 8.8003 92.3224 4.46980 7.21191 0.00311 0.07241 0.00445 19.776±0.100

775 1.27963E-14 13.2270 97.1349 4.19702 7.45831 0.00243 0.07000 0.00168 19.542±0.076

850 1.6631E-14 17.1907 95.9164 4.22814 7.52476 0.00262 0.06938 0.00278 19.441±0.072

925 1.80708E-14 18.6789 97.5558 4.16115 7.58184 0.00240 0.06886 0.00243 19.461±0.069

1000 1.15855E-14 11.9754 89.5435 4.53688 7.60745 0.00367 0.06862 0.10128 19.476±0.086

1100 9.29497E-15 9.6078 94.9919 4.25909 7.43642 0.00274 0.07021 0.00426 19.394±0.090

1250 1.53638E-14 15.8809 91.4304 4.44880 7.15277 0.00323 0.07301 0.00669 19.494±0.078

Wtd Mean Plateau age (Ma) MSWD = 1.3 19.501±0.049

Intercept = 294±14 Isochron age (Ma) MSWD = 1.9 19.493±0.061

Integrated age (Ma) 19.490±0.101
29

Appendix 1. 40Ar/39Ar incremental-heating data for samples from the Beacon Rock 7.5' quadrangle, Skamania County, Washington—

Continued.

Step (°C) mol 39Ar % 39ArK Rel % 40Ar* 40Ar/39Ar 37Ar/ 39Ar 36Ar/ 39Ar K/Ca Cl/K Age (Ma)

02BV-G55B Plagioclase IRR253-78 J = 0.00230644 

550 1.44733E-16 0.2109 1.4179 129.60535 6.47250 0.43408 0.08072 0.02245 7.664±9.290

625 9.13854E-16 1.3314 11.3694 39.57453 6.22527 0.12037 0.08394 0.00976 18.702±1.142

700 1.86467E-15 2.7167 47.6567 9.80398 6.34741 0.01909 0.08232 0.00142 19.419±0.299

775 3.86647E-15 5.6331 77.1486 6.00851 6.58324 0.00643 0.07935 0.00036 19.270±0.150

850 6.45686E-15 9.4071 84.6598 5.50484 6.89872 0.00473 0.07571 0.00025 19.376±0.118

925 9.79488E-15 14.2703 86.8374 5.39471 7.11781 0.00433 0.07337 0.00028 19.480±0.120

1000 9.17238E-15 13.3633 82.2793 5.71575 7.15626 0.00537 0.07297 0.00029 19.556±0.104

1075 1.05866E-14 15.4237 96.2676 4.86809 7.20983 0.00257 0.07243 0.00012 19.488±0.088

1150 9.19118E-15 13.3907 92.2013 5.06578 7.23264 0.00329 0.07220 0.00064 19.423±0.091

1250 1.66468E-14 24.2528 97.2912 4.84362 7.19906 0.00239 0.07254 0.00148 19.595±0.087

1400 7.62247E-15 11.1053 96.6717 4.86975 7.32712 0.00253 0.07126 0.00125 19.577±0.124

Wtd Mean Plateau age (Ma) 19.483±0.041 19.483±0.041

Intercept = 293.5±3.5 Isochron age (Ma) MSWD = 0.79 19.493±0.061

Integrated age (Ma) 19.452±0.446

06BV-G599 Plagioclase IRR253-89 J = 0.00208011

550 3.03776E-16 0.6141 27.2997 19.37521 7.53407 0.04971 0.06930 0.02821 19.840±1.148

625 9.23517E-16 1.8669 75.0825 7.02927 7.35544 0.00792 0.07099 0.01040 19.793±0.304

700 1.9772E-15 3.9969 91.1998 5.87082 7.55998 0.00379 0.06906 0.00281 20.081±0.152

775 3.78027E-15 7.6417 95.2604 5.60133 7.69396 0.00298 0.06785 0.00133 20.015±0.090

850 6.50888E-15 13.1576 93.8193 5.66446 8.75394 0.00356 0.05959 0.00224 19.948±0.079

925 9.44494E-15 19.0928 97.2517 5.49804 7.92528 0.00266 0.06586 0.00162 20.059±0.069

1000 8.49972E-15 17.1820 94.2973 5.67161 7.79143 0.00321 0.06700 0.04043 20.062±0.073

1100 1.1319E-14 22.8812 98.5153 5.45682 7.75265 0.00238 0.06733 0.00253 20.164±0.067

1200 6.71135E-15 13.5669 97.7764 5.50414 7.18733 0.00236 0.07266 0.00393 20.180±0.074

Wtd Mean Plateau age (Ma) MSWD = 0.85 20.071±0.034

Intercept = 260±39 Isochron age (Ma) MSWD = 0.70 20.172±0.081

Integrated age (Ma) 20.077±0.114

05BV-G345 Basaltic Andesite Groundmass IRR243-16 J = 0.00030841 

550 4.48138E-16 1.6661 12.9794 11.12044 3.86688 0.03381 0.13532 0.00069 0.805±0.116

625 1.95863E-15 7.2834 23.2068 8.00642 3.58676 0.02182 0.14592 0.00035 1.037±0.046

700 4.20471E-15 15.6396 34.0298 5.60834 3.24189 0.01343 0.16148 0.00025 1.064±0.021

775 6.7049E-15 24.9447 42.1895 4.57464 2.93253 0.00977 0.17855 0.00025 1.076±0.014

850 6.63988E-15 24.7070 43.7517 4.34119 2.68928 0.00902 0.19474 0.00023 1.059±0.013

925 4.05652E-15 15.0924 37.7347 5.12993 2.86958 0.01161 0.18248 0.00027 1.079±0.018

1000 1.74908E-15 6.5028 22.3457 8.78490 3.89405 0.02418 0.13437 0.00048 1.095±0.045

1075 6.57287E-16 2.4392 8.1417 21.11479 6.48643 0.06744 0.08052 0.00653 0.961±0.115

1250 4.84571E-16 1.7249 2.7612 62.31435 63.68291 0.22297 0.00787 0.00417 1.002±0.274

Wtd Mean Plateau age (Ma) MSWD = 0.43 1.068±0.008

Intercept = 294.5±3.3 Isochron age (Ma) MSWD = 0.49 1.074±0.012

Integrated age (Ma) 1.060±0.011

Recoil Age (Ma) (50.00%) MSWD = 0.570 1.067±0.008

Recoil Age (Ma) (70.00%) MSWD = 0.397 1.069±0.008
30

Appendix 1. 40Ar/39Ar incremental-heating data for samples from the Beacon Rock 7.5' quadrangle, Skamania County, Washington.—

Continued

Step (°C) mol 39Ar % 39ArK Rel % 40Ar* 40Ar/39Ar 37Ar/ 39Ar 36Ar/ 39Ar K/Ca Cl/K Age (Ma)

05BV-G345-2 Basaltic Andesite Groundmass IRR330-3 J = 0.00022190 

575 2.26682E-15 2.3274 26.6324 10.42079 3.74090 0.02689 0.13989 0.00026 1.114±0.065

650 8.06363E-15 8.2791 38.2426 7.27041 3.50392 0.01618 0.14937 0.00009 1.116±0.022

725 1.4044E-14 14.4192 50.8074 5.45717 3.13871 0.00997 0.16680 0.00019 1.112±0.013

800 2.27804E-14 23.3891 54.4607 4.80772 2.82015 0.00820 0.18568 0.00025 1.050±0.009

875 2.2373E-14 22.9708 55.0158 4.72762 2.65474 0.00794 0.19727 0.00027 1.043±0.010

950 1.48165E-14 15.2124 49.2232 5.38104 2.74542 0.01002 0.19075 -0.00007 1.062±0.012

1025 7.15785E-15 7.3491 29.8109 7.93146 3.90167 0.01994 0.13411 0.00069 0.949±0.038

1100 3.20081E-15 3.2863 16.5930 12.65444 4.78743 0.03705 0.10923 0.00244 0.843±0.081

1175 2.08366E-15 2.1393 6.5344 27.07588 7.53116 0.08775 0.06930 0.00735 0.712±0.136

1250 6.10894E-16 0.6272 2.1682 93.30018 100.19734 0.33708 0.00486 0.00859 0.871±0.609

Wtd Mean Plateau age (Ma) MSWD = 0.75 1.050±0.006

Intercept = 291±14 Isochron age (Ma) MSWD = 5.8 1.073±0.027

Integrated age (Ma) 1.044±0.009

Recoil Age (Ma) (50.00%) MSWD = 6.781 1.048±0.013

Recoil Age (Ma) (70.00%) MSWD = 6.781 1.059±0.013

02BV-G10A Basaltic Andesite Groundmass IRR212-60 J = 0.00038322 

625 -4.71679E-17 -0.1806 -40.1675 6.92678 4.86952 0.03427 0.10029 -0.00070 -1.904±0.542

700 6.44608E-15 24.6911 28.3694 4.34717 3.98931 0.01166 0.12248 -0.00001 0.855±0.012

775 3.64944E-15 13.9821 39.5963 3.80269 3.66309 0.00880 0.13342 0.00007 1.044±0.012

850 2.69357E-15 10.3180 32.6875 4.70431 3.91222 0.01182 0.12490 0.00021 1.066±0.015

925 3.22424E-15 12.3561 38.9731 4.00045 3.30721 0.00919 0.14782 0.00029 1.080±0.014

1000 4.18905E-15 16.0639 42.9300 3.54153 2.38948 0.00751 0.20472 0.00039 1.053±0.011

1075 3.18284E-15 12.1894 35.7372 4.35437 4.23341 0.01066 0.11540 0.00098 1.079±0.012

1150 9.51301E-16 3.6242 25.8962 6.05268 11.60104 0.01844 0.04189 0.00052 1.092±0.037

1400 1.82104E-15 6.9558 33.5185 4.63376 7.93638 0.01266 0.06140 0.00053 1.079±0.019

Wtd Mean Plateau age (Ma) MSWD = 1.2 1.065±0.006

Intercept = 304±11 Isochron age (Ma) MSWD = 0.69 1.013±0.032

Integrated age (Ma) 1.014±0.008

Recoil Age (Ma) (50.00%) MSWD = 1.521 1.060±0.008

Recoil Age (Ma) (70.00%) MSWD = 46.270 1.034±0.035

02BV-G10A-2 Basaltic Andesite Groundmass IRR330-99 J = 0.00022233 

675 1.159E-14 10.2956 19.3702 10.06327 3.13557 0.02834 0.16697 0.00293 0.783±0.049

750 2.24851E-14 19.9739 27.2042 7.57298 3.22673 0.01956 0.16224 0.00173 0.828±0.020

800 1.47875E-14 13.1360 36.6971 7.01274 2.85813 0.01582 0.18321 0.00020 1.034±0.017

850 1.2223E-14 10.8579 33.2327 7.79255 2.69454 0.01836 0.19436 -0.00038 1.041±0.028

900 7.87062E-15 6.9916 25.7266 10.19476 3.49162 0.02660 0.14990 -0.00005 1.054±0.029

950 5.38755E-15 4.7859 21.0792 12.72579 4.50823 0.03524 0.11602 -0.00025 1.079±0.083

1000 6.35182E-15 5.6424 24.4515 10.82402 3.96861 0.02879 0.13184 -0.00066 1.064±0.043

1075 9.41954E-15 8.3676 33.3380 7.89208 3.11838 0.01867 0.16789 -0.00043 1.057±0.024

1150 1.50414E-14 13.3616 31.9409 8.42637 2.83409 0.02020 0.18477 0.00031 1.081±0.016

1250 7.41563E-15 6.5874 32.9939 9.52940 11.84427 0.02494 0.04393 -0.00064 1.271±0.041

Wtd Mean Plateau age (Ma) MSWD = 0.39 1.066±0.011

Intercept = 296±15 Isochron age (Ma) MSWD = 0.49 1.055±0.061

Integrated age (Ma) 0.997±0.011

Recoil Age (Ma) (50.00%) MSWD = 14.538 1.025±0.038

Recoil Age (Ma) (70.00%) MSWD = 15.210 1.004±0.032
31

Appendix 1. 40Ar/39Ar incremental-heating data for samples from the Beacon Rock 7.5' quadrangle, Skamania County, Washington.—

Continued

Step (°C) mol 39Ar % 39ArK Rel % 40Ar* 40Ar/39Ar 37Ar/ 39Ar 36Ar/ 39Ar K/Ca Cl/K Age (Ma)

05BV-G420 Basalt Groundmass IRR243-22 J = 0.00034704 

575 9.23178E-16 3.2106 17.6822 7.08576 3.77328 0.02081 0.13869 0.00100 0.786±0.099

650 3.32604E-15 11.5672 34.3812 5.18220 3.75586 0.01256 0.13933 0.00036 1.118±0.025

725 5.4489E-15 18.9501 43.3623 4.10426 3.36168 0.00881 0.15571 0.00038 1.117±0.017

800 5.43163E-15 18.8900 41.1017 4.08773 2.91363 0.00897 0.17971 0.00031 1.054±0.016

875 3.89499E-15 13.5459 32.0856 5.11961 3.62127 0.01278 0.14452 0.00053 1.031±0.022

950 2.72504E-15 9.4771 27.3738 5.87961 4.21574 0.01563 0.12409 0.00071 1.010±0.029

1025 2.59253E-15 9.0163 31.2295 5.40288 3.53098 0.01356 0.14823 0.00122 1.059±0.031

1125 2.92145E-15 10.1601 20.3717 7.87580 7.96924 0.02346 0.06547 0.00369 1.010±0.038

1250 1.49025E-15 5.1828 9.6719 15.70003 11.46978 0.05121 0.04537 0.00159 0.958±0.074

Wtd Mean Plateau age (Ma) no plateau

Intercept = 291.1±4.9 Isochron age (Ma) MSWD = 0.35 1.071±0.021

Integrated age (Ma) 1.049±0.011

Recoil Age (Ma) (50.00%) MSWD = 5.022 1.051±0.022

Recoil Age (Ma) (70.00%) MSWD = 3.416 1.061±0.017

02BV-G90B Basaltic Andesite Groundmass IRR243-19 J = 0.00028544 

550 3.71263E-16 1.3868 -0.3563 9.282269 2.911145 0.032335 0.179870 0.004775 -0.017±0.155

625 1.95358E-15 7.2970 24.5779 6.536152 2.937820 0.017503 0.178229 0.000747 0.829±0.043

700 4.48549E-15 16.7539 35.1429 4.750594 2.954836 0.011259 0.177201 0.000263 0.861±0.018

775 6.39922E-15 23.9075 39.8847 4.185268 2.628741 0.009253 0.199229 0.000238 0.861±0.013

850 4.4186E-15 16.5110 30.8794 5.078650 2.360454 0.012543 0.221915 0.000373 0.808±0.017

925 3.34289E-15 12.4891 19.8148 7.209434 2.626073 0.020303 0.199432 0.000491 0.736±0.025

1000 3.14484E-15 11.7497 12.6881 11.045135 2.548776 0.033354 0.205490 0.000860 0.723±0.035

1100 1.52855E-15 5.6870 5.1832 23.176564 8.449868 0.076735 0.061723 0.003190 0.622±0.072

1250 1.14631E-15 4.2181 6.7803 24.314842 23.811172 0.083394 0.021663 0.001152 0.863±0.087

Intercept = 289.2±6.7 Isochron age (Ma) MSWD = 4.3 0.862±0.031

Integrated age (Ma) 0.793±0.010

Recoil Age (Ma) (50.00%) MSWD = 7.657 0.821±0.024

Recoil Age (Ma) (70.00%) MSWD = 8.106 0.813±0.023

02BV-G116A Basaltic Andesite Groundmass IRR275-09 J = 0.00039181 

550 3.01502E-15 4.2240 53.5690 2.37824 2.24056 0.00436 0.23381 -0.00004 0.902±0.018

600 5.65517E-15 7.9234 57.4566 2.23757 2.11826 0.00382 0.24733 0.00008 0.910±0.012

650 8.65756E-15 12.1312 59.2339 2.13471 1.98156 0.00350 0.26442 0.00011 0.895±0.009

700 1.13944E-14 15.9678 59.5421 2.12662 1.83121 0.00343 0.28616 0.00017 0.896±0.008

750 1.06674E-14 14.9499 59.9815 2.07427 1.75270 0.00330 0.29899 0.00012 0.880±0.008

800 9.67167E-15 13.5532 57.2348 2.14280 1.88112 0.00363 0.27856 0.00019 0.868±0.025

850 7.39086E-15 10.3543 51.6211 2.35777 2.25395 0.00449 0.23242 0.00021 0.861±0.011

900 4.79195E-15 6.7101 43.1920 2.81135 2.91890 0.00623 0.17939 0.00034 0.860±0.016

950 3.00585E-15 4.2080 35.2803 3.37160 3.28049 0.00831 0.15957 0.00052 0.843±0.022

1025 2.99423E-15 4.1942 27.0181 4.23068 2.45607 0.01114 0.21326 0.00087 0.809±0.024

1100 2.01366E-15 2.8129 10.9294 9.73930 6.31944 0.03113 0.08266 0.00281 0.756±0.050

1200 1.16476E-15 1.5969 6.2374 22.19434 32.33075 0.07950 0.01586 0.00168 1.001±0.109

1325 9.9543E-16 1.3743 7.1952 16.86481 22.68993 0.05933 0.02275 0.00164 0.871±0.100

Wtd Mean Plateau age (Ma) none

Intercept = 292.5±5.3 Isochron age (Ma) MSWD = 3.2 0.887±0.010

Integrated age (Ma) 0.876±0.006

Recoil Age (Ma) (50.00%) MSWD = 2.739 0.879±0.007

Recoil Age (Ma) (70.00%) MSWD = 2.454 0.879±0.006
32

Appendix 1. 40Ar/39Ar incremental-heating data for samples from the Beacon Rock 7.5' quadrangle, Skamania County, Washington.—

Continued.

Step (°C) mol 39Ar % 39ArK Rel % 40Ar* 40Ar/39Ar 37Ar/ 39Ar 36Ar/ 39Ar K/Ca Cl/K Age (Ma)

QV01-36 Basaltic Andesite Groundmass IRR189-A-6 J = 0.00035207 

550 -1.40209E-18 -0.0014 -0.0274 25.98126 4.20709 0.09151 0.11612 -- -0.447±0.112

625 1.03219E-14 10.6346 0.0684 11.02339 3.51251 0.03574 0.13916 -- 0.480±0.028

700 1.46348E-14 15.0782 0.1199 6.74436 3.01406 0.02093 0.16223 -- 0.514±0.018

775 1.92054E-14 19.7873 0.1393 5.69057 2.65258 0.01732 0.18438 -- 0.504±0.015

850 1.75851E-14 18.1178 0.1434 5.69740 2.51035 0.01722 0.19485 -- 0.519±0.015

925 1.44932E-14 14.9323 0.1161 6.56929 3.44035 0.02062 0.14208 -- 0.486±0.018

1000 8.92187E-15 9.1922 0.0802 8.83951 3.18738 0.02841 0.15339 -- 0.451±0.024

1050 5.89761E-15 6.0763 0.0711 10.33048 2.46145 0.03317 0.19872 -- 0.468±0.031

1100 3.05759E-15 3.1502 0.0441 16.91182 6.60017 0.05656 0.07389 -- 0.476±0.056

1200 1.88502E-15 1.9421 0.0121 25.59089 32.60441 0.09472 0.01468 -- 0.201±0.104

1400 1.05831E-15 1.090371075 0.02484819 35.17061 25.36495 0.12319 0.01897 -- 0.565±0.146

Wtd Mean Plateau age (Ma) MSWD = 1.2 0.498±0.007

Intercept =292.3±2.7 Isochron age (Ma) MSWD = 1.07 0.541±0.020

Integrated age (Ma) 0.490±0.006

Recoil Age (Ma) (50.00%) MSWD = 0.793 0.506±0.008

Recoil Age (Ma) (70.00%) MSWD = 1.758 0.501±0.010

QV98-17 Basaltic Andesite Whole Rock IRRCLX-B-21 J = 0.00040588 

600 2.29479E-16 0.4048 -0.1234 25.14080 3.58134 0.09659 0.13647 -- -2.248±0.541

675 3.88096E-15 6.8462 0.0146 7.99321 2.94377 0.02748 0.16611 -- 0.085±0.039

750 5.22847E-15 9.2233 0.0236 2.48434 2.70277 0.00897 0.18095 -- 0.043±0.025

825 1.00268E-14 17.6878 0.0495 1.73347 2.64366 0.00632 0.18500 -- 0.063±0.014

900 1.43362E-14 25.2899 0.0606 1.53575 2.51051 0.00559 0.19483 -- 0.068±0.011

975 1.02077E-14 18.0070 0.0208 1.57335 2.53788 0.00593 0.19273 -- 0.023±0.014

1060 6.27121E-15 11.0628 0.0443 2.52663 3.10422 0.00904 0.15750 -- 0.082±0.021

1160 4.03186E-15 7.1124 0.0216 3.89190 3.64973 0.01391 0.13391 -- 0.062±0.033

1350 2.47487E-15 4.3658 -0.0120 7.63098 22.57346 0.03248 0.02136 -- -0.067±0.069

Wtd Mean Plateau age (Ma) MSWD = 1.8 0.056±0.006

Intercept =295.7±7.2 Isochron age (Ma) MSWD = 2.1 0.055±0.021

Integrated age (Ma) 0.056±0.007

Recoil Age (Ma) (50.00%) MSWD = 3.452 0.054±0.013

Recoil Age (Ma) (70.00%) MSWD = 2.160 0.056±0.010

QV98-17-2 Basaltic Andesite Groundmass IRR189-4 J = 0.00037187 

550 3.38133E-16 0.27319 -0.04424 50.89200 3.71537 0.18089 0.13154 -- -1.494±0.390

600 2.27127E-15 1.83507 0.01214 15.93179 3.41456 0.05422 0.14316 -- 0.130±0.068

675 1.43754E-14 11.61457 0.04967 3.61449 2.83720 0.01242 0.17236 -- 0.121±0.013

750 1.15155E-14 9.30391 0.09368 1.92075 2.68445 0.00665 0.18219 -- 0.121±0.013

825 3.25922E-14 26.33280 0.08651 1.56232 2.57505 0.00555 0.18994 -- 0.090±0.007

900 2.40757E-14 19.45196 0.09182 1.29600 2.50179 0.00469 0.19551 -- 0.080±0.008

975 1.65967E-14 13.40924 0.09130 1.43477 2.79139 0.00520 0.17519 -- 0.088±0.010

1050 9.60201E-15 7.75793 0.05777 2.03125 3.85942 0.00756 0.12662 -- 0.079±0.016

1100 5.4576E-15 4.40946 0.01845 3.62588 3.57889 0.01305 0.13657 -- 0.045±0.025

1200 4.99987E-15 4.03964 0.03473 4.63396 10.09875 0.01797 0.04817 -- 0.109±0.033

1400 1.94596E-15 1.57224 0.06300 8.10564 27.27211 0.03337 0.01762 -- 0.349±0.084

Wtd Mean Plateau age (Ma) MSWD = 0.82 0.084±0.005

Intercept = 298±19 Isochron age (Ma) MSWD = 0.66 0.079±0.030

Integrated age (Ma) 0.092±0.004

Recoil Age (Ma) (50.00%) MSWD = 0.484 0.086±0.005

Recoil Age (Ma) (70.00%) MSWD = 1.896 0.090±0.006
33

Appendix 1. 40Ar/39Ar incremental-heating data for samples from the Beacon Rock 7.5' quadrangle, Skamania County, Washington.—

Continued.

Step (°C) mol 39Ar % 39ArK Rel % 40Ar* 40Ar/39Ar 37Ar/ 39Ar 36Ar/ 39Ar K/Ca Cl/K Age (Ma)

QV01-35B Basaltic Andesite Groundmass IRR203-8 J = 0.00042913 

550 1.2883E-15 1.8117 -0.0365 12.39052 3.27998 0.04438 0.14905 -- -0.346±0.232

625 8.78007E-15 12.3471 0.0194 3.78270 3.14988 0.01344 0.15522 -- 0.057±0.037

700 1.35659E-14 19.0772 0.0250 2.15958 2.63919 0.00787 0.18532 -- 0.042±0.013

775 1.97302E-14 27.7458 0.0456 1.80305 2.39012 0.00650 0.20467 -- 0.064±0.010

840 1.24489E-14 17.5065 0.0391 2.04524 2.79654 0.00744 0.17487 -- 0.062±0.014

900 6.30539E-15 8.8670 0.0243 3.01532 4.03227 0.01109 0.12117 -- 0.057±0.024

960 3.56462E-15 5.0128 0.0352 4.69351 5.14368 0.01677 0.09492 -- 0.129±0.041

1025 3.14426E-15 4.4217 0.0069 7.97078 4.18617 0.02796 0.11671 -- 0.043±0.049

1100 2.07033E-15 2.9114 -0.0029 27.31662 21.07084 0.09863 0.02291 -- -0.062±0.108

1200 2.12532E-16 0.2989 -0.0115 93.13997 34.82139 0.32861 0.01373 -- -0.84±0.696

Wtd Mean Plateau age (Ma) MSWD = 0.84 0.058±0.006

Intercept = 296.0±4.8 Isochron age (Ma) MSWD = 1.04 0.055±0.015

Integrated age (Ma) 0.047±0.009

Recoil Age (Ma) (50.00%) MSWD = 0.964 0.060±0.007

Recoil Age (Ma) (70.00%) MSWD = 0.643 0.057±0.007
34

Table 1. Chemical and modal analyses of igneous rocks of the ancestral Cascade volcanic arc, Beacon Rock 7.5’ quadrangle, Skamania 

County, Washington.

[X-ray fluorescence analyses. Map No., see figure 16 (sheet 2). Rock-type names assigned in accordance with IUGS system (Le Maitre, 2002) applied to recalculated analyses. FeO*, total Fe calculated as FeO; Mg#, atomic ratio 100 Mg/(Mg+Fe2

+) with Fe2+ set to 0.85 x Fetotal. Modal analyses, secondary minerals counted 

as primary mineral replaced. --, not present. Analyses by D.M. Johnson Cornelius and R.M. Conrey at Peter Hooper GeoAnalytical Laboratory of Washington State 

University, Pullman, Washington, using methods described in Johnson and others (1999)]

Map No. 1 2 3 4 5 6 7 8 9

Field sample No. 02BV-G76 06BV-G634A 06BV-G743 06BV-G712 06BV-G657B 06BV-G630 06BV-G766B 06BV-G692 09BV-G1280

Latitude (N) 45°43.929' 45°43.707' 45°42.502' 45°43.599' 45°44.507' 45°42.922' 45°41.954' 45°44.227' 45°44.742'

Longitude (W) 122°05.986' 122°05.218' 122°05.768' 122°04.247' 122°04.230' 122°06.179' 122°05.641' 122°03.790' 122°03.467'

Map unit Tob Tba Tba Tba Tba Tba Tba Tba Tba

Rock type Basalt Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Analyses as reported (weight percent)

SiO2 50.54 52.84 53.51 53.99 53.74 53.95 54.58 54.42 54.32

TiO2 0.90 1.22 1.70 1.53 1.90 1.62 1.73 1.74 1.49

Al2O3 17.22 17.03 15.52 16.01 15.82 18.59 15.17 15.17 16.56

FeO* 8.90 9.23 10.93 10.23 10.54 8.36 11.11 11.13 9.28

MnO 0.16 0.16 0.20 0.18 0.20 0.15 0.21 0.20 0.18

MgO 8.17 4.79 4.08 4.42 3.84 2.75 4.09 3.82 3.55

CaO 11.82 8.36 8.21 8.75 8.08 8.65 8.30 8.02 8.14

Na2O 2.10 2.93 3.45 3.26 3.46 3.25 3.35 3.40 3.43

K2O 0.24 0.98 0.86 1.00 0.67 1.08 0.93 1.12 1.11

P2O5 0.10 0.16 0.27 0.21 0.27 0.25 0.26 0.26 0.25

Total 100.14 97.69 98.73 99.58 98.52 98.65 99.74 99.28 98.30

Analyses recalculated volatile-free and normalized to 100% with all Fe as FeO (weight percent)

SiO2 50.47 54.09 54.20 54.22 54.55 54.69 54.72 54.81 55.25

TiO2 0.90 1.24 1.72 1.54 1.93 1.64 1.73 1.75 1.51

Al2O3 17.20 17.43 15.72 16.08 16.06 18.84 15.21 15.28 16.85

FeO* 8.89 9.45 11.07 10.27 10.70 8.47 11.14 11.21 9.44

MnO 0.16 0.16 0.20 0.18 0.20 0.15 0.21 0.21 0.18

MgO 8.16 4.91 4.13 4.44 3.90 2.79 4.11 3.85 3.61

CaO 11.80 8.56 8.32 8.79 8.20 8.77 8.32 8.08 8.28

Na2O 2.10 2.99 3.49 3.27 3.52 3.30 3.36 3.42 3.49

K2O 0.24 1.00 0.88 1.00 0.68 1.09 0.94 1.13 1.13

P2O5 0.09 0.16 0.27 0.21 0.27 0.25 0.26 0.26 0.26

Mg# 65.8 52.7 44.2 47.6 43.7 41.2 43.7 42.0 44.9

Modes (volume percent)

Plagioclase 6.6 20.5 1.0 0.3 0.1 27.2 1.7 0.4 10.5

Clinopyroxene 0.1 1.3 trace 0.1 trace 0.8 trace -- 0.9

Orthopyroxene -- -- -- -- -- -- -- -- 1.3

Olivine -- 5.1 trace trace -- 1.2 -- -- --

Fe-Ti Oxide -- 0.1 -- -- -- -- -- -- trace

Hornblende -- -- -- -- -- -- -- -- --

Biotite -- -- -- -- -- -- -- -- --

Quartz -- -- -- -- -- -- -- -- --

Groundmass 93.3 73.0 99.0 99.6 99.9 70.8 98.3 99.6 87.3

No. points counted 825 850 850 850 850 850 800 850 850

Texture (rock/

 groundmass)

porphyritic/

trachytic

seriate/

intergranular

~aphyric/

trachytic

aphyric/

trachytic

aphyric/

trachytic

seriate/

intergranular

sparsely

 phyric/

aphyric/

trachytic

porphyritic/

trachytic

trachytic

Trace element analyses (parts per million)

Ba 101 223 218 222 223 267 263 266 232

Rb 4 25 18 31 14 29 15 30 33

Sr 303 248 268 264 298 307 256 253 288

Y 15 32 34 34 35 36 36 38 35

Zr 47 144 146 157 156 188 168 168 167

Nb 3.6 7.3 8.2 8.3 10.2 11.0 8.7 9.4 10.9

Ni 107 29 10 16 4 5 18 17 20

Cu 108 122 169 166 70 99 169 244 166

Zn 71 88 106 96 107 87 110 110 100

Cr 635 61 13 20 3 10 33 32 40

Sc 38 31 38 37 36 27 36 38 30

V 235 228 279 287 277 216 301 301 238
35

Table 1. Chemical and modal analyses of igneous rocks of the ancestral Cascade volcanic arc, Beacon Rock 7.5’ quadrangle, Skamania 

County, Washington.—Continued

Map No. 10 11 12 13 14 15 16 17 18

Field sample No. 06BV-G719C 06BV-G686 06BV-G679B 06BV-G583 06BV-G689B 06BV-G745 06BV-G764 06BV-G706 06BV-G704

Latitude (N) 45°43.489' 45°40.851' 45°40.798' 45°44.856' 45°43.964' 45°42.850' 45°42.248' 45°42.782' 45°43.735'

Longitude (W) 122°04.018' 122°06.016' 122°07.147' 122°00.224' 122°03.651' 122°05.253' 122°05.730' 122°04.624' 122°03.723'

Map unit Tba Tba Tba Tba Tba Tba Tba Td Td

Rock type Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Andesite Dacite Dacite

Analyses as reported (weight percent)

SiO2 55.24 55.60 55.29 55.43 56.13 56.53 58.35 63.45 65.98 

TiO2 1.77 1.83 1.66 1.48 1.89 1.43 1.70 1.02 0.97 

Al2O3 15.41 15.72 16.03 16.51 14.97 15.99 15.74 15.33 14.25 

FeO* 10.32 9.72 9.37 8.69 10.39 9.39 9.13 6.54 6.29 

MnO 0.18 0.18 0.16 0.18 0.19 0.18 0.18 0.15 0.13 

MgO 3.68 3.17 3.29 3.42 3.01 3.33 2.69 1.29 1.05 

CaO 7.64 7.63 7.76 7.42 6.99 7.85 6.37 4.48 3.23 

Na2O 3.49 3.33 3.24 3.69 3.85 3.22 4.19 4.40 4.68

K2O 1.03 1.38 1.23 1.32 0.95 1.24 1.19 2.20 2.14

P2O5 0.34 0.32 0.28 0.27 0.41 0.21 0.34 0.28 0.28

Total 99.10 98.88 98.32 98.40 98.77 99.37 99.88 99.13 99.00

Analyses recalculated volatile-free and normalized to 100% with all Fe as FeO (weight percent)

SiO2 55.75 56.23 56.24 56.32 56.82 56.89 58.42 64.00 66.64

TiO2 1.78 1.85 1.69 1.50 1.91 1.43 1.71 1.03 0.98

Al2O3 15.55 15.89 16.30 16.78 15.15 16.09 15.76 15.46 14.39

FeO* 10.41 9.83 9.53 8.83 10.52 9.45 9.14 6.60 6.35

MnO 0.18 0.19 0.16 0.19 0.20 0.18 0.18 0.15 0.13

MgO 3.71 3.20 3.35 3.48 3.04 3.35 2.70 1.30 1.06

CaO 7.71 7.72 7.89 7.54 7.08 7.90 6.38 4.52 3.27

Na2O 3.52 3.36 3.29 3.75 3.90 3.24 4.19 4.44 4.73

K2O 1.04 1.39 1.25 1.34 0.96 1.25 1.19 2.22 2.16

P2O5 0.34 0.33 0.29 0.27 0.41 0.21 0.34 0.28 0.29

Mg# 43.0 40.9 42.9 45.6 38.0 42.8 38.2 29.4 26.1

Modes (volume percent)

Plagioclase 8.9 0.2 -- 19.4 0.2 0.3 1.4 9.0 3.3

Clinopyroxene 1.1 -- -- 2.5 -- -- 0.3 1.4 0.5

Orthopyroxene 0.7 trace -- 0.5 -- -- 0.2 1.4 0.4

Olivine 1.6 -- -- 1.4 -- -- trace -- --

Fe-Ti Oxide trace trace -- 0.1 trace -- 0.2 0.3 0.2

Hornblende -- -- -- -- -- -- -- -- --

Biotite -- -- -- -- -- -- -- -- --

Other -- -- -- -- -- -- -- -- --

Groundmass 87.7 99.8 100.0 76.1 99.8 99.7 97.9 87.9 95.6

No. points counted 850 850 -- 850 850 850 850 790 850

Texture (rock/

 groundmass)

seriate/

intersertal

aphyric/

microseriate

microphyric/

intersertal

porphyritic/

intergranular

aphyric/

pilotaxitic

aphyric/

microseriate

sparsely

 phyric

phyric/

pilotaxitic

seriate/

pilotaxitic

sparsely

 phyric/

micropoikilitic

Trace element analyses (parts per million)

Ba 269 307 289 298 286 258 299 408 464

Rb 28 47 47 38 39 19 32 67 65

Sr 262 264 271 354 262 247 277 223 182

Y 40 40 37 31 42 34 41 55 57

Zr 195 232 198 180 202 172 207 292 323

Nb 11.9 12.5 9.9 9.6 12.3 8.5 11.8 15.4 17.0

Ni 19 9 13 17 5 10 2 4 0

Cu 176 202 229 98 132 152 93 60 44

Zn 107 107 99 102 109 94 107 90 111

Cr 56 15 18 21 4 17 3 10 1

Sc 33 32 32 28 34 30 28 21 20

V 201 281 245 209 179 245 170 71 19
36

Table 1. Chemical and modal analyses of igneous rocks of the ancestral Cascade volcanic arc, Beacon Rock 7.5’ quadrangle, Skamania 

County, Washington.—Continued

Map No. 19 20 21 22 23 24 25 26 27

Field sample No. 06BV-G780 06BV-G792A 06BV-G719A 08BV-G1197 06BV-G600 02BV-G88 06BV-G626 06BV-G589 06BV-G6011

Latitude (N) 45°41.135' 45°40.579' 45°43.513' 45°38.354' 45°43.330' 45°41.033' 45°41.317' 45°44.569' 45°44.624'

Longitude (W) 122°06.663' 122°06.259' 122°04.054' 122°05.270' 122°03.494' 122°03.736' 122°04.789' 122°02.599' 122°02.469'

Map unit Td Ttw Tts Thba Thba Thba Thba Thba Thba

Rock type Dacite Dacite

Welded Tuff

Rhyolite

Welded Tuff

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Analyses as reported (weight percent)

SiO2 63.30 67.23 73.38 49.66 51.82 51.67 52.72 53.07 53.88

TiO2 0.77 0.54 0.48 1.35 1.08 1.04 1.07 1.17 1.08

Al2O3 14.16 15.29 12.39 18.78 19.59 18.73 18.88 18.95 19.73

FeO* 5.53 3.85 3.04 8.34 7.96 8.00 8.06 7.75 7.27

MnO 0.13 0.05 0.11 0.19 0.16 0.14 0.14 0.13 0.13

MgO 0.93 0.46 0.31 4.31 4.74 5.40 5.23 4.34 3.63

CaO 4.65 2.60 1.94 9.32 9.99 9.57 9.44 9.11 9.48

Na2O 3.79 3.89 4.46 3.08 3.07 2.98 3.09 3.36 3.36

K2O 0.91 2.86 2.23 0.30 0.50 0.73 0.77 0.73 0.72

P2O5 0.21 0.08 0.10 0.22 0.14 0.15 0.15 0.17 0.16

Total 94.39 96.84 98.43 95.57 99.05 98.39 99.54 98.78 99.45

Analyses recalculated volatile-free and normalized to 100% with all Fe as FeO (weight percent)

SiO2 67.07 69.42 74.55 51.96 52.32 52.51 52.96 53.73 54.17

TiO2 0.81 0.56 0.49 1.42 1.09 1.05 1.08 1.19 1.09

Al2O3 15.00 15.79 12.58 19.65 19.78 19.03 18.96 19.19 19.84

FeO* 5.86 3.97 3.09 8.73 8.03 8.13 8.09 7.84 7.31

MnO 0.14 0.05 0.11 0.20 0.16 0.14 0.14 0.13 0.13

MgO 0.99 0.47 0.31 4.51 4.79 5.49 5.25 4.40 3.65

CaO 4.92 2.68 1.97 9.75 10.09 9.73 9.49 9.22 9.53

Na2O 4.02 4.02 4.53 3.23 3.09 3.03 3.10 3.40 3.38

K2O 0.97 2.95 2.26 0.32 0.50 0.74 0.78 0.74 0.73

P2O5 0.23 0.08 0.10 0.23 0.15 0.15 0.15 0.17 0.16

Mg# 27.3 20.5 17.6 53.1 55.8 59.0 57.8 54.3 51.3

Modes (volume percent)

Plagioclase 1.9 18.0 2.0 31.3 23.3 31.5 27.4 24.3 25.7

Clinopyroxene 0.6 0.3 0.3 -- 0.8 0.9 1.4 0.3 0.6

Orthopyroxene 0.6 1.8 0.2 0.4 -- -- -- -- --

Olivine -- -- -- 2.4 4.3 3.8 5.5 1.9 2.9

Fe-Ti Oxide 0.1 0.1 0.1 -- -- -- -- -- --

Hornblende -- -- -- -- -- -- -- -- --

Biotite -- -- -- -- -- -- -- -- --

Other -- -- -- -- -- -- -- -- --

Groundmass 96.8 79.8 97.4 65.9 71.6 63.8 65.7 73.5 70.8

No. points counted 850 850 850 830 850 850 850 850 850

Texture (rock/

 groundmass)

sparsely

 phyric/

pilotaxitic

porphyritic/

eutaxitic

sparsely

 phyric/

eutaxitic

seriate/

intersertal

seriate/

intergranular

seriate/

intergranular

seriate/

intergranular

porphyritic/

intergranular

porphyritic/

intergranular

Trace element analyses (parts per million)

Ba 507 482 493 205 169 182 187 200 192 

Rb 86 99 65 8 12 19 20 20 23 

Sr 295 161 112 406 416 466 467 386 420

Y 47 21 48 24 18 17 19 19 19 

Zr 290 278 329 146 89 88 95 105 94 

Nb 14.8 10.1 17.2 7.6 4.6 4.4 4.5 6.3 5.4

Ni 0 5 0 13 33 30 29 18 20 

Cu 50 50 21 144 109 69 125 152 105 

Zn 88 46 85 93 79 79 79 80 74

Cr 3 8 2 39 45 49 43 43 42

Sc 17 10 12 29 26 25 26 26 23

V 18 57 4 218 210 210 212 202 192

1Block in talus deposit.
37

Table 1. Chemical and modal analyses of igneous rocks of the ancestral Cascade volcanic arc, Beacon Rock 7.5’ quadrangle, Skamania 

County, Washington.—Continued

Map No. 28 29 30 31 32 33 34 35 36

Field sample No. 06BV-G614 06BV-G618 02BV-G11B 02BV-G852 06BV-G594 06BV-G703 06BV-G615 02BV-G52A 02BV-G64B

Latitude (N) 45°42.617' 45°42.595' 45°43.298' 45°42.920' 45°43.816' 45°43.048' 45°42.386' 45°43.316' 45°40.588'

Longitude (W) 122°03.891' 122°04.958' 122°03.421' 122°04.283' 122°03.345' 122°03.824' 122°04.279' 122°03.665' 122°03.944'

Map unit Thba Thba Thba Thba Thba Thba Thba Thba Thba

Rock type Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Analyses as reported (weight percent)

SiO2 53.46 54.55 55.07 54.63 55.31 55.83 56.10 56.18 55.83

TiO2 1.10 1.12 1.47 1.03 1.03 1.05 1.71 1.13 1.25

Al2O3 18.84 17.71 17.89 18.27 18.76 18.25 16.72 18.43 17.91

FeO* 7.78 8.00 8.71 7.73 7.18 7.54 8.84 7.49 7.67

MnO 0.14 0.17 0.14 0.15 0.13 0.14 0.16 0.15 0.13

MgO 4.15 4.19 3.19 4.51 3.40 3.77 3.26 3.30 3.05

CaO 8.86 8.39 8.08 8.73 8.69 8.10 7.50 8.25 7.70

Na2O 3.28 3.51 3.45 3.34 3.53 3.63 4.00 3.61 3.69

K2O 0.81 0.75 1.18 0.72 0.89 0.78 1.05 0.81 1.10

P2O5 0.16 0.15 0.27 0.16 0.17 0.18 0.29 0.18 0.21

Total 98.58 98.54 99.45 99.26 99.09 99.27 99.61 99.53 98.55

Analyses recalculated volatile-free and normalized to 100% with all Fe as FeO (weight percent)

SiO2 54.23 55.35 55.38 55.04 55.82 56.24 56.31 56.44 56.65

TiO2 1.12 1.13 1.48 1.03 1.04 1.05 1.71 1.14 1.27

Al2O3 19.11 17.98 17.99 18.40 18.93 18.38 16.78 18.51 18.18

FeO* 7.90 8.12 8.76 7.79 7.25 7.60 8.88 7.53 7.78

MnO 0.14 0.17 0.14 0.15 0.14 0.14 0.16 0.15 0.13

MgO 4.20 4.25 3.21 4.54 3.43 3.80 3.27 3.31 3.10

CaO 8.99 8.51 8.12 8.80 8.77 8.16 7.52 8.29 7.82

Na2O 3.33 3.56 3.47 3.37 3.56 3.66 4.01 3.63 3.75

K2O 0.83 0.77 1.19 0.73 0.90 0.78 1.05 0.82 1.12

P2O5 0.17 0.15 0.27 0.16 0.17 0.18 0.29 0.18 0.22

Mg# 53.1 52.7 43.6 55.2 50.0 51.4 43.7 48.1 45.9

Modes (volume percent)

Plagioclase 1.9 18.0 2.0 31.3 23.3 31.5 27.4 24.3 25.7

Clinopyroxene 0.6 0.3 0.3 -- 0.8 0.9 1.4 0.3 0.6

Orthopyroxene 0.6 1.8 0.2 0.4 -- -- -- -- --

Olivine -- -- -- 2.4 4.3 3.8 5.5 1.9 2.9

Fe-Ti Oxide 0.1 0.1 0.1 -- -- -- -- -- --

Hornblende -- -- -- -- -- -- -- -- --

Biotite -- -- -- -- -- -- -- -- --

Other -- -- -- -- -- -- -- -- --

Groundmass 96.8 79.8 97.4 65.9 71.6 63.8 65.7 73.5 70.8

No. points counted 850 850 850 850 850 850 850 850 850

Texture (rock/

 groundmass)

sparsely

 phyric/

pilotaxitic

porphyritic/

eutaxitic

sparsely

 phyric/

eutaxitic

seriate/

intersertal

seriate/

intergranular

seriate/

intergranular

seriate/

intergranular

porphyritic/

intergranular

porphyritic/

intergranular

Trace element analyses (parts per million)

Ba 208 208 284 186 218 226 281 228 270

Rb 25 21 38 23 25 24 30 34 33

Sr 373 364 341 370 400 387 364 364 357

Y 20 24 30 18 18 18 27 19 27

Zr 113 93 174 97 109 114 153 111 156

Nb 6.5 4.4 11.7 5.1 5.0 6.0 9.0 5.5 8.3

Ni 21 12 11 26 9 20 13 8 10

Cu 59 50 152 99 97 84 167 98 59

Zn 82 82 96 84 79 82 100 78 94

Cr 33 27 23 56 23 33 27 23 22

Sc 26 28 27 25 24 23 27 24 25

V 203 234 213 198 188 181 249 198 187

2Block in landslide deposit.
38

Table 1. Chemical and modal analyses of igneous rocks of the ancestral Cascade volcanic arc, Beacon Rock 7.5’ quadrangle, Skamania 

County, Washington.—Continued

Map No. 37 38 39 40 41 42 43 44 45

Field sample No. 02BV-G83A 02BV-G70 06BV-G585 06BV-G720 06BV-G622 02BV-G69 02BV-G53 06BV-G616 06BV-G791

Latitude (N) 45°42.150' 45°41.993' 45°44.599' 45°42.672' 45°41.721' 45°41.721' 45°42.908' 45°42.285' 45°40.530'

Longitude (W) 122°05.249' 122°01.972' 122°00.717' 122°01.024' 122°04.598' 122°04.598' 122°03.356' 122°04.567' 122°06.198'

Map unit Thba Thba Thba Tha Tha Tha Tha Tha Tha

Rock type Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Andesite Andesite Andesite Andesite Andesite Andesite

Analyses as reported (weight percent)

SiO2 55.78 55.98 56.44 56.42 56.69 57.36 56.59 57.19 57.24

TiO2 1.05 0.95 1.15 1.28 1.25 1.15 1.45 1.16 0.93

Al2O3 17.73 16.45 19.11 17.53 17.46 17.73 16.61 17.06 18.49

FeO* 7.55 7.07 6.41 6.99 7.39 7.32 8.27 7.44 6.67

MnO 0.14 0.14 0.13 0.10 0.13 0.14 0.14 0.14 0.12

MgO 3.74 5.33 2.72 3.65 3.27 4.02 2.50 3.48 2.94

CaO 7.46 7.76 7.87 7.42 7.48 7.26 6.85 7.17 7.57

Na2O 3.75 3.04 3.91 3.71 3.81 3.51 3.78 3.86 3.76

K2O 0.85 1.58 1.13 1.46 1.07 1.18 1.38 1.14 0.98

P2O5 0.18 0.14 0.26 0.24 0.26 0.25 0.28 0.20 0.17

Total 98.24 98.44 99.13 98.80 98.82 99.92 97.86 98.84 98.88

Analyses recalculated volatile-free and normalized to 100% with all Fe as FeO (weight percent)

SiO2 56.78 56.86 56.94 57.11 57.37 57.41 57.83 57.86 57.89

TiO2 1.07 0.97 1.16 1.30 1.27 1.15 1.48 1.17 0.94

Al2O3 18.05 16.71 19.28 17.74 17.67 17.74 16.98 17.26 18.70

FeO* 7.68 7.19 6.46 7.08 7.47 7.33 8.45 7.53 6.75

MnO 0.15 0.14 0.13 0.10 0.13 0.14 0.14 0.14 0.13

MgO 3.81 5.41 2.75 3.69 3.31 4.02 2.55 3.52 2.97

CaO 7.59 7.88 7.94 7.51 7.57 7.27 7.00 7.26 7.66

Na2O 3.82 3.09 3.94 3.76 3.86 3.51 3.86 3.91 3.80

K2O 0.87 1.61 1.14 1.48 1.09 1.18 1.41 1.16 0.99

P2O5 0.19 0.15 0.26 0.24 0.26 0.25 0.29 0.20 0.17

Mg# 51.4 61.6 47.3 52.5 48.5 53.5 39.3 49.8 48.3

Modes (volume percent)

Plagioclase 25.4 16.4 31.2 25.5 25.2 21.8 16.1 25.2 31.2

Clinopyroxene 2.4 5.1 1.5 1.8 5.5 0.6 1.9 3.0 3.6

Orthopyroxene 1.5 3.3 0.4 2.6 2.4 4.7 0.3 2.4 4.6

Olivine 0.5 2.5 1.3 0.9 0.8 0.1 0.9 1.5 0.4

Fe-Ti Oxide 0.2 0.2 0.1 0.1 1.3 0.1 0.1 0.5 0.8

Hornblende -- -- -- -- -- -- -- -- --

Biotite -- -- -- -- -- -- -- -- --

Other -- -- -- -- -- -- -- -- --

Groundmass 70.0 72.5 65.5 69.1 64.8 72.7 80.7 67.4 59.4

No. points counted 850 850 850 850 850 850 850 850 850

Texture (rock/

 groundmass)

porphyritic/

intergranular

porphyritic/

intergranular

porphyritic/

intersertal

porphyritic/

pilotaxitic

porphyritic/

intergranular

porphyritic/

microgranular

porphyritic/

intergranular

porphyritic/

intergranular

porphyritic/

pilotaxitic

Trace element analyses (parts per million)

Ba 223 285 284 313 300 298 322 309 245

Rb 23 48 33 44 40 28 44 35 33

Sr 357 407 424 416 342 417 325 346 379

Y 19 21 25 26 28 29 41 26 19

Zr 115 181 161 217 155 176 205 157 121

Nb 5.3 7.1 8.4 10.1 7.6 12.8 10.5 7.7 6.1

Ni 14 48 14 32 10 44 4 34 12

Cu 115 89 65 125 57 89 89 72 87

Zn 88 67 77 83 87 90 95 85 77

Cr 32 149 17 54 23 70 13 62 22

Sc 20 25 20 20 22 23 26 23 20

V 182 179 132 175 176 161 196 176 160
39

Table 1. Chemical and modal analyses of igneous rocks of the ancestral Cascade volcanic arc, Beacon Rock 7.5’ quadrangle, Skamania 

County, Washington.—Continued

Map No. 46 47 48 49 50 51 52 53 54

Field sample No. 02BV-G52B 06BV-G579 09BV-G1276 02BV-G15 06BV-G625 05BV-G441 05BV-G435 09BV-G1338 02BV-G812

Latitude (N) 45°43.330' 45°43.450' 45°42.875' 45°44.067' 45°41.099' 45°42.268' 45°41.550' 45°42.055' 45°41.517'

Longitude (W) 122°03.660' 122°01.285' 122°02.492' 122°01.431' 122°05.351' 122°00.898' 122°01.020' 122°03.318' 122°04.940'

Map unit Tha Tha Tha Tha Tha Tha Tha Tha Tha

Rock type Andesite Andesite Andesite Andesite Andesite Andesite Andesite Andesite Andesite

Analyses as reported (weight percent)

SiO2 58.03 56.88 57.11 58.29 57.62 56.85 57.24 57.78 58.60

TiO2 1.01 0.97 1.44 1.18 1.18 1.73 1.45 1.20 1.02

Al2O3 18.57 16.22 16.06 17.13 17.15 15.71 15.95 16.64 17.42

FeO* 6.64 6.95 7.64 7.06 7.08 8.20 7.65 7.25 6.62

MnO 0.12 0.11 0.13 0.14 0.11 0.14 0.14 0.15 0.11

MgO 3.00 4.51 3.24 3.72 3.06 2.20 3.13 3.42 2.80

CaO 7.36 7.01 6.39 6.86 6.89 6.16 6.38 6.48 6.58

Na2O 3.95 3.20 3.70 3.47 3.97 3.99 3.70 3.46 4.11

K2O 1.09 1.79 1.75 1.61 1.22 1.72 1.71 1.78 1.18

P2O5 0.18 0.15 0.29 0.22 0.20 0.42 0.29 0.21 0.20

Total 99.96 97.80 97.75 99.68 98.49 97.12 97.63 98.37 98.64

Analyses recalculated volatile-free and normalized to 100% with all Fe as FeO (weight percent)

SiO2 58.05 58.16 58.43 58.48 58.50 58.53 58.63 58.74 59.41

TiO2 1.01 0.99 1.47 1.18 1.20 1.78 1.49 1.22 1.03

Al2O3 18.58 16.58 16.43 17.19 17.42 16.18 16.33 16.92 17.66

FeO* 6.64 7.10 7.81 7.08 7.19 8.45 7.83 7.37 6.71

MnO 0.12 0.12 0.13 0.14 0.12 0.14 0.15 0.16 0.11

MgO 3.00 4.61 3.31 3.73 3.11 2.27 3.20 3.47 2.84

CaO 7.36 7.17 6.53 6.88 6.99 6.35 6.54 6.58 6.67

Na2O 3.95 3.28 3.79 3.48 4.03 4.11 3.79 3.52 4.17

K2O 1.09 1.83 1.79 1.62 1.24 1.77 1.75 1.81 1.19

P2O5 0.18 0.16 0.30 0.22 0.21 0.43 0.30 0.22 0.21

Mg# 48.7 58.2 47.7 52.6 47.9 36.7 46.8 50.1 47.3

Modes (volume percent)

Plagioclase 24.9 15.8 20.5 29.0 23.4 6.8 19.6 26.9 22.5

Clinopyroxene 1.4 3.6 2.5 4.0 4.1 0.2 2.1 3.5 3.1

Orthopyroxene 0.3 4.2 3.6 6.8 1.8 0.1 4.1 6.2 3.7

Olivine 0.7 1.1 -- 0.1 0.4 trace -- -- 1.5

Fe-Ti Oxide 0.1 0.7 -- 0.5 0.5 -- 0.4 0.3 0.6

Hornblende -- -- -- -- -- -- -- -- --

Biotite -- -- -- -- -- -- -- -- --

Other -- -- -- -- -- -- -- -- --

Groundmass 72.6 74.6 73.4 59.6 69.8 92.9 73.8 63.1 68.6

No. points counted 850 850 850 850 850 850 850 850 850

Texture (rock/

 groundmass)

porphyritic/

intergranular

porphyritic/

intergranular

porphyritic/

pilotaxitic

porphyritic/

intersertal

porphyritic/

pilotaxitic

porphyritic/

pilotaxitic

porphyritic/

microphyric

porphyritic/

cryptocrystalline

porphyritic/

intergranular

Trace element analyses (parts per million)

Ba 270 307 391 347 300 396 373 375 287

Rb 30 57 61 43 40 58 60 62 41

Sr 357 379 320 450 342 352 349 334 353

Y 23 25 34 31 28 40 38 30 24

Zr 141 208 268 181 155 252 258 264 152

Nb 8.7 7.6 14.4 9.3 7.6 13.7 12.5 13.5 7.4

Ni 7 41 47 38 10 11 30 30 12

Cu 91 102 115 114 57 181 98 75 40

Zn 80 73 87 83 87 100 95 87 79

Cr 19 118 87 64 23 6 33 49 26

Sc 19 23 22 22 22 22 23 21 19

V 162 165 195 164 176 167 175 177 138

2 Block in landslide deposit.
40

Table 1. Chemical and modal analyses of igneous rocks of the ancestral Cascade volcanic arc, Beacon Rock 7.5’ quadrangle, Skamania 

County, Washington.—Continued

Map No. 55 56 57 58 59 60 61 62 63

Field sample No. 08BV-G1065 06BV-G577 05BV-G448 02BV-G11C 02BV-G63B 05BV-G427 06BV-G797 05BV-G432 05BV-G4332

Latitude (N) 45°39.620' 45°43.737' 45°42.605' 45°43.339' 45°40.523' 45°39.824' 45°39.440' 45°40.777' 45°40.614'

Longitude (W) 122°03.728' 122°01.571' 122°01.243' 122°03.353' 122°03.642' 122°01.243' 122°01.929' 122°01.068' 122°00.228'

Map unit Tha Tha Tha Thd Thd Teha Teha Teha Teha

Rock type Andesite Andesite Andesite Dacite Dacite

basaltic

Hornblende

basaltic

Hornblende

andesite

Hornblende

andesite

Hornblende

Analyses as reported (weight percent)

SiO2 59.33 60.54 61.03 65.99 67.38 54.72 56.05 56.17 56.39

TiO2 1.48 1.20 1.19 0.50 0.64 1.04 1.01 1.02 1.00

Al2O3 15.47 15.76 15.34 14.78 15.04 17.79 17.86 17.50 17.39

FeO* 7.38 6.54 7.18 5.99 3.88 7.00 6.85 6.69 6.82

MnO 0.18 0.09 0.17 0.08 0.05 0.13 0.14 0.13 0.17

MgO 1.69 1.73 1.57 0.19 0.25 3.74 3.59 3.53 3.30

CaO 4.96 5.00 4.55 1.94 1.89 8.43 8.14 7.80 7.38

Na2O 4.27 4.13 4.59 5.06 4.76 3.09 3.16 3.19 3.19

K2O 1.84 1.97 1.89 2.41 2.62 1.89 1.85 1.89 1.85

P2O5 0.39 0.32 0.46 0.12 0.13 0.30 0.28 0.26 0.26

Total 96.99 97.28 97.97 97.06 96.64 98.14 98.93 98.17 97.74

Analyses recalculated volatile-free and normalized to 100% with all Fe as FeO (weight percent)

SiO2 61.17 62.23 62.30 67.99 69.72 55.76 56.66 57.21 57.69

TiO2 1.53 1.23 1.21 0.52 0.66 1.06 1.02 1.04 1.02

Al2O3 15.95 16.20 15.66 15.23 15.57 18.12 18.06 17.82 17.79

FeO* 7.60 6.72 7.33 6.17 4.02 7.14 6.92 6.81 6.98

MnO 0.19 0.09 0.17 0.09 0.05 0.13 0.14 0.13 0.17

MgO 1.74 1.77 1.60 0.19 0.26 3.81 3.63 3.60 3.38

CaO 5.12 5.14 4.64 2.00 1.95 8.59 8.23 7.95 7.55

Na2O 4.40 4.25 4.69 5.21 4.92 3.15 3.20 3.25 3.26

K2O 1.90 2.02 1.92 2.48 2.71 1.93 1.87 1.92 1.89

P2O5 0.40 0.33 0.47 0.12 0.14 0.31 0.28 0.26 0.26

Mg# 33.1 36.3 31.9 6.3 12.3 53.3 52.6 53.0 50.9

Modes (volume percent)

Plagioclase 0.3 16.6 0.6 10.2 8.4 12.0 11.6 10.2 14.1

Clinopyroxene 0.1 1.5 trace -- -- 0.3 1.0 0.8 1.2

Orthopyroxene -- 1.2 trace 1.0 1.3 1.5 1.4 1.5 2.9

Olivine -- trace 0.1 -- -- -- -- -- --

Fe-Ti Oxide -- 0.5 trace -- 0.3 0.1 0.3 0.5 0.6

Hornblende -- -- -- -- -- 1.0 0.6 1.0 0.5

Biotite -- -- -- -- -- -- -- -- --

Other -- -- -- -- -- 0.2 0.8 0.6 1.0

Groundmass 99.6 80.2 99.3 88.8 90.0 84.9 84.3 85.4 79.7

No. points counted 850 850 850 850 800 850 850 850 850

Texture (rock/

 groundmass)

porphyritic/

intergranular

porphyritic/

pilotaxitic

~aphyric

pilotaxitic

porphyritic/

micropoikilitic

porphyritic/

micropoikilitic

porphyritic/

pilotaxitic

porphyritic/

pilotaxitic

porphyritic/

pilotaxitic

porphyritic/

pilotaxitic

Trace element analyses (parts per million)

Ba 403 420 434 572 611 685 618 571 611

Rb 81 68 60 77 81 28 29 30 33

Sr 303 286 300 183 167 1252 1157 1123 1060

Y 39 36 45 47 47 19 20 17 18

Zr 274 292 301 346 371 150 150 145 150

Nb 15.6 14.3 16.3 16.2 19.6 4.6 4.5 5.3 5.0

Ni 0 4 6 0 0 28 25 28 28

Cu 36 92 28 29 26 74 69 62 82

Zn 100 91 101 104 96 75 65 74 77

Cr 0 10 1 2 1 28 25 30 28

Sc 23 19 20 12 11 18 18 17 17

V 109 102 44 8 18 177 170 174 166

2

 Block in landslide deposit.
41

Table 1. Chemical and modal analyses of igneous rocks of the ancestral Cascade volcanic arc, Beacon Rock 7.5’ quadrangle, Skamania 

County, Washington.—Continued

Map No. 64 65 66 67 68 69 70 71 72

Field sample No. 04BV-G281A 11BV-G1555 08BV-G11793 05BV-G405A 05BV-G409 06BV-G599 05BV-G440A3 02BV-G61 02BV-G074

Latitude (N) 45°38.535' 45°38.518' 45°41.691' 45°40.796' 45°40.305' 45°43.463' 45°41.882' 45°40.893' 45°41.011'

Longitude (W) 122°00.919' 122°00.830' 122°00.232' 122°04.514' 122°02.928' 122°02.522' 122°00.278' 122°03.789' 122°05.062'

Map unit Teha Teha Tepa Tepa Tepa Tepa Tepa Tehd Tecu

Rock type Hornblende

andesite

Hornblende

andesite

Hbl-Pyx

andesite

Pyroxene

andesite

Pyroxene Pyroxene

andesite

Pyroxene

andesite

Hornblende

dacite

Hbl-Pyx

andesite andesite

Analyses as reported (weight percent)

SiO2 57.59 57.51 55.08 58.55 58.20 59.49 59.69 63.85 59.34

TiO2 0.98 0.99 0.98 1.00 0.99 0.91 0.88 0.70 0.97

Al2O3 17.61 17.35 18.12 16.85 16.98 16.91 16.53 17.06 17.16

FeO* 6.55 6.76 6.20 6.31 6.23 6.02 5.43 4.60 6.05

MnO 0.12 0.14 0.12 0.10 0.11 0.11 0.12 0.09 0.10

MgO 3.54 3.09 3.42 3.82 3.31 3.39 2.65 2.68 3.92

CaO 7.95 7.55 5.66 6.70 6.35 6.57 6.13 4.96 6.71

Na2O 3.34 3.27 3.10 3.74 3.71 3.40 3.44 4.00 3.35

K2O 1.33 1.74 1.37 1.37 1.39 1.98 2.10 1.69 1.91

P2O5 0.22 0.21 0.19 0.17 0.17 0.17 0.17 0.16 0.17

Total 99.24 98.61 94.25 98.60 97.43 98.96 97.13 99.78 99.68

Analyses recalculated volatile-free and normalized to 100% with all Fe as FeO (weight percent)

SiO2 58.03 58.32 58.44 59.38 59.74 60.12 61.45 63.99 59.53

TiO2 0.99 1.01 1.04 1.01 1.01 0.92 0.91 0.70 0.97

Al2O3 17.75 17.60 19.22 17.09 17.43 17.09 17.02 17.10 17.22

FeO* 6.60 6.85 6.58 6.40 6.39 6.09 5.59 4.61 6.07

MnO 0.12 0.14 0.13 0.10 0.11 0.11 0.12 0.09 0.10

MgO 3.57 3.13 3.63 3.88 3.40 3.42 2.72 2.69 3.93

CaO 8.01 7.66 6.00 6.79 6.52 6.64 6.31 4.97 6.73

Na2O 3.37 3.32 3.29 3.79 3.81 3.44 3.55 4.01 3.36

K2O 1.34 1.77 1.46 1.39 1.42 2.00 2.16 1.69 1.92

P2O5 0.22 0.22 0.20 0.17 0.17 0.18 0.18 0.16 0.17

Mg# 53.3 49.3 55.1 56.3 53.4 54.4 51.3 55.1 57.7

Modes (volume percent)

Plagioclase 16.8 17.3 15.8 17.3 18.3 13.5 18.6 12.8 14.9

Clinopyroxene 0.5 0.5 3.3 1.8 1.3 2.1 1.4 -- 3.2

Orthopyroxene 1.4 1.4 2.9 2.4 2.3 2.2 2.1 -- 3.5

Olivine -- -- -- 0.8 1.0 -- -- -- 1.4

Fe-Ti Oxide 0.3 0.3 0.5 0.2 0.3 0.6 0.4 0.4 0.4

Hornblende 1.3 0.5 0.4 -- -- 0.5 0.2 8.0 --

Biotite -- -- -- -- -- -- -- 0.3 --

Other 0.6 0.8 -- -- -- -- 0.1 -- --

Groundmass 79.1 79.2 77.1 77.5 76.8 81.1 77.2 78.5 76.6

No. points counted 850 820 830 850 850 830 850 850 770

Texture (rock/

 groundmass)

porphyritic/

pilotaxitic

porphyritic/

pilotaxitic

porphyritic/

pilotaxitic

porphyritic/

pilotaxitic

porphyritic/

pilotaxitic

seriate/

intersertal

seriate/

intersertal

porphyritic/

felsitic

seriate/

intersertal

Trace element analyses (parts per million)

Ba 455 446 408 303 307 358 377 351 305

Rb 31 31 28 31 31 44 51 24 35

Sr 1035 1015 602 510 496 694 598 556 479

Y 16 19 24 18 17 22 22 17 19

Zr 131 138 217 153 152 188 202 121 155

Nb 3.4 6.4 7.6 7.1 6.5 6.5 7.7 8.2 8.2

Ni 23 25 25 58 52 23 21 28 53

Cu 72 67 59 64 143 57 74 40 80

Zn 74 75 77 76 76 61 65 52 70

Cr 25 27 34 76 74 34 32 19 78

Sc 18 16 17 17 17 17 16 14 15

V 160 162 150 149 146 141 136 89 144 3Block in debris-avalanche deposit?

4Block in lahar deposit.
42

Table 1. Chemical and modal analyses of igneous rocks of the ancestral Cascade volcanic arc, Beacon Rock 7.5’ quadrangle, Skamania 

County, Washington.—Continued

Map No. 73 74 75 76 77 78 79 80 81

Field sample No. 02BV-G04B4 02BV-G864 02BV-G55B5 03BV-G1704 02BV-G024 02BV-G67A5 02BV-G875 06BV-G655B 06BV-G786A

Latitude (N) 45°40.316' 45°41.285' 45°41.376' 45°40.968' 45°39.061' 45°41.192' 45°41.377' 45°44.049' 45°41.680'

Longitude (W) 122°06.203' 122°03.320' 122°02.999' 122°04.528' 122°07.058' 122°01.815' 122°03.646' 122°04.428' 122°06.897'

Map unit Tecu Tecu Tecu Tecu Tecu Tecu Tecu Tiba Tiba

Rock type Pyroxene

andesite

Pyroxene

andesite

Pyroxene

andesite

Pyroxene

andesite

Hornblende

andesite

Pyroxene

dacite

Hornblende

dacite

Basaltic

andesite

Basaltic

andesite

Analyses as reported (weight percent)

SiO2 59.77 59.88 58.89 61.90 60.66 62.14 69.24 52.98 52.49

TiO2 0.88 0.90 0.91 0.81 0.85 0.76 0.46 1.56 1.35

Al2O3 17.46 17.30 16.79 16.70 17.59 17.03 16.83 17.53 17.57

FeO* 5.74 6.01 6.02 5.30 5.54 4.70 3.08 9.55 9.06

MnO 0.09 0.10 0.10 0.09 0.08 0.08 0.06 0.17 0.22

MgO 3.29 3.09 3.05 2.94 2.07 2.25 0.61 3.49 4.11

CaO 6.89 6.75 6.37 6.02 5.27 5.37 1.97 9.09 8.82

Na2O 3.72 3.40 3.27 3.76 3.92 3.81 3.29 3.29 3.28

K2O 1.51 1.67 1.68 1.80 1.32 1.74 2.23 0.79 0.44

P2O5 0.15 0.15 0.15 0.14 0.19 0.18 0.09 0.21 0.21

Total 99.50 99.26 97.23 99.45 97.48 98.04 97.87 98.66 97.54

Analyses recalculated volatile-free and normalized to 100% with all Fe as FeO (weight percent)

SiO2 60.07 60.33 60.56 62.24 62.23 63.38 70.75 53.70 53.81

TiO2 0.88 0.91 0.93 0.82 0.87 0.77 0.47 1.58 1.38

Al2O3 17.55 17.43 17.27 16.79 18.04 17.37 17.20 17.77 18.01

FeO* 5.77 6.06 6.19 5.33 5.68 4.79 3.15 9.68 9.28

MnO 0.09 0.10 0.11 0.09 0.08 0.08 0.06 0.17 0.22

MgO 3.31 3.11 3.14 2.95 2.12 2.29 0.62 3.54 4.22

CaO 6.92 6.80 6.55 6.05 5.41 5.48 2.01 9.21 9.04

Na2O 3.74 3.42 3.36 3.78 4.02 3.89 3.36 3.33 3.36

K2O 1.52 1.68 1.73 1.81 1.35 1.77 2.28 0.80 0.45

P2O5 0.15 0.15 0.15 0.14 0.20 0.18 0.10 0.22 0.22

Mg# 54.7 52.1 52.3 53.9 44.5 50.6 29.8 43.8 49.4

Modes (volume percent)

Plagioclase 17.9 20.4 21.9 28.0 22.4 19.3 6.4 14.1 18.5

Clinopyroxene 1.6 1.2 1.1 1.4 -- 0.5 -- 0.9 1.3

Orthopyroxene 3.4 4.1 2.9 3.6 2.0 2.8 -- -- 2.0

Olivine 1.2 -- -- -- -- -- -- 0.2 0.6

Fe-Ti Oxide 0.3 0.6 0.6 0.3 0.5 0.4 0.1 -- 0.1

Hornblende -- 0.4 0.5 1.1 9.0 0.1 3.8 -- --

Biotite -- -- -- -- -- -- -- -- --

Other -- 1.0 2.5 -- -- -- 0.4 -- --

Groundmass 75.6 72.3 70.5 65.6 66.1 76.9 89.3 84.8 77.5

No. points counted 850 850 850 850 830 750 850 850 850

Texture (rock/

 groundmass)

seriate/

felsitic

seriate/

pilotaxitic

seriate/

microphyric

seriate/

microphyric

seriate/

microphyric

seriate/

microphyric

seriate/

microphyric

seriate/

intergranular

seriate/

intersertal

Trace element analyses (parts per million)

Ba 305 336 339 349 412 381 604 208 155

Rb 32 30 31 40 17 25 44 20 8

Sr 510 789 738 510 713 494 280 299 313

Y 22 19 19 21 20 13 11 29 32

Zr 151 131 134 179 123 150 107 136 156

Nb 8.0 5.6 5.6 7.4 7.0 9.3 10.3 6.9 7.9

Ni 37 23 20 34 33 22 16 12 16

Cu 50 89 25 42 47 27 20 167 125

Zn 61 65 63 61 62 72 39 101 94

Cr 74 30 30 43 32 29 13 17 27

Sc 17 15 16 14 12 10 9 33 32

V 131 144 147 124 102 91 46 296 229 4Block in lahar deposit.

5Block in lithic pyroclastic-flow deposit.
43

Table 1. Chemical and modal analyses of igneous rocks of the ancestral Cascade volcanic arc, Beacon Rock 7.5’ quadrangle, Skamania 

County, Washington.—Continued

Map No. 82 83 84 85 86 87 88 89 90

Field sample No. 02BV-G77 06BV-G676 06BV-G681 06BV-G779 03BV-G71A 07BV-G820 06BV-G770A 06BV-G799 06BV-G650

Latitude (N) 45°44.339' 45°40.827' 45°40.211' 45°41.346' 45°42.000' 45°44.916' 45°43.644' 45°41.643' 45°44.829'

Longitude (W) 122°06.364' 122°06.472' 122°07.297' 122°06.519' 122°01.673' 122°03.753' 122°06.269' 122°06.082' 122°04.654'

Map unit Tiba Tiba Tiah Tia Tial Tqd Tid Tid Tidcr

Rock type Basaltic

andesite

Basaltic

andesite

Andesite Andesite Andesite

quartz diorite 

Diorite

Pyroxene Dacite Dacite Rhyolite

Analyses as reported (weight percent)

SiO2 53.45 54.43 56.46 56.45 57.44 58.00 61.51 63.94 69.03

TiO2 1.82 1.50 1.60 1.47 1.15 1.02 1.30 0.94 0.38

Al2O3 15.46 16.78 16.20 15.85 17.89 17.55 14.50 14.59 14.39

FeO* 10.38 9.28 8.99 8.29 6.55 6.61 7.61 6.26 4.42

MnO 0.18 0.19 0.16 0.14 0.10 0.13 0.15 0.14 0.09

MgO 3.69 3.46 2.75 3.16 3.48 2.55 1.65 1.23 0.26

CaO 8.66 8.47 7.50 7.17 6.60 6.38 4.89 3.92 1.59

Na2O 2.93 3.22 3.40 3.43 4.37 4.39 4.65 5.06 4.93

K2O 0.35 0.95 1.38 1.41 0.81 1.19 0.87 1.26 2.46

P2O5 0.23 0.21 0.31 0.30 0.29 0.19 0.37 0.25 0.07

Total 97.16 98.48 98.74 97.66 98.69 98.01 97.50 97.59 97.62

Analyses recalculated volatile-free and normalized to 100% with all Fe as FeO (weight percent)

SiO2 55.02 55.27 57.18 57.81 58.21 59.18 63.09 65.52 70.71

TiO2 1.87 1.52 1.62 1.50 1.16 1.05 1.33 0.96 0.39

Al2O3 15.91 17.04 16.40 16.23 18.13 17.91 14.87 14.94 14.75

FeO* 10.68 9.42 9.11 8.49 6.63 6.74 7.81 6.42 4.53

MnO 0.19 0.19 0.16 0.14 0.10 0.13 0.15 0.15 0.09

MgO 3.80 3.51 2.78 3.23 3.53 2.60 1.69 1.26 0.26

CaO 8.91 8.61 7.59 7.34 6.69 6.51 5.01 4.01 1.63

Na2O 3.01 3.27 3.44 3.51 4.43 4.47 4.77 5.19 5.05

K2O 0.36 0.96 1.40 1.44 0.82 1.21 0.89 1.30 2.52

P2O5 0.24 0.21 0.31 0.31 0.29 0.19 0.38 0.26 0.07

Mg# 43.4 44.3 39.4 45.0 53.0 45.2 31.8 29.7 11.1

Modes (volume percent)

Plagioclase trace 15.5 31.8 11.5 -- 24.5 0.9 6.7 14.7

Clinopyroxene trace 0.2 0.7 1.6 -- 1.6 0.1 1.8 0.3

Orthopyroxene -- -- 0.3 1.5 -- 1.5 0.1 1.3 0.7

Olivine trace 0.2 -- -- -- 0.7 -- trace --

Fe-Ti Oxide -- -- -- -- -- 0.4 0.1 0.2 0.1

Hornblende -- -- -- -- -- -- -- -- --

Biotite -- -- -- -- -- -- -- -- --

Other -- -- -- -- -- -- -- -- 1.7

Groundmass 100.0 84.1 67.2 85.4 100.0 71.3 98.8 90.0 82.5

No. points counted -- 850 850 850 -- 850 850 850 780

Texture (rock/

 groundmass)

aphyric/

intergran-

 ular

seriate/

intergranular

porphyritic/

intersertal

porphyritic/

intersertal

aphyric/

pilotaxitic

seriate/

hypidiomorphic

~aphyric/

pilotaxitic

sparsely

 phyric/

pilotaxitic

porphyritic/

felsitic

Trace element analyses (parts per million)

Ba 156 251 345 343 243 295 382 436 625

Rb 22 30 36 57 13 37 66 84 80

Sr 443 294 300 273 603 373 241 193 156

Y 27 31 41 40 14 24 45 54 54

Zr 120 149 231 259 132 155 281 318 357

Nb 6.3 8.1 11.2 13.1 9.4 7.4 13.6 16.8 18.6

Ni 7 8 10 21 47 2 1 1 0

Cu 201 182 163 16 62 66 106 50 20

Zn 100 95 102 92 98 82 95 93 117

Cr 12 18 19 29 57 7 3 3 2

Sc 35 34 28 25 12 20 22 19 16

V 302 274 188 189 128 147 70 44 0
44

Table 1. Chemical and modal analyses of igneous rocks of the ancestral Cascade volcanic arc, Beacon Rock 7.5’ quadrangle, Skamania 

County, Washington.—Continued

Map No. 91 92

Field sample No. 05BV-G513 11BV-G1554A

Latitude (N) 45°38.758' 45°38.591'

Longitude (W) 122°00.743' 122°00.885'

Map unit Tipd Tipd

Rock type Pyroxene

dacite

Pyroxene

dacite

Analyses as reported (weight percent)

SiO2 62.14 64.68

TiO2 0.76 0.67

Al2O3 16.11 15.67

FeO* 4.91 4.32

MnO 0.08 0.05

MgO 2.57 1.66

CaO 5.54 4.46

Na2O 3.65 3.70

K2O 1.97 2.15

P2O5 0.15 0.12

Total 97.87 97.47

Analyses recalculated volatile-free and normalized to 100% with all Fe as FeO (weight percent)

SiO2 63.49 66.36

TiO2 0.78 0.68

Al2O3 16.46 16.08

FeO* 5.02 4.43

MnO 0.08 0.05

MgO 2.62 1.70

CaO 5.66 4.58

Na2O 3.73 3.79

K2O 2.01 2.21

P2O5 0.15 0.12

Mg# 52.8 45.2

Modes (volume percent)

Plagioclase 39.4 37.9

Clinopyroxene 2.3 1.1

Orthopyroxene 5.2 3.1

Olivine -- --

Fe-Ti Oxide 0.5 0.3

Hornblende 0.2 trace

Biotite -- --

Other 4.8 --

Groundmass 47.6 53.3

No. points counted 850 720

Texture (rock/

 groundmass)

porphyritic/

cryptocrys-

 talline

porphyritic/

cryptocrys-

 talline

Trace element analyses (parts per million)

Ba 441 444

Rb 44 52

Sr 558 388

Y 17 16

Zr 167 195

Nb 6.3 9.2 

Ni 29 22

Cu 38 31

Zn 60 61

Cr 38 33

Sc 13 10

V 119 87
45

Table 2. Chemical analyses of the Columbia River Basalt Group, Beacon Rock 7.5' quadrangle, Skamania County, Washington.

[X-ray fluorescence analyses. Map No., see figure 16 (sheet 2). Rock-type names assigned in accordance with IUGS system (Le Maitre, 2002) applied to recalculated analyses. FeO*, total Fe calculated as FeO; Mg#, atomic ratio 100 Mg/(Mg+Fe2+) with Fe2+ set to 0.85 x Fetotal. Analyses by D.M. Johnson Cornelius and 

R.M. Conrey at Peter Hooper GeoAnalytical Laboratory of Washington State University, Pullman, Washington, using methods described in Johnson and others 

(1999)].

Map No. 93 94 95 96 97 98 99 100 101

Field sample No. 04BV-G323 05BV-G510 05BV-G403 09BV-G1377 09BV-G1385 05BV-G402 04BV-G282A1 04BV-G283 12BV-G1566

Latitude (N) 45°38.728’ 45°38.939’ 45°37.606’ 45°39.040’ 45°39.302’ 45°37.780’ 45°38.661’ 45°38.906’ 45°38.905’

Longitude (W) 122°01.450’ 122°00.567’ 122°06.517’ 122°01.072’ 122°00.353’ 122°06.593’ 122°00.614’ 122°00.465’ 122°00.459’

Map unit Tgwr Tgwr Tgwr Tgwr Tgwr Tggc Tgo Tgo Tgo

Rock type Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Analyses as reported (weight percent)

SiO2 54.83 54.51 54.08 54.38 54.55 55.33 55.30 54.76 55.17

TiO2 2.35 2.35 2.31 2.33 2.34 1.97 1.93 1.95 1.96

Al2O3 13.34 13.43 13.23 13.41 13.37 13.80 13.66 13.64 13.71

FeO* 12.46 12.35 12.11 12.19 12.15 11.53 11.83 11.75 10.97

MnO 0.20 0.20 0.19 0.20 0.19 0.19 0.19 0.19 0.19

MgO 3.45 3.43 3.36 3.43 3.41 3.66 3.64 3.61 3.65

CaO 7.05 7.01 6.91 7.02 7.01 7.12 7.22 7.17 7.17

Na2O 3.10 3.15 3.15 3.14 3.19 3.23 3.37 3.17 3.19

K2O 1.87 1.74 1.77 1.68 1.76 1.75 1.46 1.69 1.69

P2O5 0.41 0.40 0.40 0.41 0.41 0.34 0.32 0.34 0.34

Total 99.07 98.56 97.52 98.20 98.38 98.93 98.91 98.27 98.04

Analyses recalculated volatile-free and normalized to 100% with all Fe as FeO (weight percent)

SiO2 55.35 55.30 55.45 55.38 55.45 55.93 55.91 55.72 56.27

TiO2 2.37 2.38 2.37 2.38 2.38 1.99 1.95 1.99 2.00

Al2O3 13.47 13.63 13.57 13.65 13.59 13.95 13.81 13.88 13.98

FeO* 12.58 12.53 12.42 12.42 12.35 11.65 11.96 11.95 11.19

MnO 0.20 0.20 0.20 0.20 0.20 0.19 0.19 0.19 0.19

MgO 3.48 3.48 3.45 3.49 3.46 3.70 3.68 3.67 3.72

CaO 7.12 7.11 7.09 7.15 7.12 7.20 7.30 7.30 7.31

Na2O 3.13 3.20 3.23 3.20 3.25 3.26 3.41 3.23 3.25

K2O 1.88 1.77 1.82 1.71 1.78 1.77 1.47 1.72 1.72

P2O5 0.41 0.41 0.41 0.41 0.41 0.35 0.32 0.35 0.35

Mg# 36.9 36.8 36.8 37.5 37.0 40.0 39.5 39.6 41.1

Trace element analyses (parts per million)

Ba 698 710 697 693 710 687 655 664 683

Rb 53 51 51 51 51 49 48 50 50

Sr 331 329 321 335 332 325 324 330 328

Y 37 39 37 37 39 36 34 35 35

Zr 192 186 183 192 191 172 175 177 172

Nb 14.8 13.6 13.0 14.3 14.0 12.2 12.7 12.7 11.2

Ni 11 13 13 10 12 13 11 11 10

Cu 23 20 23 24 23 21 19 24 20

Zn 132 127 126 132 131 122 122 121 122

Cr 3 9 9 6 7 14 11 11 12

Sc 33 31 31 32 31 31 32 31 31

V 378 376 372 381 380 333 327 327 328

1Clast in pillow breccia.
46

Table 2. Chemical analyses of the Columbia River Basalt Group, Beacon Rock 7.5' quadrangle, Skamania County, Washington.—Continued

Map No. 102 103 104 105 106 107 108 109 110

Field sample No. 12BV-G1567 12BV-G1568 08BV-G1211A 08BV-G1211B1 12BV-G15691 12BV-G15701 05BV-G429A 05BV-G429B 04BV-G322

Latitude (N) 45°38.879’ 45°38.864’ 45°38.892’ 45°38.902’ 45°38.790’ 45°38.633’ 45°39.517’ 45°39.561’ 45°38.822’

Longitude (W) 122°00.460’ 122°00.460’ 122°00.285’ 122°00.281’ 122°00.492’ 122°00.598’ 122°00.733’ 122°00.721’ 122°01.459’

Map unit Tgo Tgo Tgo Tgo Tgo Tgo Tgo Tgo Tgo

Rock type Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Analyses as reported (weight percent)

SiO2 55.45 54.98 54.08 54.39 54.49 55.37 54.81 54.39 55.36

TiO2 1.97 2.03 1.99 1.91 1.91 1.93 1.94 1.95 1.93

Al2O3 13.81 13.66 13.64 13.68 13.54 13.67 13.70 13.59 13.65

FeO* 11.46 11.69 11.48 11.35 11.29 11.52 11.44 11.38 11.53

MnO 0.19 0.19 0.20 0.19 0.19 0.19 0.19 0.19 0.19

MgO 3.64 3.60 3.40 3.60 3.62 3.65 3.65 3.57 3.65

CaO 7.17 7.10 7.07 7.14 7.16 7.20 7.12 7.09 7.19

Na2O 3.21 3.15 3.27 3.23 3.17 3.38 3.20 3.15 3.12

K2O 1.70 1.80 1.45 1.49 1.55 1.41 1.68 1.73 1.78

P2O5 0.34 0.36 0.35 0.31 0.35 0.33 0.34 0.34 0.32

Total 98.94 98.56 96.92 97.29 97.27 98.65 98.07 97.38 98.73

Analyses recalculated volatile-free and normalized to 100% with all Fe as FeO (weight percent)

SiO2 56.04 55.78 55.80 55.90 56.02 56.13 55.89 55.85 56.07

TiO2 1.99 2.06 2.05 1.96 1.96 1.96 1.98 2.01 1.96

Al2O3 13.96 13.86 14.07 14.06 13.92 13.86 13.97 13.96 13.83

FeO* 11.58 11.86 11.85 11.66 11.61 11.68 11.67 11.68 11.68

MnO 0.19 0.19 0.21 0.20 0.20 0.19 0.19 0.19 0.19

MgO 3.68 3.65 3.50 3.70 3.72 3.70 3.72 3.66 3.70

CaO 7.25 7.20 7.29 7.34 7.36 7.30 7.26 7.28 7.28

Na2O 3.24 3.20 3.37 3.32 3.26 3.43 3.26 3.24 3.16

K2O 1.72 1.83 1.50 1.54 1.59 1.43 1.71 1.78 1.80

P2O5 0.34 0.37 0.36 0.32 0.36 0.33 0.34 0.35 0.33

Mg# 40.0 39.2 38.3 40.0 40.2 39.9 40.1 39.7 40.2

Trace element analyses (parts per million)

Ba 677 683 696 643 675 674 671 690 713

Rb 49 50 46 45 48 49 51 53 47

Sr 328 324 323 319 321 325 325 316 329

Y 34 35 34 33 33 34 35 35 34

Zr 175 180 175 171 169 175 177 191 170

Nb 11.4 12.7 11.1 11.4 12.1 12.5 13.3 13.7 12.1

Ni 12 12 9 11 14 11 11 9 14

Cu 23 23 24 18 20 24 19 18 21

Zn 121 125 126 120 119 123 121 128 120

Cr 12 10 12.7 11 14 12 9 10 14

Sc 32 32 31.5 32 32 31 32 31 32

V 332 346 334.4 327 329 332 329 328 326

1Clast in pillow breccia.
47

Table 2. Chemical analyses of the Columbia River Basalt Group, Beacon Rock 7.5' quadrangle, Skamania County, Washington.—Continued

Map No. 111 112 113 114 115 116 117 118 119

Field sample No. 04BV-G321A 05BV-G419 05BV-G398A 02BV-G115B-2 04BV-G201A 02BV-G116B 05BV-G401 04BV-G201B 05BV-G346A

Latitude (N) 45°38.898’ 45°39.634’ 45°37.813’ 45°37.977’ 45°38.739’ 45°37.954’ 45°37.534’ 45°38.577’ 45°38.463’

Longitude (W) 122°01.488’ 122°02.016’ 122°06.599’ 122°06.603’ 122°06.165’ 122°06.388’ 122°06.192’ 122°06.100’ 122°06.170’

Map unit Tgo Tgo Tgo Tgo Tgo Tgo Tgo Tgo Tgo

Rock type Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Analyses as reported (weight percent)

SiO2 55.53 54.63 55.01 55.75 55.56 55.75 55.58 55.36 54.98

TiO2 1.99 1.94 1.90 1.83 1.88 1.95 1.91 1.98 1.99

Al2O3 13.58 13.69 13.53 13.80 13.68 13.74 13.74 13.62 13.49

FeO* 11.87 11.09 10.96 10.88 11.62 11.17 11.05 11.26 11.34

MnO 0.19 0.19 0.19 0.19 0.19 0.19 0.19 0.19 0.19

MgO 3.49 3.58 3.48 3.51 3.72 3.50 3.55 3.49 3.51

CaO 7.06 7.09 6.97 7.10 7.26 7.04 7.07 7.01 6.87

Na2O 3.12 3.20 3.13 3.10 3.06 3.15 3.15 3.19 3.33

K2O 1.84 1.70 1.80 1.76 1.84 1.85 1.81 1.85 1.65

P2O5 0.35 0.34 0.33 0.30 0.32 0.33 0.33 0.35 0.35

Total 99.02 97.46 97.29 98.21 99.13 98.65 98.38 98.30 97.69

Analyses recalculated volatile-free and normalized to 100% with all Fe as FeO (weight percent)

SiO2 56.08 56.05 56.54 56.77 56.05 56.51 56.50 56.32 56.27

TiO2 2.01 1.99 1.95 1.86 1.90 1.97 1.94 2.02 2.03

Al2O3 13.71 14.05 13.91 14.05 13.80 13.93 13.97 13.85 13.81

FeO* 11.98 11.38 11.26 11.08 11.72 11.32 11.23 11.45 11.61

MnO 0.20 0.19 0.19 0.19 0.19 0.19 0.19 0.19 0.19

MgO 3.52 3.67 3.57 3.57 3.75 3.55 3.61 3.55 3.60

CaO 7.13 7.28 7.16 7.23 7.32 7.14 7.19 7.14 7.03

Na2O 3.16 3.29 3.22 3.15 3.09 3.19 3.21 3.24 3.41

K2O 1.86 1.75 1.85 1.79 1.86 1.87 1.84 1.88 1.69

P2O5 0.35 0.35 0.34 0.31 0.32 0.34 0.33 0.35 0.36

Mg# 38.4 40.4 39.9 40.8 40.4 40.0 40.3 39.8 39.4

Trace element analyses (parts per million)

Ba 686 717 670 697 687 694 671 727 670

Rb 50 49 50 50 50 50 45 48 44

Sr 306 311 312 312 308 307 299 333 304

Y 36 40 35 33 35 36 35 35 37

Zr 177 184 183 179 176 181 179 176 179

Nb 12.6 11.3 13.2 11.5 12.2 12.5 12.4 11.0 12.9

Ni 11 0 10 16 12 12 13 12 10

Cu 15 12 13 15 16 18 17 22 14

Zn 117 122 122 125 119 125 121 124 125

Cr 13 7 11 16 15 15 14 14 8

Sc 31 32 31 31 31 30 31 32 31

V 319 305 318 327 322 331 338 340 328
48

Table 2. Chemical analyses of the Columbia River Basalt Group, Beacon Rock 7.5' quadrangle, Skamania County, Washington.—Continued

Map No. 120 121 122 123 124 125 126 127 128

Field sample No. 05BV-G346B 05BV-G349A 04BV-G1532 05BV-G508A 05BV-G508B 09BV-G1380 09BV-G1381 04BV-G320A 04BV-G320B

Latitude (N) 45°38.504’ 45°38.540’ 45°38.368’ 45°38.973’ 45°39.054’ 45°39.160’ 45°39.004’ 45°39.140’ 45°39.034’

Longitude (W) 122°06.080’ 122°06.141’ 122°06.694’ 122°00.339’ 122°00.315’ 122°00.419’ 122°00.304’ 122°01.523’ 122°01.521’

Map unit Tgo Tgo Tgo Tgo Tgo Tgo Tgo Tgo Tgo

Rock type Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Andesite Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Analyses as reported (weight percent)

SiO2 55.04 55.50 55.15 55.68 56.49 55.58 55.94 55.71 55.44

TiO2 2.00 1.95 1.95 1.85 1.97 1.90 1.97 1.92 1.99

Al2O3 13.84 13.43 13.47 13.64 13.85 13.64 13.63 13.69 13.48

FeO* 11.33 11.22 11.33 10.98 10.58 11.51 11.93 11.52 11.83

MnO 0.19 0.18 0.20 0.19 0.16 0.17 0.19 0.19 0.19

MgO 3.56 3.46 3.54 3.52 3.44 3.57 3.52 3.48 3.51

CaO 7.20 6.87 7.10 7.06 6.72 6.78 7.16 7.13 6.82

Na2O 3.20 3.32 3.08 3.19 3.36 3.30 3.27 3.07 3.35

K2O 1.66 1.63 1.85 1.71 1.73 1.68 1.65 1.61 1.66

P2O5 0.40 0.35 0.34 0.33 0.34 0.34 0.35 0.33 0.36

Total 98.42 97.90 98.02 98.15 98.63 98.48 99.61 98.65 98.63

Analyses recalculated volatile-free and normalized to 100% with all Fe as FeO (weight percent)

SiO2 55.92 56.69 56.27 56.72 57.27 56.43 56.15 56.48 56.21

TiO2 2.03 1.99 1.99 1.89 2.00 1.93 1.98 1.95 2.02

Al2O3 14.06 13.72 13.74 13.90 14.04 13.85 13.69 13.87 13.67

FeO* 11.51 11.46 11.56 11.19 10.73 11.69 11.98 11.68 12.00

MnO 0.20 0.18 0.20 0.19 0.16 0.18 0.19 0.19 0.19

MgO 3.62 3.54 3.61 3.59 3.49 3.63 3.54 3.53 3.56

CaO 7.32 7.02 7.24 7.19 6.82 6.89 7.19 7.22 6.91

Na2O 3.26 3.39 3.14 3.25 3.41 3.35 3.28 3.11 3.40

K2O 1.69 1.66 1.89 1.75 1.75 1.70 1.66 1.63 1.68

P2O5 0.40 0.35 0.35 0.33 0.34 0.35 0.35 0.34 0.36

Mg# 40.1 39.3 40.1 40.2 40.6 39.4 38.2 39.1 38.7

Trace element analyses (parts per million)

Ba 667 686 691 670 682 718 673 688 719

Rb 48 50 49 46 49 45 47 49 50

Sr 309 311 324 307 318 303 317 309 298

Y 34 36 36 36 37 37 38 35 35

Zr 180 181 194 187 189 171 191 176 170

Nb 12.1 12.8 12.4 13.3 14.2 12.4 14.8 12.1 11.2

Ni 13 9 8 6 9 5 9 12 6

Cu 13 12 12 14 16 12 18 17 12

Zn 123 120 123 124 127 121 129 122 121

Cr 7 8 6 6 5 8 5 14 12

Sc 31 33 30 31 32 32 32 31 30

V 330 310 312 321 329 323 334 320 321

2Float.
49

Table 2. Chemical analyses of the Columbia River Basalt Group, Beacon Rock 7.5' quadrangle, Skamania County, Washington.—Continued

Map No. 129 130 131 132 133 134 135 136 137

Field sample No. 05BV-G434 04BV-G321B 09BV-G1383 05BV-G507 05BV-G509 09BV-G1378 05BV-G418 05BV-G430 05BV-G439

Latitude (N) 45°39.843’ 45°38.952’ 45°39.000’ 45°39.374’ 45°38.950’ 45°39.228’ 45°39.665’ 45°40.181’ 45°42.684’

Longitude (W) 122°00.855’ 122°01.511’ 122°00.199’ 122°00.586’ 122°00.371’ 122°00.574’ 122°01.903’ 122°00.933’ 122°00.524’

Map unit Tgo Tgo Tgo Tgo Tgo Tgo Tgo Tgo Tgo

Rock type Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic Basaltic Basaltic

andesite andesite andesite

Analyses as reported (weight percent)

SiO2 54.94 55.96 55.69 55.75 55.73 55.57 55.31 54.50 54.91

TiO2 1.92 1.94 1.94 1.93 1.95 1.93 1.93 1.99 1.96

Al2O3 13.57 13.65 13.68 13.73 13.80 13.66 13.69 13.51 13.73

FeO* 11.19 11.47 11.64 11.23 10.77 11.61 11.35 11.72 10.87

MnO 0.19 0.19 0.19 0.19 0.19 0.19 0.18 0.20 0.19

MgO 3.46 3.39 3.51 3.52 3.47 3.45 3.49 3.60 3.58

CaO 6.94 6.96 7.03 7.02 7.00 7.00 7.02 7.08 7.11

Na2O 3.19 3.06 3.20 3.25 3.16 3.19 3.14 3.11 3.22

K2O 1.72 1.76 1.71 1.76 1.80 1.74 1.69 1.79 1.70

P2O5 0.33 0.34 0.33 0.32 0.33 0.33 0.32 0.35 0.34

Total 97.45 98.70 98.92 98.70 98.20 98.66 98.13 97.86 97.59

Analyses recalculated volatile-free and normalized to 100% with all Fe as FeO (weight percent)

SiO2 56.38 56.70 56.30 56.48 56.76 56.33 56.36 55.70 56.27

TiO2 1.97 1.96 1.96 1.95 1.99 1.96 1.97 2.03 2.00

Al2O3 13.93 13.83 13.83 13.91 14.05 13.84 13.95 13.80 14.07

FeO* 11.48 11.62 11.77 11.38 10.96 11.76 11.57 11.98 11.14

MnO 0.19 0.19 0.19 0.19 0.19 0.19 0.19 0.21 0.19

MgO 3.55 3.43 3.55 3.56 3.54 3.50 3.55 3.68 3.66

CaO 7.12 7.06 7.11 7.11 7.13 7.10 7.16 7.24 7.28

Na2O 3.28 3.10 3.24 3.29 3.21 3.23 3.20 3.18 3.29

K2O 1.77 1.78 1.73 1.79 1.84 1.76 1.73 1.83 1.74

P2O5 0.33 0.34 0.33 0.33 0.34 0.33 0.33 0.36 0.35

Mg# 39.4 38.5 38.7 39.6 40.4 38.4 39.2 39.2 40.8

Trace element analyses (parts per million)

Ba 685 693 723 686 727 674 713 715 669

Rb 51 50 52 52 50 47 49 54 49

Sr 312 307 322 312 312 307 328 319 329

Y 35 35 36 35 36 36 34 36 35

Zr 185 179 181 185 180 180 171 188 175

Nb 13.1 12.7 12.4 13.0 12.2 12.7 12.4 13.9 12.4

Ni 8 12 11 8 10 12 14 10 11

Cu 17 15 15 15 14 17 22 16 23

Zn 123 121 123 122 122 125 121 125 122

Cr 11 14 15 10 14 16 16 11 11

Sc 31 31 31 31 32 32 32 32 32

V 328 322 323 322 324 334 331 328 332
50

Table 2. Chemical analyses of the Columbia River Basalt Group, Beacon Rock 7.5' quadrangle, Skamania County, Washington.—Continued

Map No. 138 139 140 141 142 143 144 145 146

Field sample No. 09BV-G1388 09BV-G1379A 09BV-G1379B 09BV-G1384 09BV-G1382 09BV-G1376 06BV-G572 05BV-G417 02BV-G115A

Latitude (N) 45°39.185’ 45°39.234’ 45°39.235’ 45°39.197’ 45°39.007’ 45°39.085’ 45°42.748’ 45°39.639’ 45°37.962’

Longitude (W) 122°01.630’ 122°00.638’ 122°00.667’ 122°00.255’ 122°00.226’ 122°01.184’ 122°00.473’ 122°01.731’ 122°06.712’

Map unit Tgo Tgo Tgo Tgo Tgo Tgo Tgo Tgww Tgww

Rock type Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Analyses as reported (weight percent)

SiO2 55.93 55.25 55.15 55.73 55.69 54.43 55.11 53.71 54.35

TiO2 1.94 1.96 1.97 1.94 2.03 2.06 1.92 2.00 2.03

Al2O3 13.76 13.75 13.76 13.82 13.54 13.52 13.49 13.43 13.58

FeO* 11.41 11.67 11.72 11.65 11.81 12.24 11.29 12.38 12.44

MnO 0.19 0.19 0.19 0.19 0.18 0.20 0.19 0.20 0.21

MgO 3.44 3.66 3.59 3.73 3.50 3.69 3.48 3.78 3.75

CaO 7.00 7.17 7.11 7.19 6.91 7.27 7.04 7.51 7.52

Na2O 3.14 3.26 3.19 3.26 3.34 3.03 3.15 3.07 3.02

K2O 1.79 1.67 1.76 1.65 1.65 1.89 1.71 1.49 1.63

P2O5 0.34 0.35 0.35 0.34 0.37 0.37 0.34 0.31 0.32

Total 98.80 99.50 99.01 98.70 97.71 97.89 98.84 97.97 98.28

Analyses recalculated volatile-free and normalized to 100% with all Fe as FeO (weight percent)

SiO2 56.53 55.85 55.82 56.02 56.24 55.15 56.40 54.87 54.99

TiO2 1.96 1.98 2.00 1.95 2.05 2.09 1.97 2.04 2.05

Al2O3 13.90 13.90 13.93 13.89 13.68 13.70 13.81 13.72 13.74

FeO* 11.53 11.80 11.87 11.70 11.93 12.40 11.55 12.65 12.59

MnO 0.19 0.19 0.19 0.19 0.18 0.20 0.20 0.21 0.21

MgO 3.48 3.70 3.63 3.75 3.54 3.74 3.56 3.86 3.79

CaO 7.08 7.25 7.20 7.22 6.98 7.36 7.20 7.68 7.61

Na2O 3.18 3.30 3.23 3.27 3.38 3.07 3.22 3.14 3.05

K2O 1.81 1.68 1.78 1.66 1.66 1.91 1.75 1.52 1.65

P2O5 0.34 0.35 0.35 0.34 0.37 0.37 0.35 0.32 0.32

Mg# 39.0 39.7 39.1 40.2 38.3 39.1 39.8 39.0 39.0

Trace element analyses (parts per million)

Ba 692 671 677 654 667 696 656 542 587

Rb 50 49 46 49 48 46 45 40 40

Sr 329 329 308 318 313 323 317 309 314

Y 35 34 37 37 36 34 33 36 35

Zr 177 175 189 186 181 175 171 170 174

Nb 13.3 13.0 13.6 13.5 12.2 11.1 10.5 12.3 12.6

Ni 10 13 9 11 9 9 9 9 14

Cu 22 23 19 21 15 24 19 9 10

Zn 123 121 129 128 124 126 121 124 127

Cr 12 15 12 14 6 13 11 11 12

Sc 32 33 32 32 33 32 32 35 35

V 331 330 340 363 328 334 329 367 368
51

Table 2. Chemical analyses of the Columbia River Basalt Group, Beacon Rock 7.5' quadrangle, Skamania County, Washington.—Continued

Map No. 147 148 149 150 151 152 153 154 155

Field sample No. 14BV-G1702B 05BV-G415 05BV-G416 02BV-G012 05BV-G413 05BV-G424B 06BV-G573 06BV-G576 14BV-G1702A

Latitude (N) 45°38.623’ 45°39.961’ 45°39.715’ 45°39.000’ 45°40.371’ 45°40.655’ 45°43.171’ 45°42.666’ 45°38.614’

Longitude (W) 122°06.593’ 122°01.710’ 122°01.690’ 122°06.642’ 122°01.895’ 122°01.321’ 122°00.172’ 122°00.038’ 122°06.636’

Map unit Tgww Tgsb Tgsb Tgsb Tgsb Tgsb Tgsb Tgsb Tgsb

Rock type Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Analyses as reported (weight percent)

SiO2 54.58 52.75 52.89 53.92 53.35 52.97 52.92 53.01 53.54

TiO2 2.07 1.93 1.94 1.95 1.92 1.90 1.91 1.90 1.97

Al2O3 13.57 13.74 13.73 14.03 13.82 13.70 13.67 13.73 14.00

FeO* 12.35 11.69 11.84 11.76 11.83 11.95 11.64 11.75 11.82

MnO 0.22 0.21 0.21 0.21 0.21 0.21 0.21 0.21 0.22

MgO 3.73 4.73 4.75 4.77 4.79 4.75 4.73 4.78 4.66

CaO 7.51 8.45 8.45 8.58 8.51 8.45 8.40 8.44 8.52

Na2O 2.98 3.00 2.99 2.96 2.95 2.92 2.87 2.91 2.88

K2O 1.70 1.16 1.15 1.25 1.18 1.15 1.24 1.12 1.25

P2O5 0.33 0.32 0.32 0.29 0.28 0.28 0.29 0.28 0.29

Total 99.03 97.97 98.28 99.72 98.83 98.29 97.87 98.14 99.15

Analyses recalculated volatile-free and normalized to 100% with all Fe as FeO (weight percent)

SiO2 55.11 53.84 53.82 54.08 53.99 53.89 54.07 54.01 54.00

TiO2 2.09 1.97 1.97 1.96 1.94 1.94 1.95 1.94 1.99

Al2O3 13.70 14.02 13.97 14.07 13.98 13.94 13.96 13.99 14.12

FeO* 12.47 11.93 12.05 11.79 11.97 12.15 11.90 11.97 11.93

MnO 0.22 0.21 0.21 0.21 0.21 0.21 0.22 0.21 0.22

MgO 3.76 4.83 4.83 4.79 4.84 4.83 4.83 4.87 4.70

CaO 7.58 8.62 8.60 8.60 8.61 8.60 8.58 8.60 8.59

Na2O 3.01 3.06 3.04 2.97 2.98 2.97 2.93 2.97 2.90

K2O 1.72 1.19 1.17 1.25 1.19 1.17 1.27 1.14 1.26

P2O5 0.33 0.33 0.33 0.29 0.29 0.29 0.30 0.29 0.29

Mg# 39.0 45.9 45.7 46.1 46.2 45.5 46.5 46.5 45.5

Trace element analyses (parts per million)

Ba 578 524 480 496 453 456 468 456 491

Rb 41 27 27 28 30 30 30 29 29

Sr 317 305 306 318 314 311 315 317 321

Y 39 36 36 35 33 34 33 33 54.5

Zr 184 160 159 157 154 155 155 156 164

Nb 13.8 12.0 12.3 11.3 10.8 10.9 9.5 10.1 12.1

Ni 5 16 15 20 13 14 13 13 12

Cu 14 28 27 30 26 27 26 26 29

Zn 128 118 118 119 114 115 115 115 120

Cr 8 36 35 25 25 21 22 22 20

Sc 33 38 38 38 37 37 37 37 37

V 367 317 316 342 332 332 332 336 336

2Float.
52

Table 2. Chemical analyses of the Columbia River Basalt Group, Beacon Rock 7.5' quadrangle, Skamania County, Washington.—Cotinued

Map No. 156 157 158 159 160 161 162 163 164

Field sample No. 05BV-G414 05BV-G424A 02BV-G05-2 02BV-G56A 12BV-G1562 12BV-G1563 04BV-G202 08BV-G1067 10BV-G1403

Latitude (N) 45°40.166’ 45°40.776’ 45°40.338’ 45°41.077’ 45°41.173’ 45°41.463’ 45°38.967’ 45°37.600’ 45°37.844’

Longitude (W) 122°01.809’ 122°01.401’ 122°06.090’ 122°02.475’ 122°02.387’ 122°02.592’ 122°06.212’ 122°07.310’ 122°06.786’

Map unit Tgsb Tgsb Tgsb Tgsb Tgsb Tgsb Tgsb Tgsb Tgsb

Rock type Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Analyses as reported (weight percent)

SiO2 53.54 52.97 54.18 54.09 54.00 54.30 54.49 52.92 53.59

TiO2 1.79 1.76 1.83 1.76 1.77 1.82 1.80 1.86 1.76

Al2O3 14.13 14.05 14.18 14.12 14.24 14.02 14.11 14.54 13.97

FeO* 11.30 11.43 11.31 11.14 11.30 11.27 10.94 11.10 10.97

MnO 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20

MgO 5.10 5.13 4.48 4.82 4.80 4.65 4.54 4.32 4.66

CaO 8.77 8.86 8.36 8.39 8.46 8.31 8.32 8.52 8.50

Na2O 2.90 2.85 2.93 3.02 2.93 2.98 2.94 2.82 2.92

K2O 1.23 1.08 1.41 1.31 1.27 1.38 1.43 1.18 1.35

P2O5 0.28 0.27 0.36 0.32 0.33 0.35 0.35 0.35 0.32

Total 99.24 98.61 99.23 99.16 99.31 99.27 99.13 97.80 98.25

Analyses recalculated volatile-free and normalized to 100% with all Fe as FeO (weight percent)

SiO2 53.95 53.72 54.60 54.55 54.38 54.70 54.97 54.11 54.55

TiO2 1.80 1.78 1.85 1.78 1.79 1.83 1.81 1.90 1.79

Al2O3 14.24 14.25 14.29 14.24 14.34 14.12 14.24 14.87 14.22

FeO* 11.39 11.59 11.39 11.23 11.38 11.35 11.04 11.35 11.17

MnO 0.20 0.21 0.20 0.20 0.20 0.20 0.20 0.21 0.20

MgO 5.13 5.20 4.52 4.86 4.84 4.68 4.58 4.41 4.75

CaO 8.84 8.99 8.42 8.46 8.52 8.37 8.40 8.71 8.65

Na2O 2.92 2.89 2.95 3.04 2.95 3.00 2.97 2.88 2.98

K2O 1.24 1.10 1.42 1.33 1.28 1.39 1.45 1.21 1.37

P2O5 0.28 0.27 0.36 0.32 0.33 0.36 0.36 0.36 0.33

Mg# 48.6 48.5 45.6 47.8 47.3 46.6 46.7 45.5 47.6

Trace element analyses (parts per million)

Ba 500 441 575 540 534 561 561 634 577

Rb 29 29 36 33 32 35 34 29 34

Sr 308 308 321.7 310 315 316 300 327 325

Y 35 32 36 32 39 35 34 43 34

Zr 153 149 173 161 165 169 156 173 165

Nb 10.7 10.3 11.8 11.7 11.1 11.5 11.1 10.9 11.1

Ni 19 19 15 19 15 14 10 11 16

Cu 35 36 26 25 26 27 23 26 27

Zn 112 109 124 117 119 120 116 126 118

Cr 46 46 33.8 41 39 34 38 34 35

Sc 36 37 36 35 35 35 34 37 35

V 327 320 299 306 308 299 299 312 310
53

Table 2. Chemical analyses of the Columbia River Basalt Group, Beacon Rock 7.5' quadrangle, Skamania County, Washington.—Continued

Map No. 165 166 167 168 169 170 171 172 173

Field sample No. 06BV-G575 06BV-G574A 05BV-G397 06BV-G574B 10BV-G1402 04BV-G1982 09BV-G1308 10BV-G1404 14BV-G1690A

Latitude (N) 45°42.782’ 45°43.078’ 45°37.570’ 45°42.963’ 45°37.849’ 45°38.046’ 45°37.822’ 45°37.990’ 45°38.030’

Longitude (W) 122°00.076’ 122°00.034’ 122°06.917’ 122°00.128’ 122°06.824’ 122°07.150’ 122°06.857’ 122°06.836’ 122°06.994’

Map unit Tgsb Tgsb Twfsh Twfsh Twfsh Twfss Twfss Twfss Twfss

Rock type Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basalt Basalt Basaltic

andesite

Basaltic

andesite

Basalt Basaltic

andesite

Analyses as reported (weight percent)

SiO2 53.10 53.64 51.74 50.86 50.73 51.36 51.02 50.28 51.76

TiO2 1.73 1.80 3.03 2.95 2.93 2.96 2.96 3.04 3.02

Al2O3 13.97 13.94 13.63 13.33 13.11 13.23 12.85 13.24 13.00

FeO* 10.75 10.97 12.39 13.31 14.15 13.91 14.30 14.31 14.47

MnO 0.19 0.20 0.22 0.23 0.23 0.23 0.31 0.24 0.24

MgO 4.78 4.54 4.22 4.34 4.39 3.95 3.97 4.02 4.03

CaO 8.37 8.26 8.47 8.22 8.25 7.91 7.84 8.01 7.95

Na2O 2.82 2.88 2.85 2.74 2.77 2.76 2.82 2.87 2.85

K2O 1.24 1.47 1.41 1.37 1.38 1.49 1.58 1.35 1.60

P2O5 0.31 0.35 0.58 0.57 0.60 0.68 0.66 0.71 0.70

Total 97.28 98.06 98.53 97.91 98.54 98.48 98.31 98.07 99.61

Analyses recalculated volatile-free and normalized to 100% with all Fe as FeO (weight percent)

SiO2 54.58 54.70 52.51 51.94 51.48 52.15 51.90 51.27 51.96

TiO2 1.78 1.84 3.07 3.01 2.98 3.00 3.01 3.10 3.03

Al2O3 14.36 14.22 13.83 13.62 13.31 13.44 13.07 13.51 13.05

FeO* 11.05 11.19 12.57 13.59 14.36 14.13 14.55 14.59 14.52

MnO 0.20 0.20 0.23 0.23 0.24 0.24 0.32 0.24 0.24

MgO 4.92 4.63 4.28 4.43 4.45 4.01 4.03 4.10 4.05

CaO 8.61 8.43 8.59 8.39 8.37 8.03 7.98 8.17 7.98

Na2O 2.90 2.93 2.89 2.80 2.81 2.81 2.87 2.93 2.86

K2O 1.28 1.50 1.44 1.40 1.40 1.51 1.60 1.37 1.61

P2O5 0.32 0.36 0.59 0.58 0.61 0.69 0.67 0.72 0.70

Mg# 49.0 46.9 42.0 41.1 39.8 37.7 37.2 37.5 37.0

Trace element analyses (parts per million)

Ba 509 556 632 578 551 680 626 724 649

Rb 33 36 34 34 37 33 41 33 40

Sr 312 319 322 318 318 300 310 321 315

Y 32 35 46 43 43 47 46 51 47

Zr 157 166 195 191 194 197 213 223 220

Nb 9.8 11.5 15.4 14.3 14.7 15.1 16.8 16.0 17.4

Ni 15 14 29 22 20 12 18 18 14

Cu 25 23 28 27 29 18 26 22 25

Zn 112 117 144 140 143 146 152 161 155

Cr 39 36 44 40 37.3 19 14 14 14

Sc 35 36 38 37 37 36 37 38 35

V 303 296 429 420 416 400 395 400 392

2Float.
54

Table 2. Chemical analyses of the Columbia River Basalt Group, Beacon Rock 7.5' quadrangle, Skamania County, Washington.—Continued

Map No. 174

Field sample No. 14BV-G1690B

Latitude (N) 45°38.077’

Longitude (W) 122°06.979’

Map unit Twfss

Rock type Basaltic

andesite

Analyses as reported (weight percent)

SiO2 51.63

TiO2 2.99

Al2O3 13.06

FeO* 13.90

MnO 0.23

MgO 3.97

CaO 7.92

Na2O 2.84

K2O 1.65

P2O5 0.69

Total 98.88

Analyses recalculated volatile-free and normalized to 100% with all Fe as FeO (weight percent)

SiO2 52.21

TiO2 3.02

Al2O3 13.21

FeO* 14.06

MnO 0.23

MgO 4.01

CaO 8.01

Na2O 2.87

K2O 1.66

P2O5 0.70

Mg# 37.7

Trace element analyses (parts per million)

Ba 652

Rb 40

Sr 316

Y 48

Zr 222

Nb 17.7

Ni 13

Cu 23

Zn 157

Cr 15

Sc 36

V 385
55

Table 3. Chemical and modal analyses of Quaternary volcanic rocks, Beacon Rock 7.5' quadrangle, Skamania County, Washington.

[X-ray fluorescence analyses. Map No., see figure 16 (sheet 2). Rock-type names assigned in accordance with IUGS system (Le Maitre, 2002) applied to recalculated analyses. FeO*, total Fe calculated as FeO; Mg#, atomic ratio 100 Mg/(Mg+Fe2+) with Fe2+ set to 0.85 x Fetotal. Modal analyses, secondary minerals counted 

as primary mineral replaced. -, not present. Analyses by D.M. Johnson Cornelius and R.M. Conrey at Peter Hooper GeoAnalytical Laboratory of Washington State 

University, Pullman, Washington, using methods described in Johnson and others (1999)]

Map No. 175 176 177 178 179 180 181 182 183

Field sample No. 02BV-G10A 02BV-G10B 02BV-G14 06BV-G5801 02BV-G591 05BV-G407 05BV-G408 05BV-G410 05BV-G412A

Latitude (N) 45°43.942’ 45°43.922’ 45°43.979’ 45°43.866’ 45°40.563’ 45°40.722’ 45°40.326’ 45°39.967’ 45°40.649’

Longitude (W) 122°02.970’ 122°02.942’ 122°02.269’ 122°01.547’ 122°03.229’ 122°02.460’ 122°02.308’ 122°02.187’ 122°02.243’

Map unit Qtcr Qtcr Qtcr Qtcr Qmwc Qmwc Qmwc Qmwc Qmwc

Rock type Basaltic

andesite

Basalt Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Analyses as reported (weight percent)

SiO2 52.23 51.62 52.91 53.22 54.22 53.32 53.84 53.46 53.36

TiO2 1.18 1.23 1.15 1.18 1.23 1.22 1.22 1.28 1.28

Al2O3 17.79 17.68 17.65 17.55 18.15 18.06 18.26 18.65 19.08

FeO* 7.84 8.08 7.85 7.68 7.34 7.57 7.47 7.64 7.57

MnO 0.13 0.14 0.13 0.13 0.12 0.13 0.13 0.13 0.13

MgO 7.17 7.74 7.28 7.60 5.38 6.46 6.00 5.68 5.64

CaO 8.45 9.18 8.82 8.95 8.34 8.45 8.51 8.08 8.44

Na2O 3.32 3.30 3.35 3.35 3.72 3.47 3.66 3.61 3.78

K2O 0.73 0.72 0.76 0.77 0.83 0.83 0.83 0.84 0.71

P2O5 0.22 0.21 0.21 0.22 0.24 0.23 0.23 0.24 0.24

Total 99.06 99.90 100.12 100.65 99.58 99.73 100.16 99.61 100.22

Analyses recalculated volatile-free and normalized to 100% with all Fe as FeO (weight percent)

SiO2 52.73 51.67 52.85 52.88 54.45 53.46 53.75 53.67 53.24

TiO2 1.19 1.23 1.15 1.17 1.24 1.22 1.22 1.28 1.28

Al2O3 17.96 17.69 17.63 17.44 18.23 18.11 18.23 18.72 19.04

FeO* 7.91 8.09 7.84 7.63 7.37 7.59 7.46 7.67 7.55

MnO 0.13 0.14 0.13 0.13 0.12 0.13 0.13 0.13 0.13

MgO 7.24 7.75 7.27 7.55 5.41 6.48 5.99 5.71 5.62

CaO 8.53 9.19 8.81 8.89 8.38 8.47 8.50 8.11 8.42

Na2O 3.35 3.31 3.35 3.33 3.74 3.48 3.65 3.63 3.77

K2O 0.74 0.72 0.76 0.77 0.83 0.84 0.83 0.85 0.71

P2O5 0.22 0.21 0.21 0.22 0.24 0.23 0.23 0.24 0.24

Mg# 66.0 66.8 66.0 67.3 60.7 64.2 62.7 61.0 60.9

Modes (volume percent)

Plagioclase -- -- -- -- -- 0.1 -- -- --

Clinopyroxene -- -- -- -- 0.7 1.0 0.2 trace trace

Orthopyroxene -- -- -- -- 0.6 0.3 0.4 0.3 0.1

Olivine -- -- -- -- -- -- -- -- --

Fe-Ti Oxide -- -- -- -- -- -- -- -- --

Hornblende -- -- -- -- -- -- -- -- --

Biotite -- -- -- -- -- -- -- -- --

Quartz -- -- -- -- -- -- -- -- --

Groundmass 100.0 100.0 100.0 100.0 98.7 98.6 99.4 99.7 99.9

No. points

 counted

-- -- -- -- 850 850 850 850 850

Texture (rock/

 groundmass)

aphyric/

intergranular

seriate/

trachytic

aphyric/

intergranular

aphyric/

intergranular

microphyric/

intergranular

microphyric/

trachytic

microphyric/

trachytic

microphyric/

trachytic

aphyric/

trachytic

Trace element analyses (parts per million)

Ba 182 179 176 196 144 146 143 150 141

Rb 5 5 6 6 4 4 4 4 3

Sr 620 684 631 605 733 671 696 740 732

Y 18 18 18 17 15 16 16 16 17

Zr 123 113 121 119 120 115 117 120 119

Nb 9.2 8.6 8.3 7.8 7.4 7.2 6.7 7.0 6.7

Ni 138 140 144 144 83 106 89 83 75

Cu 122 66 47 71 66 82 61 69 58

Zn 76 73 72 70 79 72 75 77 76

Cr 202 242 250 254 141 197 174 125 115

Sc 25 25 25 26 19 23 22 20 21

V 190 199 187 195 170 181 174 171 174

1

Block in landslide deposit.
56

Table 3. Chemical and modal analyses of Quaternary volcanic rocks, Beacon Rock 7.5' quadrangle, Skamania County, Washington.—

Continued

Map No. 184 185 186 187 188 189 190 191 192

Field sample No. 05BV-G422 05BV-G342 05BV-G345 05BV-G423 05BV-G343 02BV-G65 09BV-G1387 04BV-G3182 09BV-G1389

Latitude (N) 45°41.057’ 45°40.800’ 45°40.746’ 45°41.184’ 45°40.995’ 45°41.281’ 45°39.172’ 45°38.773’ 45°38.859’

Longitude (W) 122°02.854’ 122°02.726’ 122°03.199’ 122°02.825’ 122°03.255’ 122°03.092’ 122°01.712’ 122°01.717’ 122°01.663’

Map unit Qmwc Qmwc Qmwc Qmwc Qmwc Qmwc Qmwc Qmwc Qmwc

Rock type Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Analyses as reported (weight percent)

SiO2 53.44 52.81 53.33 53.28 52.57 52.11 53.51 53.87 53.98

TiO2 1.27 1.25 1.25 1.19 1.20 1.20 1.15 1.16 1.19

Al2O3 19.04 18.27 17.73 17.23 17.21 17.59 17.84 18.90 18.85

FeO* 7.60 7.39 7.69 7.52 7.60 7.82 7.62 7.54 7.75

MnO 0.13 0.12 0.13 0.13 0.13 0.14 0.13 0.13 0.13

MgO 5.64 5.50 6.79 7.75 7.70 7.69 6.94 5.36 5.35

CaO 8.13 8.00 8.65 9.01 8.70 8.67 8.68 8.38 8.32

Na2O 3.69 3.58 3.48 3.39 3.32 3.24 3.55 3.69 3.75

K2O 0.80 0.79 0.85 0.84 0.87 0.74 0.79 0.83 0.85

P2O5 0.23 0.23 0.24 0.23 0.25 0.24 0.23 0.24 0.25

Total 99.98 97.94 100.14 100.56 99.54 99.45 100.45 100.08 100.42

Analyses recalculated volatile-free and normalized to 100% with all Fe as FeO (weight percent)

SiO2 53.45 53.92 53.26 52.98 52.81 52.40 53.27 53.82 53.76

TiO2 1.27 1.27 1.25 1.18 1.20 1.21 1.15 1.16 1.18

Al2O3 19.05 18.66 17.71 17.13 17.29 17.69 17.76 18.88 18.77

FeO* 7.60 7.54 7.68 7.48 7.64 7.87 7.59 7.53 7.72

MnO 0.13 0.12 0.13 0.13 0.13 0.14 0.13 0.13 0.13

MgO 5.64 5.62 6.78 7.71 7.73 7.73 6.91 5.36 5.33

CaO 8.13 8.17 8.64 8.96 8.74 8.71 8.64 8.37 8.29

Na2O 3.70 3.66 3.48 3.37 3.33 3.25 3.53 3.69 3.73

K2O 0.80 0.81 0.84 0.83 0.87 0.75 0.79 0.83 0.85

P2O5 0.23 0.23 0.24 0.23 0.25 0.24 0.23 0.24 0.25

Mg# 60.9 61.5 64.9 68.3 68.1 67.5 65.5 59.8 59.1

Modes (volume percent)

Plagioclase -- -- -- -- -- -- -- -- --

Clinopyroxene trace 0.5 0.1 0.5 0.3 trace 0.1 -- --

Orthopyroxene trace 0.2 0.1 1.2 0.3 trace trace -- --

Olivine -- -- -- -- -- -- -- -- --

Fe-Ti Oxide -- -- -- -- -- -- -- -- --

Hornblende -- -- -- -- -- -- -- -- --

Biotite -- -- -- -- -- -- -- -- --

Other -- -- -- -- -- -- -- -- --

Groundmass 100.0 99.3 99.8 98.3 99.4 100.0 99.9 100.0 100.0

No. points

 counted

850 850 850 850 850 -- 850 -- --

Texture (rock/

 groundmass)

aphyric/

trachytic

microphyric/

trachytic

aphyric/

trachytic

microphyric/

trachytic

microphyric/

intergranular

microphyric/

trachytic

microphyric/

trachytic

aphyric/

trachytic

aphyric/

trachytic

Trace element analyses (parts per million)

Ba 140 130 145 153 167 171 163 160 167

Rb 3 5 5 6 5 4 7 6 8

Sr 735 716 650 623 582 616 602 641 683

Y 17 15 17 16 17 18 17 17 18

Zr 117 115 123 114 119 124 123 114 130

Nb 7.1 5.5 9.1 9.1 9.9 10.2 9.3 7.7 9.5

Ni 77 84 112 146 152 156 126 75 73

Cu 65 40 175 50 56 62 64 31 34

Zn 73 76 72 71 69 76 74 73 76

Cr 115 139 211 332 336 334 241 112 106

Sc 20 18 25 26 27 24 24 20 21

V 172 171 191 184 191 186 182 158 169

2

Block in talus deposit.
57

Table 3. Chemical and modal analyses of Quaternary volcanic rocks, Beacon Rock 7.5' quadrangle, Skamania County, Washington.—

Continued

Map No. 193 194 195 196 197 198 199 200 201

Field sample No. 02BV-G90B 02BV-G116A 05BV-G350 QV01-361 QV03-212A1 QV03-212D1 QV01-372 04BV-G319 05BV-G4112

Latitude (N) 45°39.088’ 45°37.987’ 45°38.252’ 45°38.000’ 45°37.998’ 45°38.014’ 45°38.565’ 45°38.803’ 45°40.029’

Longitude (W) 122°05.588’ 122°06.405’ 122°06.374’ 122°01.345’ 122°01.288’ 122°01.290’ 122°01.943’ 122°01.872’ 122°01.988’

Map unit Qmdc Qmdc Qmdc Qlbr Qlbr Qlbr Qlbr Qlbr Qmlc

Rock type Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basalt Basalt Basalt Basalt Basalt Basaltic

andesite

Analyses as reported (weight percent)

SiO2 52.14 53.57 54.31 52.17 51.32 51.24 50.61 51.42 53.00

TiO2 1.46 1.36 1.45 1.26 1.26 1.31 1.22 1.27 1.13

Al2O3 17.68 17.82 17.97 17.22 17.26 17.30 17.06 17.19 19.81

FeO* 8.10 7.77 7.73 7.93 8.25 8.29 8.27 7.89 7.95

MnO 0.14 0.13 0.13 0.14 0.14 0.14 0.14 0.14 0.14

MgO 6.67 5.82 5.44 6.96 7.76 7.29 7.71 7.32 4.31

CaO 7.97 8.07 7.84 8.79 9.01 8.36 8.92 9.46 7.62

Na2O 3.79 3.94 4.16 3.58 3.46 3.38 3.47 3.45 3.67

K2O 0.91 0.96 1.07 0.88 0.80 0.93 0.81 0.98 0.75

P2O5 0.36 0.35 0.38 0.33 0.33 0.31 0.31 0.33 0.24

Total 99.22 99.80 100.46 99.25 99.60 98.54 98.51 99.45 98.62

Analyses recalculated volatile-free and normalized to 100% with all Fe as FeO (weight percent)

SiO2 52.55 53.68 54.06 52.56 51.52 51.99 51.37 51.70 53.74

TiO2 1.47 1.36 1.44 1.27 1.27 1.33 1.23 1.28 1.14

Al2O3 17.81 17.86 17.89 17.35 17.33 17.56 17.32 17.28 20.09

FeO* 8.17 7.79 7.69 7.99 8.29 8.41 8.40 7.94 8.06

MnO 0.15 0.13 0.13 0.14 0.14 0.15 0.14 0.14 0.14

MgO 6.73 5.83 5.41 7.01 7.79 7.40 7.83 7.37 4.37

CaO 8.03 8.09 7.80 8.86 9.05 8.49 9.05 9.51 7.72

Na2O 3.82 3.94 4.14 3.61 3.48 3.43 3.52 3.47 3.72

K2O 0.92 0.96 1.06 0.89 0.81 0.94 0.82 0.98 0.76

P2O5 0.36 0.35 0.38 0.33 0.33 0.32 0.31 0.33 0.24

Mg# 63.5 61.2 59.5 65.0 66.4 65.2 66.5 66.2 53.6

Modes (volume percent)

Plagioclase -- 0.4 0.1 trace -- -- trace 0.1 --

Clinopyroxene 0.1 -- -- -- -- -- -- -- --

Orthopyroxene -- -- -- -- -- -- -- -- --

Olivine 3.7 4.5 4.0 4.7 5.7 6.4 5.6 8.5 --

Fe-Ti Oxide -- -- -- -- -- -- -- -- --

Hornblende -- -- -- -- -- -- -- -- --

Biotite -- -- -- -- -- -- -- -- --

Other -- -- -- -- -- -- -- -- --

Groundmass 96.2 95.1 95.9 95.3 94.3 93.6 94.4 91.4 100.0

No. points

 counted

850 850 850 810 850 850 850 850 --

Texture (rock/ porphyritic/

trachytic

porphyritic/

trachytic

porphyritic/

trachytic

microphyric/

trachytic

microphyric/

trachytic

seriate/

trachytic

seriate/

trachytic

seriate/

trachytic

microseriate/

 groundmass) trachytic

Trace element analyses (parts per million)

Ba 282 273 299 420 451 361 376 484 146

Rb 8 9 11 8 6 9 5 8 5

Sr 610 624 630 875 918 714 811 886 709

Y 23 22 23 21 21 21 21 20 17

Zr 165 165 172 135 139 133 138 125 119

Nb 18.3 16.4 19.0 10.8 10.4 8.1 10.8 9.0 6.3

Ni 132 96 87 131 165 167 168 129 32

Cu 50 53 23 54 51 57 34 28 140

Zn 85 246 86 83 87 81 81 72 74

Cr 186 133 110 229 271 268 287 262 41

Sc 22 20 21 22 24 26 24 27 16

V 172 166 158 187 190 197 187 202 144 1

Block in landslide deposit.

2

Block in talus deposit.
58

Table 3. Chemical and modal analyses of Quaternary volcanic rocks, Beacon Rock 7.5' quadrangle, Skamania County, Washington.—

Continued

Map No. 202 203 204 205 206 207 208 209 210

Field sample No. 05BV-G420 QV02-64A QV02-64B 03BV-G120 QV97-13A QV97-14 QV98-17 QV01-35B QV03-211B2

Latitude (N) 45°39.793’ 45°38.558’ 45°38.446’ 45°40.073’ 45°39.419’ 45°39.894’ 45°37.699’ 45°37.612’ 45°37.654’

Longitude (W) 122°02.014’ 122°07.065’ 122°07.252’ 122°07.225’ 122°07.453’ 122°07.384’ 122°01.313’ 122°01.195’ 122°01.146’

Map unit Qmlc Qmum Qmum Qmus Qmbc Qmhs Qmbr Qmbr Qmbr

Rock type Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Basaltic

andesite

Analyses as reported (weight percent)

SiO2 53.77 51.81 51.99 54.47 53.20 52.68 55.36 55.09 53.93

TiO2 1.16 1.18 1.19 0.93 1.21 1.21 1.12 1.13 1.14

Al2O3 19.91 18.06 18.25 17.24 17.73 18.29 17.93 17.93 17.68

FeO* 7.96 7.68 7.37 7.45 7.36 7.36 7.16 7.27 7.03

MnO 0.14 0.12 0.13 0.13 0.12 0.12 0.12 0.12 0.12

MgO 4.49 6.51 6.63 6.77 5.56 4.98 5.05 4.97 4.57

CaO 7.61 7.89 8.28 8.31 8.03 7.20 7.61 7.52 7.59

Na2O 3.76 3.94 3.97 3.77 3.85 3.72 4.16 4.22 4.22

K2O 0.79 0.68 0.60 0.57 1.21 1.38 0.86 0.85 0.84

P2O5 0.25 0.28 0.29 0.22 0.33 0.32 0.27 0.27 0.29

Total 99.84 98.16 98.70 99.85 98.60 97.25 99.65 99.36 97.40

Analyses recalculated volatile-free and normalized to 100% with all Fe as FeO (weight percent)

SiO2 53.86 52.78 52.68 54.55 53.96 54.17 55.56 55.44 55.37

TiO2 1.16 1.20 1.20 0.93 1.22 1.24 1.12 1.14 1.17

Al2O3 19.94 18.40 18.49 17.26 17.98 18.80 17.99 18.04 18.16

FeO* 7.97 7.82 7.47 7.46 7.46 7.57 7.19 7.32 7.21

MnO 0.14 0.13 0.13 0.13 0.12 0.13 0.12 0.12 0.12

MgO 4.50 6.64 6.72 6.78 5.64 5.12 5.07 5.00 4.69

CaO 7.62 8.04 8.39 8.33 8.15 7.40 7.64 7.57 7.80

Na2O 3.77 4.02 4.02 3.77 3.90 3.82 4.17 4.24 4.33

K2O 0.79 0.70 0.61 0.57 1.23 1.42 0.86 0.85 0.86

P2O5 0.25 0.28 0.29 0.22 0.34 0.33 0.27 0.28 0.29

Mg# 54.2 64.5 65.7 65.6 61.7 59.3 59.1 57.4 58.4

Modes (volume percent)

Plagioclase -- trace 0.1 -- -- -- -- -- 0.3

Clinopyroxene -- -- -- -- 3.7 -- trace 0.1 0.3

Orthopyroxene -- -- -- -- -- -- -- -- --

Olivine -- 4.4 3.5 4.8 3.9 -- 0.5 1.8 1.4

Fe-Ti Oxide -- -- -- -- -- -- -- -- --

Hornblende -- -- -- -- -- -- -- -- --

Biotite -- -- -- -- -- -- -- -- --

Other -- -- -- -- -- -- -- -- --

Groundmass 100.0 95.6 96.4 95.2 92.4 100.0 99.5 98.1 98.0

No. points 

 counted

-- 830 775 850 820.0 -- 810 850 850

Texture (rock/

 groundmass)

microseriate/

trachytic

microphyric/

trachytic

microphyric/

trachytic

microphyric/

trachytic

porphyritic/

intersertal

microphyric/

trachytic

~aphyric/

intergranular

sparsely

 phyric/

intergranular

sparsely

 phyric/

trachytic

Trace element analyses (parts per million)

Ba 153 333 337 175 420 571 253 253 255

Rb 5 3 1 5 9 10 10 9 9

Sr 718 874 910 617 1029 860 684 749 691

Y 18 19 17 15 19 22 17 17 17

Zr 122 134 130 102 166 185 147 145 142

Nb 7.8 9.4 7.8 6.7 10.6 10.2 10.6 10.4 10.7

Ni 32 130 131 102 100 71 83 65 63

Cu 25 52 42 60 60 55 27 35 41

Zn 76 79 80 85 85 96 77 87 82

Cr 44 189 195 344 147 110 92 86 80

Sc 18 20 20 21 19 19 17 16 19

V 153 333 337 175 420 571 253 253 255

2

Block in talus deposit.
59

Table 3. Chemical and modal analyses of Quaternary volcanic rocks, Beacon Rock 7.5' quadrangle, Skamania County, Washington.—

Continued

Map No. 211

Field sample No. QV01-403

Latitude (N) 45°40.172’

Longitude (W) 122°07.513’

Map unit Qmhs

Rock type Basaltic

andesite

Analyses as reported (weight percent)

SiO2 54.99

TiO2 1.19

Al2O3 17.74

FeO* 7.09

MnO 0.12

MgO 4.53

CaO 7.48

Na2O 3.96

K2O 1.63

P2O5 0.33

Total 99.06

Analyses recalculated volatile-free and normalized to 100% with all Fe as FeO (weight percent)

SiO2 55.51

TiO2 1.20

Al2O3 17.91

FeO* 7.15

MnO 0.12

MgO 4.57

CaO 7.55

Na2O 4.00

K2O 1.65

P2O5 0.33

Mg# 57.5

Modes (volume percent)

Plagioclase --

Clinopyroxene --

Orthopyroxene --

Olivine --

Fe-Ti Oxide --

Hornblende --

Biotite --

Other --

Groundmass 100.0

No. points

 counted

--

Texture (rock/

 groundmass)

microphyric/

trachytic

Trace element analyses (parts per million)

Ba 509

Rb 20

Sr 973

Y 20 

Zr 183

Nb 10.1

Ni 60

Cu 38

Zn 87

Cr 89

Sc 17

V 162

3

Locality in adjacent Bobs Mountain quadrangle.
60

Table 4. Summary of 40Ar/39Ar incremental-heating age determinations, Beacon Rock 7.5' quadrangle, Skamania County, Washington.

[See figure 17 (sheet 2) for sample locations. Analytical data provided in appendix 1. Ages calculated to a monitor age equivalent to an age of 28.02 Ma for the Fish 

Canyon Tuff sanidine (FCs) standard (Renne and others, 1998), as described in Fleck and others (2014)]

Field sample no. Location (NAD 83) Map unit Rock type Material dated Age (±1σ error) Comments

Latitude (N) Longitude (W)

06BV-G633 45°42.627’ 122°06.590’ Tba Andesite Plagioclase 25.78±0.55 Ma This report

06BV-G700B 45°43.051’ 122°04.564’ Tts Welded tuff Plagioclase 24.30±0.11 Ma This report

06BV-G685 45°40.913’ 122°05.914’ Ttw Welded tuff Plagioclase 24.04±0.06 Ma This report

03BV-G122 45°40.271’ 122°07.166’ Tiah Andesite Plagioclase <23.14±0.18 Ma Maximum age; this report

02BV-G51A 45°44.699’ 122°01.869’ Tt Welded tuff Plagioclase 22.79±0.13 Ma This report

06BV-G650 45°44.829’ 122°04.654’ Tidcr Rhyolite Plagihoclase 22.60±0.06 Ma Tis report

05BV-G437 45°41.372’ 122°00.772’ Tha Andesite Plagioclase 21.94±0.06 Ma This report

02BV-G52A 45°43.317’ 122°03.664’ Thba Basaltic andesite Plagioclase 21.59±0.31 Ma This report

04BV-G281A 45°38.535’ 122°00.919’ Teha Andesite Plagioclase 19.50±0.05 Ma This report

02BV-G55B 45°41.376’ 122°02.999’ Tecu Dacite block Plagioclase 19.48±0.04 Ma This report

06BV-G599 45°43.463’ 122°02.522’ Tepa Andesite Plagioclase 20.07±0.03 Ma This report

05BV-G345 45°40.746’ 122°03.199’ Qmwc Basaltic andesite Groundmass 1,068±8 ka Fleck and others, 2014

1,050±6 ka This report

1,056±9 ka Weighted mean age

02BV-G10A 45°43.942’ 122°02.970’ Qtcr Basaltic andesite Groundmass 1,065±6 ka Fleck and others, 2014

1,066±11 ka This report

1,065±5 ka Weighted mean age

05BV-G420 45°39.793’ 122°02.014’ Qmlc Basaltic andesite Groundmass 1,061±17 ka Corrected from Fleck and

 others, 2014

02BV-G90B 45°39.088’ 122°05.588’ Qmdc Basaltic andesite Groundmass 821±24 ka Fleck and others, 2014

02BV-G116A 45°37.987’ 122°06.405’ Qmdc Basaltic andesite Groundmass 879±7 ka Preferred age of unit; Fleck

 and others, 2014

QV01-36 45°38.000’ 122°01.345’ Qlbr Basaltic andesite Groundmass 498±7 ka Fleck and others, 2014

QV98-17 45°37.700’ 122°01.313’ Qmbr Basaltic andesite Whole rock 55.8±6.1 ka Fleck and others, 2014

QV98-17 45°37.700’ 122°01.313’ Qmbr Basaltic andesite Whole rock 84.2±5.1 ka Considered spurious; Fleck 

 and others, 2014

QV01-35B 45°37.612’ 122°01.195’ Qmbr Basaltic andesite Groundmass 58.4±6.4 ka Preferred age of unit; Fleck 

 and others, 2014
61

Table 5. Paleomagnetic data for Columbia River Basalt flows and Quaternary volcanic rocks, Beacon Rock 7.5' quadrangle, Skamania 

County, Washington.

[See figure 18 (sheet 2) for sample locations. λSITE and φSITE are latitude and west longitude of site (degrees). Strike/Dip, strike and dip of host rock (strike 90° 

counterclockwise of dip direction). Ic, Dc, paleomagnetic directions corrected for structural tilt. N/N0 is number of samples analyzed/number of samples collected. 

R is vector sum of N unit vectors. k is the concentration parameter (Fisher, 1953) for means calculated using Bingham statistics, R is not calculated. α95, the radius 

of 95% confidence (degrees). λP and φP, latitude and east longitude of virtual geomagnetic pole (degrees) corrected for structural tilt. --, not applicable. Analyses by 

J.T. Hagstrum, U.S. Geological Survey, Menlo Park, California, using techniques described in Fleck and others (2014). Latitudes and longitudes in North American 

Datum of 1983 coordinates]

Fieldsample No Laboratory 

Sample No.

Map 

Unit

λSITE φSITE Strike/

Dip

lc Dc N/N0 R k α95 λP φP

02BV-G115B CB9199 Tgo 45.63295 -122.11005 135/2 52.0 18.5 8/8 7.9692 227 3.7 70.7 3.9

02BV-G116B T5065 Tgo 45.63257 -122.10646 135/2 68.2 18.0 7/8 5.9685 159 5.3 76.8 295.7

05BV-G429B T6239 Tgo 45.65935 -122.01201 135/2 71.5 349.7 6/8 5.8874 44 11.3 77.7 210.5

05BV-G507 T6247 Tgo 45.65624 -122.00977 135/2 75.9 43.2 8/8 7.8990 69 6.7 60.2 276.2

05BV-G398A T6311 Tgo 45.63022 -122.10998 135/2 52.0 89.1 7/8 6.9555 135 5.2 23.2 304.3

04BV-G321A1 -- Tgo 45.64830 -122.02480 135/2 61.8 85.5 -- -- -- -- -- --

04BV-G3221 -- Tgo 45.64704 -122.02432 135/2 70.8 17.7 -- -- -- -- -- --

QV03-211A T3374 Qmbr 45.62638 -122.01972 0/0 72.0 27.3 9/9 8.9597 198 3.7 69.7 284.2

QV03-212A2 T3383 Qlbr 45.63361 -122.02183 0/0 54.7 353.6 8/8 7.9061 75 6.5 78.5 85.4

02BV-G10A 4T048 Qtcr 45.73237 -122.04950 0/0 46.8 19.9 7/8 6.7788 27 11.8 66.3 9.6

02BV-G90A 4T040 Qmdc 45.65117 -122.09190 0/0 -66.4 174.9 8/8 7.9801 352 3.0 85.3 192.7

04BV-G319 T6319 Qmlc 45.64672 -122.03120 0/0 64.9 12.6 8/8 7.9653 202 3.9 81.2 315.5

05BV-G345 T6217 Qmwc 45.68325 -122.05425 0/0 48.5 28.1 8/8 7.9785 326 3.1 62.7 354.6

03BV-G1201 -- Qmus 45.66788 -122.12042 0/0 2.7 4.8 -- -- -- -- -- --

1 Oriented hand specimen; considered reliable for polarity only.

2

 Locality is in large slump block; considered reliable for polarity only.