2011

U.S. Department of the Interior

U.S. Geological Survey

Lidar-Revised Geologic Map of the Uncas 7.5’

Quadrangle, Clallam and Jefferson Counties, Washington

By Rowland W. Tabor, Peter J. Haeussler, Ralph A. Haugerud, and Ray E. Wells

Pamphlet to accompany

Scientific Investigations Map 3160
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Viginia: 2011

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Suggested citation:

Tabor, R.W., Haeussler, P.J., Haugerud, R.A., and Wells, R.E., 2011, Lidar-revised geologic map of the Uncas 7.5’ quadrangle, Clallam and Jefferson Counties, Washington: U.S. Geological Survey Scientific Investigations Map 3160, 25 p., 

scale 1:250,000 [http://pubs.usgs.gov/sim/3160/].

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endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual 

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iii

Contents

About Revision from Lidar Topography .....................................................................................................1

Acknowledgments ........................................................................................................................................1

Geologic Summary ........................................................................................................................................1

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

Stratigraphy ..........................................................................................................................................2

Bedrock .........................................................................................................................................2

Consolidated Deposits ...............................................................................................................3

Unconsolidated Deposits ..........................................................................................................3

Structural History .................................................................................................................................4

DESCRIPTION OF MAP UNITS ...................................................................................................................5

SURFICIAL DEPOSITS .........................................................................................................................5

NONGLACIAL DEPOSITS ...........................................................................................................5

GLACIAL DEPOSITS ....................................................................................................................6

SURFICIAL DEPOSITS AND BEDROCK .............................................................................................7

BEDROCK ...............................................................................................................................................7

References Cited ...........................................................................................................................................8

About Revision from Lidar Topography

In 2000 and 2001, the Puget Sound Lidar Consortium 

obtained 1 pulse/m2 lidar data for about 65 percent of the 

Uncas 7.5′ quadrangle. For a brief description of LIDAR (LIght 

Detection And Ranging) and this data acquisition program, 

see Haugerud and others (2003). This map combines geologic 

interpretation (mostly by Haugerud and Tabor) of the 6-ft (2-m) 

lidar-derived digital elevation model (DEM) with the geology 

depicted on the Preliminary Geologic Map of the Uncas 7.5′ 

Quadrangle, Clallam and Jefferson Counties, Washington, by 

Peter J. Haeussler, and others (1999).

Interpretation of the lidar DEM began with a geomorphic 

interpretation following the methods outlined by Haugerud 

(2009). We did no additional field work to make this map, and, 

although the lidar DEM provides little insight to the bedrock 

geology, we revised the extent of bedrock outcrops based on 

geomorphic interpretation of the DEM.

We also revised the stratigraphy of the Tertiary bedrock 

units to conform to changes in the Eocene time scale (Gradstein and others, 2005) and revised the interpretation of some 

structure and identification of a major outcrop area. We also 

smoothed a few lengthy buried faults and moved them slightly 

to accommodate changes in the revised bedrock extent. We 

added a cross section to illustrate the revised structural interpretation.

The absolute positions of geographic features in Haeussler 

and others (1999), based on the 1953 (revised 1973) 1:24,000-

scale topographic map, do not match well with the lidar DEM. 

For this reason, contacts adopted directly from the original map 

are shown as approximately located. We reproduced structural 

data directly from Haeussler and others (1999) but moved a few 

features slightly to accommodate the revised base. For instance, 

a bedding attitude that is west of a creek on the old map might 

plot east of the creek on the new base; we have moved it to the 

west side.

Landforms revealed by lidar for most of the Uncas quadrangle are different in character than those portrayed on the 

1953 topographic map. Alluvial flats—both Holocene and late 

Pleistocene—are easily distinguished from the fluted, rolling 

glaciated terrain of most of the quadrangle. Most stream valleys 

are steep-sided canyons with a distinct break in slope to more 

gentle terrain of the interfluves. This canyon form is depicted on 

the 1953 topographic map for Snow Creek and the lower ends 

of major drainages but not for other valleys.

Much of the text, especially in the descriptions of materials, in this revision has been taken verbatim from Haeussler and 

others (1999) with only minor editing.

There are no digital representations of drainage or road 

networks that are sufficiently accurate to match the lidar DEM. 

We have digitized both from the DEM. Many logging roads are 

apparent on the DEM that are not shown on other large-scale 

road maps. These roads may not be maintained, and we referred 

to 1:24,000-scale road depictions on the 1953 topographic map 

and Google Earth to select the roads we digitized. We selected 

streams to digitize based on 1:24,000-scale drainage depictions. 

Acknowledgments

We benefited greatly from reviews by Dori Kovanen and 

Russell Evarts. The metadata accompanying this report was 

much improved by Karen Wheeler.

Geologic Summary

Introduction

The Uncas quadrangle in the northeastern Olympic Peninsula covers the transition from the accreted terranes of the 

Olympic Mountains on the west to the Tertiary and Quaternary 

basin fills of the Puget Lowland to the east. Elevations in the 

map area range from sea level at Port Discovery to 4,116 ft 

(1,255 m) on the flank of the Olympic Mountains to the southwest. Previous geologic mapping within and marginal to the 

Uncas quadrangle includes reports by Cady and others (1972), 

Brown and others (1960), Tabor and Cady (1978a), Yount and 

Gower (1991), and Yount and others (1993). Paleontologic 

and stratigraphic investigations by University of Washington 

graduate students (Allison, 1959; Thoms, 1959; Sherman, 1960; 

Hamlin, 1962; Spencer, 1984) also encompass parts of the 

Uncas quadrangle. Haeussler and Wells mapped in February 

1998, following preliminary mapping by Yount and Gower in 

1976 and 1979. The description of surficial map units follows 

Yount and others (1993) and Booth and Waldron (2004). Bedrock map units are modified from Yount and Gower (1991) and 

Spencer (1984). We used the geologic time scale of Gradstein 

and others (2005).

The Uncas quadrangle lies in the forearc of the Cascadia 

subduction zone, about 6.25 mi (10 km) east of the Cascadia 

accretionary complex exposed in the core of the Olympic 

Mountains (Tabor and Cady, 1978b). Underthrusting of the 

accretionary complex beneath the forearc uplifted and tilted 

eastward the Coast Range basalt basement and overlying 

marginal basin strata, which comprise most of the rocks of the 

Uncas quadrangle. The Eocene submarine and subaerial tholeiitic basalt of the Crescent Formation on the Olympic Peninsula is thought to be the exposed mafic basement of the Coast 

Range, which was considered by Snavely and others (1968) to 

be an oceanic terrane accreted to the margin in Eocene time. 

In this interpretation, the Coast Range basalt terrane may have 

originated as an oceanic plateau or by oblique marginal rifting, 

but its subsequent emplacement history was complex (Wells 

and others, 1984). Babcock and others (1992) and Haeussler 

and others (2003) favor the interpretation that the basalts were 

the product of an oceanic spreading center interacting with the 

continental margin. Regardless of their origin, onlapping strata 

in southern Oregon indicate that the Coast Range basalts were 

attached to North America by 50 Ma; but on southern Vancouver Island, where the terrane-bounding Leech River Fault is 

exposed, Brandon and Vance (1992) concluded that suturing to 

North America occurred in the broad interval between 42 and 

24 Ma. After emplacement of the Coast Range basalt terrane, 

the Cascadia accretionary wedge developed by frontal accretion 
2

and underplating (Tabor and Cady, 1978b; Clowes and others, 

1987). Domal uplift of the part of the accretionary complex 

beneath the Olympic Mountains occurred after ~18 Ma (Brandon and others, 1998). Continental and alpine glaciation during 

Quaternary time reshaped the uplifted rocks of the Olympic 

Mountains.

Stratigraphy

Bedrock

The oldest rocks in the quadrangle belong to the lower(?), 

middle, and late(?) Eocene Crescent Formation, which consists 

of submarine and subaerial basalt flows, breccia, and interbedded sedimentary rocks. Pillow lavas, pillow and lapilli breccia, 

amygdaloidal lava flows, dark-gray calcareous mudstone, basaltic siltstone, and sandstone make up the lower submarine unit 

(unit Tcs) exposed in the southwest corner of the quadrangle. 

Calcareous nannoplankton from interbedded turbidite siltstone 

and sandstone at the top of the submarine unit at Bon Jon Pass, 

1 km west of the quadrangle, are referred to the CP 11 zone, or 

early Eocene (about 50 Ma), by D. Bukry (written commun., 

1998). Most of the upper part of the Crescent Formation 

consists of amygdaloidal basaltic flows and breccia (units Tcf, 

Tcb, Tcfb, and Tcs). The breccia is monolithologic, and the 

flows in some places have rubbly reddish weathering surfaces 

that indicate subaerial eruption. However, about 9 mi (14.4 

km) south of the map area on the west shore of Quilcene Bay, 

Spencer (1984) noted massive basaltic flows interbedded with 

pillow lavas, indicating complex depositional environments 

typical of oceanic islands. No ages have been determined from 

the subaerial part of the Crescent Formation within the quadrangle. In the Bremerton area, 25 mi (40 km) to the south, Duncan 

(1982) reported an 40Ar/39Ar age of 55.3±3.1 Ma, and Babcock 

and others (1992) report an 40Ar/39Ar age of 50.3±1.5 Ma. East 

of the quadrangle, Yount and Gower (1991) report foraminifera 

referable to the Ulatisian benthic foraminferal stage (early 

middle Eocene) within marine siltstones and sandstones of the 

Crescent Formation.

Marine basaltic conglomerate, sandstone, and mudstone 

(unit Tebs) overlie the basaltic flows and breccia of the Crescent Formation. These basaltic sediments are found in the northeastern part of the quadrangle along the Salmon Creek drainage 

and along Highway 101 on the west side of Port Discovery. 

Spencer (1984) measured the section in Salmon Creek as more 

than 232 m thick and argued that its rocks are sourced from 

the Crescent Formation. Spencer (1984) found that the contact 

with the underlying basalt was irregular and undulatory, with 

limestone pods in basaltic breccia at the base of the section, 

suggesting some erosion between deposition of the two units. 

Subaerial basalt flows and breccia stratigraphically beneath the 

marine basaltic sandstones indicate subsidence following the 

end of basaltic volcanism. The basaltic sediments contain upper 

Ulatisian to lower Narizian benthic foraminiferal stage taxa and 

mollusks indicative of middle Eocene age. Macrofossils, microfossils, and trace fossils indicate subtropical to temperate water 

temperatures at neritic water depths <200 m (Spencer, 1984). 

Based on fossils, Tabor and Cady (1978a) cite a late Eocene age 

for basaltic sandstones now included in this unit, but subsequent 

revision of the time scale makes the age more likely middle 

Eocene (Gradstein and others, 2005).

The middle and late Eocene Aldwell Formation (unit Ta) 

crops out in the southeastern part of the quadrangle, near the 

Snow Creek road. Regionally, the Aldwell Formation consists 

of thin-bedded slope mudstone and thin-bedded turbidite sandstone (Tabor and Cady, 1978a). Massive to well-bedded basaltic 

conglomerate and lithic sandstone occur locally. Along Quilcene 

Bay, south of the map area, basal Aldwell Formation interfingers with basaltic sedimentary rocks (Tebs?), and the contact 

is placed where coarse, massive to thick-bedded volcaniclastic rocks give way to the more thinly bedded sandstone and 

siltstone of the Aldwell (Spencer, 1984, p. 24–25). The upper 

contact of the Aldwell Formation is not exposed in the map 

area, but it may be unconformable based on relations with the 

overlying Lyre Formation elsewhere on the Olympic Peninsula. 

Foraminifera from the Aldwell Formation are referable to the 

lower Narizian or upper Ulatisian benthic foraminiferal stages. 

At Quilcene Bay, south of the map area, calcareous nannoplankton from the Aldwell are referable to calcareous nannofossil CP 

14a zone (upper middle Eocene, about 42–40 Ma; D. Bukry, 

written commun., 1998). These fossil data indicate that the 

Aldwell Formation is coeval with the middle Eocene basaltic 

sedimentary rocks (Tebs) described above. The paleoecology 

of faunas collected from the Aldwell Formation indicates cool 

to cold water temperatures at bathyal depths (Spencer, 1984), 

which is quite different from the neritic water depths and warm 

temperatures during deposition of basaltic sedimentary rocks 

overlying the Crescent Formation.

The upper Eocene Lyre Formation (unit Tl) is about 300 

m thick in the map area (Spencer, 1984) and consists mostly of 

conglomerate, with some interbedded andesite tuff and breccia 

(unit Tlav). Locally, the conglomerates are either clast or matrix 

supported. Commonly there is no preferred orientation for the 

clasts, which suggests deposition by debris flows in a submarine 

fan environment. In the southern part of the Uncas quadrangle, 

the Lyre Formation lies on the Aldwell Formation. Along the 

west edge of the quadrangle and northeast of the quadrangle, the 

Lyre Formation lies directly on the Crescent Formation. Spencer (1984) observed the contact in the upper part of the Snow 

Creek drainage and described it as irregular and undulatory. Near 

the contact northeast of the quadrangle, the formation contains 

basaltic cobble and boulder conglomerate. From these observations, Spencer (1984) inferred that there was considerable relief 

on the surface beneath the Lyre Formation. Spencer (1984) 

measured 418 paleocurrent indicators in the Lyre Formation and 

found a consistent south-southwest direction of transport. The 

base of the section has a higher percentage of basaltic clasts, 

which indicates erosion of the underlying Crescent Formation. 

Chert and phyllite clasts are also common in the conglomerates 

(units Tllc and Tluc), and Snavely and others (1993) and Garver 

and Brandon (1994) concluded that similar clasts in the Lyre 

Formation at the northwest tip of the Olympic Peninsula were 

derived from Mesozoic sediments on southern Vancouver Island.

There is no direct fossil control on the age of the Lyre 

Formation in the map area, but to the west it contains foraminifera referable to the upper benthic foraminiferal Narizian stage 
3

(Snavely and others, 1993). The interbedded andesitic volcanic 

rocks (unit Tlav), near the bottom of the unit on Mount Zion, 

just west of the quadrangle, may represent post-Crescent Formation rejuvenation of Coast Range volcanism or may represent 

the distal parts of the earliest lavas derived from the Cascade 

Volcanic Arc. Hornblende separates from an andesite boulder in 

this unit collected just west of the quadrangle boundary, near the 

summit of Mount Zion and near the bottom of the Lyre Formation, yielded K-Ar ages of 36.4±1.3 and 42.1±6.9 Ma (Engels 

and others, 1976; ages corrected for new constants). Although 

Spencer’s (1984) biostratigraphic reasoning indicates that the 

unit is middle Eocene in age, the analytically more secure 

isotopic age of about 36 Ma suggests that the Lyre Formation in 

the Uncas quadrangle is mostly late Eocene. Possibly correlative silicic volcanic rocks have been described in the Sequim 

quadrangle to the northwest, where they were included in the 

Crescent Formation by Schasse and Logan (1998).

Silty sandstone and sandstone turbidites in the Snow Creek 

drainage form the sandstone of Snow Creek. Spencer first used 

the informal name “sandstone of Snow Creek” and we follow 

his usage. The section in the upper Snow Creek drainage is 

567 m thick; foraminifera are referable to the upper Narizian 

stage and are “indicative of cool to cold water temperatures and 

water depths in the lower bathyal zone” (Spencer, 1984, p. 135). 

Sandstone grains are predominantly chert and lesser quartz 

(Spencer, 1984). Spencer (1984) notes that the sandstone of 

Snow Creek (Tss) and the Lyre Formation are related because 

of a conformable contact and because there is an abundance 

of chert and metamorphic clasts in both units, which indicates 

a similar source area. The sandstone of Snow Creek, at least 

locally, overlies conglomerates of the Lyre Formation, but the 

upper Narizian benthic foraminiferal stage age suggests that it 

is in part contemporaneous with the Lyre Formation. Deposition 

of the sandstone of Snow Creek appears to be localized, because 

it is not found northeast of the quadrangle, where the Townsend 

Shale lies unconformably on the Lyre Formation (Thoms, 

1959).

Yount and Gower (1991) and Tabor and Cady (1978a) map 

the Snow Creek unit as the Twin River Formation of Brown and 

Gower (1958), and Spencer (1984) considers it to be a member 

of the Lyre Formation. We prefer to map it as a separate unit. 

We thus avoid expanding the Lyre Formation, and we avoid 

using the term “Twin River,” which has been used as a Group 

name to refer to several formations that range in age from 

uppermost middle Eocene (upper Narizian) to Miocene time 

(for example, Garver and Brandon, 1994).

The stratigraphic position of the Eocene(?) micaceous sedimentary rocks on the north side of Big Skidder Hill (unit Tems) 

is problematic, because there are no ages from this unit and 

contacts with the adjacent Lyre and Crescent Formations are 

not clear. Massive micaceous siltstone and fine sandstone predominate. They could represent one of three known micaceous 

sequences in the forearc: Eocene Tyee Formation equivalents 

(Sandstone of Scow Bay, Armentrout and Berta, 1977); middle 

and late Eocene Puget Group; or late Oligocene and early Miocene Pysht Formation and the early Miocene Clallam Formation 

(Garver and Brandon, 1994). Because the micaceous sedimentary rocks apparently rest on Crescent Formation and are in an 

upthrown block against the Lyre Formation on Big Skidder Hill, 

they may be one of the pre-Lyre Formation units.

Consolidated Deposits

Overlying all of the Tertiary units are surficial deposits related to the Vashon Stade of the Fraser Glaciation of 

Armstrong and others (1965). The only surficial deposits that 

we found that may predate this stade are indurated colluvial 

deposits on the flank of the Olympic Mountains in the southwestern part of the map area (unit Qci) and possibly deposits 

represented by lidar-mapped pre-Vashon stade till (unit prevt). 

The colluvial deposits lie directly on bedrock. They contain 

locally derived clasts only and, thus, predate the culmination of 

the Vashon-age ice, which delivered considerable amounts of 

foreign debris to all the deposits of its time period.

Unconsolidated Deposits

During the Vashon stade advance, the Vashon ice sheet 

bifurcated northeast of the map area into the Puget lobe, which 

occupied the Puget Lowland, and the Juan de Fuca lobe, which 

occupied the Strait of Juan de Fuca (Waitt and Thorson, 1983). 

The Puget lobe blocked northward drainage from the Puget 

Lowland and formed proglacial lakes that drained southward 

to the Chehalis River valley and thence to the Pacific Ocean. 

The Juan de Fuca lobe presumably blocked drainage of Port 

Discovery (Discovery Bay) before occupying it. Pebbly and 

stony silts (unit Qval) near Port Discovery are interpreted as 

deposits of these proglacial lakes. These sediments are overlain 

by gravel-dominated recessional outwash (unit Qvrg) that was 

deposited by streams and rivers issuing from the front of the 

retreating ice sheet. The ice sheet first covered the map area 

around 18,000–17,500 cal yr B.P., reached its maximum extent 

in southern Puget Sound around 16,950 cal yr B.P. (Porter and 

Swanson, 1998), and covered the region with as much as 1,500 

m of ice (Booth, 1987). Till (unit Qvt) marks the presence of the 

ice sheet in the map area and mantles most surfaces. Abundant 

granitic clasts within the till attest to a northern source, either 

Vancouver Island or the Coast Mountains of British Columbia.

The canyon topography revealed by the lidar DEM suggest 

that the bottom of the thick Vashon stade till blanketing much of 

the terrain should be exposed on the steep canyon walls. Assuming that the till is no more than 10–15 m thick, we have mapped 

a lower contact in most deep canyons and depicted an unknown 

pre-Vashon stade till unit (prevt). In some parts of the canyons, this contact is characterized by a break in slope from the 

erosionally resistant till above more friable sands and gravels. 

Most likely the underlying deposits are advance outwash such 

as Qvas and (or) Qval.

The Puget lobe rapidly retreated from its maximum extent 

and a series of proglacial lakes formed. Initially, the lakes 

drained south to the Chehalis River. The lowermost, and largest, 

of these lakes is known as “Lake Russell” (Bretz, 1910; Thorson, 1989). When the ice front retreated to the latitude of the 

map area around 16,500 cal yr B.P. (Porter and Swanson, 1998), 

a new, lower-elevation outlet to the Strait of Juan de Fuca 
4

was opened. Thorson (1979, 1980) named the younger, northdraining lake “Lake Bretz”. Thorson (1979, 1980) and Waitt and 

Thorson (1983) interpreted the Port Discovery-Crocker LakeLeland Lake valley on the east side of the Uncas quadrangle 

to be the Lake Bretz outlet. Lidar topography does not support 

this interpretation; although such evidence might be covered by 

younger deposits, this valley shows no sign of through-going, 

large-discharge subaerial flow. The Chimacum Valley, about 10 

km east of the Uncas quadrangle, was the outlet of Lake Bretz 

(Haugerud, 2009).

Extensive high alluvial terraces (units Qa1 and Qa2) 

revealed by the lidar DEM probably formed originally by 

ice-marginal drainage flowing south into glacial Lake Russell. 

Most sediment in these terraces was derived from bedrock along 

the mountain front to the west. When the ice retreated enough 

to open drainage to the north and through the Strait of Juan 

de Fuca, the lateral system was disrupted as streams from the 

mountains drained eastward into Snow Creek valley and Port 

Discovery. The major east-draining streams, Salmon Creek, 

Snow Creek, and Andrews Creek, cut canyons through the 

lateral stream deposits, Vashon Stade till (Qvt) that blanketed 

the area, and older outwash deposits and bedrock. The youngest 

terraces (Qa3 and Qa4) may record stages of falling base level 

during late- and post-glacial isostatic rebound.

Details of this terrace history are elusive within the map 

area, but significant isostatic rebound is required to explain 

the presence of glaciomarine deposits at elevations above sea 

level (Thorson, 1989; Dethier and others, 1995). Around Port 

Discovery and in the lower Snow Creek valley, we mapped 

a subtle maximum-elevation, late-glacial marine shoreline at 

elevations of about 121 ft (37 m) to 141 ft (43 m). Thorson 

(1989) observed the late-glacial marine shoreline at an elevation 

of 131 ft (40 m) at Gardiner, about 2 mi (3.2 km) north of the 

map area on the west shore of Discovery Bay, and added 328 

ft (100 m) of global sea level rise since the Late Pleistocene to 

obtain an estimate of 459 ft (140 m) of total isostatic rebound at 

this locale.

The lidar DEM reveals multiple episodes of alluvial fan 

construction, shown by map units Qf1, Qf3, Qf4, and Qf5. 

These deposits may reflect the same changes in base level 

responsible for the terrace development described above, but 

exact correlation, as implied by the numbers in unit labels, with 

the terraces is uncertain. Where the change from a sloping fan to 

an alluvial flat is gradual, the contact between the two deposits 

is somewhat arbitrary, and we give the units the same number 

unless other evidence, such as a local scarp, indicates a greater 

age difference.

Following Vashon stade glaciation, landslides developed 

on some steep hillsides (unit Qls). Perhaps the most interesting 

of these is a landslide about 2.2 mi (3.5 km) south of the head 

of Port Discovery, where a massive jumble of Lyre Formation 

conglomerate, with blocks as large as houses, slumped off the 

east-facing hillside. As a result, the landslide deflected Snow 

Creek eastward. The modern depositional environment includes 

alluvium (unit Qa) and alluvial fan deposits (unit Qf) along 

modern streams and beach deposits (unit Qb) along the shores 

of Port Discovery. Human modification of the land surface is 

mostly seen as artificial fill (unit af) along Highway 101.

Structural History

The structure of the Uncas quadrangle is difficult to interpret, because faults strike north, east, northeast, and northwest. 

Overall, the area has been tilted eastward on the flank of the 

growing Olympic uplift, which was accompanied by folding 

and faulting of the bedrock strata into an eastward-plunging 

synclinorium. Although attitudes in the Crescent Formation 

are similar to those in the Lyre Formation, an unconformable 

contact is inferred from the general absence of the Aldwell 

Formation and the deposition of Lyre Formation conglomerate 

directly on the Crescent Formation. This suggests deformation 

began in late middle Eocene time. In the Cape Flattery area, at 

the northwest tip of the Olympic Peninsula, the Lyre Formation 

overlies the main thrust at the base of the Crescent Formation, 

suggesting that it postdates the beginning of accretionary wedge 

formation (Snavely and others, 1993). On Big Skidder Hill, the 

attitudes in the Lyre Formation are variable. The attitudes of 

the overlying sandstone of Snow Creek generally strike north 

or north-northeasterly, which suggests an angular unconformity between the units. However, Spencer (1984) argues for a 

conformable contact at the base of the sandstone of Snow Creek 

(unit Tss), and outcrops of the Snow Creek unit along Snow 

Creek can be interpreted as being near the nose of an eastwardplunging syncline. The eastward plunge of the fold along upper 

Snow Creek, the eastward dips of most units, and the northweststriking depositional contacts between the Lyre and the Crescent Formations in the southwestern part of the quadrangle are 

caused by eastward tilting of the entire area during uplift of the 

Olympic Mountains since about 18 Ma (Brandon and others, 

1998).

We interpret the east-west-striking fault on the south 

flank of Big Skidder Hill to be a reverse fault where the Lyre 

Formation is faulted over sandstone of Snow Creek. We did 

not observe fault-scarp breccias in the sandstone of Snow 

Creek that would indicate uplifted rocks during deposition, 

and this lack suggests that the fault formed later, due to northsouth shortening of rocks exposed on the Olympic Peninsula. 

Another east-west-striking fault with south-side-up motion may 

exist about a mile north of Big Skidder Hill. Lacking strong 

evidence of the dip of the fault on the north side of Big Skidder 

Hill, we map it as an irregular vertical fault with the north side 

up. A concealed fault to the north helps to explain the presence 

of the Eocene(?) micaceous sedimentary rocks on the north 

side of Big Skidder Hill, topographically above the Aldwell 

Formation-age basaltic rocks (unit Tebs) in the Salmon Creek 

drainage; alternatively, the geometry is consistent with a syncline, but there is little structural data to support the syncline 

interpretation.

In the southwest corner of the quadrangle, the right-lateral 

Bon Jon Pass Fault trends northwest along Deadfall Creek and 

the Little Quilcene River. This fault is exposed in a quarry near 

the summit of the pass (just west of the quadrangle) where 

a 33-ft-wide (10-m-wide) shear zone with strong horizontal 

slickensides juxtaposes Crescent Formation submarine deposits 

on the southwest with Crescent Formation basaltic flows on 

the northeast. Locally, these slickensides indicate right-lateral 

offset. Northeast-trending faults cutting through Cedar Flat 
5

juxtapose Lyre Formation with the Crescent Formation and may 

be normal or left-lateral faults kinematically related to the Bon 

Jon Pass Fault.

A north-northwest-trending linear trough in the center of 

the quadrangle extends along Ripley Creek and the north fork of 

Andrews Creek and through the valley west of Big Skidder Hill. 

West of Little Skidder Hill, unconsolidated deposits conceal 

the contact between the sandstone of Snow Creek and the Lyre 

Formation. Outcrops along the walls of the Snow Creek drainage reveal several near-vertical fractures, some that indicate 

right-lateral offset, but the deposits along the linear trough 

probably conceal a fault with a major west-side-up component 

of movement.

An apparently high-angle, north-south-trending fault truncates the east-west fault pair bounding the Lyre Formation on 

Big Skidder Hill (cross section A′—A′′). These bounding faults 

reappear on the east side of the Snow Creek valley about 0.5 mi 

north of Big Skidder Hill, suggesting some sinistral movement 

on the mostly concealed, north-trending fault, but major movement is up to the west. Both north-trending faults reflect the 

uplift of the Olympic Mountains to the west.

Overall, observed and inferred folds and the fault pattern generally suggest north-south shortening in post-Snow 

Creek time, a conclusion consistent with underthrusting of the 

Crescent Formation beneath Vancouver Island and continued 

northward migration of the Coast Range in response to oblique 

subduction (Clowes and others, 1987; Wells and others, 1998).

On the west end of Big Skidder Hill, the lidar DEM reveals 

a northeast-trending linear scarp that has about 3 ft (1 m) of 

displacement, northwest side down, demonstrating post-17-ka 

faulting. The scarp clearly cuts the major north-trending glacial 

grooves.

DESCRIPTION OF MAP UNITS

SURFICIAL DEPOSITS

NONGLACIAL DEPOSITS

af Artificial fill (Holocene)—Mud, sand, and gravel of varying proportions, possibly including debris such as concrete, logs, timbers, or brick. Thickness generally greater than 2 m. 

Mapped where fill substantially obscures or has altered original geologic deposit. Includes 

some small areas of extensively modified older deposits

Qb Beach deposits (Holocene)—Soft sand, silt, and mud deposited or reworked by wave or tidal 

action at head of Port Discovery. Contains logs and timbers at or above high tide. Elevation 

of deposits is within 2 m of mean higher high water

Qf Alluvial fan deposits (Holocene and Pleistocene)—Moderately sorted to unsorted sand, gravel, 

cobbles, and silt with moderate to low surface slopes. Includes debris flow and fluvial 

deposits. Numbers in subunits relate fans to stratigraphic position of alluvial terrace and 

alluvial deposits (Qa1, Qa3, Qa4, and Qa5). Fan matched with alluvium by smooth transition; contact somewhat arbitrary. Older fan deposits commonly separated from younger by 

low scarp. Alluvial fan deposits are mostly divided into the following subunits:

Qf5 Unit 5 (Holocene)

Qf4 Unit 4 (Holocene and Pleistocene)

Qf3 Unit 3 (Pleistocene)

Qf1 Unit 1 (Pleistocene)

Qa Alluvium (Holocene and Pleistocene)—Moderately sorted, unconsolidated deposits of cobble 

gravel, pebbly sand, and sandy silt along the floodplain of Snow and Salmon Creeks south of 

Port Discovery and along Snow Creek. Gradational with and includes sediment equivalent 

to unit Qb. Relative age position in terrace subunits (Qa1–Qa4) not known. Correlation of 

subunits in northern tributary of Salmon Creek with units in main Salmon Creek is uncertain. Alluvium is mostly divided into the following subunits:

Qa5 Unit 5 (Holocene)

Qa4 Unit 4 (Holocene and Pleistocene)

Qa3 Unit 3 (Pleistocene)

Qa2 Unit 2 (Pleistocene)—Probably marginal drainage deposits to Fraser Glaciation ice along the 

east side of mostly bedrock terrane on the west side of the quadrangle and include recessional outwash
6

Qa1 Unit 1 (Pleistocene)—Probably marginal drainage deposits to Fraser Glaciation ice along the 

east side of mostly bedrock terrane on the west side of the quadrangle and include recessional outwash

Qw Wetland deposits (Holocene and upper Pleistocene)—Peat, marsh, and bog deposits, with 

some intermixed sand, silt, and clay. Unit is very soft, water saturated, and occupies shallow 

depressions in till (Qvt) or other low-lying areas. South of Crocker and Leland Lakes, this 

material was previously mapped (Haeussler and others, 1999) as Vashon stade recessional 

sand (Qvrs) and may include some recessional sand

Qls Landslide deposits (Holocene and upper Pleistocene)—Diamictons of angular clasts of 

bedrock and surficial deposits derived from upslope and commonly includes trees. Includes 

areas of irregular, hummocky topography. Poorly consolidated. Landslide about 2.2 mi (3.5 

km) south of Port Discovery consists of house-sized blocks of Lyre Formation conglomerate

Qci Indurated colluvium (Pleistocene?)—Indurated, unsorted, iron-stained angular colluvium on 

flanks of Olympic Mountains in southwestern part of map area. Inferred to predate Vashon 

stade, because sediment is indurated and lying on bedrock

GLACIAL DEPOSITS

Deposits of Vashon Stade of Fraser Glaciation of Armstrong and others (1965) (Pleistocene)

Qvrg Gravel-dominated recessional outwash deposits (upper Pleistocene)—Stratified, unconsolidated, gravel-dominated sand and gravel deposits. May include interbeds of sand-dominated 

recessional outwash

Qvrf Fine-grained recessional glaciomarine or glaciolacustrine deposits (upper Pleistocene)—

Stratified firm fine silt with rounded dropstones of pebbles and cobbles along Highway 20, 

southeast of Port Discovery. Coarse silt and sand are interbedded. Beds are greater than 1–3 

in (3–10 cm) thick. Some contacts between beds are convoluted, and sand dikes and blobs 

are present. Local cross bedding. Overlies probable till

Qvri Ice contact deposits (upper Pleistocene)—Sand, gravel, and silt, commonly with disrupted 

or no stratification, characterized by surface morphology of steep-walled channels, collapse 

features, eskers, and common alluvial flats

Qvt Till (upper Pleistocene)—Compact and firm, light- to dark-gray, nonstratified diamict containing subangular to well-rounded clasts, glacially transported and deposited. Clasts are 

commonly granitic and were derived from the Cascade Range or Canada. Commonly overlies advance outwash deposits (units Qval and Qvas) in the eastern part of the quadrangle 

and bedrock in the western part of the quadrangle. Generally forms an undulating surface 

with a few meters to a few tens of meters relief. However, west of Crocker Lake, more than 

246 ft (75 m) of till is exposed

Qrg Rilled ground (upper Pleistocene)—Deposits with many small parallel gullies. Where above 

about 400 ft (122 m), may represent zones of sandy outwash sediments (Qvas) interbedded 

or intermixed with Qvt or in landslides (Qls) derived from such sandy till. Rilled ground is 

not shown separately where formed in mapped landslides. Area below 400 ft (122 m) east of 

Leland Lake could be thin Qvt over Qvas

Qvrf Fine-grained recessional glaciomarine or glaciolacustrine deposits (upper Pleistocene)—

Stratified firm fine silt with rounded dropstones of pebbles and cobbles along Highway 20, 

southeast of Port Discovery. Coarse silt and sand are interbedded. Beds are greater than 1–3 

in (3–10 cm) thick. Some contacts between beds are convoluted, and sand dikes and blobs 

are present. Local cross bedding. Overlies probable till

Qvas Sand-dominated advance outwash deposits (upper Pleistocene)—Well-bedded, sand-dominated, compact sand and gravel deposits. May have interbeds of silt or clay. Deposited by 

streams and rivers issuing from front of advancing ice sheet. In the vicinity of Crocker Lake 

and south, Qw was previously mapped as sand-dominated recessional outwash

Qvv Valley-fill glacial and glaciofluvial deposits (upper Pleistocene)—Bedded, sandy-matrix, 

clast-rich, compact diamict with angular clasts and a few large boulders, interbedded with 

firm fine to coarse sand and silt and clast-poor clayey matrix diamict, which we consider as 

till. Clasts are commonly basaltic with probable local provenance. Occupies the headwaters 

of the Little Quilcene River drainage in the southwestern part of the map area

Qval Advance outwash lake deposits (upper Pleistocene)—Laminated to massive silt, clayey silt, 

and silty clay deposited in a lowland lake south of Port Discovery. Sediment is firm
7

SURFICIAL DEPOSITS AND BEDROCK

prevt Pre-Vashon-stade till deposits (upper Pleistocene and (or) older)—Probable older deposits 

of unknown composition. May include some Vashon stade advance outwash and bedrock. 

Mapped on assumption that thickness of Qvt is 30–50 ft (10–15 m) or less

BEDROCK

Tss Sandstone of Snow Creek (upper Eocene)—Sandstone and silty sandstone turbidite. Beds 5 

cm to 1 m thick. Contains channelized interbeds of pebble and granule conglomerate and 

siltstone as much as 3 ft (1 m) thick. Clasts are dominantly quartz and chert

Tl Lyre Formation (upper Eocene)—Pebble and cobble conglomerate in a matrix of medium- to 

coarse-granule sand. Very similar to unit Tluc

Tluc Upper conglomerate member—Pebble and cobble conglomerate in a matrix of medium- to 

coarse-granule sand. Contains some sandstone interbeds. Dominantly chert, basalt, and conglomerate clasts in decreasing order of abundance (Spencer, 1984). Clasts to 3 ft (1 m)

Tlav Andesite tuff and breccia—Andesite and hornblende-andesite tuff and breccia, white to light 

gray. Locally contains rare leaves and carbonized wood. Commonly massive, but some tuffs 

are thin bedded

Tllc Lower conglomerate member—West of Cedar Flat, unit is predominantly thick-bedded to 

massive conglomerate; to the south, thin- to thick-bedded sandstone with minor conglomerate is most common. Conglomerates are composed of well-rounded pebbles and cobbles of 

chert with lesser amounts of metasedimentary and igneous rocks, quartz, and graywacke. 

Lower part of unit south of Snow Creek locally includes some angular to rounded basalt 

clasts. Sandstones are fine to coarse grained and commonly contain scattered pebbles. Siltstone is sandy and thinly to faintly bedded

Tems Micaceous sediments (Eocene?)—Massive, buff-brown micaceous siltstone and fine sandstone 

on the north side of Big Skidder Hill. Bedding commonly indistinct or not present

Tebs Basaltic sediments (middle Eocene)—Basaltic conglomerate and breccia and fine- to mediumgrained, dark-gray to black, massive sandstone, with thin local interbeds of dark siltstone. 

Lowermost few meters are gray-green argillaceous limestone pods and basaltic sedimentary 

breccia. Total thickness is approximately 755 ft (230 m) along Salmon Creek

Ta Aldwell Formation (middle and upper Eocene)—Massive to bedded mudstone and turbidite 

sandstone, basaltic conglomerate, lithic sandstone, and siltstone

Crescent Formation (lower, middle, and upper Eocene)

Tcb Basalt breccia—Rubbly, reddish-weathering basaltic breccia. No silty or sandy matrix, indicating subaerial eruption of lava. Minor basalt flows

Tcf Basalt flows—Massive basaltic lava flows. Locally with calcite or zeolite-filled vesicles and 

pyroxene phenocrysts. Some flows have red oxidized tops indicating subaerial eruption. 

Minor basaltic breccia

Tcfb Basalt flows and breccia—Rubbly, reddish-weathering basaltic breccia and massive basaltic 

lava flows in roughly equal proportions. Breccia contains no silty or sandy matrix. Some 

flows have red oxidized zones

Tcs Basaltic submarine deposits—Pillow lava and pillow and lapilli breccia, amygdular lava 

flows, dark-gray calcareous mudstone, basaltic siltstone and sandstone. Intruded by basaltic 

sills. At head of Little Quilcene River, sandstone makes up approximately half of section
8

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