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Kittitas Valley Field Trip
Of all the geologic field trips offered out of the Seattle area, none is more popular than the trip over the
mountains to the Kittitas Valley around the town of Cle Elum. This east-side setting offers a refreshing
break from the damp climate of the Puget Sound region, and is popular in the spring and fall when the
weather is less dependable on the west side. The region features an instructive mix of igneous,
sedimentary and metamorphic rocks, in a setting which illustrates both the principles of stratigraphy
and the fundamentals of structural geology. It is an excellent trip for introductory students, yet offers
enough variety to be interesting to those with a greater depth of experience. Of no minor significance,
it also includes a stop at the renowned Cle Elum Bakery, one of the oldest and best bakeries in the
northwest.
Given the variety of rocks and the generally good outcrop locations across this region, it would be
possible to engineer a number of potential field trips to suit a range of different instructional purposes.
Introductory trips are usually characterized by a larger number of stops, designed to illustrate largescale features and relationships. More advanced trips usually involve fewer stops, with a greater
attention to the details offered in the rock record. This version is designed as an introductory – level
trip, suitable for students in the latter half of a college-level introductory course. It serves to illustrate
the variety of rock types, the basics of stratigraphy and structure, and how the course of geologic
evolution can be traced through the rock record. It consists of a dozen stops, making it about a 12-hour
round trip from Seattle. Two hours are invested at the last stop, which can be considered optional,
however desirable.
Geology of the Kittitas Valley
The geology of the Kittitas Valley includes metamorphic basement rocks which were added to the
continental margin in Late Cretaceous time, perhaps something like 90 – 95 million years ago. Those
older rocks are overlain by a sequence of sedimentary and volcanic rocks which date from Eocene
time, and which extend discontinuously to as recently as 4 million years ago. Those sedimentary and
volcanic rocks can be assigned to two distinct sequences. The first dates from Eocene time (~53 – 38
Ma), and is known as the “Challis” sequence. The second dates from 37 – 4 Ma, and is known as the
“Cascade” sequence. The two sequences are separated by a major regional unconformity, and represent
two distinct regimes of regional plate-tectonic relationships.
The basement rocks in this area are a suite of phyllite and greenschist which represent the
metamorphosed equivalent of oceanic crust and ocean-floor sediments. The greenschist is properly
known as the Easton Greenschist, but the two rocks are better known as the Shuksan Greenschist and
the Darrington Phyllite, for their occurrence on the west side of the Cascades. Together, they are
commonly referred to as the Shuksan Metamorphic Suite. They are part of the Northwest Cascades
Belt of terranes, which was added to the continental margin in Late Cretaceous time.
The sedimentary and volcanic “Challis” sequence of formations probably dates from an early horizon
of perhaps 53 million years ago. The oldest of these is a localized basal section of felsic volcanic rocks
and arkose sandstones known as the Taneum Formation. These may date from as early as 53 Ma. They
are overlain by a regionally thick section of fluvial arkose sandstone, siltstone and conglomerate
known as the Swauk Formation. The Swauk Formation is unconformably overlain by the largely
basaltic Teanaway Formation, dated at about 47 Ma. The Teanaway basalts are in turn overlain by the
Roslyn Formation of arkose sandstone, noted for the local abundance of coal beds, the former
economic mainstay of this region. The uppermost elements of the “Challis” sequence are absent in this
area, simplifying the picture.
Three formations of the “Cascade” sequence are preserved overlying the Roslyn Formation in this
area. They include the <25 Ma Ellensburg Formation of tuffaceous sandstone and conglomerate, the
17-15 Ma Grand Ronde Member of the Columbia River Basalt Group, and the ~4 Ma Thorp Gravel.
The informally-designated Ellensburg Formation includes all sedimentary interbeds below and
between the various flows of the Grand Ronde Basalts. These are tuffaceous sandstones, siltstones and
conglomerate, derived from the Cascade Arc volcanoes to the west. They are dominantly andesitic in
character, and include significant lahar deposits in their mix.
The flows of the Grand Ronde member of the Columbia River Basalts are basalt. Locally, flows
include a pillow-palagonite complex at their base, a reflection of the wet landscape which persisted
between eruptive events. Elsewhere these flows display classic columnar structure, a reflection of the
cooling process in lava flows. The youngest rocks here are a section of gravel known as the Thorp
Formation. The Thorp gravels date from about 4 million years ago, and form a thick belt in the middle
portion of the Kittitas Valley. This accumulation broadly dates from the onset of the uplift which has
produced the modern Cascade Mountains.
The rocks in this area are preserved in a northwest - trending fold with a broad syncline (the Kittitas
Syncline) on the north side, and a narrower anticline (the Ainsley Canyon Anticline) on the south side.
This is part of the Yakima Fold Belt, which extends southeast along this strike. These folds are
accommodating northeasterly compression produced by the northward shearing of California by the
Pacific Plate, and the eastward compression produced by the local Juan De Fuca Plate. This regime
first developed between 25 and 20 million years ago, and persists into the present.
While these rocks are fairly abundantly exposed in the Kittitas Syncline to the north, many of the
important contacts are not particularly well illustrated. The first half of this field trip illustrates the
structure as progressively younger rocks appear toward the center of the syncline, but those
relationships are not particularly evident in the field. The latter part of this trip ascends the Ainsley
Canyon Anticline up Taneum Canyon, which more clearly delineates the local stratigraphy.
Along the southern margin of the Kittitas Valley there is a low-angle thrust fault which occupies the
inflection point between the Ainsley Canyon Anticline and the Kittitas Valley Syncline. This is known
as the Easton Thrust Fault. Like much of the rest of the Yakima Fold Belt, this is a fold-and-thrust belt,
where folding accumulates stress which is periodically relieved by low-angle faulting. Not
coincidentally these are the same characteristics as are found in the Seattle Fault, which lies broadly on
strike to the west. Between these two locales, uplift of the the north-south striking Cascade Anticline
over the last five million years has served to obscure the original connections across this region.
A Brief History of the Kittitas Valley
Prior to the 1870’s, the Kittitas Valley was the exclusive domain of native tribes which had inhabited
the region for millennia. Army patrols and prospecting parties passed through the region, but found
little of particular interest. This changed in 1873 when gold was discovered on the Swauk River. This
caused a modest rush to the region, and by 1879, a rough road extended over Blewett Pass from
modern-day Cle Elum. Mining was a going concern here into the 1890’s, when returns began to
diminish.
While gold proved a profitable venture for a number of local interests, the larger history of this area
centered on a less-glamorous commodity: coal. Coal was fuel for the railroads, which determined the
course of development over the last half of the 19th Century. Absent the coal fields in Centralia,
Renton, Bellingham and Cle Elum, the history of this state would have taken a much different course.
Explorers for the Great Northern Railroad discovered the extensive coal deposits in the Roslyn
Formation north of Cle Elum in the mid 1880’s, and developed it as a major fueling station for their
regional network.
The railroad established the town of Roslyn to develop the mines, named for Roslyn, New York – the
home town of the mining superintendent. The mines were producing for the railroad by 1886, boosting
production with the opening of the Stampede Pass tunnel in 1888. Roslyn grew to a population of
several thousand and the mines continued to expand. Labor unrest resulted in a general strike in 1888,
an event of regional significance. The owners brought in 300 young African-American men from the
south to serve as strike breakers, and employed a private militia to maintain the peace. The governor
took exception to the notion of private law enforcement, with the effect that an article in the State
Constitution now prohibits it. The strike was resolved and the strike-breakers were absorbed into the
workforce. This was a huge increase in the African-American population of the state at that date, and
was an important historical event in that context. As a thriving mining town, Roslyn had a reputation
for respecting ethnic and cultural differences. The local cemetery is said to have names from some 24
different countries.
The mines were also the site of the state’s worst mining accident, when 45 died in a gas explosion on
the lowest level of the mine in 1892. This was seven levels down, 2700 feet directly beneath the town.
Production continued into the 1950’s, but shut down when the railroads converted to diesel fuel. While
huge amounts were mined, it was only 20% of the amount available. The town went on to be the set
for a 1990’s-era sitcom called “Northern Exposure”, where it played the role of the fictitious town
Cicely, Alaska. In appreciation, the production company furnished new metal roofs for the town
residents. The town is home to The Brick Tavern, the oldest continuously-operating saloon in the state
under the same name. It dates from 1898.
The town of Cle Elum is a few years younger than Roslyn, situated along the main east-west rail line.
It was founded as a more refined community than the rough-and-tumble mining camp that was Roslyn.
It featured quality hotels, eating establishments and general stores, and catered to travelers along this
major cross-country route. Its commercial district was decorated in ornate woodwork, and many of the
buildings were quite opulent in their architecture. Unfortunately it was wooden architecture, and 30
blocks of the downtown business district burned to the ground in 1918. The town never really
recovered from that disaster.
By the 1930’s automobile travel was becoming popular, and Cle Elum enjoyed a position along
Highway 10, the major east-west route over Snoqualmie Pass to Seattle. The Kittitas Valley was
developed as agricultural land, based on water supplied by the Yakima River, and local artesian wells.
The Yakima, Kechelus and Cle Elum valleys were dammed to form lakes, as a water supply for
irrigation. In post-war time, the local economy also benefited from the expansion of the ski areas at
Snoqualmie Pass, and other recreational opportunities in the area. As automobile and truck traffic
increased, Cle Elum became a major stop along the main east-west corridor. That status declined in
1968, when the Interstate 90 by-pass route was finally completed. For many years prior, it had the
distinction of being the only stoplight on the interstate between Seattle and Boston.
Several years ago the town of Cle Elum permitted a large development on the west side of the
community, an exclusive private residential / recreational complex which will ultimately be larger than
the town itself. “Suncadia” is a secure gated community with modest vacation homes starting in the
low 400’s. It features exclusive river access, championship golf courses, miles of private hiking trails,
patrolled by private security guards and maintained by professional service staff. The people of Cle
Elum (mistakenly) saw this as their economic salvation, conveniently financed by outside investment.
The people of Roslyn looked upon it as a serious threat to their way of life. Since that decision, dozens
of new “vacation communities” have been platted up the valley, marketed to affluent “coasties” from
the Puget Sound region, and to retirees from across the country. The entire region is undergoing a
development boom, everywhere except in the town of Cle Elum itself. Not everyone is happy about
this.
Cle Elum:
Cle Elum takes its name from Tie-el-lum, a Kittitas term for “swift water”. It was incorporated in
1902, and reached its zenith between 1900 and 1910, when it was home to over 2500 people. A major
fire in the business district in 1918 was an event the town never really recovered from. It long enjoyed
a position on the major east-west roadway, first as the Sunset Highway, then Highway 10, and finally
Interstate 90. It lost that status in 1968, when the I-90 bypass was completed. Some 1755 people make
their homes here.
The town lacks something from an esthetic standpoint. Gas stations, truck service yards, rail yards and
other amenities give it a somewhat utilitarian flavour. Except for the new mall (Safeway) on the west
side of the town, nothing here has changed much since the late 1960’s. The best thing about the town is
unequivocably the bakery. The Cle Elum Bakery has been baking for over a hundred years, and is
widely acknowledged as one of the best in the state.
Stop 1: The Easton Greenschist
Take the Lake Easton State Park exit (71) from Interstate 90 and follow the signs to Lake Easton State
Park. Inside the park, take a right turn at the intersection and continue several miles to the swimming
beach.
Lake Easton is a man-made feature designed to provide water for irrigation in the Yakima Valley. It is
impounded by a dam, and the level of the lake varies seasonally. It is a popular recreation area, just an
hour east of Seattle.
From the swimming beach, the outcrop lies along the shoreline of the lake about 100 yards to the west.
When the water is low you can walk the shoreline, but a trail leads to the area above, and generally
provides easier access. Leave the trail at the switchback, and continue 100 feet to a point above the
outcrop. A short path leads down to the lake here. The outcrop is modest, but provides good
exposures.
The rock here is greenschist, a metamorphic variety. It is the metamorphic equivalent of basalt, where
the original olivine, pyroxene and feldspar minerals have been changed to chlorite, actinolite and
epidote. This happens at temperatures of ~250 C, and under several thousand atmospheres of pressure.
It is a fine-grained species with a well-developed foliation, here dipping almost vertically. On a clean
exposure, you can see a distinctive banding in the rock. This unit is called the Easton Greenschist,
named after exposures in this area. It is however part of a larger belt of rocks better preserved in the
Mt. Baker area well to the northeast. There, the rock is known as the Shuksan Greenschist (after Mt.
Shuksan). The “Easton” name is more proper, but the “Shuksan” name is more common.
When the lake level is low (late fall), exposures of phyllite outcrop south of the greenschist. Phyllite is
a somewhat silvery fine-grained metamorphic rock characterized by microscopic muscovite (mica)
minerals. It develops from a mudstone, under conditions similar to those which produce greenschist
from basalt. Again, this rock is best known for its occurrences well to the northeast, where it is known
as the Darrington Phyllite (Darrington is a town on the Stilliguamish River).
Because both rocks were produced at the same metamorphic grade, they were likely part of a common
“package” of rocks, a “suite” as they are known. The most common “suite” of rocks on our planet is
basalt with a section of mud on top. This is the character of our ocean floors, but is an uncommon
combination on the continents. Accordingly, it is most likely that this represents a section of oceanic
crust, which has been suitably metamorphosed and somehow ended up as part of the continent. The
Shuksan and Darrington rocks are known collectively as the “Shuksan Metamorphic Suite.”
These are the “basement” rocks in this area, the deepest levels exposed. All of the “basement” rocks of
Washington (west of Spokane) consist of sections of oceanic crust and the remains of Pacific island
groups which have been added (“accreted”) to the margin of the continent over the last 200 million
years. This has happened under an evolving set of plate-tectonic relationships, which have developed
as North America has progressively moved to the west over this expanse of time. Much of this has
happened under convergent margin conditions, much as exist today. In this setting, North America
effectively collided with two large islands chains at about 170 and 115 million years ago, which added
distinctive belts of rock which are the “basement” to most of British Columbia and northern
Washington east of the Puget Sound. These “belts” are known as the Intermontane and Insular Belts,
and they are not exposed in this area.
The rocks of the Shuksan Suite are part of a larger group of similar rocks known as the “Melange
Belts”. The term “mélange” is French for a “mix” of rocks, as is their general character. While the
Shuksan suite preserves coherent sections of oceanic crust, most of the rocks of the Melange Belts
have been disrupted, fragmented, sheared, jumbled and juxtaposed into an indecipherable mix. Rock
lithologies like this typically accumulate in an “accretionary wedge” of material scraped off the
descending oceanic plate as it is subducted beneath the continent. Such accumulations are common in
areas well south of here, and we suspect that these rocks have their origins in what is now southern
Oregon or northern California. Along with the rocks of the Shuksan Suite (the subducting oceanic
plate), these rocks appear to have been rifted off that southern coastline during a change in regional
plate-tectonics, marked by the inception of a new east-west trending spreading center intersecting the
continent in what is now northern California. This new spreading center produced a northerly sense of
plate motion to the north, transporting these rocks toward our region.
In our area, that northerly plate motion appears to have concluded along an east-west trending
subduction zone, across what is now the southern half of the state. As these sections of displaced
mélange rocks arrived at the subduction zone, they were obducted (thrust over) across the top of the
continent along low-angle thrust faults. Some of these great “thrust sheets” may have been thrust for
hundreds of kilometers. We suspect that this happened about 90 million years ago. These rocks
overthrust the southern end of the Insular Belt, which had been accreted about 25 million years earlier.
In the end, these rocks are probably a displaced section of southern Oregon or northern California
coastline, the remains of an old subduction zone which was rifted off the coastal margin, exhumed
from depth and transported northward, then thrust across the edge of the continent here about 90
million years ago. Along with the rocks of the Intermontane and Insular Belts, and those of the
Olympic Coast Belt, they comprise the deepest “basement” rocks of Washington.
Stop 2: The Swauk Formation and the Teanaway Basalt
Return to Interstate 90 and continue east to the exit for the town of Roslyn (Exit ). Take the exit and
continue north through the towns of Roslyn and Ronald. From downtown Ronald (the # 3 Tavern)
continue
miles north, eventually along the shores of Lake Cle Elum, to the distinctive outcrop.
There is abundant parking on the west side of the road.
Lake Cle Elum is an artificial lake, impounded by a dam on the south end. The lake stores water for
irrigation, and thus its shoreline varies seasonally. The dam was built in
, enlarging a small lake
which used to mark the valley here. This is a very popular recreation area. The road extends north into
the heart of the Alpine Lakes Wilderness Area.
There are two rocks exposed here, the dark basalt of the Teanaway Formation and the tan-colored
sandstone of the Swauk Formation. A large block of the Swauk sandstone is preserved as a prominent
xenolith (inclusion) in a flow of Teanaway Basalt. By the stratigraphic principle of inclusion, the
sandstone is thus older than the basalt.
The Swauk Formation is the dominant unit for some distance north of this location, on the border of
the Teanaway Basalt. It takes its name from the Swauk (Blewett) Pass area, well to the northeast of
here. Much of the formation is sandstone, as seen here, but significant proportions are conglomerate
and finer-grained rocks. Collectively, they reflect deposition in a river-basin setting, probably by a
substantial river system. Based on the minerals which make up these sediments, their source area was
likely well to the east or northeast, along the front of the Rocky Mountains. It is a uniquely thick
assemblage, accumulating to as much as 5 km of sediment. It contains significant coal beds and is
notably fossiliferous, the most common species being palm trees. The fossil assemblages reflect a
significantly warmer and more equitable climate than exists now.
Adjacent to the Teanaway Basalt, these sandstones have been contact metamorphosed to quartzite. Up
the road (to the north), this character diminishes. Bedding is well-displayed in some of these rocks,
ranging from millimeter to meter scale. While much of the bedding is planar, it is possible to discern
crossbeding in some sections, a reflection of deposition in a river system. The sandstone is largely
comprised of moderately well-sorted medium-grade sand of quartzo-feldspathic composition. Quartz
grains are typically sub-rounded in aspect. The rock also contains beds of distinctly finer material,
comprising mudstones of various composition.
Basalt is the dominant rock of the Teanaway Formation, although there are minor sedimentary beds to
be found. The basalt is black, aphanitic, and phenocrysts are relatively rare. Chemically, they are
closest to MORB (mid-ocean ridge) basalts, those which erupt from mid-ocean spreading centers.
Here, these rocks have erupted through fissures, which can be seen cutting the Swauk Formation.
Above the Swauk Formation, they accumulated to several hundred meters of lava. It covered an area at
least twenty miles in diameter, in an event dated at about 47 million years ago.
The Teanaway Basalt is probably related to the Crescent Basalts, which make up much of the Olympic
Peninsula. Collectively they represent an immense outpouring of basalt, erupted through fissures in the
crust. These may have erupted off a “stalled” spreading center, along transform faults which riddled
the plate to the west. The Teanaway Basalts appear to be the easternmost expression of this regime,
erupting along faults which extended to the west. There is evidence for more than one eruptive event,
but all appear to have been voluminous extrusions.
Stop 3: The Swauk Formation
Turn around and return south on the road toward Roslyn. After miles, pull off at the driveway to a
gated real-estate development and park along side the road. The outcrop is just north of the driveway.
The rock here is part of the Swauk Formation, but with more varied lithologies than were present at the
last stop. The outcrop here is a mix of sandstone and conglomerate, all of which dip to the southeast.
The lowermost strata are largely sandstone, consisting of floodplain and riverbank deposits. The
conglomerate layer above lies unconformably on the sandstone, and represents channel deposits which
have cut into those older layers. Consistent with those seen before, these sediments reflect deposition
by a river system migrating across its floodplain.
The clasts in the conglomerate are polymictic, accumulated from a variety of source areas. Most are
sub-rounded to rounded, but local varieties (e.g. the Stuart granodiorite, the Ingalls peridotite) are more
angular in aspect. These rocks are exposed to the north of this area. The character of the Swauk
Formation isn’t consistent with much local relief on the landscape, so these probably came from
modest outcrops rather than large exposures.
Notable in the mix are angular fragments of coal, clearly of local origins. Coal layers are not
uncommon in the Swauk formation. Looking in the prominent cleft in the outcrop, you can see a
coalified section of tree trunk. This is the source of part of this material. The wood has undergone
fossilization to a degree, preserving enough detail to see annular rings in its cross-section. As noted
earlier, the Swauk Formation is locally fossiliferous, containing abundant leaf impressions and woody
material in fine-grained sediments. This tree trunk fell into the river channel here some fifty million
years ago, and was subsequently buried by accumulated sediments.
Stop 4: Roslyn
Return down the road to the town of Roslyn. Turn left on Railroad Avenue and continue to its end at
the site of the old coal mines. An interpretive trail runs though this site.
The town of Roslyn was founded by the Great Northern Railway, to provide coal for its railroad
empire. It is the only significant coal deposit in Eastern Washington, and was thus a strategic coup for
the railroad. Coal from the Roslyn mines powered locomotives over Stampede Pass to the Puget
Sound, and across Eastern Washington to Spokane and points beyond. This stop is located at the head
of the main shafts, at the end of the main rail spur. From here, the mines extended some 2700 feet
beneath the town.
The Roslyn mines produced coal for the railroad from the mid 1880’s through the 1950’s, when
locomotives changed to diesel fuel. At its height, the mines supported a town of several thousand
people. As always, the work was dirty, strenuous and dangerous. Labor unrest sparked a general strike
in 1888, and some 300 African-American men were brought in from the south as strikebreakers. After
the strike those men were absorbed into the workforce, a significant event in regional AfricanAmerican history. The strike was resolved, but safety concerns persisted as tunneling progressed to
deeper depths. Worker’s concerns about methane gas accumulations were verified in 1892, when a
large explosion ripped through the lowest level. Some 45 men died in the explosion, the worst mining
accident in state history.
The closure of the mines was a major economic blow to the town, which had little other sources of
employment. To a certain degree, expanding recreational interests provided some measure of relief.
The town went on to be the set for a 1990’s-era sitcom called “Northern Exposure”, where it played
the role of the fictitious town Cicely, Alaska. In appreciation, the production company furnished new
metal roofs for the town residents.
Over the last two decades the community has adopted something of an artisan image, building on its
popularity from the television series. It is a charming community displaying over a century of history,
and draws a healthy stream of tourists over the summer months. The town is home to The Brick
Tavern, the oldest continuously-operating saloon in the state under the same name. It dates from 1898.
The Roslyn Mines were, appropriately, in the Roslyn Formation. The Roslyn Formation is largely
sandstone and siltstone, and (obviously) includes extensive coal beds. Here, the strata of the Roslyn
Formation dip to the east, as do those in the Swauk Formation. The Swauk, the Teanaway and the
Roslyn Formations are all folded into a broad northwest-southeast striking syncline, dipping to the
southeast. This is known as the Kittitas Syncline. At this latitude, the Roslyn Formation occupies the
central portion of that syncline, which probably formed between 40 and 38 million years ago. By the
map distribution of these rocks, one can conclude that the Roslyn Formation overlies the Teanaway
Formation.
Stop 5: The Roslyn Formation
Near the east end of town, turn left on Columbia Avenue. Columbia turns right onto a dirt road just
past a church. This dirt road rises and then reaches a fork. Take the right fork and park at the prominent
switchback in the road.
While the Roslyn Formation is abundantly exposed in the slopes above town to the north(see above),
these are on private property, and are heavily weathered exposures. The outcrop here is small, but
provides an convenient opportunity to examine fresh samples.
The Roslyn Formation is largely a sandstone unit, similar to the sandstones of the Swauk Formation in
its general character. It is frequently a bit finer-grained, but the major difference is in the color. While
the Swauk Sandstone is tan to buff in color, the Roslyn is distinctly white. This reflects the presence of
an alteration product (laumonite) in the rock. Of equal significance, the Roslyn displays much largerscale bedding than does the Swauk. The beds in the Swauk Formation are typically millimeter to
centimeter in scale, reflecting events of limited duration. By contrast, the bedding in the Roslyn is
typically meter-scale, reflecting events of longer duration. During Roslyn time, depositional conditions
did not change as often as seen in Swauk time.
Rocks of the Roslyn Formation were deposited in a river basin setting, but an environment featuring
more boggy conditions and a more stable depositional regime. The scale of bedding cannot be seen
here, but can be observed at the old Red Bridge Crossing off the Teanaway River Road, to the
northeast. More advanced students will want to make this side-trip. For introductory purposes, this
outcrop illustrates the fundamental character of the rock itself.
Stop 6
Return to main street (SR 970) and continue east out of town. The road meets with SR 97, heading
north. After a short distance, turn right onto US 10, signed for Ellensburg. Take this road for miles
to a prominent turn-out above the Yakima River.
These are rocks of the Ellensburg Formation, which locally overlie the Roslyn Formation. Here, they
occupy the center of the Kittitas Syncline. The Ellensburg is an informally-designated formation,
consisting of more than one mappable unit of rock. This is a good representative exposure of these
rocks, illustrating their essential character.
The rocks here vary from very fine-grained sediments to conglomerates, but all share in one
characteristic – they are all volcanic (largely, andesitic) rocks. The finest material is volcanic ash (tuff),
while the sandier material is a tuffaceous sandstone. The conglomerates consist of matrix-supported,
well-rounded cobbles of andesite, often in meter-scale beds. These deposits likely date from 20-25 Ma.
They are sediments which were eroded off the ancestral Cascade Volcanoes to the west, carried by
rivers flowing to the east. Most of the finer sediments were deposited by rivers, but some of the
conglomerates represent debris flows and lahars which periodically coursed down the river valleys.
The rivers carrying these sediments were overburdened in their capacity, rapidly accumulating
sediment and constantly changing course. Elsewhere, the formation displays a classic pattern of trough
cross-bedding, characteristic of heavily overburdened rivers.
The “Ellensburg” formation refers to all tuffaceous sediments which accumulated below and inbetween flows of the Columbia River Basalts along the east slopes of the Cascades. They reflect active
volcanism in the Cascade Arc to the west, and rapid erosion and transport of that material east by local
rivers. Plant fossils can often be found in the finer sediments, reflecting a temperate climate, but a
much wetter setting than exists today. While the ancestral Cascade Volcanoes rose to the west, the
modern Cascade Range did not start to rise until much more recently, over the last five million years.
Prior to that date, the moisture from Pacific weather systems flowed unimpeded into what is now
Eastern Washington.
The other feature of note here is a prominent high-angle fault. It can be seen cutting and offsetting beds
of fine-grained sediments and conglomerate. This is what is known as a “normal” fault, which results
from extending or depressing the crust. The diagnostic characteristic of this sense of motion is that the
“hanging wall” of rock lying above the fault line has been down-dropped, relative to the “footwall” of
rock lying below the fault. This is characteristic of an extensional tectonic regime or a subsiding
setting. In this case, it likely reflects the subsidence of the Columbia Basin to the east, as it
accumulated up to 5 km of the Columbia River Basalts, largely between 19 and 15 million years ago.
This is one of many faults which developed as that basin sank under the massive weight of these lava
flows.
Continue east on US 10
In short order, the road enters the province of the Columbia River Basalt Flows. The Columbia River
Basalts are a regional-scale feature which covers much of Eastern Washington.
In miles, note the layer of gravel which lies on top of the Columbia River Basalts. These are the
Thorp Gravels, which we will consider later. The important observation here is that they are the
youngest strata in the area.
In
miles, pull over onto the side of the road along a narrow strip next to the guard rail.
Stop 7 The Columbia River Basalts
Yakima River
These are flows of the Columbia River Basalts. These flood basalts erupted from fissures, principally
in southeastern Washington and northeastern Oregon. They erupted between 17 and 6 million years
ago, but most of the material was erupted about 16 million years ago, as part of the Grand Ronde
series. These rocks date from that episode. The Columbia River Basalts appear to have erupted as the
continent moved over the Yellowstone Hot Spot. The area in which they erupted is a zone along which
the continent is spreading apart, creating a weakness in the crust.
While now restricted to eastern Washington, Oregon and the Columbia River Gorge, these flows were
probably much more extensive when they first erupted. Since that time, the modern Cascade Range has
risen, likely resulting in the loss of considerable exposure. In this area, an east-dipping paleoscope is
reflected in the flow of the rivers depositing the Ellensburg Formation, so at least a modest topographic
high existed to the west when the flows first erupted. Their original western margin in this area
remains uncertain.
Erupting prior to the uplift of the modern Cascade Range, the Columbia River Basalts flowed over a
landscape which enjoyed a damp maritime climate, marked by temperate forests and numerous small
lakes and boggy areas. This setting is reflected in the fossils from the Gingko Petrified Forest, near
Vantage. In the Grand Coulee area, the cast of a small rhinoceros was preserved at the base of a flow,
known as the “Blue Lake Rhino”. Its habitat is consistent with such a setting.
The rocks at this field stop are also consistent with such a setting. In contrast to the clean (often
columnar) appearance of the surrounding flows, these are a lumpy mix of basalt and a yellow mineral
known as palagonite. Palagonite is a clay mineral is formed as basalt flows over water or damp areas.
It is interspersed with large “pillows” of dark basalt, the characteristic form of lavas erupted in a
submarine setting. Together, they are known as a “pillow-palagonite” complex, and it develops where
hot lava contacts water.
There is additional evidence for this at this stop. At the base of the pillow-palagonite complex, there is
a bed of white clay. This clay accumulated in the bed of a small lake which existed here, a lake which
the lava subsequently flowed over.
Continue east on US 10 to the Thorp Mill Road, and turn right on that road. Continue miles to the
intersection with the Taneum Road. Turn right on the Taneum road and continue across the Thorp
Prairie to where it crosses Interstate 90. On the far side, park at the entrance to Taneum Canyon.
Stop 8 Taneum Canyon and the Thorp Gravel
Taneum Creek broadly parallels the course of the Yakima River to the north, separated from that
drainage by the South Cle Elum Ridge. The remainder of the trip ascends Taneum Canyon, to a point
where one can cut across the intervening ridge, and descend back down to Cle Elum. The high point of
the trip is Peoh Point, a spectacular vantage on the Kittitas Valley below.
Taneum Canyon is cut into a structural feature called the Ainsely Canyon Anticline. This is a
southeasterly-plunging anticline which strikes northwest – southeast. It is the anticlinal counterpart to
the Kittitas Valley Syncline to the north. In contrast to the relatively gentle character of the latter, the
Ainsley Canyon Anticline is a more steeply-folded structure. These features reflect a regime of northdirected compression which has been in effect for the last twenty million years. On a larger scale, these
are northern elements in a structural zone known as the Yakima Fold Belt.
Because this feature plunges to the southeast, the route up the canyon exposes progressively older
strata as one makes their way up the valley. For whatever ambiguity persists over the stratigraphic
order of the rocks we have visited, this section serves to resolve it.
The Thorp Gravel
The Thorp Gravel is the highest member in this package (not including the Pleistocene Palouse Loess
or local glacial tills), and is thus the first exposed in this section. The large rounded hill to the west is
comprised of the Thorp Gravel. In this part of the Kittitas Valley, these coarse-grained sediments have
accumulated to appreciable thicknesses. Topographically, they give a rounded aspect to a landscape
otherwise largely cut from blocky flows of the Columbia River Basalts.
The Thorp Gravel has been dated at about 4 million years in age, based on radiometric dating of ash
layers found near its base. This makes its accumulation coeval with the rise of the modern Cascade
Range to the west. Paleocurrent indicators in the unit reflect a western source area for this gravel. It
may well have been eroded off the rising Cascades. It is a polymictic assemblage, displaying a wide
diversity of rock types.
The prairie at the entrance to Taneum Canyon is part of an elk reserve, managed by the Rocky
Mountain Elk Foundation. Elk are the largest members of the deer family, and can often be seen
grazing here.
Continue up the road
miles to the next stop.
Stop 9 The Ellensburg Formation and the Columbia River Basalts
There are two rock types here, recognizable as the Columbia River Basalts and the Ellensburg
Formation. The Ellensburg is easily distinguished by its tuffaceous character, and the presence of
andesite pebbles in the mix. The position of the basalt above the Ellensburg eliminates the possibility
that this is the Teanaway Basalts. Above, it can be seen that this is two distinct flows of the Columbia
River Basalts, with the contact showing the effects of weathering. These are the Grand Ronde flows,
dating from about 16 million years ago.
The texture of the basalt flows here is described as a “hackely” one. It develops near the top of such
flows, in the part called the “entablature.” Because of its well-broken texture, water percolates down
through flows like this, and often emanates at contact points. For engineers building roadways, this is
the preferred material for road-beds. Its texture leaves it to break easily, but produces smaller
fragments which are angular and durable. It is considerably easier to process than massive basalt.
These flows lie beneath the Thorp Gravels, and over the Ellensburg Formation. The contact with the
Ellensburg has been eroded, by water draining down through the rock, and emanating at the contact
point. Because it will be seen that the basalts do not occur below this section of the Ellensburg, this
section is older than the basalt flows. The earliest Ellensburg sediments may date from about 25 Ma,
about 9 million years before the Grand Ronde flows. Elsewhere, these sediments also occur as
interbeds between various flows.
Continue
miles to the next stop, at a prominent bend in the road.
Stop 10 The Roslyn Formation
A recent slide on this bend in the road has exposed the outcrop above. On inspection, these are found
to be rocks of the Roslyn Formation. While not as well lithified as found elsewhere, the distinctive
white color to the sediments easily identifies it as the Roslyn. A few smaller outcrops down the road
are also consistent with this interpretation. They also confirm that the previous stop was the basal
section of the Ellensburg Formation, and that no lower basalt flows intervene between the Ellensburg
and the Roslyn Formations in this area.
The contact between the Ellensburg and the Roslyn Formations is an unconformable one. A major
regional unconformity at about 38 Ma marks the division beween the “Challis” episode rocks of the
Swauk, Teanaway and Roslyn Formations from the “Cascade” episode rocks of the Ellensburg,
Columbia River and Thorp Formations.
Continue miles to the next stop.
Stop 11 The Swauk Formation
The rocks here are the Swauk Formation, easily identified by their tan to buff color. Based on our
earlier stops, we are disposed to wonder what happened to the Teanaway Basalts? The answer is that a
small normal fault cuts the valley east of here, and the area to the east has been down-dropped relative
to that on the west. In this process, the Teanaway Basalts have been faulted out of the sequence here.
They are present on the slopes above, but do not appear at the river level where the road is.
This is a fairly good location to dig for fossils, if enough fresh material is available. Over the years we
have found fossils of palm fronds, woody debris, and the leaves of a variety of species at this outcrop.
Collectively, they reflect a lowland river-basin setting, a paratropical environment where the average
temperature was about 70 F, and varied no more than 5 degrees year - round. The dominant species
were palm trees, but a wide variety of other foliage shared that setting.
The west-side equivalent of the Swauk Formation is the Chuckanut Formation around Bellingham,
some 90 miles north of Seattle. These sediments are identical in all respects, and share the same fossil
assemblages. They represent the deposits of a continuous river basin between ~53 and 48 Ma. Starting
at about 48 Ma, a large north-south striking fault developed not far west of here, extending north to the
Canadian arctic. Over the next ten million years, the rocks on the west side of that fault (known as the
Fraser Fault) were displaced some 90 miles (145 km) to the north. The Chuckanut formation, now at
the latitude of Bellingham, was originally deposited at the latitude of Seattle. There, it was the
downstream equivalent of the Swauk Formation. This same tectonic event was responsible for
emplacing the Olympic Coast Belt into the continental margin.
Stop 12 The Darrington Phyllite
These rocks are the Darrington Phyllite, part of the Shuksan Suite, as mentioned at stop 1. Late in the
season, when the water is low, you can see these rocks at Lake Easton. Along with the Easton
Greenschist, these rocks comprise the “basement” rocks of this region, and occupy the lowest
stratigraphic position.
Phyllite is a low-grade metamorphic rock which develops from a mudstone. In this case, it was the
mud covering a section of oceanic crust. The minerals are microscopic, but the distinctive sheen on the
rock results from microscopic crystals of muscovite, a mica mineral. While homogeneous, the rock
has a distinctive structural sense of layering, or foliation, to it.
The Darrington Phyllite is distinctive because it contains abundant graphite. This gives the rock a
decidedly silvery appearance. You will find that it will make a visible mark on a piece of paper
(graphite is the material used to make pencil lead). Graphite is a carbon compound, and prompts the
question of how so much carbon was deposited in this mud. Carbon comes from organic sources, and
this quantity suggests that it was probably derived from a terrestrial source. Large amounts of organic
material are brought by rivers draining into the ocean, particularly after fires and other events. This
suggests that this section of oceanic plate, as it was being subducted, lay not far from the shoreline of
the continent.
Like the Swauk Formation (but not the younger formations), the Shuksan Suite here has been displaced
from its western counterpart to the northwest. These rocks are typical of the region north of the
Stilliguamish River, and have been displaced from their western counterparts along the Fraser Fault.
As discussed at the last stop, this happened between 48 and 38 Ma, during the Challis Episode.
Stop 13 Peoh Point
Peoh Point provides a spectacular vantage on the Kittitas Valley, and on the dramatic Stuart Range to
the north. The Stuart Range is composed of granodiorite, an intrusion dated at about 90 million years
ago. This was about the same time that the Melange Belts were being added to the continent here. It
was a time of great mountain-building along the coast, as the ancestral Coast Range Mountains
developed over this period. The granodiorite that makes up Mt. Stuart was intruded over a dozen
kilometers deep in the crust, at the root of an ancient volcano which rose on the surface at that time.
As discussed earlier, the Kittitas Valley area is a large southeast-plunging syncline, known as the
Kittitas Valley Syncline. To the south, the field trip route has just ascended the Ainsley Canyon
Anticline, the southern half of that fold system. As it turns out however, there is a break between the
Kittitas syncline and the Ainsley Canyon Anticline. That break is a low-angle thrust fault, known as
the Easton Thrust.
These rocks comprise what is known as a fold-and-thrust belt. Compressed from the south, the rocks
here fold until they periodically yield along the fault, thrusting to the north. This is the character of
most of the structures in the Yakima Fold Belt, as well as similar features to the west. While recent
uplift of the Cascade Mountains has obscured connections between the two, these faults extend west to
the Puget Sound region. The Seattle Fault, for example, shares these same characteristics. Stress
accumulates in folding the Newcastle Anticline along the south end of the fault, and is periodically
released by displacement on the Seattle Fault at its base.
This fold-and-thrust regime has been going on for some twenty million years, driven by the northward
shearing of the California coast along the transform margin in that region. As California moves north,
it drives Oregon into Washington along a northeast-southwest axis. Because the northern half of
Washington includes considerable granitic rock, it acts as a “backstop” against this northeastern
compression. As a result, a northwest-southeast set of folds (locally, the Yakima Fold Belt) has
developed to accommodate this compression. As noted, this is the same regime which causes periodic
movement on the Seattle Fault to the west.