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Kittitas Valley
Field Trip
The Structure and Stratigraphy
of the Eastern Central Cascades
J Figge, 2009
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Kittitas Valley
Field Trip
The Structure and Stratigraphy
of the Eastern Central
Cascades
J. Figge
2009
North Seatt;e Community College
This document was prepared for the exclusive use of students enrolled in Geology 101 at North Seattle Community College, to whom it is provided as part of that body of academic coursework.This document is not available for
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© John Figge 2009
Kittitas Valley Field Trip
The Structure and Stratigraphy of the Eastern Central Cascades
John Figge 2009
Published by
The Northwest Geological Institute
P.O. Box
Seattle, Washington USA
98115
Kittitas Valley Field Trip
The Structure and Stratigraphy of the Eastern Central Cascades
J. Figge 2009
Kittitas Valley Field Trip
The Structure and Stratigraphy of the Eastern Central Cascades
Introduction
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 to the west. 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 possible field trips to suit a range of different instructional purposes. Introductory trips are usually
characterized by a larger number of stops, designed to illustrate large-scale 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.
This trip is broadly centered around the town of Cle Elum, some 80 miles east of Seattle on Interstate 90. It includes
stops at Lake Easton State Park, along the shores of Lake Cle Elum, along the Yakima River, along Taneum Creek,
and at the Peoh Point lookout. It also includes one stop in the town of Roslyn, and one in Cle Elum, the latter conveniently close to the bakery. Restrooms are available at Lake Easton, in Cle Elum (City Park or Safeway), at Indian
John Hill (I-90) and (in season) at Taneum camp in Taneum Canyon.
Perhaps the best time to take this trip is early May, when the wildflowers are in full bloom. Yellow arrowroot (above)
covers the slopes of Taneum Canyon, while blue lupine carpets the prairies of the Kittitas Valley. It is a refreshing
break from the gray skies of the west side, and a beautiful setting for examining the local geology.
Image (Left)
Yellow arrowroot colors slopes above the Yakima River, below the town of Cle Elum. Here, the Yakima cuts into flows of the
Columbia River Basalts, forming a distinct canyon. In the summer, this is a very popular stretch for rafting. The rail lines serve
the Burlington Northern - Santa Fe Railroad, and are the main east-west route across the state. Trains are a common sight
along this stretch of the river.
Images:
(Above) Rocks of the Roslyn Formation outcrop above the town of Cle Elum. These distinctly white slopes can be seen from the
freeway.
(Avove Right) Peaks of the Stuart Range rise above the meadows of the Thorp Prairie.
Table of Contents
Geology of the Kittitas Valley .........................................................................1
A Brief History of the Kittitas Valley................................................................4
The Kittitas Valley Field Trip
Stop 1 The Easton Greenschist, Lake Easton........................................................9
Stop 2 The Swauk and Teanaway Formations, Lake Cle Elum............................12
Stop 3 The Swauk Formation, Lake Cle Elum.......................................................15
Stop 4 The Roslyn Coal Mines, Roslyn................................................................17
Stop 5 The Roslyn Formation, Cle Elum..............................................................20
Stop 6 The Ellensburg Formation, Yakima River..................................................21
Stop 7 The Columbia River Basalt Flows, Yakima River......................................25
Stop 8 The Thorp Gravel, Taneum Canyon............................................................27
Stop 9 The Columbia River Basalts and Ellensburg Formations, Taneum Cyn.....30
Stop 10 The Roslyn Formation, Taneum Canyon...................................................32
Stop 11 The Swauk Formation, Taneum Canyon....................................................33
Stop 12 The Darrington Phyllite, Taneum Canyon.................................................37
Stop 13 Peoh Point.................................................................................................39
A Final Last Word From Your Instructor............................................................................44
Kittitas
Swauk Formation
Syncline
Teanaway
Formation
2
3
Roslyn
Formation
4
Fraser
Fault
5
6
Cle Elum
13
12
11
Ainsley Canyon
7
10
Anticline
9
8
Columbia River BAsalts
Figure 1 Simplified geologic map of the Kittitas Valley area. The two main structural features are the Kittitas Syncline and the
Ainsley Canyon Anticline, both of which plunge to the southeast. The Columbia River Basalts erupted after these features were
initially formed, but have been similarly deformed along these lines. The various formations along Taneum Canyon and the Yakima River are not detailed, as the outcrops are too small at this scale. See figure 59 for additional details. Red dots indicate field
stops
1
The 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.
Map area
to the left
Figure 2 Regional map showing the locale of the Kittitas Valley
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 andesitic to dacitic in character, and include significant lahar deposits in their mix.
2
Palouse Loess
1 MA
Thorp Formation
4 MA
Columbia
River
Basalts
Ellensburg
Formation
Cascade
Episode
17 MA
25 MA
40 MA
Roslyn
Formation
46 MA
Teanaway
Formation
Challis
Episode
48 MA
Swauk
Formation
53MA
Darrington
Phyllite
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 lowangle 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.
Figure 3 (Above)
Simplified stratigraphic column for the Kittitas Valley region. All contacts are unconformable. Not to scale.
The Tertiary formations here can be divided
into two groups, based on two distinct episodes of regional plate tectonic relationships.
3
A Brief History of the Kittitas Valley
Figure 4 The town of Roslyn, circa 1889. Early mining operations are pictured here. Over the ensuing years, they were expanded considerably.
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 lessglamorous 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.
4
Figure 5 Coal mining in Roslyn in the early 1900’s. The mineshafts extended some 2700 feet beneath the town, in seven levels.
One set of tracks brought miners and ore carts down, the other brought full carts up. The work was dirty, hazardous and strenuous, but provided steady employment for over half a century. Historical images from the Kittitas County Historical Society.
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, Keechelus and
5
Figure 6 A saloon in the town of Ronald, just north
of Roslyn, circa 1898. The Roslyn mines stretched
from Ronald to Cle Elum, the # 3 mine being in the
town of Ronald. Ronald took its name from the mining superintendant, with the thought that it might
gain some advantage from that patronage.
Ronald’s current claim to fame is the “Old # 3 Tavern”, advertised on bumper stickers seen around the
state. It stands to benefit from increased development
underway in the region.
Figure 7: The hotel in Cle
Elum, circa 1910. Cle Elum
distinguished itself as a more
refined and cultured setting than the mining town of
Roslyn. Located on the main
rail line, it hoped to prosper by providing services to
travelers.
The town adorned its central
business district with opulent
architecture, as pictured
here. Unfortunately they
were of wooden construction, and most of the business
district burned to the ground
in a fire in 1918.
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.
Figure 8: Cle Elum today. Main street here hasn’t changed much since 1968, when the freeway by-pass was completed. The
faded patchwork of buildings, gas stations and truck service yards gives it a somewhat utilitarian sense of decor.
6
Snoqualmie Pass
Stuart Range
Keechelus
Lake
Kachess
Lake
2
Cle Elum
Lake
3
1
Lake Easton
Yakima
River
4
Roslyn
Cle
Elum 5
13
6
7
Taneum Creek
12
11
10
9
8
Figure 9 Area map showing location of the field stops. Heavy red line is Interstate 90. Seattle lies roughly 60 miles (100 km)
west of Snoqualmie Pass
7
Kittitas Valley Field Trip
A Few Notes on Field Guide Organization:
Travel directions are printed flush to the margin, in italic type
Field stops and other geologic notes are printed flush to the margin, in regular type
Other notes of interest are intented, and printed in this type.
Travel distances are given in miles
General Description:
This trip starts out at Lake Easton State Park, about 70 miles east of Seattle. From there it makes its way north to
the shores of Lake Cle Elum, for two stops in the Swauk Formation. Returning to the south, it makes one stop in the
town of Roslyn, and one in the town of Cle Elum, to consider the Roslyn Formation. It then continues east to examine
outcrops of the Ellensburg Formation, the Thorp Formation, and the Columbia River Basalt flows.
To route then crosses to the south side of the valley, and heads up the canyon of Taneum Creek. Here are included
stops in the Thorp Formation, the Columbia River Basalts, the Ellensburg Formation, the Roslyn Formation, the
Swauk Formation, and finally the Darrington Phyllite. The trip concludes with a visit to Peoh Point, a spectacular
viewpoint on the Kittitas Valley, and an excellent setting for considering the structure of the region.
Figure 10 (Above) Rocks of the Roslyn Formation, outcropping above the town of Cle Elum. Note the distinctly white color to
the rocks
8
Figure 11 Lake Easton
This is an artificial lake,
impounded to provide irrigation water for the Yakima Basin. It takes its name from the
small community of Easton,
just to the east. The town was
so named because it was at
the eastern end of the Great
Northern Railway tunnel
under Stampede Pass.
Just 70 miles east of Seattle,
this is a popular recreation
site over the summer. The
lake is a state park, and
features camping, boating,
hiking, and a host of other
activities.
Stop 1: The Easton Greenschist,
Lake Easton
Take the Lake Easton State Park exit (70) from Interstate 90 and follow the signs to Lake Easton State Park. Inside
the park, take a right turn at the intersection and continue .9 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 feet 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
9
Figure 12 (Right) Outcrop of
the Easton (Shuksan) greenschist along the shore of Lake
Easton. Hammer provides
scale. In late season the lake
level is lower, exposing sections of phyllite below.
Figure 13 (Below) Detail
from the outcrop above. Here
you can see the distinctly
green color of the rock, and
the near-vertical foliation.
Hammer provides scale.
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.
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
10
Melange Belt Teranes
New (~120 Ma)
Kula-Farallon
spreading center
Older (>120 Ma)
Subduction Zone
Columbia Embayment
Figure 14 Simplified diagram illustrating the suspected origins of the Melange Belt Terranes. These appear to have have been
accretionary deposits accumulated above a subduction zone along the coast south of here. That subduction zone and the associated coastal rocks were rifted off the margin as a new spreading center (the Kula-Farallon Ridge) formed here. The northward
sense of plate motion north of that new spreading center transported these rocks to our area.
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.
11
Figure 15: Xenolith (inclusion) of a block of Swauk sandstone in the Teanaway Basalt. Person gives scale. By the stratigraphic
principle of inclusion, the Swauk must be older than the Teanway.
Stop 2 The Swauk and Teanaway Formations,
Lake Cle Elum
Return to Interstate 90 and continue east to the exit for the town of Roslyn (Exit 80). Take the exit and continue north
through the towns of Roslyn and Ronald. From downtown Ronald (the # 3 Tavern) continue about 7 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. The location is 47 20 19.88 N by 121 06 17.42 W Elevation 2261 feet
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 1933,
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. It likely dates from ~53 - 48 million years ago.
12
Figure 16: Cross-bedded sandstone, just north of the locale pictured in figure 15. These have been somewhat metamorphosed by
the adjacent basalt. Note that these beds dip steeply to the rear. Pipe gives scale.
Figure 17: The Swauk
sandstone, at an outcrop
well up the road from the
field stop. These are planar
and crossbedded sandstones, typical of a floodplain setting.
The Swauk is a voluminous
formation, encompassing
up to 5 km of strata. This
makes it one of the thickest non-marine sedimentary formations in North
America. These sediments
were likely transported by a
large river system heading
on the Rocky Mountains.
13
Figure 18: The Swauk Formation with an intruding dike of the Teanaway Basalt, at a location near Blewett Pass. This illustrates
that the basalts intruded vertically through previously-tilted strata. The Swauk Formation was folded into a series of NW-SE striking folds, and then eroded back to a relatively flat landscape prior to the eruption of the Teanaway Basalts.
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 quartzofeldspathic 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 multiple eruptive events, but all appear to have been voluminous extrusions.
It is important to recognize that the Teanaway Basalts intruded Swauk Formation rocks which had been folded prior
to that event. Between the ~50 - 48 Ma conclusion of deposition in the Swauk Formation, and the ~47 Ma eruption of
the Teanaway Basalts, these rocks were folded into a series of NW-SE striking folds. Those folded rocks were eroded
back to a relatively flat landscape prior to the eruption of the Teanaway Basalts.
14
Figure 19 Stop 3 outcrop, Dr. Furutani lecturing.
Note the mixed character of this rock, including both fine-grained and coarse-grained components. This combinaton reflects the
actions of a river meandering across its floodplain, some 50 million years ago. Students look on in absolute amazement.
Stop 3: The Swauk Formation,
Lake Cle Elum
Turn around and return south on the road toward Roslyn. After about 2 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 location is 47 19
46.82 N by 121 06 20.48 W Elevation 2307 feet
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
15
Crevasse Splay
sandstone
Floodplain
siltstone, shale, coal
Main Channel
conglomerate
Levee
sandstone, siltstone
Oxbow Lake
siltstone, shale
Point Bar
cross-bedded sandstone
Figure 20 (Above) Depositional setting for the Swauk Formation, showing where various sediments are deposited in a river
basin setting. The rocks of stop 3 are river-channel deposits, incised into older riverbank and floodplain sediments. These are
common lithologies in the Swauk Formation. Adapted from Mustoe, 1997.
Figure 21: (Right) Outcrop detail. The black line
highlights the unconformable contact between the
conglomerate above and the finer sediments below.
The conglomerate is a channel deposit, while the
fine-grained materials are levee and floodplain
deposits. The unconformable contact is created as
the river channel migrates across its floodplain over
time.
The vretical features are drill holes, used to blast
rock during highway construction. They are not,
contrary to popular belief, the tunnels of hortas.
Hortas are a silicon-based lifeform capable of boring through rocks by secreting a strong acid. Hortas
only live on Janus VI.
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.
16
Figure 22: The modern-day town of Roslyn.
Stop 4: The Roslyn Coal Mines
Roslyn, WA
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 African-American 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 few 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 of Cicely, Alaska. In appreciation,
the production company furnished new metal roofs for the town residents.
17
Figure 23 Roslyn,
circa 1900. Stop 4 is
at the entrance to the
mines, the terminus
of the railroad spur.
Note the well-ordered appearance
of the town, with
boardwalks, telegraph poles and a
baseball field (front
left). Paint seems to
be a scarce commodity.
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.
Figure 24 The historic Brick Tavern, the oldest continuously operating saloon under the same name in the State of Washington. It was completed in 1889, and still stands today. It is said
that construction required some 20,000 bricks. Unfortunately,
the brickwork has since been painted over.
18
Return to the highway and head south to the town of Cle Elum
“Suncadia” is a residential / recreational development which was approved by the town of Cle
Elum some years ago, in hopes that it would boost the local economy. It is a secured private
community featuring a championship golf course, private river access, private hiking and
biking trails, tennis courts, swimming pools, and a host of other amenities. Modest vacation
homes start in the low $400’s. Ultimately, it will be larger than the town itself.
Rather than spuring development in Cle Elum, this project has fostered rapidly-increasing
development of the corridor through Roslyn to Lake Cle Elum. Vacation lots have been platted
across the landscape, many of them gated developments designed to appeal to security-conscious west-siders. They are being marketed to “coasties” from the Puget Sound region, and
to retirees across the country.
The residents of Cle Elum haven’t seen any benefit, and those in Roslyn see this as a serious
threat to their historic community. The advisability of such development is dubious. The entire
region is ponderosa pine forest, a landscape periodically subject to fires.
At the round-about, turn left (east) toward Cle Elum
Between the towns of Roslyn and Cle Elum are a whole string of coal mines, the talings piles can be seen along the
north side of the road.
Continue through the town of Cle Elum to Peoh Street (Bakery) or Columbia Avenue (Stop 5)
Cle Elum is a somewhat non-descript town, a patchwork of faded buildings from different
eras marking the main street of commercial activity. The number of vacant store-fronts suggests a less-than-thriving local economy. The rail yards, sas stations, truck service yards and
other utilitarian enterprises give it a unique ambiance.
The town is none-the-less the commercial center of the upper Kittitas Valley, and provides
services to a broad region. It’s most redeeming quality is that it hosts the Cle Elum Bakery,
widely acknowledged as one of the best in the state.
Figure 25 The historic Cle
Elum Bakery. The brick ovens here have not gone cold
since first fired up in 1906.
It is a full-service bakery,
providing a range of breads,
cookies, pastries and other
products.
Cross-state travelers often
stop here to stock up on
baked goods.
19
Figure 26: Exposures of the
Roslyn Formation, above the
town of Cle Elum. Unfortunately, these outcrops are extremely weathered, and show
few details of the rock. They
are also on private property,
and not available for public
access.
What can be seen here is the
distinctly white color of the
rock, in contrast to the tan
or buff color of the Swauk
Formation. Note also that
these rocks are not cut by
dikes of the Teanaway Basalt,
which helps to distinguish the
Roslyn from the Swauk where
the rock types are similar.
Stop 5 The Roslyn Formation
Cle Elum, WA
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.
Figure 27 The Roslyn
Formation, stop 5. This is
not much of an outcrop, but
provides an opportunity to
examine fresh samples of the
rock.
This is on one of the many
new roads being cut into
the hills behind the town of
Cle Elum, for new housing
developments and vacation
communities. Development
is happening all across the
area, except in the town of
Cle Elum itself.
20
Figure 28 The Roslyn Formation, Stop 5. The scale of bedding in the Roslyn Formation is too large to be seen at this outcrop,
but it is a good site for examining the rock itself. In contrast to the tan and buff-colored sandstones of the Swauk Formation, this
is a distinctly white rock. The white color comes from an alteration product (laumontite).
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 larger-scale 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.
Figure 29 (Right) A hand-sample of sandstone from the Roslyn
Formation. Note the distinctly white color of the rock. Dime
provides scale.
21
Figure 30 The Ellensburg Formation, along the Yakima River (Stop 6). The Ellensburg is almost exclusively volcanic sediments,
shed from the ancestral Cascade volcanoes to the west. Note the prominent fault in the center of the image.
Stop 6: The Ellensburg Formaton
Yakima River
Return to main street (SR 970) and continue east out of town. The road is SR 970, heading north. After a short distance, turn right onto US 10, signed for Ellensburg. Take this road for about 5 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 poorly-sorted, matrix-supported, well-rounded cobbles of andesite and
dacite, 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.
22
Figure 31 Normal (extensional) fault in the Ellensburg Formation. Image on the right shows pre-fault configuration. This fault
probably developed as the Columbia Basin to the east sank under the accumulated weight of up to 5 km of basalt flows from the
Columbia River Basalts.
The “Ellensburg” formation refers to all tuffaceous sediments which accumulated below and in-between 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 Highway 10, toward Ellensburg. Continue about 2.5 miles to the next stop.
In short order, the route enters the province of the Columbia River Basalt Flows. These are a regional-scale feature
covering much of eastern Washington and Oregon.
23
Figure 32 (Above) The Thorp Gravels, along Highway 10. These overlie the Columbia River Basalt Flows. Above the gravel can
be seen a thin layer of the Palouse Loess. Both of these units become thicker to the east.
Figure 33 (Below right) The Yakima River, as it cuts into the Columbia River Basalts. From near stop 7.
In 2 miles, note the layer of gravel which
lies on top of the Columbia River Basalt
Flows. This is gravel of the Thorp Formation, known as the Thorp Gravels. The
Thorp Gravels accumulated about 4 million years ago. Atop the Thorp Gravels
lies the Palouse Loess, the dominant soil
of Eastern Washington.
The Palouse Loess is a glacially-produced
silt, and was deposited over a million
years ago, during the early ice ages of the
Pleistocene. It is an outstanding soil for
agricultural purposes, and is the economic
foundation of Eastern Washington.
Continue a total of miles to a slight turnout on the right side of the road. The rocks
here are a distinctly yellow color.
24
Figure 34 Pillow-palagonite complex, along US 10 (Stop 7). These developed as basalt flowed over moist ground or water. They
are relatively common appearances across the basalt province of Eastern Washington, illustrating that this region enjoyed a wet
climate prior to the rise of the modern Cascade Range.
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.
25
Figure 35 Clay layer
(arrow) at the base of the
palagonite complex, stop 7.
This clay accumulated in the
bottom of a small lake, a lake
which the basalt subsequently flowed over.
Clay Layer
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.
Figure 37 (Below) Fossilized tree trunks, at the Gingko Petrified Forest Interpretive Center. These trees reflect a moist and
temperate setting here some 16 million years ago, before the
rise of the modern Cascade Range.
Figure 36 (below) A model of the Blue Lake Rhino. The body
cast of this animal was found at the base of basalt flows near
Blue Lake, in the Grand Coulee. This model is on display at
the Dry Falls Interpretive Center
26
Swauk Formation
Teanaway
Formaton
2
1
3
Roslyn
Formation
Kittitas
Syncline
4
5
Cle Elum
13
12
Ainsley
Canyon
Anticline
11
6
7
10
9
.
Columbia
River Basalts
8
Figure 38 Geologic map of the Kittias Valley region, showing field trip stops. Rocks of the Ellensburg Formation, along with the
outcrops along Taneum Creek, are not detailed at this scale. See figure 9 for regional view.
Continue east on US 10 to the Thorp Mill Road, and turn right on that road. Continue 2.5 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. Reset your trip odometer here.
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 north-directed compression which has been in effect for the last twenty
27
Taneum Canyon
Thorp Gravel
Figure 39 Looking west into the entrance of Taneum Canyon. The hillside on the right is mantled in the Thorp Gravels, which
accumulated starting about 4 million years ago. They likely reflect the rise of the modern Cascade Range.
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 order of the rocks we have visited, this section
serves to remove 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 broad diversity of rock types.
28
Figure 40 (Right)
Elk graze in the
meadows of Taneum
Canyon. Part of
this region is an Elk
reserve.
Figure 41 (below)
Near the entrance to
the canyon. The rock
outcrops are basalt.
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 to the next stop at 2.3 miles.
29
Figure 42
The Columbia
River Basalt Flows
and the Ellensburg Formation,
in lower Taneum
Canyon. This view
illustrates that the
basalts consist of
two distinct flows.
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.
30
Figure 43 The Ellensburg Formation, below the Columbia River Basalt Flows. The Ellensburg is easily discerned by the tuffaceious character of the fine sediments and the volcanic character of larger clasts.
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. It breaks
relatively easily, and 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.
Looking up the road, one can discern than the contact between these two rocks dips to the southeast. This is consistent
with the southeast-plunging character of the Ainsely Canyon Anticline. Accordingly, we will be advancing through
progressively older rocks as we progress up the valley.
Continue to the next stop at 4.2 miles.
31
Figure 44 The Roslyn Formation, exposed by a landslide on a prominent bend in the road. Note the distinctly white color.
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 FormaFigure 45 Detail of the site above. These sediments are not
tions is an unconformable one. A major regional unconfor- as well lithified as those seen earlier, but are clearly of the
mity at about 38 Ma marks the division beween the “Chal- Roslyn Formation. Hammer provides scale.
lis” episode rocks of the Swauk, Teanaway and Roslyn
Formations and the “Cascade” episode rocks of the Ellensburg, Columbia River and Thorp Formations.
Continue to the next stop at 5.2 miles.
32
Figure 44
Outcrop of the
Swauk Formation,
along the Taneum
Road. The outcrop is
not exactly obvious,
so it is worth watching your mileage to
see that you don’t
pass it.
It is a fairly productive site for fossil
hunting, given a
continuing supply of
new material.
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 left 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.
33
Figure 45
Leaf imprints from
the Swauk Formation.. These appear
to be a cinnamonium species, thought
to be an extant
member of the Laural family. Hammer
provides scale.
The fossil foliage of
the Swauk Formation is typical of a
warm paratropical
environment, dominated by medium to
large-leaved species
suited for a warm
damp climate.
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
Figure 46
Leaf imprints from
the Swauk Formation. This appears
to be a maple leaf,
a common species.
Hammer provides
scale.
34
Figure 47 Leaf imprints
(Palm fronds) in the Chuckanut Formation, east of
Bellingham. The Swauk and
the Chuckanut Formations
were originally coextensive.
Based on the fossil record,
palm trees were the dominant vegetation during this
period, reflecting a warmer
and more equitable climate.
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.
Figure 48
Generalized paleogeography of the northwest over Eocene
time (ca 50 Ma).
Note the large river
system draining from
the east. Note also
the absence of the
Olympic Peninsula,
and the large embayment in the coastline
to the south. Dashed
line is state outline.
*
Columbia Embayment
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* locates Swauk
Formation
Fraser Fault
Leech River Fault
C
C
S
S
Fraser
Fault
50 Ma
40 Ma
Figure 49 (Above)
Really complicated-looking map depicting the Mid - Eocene (50-40 Ma) tectonic setting in this region. The different-colored
areas represent different “terrane” units which make up the basement rocks. The figure on the left reflects conditions at about
50 Ma, while the figure on the right reflects conditions at about 40 Ma, after faulting on the Fraser and Leech River Faults.
In the figure on the left, the red dots represent the Chuckanut Formation (C) around Bellingham, and the Swauk Formation
(S) around Cle Elum. These were originally a coextensive formation. In the figure on the right, note how right-lateral movement on the Fraser Fault has displaced the west side to the north by about 150 km. This same regime was responsible for
transporting the oceanic rocks of the Olympic Coast Belt to the north, underthrusting the southern end of Vancouver Island
along the Leech River Fault. In this process, this last “terrane” was implaced on the continental margin.
These events happened between 50 and ~40 Ma, and reflect the north-directed transform tectonics which was characteristic
of the 58 - 38 Ma “Challis” Episode. By 37 Ma, a convergent margin had been re-established along the coast here. That
tectonic setting is characteristic of the modern (37 Ma - present) “Cascade” Episode.
Continue up the road to a major fork, and take the right fork signed for Peoh Point. Stop a few hundred feet up the
road at this outcrop. Total mileage 9.6 miles.
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Figure 50 An outcrop of the Darrington Phyllite, above Icewater Creek Camp. On a sunny day, it is easy to spot as it glitters in
the sun.
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.
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Figure 51
The Darrington
Phyllite, detail from
the image to the left.
Hammer provides
scale.
The luster of this
rock is a combination of the microscopic muscovite
(white mica) typical
of phyllite, with
graphite, which
imparts a silvery
color.
Continue up the road toward Peoh Point. At the top of the ridge, there is a parking area where the road makes several
forks. Park here and continue on foot. The road to Peoh Point can be traveled in 4-wheel drive vehicles under good
conditions, but it is a somewhat nerve-wracking affair. It is easier to park the vehicles here, and walk the mile to the
point.
As you start up the road, note the rock it is cut into . This is the Swauk Formation. It is a silty, clayey section of the
Swauk, and makes for a very slick road when wet. Occasionally, fossils can be found in the outcrops here.
Follow the road, as signed, to the ridge crest and the turnoff for Peoh Point. Park here and walk this road for about a
mile and a quarter to the Peoh Point Overlook.
Figure 52 (Right)
A hand-sample of Darrington Phyllite, a somewhat
less graphitic example than seen in the field. Phyllite
develops from mudstone, and is characterized by the
development of microscopic muscovite. The Darrington Phyllite also includes knobs and ribbons of
quartz, as seen here,
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Figure 53 Looking north from Peoh Point. In the background are peaks of the Stuart Range, culminating in 9415 - foot Mt. Stuart. Below is Cle Elum Lake, an artifically-impounded reservoir which stores water for irrigation. The town of Roslyn can be seen
in the foreground. This is a truly spectacular vantage on the Kittitas Valley, well-worth the trip when the weather is clear. Highly
recommended on clear spring days, when snow still mantles the mountains.
Stop 13 Peoh Point
As you start up the road, you will notice that the road-cut is in fine-grained sediments of the Swauk Formation. When
wet, this is a slick bit of roadway. This is the same belt of the Swauk Formation as was visited below. On reaching
Peoh Point, you will notice that the rock is a light-colored volcanic rock (felsite). These are rocks of the Taneum
Formation, which locally appears between the Swauk Formation and the Teanaway Formation. This magma was quite
different from the Teanaway Basalt, and came from a different source. It reflects a continental - arc setting (the Challis
Arc) which existed east of the Fraser Fault from 58 - 40 Ma. The Taneum Formation is not a wide-spread unit.
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.
The overlook provides a clear view of Cle Elum Lake, which is impounded by a dam for irrigation purposes. Prior to
being artificially dammed, two smaller lakes occupied this valley, impounded by moraines. On careful inspection, you
can discern moraine features on the landscape here.
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.
39
Figure 54 Looking east from Peoh Point. Note the light-colored volcanic rock in the foreground, part of the Taneum Formation.
Consider the basalt flows which make up the top of the mountain in the center of the image. The tilt of these flows (down to the
east) reflects uplift of the modern Cascade Range to the west. Projected westward, the top of this flow extends over the top of Mt.
Stuart to the west (see figure 53, left).
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.
40
Figure 55 (Above)
“Fixed” North American Block
The Easton Thrust Fault, cutting across the
southern end of the Cle Elum Valley. Rocks on the
foreground side have been uplifted and moved
northward along this feature.
North-South
Compression
Yakima
Fold Belt
Figure 56 (Right)
Diagram illusrating how the combination of
northward shear along the California margin and
eastward compression along the Washington-Oregon margin produce northeast-directed compression into Eastern Washington, and northward
compression in Western Washington. Based on a
diagram by the US Geologic Survey.
The Yakima Fold Belt is a series of northweststriking folds which have accomodated this
compression. It is a fold-and-thrust belt, where
folding has yielded to low-angle thrust faulting
along the base of the folds.
In Western Washington, northward compression is
accomodated on fold-and-thrust features like the
Seattle Fault and the South Whidbey Island Fault,
among others. These share the same architecture
as the folds in the Yakima area.
Rotating
Coastal
Block
Juan De
Fuca Plate
Pacific
Plate
41
Translating
Sierra Block
Figure 57 (Above)
Northeast
Southwest
Kittitas
Syncline
The Easton Thrust Fault, cutting across the
base of the Cle Elum Ridge. This view is
looking east from Peoh Point. Rocks in the
foreground have been moved up and north
relative to those behind.
Ainsley
Canyon
Anticline
Figure 58 (Right)
Easton Thrust
Fault
Diagram illustrating the development of a
fold-and-thrust belt. Rocks are initially folded
by compression, but eventually yield to lowangle thrust faulting along the base of the
anticline. This is the dominant character of
the Yakima Fold Belt.
42
Kittitas Valley
Syncline
Easton
Thrust
Fault
Ainsley
Canyon
Anticline
Figure 49 Simplified geologic map of the Kittitas Valley, showing the major structural features. The Easton Thrust Fault lies at
the base of the Ainsley Canyon Anticline, moving those rocks up and northeast over rocks in the Kittitas Valley Syncline. This is
the character of the Yakima Fald Belt, accomodating compression to the northeast.
43
A Last Final Word From Your Instructor:
The Pacific Northwest is home to some of the most remarkable geology to be found anywhere on the planet.
No region can claim to a greater variety of rock types, or features them in more spectacular settings. More
significantly, no region affords such a remarkable venue on the truly collosal forces which drive the dynamics
of our planet, or such graphic illustrations on the variety of geologic processes which they support. There is
simply no better place on the planet to see how the Earth works. There is certainly no better place on Earth
to learn and experience geology.
If you are planning on living in this area, you should know that you are living in the midst of some of the
most incredible geology in the world. You should know that the modern landscape that surrounds you is the
product of a truly amazing course of geologic history, one that stretches back hundreds of millions of years.
You should recognize that you occupy a unique point in time and space in that course of history, amidst the
ongoing geologic processes which will continue to shape this region into the future.
Finally, you should appreciate that you are the very first generation of people to enjoy the privilage of walking this landscape with a knowledge of the story behind it. In this respect, yours is a truly privilaged position
in that course of Earth history.
John
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