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 KittitasValley 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 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.
Geology of the KittitasValley
The geology of the KittitasValley 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 KittitasValley. 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 TaneumCanyon, which more clearly delineates the local stratigraphy.
Along the southern margin of the KittitasValley 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 KittitasValley
Prior to the 1870’s, the KittitasValley 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 SwaukRiver. This caused a modest rush to the region, and by 1879, a rough road extended over BlewettPass 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 StampedePass 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 townCicely, 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 SnoqualmiePass to Seattle. The KittitasValley 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 SnoqualmiePass, 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 Elumpermitted 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 LakeEastonState Park exit (71) from Interstate 90 and follow the signs to LakeEastonState Park. Inside the park, take a right turn at the intersection and continue several miles to the swimming beach.
LakeEaston is a man-made feature designed to provide water for irrigation in the YakimaValley. 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 StilliguamishRiver).
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.