Drive from UW to Snoqualmie Pass
Glaciation and recent tectonics dominate the landscape from UW to Snoqualmie Pass.
Glaciation ended in the Puget Sound area a scant 10,000 years ago (end of the Pleistocene
or Quaternary). The upland (relatively speaking) of the main part of campus, about 200500 feet above sea level is the base of the continental glaciers the entered Puget Sound
from the north. These glaciers blocked the Straits of Juan de Fuca forcing the meltwaters
to travel uphill from the Seattle area to their outlet south of the Olympic Mountains near
Aberdeen. As a result, the waters underneath the glaciers were under considerable
pressure and large volumes of meltwater carve immense channels that we see today as
Puget Sound, Lake Washington, and Lake Sammamish, among others. We will see many
effect of glaciation along the I-90 route. Look for:
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The “plain” that represents the continental glacier base
Steeply dipping delta “foreset” beds (front edge of a delta) in the large sand and
gravel quarry north of I-90 at Issaquah
Terminal moraines of the alpine glaciers east of North Bend (a large ridge of sand
and gravel crossing the Snoqualmie valley). These accumulations built up at the
front of the glaciers where melting is strongest – this is where I-90 starts climbing
into Snoqualmie Pass
A contrast of the relative smooth base of Mt Si just beyond North Bend and it
more jagged top – this smooth part represents the part of the mountain exposed to
glacial carving (and thus smoothed).
The second major topographic feature of the first part of our trip is the “Issaquah Alps”
so named by Harvey Manning who was successful in promoting land preservation in the
face of urban sprawl on the Eastside. The Issaquah Alps are folded Eocene-age
sandstones, mudstones and coal deposits. We will see these close up on Field Trip #2
when we look at the effects of coal mining subsidence on the terrain.
When I first came to Washington 18 years ago, these “alps” were thought to be eroded
core of ancient mountains. We now know they are part of an active tectonic system
associated with the Seattle Fault. The fault runs parallel to I-90 from west of North Bend
across Seattle’s Sodo area, and over to Bainbridge Island. It has thousands of meters of
offset bringing Eocene bedrock to surface on its south, upthrown side. The Seattle Fault
was recognized in the 1990’s and it had very major earthquake around 1000 AD. We
know this in part from sunken forests in many of the lakes; include one at the south end o
Lake Sammamish (Field Trip #2). The Seattle Fault is part of a system of E-W trending
compressional/thrust faults. The faults accommodate deformation in crust of the upper
surface of the Subduction zone. The earthquakes may not be as big as ones on the zone
itself, but are likely to cause as much or more damage because of their shallow foci.
The final “big” feature is the Cascade Range itself which appears as an imposing front.
The range is a product of the interaction of the Juan de Fuca Plate and the North
American Plate.
Geological History Simplified
There are some Paleozoic rocks in the core of the San Juan Islands, however the oldest
Cascade Range of in western Washington are from the latest Paleozoic (Permian) or early
Mesozoic (Triassic). These cannot be dated with great confidence because there is scant
fossil evidence and metamorphism has reset the radioactive “clocks” through
recrystallization.
The basic subduction setting of western Washington becomes set with the opening of the
Atlantic Ocean at the end of the Paleozoic and beginning of the Mesozoic (Triassic) starts
the westward motion of the North American continent. The Cascade geology is one of
classic andesitic to rhyolitic volcanic activity with associated sediment deposition that is
primarily continental with shales, sandstones and coal deposits, or deep marine, the latter
being accreted (or attached) to North America in association with the plate movements.
We see the evidence of the subduction zone activities through the volcanic rocks left
behind as well as the exposed granitic plutons that were feeding the igneous activity.
Also associated with subduction zones, is the combination of deep sea pillow basalts,
cherty sediments, and mantle-derived ultramafic rocks of the Ingalls tectonic complex
along US-97.
We will see two of these plutons (or at least material from them) specifically the
Cretaceous-age Mount Stuart batholith (which lies along US 2) and the Miocene age
Snoqualmie Batholith which straddles I 90. The Mount Stuart Batholith lies within a
well developed metamorphic belt of schists and gneisses.
The geology of the Cascades is very complex and not the best place to begin geologic
studies if simplicity is one’s goal. The complexity arises in part from the jumble of
accreted terranes, tectonic blocks, or mélanges (from French for really mixed up stuff).
Correlation is difficult between these units as block or terrane boundaries are faults the
separate rock assemblages that appear to have little relationship to their neighbors.
In terms of chronological order here is what we will see:
Pre-Cretaceous: Serpentines and deep sea sediments/basalts of Ingalls Tectonic complex,
which are intruded by Mount Stuart Batholith (se see sediments from unroofing)
Cretaceous: Mount Stuart Batholith. Metamorphism of sediments and volcanics of the
Easton Metamorphic Complex
Eocene: Lots! Renton formation sandstones, volcanics and coal (drive by in Eastgate
area along I-90), Swauk Formation – conglomerates from Mt. Stuart batholith; Teanaway
block basalt and sedimentary rocks as well as dikes (first sediments then basalts)
Oligocene: Ohanapecosh Formation, Andesites and other volcanics along I-90 east of
North Bend; intruded by Snoqualmie Batholith; also Naches formation in Snoqualmie
Pass
Miocene: early Miocene- Snoqualmie Batholith; middle to late Miocene: Columbia River
Basalts
Quaternary (Pleistocene): Puget Sound glacial deposits and features, Bretz flood deposits
and features.
Snoqualmie Pass
We will have up to three stops in Snoqualmie Pass. We will stop to look at granitic rocks
of the Snoqualmie Batholith at Denny Creek. We will hike to the Denny Creek water
slide which is located in rock close to the batholith margin, and look at structures and
country rock inclusions in the granite. We will stop at a road cut and take some
orientation measurements on joints and fractures. Finally, we will look at some roadcuts
at Hyak near I-90 for an exercise in rock identification and to look at shear/fault features.
Denny Creek Questions:
Sketch the a portion of the exposure showing scale, inclusions, any intrusions into the
granite, and joints/fractures.
Road outcrop: Form teams of three with compasses, and take measurement of orientation
on 10 fractures. Plot the results on a stereoplot, one plot as poles and one plot as dip
vectors.
Hyak Questions
Exposure 1 – What is the rock type here and give your reasoning? Try to locate any
bedding or foliation features. What is their orientation? Locate a possible fault and
sketch. How do striations (slickensides) indicate fault movement direction?
Exposure 2 – What are the features affecting rock cut stability? Locate and sketch some
rock wedges.
South Cle Elum Ridge
We have two stops here to look at metamorphic rocks. One to inspect a phyllite, the
other to look at banded gneisses. These rocks are part of the Easton metamorphic
complex that grades from moderate to quite high metamorphic grade.
What are some geotechnical properties of the rocks in these outcrops?
For the gneiss stop (be very careful of logging truck traffic) note the sketch the
deformational features.
Teanaway Block and Ingalls Tectonic Complex
This material is described in the Ruby Creek guidebook from the geological society of
America.
This portion of the trip follows a stratigraphic section of sedimentary rocks and basalts,
crosses a major fault zone, and includes a sliver of mantle derived rock. We do not see
the Mount Stuart batholith in this trip, but we certainly feel its presence in the sediments
shed from the body as it was being unroofed by erosion and sedimentation.
At Ruby Creek we see the fault contact on the Leavenworth Fault zone between Swauk
conglomerates shed from the batholith. These are juxtaposed against ultramafic rocks of
the Ingalls Tectonic complex.
Questions
Walk the outcrop on the Ruby Creek road. Identify the major rocks types and locate the
fault zone. Which is the upthrown side? Look for secondary faulting and shearing
features in the conglomerate and sketch. Show some evidence for shearing.
Along the trip of Hwy 97, we travel down section (from younger to older). We start in
the basalts of the complex. They look like Columbia River basalts but they are older
(Eocene). Look for evidence for the source of the basalts as we travel along the route.
We will make periodic stops along the way to measure bed orientation. What to the
measurements tell us about the geologic structure here?
Frenchman Coulee
The Vantage area provides us with a look at two major floods – the flood of Columbia
River basalt of the middle to late Miocene and the so-called Bretz, or Spokane floods that
occurred late in the ice-age period (Pleistocene). In addition, these stops give a look at
the Yakima fold belts, which are caused by a strong north-south compression in the
Columbia Basin. These folds shape the topography and control the course of the Bretz
flood here at the western margin of the basin.
If time allows we will make a stops at Frenchman coulee where we will take an overland
hike to look at the dry falls and plunge pools formed by the Bretz floods. The floods (or
jökulhualps from such phenomena in Iceland) occurred when an ice dam in the Montana
Rockies failed during the ice age. This dam was formed by the Spokane lobe of the
Cordilleran ice sheet that dammed the valley of the Clark Fork River forming glacial
Lake Missoula. This dam failed repeated from about 15,000 years ago to 10,000 years
ago in a series of about 40 floods that had devastating effects on eastern Washington.
For a short time, the floods carried flows greater than all present rivers of the world
combined. They cut channels in the loess (wind-deposited glacially-derived sediment)
down to the basalt bedrock to form the channeled scab lands. The waters contained
icebergs that dropped large boulders as they melted, as well as depositing tell-tale
sediments in the backwater valleys that were not part of the direct flow path. Where the
flood passed over cliffs, the waters formed immense water falls with plunge pools that
are now known as dry falls.
At Frenchman coulee we are at the western edge of the Columbia Basin. Waters passing
through the northern part of the basin become concentrated here as the flow is prevented
from continuing westward by the rising slopes on the west bank of Columbia River and
the damming effects of the folded, E-W anticlinal ridges. These two effects serve to
focus the erosive energy of the flood to cut Frenchman Coulee.
We also see here the spectacular examples of basalt flow stratigraphy. Flow have a clear
stratification that is related the flow and the cooling of the flow. Remember that these are
very low viscosity flows that likely originated in southeaster Washington and continued
all the way to the mouth of the Columbia River. A typical flow consists of four zones.
The upper and lower parts are chilled quickly and may be highly fractures and fill of
large pores from escaping gasses. These are collectively known as flow tops or flow
bottoms, and can be high porous, extremely permeable to water flow, and rather weak
mechanically. In the core of the flow, where cooling proceeded more slowly one has
cooling joints that are named for the architectural elements of classical Greco-Roman
buildings, specifically a set of vertical columnar joints (colonnade) and an overlying
thickness of more chaotic cooling joints known as the entablature.
If we have time for a stop at Gingko State Park here are some questions:
Why is the petrified forest here – at this stratigraphic level?
What is the dominant rock type present?
Are there other rock types here? What are they? How did they get here?
Think about “hot spots”. What may Columbia basalts have in common with the
Hawaiian chain?
CEE 437
Field Trip Health and Safety
Traffic! Our biggest potential source of injury is traffic. It is important to:
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Follow all instructions
Stay out of the roadway as much as possible
Check for traffic before leaving vehicles
Check for drop offs when leaving vehicles
Stay with the group – do not wander off
Look carefully before crossing any roads
On blind curves always be aware of possible traffic – listen for traffic and be
prepared to get out of the way
Rock Fall and Climbing Hazards
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Avoid working under tall unstable rock cuts and slopes
Stay in the areas instructed
Do not climb slopes unless instructed to do so
Do not roll or loosen rocks intentionally – people could be working below you
Stay clear of cliffs and other falling hazards
Rock Hammers
 No use unnecessary use of rock hammers – no use unless instructed and none
without use of eye protection unless on soft or unconsolidated material
Driving
 Drivers will observe speed limits and all traffic laws
 Passengers will wear seat belts
 Radios and cell phones are not for use by drivers while driving
 Drivers will check for all passengers before leaving a stop
Snakes, etc
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Be aware of snakes along trails
Avoid rock climbing where snakes may be present
General Precautions and Safety Consciousness
 Wear shoes and clothes appropriate to the weather conditions
 Look out for yourself and others
 Point out any other unsafe behaviors to trips leaders or other students