NORTH SEATTLE COMMUNITY COLLEGE
Geology 101
Summer 2011
Linda Khandro
From Seattle to the Columbia River; a trip through time and terrain.
FIELD TRIP DATA AND ROAD LOG:
Introduction:
This field trip will take us across a variety of geologic events and times, beginning with glacial
sediments, marine (coastal) sedimentary rocks and fossils with evidence of volcanic activity in
the sediments; we continue east on I 90 past Snoqualmie Pass, Cle Elum, and Ellensburg to
Vantage on the Columbia River and beyond. En route we will be driving along the river valley
of the glacial Snoqualmie River, noticing the terraces and deltas on either side of the highway,
along with the remnants of ponded glacial lakes, and river deposits. At the Snoqualmie region,
we will be crossing the Snoqualmie batholith, a granodiorite intrusion of younger age than the
Eocene marine and terriginous sediments and volcanics to the west and east. After Cle Elum,
the geology and topography is dominated by the Columbia River Basalts (CRB’s). The
following guide consists of a general outline of the rock types and ages, along with a road log
with approximate mileages to each feature.
Field Trip Report (50 points):
You will have until the end of the quarter to submit a completed Field Trip Report; this will
consist of a summary of the geology and your experience, in addition to the questions that
you’ll find in the Field Guide. Please write your answers (in pencil) in the Field Guide as we go
along, but then submit a separate page(s) with those questions and answers with your Field
Trip Report.
General Geologic Descriptions:
In our drive from west to east across about 1/3 of Washington State, we go back and forth in
time. The first bedrock we come to after we cross Lake Washington is sedimentary and
volcanic rock of Eocene age and younger (post 53 mya); it is called “The Puget Group”. A little
further along (inland) we find the volcanic Naches Formation of similar age. All along this part
of the route, the landscape has been transformed by glacial erosion and deposition, with the
most recent age about 10,000 years. The next bedrock is the Snoqualmie Batholith (17 my),
which is followed by the approximately 53 – 44 mya “Challis Sequence”. Our last bedrock will
be the Columbia Flood Basalts, also called Columbia River Basalts (CRB’s) at 15-5 my old.
Finally, we will see more glacial evidence in the form of the results of the Ice Age Flood events
between about 1 million and 10,000 years ago. The guide to the rock types below follows our
route, so you will see the ages go back and forth in time! (Note: "Ma" after a geologic age
means, Mega Anum, or million years.)
ROCKS AND FEATURES CROSSED BY OUR FIELD TRIP ROUTE
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1. The Puget Group and the "Issaquah Alps" (53-25 Ma)
A. Raging River Formation: The oldest rocks exposed in the Issaquah Alps are a series of
siltstones, sandstones and some coarser sedimentary rocks (conglomerates), which were
deposited as marine sediments in early Eocene time (prior to 53 Ma) Particles in these
sediments were mostly of volcanic origin, indicating that volcanoes were part of the
environment then. In early Eocene time the area was part of a slowly subsiding coastal plain,
extending at least as far east as the present Cascade Mountains, which did not exist in their
present form yet. The Raging River formation fossils include clams, snails, and sea urchins.
B. Tiger Mountain Formation: As the sediments accumulated and/or the sea retreated, nonmarine sediments were deposited. Land-plant fossils and some coal beds are found in the
shale, siltstone and sandstone of the 46-44 Ma. Tiger Mountain formation.
C. Tukwila Formation: Later in Eocene time (44-38 Ma), volcanic activity must have
increased in the area. Igneous rocks (lavas and intrusives) of this age are found locally, south
of Issaquah. 3000 feet of volcanic detritus (conglomerate, ash and tuff sediments) derived
from and associated with the newly erupted lava constitute the Tukwila formation. Interbedded
with the volcanic deposits of the Tukwila formation, are some fine-grained sediments that were
deposited in, and preserve a record of, short, quiet intervals between eruptions. Fossil leaf
impressions are common here
D. Renton Formation: Towards the close of the Eocene, early Oligocene (38-36 Ma), the
volcanic activity that produced many of the Tukwila rocks ceased, at least temporarily. The
Issaquah area was part of a flat, coastal plain almost at sea level. Over this plain, slow-moving
rivers deposited very fine-grained sediment. Marshes and swamps, some with brackish water,
contributed to the sedimentary load; vegetation must have been lush and abundant. The
siltstone, shale, carbonaceous shale and coal, up to 1500 - 3000 feet thick in all, is called the
Renton formation, and it preserves a record of the sediments and masses of vegetation that
covered the Tukwila and earlier deposits at that time.
E. Blakely Formation: In Oligocene time (40-25 Ma) the sea had risen over the
predominantly continental sediments of the Renton formation, covering them with sand and
silt. Marine clams and snails, now fossils in sandstone and siltstone, are similar to those in
sedimentary rocks found in the Blakely - Bainbridge Island area. This is thus called the Blakely
formation. Several zones in the formation are noticeably white with seashells. The Blakely
formation is the youngest of the pre-glacial deposits found in the Issaquah area.
During the following Miocene Epoch (25-5 Ma) erosion rather than deposition dominated the
landscape. Western Washington was uplifted and folded along northwesterly-trending axes.
Subsequent erosion during the last 20 Ma has differentially carved and exposed the more
resistant Tukwila-age rocks as the dominant cliffs and high points of the Issaquah Alps. The
Ice Age of 1.6 Ma hastened this erosion, both by the abrasive action of embedded rock
particles in moving ice, and by the torrents of sediment-laden meltwater that flowed between
and around each of the Issaquah Alps.
(Pistrang, 1981; Galster & Laprade, 1991)
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2. Naches Formation (44-38 Ma)
The Naches Formation extends from near Snoqualmie Pass to south of Easton. This later
Eocene unit consists of rhyolite, andesite, basalt, feldspathic sandstone, siltstone, and rare
coal. We will only be observing rhyolite on this field trip. The rhyolite is typically aphanitic and
white, with limonitic weathering. Some of the rhyolites were probably pyroclastic flows, though
no source is yet known for such flows.
(Frizzell et al, 1984; Cheney, 1994; Cheney, 1986)
3. Snoqualmie Batholith (16-18 Ma)
The Snoqualmie Batholith is one of several major intrusions of igneous rock in Washington.
While some date as old as about 90 Ma (Mount Stuart Batholith), the Snoqualmie event has
been dated radiometrically as between 16 - 18 Ma and is thus the youngest. Dates as young
as Miocene are rare for large, exposed batholiths, since it means that considerable uplift and
erosion must have taken place in fairly recent times. Young batholiths exposed at the surface
mean recent vigorous mountain building. The emplacement of these batholiths roughly
coincides with the folding and faulting of other rocks in Oregon and Washington, the Coast
Range, and the Blue Mountains, and the extensive lava flows of the Columbia Flood Basalts.
The Snoqualmie Batholith is a granodiorite, with abundant, well-formed hornblende crystals,
and large phenocrysts of quartz and feldspar. It has intruded older volcanic and sedimentary
rocks.
(McKee, 1966, and other sources)
4. Challis Sequence - Western Succession (53-44 Ma)
A. Swauk Formation: Best exposed between Leavenworth and Cle Elum, the prior to 53Ma
Swauk Formation consists of several stream-dominated sediments, including sandstone,
conglomerate, and arkose. In the region of Swauk Pass, this rock was intruded by basalt dikes
of the Teanaway Formation (Teanaway Dike Swarm).
B. Silver Pass Volcanic Rocks: These volcanics (53-50 Ma) consist of dacitic to andesitic
tuff and breccia, interbedded with arkosic sandstone like the Swauk Formation.
C. Teanaway Basalt: Intermittently exposed as ridges in the Cle Elum area, and as the
Teanaway Dike Swarm cutting Swauk sedimentary rocks, the Teanaway Basalt (50-46 Ma)
consists of basalt, basaltic tuff and breccia, but also includes andesite, dacite, and rhyolite.
D. Roslyn Formation: Another sedimentary group of Eocene-age rocks (46-44 Ma), the
Roslyn contains non-marine micaceous, feldspathic sandstone, with lesser amounts of
siltstone and conglomerate. Some coal seams are found in the upper regions.
5. Columbia River Basalt Group (CRB's) (17-6Ma)
The Columbia River Basalt Group (colloquially called "CRB's") consists of a thick sequence of
about 300 continental basalt flows erupted over an 11 million year period, between about 17
and 6 Ma. These flood-basalt flows cover an area over 164,000 km 2 in Washington, Oregon,
and western Idaho, and have a total estimated volume of more than 174,000 km 3. The source
for these flows was a series of north-northwest trending linear fissure systems in eastern
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Washington, northeastern Oregon, and western Idaho. Although the eruption of CRB's spans
an 11 million year period, most of the basalt (>96% of volume) was erupted over a period of
about 2.5 million years, between 17 and 145.5 Ma. During this peak period of activity, many of
the erupting flows were of extraordinary size, exceeding 1,000 km3 in volume, and traveled
many hundreds of miles from their vent systems. Detailed study and mapping of the CRB
demonstrate that significant variations in lithologic, geochemical, and paleomagnetic polarity
properties exist among flows (and packets of flows). These variations have enabled the
establishment of stratigraphic units that can be reliably identified and correlated on a regional
or large scale basis.
6. Ice Age Floods or Glacial Lake Missoula Flooding (2 Ma-10,000 ya)
About 2 million years ago, a cooling global temperature provided conditions in the Northern
Hemisphere favorable for the creation of great mile-thick sheets of moving ice called glaciers.
Thus began the Pleistocene Ice Age. In western North America, glaciers flowed south out of
Canada, damming rivers and creating temporary lakes in Washington, Idaho and Montana.
One especially large lake, covering a portion of northwest Montana grew in size until its ice
dam broke and its impounded waters rushed across northern Idaho, eastern and central
Washington, ultimately reaching the Pacific Ocean. And this event was repeated between 40100 times, before the mass of ice was insufficient to cause further impoundment. The resulting
effect on the landscape of Washington was unprecedented, and unique in the world. Great
channels of the tilted and gently folded basalt bedrock were ripped out, forming the Channeled
Scablands we see today, including the massive, now bone-dry coulees. In other areas, ripple
marks from the flowing water can be seen as rolling hills hundreds of meters apart and dozens
of meters high; enormous gravel bars in slack-water areas, plunge pools, abandoned
waterfalls, and other features formed by running water where there is now no water flowing
can be seen, and all of it creates a sere, spectacular landscape of unparalleled violence
ROCKS AND FEATURES OBSERVABLE ONLY FROM A DISTANCE
1.Ingalls Tectonic Complex
This fragment of accreted terrane is a slice of Jurassic to Cretaceous crustal and basement
rocks intruded by the 93-90 Ma Mount Stuart Batholith. It contains foliated and massive
serpentinite, gabbro, metabasalt (greenstone), amphibolite, argillite, and other deep-sea
crustal and sedimentary rocks. The ultramafic units are an ophiolite assemblage.
2. Easton Metamorphic Suite
There are two subdivisions of the Easton Metamorphic Suite, greenschist and phyllite. It is
interpreted to be the offset equivalent of the Shuksan Greenschist and Darrington Phyllite
found west of the Straight Creek Fault in the Skagit Valley. These units have a late Jurassic
protolith age, and an Early Cretaceous metamorphic age. The relationship between the Ingalls
and Easton rocks is obscured by the wide belt of Challis rocks north of Cle Elum.
ROAD TRIP LOG
Actual driving mileages are as accurate as possible, but where possible, we refer to
road-side Mile Posts (MPs) and Exit numbers.
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STOP 1 Puget Group:
Exit 13 off Interstate 90. Turn right after the exit, then right again onto Newport Way. Drive
about 100 yards to the end of the concrete wall. Park on the right side, cross Newport Way
carefully and cross the ditch at the bottom of the outcrop.
This is the Blakely Formation. Look for the following features:
1. Bedding planes. Which way are the beds dipping? This exposure of the Blakely is on the
north limb of the NW/SE Newcastle anticline. Walk up and down the outcrop from the
roadside, and find other evidence of folding and deformation.
2. Look at the grain sizes. Is this coarse grained sand, or finer grained silt and clay? Are the
grains well rounded, or sharp and angular? The formation is young and soft, so it has eroded
and continues to erode easily. There are varying degrees of solidity to sedimentary rocks!
3. Look for fossils. Some of the marine shells will still show their calcite shells. Can you find
clams (concentric rings in the shells), and mussels (elongated smooth shells)? Can you find
snails? Especially, break open some of the talus pieces loose on the ground, and see if you
can find bits of wood. If you find wood, what does that tell you about how far from land this
sediment was deposited?
Set Odometer or Trip Meter to 0 here
Mile 0.0 Enter I 90 at Exit 13 (which is actually at Mile Post (MP) 14):
As we drive east on I 90 towards Snoqualmie Pass, watch for the following features, visible
from the car:
Mile 0.7 Lake Sammamish on the left occupies a former glacial trough, gouged out of the
landscape by the glacier(s) here; as was Lake Washington. Look ahead to see a flat terrace
on the east side of the valley. It is a delta, built in a lake that was adjacent to the glacial ice
that occupied the trough now filled with Lake Sammamish.
Mile 3.1 East of the trough, the highway to Preston follows the channel of the stream which
built the delta. The delta consists of outwash gravels from the glacier and you may see the
mining operation here. The sand and gravel sediments are already sorted and relatively easy
to mine.
What caused the sorting of these sediments into different sizes of sand and gravel?
_______________________________________________________________________
Mile 6.6/MP 17+ Outcrops along the left side of the road are Eocene to Oligocene volcanic
(andesite) and sedimentary rocks, possibly Tukwila Formation. Look for further volcanics also
at Mile 7.8, 12.6, MP 26, Mile 16.1 and 22.9.
What is the age range for the Eocene?__________The Oligocene?____________
Mile 12.6/MP 27+ Mount Si is on the skyline at 12 o’clock. Mt. Si is a massive volcanic
breccia, with deep-sea sediments of Mesozoic age.
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What is the age range for the Mesozoic Era?_________________. Did Mount Si form “in
place” here? _____How do we know one way or the other?
________________________________________________________________________
If you can see the mountains in the distance, they are largely granodiorites of the Snoqualmie
Batholith. The low hills ahead and to the right of the highway are terminal moraines deposited
by the last of 4 glacial episodes. The Middle and South Forks of the Snoqualmie River have
since breeched the moraines, and the highway goes through one of those gaps.
What is a “terminal moraine”?______________________________________________
From MP 30 to about MP 37 there are many exposures of volcanic rock, especially on the
north side of the highway. Volcanic rock can often be recognized from a distance by its brickreddish color due to weathering.
Which of the chemical weathering processes (hydrolysis, dissolution, oxidation) often
creates this reddish color?______________Name one common silicate mineral often
present in volcanic rock that would be the source of this weathered color?__________
Mile 21.6+ The highway is climbing the ice contact front of the terraced moraine
embankment. The Vashon glacier dammed off the valley during the early part of its advance.
The resulting lake that was impounded up-valley acted as a depositional basin for the debris
being washed off the glacier; sand and gravel was deposited close to the ice, and silt and clay
farther up the valley. Fine-grained silt and clay-sized deposits from glacial outwash are often
very finely layered, and thus called “varved”. As the glacier advanced and spread out laterally,
it overrode earlier deposits, and the streams discharging off the ice built new gravel terraces at
higher elevations.
Mile 23.2/MP 37+ The finely layered clay and silt beds on the left side of the road, high up on
the bank, are varved beds, originating from the sides and terraces of the lake which was
formed when the Vashon glacier dammed off the valley.
Mile 26.2/MP 40 We are entering the granodiorite of the Snoqualmie Batholith now, although
this rock will alternate with Naches Formation volcanics, into which the batholith intruded.
Exposures in the granodiorite outcrop will be of massive, light colored, well-jointed, coarsegrained dioritic type rock.
Define granodiorite in general terms of its texture and composition________________
________________________________________________________________________
Why do we say that the granodiorite intruded the volcanic rock? In other words,
couldn’t the volcanic lava have flow over the granodiorite? Explain:
_______________________________________________________________________
STOP 2 Snoqualmie Pass:
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Mile 39.7/Exit #53 East Summit Snoqualmie Pass Turn off I 90 here for a brief pit stop.
East of the ski area, we will look at a small exposure of the phyllite subdivision of the Easton
Metamorphic Suite.
Mile 40 to Mile 44 To the south of I-90, Lake Keechelus is the dammed upper region of the
Yakima River. Additional reservoirs in this area are Lake Kachess, and Cle Elum Lake. On the
north side of I-90 is Keechelus ridge, made of various units of the Naches Formation volcanic
units.
MP 61 The rocks here consist of aphanitic, white-weathering volcanic rocks of the Naches
Formation. In this area we cross an important N/S trending right lateral strike-slip fault, the
Straight Creek Fault, which extends into B.C. as the Fraser River Fault. The Straight Creek
Fault separates the Naches Formation from the Challis Group.
MP 66-67 The outcrops here are a succession of northwest-pointing rocks dipping steeply to
the southwest. See if you can see this orientation. The most westerly unit is structurally
uppermost, and is probably Teanaway Basalt; the easterly, lower unit is probably Silver Pass
volcanics. Along with the Roslyn Formation, all are units of the Challis group. Note that when
we say “structurally” uppermost, we mean in terms of “which came first”. So this is regardless
of what later changes happened to the landscape, such as uplift, erosion, folding, etc.
Consider the ages and geometry here: Teanaway Basalt of age_____ is above Silver
Pass Volcanics of age _____, and both are slanting steeply down to the SW. What could
have created this slant?_____________________________________________________.
If we wanted to find the Roslyn Formation nearby, we might look further to the SW and
structurally above the Teanaway Basalt. Or we might look to the NE and above the
Teanaway. Explain why we would look in either of these directions.
_______________________________________________________________________
MP 68 On the north side of the highway, note the green, volcaniclastic rock.
What does “volcaniclastic” mean?_________________________________________
Mile 70 Now the ridge NE of I-90 (Easton Ridge) is made of Teanaway Basalt, but it is
followed by Cle Elum Ridge, composed of Roslyn Formation sedimentary rocks (look north of
Cle Elum).
Which rock is older, Teanaway Basalt or Roslyn Formation?___________. We have just
crossed a fold axis! What kind of fold is it, an anticline or a syncline?______________
Explain your reasoning____________________________________________________
(Go back to the questions for MP 66-67 for help)
Mile 70/Exit 84 Turn off I90 at Exit 84, enter Cle Elum, and stop at the Cle Elum bakery on the
north side of the main road about ½ way hrough the town. This is a very BRIEF stop for
yumminess only; it does not count as a field stop!
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STOP 3 Teanaway River:
Mile 81 (approximately) Here we are crossing a glacial moraine, marking the terminus of the
glacier which advanced southeastward down Teanaway Valley. Cross the Teanaway River
and pull over to the south side of the road at the chained driveway. Park cars so that residents
of the driveway have access. Walk back down the road to the bridge, and descend the bank to
the streamside. Look SE along the stream to the cut-bank opposite the bridge. Notice the old
stream channel about 1/2 way up the cut-bank. The channel contains boulders, in contrast to
the finer sediments of the rest of the bank. Speculate on the origin of this channel, given the
fact that a lobe of ice filled the valley to the north, and likely caused a pond or lake in this area,
much the same as the ponded drainage lakes in the Issaquah/Snoqualmie area we saw
earlier. Whether or not there is evidence of a glacial lake, this is evidence of an earlier stream
flow.
Walk back to the cars on the north side of the road to see the meandering stream processes
there. The “cut banks” are where the stream flow aims directly into the bank, eroding them into
steep slopes. The “point bars” are on opposite sides of the cut banks, where the velocity of the
stream is much slower, allowing sediment to be deposited there.
Turn back west on SR 970, toward Ellensburg. At the junction with SR10, bear right onto
SR 10 toward Ellensburg.
RE-SET THE TRIP METER HERE TO 0. We will make unscheduled photo opp stops
along this beautiful stretch of road as we find them and as time permits.
East of Cle Elum, look back to the northwest to see the 90 Ma Mount Stuart batholith at the
core of the Mount Stuart Range. The Challis volcanic and sedimentary rocks which abut Mt.
Stuart are Eocene (57 - 36 Ma) in age. Think now on the resistant granite of the batholith
poking up through this carapace of surrounding younger volcanic and sedimentary layers.
Which came first, the batholith or the surrounding rock? __________________
If the batholith came first, crystallizing at deep depths beneath the surface, then why
does it appear as higher relief than the Challis rocks?
______________________________________________________________________
Is the Cascade Range itself in motion somehow? Think about what happens to the crust
when compressive stress occurs during mountain building (remember the rug???).
_________________________ and _________________________
Along SR 10, we begin to see the gently dipping and folded, but relatively flat-lying Columbia
River Basalts (CRB's). Note the flat topography to the east and south as we exit the hills of
the Cascades. The rock under us is CRB, which may originally (Miocene age) have flowed up
and over portions of the pre-uplifted Cascade area behind us.
Thus, what is it we are driving on? An ______________ surface.
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Now look east to the eastward dipping surfaces of CRB's. It has been suggested that the
CRB's flowed over the terrain to the west before the uplift of the older Cascade volcanic and
sedimentary structures began; some CRB rock can even be found at the coast near Astoria.
Why don't we see any (or enough) CRB's on the tops of the Cascade peaks, such as Mt.
Stuart to confirm this idea?_______________________________________________
SR 10 skirts the edge of the canyon here containing the Yakima River. Look for occurrences
of point bars and cut banks.
Which of these comes from deposition on the inside of a meander?_______________;
from erosion on the outside of a meander?_________________.
Mile 15 (MP 98) Note the white sandstone cropping out on the left side of the road.
STOP 4 Ellensburg Formation:
Mile 15.3 (MP 98.3) Pull off on the right side, just after the guard rail, across from the outcrop.
This is the Ellensburg (formation), which has a peculiar relation to the CRB's. There is no
ONE Ellensburg Formation in time or space. It is a generic name for various interbedded,
partially fluvial (stream origin) sandy sediments which occur between some of the CRB flows.
Here, we have a thick Ellensburg section; in other places on the Columbia Plateau, there is no
Ellensburg at all. In some places the Ellensburg has pumice fragments in it.
What is the significance of finding pumice fragments in sediment?_________________.
What is the significance of finding any kind of sediment at all interbedded between
basalt flows?
__________________________________________________________________________
There is no mud or shale in these sediments; instead there are clean, quartz-rich sands. Put
yourself here during the deposition of Ellensburg:
Are we in a land (dry) or ocean (underwater) environment? Explain:_____________
________________________________________________________________________
If these sediments are eroded off nearby higher elevations and deposited by streams,
explain why the dominant mineralogy is quartz-rich rather than feldspar-rich.
_______________________________________________________________________
Mile 32 Junction of SR 10 with SR 97. Turn right on SR 97 toward I-90 east. At the
junction of SR 97 and I-90 east, enter I-90 toward Ellensburg. Continue eastbound on I90.
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Mile 65-72 (MP 125 - MP 132) Look to both sides of I-90 here for pillow basalts. Recall from
the discussion of basalt formation that pillows form as the basalt extrudes and is chilled by
contact with water.
In this case, is this sea water?_____Are we at an ancient continental margin?_____
If not, explain the presence of pillow basalts here_____________________________
_______________________________________________________________________
Remember that the age of CRB's is younger than the Eocene deposition of marine sediments.
Mile 60-61 (MP 132-133) Look on the right (south) side of I-90 for pillows, and holes from
former fossilized trees, which were living here, died, and were fossilized by the basalt flow. .
The green or yellow-brown coloring in between many of the pillows is the mineral palagonite,
which forms as hot basalt reacts with the water into which it is flowing.
Does this basalt form strong, upright columns or cliffs? Or does it form
slopes?__________
We find a variety of cliff-forming basalts, and slope-forming basalts in the CRB. What
does the nature of the landscape (cliff vs. slope) tell you about the resistance the rock
has to being weathered and eroded?_______________________________________
______________________________________________________________________
Mile 64, Exit 136 Take Exit 136 toward Gingko State Park. Turn left onto Museum Road, then
right into Gingko State Park and Gift Shop. Hang onto your wallets, checkbooks and credit
cards! This place is highly seductive. Or just go for it! But make this the END of your visit to
the Information Center up ahead - we are only allotting a very short time for this entire stop.
STOP 5 Gingko State Park and Fossilized Trees
Gingko State Park Information Center has excellent exhibits detailing the CRB's, the Ice Age
Floods, fossilization, books, maps, and Indian petroglyphs which are around the southeast
side of the building. These were moved up to this height when the river was flooded to create
the Wanapum Lake reservoir.
Drive back down the Museum road to the entrances to I-90 east. Enter I-90 east, cross
the Columbia River, bear left to stay on I-90 east.
STOP 6: Columbia River and CRBs
Mile 67 (MP 139) Pull into the scenic view point on the east side of the road for a good photo
opportunity of the Columbia River, the CRB's on the west side, and especially, the shallowdipping structure of the anticline to the northwest.
Mile 72 (EXIT 143) Take this exit toward the Gorge Amphitheater. Turn left toward the river
after the exit. Go straight at the first intersection (toward Gorge). Drive this road to the
intersection with a road named 2.5 SW. Drive toward the white diatomite quarry and park.
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STOP 7 Diatomite Quarry with Basalt Intrusions:
Diatomite is made up of the remains of millions of skeletons of diatoms, microscopic
organisms which make shells, called tests, from silica in lake and sea water. Here it is being
mined to be used for filtration systems in making wine and beer. The hot, intruding basalt
turned the diatomite into low grade opal by “cooking” it upon contact. Think about how glass is
made…sand (silica) is melted and quenched by dunking it into water.
Return to the Amphitheater road, turn left toward I-90 to the junction with Vantage Road.
Turn right onto Vantage Road, toward the river and Frenchman Coulee. Park at the
portapotties.
STOP 8 CRB and the Ice Age (Missoula) Megaflood!
This region of CRB units form large columns and entablature, as well as vesicles where the
basalt came into contact with either water, or moist soils. The coulee to our right was ripped
into being by side channels of the huge flood waters racing downstream; the basalt columns
are jointed, thus can be pulled apart.
STOP 9 (pointing out only) Glacial flood waters plunge pool and ecosystem
Drive a few hundred yards toward the Columbia River to a small pull-out on the right side of
the road. Park and walk back east from here into a small, unnamed coulee south of
Frenchman's Coulee (½ hr or longer walk). At the easternmost end of this coulee is a deep
plunge pool, where ice age flood waters cascaded over the edge of the basalt, ripping it away
as it fell. The pool and immediate surroundings form a miniature ecosystem with cattails,
willows and other moisture loving plants. Swallows swoop, dive, and tunnel in the vesicles,
feasting on rich insects over the water.
STOP 10 (this is our LAST stop!)
Return to the cars, and drive the rest of the way to the Columbia River. Wash your hands,
face, feet; take a moment to savor this extraordinary environment, and make yourself a
promise to come back again, next time with other friends and family!
NOTE: this is rattle snake country. Exercise intelligence and caution. Watch your step
and keep your hands out of dark places.
References
Cheney, Eric, 1994; Cenozoic Unconformity-Bounded Sequences of Central and Eastern Washington, WA Division of
Geology and Earth Resources
Cheney, Eric, & Bush, Tom, 1996; Guide to the Geology in the Vicinity of Swauk and Snoqualmie Passes, Central Cascade
Mountains, Washington, NW Geological Society
Frizzell, Virgil et al, 1984; Preliminary Geologic Map of the Snoqualmie Pass 1:100,000 Quadrangle, Washington, US
Geological Survey
Galster, Richard & Laprade, William, 1991; Geology of Seattle, Washington, USA, Bulletin of the Association of Engineering
Geologists
McKee, Bates, 1966; Cascadia
Pistrang, Martin, 1981; Bedrock and Bootsoles, an Introduction to the Geology of the Issaquah Alps,
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