A Report on the Geology of Mirabeau Point Park

A Report on the Geology of Mirabeau Point Park: an
Undergraduate Service-Learning Research Project
Justin Vaughan, Spokane Community College
Travis Taylor, Spokane Community College
Andy Buddington, Spokane Community College, [email protected]
INTRODUCTION
Mirabeau Point Park is located in the central Spokane Valley area along the southwest bank of the
Spokane River. Comprised of approximately 56 acres, the park occupies the old Walk in the Wild Zoo site
and is presently administered by the City of Spokane Valley. This project was conceived as a cooperative
between the City of Spokane Valley and Spokane Community College as a service-learning opportunity
for geology students.
Although the site is located within the heart of the urban Spokane Valley, this area has remained
relatively undeveloped and offers a scenic rural forest setting. The park has numerous walking trails that
weave through abundant hilly outcrops, which ultimately make for an excellent field study site for both
students and the public.
Mirabeau Point Park has a long-lived and complex geologic history. The park occurs in the southern part
of the Spokane Dome of the Priest River metamorphic complex (PRMC), (Doughty, et al. 1997). The
bedrock geology here is composed primarily of Newman Lake gneiss, a Mesozoic-aged granitic
orthogneiss (Derkey, et al., 2004, Armstrong, et al., 1987). Extensive pegmatite veining occurs
throughout the orthogneiss. In essence, the park is part of a larger bedrock “island” within the Spokane
Valley, surrounded by a thick sequence of Missoula flood deposits. Throughout the park, the Newman
Lake gneiss exhibits strong foliation, mineral lineation, and pervasive mylonitization. The mylonitization
(blastomylonite) is characteristic of the upper part of the PRMC within the Spokane Dome mylonite
zone. Bedrock knobs and adjacent gullies have a consistent NNW-trend suggesting possible structural
control to topography. Scattered throughout the park are cobble to boulder-sized Missoula flood
deposits and possible dropstones. Outcrops adjacent to the Spokane River exhibit various fluvial
erosional features.
Mirabeau Point Park has numerous quality examples of basic and complex geology that are easily
accessible to the public. This undergraduate service-learning research project was developed to create
earth science awareness and education opportunities for the general public that utilize the park. Field
studies and interpretations will allow for the development of informational displays, a guided walking
tour, and potential lesson plans for K-12 field trips.
GENERAL BEDROCK GEOLOGY
The major bedrock type throughout the park is the Newman Lake Gneiss, which is a light-colored,
moderately to well-foliated orthogneiss of granodiorite composition. Gneiss is the term used to describe
metamorphic rocks that form from high temperature and high pressure recrystallization of some
preexisting rock (parent rock). The parent rock could have been either sedimentary or igneous. An
orthogneiss is a specific type of gneiss that is the result of metamorphism of an igneous parent. The
dominant minerals within the Newman Lake Gneiss include quartz (glassy, gray), orthoclase (milky
white), biotite (shiny, black), and plagioclase (white to gray). Regionally, outside the park, the Newman
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Lake Gneiss is characterized by very large, coarse megacrysts (crystals) of orthoclase; however, within
the park megacrysts are rare.
Newman Lake Gneiss from Mirabeau Point Park. Note foliation of dark biotite with light-gray quartz and
milky white feldspar.
“Typical” Newman Lake Gneiss (with large, orthoclase megacrysts) that occurs outside the park, north of
the Spokane Valley.
Pegmatite bodies intrude the gneiss sporadically throughout the park. Pegmatite is a coarse-grained
(crystals larger than 2 cm.) igneous rock that is typically light-colored and occurs as veins such as dikes
and sills (see Site 2, Guided Walking Tour below). The pegmatite at Mirabeau Park is white to light-gray
in color and consists primarily of coarse-grained orthoclase and quartz, with some plagioclase and mica.
The intrusive bodies of pegmatite range in thickness from a few centimeters to several meters, and
occur as crosscutting dikes, lenticular pods (that are parallel to foliation), and boudins. Boudins are lensshaped masses containing light-colored crystals. One boudin (“The Boudin”) in particular is almost five
feet in length and two and a half feet wide (see Site 6, Guided Walking Tour below).
The bedrock geology map (see below) was created to illustrate bedrock outcroppings identified
throughout the park. Included on the map are strike & dip measurements which indicate the orientation
of the rock foliation along with the trend and plunge of mineral lineations.
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ROCK TEXTURES, FABRICS, AND STRUCTURE
The most notable characteristic of the Newman Lake Gneiss within Mirabeau Point Park is the
moderately to strongly foliated texture and mylonitic fabric. Foliation is a texture of aligned crystals,
which can form most often during high pressure metamorphism. Foliation within the Newman Lake
Gneiss is generally defined by aligned biotite and bands of quartz and feldspar (see photo above).
Locally, foliation occurs as alternating felsic-mafic bands consisting of orthoclase and quartz (felsic, lightcolored) paired with biotite-rich bands (mafic, dark-colored). Foliations along the western side of the
park dip to the west. Along the eastern side of the park, within outcrops along the Spokane River,
foliations dip to the east, suggesting possible folding. Lineations are defined by elongated minerals (or
mineral aggregates) within the plane of foliation and are the result of high-pressure metamorphism and
shearing during metamorphism. Within Mirabeau Point Park, lineations occur on foliation surfaces as
elongated quartz stringers or ribbons, and generally trend ENE-WSW with both westerly and easterly
plunge (tilt) directions. The lineations record significant shearing, which occurred as a form of
deformation resulting from shearing stress (both squeezing and stretching of the rocks) during
metamorphism. Shearing stress is common in large fault zones or deep tectonic zones where the crust is
being subjected to intense and complex pressures due to plate movements.
Mylonitization of the Newman Lake Gneiss is moderate. Mylonite is a fabric that develops in some rocks
that undergo significant shear-related deformation due to squeezing and flattening. At Mirabeau, the
original igneous textures (large megacrysts) are difficult to discern because the variable amounts of
cataclasis (grinding and crushing) and recrystallization ultimately developed blastomylonite. The
blastomylonite is characterized by well-developed quartz ribbons, stringers of ground orthoclase, and
recrystallized biotite-rich bands (see Site 8, Guided Walking Tour below).
Late fractures crosscut the foliated bedrock. Derkey et al. (2004) describe NNW-trending fractures and
associated topography just west of the park and north of the Spokane Valley. Within the east-central
portion of the park, the study recognized defined ravines trending NNW. These ravines are interpreted
as fractures or possible fault structures parallel to those mapped by Derkey to the west.
TEXTURE, FABRIC, AND STRUCTURE DISCUSSION
At Mirabeau Point Park, the Newman Lake Gneiss exhibits a well-developed foliation as well as mineral
lineations and mylonite fabric. The combination of these three features is a good indication that the
rocks have undergone a significant amount of deformation as a result of being subjected to intense
stress. As mentioned above, outside the park, the Newman Lake Gneiss exhibits characteristic large
megacrysts of feldspar. But at Mirabeau, the original igneous textures and megacrysts have been for the
most part, obliterated, as a result of intense and complex pressures. These features are also seen in
other parts of the Spokane Dome mylonite zone, which is part of the larger Priest River metamorphic
complex (McCallum and Buddington, 2009, Derkey, et al., 2004, Doughty, et al., 1997). The Spokane
Dome mylonite is a diffuse zone of mylonitization that formed during uplift and detachment of the
Priest River complex. In essence, the mylonite zone represents a large shear (fault) zone that developed
deep in the crust as a result of intense squeezing and sliding pressures that the region was subjected to
between 30 to 50 million years ago. So, the rocks at Mirabeau represent a portion of the crust that was
once a large igneous mass (granodiorite) that got entrained into a large, deep shear (fault) zone
(Spokane Dome mylonite zone). The foliation, lineations, and mylonite developed as the granodiorite
was being deformed within the shear zone. This was all later cooled and uplifted to the surface, where it
was eroded and exposed to what we see today.
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Bedrock map of Mirabeau Point Park
SURFICIAL GEOLOGY
Along the eastern side of the park, outcrops (up to 60 feet high) of gneiss dominate the topography. In
one area, named “The Gorge”, several large angular blocks (up to 40-50 feet high) have separated (up to
30 feet) from the main outcrop along large fractures (see Site 5 of the Guided Walking Tour below).
Measured foliations of the gneiss blocks are discordant to the main outcropping due to displacement.
We interpret these blocks to have separated soon after undercutting erosion during the Missoula
Floods, which was then followed by pervasive fracturing due to freeze-thaw. The undercutting and
fracturing promoted collapse and displacement.
Numerous erratic boulders (“The Erratic” and “The Cannon Balls”, see Sites 3 and 7 in the Guided
Walking Tour below), ranging in size from a few feet up to fifteen feet in diameter, have been identified.
The boulders are rounded and composed of non-foliated, coarse-grained hornblende and biotitebearing granitic rock. The nearest potential source of the granitic rock is likely northern Idaho. The
boulders were either carried as ice-rafted erratics or were moved as bed material during the Missoula
Floods.
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Outcrops of gneiss along the Spokane River boast numerous potholes (see Site 9, “The Potholes”,
Guided Walking Tour below). The potholes range up to two feet in diameter and three feet deep. They
contain rounded gravel and cobble-sized materials. The potholes do not occur above the recognized
high water level of the river. We interpret the potholes to be the result of modern, post-Missoula Flood
erosion by the Spokane River. The cobble-gravel aggregates within the potholes acted as abrasives
during high flow water circulation over and about the outcrops.
GEOLOGIC HISTORY OF THE SPOKANE VALLEY AREA
The known geologic history of Mirabeau Point Park and its immediate surroundings can be traced back
to about 1.1-1.4 billion years ago. At this point in Earth’s past, an ancient time period geologists term
the Precambrian, the entire area was part of a massive basin that became a large inland sea. During this
time sea levels rose enough to inundate large portions of northeastern Washington, northern Idaho, and
western Montana. Within this large inland sea, known as the Belt Sea, thick layers of sediment were
deposited over a very long period of time. These sediments were then lithified (cemented) into
sedimentary rock.
About 140-200 million years ago, during Earth’s Mesozoic time period, a large magma chamber intruded
(invaded) the sedimentary layers deep underground. As it cooled, the magma turned into a coarsegrained igneous rock called granodiorite. Igneous is a term used to describe rocks that are formed from
the complete melting of rock, then the cooling of magma. Granodiorite is very similar to granite, a rock
commonly used for countertops. This granitic intrusion would one day become the rock that you are
walking on here at Mirabeau Point Park.
Approximately 100 to 150 million years ago, during Mesozoic time, collision of the plates beneath the
earth’s crust caused the deep burial of the sedimentary layers and granodiorite. This tectonic activity led
to metamorphism of the rock here at the park. Metamorphism occurs when minerals in a rock are
changed due to contact with a lot of heat, plus or minus pressure. The change from one type of rock to
another occurs without completely melting the original rock, such as occurs in the formation of igneous
rock. There are two basic types of metamorphism, regional and contact. Contact metamorphism occurs
at high temperatures, with no pressure. Regional metamorphism also occurs at high temperatures.
However, this type of metamorphism is characterized by immense pressure when tectonic plates of
crust collide. This pressure causes the visible parallel alignment of minerals (foliation) in the rock.
Locally, the deep burial of the granodiorite caused it to be regionally metamorphosed into gneiss, the
well-foliated metamorphic rock we see throughout the park. In 1968, a geologist named Paul Weiss
identified and named this particular unit of rock the “Newman Lake Gneiss”.
Eventually, the collision of the plates ceased, and about 40 million years ago, a very interesting thing
happened. The plates began to relax and extend apart! This is the very nature of plate tectonics. The
plates, miles beneath our feet, are constantly moving, shifting course, and reshaping our earth. This
change in tectonic activity led to a regional uplift of the earth’s crust, including the Newman Lake
Gneiss. Extensive faulting (fracturing and displacement) of the uplifted crust began to occur at that time.
A large fault, termed a detachment fault, developed deep in the crust as it was being uplifted. The
detachment fault allowed the crust to separate and slide during uplift, thus causing the mineral
lineations and mylonite to form.
Close to 17.5 million years ago, during the Miocene Epoch, huge amounts of lava erupted and flowed
over much of the Pacific Northwest, including the Spokane Valley and Mirabeau Point Park area. The
lavas, referred to as “Columbia River Basalts”, cooled into a fine-grained, dark-colored igneous rock
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called basalt. Thick layers of this basalt can be observed throughout Spokane and eastern Washington.
The basalt can be seen in the prominent cliffs along the north bank of the river to the north of the park.
In more recent times, approximately 15-20 thousand years ago, numerous massive floods marked the
end of the last ice age. These floods are thought to be some of the largest floods ever in Earth’s history.
The floods were caused by the periodic emptying of glacial Lake Missoula, a 3000 square mile lake in
Montana that held about half the volume of water of present-day Lake Michigan. When the flood
waters reached this area, they were traveling at about 60 miles per hour, and were several miles wide
and 300 to 400 feet deep! The raging torrent scoured out the valley we see here today, eroding it into a
much deeper and wider valley than had been here before the floods. These “Missoula Floods” were
slowed somewhat by the fact that this area was at the bottom of another glacial lake, Lake Columbia. As
this towering force of nature met Lake Columbia, it dropped a lot of its bedload, sediment consisting of
coarse sands, gravels, and boulders. The combined mass of water then sped off to the west and
southwest as one herculean flood, ripping apart the landscape in a frenzy of torrential violence on its
way to the ocean.
Present-day Mirabeau Point Park is located right on top of a 56 acre section of the Newman Lake Gneiss.
The gneiss is exposed in numerous outcrops throughout the park. This durable metamorphic rock has
withstood the forces of millions of years of violent tectonic activity, and several thousand years of
catastrophic flooding and erosion. The park is part of an “island” of bedrock that is surrounded by the
gravels and boulders deposited by those giant floods so long ago. To the north of the park, the Spokane
River continues to erode into the valley, and landscape development all around the park has
permanently redefined the surface geology. Still, inside the park, surrounded by the quiet beauty of
nature, very little of our earth’s history has been altered, except that which has been altered by the
earth itself.
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GUIDED WALKING TOUR
The Guided Walking Tour was developed to show some of the interesting features observed at the park.
The sites corresponding to the Walking Trail Map can be seen below.
Site 1 - Entrance Overview. Here in Mirabeau Park you are standing on a “bedrock island” which
protrudes from the gravels that underlie the valley and make up the Spokane Aquifer. If you look to the
north of the park you will notice some dark, prominent basalt cliffs. In the distant past, this basalt
covered the area you are now standing on. Great outburst floods from Montana, known to geologists as
the “Missoula Floods”, swept through this area, after being unleashed from the ancient glacial Lake
Missoula, following the collapse of a gigantic ice dam. Imagine a 300 foot wall of water, blasting through
like a tsunami, sweeping house-sized boulders away as if they were little pebbles, stripping the basalt
down until it reached the bedrock which you now stand on.
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Site 2 - Pegmatite Dike. As you head up the trail you will notice a prominent rock outcrop to your right.
This rock is the Newman Lake Gneiss. Gneiss is a type of metamorphic rock that forms deep
underground, under intense pressure. If you look closely, you will see foliation, the aligned mineral
pattern typical of metamorphic rocks. The rock outcrop displays many fractures, faults, and light-colored
pegmatite dikes. Movement along faults is inferred easily from the occurrence of offset pegmatite dikes.
This happens when a fault (or vein) cuts through the dike. Movement displaces the dike above and
below the fault.
Site 2: Newman Lake Gneiss with small, vertical dike offset and crosscut by a foliation parallel pegmatite
dike. Both are composed of feldspar and quartz.
Site 3 - The Erratic. Farther up the trail you will see what looks to be a plain, ordinary boulder. This
“plain, ordinary boulder” actually has some exciting geologic history. The boulder is made of granite
from Idaho, meaning it formed somewhere in Idaho, not Spokane. How did it get here? This boulder is
an erratic that was either swept here by the Missoula Floods, or floated here, lodged within an iceberg.
Once the iceberg melted, the boulder would have been dropped in its current location.
Site 3: “The Erratic”: A large erratic boulder composed of non-foliated granite.
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Site 4 - The Wall of Mosses. Here you can see another outcrop of Newman Lake Gneiss. This outcrop
consists of gneiss that has been heavily weathered and overgrown by lush vegetation. The trees around
this site block out the sun and allow moisture to collect, providing the perfect moist forest setting where
vegetation can grow and flourish. This wall of gneiss has been heavily overgrown by mosses, which
would dry up if exposed to too much sunlight.
Site 5 - The Gorge. On the back side of The Wall of Mosses you will find The Gorge. The Gorge is a deep
crevice in the rock that formed as a giant block of rock was detached and slid west of the main outcrop.
This is similar to how giant ice blocks will detach from ice sheets and fall into the ocean, except in this
case, the block was rock and fell onto hard ground, so that now it rests about ten feet from where it
detached. Because of its close proximity to the original rock body, the original contour of the fracture is
preserved very well. On the original rock structure you will notice a prominent overhang. Slightly
underneath and west of this overhang, you will notice a corresponding platform on the detached block.
The contours of the original rock body and the detached block line up like pieces of a jigsaw puzzle.
Site 5: one of several large detached blocks of bedrock within “The Gorge”.
Site 6 - The Boudin. As you walk just south of The Gorge, or The Wall of Mosses, you will see The
Boudin. Boudins are metamorphic geologic structures that form during shearing stress. As shearing
takes place, some more resistant minerals will tend to “glob up”. As the minerals around them are
sheared, stretched, and flattened out, these boudins will form resistant “globs” that are only slightly
elongated from the shearing stress. The result of this elongation is a geologic structure that resembles
the shape of an giant eye. Notice how the normally parallel layers warp around the boudin and meet
back up with one another.
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Site 6: “The Boudin”: A mega-boudin composed of quartz and feldspar can be found just south of “The
Gorge”.
Site 7 - The Cannonballs. On your way from The Boudin to The Waterfall, keep an eye out for The
Cannonballs. The Cannonballs are glacial erratics, which originated east of Washington and were swept
into this area by the Great Missoula Floods. The Cannonballs have been rounded from the floodwaters
tumbling them across the land, from their origin, all the way to their final resting stop. The way in which
The Cannonballs were rounded is a large scale version of what happens when you polish a rock in a rock
tumbler or what happens to boulders at the bottom of a normal stream bed.
Site 7: “The Cannonballs”. Two large rounded erratics.
Site 8 - The Waterfall and Quartz Ribbons. Although the Waterfall at Mirabeau Park is a man-made
waterfall, it still displays some interesting geology. The Waterfall pours over a giant wall of Newman
Lake Gneiss. Because the wall is wet, and not obscured by lichens and weathering, it allows the
fractures, faults, and dikes to be seen more easily. During the winter, water will freeze inside the
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fractures of the gneiss wall. This causes the cracks to expand. In the spring, when the ice melts, the
structure of the gneiss wall is weakened. With the liquid water from the falls acting as a lubricant, loose
pieces of rock will slide off the wall to the bottom of the falls. Several of these rocks, freed loose by the
action of the freeze/thaw cycle, can be viewed at the base of The Waterfall. In the rocks along the path
in front of the pavilion, look closely for quartz ribbons. Here the gneiss has ribbons 2 to 3 inches long of
gray quartz. These ribbons formed and became aligned during the metamorphism that created the
gneiss. Quartz ribbons are rare and indicate very intense metamorphic conditions from deep in the
crust.
Site 8: quartz ribbons in Newman Lake Gneiss.
Site 9 -The Potholes. If you walk toward the far, eastern side of Mirabeau Park, after you cross
Mirabeau Parkway, you will enter The Potholes. These potholes have been drilled out slowly by the
erosive power of the Spokane River and the sediments that scrape along its riverbed. As sediment is
swept downstream, some will inevitably collect in the depressions and low points in the riverbed. If the
depression is too deep for the river to sweep the sediments out, then the sediments will be trapped,
and will be pushed around in circles along the bottom of the depression. Over time, these loose
sediments act as a dremel tool, gradually eroding away at the depression, causing it to widen and
deepen. If you walk along the banks of the Spokane River you will see several of these interesting
geologic features.
Site 9: pothole in bedrock adjacent the Spokane River. Note the coarse aggregate at the bottom of the
pothole.
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Site 10 – The Gneiss Wall. Here is an excellent exposure of the main bedrock formation seen
throughout the park, the Newman Lake Gneiss formation. Gneiss is a metamorphic rock that forms as a
result of high temperatures and pressures. When preexisting (parent) rocks are subjected to extreme
temperature and pressure conditions, a variety of chemical reactions occur that cause new minerals to
form without actually melting. As a result of the intense pressure (and heat), the new minerals will form
in an aligned pattern called foliation. Foliation, or aligned mineral crystals, is a hallmark of metamorphic
rocks. In this outcrop you can see a variety of minerals such as quartz (gray), feldspar (milky white), and
biotite mica (black). If you look closely, you can also see foliation. What other interesting features can
you see in this outcrop? What do you think this rock was before it became metamorphosed?
Site 11 – The Forest. Our tour ends in the peace and quiet of the Mirabeau forest. Look around you and
note the topography; the small hills and low places. Note that there is not much rock exposed here. Why
is that? Weathering of rocks by the elements, rain, snow, and air, cause the rocks to decompose and
eventually develop into soil. Once soil starts to form, a variety of plants begin to take hold and
eventually a forest takes over the landscape. But keep in mind that not deep below the surface are the
rocks we have observed on the tour. Look around this forest and note all the different types of plants,
from the taller trees to the low-lying bushes and ground hugging mosses and flowers. Then think of all
the places bugs and animals can hide and live here at Mirabeau. Mirabeau Point Park is a special place
where the geology and biology have created a natural and undisturbed landscape. We hope that you
have enjoyed this tour and learned a little more about the earth and processes that led to the formation
of this park and our beautiful region.
SUMMARY
The geology and natural history of Mirabeau Point Park is both long and complex. Yet, because of
numerous bedrock exposures and excellent access by walking trails, the geology can be easily viewed
and experienced by park visitors. This project has mapped and documented many interesting features
seen throughout the park. The potential for educational displays and/or a guided walking tour would
create learning opportunities for the general public as well as local school groups. Mirabeau Point Park
offers a unique opportunity for the public to experience the natural history and beauty of our region.
ACKNOWLEDGEMENTS
We would like to thank Dr. Ted Doughty of Prism Geoscience for his thoughtful comments on our field
observations. We would also like to thank Laura Mincks of Spokane Falls Community College for help in
drafting the maps in this report.
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REFERENCES
Armstrong, R.L., Parish, R.R., Van der Hayden, P., Reynolds, S.J., and Rehrig, W.A., 1987, Rb-Sr and U-Pb
geochronometry of the Priest River metamorphic complex-Precambrian basement and its MesozoicCenozoic plutonic-metamorphic overprint, northeastern Washington and northern Idaho, in Schuster,
J.E., ed., Selected Papers on the Geology of Washington: Washington Division of Geology and Earth
Resources Bulletin, v. 77, p. 15-40.
Derkey, R.E., Hamilton, M.M., and Stradling, D.F., 2004, Geologic Map of the Greenacres 7.5-minute
Quadrangle, Spokane County, Washington: Washington Division of Earth Resources, Open File Report,
2004-11.
Doughty, P.T., Price, R.A., and Parrish, R.R., 1997, Geology and U-Pb geochronology of Archean
basement and Proterozoic cover in the Priest River complex, northwestern Unite States, and their
implications for Cordilleran structure and Precambrian continent reconstructions: Canadian Journal of
Earth Science, v. 35, p. 39-54.
McCallum, L.R., Buddington, A.M, and Doughty, P.T., 2009, GSA Cordilleran Section Meeting, Kelowna,
British Columbia, GSA abs. w/prog.
Weiss, P.L., 1968, Geologic map of the Greenacres quadrangle, Washington and Idaho, 1968, Weis, P.L.,
United States Geological Survey, GQ-734, scale 1:62,500.
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